Optimization of AA2024 Anodization for Oxy-Fuel Welding with PEI/Glass Fiber Composite

Optimization of AA2024 Anodization for Oxy-Fuel Welding with PEI/Glass Fiber Composite | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Optimization of AA2024 Anodization for Oxy-Fuel Welding with PEI/Glass Fiber Composite Marcos Paulo Souza Ribeiro, Edson Cocchieri Botelho, Ana Beatriz Ramos Moreira Abrahão, and 5 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8957869/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 5 You are reading this latest preprint version Abstract This study investigates the influence of anodizing parameters on the AA2024 aluminum alloy for joining with a PEI/glass fiber composite using the oxy-fuel welding (OFW) method. AA2024 samples were anodized in a 10% phosphoric acid solution under different voltage (10–30 V) and time (5–25 min) conditions. The hybrid joints were produced by OFW and evaluated by lap shear strength (LSS) tests using a full factorial experimental design. The anodizing condition of 10 V for 20 min exhibited the best mechanical performance, reaching 14 MPa. Electrochemical impedance spectroscopy (EIS), combined with wettability, roughness, optical microscopy, and scanning electron microscopy analyses, showed that anodizing promotes the formation of a porous and protective oxide layer, suitable for anchoring the polymer matrix. EIS results revealed a significant increase in corrosion resistance for anodized samples compared to the control sample, with the electrochemical response dominated by the overall behavior of the anodic film. Equivalent electrical circuit modeling confirmed the effectiveness of the barrier layer formed under optimized conditions. Overall, the results demonstrate the potential of anodizing as an effective surface treatment to enhance metal–composite bonding and improve the performance of hybrid joints produced by the OFW process. Anodizing Oxy-Fuel Welding (OFW) AA2024 Aluminum Alloy PEI/Glass Fiber Composite Lap Shear Strength (LSS) Corrosion Resistance Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 1. Introduction The growing demand for lightweight and high-strength materials has driven the use of aluminum alloys and fiber-reinforced polymer composites in structural applications, especially in the aerospace and automotive sectors. The AA2024-T3 aluminum alloy is widely used due to its high mechanical strength resulting from copper addition, but it exhibits low resistance to localized corrosion [ 1 ] [ 2 ] [ 3 ]. To overcome this limitation, anodizing is applied as an electrochemical process that forms an aluminum oxide (Al₂O₃) layer, enhancing corrosion resistance and adhesion in joining processes [ 4 ]. The PEI (polyetherimide)/glass fiber composite has gained attention in the aerospace industry due to its low density, chemical resistance, and thermoformability [ 5 ] [ 6 ] [ 7 ]. However, when exposed to humid and saline environments, such composites may undergo mechanical degradation, highlighting the importance of studying environmental aging and joint integrity [ 8 ]. Acid anodizing has proven effective in enhancing the adhesion between aluminum and composites by generating a rough, moisture-resistant surface [ 9 ] [ 10 ] [ 11 ]. Recent studies show that anodizing parameters such as voltage and time significantly influence film properties: higher porosity increases adhesion but reduces hardness and thermal resistance [ 12 ], while stepped voltage leads to denser films with improved electrochemical performance [ 13 ]. In this context, oxy-fuel welding (OFW), a low-cost and practical method for joining dissimilar materials, was adopted in this study [ 14 ]. This work aims to evaluate the influence of anodizing voltage and time on the corrosion resistance and mechanical performance of AA2024-T3 and PEI/glass fiber composite hybrid joints, using experimental design methodology. 2. Experimental 2.1 Anodization of AA2024-T3. The anodizing process of the AA2024 aluminum alloy with dimensions of 25 mm × 100 mm × 2.6 mm (Fig. 1 ) was carried out in three main steps. First, the samples were cleaned by washing with water and neutral detergent, followed by drying with an air jet at room temperature and degreasing with isopropyl alcohol to remove organic and oily surface contaminants. Next, chemical etching was performed by immersing the samples in an aqueous sodium hydroxide solution (NaOH at 10 wt.%, analytical grade 97%) at 60°C for 60 seconds. After rinsing with deionized water, the samples were immersed for 90 seconds in a nitric acid solution (HNO₃ at 35 wt.%, analytical grade 65%) to neutralize the surface. The procedure was followed by rinsing with tap water, deionized water, and isopropyl alcohol, and then drying with an air jet at room temperature.Anodizing was subsequently conducted in a glass container (Fig. 2 ) with an approximate volume of 900 mL, containing a phosphoric acid solution (H₃PO₄ at 10 wt.%, 85%). Two stainless steel counter-electrodes (cathodes) and the AA2024 sample (anode) were connected to a direct current power supply (AGILENT, model E3634A, 25 V / 7 A). During the process, air agitation was provided through a submerged aquarium pump to ensure uniform electrolyte circulation and to promote the homogeneous growth of the oxide layer, as described by Santos [ 10 ]. After anodization, the samples were rinsed with deionized water and dried in an oven at 60°C for 30 minutes. 2.2 Experimental Design To optimize the anodizing process, a statistical approach known as experimental design was adopted. This methodology is an essential tool for improving procedures and developing new processes. Optimizing a process requires understanding the independent variables involved so that the system under study yields the best possible response [ 14 ]. According to Galdámez [ 15 ], experimental design should be carried out randomly in order to minimize systematic errors and reduce the likelihood of incorrect conclusions. This methodology applies statistical principles to analyze data and predict system behavior within a defined experimental domain, providing valuable information with a minimum number of trials. It is, therefore, a mathematical tool used to assess the influence of processing variables and identify how to control them to optimize process performance. In this study, a full factorial design 2² was employed to determine the anodizing conditions that would provide better adhesion between the AA2024 aluminum alloy and the PEI/glass fiber composite. The independent variables selected were anodizing voltage and time. The selection of initial anodizing conditions for the AA2024 alloy was based on findings from the literature. Santos et al. [ 10 ] studied metal/carbon fiber/epoxy laminates prepared with and without phosphoric anodization on AA2024 and observed that anodizing at 10 V for 10 min resulted in an approximately 12% increase in shear strength compared to untreated material. Jothi et al. [ 16 ] reported that sulfuric anodizing of AA2024 at 15 V for 30 min produced improved adhesion of epoxy-based coatings (diglycidyl ether of bisphenol A) and enhanced corrosion resistance in a 3.5% NaCl solution. Based on these studies, the minimum and maximum values for anodizing parameters, voltage and time, were established in this work. Table 1 presents the anodizing conditions adopted in the experimental design. Table 1 Minimum and maximum parameters for anodizing Parameter Minimum level Maximum level Voltage (V) 10 30 Time (min) 5 25 With the previously established minimum and maximum parameters, the Design Expert 6.0.6 statistical software was used to define the experimental combinations to be applied in the anodizing process. To ensure the reliability of the adopted experimental model, Analysis of Variance (ANOVA) was employed, allowing the individual evaluation of each factor’s influence on the treatment. This statistical approach is widely recommended for validating experimental models, as highlighted by Panneerselvam [ 17 ]. 2.3 Oxy Fuel Gas Welding To enable joining between the AA2024 aluminum alloy and the PEI/glass fiber composite, an oxy-fuel welding (OFW) process adapted from Reis et al. [ 14 ] was employed. The OFW setup consisted of a propane gas cylinder equipped with a pressure regulator and a torch capable of delivering a focused flame directly to the aluminum surface, capitalizing on its high thermal conductivity. The torch was securely mounted on an adjustable support, allowing precise control over the flame height and orientation during welding operations. Positioning and stabilization of the torch were achieved using C-clamps affixed to a metallic rod, ensuring reproducible alignment throughout the procedure. Refractory bricks were incorporated to provide a stable, heat-resistant base, improving operator safety and consistency in thermal delivery. The selection of this process was based on its equipment versatility, cost-effectiveness, and the ability to precisely control the heat input to the specimens. The specific welding parameters utilized in this study are summarized in Table 2 . Table 2 Welding process parameters Flame Distance (mm) Exposure Time (s) Propane Gás Pressure (psi) 35 105 5 The OFW technique involves a torch connected to a liquefied petroleum gas (LPG) cylinder with the regulator set to 5 psi, producing a concentrated flame applied to the aluminum alloy surface (Fig. 3 ). During heating, thermal energy is effectively transferred to the aluminum substrate, raising its temperature above the glass transition temperature of the thermoplastic polymer matrix (approximately 220°C). For each welded specimen, surface temperature was recorded immediately after welding by positioning the pyrometer on the aluminum area directly exposed to the flame, thereby ensuring adherence to the specified thermal range. Following cooling and solidification, a metallurgical bond was achieved between the composite and the AA2024 alloy. To ensure uniform pressure during the welding process, steel bars were placed in direct contact with both components being joined, as depicted in Fig. 4. 2.4 Electrochemical Tests Electrochemical tests were performed using an Autolab potentiostat, model PGSTAT302, controlled by NOVA software. The samples were mounted in a flat-type electrochemical cell, employing the AA2024 alloy specimen as the working electrode with an exposed area of 1 cm², an Ag/AgCl (saturated KCl) reference electrode, and a platinum counter electrode. The experiments were conducted in naturally aerated 0.1 M Na₂SO₄ solution at ambient temperature (25 ± 2) °C. Prior to measurements, the samples were immersed in this solution for 24 hours. Initially, open circuit potential (OCP) measurements were recorded for 5 minutes to assess the initial stability of the oxide layer. Subsequently, electrochemical impedance spectroscopy (EIS) measurements were performed to evaluate the stability and characteristics of the passive layer formed after anodizing. The EIS tests were carried out at the open circuit potential, applying a 10 mV and frequency sweeps from 100 kHz to 0.01 Hz, with ten data points per frequency decade. 2.5 Lap Shear Test (LSS) The Lap Shear (LSS) test was performed after the oxy-fuel welding process to determine the failure strength of the welded joints between the AA2024 alloy and the composite material. The tests were conducted using a SHIMADZU universal testing machine, model AG-X, equipped with a 50 kN load cell, operating at a crosshead speed of 1.5 mm/min.The experiments were carried out in accordance with ASTM D1002-10 (2019) and ASTM D5868-01 (2014) 2.7 Microscopy To assess the anchorage of the composite on the anodized AA2024 alloy surface, fractured surfaces from the Lap Shear (LSS) tests were examined. Initial observations were performed using a ZEISS STEMI 2000 stereomicroscope with up to 50× magnification, with images processed in ImageJ.For optical microscopy, samples were cold-mounted, ground with 220–3000 mesh papers, and polished with 1 µm diamond paste. Analyses were carried out using an OLYMPUS microscope at 50×–1000× magnification. To enable higher-resolution analysis of anodized surfaces under different voltages and times, as well as of corrosion products, Scanning Electron Microscopy (SEM) was employed. SEM imaging was conducted with a TESCAN VEGA 3 XMU microscope at the Department of Materials and Processes, Division of Mechanical Engineering, ITA. Images were acquired at various magnifications with 10 kV accelerating voltage, 2.6 × 10⁻² Pa vacuum, a secondary electron detector, and working distances of 13–27 mm. 2.8 Wettability Analysis The wettability of anodized AA2024 alloy surfaces was assessed through contact angle measurements to characterize the hydrophilic or hydrophobic nature of the resulting oxide layer. This evaluation is crucial for understanding the adhesion potential between the metallic substrate and the polymer matrix, as surfaces with lower contact angles (< 90°) generally enable improved adhesive spreading and resin infiltration, enhancing joint strength. Measurements were performed using an ADVANCED goniometer, model 300-F1. Standardized drops of distilled water were applied to the anodized surfaces, and images were digitally analyzed to obtain average contact angle values. This characterization enabled correlation between the anodizing conditions and surface wettability, serving as an additional criterion for assessing the quality of surface treatment in metal–composite bonding applications. 3. Results and Discussion 3. 1 Anodizing Anodizing is a critical surface preparation step for aluminum alloys intended for joining with composite materials, as the morphology, thickness, and quality of the oxide layer directly influence interfacial adhesion and joint durability. In this study, anodizing of the AA2024-T3 alloy was carried out under different combinations of voltage and time to explore a range of conditions that would help identify optimal parameters for oxy-fuel welding (OFW) with PEI/glass fiber composite.Initially, three voltage levels (10, 15, and 20 V) and times of 5, 12.5, and 25 minutes were selected, as detailed in Table 3 . All anodizing experiments were performed in triplicate to ensure statistical reliability. After preliminary analyses, it was observed that higher voltages could yield thicker and more porous oxide films, potentially enhancing mechanical anchorage of the composite. Consequently, a fourth voltage level (30 V) was included, maintaining a time of 25 minutes, to investigate the effect of an even higher voltage on the anodic layer characteristics. Anodizing is a critical surface preparation step for aluminum alloys intended for joining with composite materials, as the morphology, thickness, and quality of the oxide layer directly influence interfacial adhesion and joint durability. In this study, anodizing of the AA2024-T3 alloy was carried out under different combinations of voltage and time to explore a range of conditions that would help identify optimal parameters for oxy-fuel welding (OFW) with PEI/glass fiber composite. Initially, three voltage levels (10, 15, and 20 V) and anodizing times of 5, 12.5, and 25 minutes were selected, as detailed in Table 3 . All anodizing experiments were performed in triplicate to ensure statistical reliability. After preliminary analyses, it was observed that higher voltages could yield thicker and more porous oxide films, potentially enhancing the mechanical anchorage of the composite. Consequently, a fourth voltage level (30 V) was included, maintaining a time of 25 minutes, to investigate the effect of an even higher voltage on the anodic layer characteristics. Literature supports this strategy demonstrated that increasing anodizing voltage promotes faster growth and greater densification of the initial barrier layer, followed by increased porosity and larger pore diameters[ 3 ]. Such higher porosity can improve mechanical interlocking with polymer matrices by providing greater contact area and anchoring sites, which is beneficial for hybrid joint adhesion. However, excessive porosity may reduce film microhardness and create preferential pathways for aggressive electrolyte penetration, compromising corrosion resistance. Table 3 voltage and time parameters for the anodizing process Sample 1 2 3 4 5 6 7 8 9 10 11 12 Time (min) 5 5 5 12.5 12.5 12.5 25 25 25 25 25 25 Voltage (V) 10 10 10 15 15 15 20 20 20 30 30 30 Further highlighted that controlled voltage regimes, including optimized anodizing times, lead to more homogeneous oxide layers with fewer cracks or structural defects, resulting in superior electrochemical performance. In metal–composite joining applications, this greater uniformity is desirable to ensure a more stable stress distribution and to limit interfacial failure propagation under aging or harsh environmental conditions [ 13 ]. Thus, the experimental design, which expanded the range of anodizing voltages and times, aimed not only to identify the most promising conditions for maximizing composite adhesion but also to balance factors such as oxide layer thickness, porosity, and mechanical integrity. This systematic approach enables a comprehensive evaluation of how electrochemical parameters influence the quality of the metal–composite interface, providing a solid basis for optimizing the OFW process and achieving hybrid joints with improved mechanical performance and corrosion resistance. 3.2 OFW Process The bonding between anodized AA2024-T3 aluminum alloy and PEI/glass fiber composite via oxy-fuel welding (OFW) proved to be highly dependent on the surface condition of the aluminum, particularly on the oxide layer generated through anodization. This oxide layer plays a crucial role in promoting an effective interface with the polymer matrix, enhancing the mechanical anchoring of the resin into the oxide pores and improving the strength of the hybrid joint, as shown in Fig. 6 . To ensure effective adhesion, the composite must reach a temperature above its glass transition temperature (Tg), which is approximately 220°C [ 18 ]. During the welding process, the surface temperature of the AA2024 samples was monitored using a pyrometer for each specimen to confirm that the thermal conditions were appropriate for partial melting of the thermoplastic matrix. After complete cooling, the joint quality was verified through visual inspection and manual handling tests. The mechanical strength of the welded joints was assessed through Lap Shear Strength (LSS) tests, as shown in Table 4 . The results indicated that the anodization process significantly affected joint performance. Samples anodized at 10 V for 5 min exhibited the lowest shear strength values, with sample 1 reaching only 9.44 MPa. In contrast, the condition of 30 V for 25 min yielded a maximum strength of 14.52 MPa in sample 10, with an average value of (13.0 ± 1.3) MPa across samples 10, 11, and 12. However, this increase was not substantially higher than the average value of (12.4 ± 0.3) MPa obtained for samples anodized at 15 V for 12.5 min. These findings suggest that although higher voltages may enhance oxide layer thickness, joint strength does not increase linearly, possibly due to the formation of microcracks or structural instabilities in thicker anodic films. Table 4 Welding Temperature and Lap Shear Strength for Each Sample Amostra 1 2 3 4 5 6 7 8 9 10 11 12 Welding Temperature (°C) 437 374 476 433 444 414 426 411 391 398 407 431 Lap Shear Strength (MPa) 9,44 11,9 12,5 12,0 12,2 12,7 10,5 13,1 13,1 14,5 11,9 12,6 Average (MPa) (11,3 ± 1,3) (12,4 ± 0,3) (12,2 ± 1,2) (13,0 ± 1,3) The OFW process, due to its simplicity and low cost, proved viable for hybrid joint fabrication. The direct flame applied to the metallic surface promotes localized heating, allowing the molten polymer matrix to penetrate the oxide pores. The roughness and porosity of the anodized surface play a decisive role in adhesion quality, as previously reported [ 18 ] [ 19 ]. The bonding mechanism is strengthened by capillarity, mechanical anchoring, and surface compatibility between the aluminum oxide and the composite. Furthermore, these results are consistent with previous studies, which reported increased shear strength for phosphoric acid anodization at moderate voltages and times, as well as improved adhesion of polymer-based coatings on anodized AA2024 surfaces [ 4 , 5 ]. Other investigations have highlighted that highly porous anodic films enhance adhesion but reduce microhardness and thermal crack resistance, which may explain the non-linear mechanical performance observed under more extreme anodization conditions . Furthermore, these results are consistent with previous studies [ 10 ] reported a 12% increase in shear strength in joints involving phosphoric acid anodization at 10 V for 10 min, while [ 16 ] observed a significant improvement in epoxy coating adhesion on AA2024 anodized at 15 V for 30 min. [ 12 ] [ 13 ] also highlighted that more porous anodic films enhance adhesion but reduce microhardness and thermal crack resistance, which may explain the non-linear mechanical performance observed under more extreme anodization conditions. Furthermore, these results are consistent with previous studies, which reported increased shear strength for phosphoric acid anodization at moderate voltages and times, as well as improved adhesion of polymer-based coatings on anodized AA2024 surfaces [ 10 ]. Other investigations have highlighted that highly porous anodic films enhance adhesion but reduce microhardness and thermal crack resistance, which may explain the non-linear mechanical performance observed under more extreme anodization conditions . 3.3 experimental planning To optimize the anodizing process of the AA2024 alloy and improve adhesion to the PEI/glass fiber composite, a full factorial experimental design 2² was adopted, considering anodizing potential (levels: 10 V–30 V) and anodizing time (levels: 5 min–25 min) as independent variables. This statistical approach enables an efficient evaluation of both the individual and interaction effects of the selected factors on corrosion resistance and on the mechanical strength of the welded joints. The application of a structured and randomized experimental design is essential to minimize systematic errors and ensure data reproducibility [ 21 , 16 ]. The full factorial model was implemented using Design Expert software version 6.0.6, which was also employed to perform analysis of variance (ANOVA) in order to assess the statistical significance of the individual and combined effects of the process variables. The initial anodizing conditions were defined based on previously reported studies. Phosphoric acid anodization at 10 V for 10 min applied to AA2024 alloys has been shown to promote a significant increase in shear strength when bonded to fiber-reinforced polymer composites [ 16 ]. In addition, sulfuric acid anodization at 15 V for 30 min has been reported to enhance epoxy coating adhesion and improve corrosion resistance [ 4 ]. Based on these findings, the minimum and maximum values for anodizing voltage and time were established in the present study. The anodizing conditions adopted in the experimental plan are summarized in Table 5 Table 5 - Experimental conditions and corresponding Lap Shear strength results. Experiment 1 2 3 4 5 6 7 8 9 10 11 12 13 Time (min) 5 20 5 20 1,89 23,1 12,5 12,5 12,5 12,5 12,5 12,5 12,5 Potential (V) 10,0 10,0 20,0 20,0 15,0 15,0 7,93 22,1 15,0 15,0 15,0 15,0 15,0 Joint Strength (Mpa) 11 14 10 11 11 11 13 10 12 12 12 12 12 Based on the minimum and maximum values of anodizing time and potential defined during the preliminary stage, a Central Composite Rotatable Design (CCRD) was developed using Design Expert® software version 6.0.6 to investigate the influence of anodizing parameters on the mechanical strength of the welded joints. The objective was to establish a statistical model capable of predicting joint performance under different anodizing conditions. Thirteen experiments were carried out, distributed among axial, central, and factorial points. The coded levels of the factors (time and voltage) were adjusted to allow the generation of a reliable response surface, enabling the identification of optimal anodizing conditions. Joint strength, obtained from Lap Shear Strength (LSS) tests, was adopted as the response variable for model development. Analysis of variance (ANOVA) was applied to evaluate the statistical significance of the main effects, their interactions, and the adequacy of the proposed model. Model validation was performed based on the coefficient of determination (R²), residual analysis, and the statistical significance of the model terms, ensuring the robustness of the experimental approach, as presented in Table 6 Table 6 Analysis of Variance (ANOVA) for the Anodizing Process of AA2024 Alloy. Source Sum of squares DF Mean Square F-Value Prob > F Model 12,94456376 5 2,588912753 8,498750262 0,0069 A (time) 2 1 2 6,565497623 0,0374 B (Voltage) 8,492640687 1 8,492640687 27,87920612 0,0011 A² 1,331521739 1 1,331521739 4,371051407 0,0749 B² 0,244565217 1 0,244565217 0,802846177 0,4000 AB 1 1 1 3,282748811 0,1129 Residual 2,132359313 7 0,304622759 Lack of Fit 2,132359313 3 0,710786438 Erro 0 4 0 Cor. total 15,07692308 12 R² 0,858568005 * Significant value < 0,05 The statistical analysis derived from the CCRD allowed the evaluation of the influence of anodizing parameters—time and voltage—on the mechanical strength of joints between the AA2024 aluminum alloy and the PEI/glass fiber composite. The quadratic model fitted to the response variable exhibited a satisfactory coefficient of determination (R² = 0.8586), indicating that 85.86% of the variability in joint strength could be explained by the model. Both anodizing time (A) and voltage (B) were identified as statistically significant terms, confirming their direct influence on joint strength. However, the quadratic terms associated with time and voltage were not statistically significant, suggesting a predominantly linear behavior within the investigated experimental range. According to the ANOVA results, p-values (Prob < F) lower than 0.0500 indicate statistically significant model terms. Therefore, in the conventional anodizing process, both anodizing time and voltage were confirmed as critical parameters affecting bonding strength. The response surface generated by the model indicated an optimal anodizing region near 12.5 minutes and 15 V, which is consistent with the favorable results observed in preliminary tests. Lower anodizing times and voltages resulted in reduced joint strength, likely due to the formation of thinner and less stable anodic oxide layers. Conversely, although higher voltages such as 30 V promote thicker oxide films, they may also induce surface microcracks or structural inhomogeneities that compromise joint integrity [ 12 , 13 ]. The absence of a statistically significant lack of fit further confirms the reliability of the proposed model, which can be applied as a predictive tool for optimizing the anodizing process and improving the mechanical performance of hybrid joints. LapShear = + 12.00 + 2.00*A-0.94*B-0.44*A2-0.19*B2-0.50*A*B-1.00*A3-0.061*B3 ......................... (1) The model coefficients were estimated using the Ordinary Least Squares (OLS) method, which minimizes the difference between experimental and predicted response values. Based on the Design-Expert® analysis, response surface plots were generated (Fig. 7 ), illustrating the relationship between anodizing conditions and the resulting Lap Shear Strength (LSS). The model predicted a maximum joint strength of approximately 14 MPa at anodizing conditions of 10 V and 18 minutes, identifying this region as the most effective for enhancing adhesion between anodized aluminum and the PEI/glass fiber composite. 3.4 Contact Angle Analysis In this analysis, a water droplet was deposited onto the surface of the anodized AA2024 alloy to observe its shape and the angle formed with the surface (contact angle). When the measured angle is greater than 90°, the material is considered hydrophobic; when it is less than 90°, it is classified as hydrophilic, indicating a greater tendency for water penetration into the surface. This test was applied to evaluate the hydrophilicity of anodized AA2024 under different anodizing conditions. A comparison was performed between the preliminary conditions (10 V / 5 min, 15 V / 12.5 min, 20 V / 25 min, and 30 V / 25 min) and the optimal condition identified (10 V / 20 min). The contact angle values obtained for each anodizing condition are presented in Table 7 . Under the optimal anodizing condition (10 V / 20 min), the lowest contact angle value (8.21°) was observed, indicating a highly hydrophilic surface. Previous studies have reported a significant improvement in wettability starting from anodizing potentials of 10 V, with contact angles around 13° observed at 15 V for 30 minutes [ 15 ]. All anodized samples exhibited hydrophilic behavior, which is a critical factor for promoting improved adhesion during the welding process between the composite and the aluminum alloy. Similar behavior has been reported in the literature, where anodized aluminum surfaces present high porosity and roughness, characterized by peaks and valleys that increase surface energy and hydrophilicity, with average contact angle values close to 15° [ 22 , 23 ]. Table 7 Contact angle values under different anodization conditions Samples 10 V/5min 15 V/12,5min 20 V/25min 30 V/25min 10 V/20min* Contact Angle (º) 19,14 19,21 9,29 10,34 8,21 Standard Deviation 1,44 3,65 0,82 1,37 0,90 *Optimal anodization condition A noticeable difference was observed between the contact angle values of samples anodized at higher potentials and those treated under optimal conditions. This behavior is commonly associated with increased surface roughness, which leads to higher surface free energy and enhanced hydrophilicity. Surface free energy plays a fundamental role in intermolecular interactions and strongly influences adsorption and adhesion mechanisms with other materials [ 18 ]. The droplet profile analysis reinforces the previous results and helps explain the performance of the welded joints, as the joining process between the composite and the anodized aluminum relies on the penetration of the PEI matrix into surface pores through capillary action. Therefore, higher surface hydrophilicity favors greater polymer infiltration into these regions. Figure 8 illustrates the interaction between the anodized surface and the water droplet. 3.5 Microscopic Analysis The surfaces of the AA2024 alloy were examined after the Lap Shear Strength (LSS) tests using a stereomicroscope in order to evaluate the quality of the interfacial anchorage between the PEI/glass fiber composite and the anodized aluminum surface. The analysis focused on the fracture regions, aiming to identify residual polymer matrix adhered to the metallic substrate, which is indicative of effective mechanical interlocking. Figure 9 presents the fractured area, highlighted in red, on the aluminum surface selected for assessing metal–composite adhesion. For the sample anodized at 10 V for 5 minutes (Fig. 10 a), only a limited amount of residual composite was observed. This condition, associated with low anodizing potential and short treatment time, resulted in reduced interaction between the composite and the alloy surface. Regions of exposed aluminum substrate (indicated by red arrows) coexist with isolated areas containing residual composite (blue arrows), revealing a non-uniform bonding mechanism. The left side of the sample, corresponding to the edge region, exhibited a larger exposed substrate area, while the central region showed greater composite impregnation. This behavior is attributed to the direction of flame application during the OFW process, which progressed from the edge toward the center, leading to higher thermal input in the central zone. At 10× magnification, discontinuous fiber imprints were observed on the aluminum surface, indicating insufficient anchorage under this anodizing condition. In contrast, samples anodized at higher potentials and longer times exhibited a substantially greater amount of residual composite on the aluminum surface, reflecting improved adhesion. This enhancement is associated with the increased thickness and roughness of the anodic oxide layer formed under these conditions, which promotes more effective mechanical interlocking [ 22 , 23 ]. In particular, samples anodized at 30 V for 25 minutes showed well-defined and continuous fiber imprints across the interface, suggesting strong and homogeneous anchorage. The most pronounced anchoring behavior was observed for the optimal anodizing condition (10 V for 20 minutes). In this case, uniform composite impregnation was evident over the entire aluminum surface, with continuous fiber imprints clearly transferred to the metallic substrate. This result indicates that, despite the lower anodizing potential, the extended anodizing time enabled the formation of a porous and well-adhered oxide layer, facilitating resin infiltration through capillary action and promoting the development of a cohesive metal–composite interface [ 16 , 18 , 24 ]. Overall, the microscopy results confirm that both anodizing potential and time play a decisive role in controlling the quality of the metal–composite interface. The optimal condition identified through experimental design provided the most favorable balance between oxide morphology and interfacial adhesion, directly correlating with the highest joint strength values and reinforcing the importance of surface parameter optimization for hybrid structural applications. 3.6 Scanning Electron Microscopy (SEM) In order to gain a deeper understanding of the factors influencing the bonding between the AA2024 aluminum alloy and the PEI/glass fiber composite, Scanning Electron Microscopy (SEM) analyses were performed on the anodized alloy surfaces under different voltage and time conditions (Fig. 11 ). The micrographs acquired at 1000× magnification allowed a detailed evaluation of the oxide layer morphology, which plays a critical role in the mechanical anchoring of the composite and, consequently, in the mechanical strength of the welded joints. Since adhesion between dissimilar materials occurs predominantly through capillary action, surface topography becomes a key parameter governing interfacial performance. Under the anodizing condition of 10 V for 5 minutes, the surface exhibited a relatively smooth and uniform appearance, with a low density of visible pores, indicating the formation of a denser and more compact oxide film. Such morphology limits mechanical interlocking with the polymer matrix, which is consistent with the reduced joint strength observed under this condition. Increasing the anodizing parameters to 15 V for 12.5 minutes resulted in a noticeable increase in surface roughness, characterized by well-defined grooves and cavities distributed across the surface. This morphology favors resin penetration and mechanical anchoring, in agreement with the improved mechanical resistance obtained. A further increase to 20 V for 25 minutes intensified this effect, producing an even more irregular surface with a higher density of deep cavities, suggesting enhanced electrolyte attack and increased oxide layer porosity. However, under the most aggressive condition of 30 V for 25 minutes, an opposite trend was observed, with the surface appearing visually smoother and less porous. This behavior is attributed to partial degradation of the oxide layer, likely associated with the accumulation of reaction by-products, such as aluminum hydroxides, which can block pores and reduce surface roughness [ 25 ]. The anodizing condition of 10 V for 20 minutes—identified as optimal through experimental design—exhibited a homogeneous and well-developed morphology, with uniform pore distribution and clearly defined surface features. Although less rough than surfaces formed at intermediate voltages, this condition provided effective mechanical interlocking, indicating that oxide layer homogeneity is as important as absolute roughness for achieving strong interfacial bonding. These observations reinforce the hypothesis that both surface roughness and oxide layer uniformity are critical parameters for successful bonding between thermoplastic composites and anodized aluminum substrates. Similar trends reported in the literature indicate that increased anodizing potential enhances surface roughness up to a critical limit, beyond which oxide degradation or saturation effects may occur [ 25 ] [ 26 ]. For a more detailed evaluation of the surface microstructure, additional SEM analyses were conducted at 40,000× magnification (Fig. 12 ). At this scale, microstructural features associated with oxide formation under different anodizing conditions become more evident. For the sample anodized at 10 V for 5 minutes (Fig. 12 a), the surface displayed a granular morphology composed of relatively large and shallow oxide “islands,” widely spaced from one another. This configuration indicates a low-porosity surface with limited specific surface area, which may restrict polymer penetration during bonding. When the anodizing parameters were increased to 15 V for 12.5 minutes (Fig. 12 b), the surface exhibited a more compact and interconnected structure, with a well-distributed network of pores. This morphology enhances mechanical anchoring and correlates well with the higher shear strength values obtained. The sample anodized at 20 V for 25 minutes (Fig. 12 c) showed a marked increase in porosity and surface roughness, with smaller and more densely packed oxide features, reflecting intensified electrolyte action. While such morphology may favor capillary-driven adhesion, excessive roughness can also compromise interfacial homogeneity. At 30 V for 25 minutes (Fig. 12 d), the surface consisted of extremely fine and uniformly distributed particles, but with a flatter and more compact appearance. This suggests pore saturation or blockage, likely caused by reaction products filling the pore structure, which may reduce the functional effectiveness of the oxide layer. Finally, the optimal condition of 10 V for 20 minutes (Fig. 12 e) exhibited a balanced morphology, characterized by a homogeneous distribution of small oxide features forming a continuous porous network. This combination of uniformity and adequate porosity provides an optimal balance between roughness and structural integrity, directly supporting the superior joint performance observed in the mechanical tests. 3.7 Electrochemical Impedance Spectroscopy (EIS) The electrochemical behavior of the AA2024 alloy—including the mechanically polished, non-anodized sample used as a control and the samples anodized under different voltage and time conditions—was evaluated by electrochemical impedance spectroscopy (EIS). The objective was to assess the protective efficiency of the anodic films formed under different anodizing parameters. The measurements were carried out in a 0.1 mol L⁻¹ sodium sulfate (Na₂SO₄) solution, chosen as a neutral and non-aggressive electrolyte. The experimental impedance data, presented in Nyquist and Bode representations (Figs. 13 and 14 ), were fitted using equivalent electrical circuit models to describe the electrochemical response of the system. Several circuit configurations were tested based on established approaches reported in the literature [27,28]. Among them, the models shown in Figs. 15 (a) and 15(b) provided the best fitting quality and physical relevance. The circuit in Fig. 15 (a) consists of the solution resistance (Rs) in series with a parallel combination of a polarization resistance (Rp) and a constant phase element (CPE). The use of a CPE instead of an ideal capacitor accounts for the non-ideal capacitive behavior arising from surface heterogeneity, roughness, and non-uniform current distribution typically observed on corroding metallic surfaces. To better represent the electrochemical response of anodized samples, a more complex circuit (Fig. 15 (b)) was adopted, incorporating two time constants. This approach is widely used for metallic substrates covered by anodic or protective films [ 29 ]. In this configuration, two CPE–Rp elements are arranged in series, representing the contribution of different layers within the anodic oxide film. The first branch (CPE₁–Rp₁) is associated with the outer porous layer and the immediate electrolyte/oxide interface, reflecting the influence of surface roughness, pore distribution, and electrolyte penetration. The second branch (CPE₂–Rp₂) corresponds to the inner barrier-type oxide layer, which is primarily responsible for corrosion protection at low frequencies. The fitting parameters are summarized in Table 8 . The solution resistance (Rs) remained approximately constant at ~ 50 Ω for all samples, including the control, as expected since the electrolyte composition was identical in all experiments. In contrast, the anodized samples exhibited significantly higher polarization resistance values compared to the pickled control sample, confirming the formation of anodic oxide films with a barrier effect against charge transfer. Despite this general improvement, no monotonic increase in polarization resistance with increasing anodizing voltage or time was observed. This behavior reflects the complex relationship between anodizing parameters and oxide film quality. Under mild conditions (low voltage and short time), oxide formation may be incomplete, resulting in thinner or discontinuous films. Conversely, more aggressive conditions, such as anodizing at 30 V for extended times, can lead to excessive pore growth, increased defect density, or partial collapse of the oxide structure, ultimately compromising its protective performance. This interpretation is consistent with the SEM observations discussed previously, which revealed surface smoothing and pore blockage under severe anodizing conditions. The incorporation of two constant phase elements was essential to describe the non-ideal capacitive behavior associated with film heterogeneity. The admittance parameter (Y₀) of the control sample was significantly higher (4.40 µS·sⁿ) than those of the anodized samples (0.12–0.57 µS·sⁿ), indicating a decrease in effective interfacial capacitance due to the formation of thicker and more insulating oxide layers. The CPE exponent values (n₁ and n₂) ranged between 0.90 and 0.99, suggesting a response close to that of an ideal capacitor and indicating relatively homogeneous electrical behavior, despite the morphological variations observed by SEM. According to the results presented in Table 9, the sample anodized at 10 V for 20 minutes exhibited the highest Rp₂ value among all anodized conditions, demonstrating superior barrier layer performance. This finding correlates well with SEM analyses, which showed a homogeneous and well-developed porous morphology under this condition, favoring both corrosion protection and mechanical anchoring. In contrast, the sample anodized at 30 V for 25 minutes showed lower Rp₂ and n₂ values, indicating reduced protection due to increased roughness, heterogeneity, and possible pore blockage. Overall, the EIS results confirm that anodizing significantly enhances the corrosion resistance of the AA2024 alloy compared to the non-anodized condition. However, the protective efficiency of the anodic films strongly depends on the anodizing parameters, which directly affect oxide morphology, uniformity, and defect density. The condition of 10 V for 20 minutes provided the most favorable balance between oxide homogeneity and barrier properties, in excellent agreement with the SEM observations and mechanical performance results. Equivalent circuit models with two Rp–CPE elements in series, similar to those proposed in previous studies [ 30 , 12 ], were also evaluated. Nevertheless, under the present experimental conditions, the adopted models (Fig. 15 ) yielded superior fitting quality, with very low χ² values (10⁻³–10⁻⁴), confirming their adequacy in describing the electrochemical behavior of the anodized AA2024 alloy. Table 8 EIS fitting parameters obtained for different AA2024 samples (control and anodized) measured in 0.1 mol L⁻¹ sodium sulfate (Na₂SO₄) solution. Samples R s (Ω) R p1 (kΩ) CPE 1 (µMho. S n ) n 1 R p2 (kΩ) CPE 2 (µMho. S n ) n 2 χ 2 Control 49,4 354 4,40 0,94 --- --- --- 0,03 10 V, 5 min 54,8 220 0,57 0,97 736 0,14 0,99 0,006 15 V, 12,5 min 55,0 707 0,64 0,97 225 0,15 0,86 0,02 20 V, 25 min 51,4 581 0,30 0,97 840 0,22 0,90 0,01 30 V, 25 min 51,3 630 0,31 0,97 163 0,45 0,65 0,0004 10 V, 20 min 48,8 155 0,12 0,95 921 0,15 0,95 0,002 Previous studies have demonstrated that the electrochemical performance of anodic films on the AA2024 alloy is strongly influenced by the electrolyte composition, oxide layer morphology, and the use of modifying agents or post-treatment sealing steps. Significant improvements in barrier layer resistance have been reported when mixed electrolytes and corrosion inhibitors are employed, particularly when combined with sealing treatments, which can increase impedance values by several orders of magnitude [ 31 ]. Other investigations have shown that, although hard anodizing conditions promote thicker oxide layers, long-term electrochemical stability is more closely related to pore morphology, connectivity, and film homogeneity than to barrier thickness alone [ 26 ]. Additionally, the incorporation of inhibitors or sealing agents after anodizing has been shown to substantially enhance impedance modulus values during prolonged immersion tests, reinforcing the role of pore blocking and chemical stabilization in corrosion protection [ 27 ]. In the present work, no corrosion inhibitors, sealing steps, or additional surface modifications were applied after anodizing. Even under these simplified conditions, the EIS results revealed a clear improvement in corrosion resistance relative to the non-anodized substrate, with the electrochemical response being dominated by the overall behavior of the anodic film. This observation supports the use of a simplified equivalent electrical circuit, as the contributions of the porous outer layer and the inner barrier layer appear to be strongly coupled under the investigated anodizing conditions. This coupled response is consistent with the SEM observations, which showed continuous and relatively homogeneous oxide morphologies, particularly for the condition of 10 V for 20 minutes, suggesting that film uniformity and controlled porosity play a more decisive role in electrochemical performance than maximum oxide thickness alone. 4. Conclusion The experimental design strategy proved to be effective for optimizing the anodizing parameters of the AA2024 alloy for joining with PEI/glass fiber composite by oxy-fuel welding (OFW), yielding a reliable statistical model with a coefficient of determination (R²) of 85.25%. Among the investigated conditions, anodizing at 10 V for 20 min was identified as the most suitable, providing an average lap shear strength of approximately 14 MPa. This result was consistently supported by the response surface analysis, experimental validation, and Lap Shear Strength (LSS) testing. Macroscopic examination of the fractured joints revealed a strong dependence of adhesion quality on the anodizing condition. Under less favorable parameters (10 V for 5 min), limited polymer matrix impregnation and extensive exposed aluminum surface were observed, indicating weak interfacial bonding. Conversely, increasing anodizing potential and time (20 V for 25 min and 30 V for 25 min) promoted greater matrix impregnation. The optimized condition (10 V for 20 min) exhibited the highest and most uniform adhesion, reflecting an effective balance between oxide layer development and interfacial compatibility. These observations were corroborated by SEM analyses, which showed that low potential and short anodizing time resulted in a compact and relatively smooth oxide layer, while excessively severe conditions led to oxide degradation and aluminum hydroxide deposition, reducing effective roughness. In contrast, the optimized condition produced a homogeneous anodic morphology with nodular features and controlled roughness, favoring mechanical anchoring and capillary-driven polymer infiltration. Electrochemical characterization further confirmed the benefits of anodizing on the corrosion performance of the AA2024 alloy. Electrochemical impedance spectroscopy demonstrated a significant increase in polarization resistance and impedance modulus at low frequencies for all anodized samples compared to the control, evidencing the formation of a protective oxide layer with a barrier effect against charge transfer. The EIS spectra were satisfactorily fitted using equivalent electrical circuit models, indicating that the electrochemical response is dominated by the overall behavior of the anodic film and can be described by a single dominant capacitive process. Among the tested conditions, anodizing at 10 V for 20 min exhibited the highest barrier layer resistance and near-ideal capacitive behavior, indicating a more homogeneous and protective oxide film. Overall, the mechanical, morphological, and electrochemical results are in strong agreement and demonstrate that precise control of anodizing parameters is essential to optimize both adhesion performance and corrosion resistance in hybrid joints produced by OFW. The anodizing condition of 10 V for 20 min represents an optimal compromise between oxide layer morphology, interfacial bonding efficiency, and electrochemical protection, highlighting its suitability for the fabrication of durable aluminum–thermoplastic composite hybrid structures. Declarations Funding The author are grateful to the Brazilian Funding Institution CAPES fundation (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) for financial support. All authors contributed to the conception and planning of the study. The preparation of materials, data collection, and data analysis were carried out by Marcos Paulo Soza Ribeiro, Rafael Resende Lucas, Luis Felipe Barbosa Marques, Heloisa Andrea Acciari, and Rita de Cassia Sales. Edson Coccheri Botelho and Ana Beatriz Moreira Abrahão contributed to project development and provided methodological support. The project and manuscript was written by Marcos Paulo Soza Ribeiro under the supervision and guidance of Roberto Zenhei Nakazato. The author declare that they have no conflicts of interest related to the research, authorship, and/or publication of this article. References PEREIRA MC et al (2000) Efeito do tratamento térmico na corrosão das ligas de alumínio 2024 e 7050 em meio salino. 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Trans Nonferrous Met Soc China v 27:711–721. 10.1016/S1003-6326(17)60079-7 Cite Share Download PDF Status: Under Review Version 1 posted Reviewers agreed at journal 02 May, 2026 Reviewers invited by journal 20 Apr, 2026 Editor invited by journal 07 Mar, 2026 Editor assigned by journal 27 Feb, 2026 First submitted to journal 26 Feb, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8957869","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":626029936,"identity":"d198f8fd-8099-4eb0-93db-62ba5483e664","order_by":0,"name":"Marcos Paulo Souza 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fiber samples\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8957869/v1/72f514c7c03cfa6c84da1342.png"},{"id":108181397,"identity":"361c71f0-96cb-4716-a97e-038bbc8d59da","added_by":"auto","created_at":"2026-04-30 08:58:36","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":87312,"visible":true,"origin":"","legend":"\u003cp\u003eThe anodizing process of the AA2024 aluminum alloy\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8957869/v1/18a1d7cb64350bf6b273a403.png"},{"id":108181288,"identity":"75c74692-1fef-4824-b0d4-8c45c448a218","added_by":"auto","created_at":"2026-04-30 08:58:30","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":11987,"visible":true,"origin":"","legend":"\u003cp\u003eOxy-fuel welding process\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8957869/v1/e8eecf348899e9d406dd0040.jpg"},{"id":108014387,"identity":"215a22f0-7df2-4dd3-9b8a-a3c8a0baaf77","added_by":"auto","created_at":"2026-04-28 13:29:38","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":4021,"visible":true,"origin":"","legend":"\u003cp\u003eAlloy fixing device\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8957869/v1/52e9242bbf4385150fdd52f3.jpg"},{"id":108014388,"identity":"ac1bb2fe-aec9-452b-adf9-c5f68adf4dd1","added_by":"auto","created_at":"2026-04-28 13:29:38","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":57878,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFigure 6\u003c/strong\u003e a) Top view of the PEI/fiberglass and AA2024 welded sample. (b) Side view of the welded joint\u003c/p\u003e","description":"","filename":"6.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8957869/v1/3992a6361cef4140e7620a91.jpg"},{"id":108014389,"identity":"6c2a0c48-fb76-4592-91ae-a9306f3508e1","added_by":"auto","created_at":"2026-04-28 13:29:38","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":107927,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFigure 7\u003c/strong\u003e – (a) Joint strength contour plots; (b) Anodizing response surface for AA2024/PEI-glass fiber bonding.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-8957869/v1/f2b94146ad94268784ed4098.png"},{"id":108490805,"identity":"8af2375e-e0ce-4f3a-bcac-d8a6b8e0d496","added_by":"auto","created_at":"2026-05-05 09:48:48","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":42785,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFigure 8\u003c/strong\u003e Water droplet profiles on the surface of anodized AA2024 alloy under diferente conditions\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-8957869/v1/2338d65cf4a431fbffd4b8f4.png"},{"id":108181306,"identity":"07753054-8abe-4882-b794-af3bcb58eb8f","added_by":"auto","created_at":"2026-04-30 08:58:32","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":485561,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFigure 9\u003c/strong\u003e – Fractured surface with red circle showing the polymer matrix impregnated in the aluminium\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-8957869/v1/0e4a8c4bcf4489280c69b358.png"},{"id":108491046,"identity":"c2b42c77-a777-4d88-af0d-992c2ae82634","added_by":"auto","created_at":"2026-05-05 09:51:32","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":198775,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFigure 10 - \u003c/strong\u003e(a) Fractured surface AA 2024 alloy sample anodized at 10V/5 min, 6.5x approximation; (b) Fractured surface AA 2024 alloy sample anodized at 30V/52 min, 6.5x approximation; (c) Fractured surface AA 2024 alloy sample anodized at 10V/20 min, 6.5x approximation .\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-8957869/v1/a6682e09cbc0228a82e94487.png"},{"id":108014391,"identity":"996f5daf-c987-4df4-b55d-084e0ababe5b","added_by":"auto","created_at":"2026-04-28 13:29:38","extension":"jpg","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":61095,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFigure 11\u003c/strong\u003e – SEM micrograph (1000x) of anodized AA2024 alloy surfaces under different voltage and time conditions, illustrating the morphological evolution of the oxide layer.\u003c/p\u003e","description":"","filename":"11.jpg","url":"https://assets-eu.researchsquare.com/files/rs-8957869/v1/dc3006adce8826489f43104c.jpg"},{"id":109221722,"identity":"3d65198b-57c5-4d4b-adf0-c22f0a811925","added_by":"auto","created_at":"2026-05-13 20:54:14","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":293746,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFigure 12\u003c/strong\u003e SEM micrographs at 40,000× magnification of the anodized AA2024 alloy under different voltage and time conditions: (a) 10 V / 5 min, (b) 15 V / 12.5 min, (c) 20 V / 25 min, (d) 30 V / 25 min, (e) 10 V / 20 min.\u003c/p\u003e","description":"","filename":"12.png","url":"https://assets-eu.researchsquare.com/files/rs-8957869/v1/437315ffc5830be702dafff4.png"},{"id":108014395,"identity":"a479d37d-5988-4b2b-8e64-b83aa4bf1e4a","added_by":"auto","created_at":"2026-04-28 13:29:38","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":115099,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFigure 13.\u003c/strong\u003e Nyquist plots (EIS) of control and anodized AA2024 samples measured in 0.1 mol L⁻¹ Na₂SO₄ solution.\u003c/p\u003e","description":"","filename":"13.png","url":"https://assets-eu.researchsquare.com/files/rs-8957869/v1/b9c1d6750e3c559ed5009c0b.png"},{"id":108181005,"identity":"27868f42-9d5d-42da-a6f4-0a916f1ac92d","added_by":"auto","created_at":"2026-04-30 08:56:08","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":166432,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFigure 14.\u003c/strong\u003e Bode plots (|Z| and phase angle) of control and anodized AA2024 samples measured in 0.1 mol L⁻¹ Na₂SO₄ solution\u003c/p\u003e","description":"","filename":"14.png","url":"https://assets-eu.researchsquare.com/files/rs-8957869/v1/9a83c3acd065fb31d810ba25.png"},{"id":108014397,"identity":"f50f1be0-105c-4cb7-b032-aca092d03fa2","added_by":"auto","created_at":"2026-04-28 13:29:38","extension":"png","order_by":14,"title":"Figure 14","display":"","copyAsset":false,"role":"figure","size":21174,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eFigure 15\u003c/strong\u003e. Equivalent electrical circuit models used to fit the EIS of AA2024 samples: (a) control, (b) anodized.\u003c/p\u003e","description":"","filename":"15.png","url":"https://assets-eu.researchsquare.com/files/rs-8957869/v1/7720cfd8769bd70ed1b86289.png"},{"id":109249459,"identity":"867984f0-6f4f-4cb3-8e0c-a6e5139bea3d","added_by":"auto","created_at":"2026-05-14 08:53:18","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2110727,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8957869/v1/1c61abec-5f70-4794-8a35-2c3a93052757.pdf"}],"financialInterests":"","formattedTitle":"Optimization of AA2024 Anodization for Oxy-Fuel Welding with PEI/Glass Fiber Composite","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eThe growing demand for lightweight and high-strength materials has driven the use of aluminum alloys and fiber-reinforced polymer composites in structural applications, especially in the aerospace and automotive sectors. The AA2024-T3 aluminum alloy is widely used due to its high mechanical strength resulting from copper addition, but it exhibits low resistance to localized corrosion [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e] [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e] [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. To overcome this limitation, anodizing is applied as an electrochemical process that forms an aluminum oxide (Al₂O₃) layer, enhancing corrosion resistance and adhesion in joining processes [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eThe PEI (polyetherimide)/glass fiber composite has gained attention in the aerospace industry due to its low density, chemical resistance, and thermoformability [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e] [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e] [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. However, when exposed to humid and saline environments, such composites may undergo mechanical degradation, highlighting the importance of studying environmental aging and joint integrity [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eAcid anodizing has proven effective in enhancing the adhesion between aluminum and composites by generating a rough, moisture-resistant surface [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e] [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]. Recent studies show that anodizing parameters such as voltage and time significantly influence film properties: higher porosity increases adhesion but reduces hardness and thermal resistance [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e], while stepped voltage leads to denser films with improved electrochemical performance [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. In this context, oxy-fuel welding (OFW), a low-cost and practical method for joining dissimilar materials, was adopted in this study [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. This work aims to evaluate the influence of anodizing voltage and time on the corrosion resistance and mechanical performance of AA2024-T3 and PEI/glass fiber composite hybrid joints, using experimental design methodology.\u003c/p\u003e"},{"header":"2. Experimental","content":"\u003cp\u003e\u003cstrong\u003e2.1 Anodization of AA2024-T3.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe anodizing process of the AA2024 aluminum alloy with dimensions of 25 mm \u0026times; 100 mm \u0026times; 2.6 mm (Fig. \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) was carried out in three main steps. First, the samples were cleaned by washing with water and neutral detergent, followed by drying with an air jet at room temperature and degreasing with isopropyl alcohol to remove organic and oily surface contaminants. Next, chemical etching was performed by immersing the samples in an aqueous sodium hydroxide solution (NaOH at 10 wt.%, analytical grade 97%) at 60\u0026deg;C for 60 seconds. After rinsing with deionized water, the samples were immersed for 90 seconds in a nitric acid solution (HNO₃ at 35 wt.%, analytical grade 65%) to neutralize the surface. The procedure was followed by rinsing with tap water, deionized water, and isopropyl alcohol, and then drying with an air jet at room temperature.Anodizing was subsequently conducted in a glass container (Fig. \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) with an approximate volume of 900 mL, containing a phosphoric acid solution (H₃PO₄ at 10 wt.%, 85%). Two stainless steel counter-electrodes (cathodes) and the AA2024 sample (anode) were connected to a direct current power supply (AGILENT, model E3634A, 25 V / 7 A). During the process, air agitation was provided through a submerged aquarium pump to ensure uniform electrolyte circulation and to promote the homogeneous growth of the oxide layer, as described by Santos [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. After anodization, the samples were rinsed with deionized water and dried in an oven at 60\u0026deg;C for 30 minutes.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.2 Experimental Design\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTo optimize the anodizing process, a statistical approach known as experimental design was adopted. This methodology is an essential tool for improving procedures and developing new processes. Optimizing a process requires understanding the independent variables involved so that the system under study yields the best possible response [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]. According to Gald\u0026aacute;mez [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e], experimental design should be carried out randomly in order to minimize systematic errors and reduce the likelihood of incorrect conclusions. This methodology applies statistical principles to analyze data and predict system behavior within a defined experimental domain, providing valuable information with a minimum number of trials. It is, therefore, a mathematical tool used to assess the influence of processing variables and identify how to control them to optimize process performance.\u003c/p\u003e\n\u003cp\u003eIn this study, a full factorial design 2\u0026sup2; was employed to determine the anodizing conditions that would provide better adhesion between the AA2024 aluminum alloy and the PEI/glass fiber composite. The independent variables selected were anodizing voltage and time. The selection of initial anodizing conditions for the AA2024 alloy was based on findings from the literature. Santos et al. [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] studied metal/carbon fiber/epoxy laminates prepared with and without phosphoric anodization on AA2024 and observed that anodizing at 10 V for 10 min resulted in an approximately 12% increase in shear strength compared to untreated material. Jothi et al. [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] reported that sulfuric anodizing of AA2024 at 15 V for 30 min produced improved adhesion of epoxy-based coatings (diglycidyl ether of bisphenol A) and enhanced corrosion resistance in a 3.5% NaCl solution. Based on these studies, the minimum and maximum values for anodizing parameters, voltage and time, were established in this work. Table \u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e presents the anodizing conditions adopted in the experimental design.\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003e\u0026nbsp;Minimum and maximum parameters for anodizing\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"3\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eParameter\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003eMinimum level\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003eMaximum level\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e\u003cstrong\u003eVoltage (V)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\n \u003cp\u003e30\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e\u003cstrong\u003eTime (min)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\n \u003cp\u003e5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\n \u003cp\u003e25\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n\u003cp\u003eWith the previously established minimum and maximum parameters, the Design Expert 6.0.6 statistical software was used to define the experimental combinations to be applied in the anodizing process. To ensure the reliability of the adopted experimental model, Analysis of Variance (ANOVA) was employed, allowing the individual evaluation of each factor\u0026rsquo;s influence on the treatment. This statistical approach is widely recommended for validating experimental models, as highlighted by Panneerselvam [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e].\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.3 Oxy Fuel Gas Welding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eTo enable joining between the AA2024 aluminum alloy and the PEI/glass fiber composite, an oxy-fuel welding (OFW) process adapted from Reis et al. [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e] was employed. The OFW setup consisted of a propane gas cylinder equipped with a pressure regulator and a torch capable of delivering a focused flame directly to the aluminum surface, capitalizing on its high thermal conductivity. The torch was securely mounted on an adjustable support, allowing precise control over the flame height and orientation during welding operations. Positioning and stabilization of the torch were achieved using C-clamps affixed to a metallic rod, ensuring reproducible alignment throughout the procedure. Refractory bricks were incorporated to provide a stable, heat-resistant base, improving operator safety and consistency in thermal delivery. The selection of this process was based on its equipment versatility, cost-effectiveness, and the ability to precisely control the heat input to the specimens. The specific welding parameters utilized in this study are summarized in Table \u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003e\u0026nbsp;Welding process parameters\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"3\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\n \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003cbr\u003e\u003c/div\u003e\u0026nbsp;\u003ctable float=\"No\" id=\"Taba\" border=\"1\"\u003e\n \u003ccolgroup cols=\"1\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eFlame Distance (mm)\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n \u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\n \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003cbr\u003e\u003c/div\u003e\u0026nbsp;\u003ctable float=\"No\" id=\"Tabb\" border=\"1\"\u003e\n \u003ccolgroup cols=\"1\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eExposure Time\u003c/p\u003e\n \u003cp\u003e(s)\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n \u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\n \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003cbr\u003e\u003c/div\u003e\u0026nbsp;\u003ctable float=\"No\" id=\"Tabc\" border=\"1\"\u003e\n \u003ccolgroup cols=\"1\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003ePropane G\u0026aacute;s Pressure (psi)\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n \u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e35\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\n \u003cp\u003e105\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\n \u003cp\u003e5\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n\u003cp\u003eThe OFW technique involves a torch connected to a liquefied petroleum gas (LPG) cylinder with the regulator set to 5 psi, producing a concentrated flame applied to the aluminum alloy surface (Fig. \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). During heating, thermal energy is effectively transferred to the aluminum substrate, raising its temperature above the glass transition temperature of the thermoplastic polymer matrix (approximately 220\u0026deg;C). For each welded specimen, surface temperature was recorded immediately after welding by positioning the pyrometer on the aluminum area directly exposed to the flame, thereby ensuring adherence to the specified thermal range. Following cooling and solidification, a metallurgical bond was achieved between the composite and the AA2024 alloy. To ensure uniform pressure during the welding process, steel bars were placed in direct contact with both components being joined, as depicted in Fig. 4.\u003c/p\u003e\n\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n \u003ch2\u003e2.4 Electrochemical Tests\u003c/h2\u003e\n \u003cp\u003eElectrochemical tests were performed using an Autolab potentiostat, model PGSTAT302, controlled by NOVA software. The samples were mounted in a flat-type electrochemical cell, employing the AA2024 alloy specimen as the working electrode with an exposed area of 1 cm\u0026sup2;, an Ag/AgCl (saturated KCl) reference electrode, and a platinum counter electrode. The experiments were conducted in naturally aerated 0.1 M Na₂SO₄ solution at ambient temperature (25\u0026thinsp;\u0026plusmn;\u0026thinsp;2) \u0026deg;C. Prior to measurements, the samples were immersed in this solution for 24 hours. Initially, open circuit potential (OCP) measurements were recorded for 5 minutes to assess the initial stability of the oxide layer. Subsequently, electrochemical impedance spectroscopy (EIS) measurements were performed to evaluate the stability and characteristics of the passive layer formed after anodizing. The EIS tests were carried out at the open circuit potential, applying a 10 mV and frequency sweeps from 100 kHz to 0.01 Hz, with ten data points per frequency decade.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n \u003ch2\u003e2.5 Lap Shear Test (LSS)\u003c/h2\u003e\n \u003cp\u003eThe Lap Shear (LSS) test was performed after the oxy-fuel welding process to determine the failure strength of the welded joints between the AA2024 alloy and the composite material. The tests were conducted using a SHIMADZU universal testing machine, model AG-X, equipped with a 50 kN load cell, operating at a crosshead speed of 1.5 mm/min.The experiments were carried out in accordance with ASTM D1002-10 (2019) and ASTM D5868-01 (2014)\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n \u003ch2\u003e2.7 Microscopy\u003c/h2\u003e\n \u003cp\u003eTo assess the anchorage of the composite on the anodized AA2024 alloy surface, fractured surfaces from the Lap Shear (LSS) tests were examined. Initial observations were performed using a ZEISS STEMI 2000 stereomicroscope with up to 50\u0026times; magnification, with images processed in ImageJ.For optical microscopy, samples were cold-mounted, ground with 220\u0026ndash;3000 mesh papers, and polished with 1 \u0026micro;m diamond paste. Analyses were carried out using an OLYMPUS microscope at 50\u0026times;\u0026ndash;1000\u0026times; magnification. To enable higher-resolution analysis of anodized surfaces under different voltages and times, as well as of corrosion products, Scanning Electron Microscopy (SEM) was employed. SEM imaging was conducted with a TESCAN VEGA 3 XMU microscope at the Department of Materials and Processes, Division of Mechanical Engineering, ITA. Images were acquired at various magnifications with 10 kV accelerating voltage, 2.6 \u0026times; 10⁻\u0026sup2; Pa vacuum, a secondary electron detector, and working distances of 13\u0026ndash;27 mm.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\n \u003ch2\u003e2.8 Wettability Analysis\u003c/h2\u003e\n \u003cp\u003eThe wettability of anodized AA2024 alloy surfaces was assessed through contact angle measurements to characterize the hydrophilic or hydrophobic nature of the resulting oxide layer. This evaluation is crucial for understanding the adhesion potential between the metallic substrate and the polymer matrix, as surfaces with lower contact angles (\u0026lt;\u0026thinsp;90\u0026deg;) generally enable improved adhesive spreading and resin infiltration, enhancing joint strength. Measurements were performed using an ADVANCED goniometer, model 300-F1. Standardized drops of distilled water were applied to the anodized surfaces, and images were digitally analyzed to obtain average contact angle values. This characterization enabled correlation between the anodizing conditions and surface wettability, serving as an additional criterion for assessing the quality of surface treatment in metal\u0026ndash;composite bonding applications.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"3. Results and Discussion","content":"\u003cp\u003e\u003cstrong\u003e3. 1 Anodizing\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAnodizing is a critical surface preparation step for aluminum alloys intended for joining with composite materials, as the morphology, thickness, and quality of the oxide layer directly influence interfacial adhesion and joint durability. In this study, anodizing of the AA2024-T3 alloy was carried out under different combinations of voltage and time to explore a range of conditions that would help identify optimal parameters for oxy-fuel welding (OFW) with PEI/glass fiber composite.Initially, three voltage levels (10, 15, and 20 V) and times of 5, 12.5, and 25 minutes were selected, as detailed in Table\u0026nbsp;\u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. All anodizing experiments were performed in triplicate to ensure statistical reliability. After preliminary analyses, it was observed that higher voltages could yield thicker and more porous oxide films, potentially enhancing mechanical anchorage of the composite. Consequently, a fourth voltage level (30 V) was included, maintaining a time of 25 minutes, to investigate the effect of an even higher voltage on the anodic layer characteristics.\u003c/p\u003e\n\u003cp\u003eAnodizing is a critical surface preparation step for aluminum alloys intended for joining with composite materials, as the morphology, thickness, and quality of the oxide layer directly influence interfacial adhesion and joint durability. In this study, anodizing of the AA2024-T3 alloy was carried out under different combinations of voltage and time to explore a range of conditions that would help identify optimal parameters for oxy-fuel welding (OFW) with PEI/glass fiber composite.\u003c/p\u003e\n\u003cp\u003eInitially, three voltage levels (10, 15, and 20 V) and anodizing times of 5, 12.5, and 25 minutes were selected, as detailed in Table \u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. All anodizing experiments were performed in triplicate to ensure statistical reliability. After preliminary analyses, it was observed that higher voltages could yield thicker and more porous oxide films, potentially enhancing the mechanical anchorage of the composite. Consequently, a fourth voltage level (30 V) was included, maintaining a time of 25 minutes, to investigate the effect of an even higher voltage on the anodic layer characteristics.\u003c/p\u003e\n\u003cp\u003eLiterature supports this strategy demonstrated that increasing anodizing voltage promotes faster growth and greater densification of the initial barrier layer, followed by increased porosity and larger pore diameters[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Such higher porosity can improve mechanical interlocking with polymer matrices by providing greater contact area and anchoring sites, which is beneficial for hybrid joint adhesion. However, excessive porosity may reduce film microhardness and create preferential pathways for aggressive electrolyte penetration, compromising corrosion resistance.\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003evoltage and time parameters for the anodizing process\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"13\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eSample\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e2\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e3\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003e5\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c7\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c8\"\u003e\n \u003cp\u003e7\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c9\"\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c10\"\u003e\n \u003cp\u003e9\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c11\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c12\"\u003e\n \u003cp\u003e11\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c13\"\u003e\n \u003cp\u003e12\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e\u003cstrong\u003eTime (min)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\n \u003cp\u003e5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\n \u003cp\u003e5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\n \u003cp\u003e5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003e12.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003e12.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c7\"\u003e\n \u003cp\u003e12.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\n \u003cp\u003e25\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e\n \u003cp\u003e25\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c10\"\u003e\n \u003cp\u003e25\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c11\"\u003e\n \u003cp\u003e25\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c12\"\u003e\n \u003cp\u003e25\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c13\"\u003e\n \u003cp\u003e25\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e\u003cstrong\u003eVoltage (V)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003e15\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003e15\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c7\"\u003e\n \u003cp\u003e15\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c8\"\u003e\n \u003cp\u003e20\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c9\"\u003e\n \u003cp\u003e20\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c10\"\u003e\n \u003cp\u003e20\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c11\"\u003e\n \u003cp\u003e30\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c12\"\u003e\n \u003cp\u003e30\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\" char=\".\" colname=\"c13\"\u003e\n \u003cp\u003e30\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n\u003cp\u003eFurther highlighted that controlled voltage regimes, including optimized anodizing times, lead to more homogeneous oxide layers with fewer cracks or structural defects, resulting in superior electrochemical performance. In metal\u0026ndash;composite joining applications, this greater uniformity is desirable to ensure a more stable stress distribution and to limit interfacial failure propagation under aging or harsh environmental conditions [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. Thus, the experimental design, which expanded the range of anodizing voltages and times, aimed not only to identify the most promising conditions for maximizing composite adhesion but also to balance factors such as oxide layer thickness, porosity, and mechanical integrity. This systematic approach enables a comprehensive evaluation of how electrochemical parameters influence the quality of the metal\u0026ndash;composite interface, providing a solid basis for optimizing the OFW process and achieving hybrid joints with improved mechanical performance and corrosion resistance.\u003c/p\u003e\n\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\n \u003ch2\u003e3.2 OFW Process\u003c/h2\u003e\n \u003cp\u003eThe bonding between anodized AA2024-T3 aluminum alloy and PEI/glass fiber composite via oxy-fuel welding (OFW) proved to be highly dependent on the surface condition of the aluminum, particularly on the oxide layer generated through anodization. This oxide layer plays a crucial role in promoting an effective interface with the polymer matrix, enhancing the mechanical anchoring of the resin into the oxide pores and improving the strength of the hybrid joint, as shown in Fig. \u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e6\u003c/span\u003e. To ensure effective adhesion, the composite must reach a temperature above its glass transition temperature (Tg), which is approximately 220\u0026deg;C [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. During the welding process, the surface temperature of the AA2024 samples was monitored using a pyrometer for each specimen to confirm that the thermal conditions were appropriate for partial melting of the thermoplastic matrix. After complete cooling, the joint quality was verified through visual inspection and manual handling tests.\u003c/p\u003e\n \u003cp\u003eThe mechanical strength of the welded joints was assessed through Lap Shear Strength (LSS) tests, as shown in Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. The results indicated that the anodization process significantly affected joint performance. Samples anodized at 10 V for 5 min exhibited the lowest shear strength values, with sample 1 reaching only 9.44 MPa. In contrast, the condition of 30 V for 25 min yielded a maximum strength of 14.52 MPa in sample 10, with an average value of (13.0\u0026thinsp;\u0026plusmn;\u0026thinsp;1.3) MPa across samples 10, 11, and 12. However, this increase was not substantially higher than the average value of (12.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.3) MPa obtained for samples anodized at 15 V for 12.5 min. These findings suggest that although higher voltages may enhance oxide layer thickness, joint strength does not increase linearly, possibly due to the formation of microcracks or structural instabilities in thicker anodic films.\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eWelding Temperature and Lap Shear Strength for Each Sample\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"13\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eAmostra\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e2\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e3\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003e5\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c7\"\u003e\n \u003cp\u003e6\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c8\"\u003e\n \u003cp\u003e7\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c9\"\u003e\n \u003cp\u003e8\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c10\"\u003e\n \u003cp\u003e9\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c11\"\u003e\n \u003cp\u003e10\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c12\"\u003e\n \u003cp\u003e11\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c13\"\u003e\n \u003cp\u003e12\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eWelding Temperature (\u0026deg;C)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e437\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e374\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e476\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003e433\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003e444\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c7\"\u003e\n \u003cp\u003e414\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c8\"\u003e\n \u003cp\u003e426\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c9\"\u003e\n \u003cp\u003e411\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c10\"\u003e\n \u003cp\u003e391\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c11\"\u003e\n \u003cp\u003e398\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c12\"\u003e\n \u003cp\u003e407\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c13\"\u003e\n \u003cp\u003e431\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eLap Shear Strength (MPa)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e9,44\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e11,9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e12,5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003e12,0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003e12,2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c7\"\u003e\n \u003cp\u003e12,7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c8\"\u003e\n \u003cp\u003e10,5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c9\"\u003e\n \u003cp\u003e13,1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c10\"\u003e\n \u003cp\u003e13,1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c11\"\u003e\n \u003cp\u003e14,5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c12\"\u003e\n \u003cp\u003e11,9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c13\"\u003e\n \u003cp\u003e12,6\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eAverage (MPa)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"3\" nameend=\"c4\" namest=\"c2\"\u003e\n \u003cp\u003e(11,3 \u0026plusmn; 1,3)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"3\" nameend=\"c7\" namest=\"c5\"\u003e\n \u003cp\u003e(12,4 \u0026plusmn; 0,3)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"3\" nameend=\"c10\" namest=\"c8\"\u003e\n \u003cp\u003e(12,2 \u0026plusmn; 1,2)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colspan=\"3\" nameend=\"c13\" namest=\"c11\"\u003e\n \u003cp\u003e(13,0 \u0026plusmn; 1,3)\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n \u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n \u003cp\u003eThe OFW process, due to its simplicity and low cost, proved viable for hybrid joint fabrication. The direct flame applied to the metallic surface promotes localized heating, allowing the molten polymer matrix to penetrate the oxide pores. The roughness and porosity of the anodized surface play a decisive role in adhesion quality, as previously reported [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e] [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]. The bonding mechanism is strengthened by capillarity, mechanical anchoring, and surface compatibility between the aluminum oxide and the composite. Furthermore, these results are consistent with previous studies, which reported increased shear strength for phosphoric acid anodization at moderate voltages and times, as well as improved adhesion of polymer-based coatings on anodized AA2024 surfaces [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Other investigations have highlighted that highly porous anodic films enhance adhesion but reduce microhardness and thermal crack resistance, which may explain the non-linear mechanical performance observed under more extreme anodization conditions .\u003c/p\u003e\n \u003cp\u003eFurthermore, these results are consistent with previous studies [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e] reported a 12% increase in shear strength in joints involving phosphoric acid anodization at 10 V for 10 min, while [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e] observed a significant improvement in epoxy coating adhesion on AA2024 anodized at 15 V for 30 min. [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e] [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e] also highlighted that more porous anodic films enhance adhesion but reduce microhardness and thermal crack resistance, which may explain the non-linear mechanical performance observed under more extreme anodization conditions.\u003c/p\u003e\n \u003cp\u003eFurthermore, these results are consistent with previous studies, which reported increased shear strength for phosphoric acid anodization at moderate voltages and times, as well as improved adhesion of polymer-based coatings on anodized AA2024 surfaces [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Other investigations have highlighted that highly porous anodic films enhance adhesion but reduce microhardness and thermal crack resistance, which may explain the non-linear mechanical performance observed under more extreme anodization conditions .\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\n \u003ch2\u003e3.3 experimental planning\u003c/h2\u003e\n \u003cp\u003eTo optimize the anodizing process of the AA2024 alloy and improve adhesion to the PEI/glass fiber composite, a full factorial experimental design 2\u0026sup2; was adopted, considering anodizing potential (levels: 10 V\u0026ndash;30 V) and anodizing time (levels: 5 min\u0026ndash;25 min) as independent variables. This statistical approach enables an efficient evaluation of both the individual and interaction effects of the selected factors on corrosion resistance and on the mechanical strength of the welded joints. The application of a structured and randomized experimental design is essential to minimize systematic errors and ensure data reproducibility [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. The full factorial model was implemented using Design Expert software version 6.0.6, which was also employed to perform analysis of variance (ANOVA) in order to assess the statistical significance of the individual and combined effects of the process variables.\u003c/p\u003e\n \u003cp\u003eThe initial anodizing conditions were defined based on previously reported studies. Phosphoric acid anodization at 10 V for 10 min applied to AA2024 alloys has been shown to promote a significant increase in shear strength when bonded to fiber-reinforced polymer composites [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]. In addition, sulfuric acid anodization at 15 V for 30 min has been reported to enhance epoxy coating adhesion and improve corrosion resistance [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. Based on these findings, the minimum and maximum values for anodizing voltage and time were established in the present study. The anodizing conditions adopted in the experimental plan are summarized in Table \u003cspan refid=\"Tab5\" class=\"InternalRef\"\u003e5\u003c/span\u003e\u003c/p\u003e\n \u003cp style='margin-top:3.0pt;margin-right:0cm;margin-bottom:3.0pt;margin-left:0cm;text-align:justify;font-size:13px;font-family:\"Arial\",sans-serif;'\u003e\u003cstrong\u003e\u003cspan style='font-family:\"Times New Roman\",serif;'\u003eTable 5 - \u003c/span\u003e\u003c/strong\u003e\u003cspan style='font-family:\"Times New Roman\",serif;'\u003eExperimental conditions and corresponding Lap Shear strength results.\u003c/span\u003e\u003c/p\u003e\n \u003ctable style=\"width:487.35pt;border-collapse:collapse;border:none;\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"width:75.2pt;border-top:solid windowtext 1.0pt;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:none;padding:0cm 5.4pt 0cm 5.4pt;height:9.1pt;\"\u003e\n \u003cp style='margin:0cm;text-align:center;line-height:normal;font-size:16px;font-family:\"Times New Roman\",serif;color:black;margin-right:.55pt;border:none;'\u003e\u003cspan style=\"font-size:13px;\"\u003eExperiment\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:30.3pt;border-top:solid windowtext 1.0pt;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:none;padding:0cm 5.4pt 0cm 5.4pt;height:9.1pt;\"\u003e\n \u003cp style='margin:0cm;text-align:center;line-height:normal;font-size:16px;font-family:\"Times New Roman\",serif;color:black;margin-right:.55pt;border:none;'\u003e\u003cspan style=\"font-size:13px;color:windowtext;\"\u003e1\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:30.3pt;border-top:solid windowtext 1.0pt;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:none;padding:0cm 5.4pt 0cm 5.4pt;height:9.1pt;\"\u003e\n \u003cp style='margin:0cm;text-align:center;line-height:normal;font-size:16px;font-family:\"Times New Roman\",serif;color:black;margin-right:.55pt;border:none;'\u003e\u003cspan style=\"font-size:13px;color:windowtext;\"\u003e2\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:30.3pt;border-top:solid windowtext 1.0pt;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:none;padding:0cm 5.4pt 0cm 5.4pt;height:9.1pt;\"\u003e\n \u003cp style='margin:0cm;text-align:center;line-height:normal;font-size:16px;font-family:\"Times New Roman\",serif;color:black;margin-right:.55pt;border:none;'\u003e\u003cspan style=\"font-size:13px;color:windowtext;\"\u003e3\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:30.3pt;border-top:solid windowtext 1.0pt;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:none;padding:0cm 5.4pt 0cm 5.4pt;height:9.1pt;\"\u003e\n \u003cp style='margin:0cm;text-align:center;line-height:normal;font-size:16px;font-family:\"Times New Roman\",serif;color:black;margin-right:.55pt;border:none;'\u003e\u003cspan style=\"font-size:13px;color:windowtext;\"\u003e4\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:32.2pt;border-top:solid windowtext 1.0pt;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:none;padding:0cm 5.4pt 0cm 5.4pt;height:9.1pt;\"\u003e\n \u003cp style='margin:0cm;text-align:center;line-height:normal;font-size:16px;font-family:\"Times New Roman\",serif;color:black;margin-right:.55pt;border:none;'\u003e\u003cspan style=\"font-size:13px;color:windowtext;\"\u003e5\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:32.2pt;border-top:solid windowtext 1.0pt;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:none;padding:0cm 5.4pt 0cm 5.4pt;height:9.1pt;\"\u003e\n \u003cp style='margin:0cm;text-align:center;line-height:normal;font-size:16px;font-family:\"Times New Roman\",serif;color:black;margin-right:.55pt;border:none;'\u003e\u003cspan style=\"font-size:13px;color:windowtext;\"\u003e6\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:32.2pt;border-top:solid windowtext 1.0pt;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:none;padding:0cm 5.4pt 0cm 5.4pt;height:9.1pt;\"\u003e\n \u003cp style='margin:0cm;text-align:center;line-height:normal;font-size:16px;font-family:\"Times New Roman\",serif;color:black;margin-right:.55pt;border:none;'\u003e\u003cspan style=\"font-size:13px;color:windowtext;\"\u003e7\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:35.25pt;border-top:solid windowtext 1.0pt;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:none;padding:0cm 5.4pt 0cm 5.4pt;height:9.1pt;\"\u003e\n \u003cp style='margin:0cm;text-align:center;line-height:normal;font-size:16px;font-family:\"Times New Roman\",serif;color:black;margin-right:.55pt;border:none;'\u003e\u003cspan style=\"font-size:13px;color:windowtext;\"\u003e8\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:32.2pt;border-top:solid windowtext 1.0pt;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:none;padding:0cm 5.4pt 0cm 5.4pt;height:9.1pt;\"\u003e\n \u003cp style='margin:0cm;text-align:center;line-height:normal;font-size:16px;font-family:\"Times New Roman\",serif;color:black;margin-right:.55pt;border:none;'\u003e\u003cspan style=\"font-size:13px;color:windowtext;\"\u003e9\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:32.2pt;border-top:solid windowtext 1.0pt;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:none;padding:0cm 5.4pt 0cm 5.4pt;height:9.1pt;\"\u003e\n \u003cp style='margin:0cm;text-align:center;line-height:normal;font-size:16px;font-family:\"Times New Roman\",serif;color:black;margin-right:.55pt;border:none;'\u003e\u003cspan style=\"font-size:13px;color:windowtext;\"\u003e10\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:32.2pt;border-top:solid windowtext 1.0pt;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:none;padding:0cm 5.4pt 0cm 5.4pt;height:9.1pt;\"\u003e\n \u003cp style='margin:0cm;text-align:center;line-height:normal;font-size:16px;font-family:\"Times New Roman\",serif;color:black;margin-right:.55pt;border:none;'\u003e\u003cspan style=\"font-size:13px;color:windowtext;\"\u003e11\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:32.2pt;border-top:solid windowtext 1.0pt;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:none;padding:0cm 5.4pt 0cm 5.4pt;height:9.1pt;\"\u003e\n \u003cp style='margin:0cm;text-align:center;line-height:normal;font-size:16px;font-family:\"Times New Roman\",serif;color:black;margin-right:.55pt;border:none;'\u003e\u003cspan style=\"font-size:13px;color:windowtext;\"\u003e12\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:30.3pt;border-top:solid windowtext 1.0pt;border-left:none;border-bottom:solid windowtext 1.0pt;border-right:none;padding:0cm 5.4pt 0cm 5.4pt;height:9.1pt;\"\u003e\n \u003cp style='margin:0cm;text-align:center;line-height:normal;font-size:16px;font-family:\"Times New Roman\",serif;color:black;margin-right:.55pt;border:none;'\u003e\u003cspan style=\"font-size:13px;color:windowtext;\"\u003e13\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width:75.2pt;border:none;padding:0cm 5.4pt 0cm 5.4pt;height:9.1pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003eTime (min)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:30.3pt;border:none;padding:0cm 5.4pt 0cm 5.4pt;height:9.1pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:30.3pt;border:none;padding:0cm 5.4pt 0cm 5.4pt;height:9.1pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e\u003cspan style=\"color:red;background:lightgrey;\"\u003e20\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:30.3pt;border:none;padding:0cm 5.4pt 0cm 5.4pt;height:9.1pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:30.3pt;border:none;padding:0cm 5.4pt 0cm 5.4pt;height:9.1pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New 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Roman\",serif;text-align:center;'\u003e12,5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:32.2pt;border:none;padding:0cm 5.4pt 0cm 5.4pt;height:9.1pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e12,5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:32.2pt;border:none;padding:0cm 5.4pt 0cm 5.4pt;height:9.1pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e12,5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:32.2pt;border:none;padding:0cm 5.4pt 0cm 5.4pt;height:9.1pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e12,5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:32.2pt;border:none;padding:0cm 5.4pt 0cm 5.4pt;height:9.1pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e12,5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:30.3pt;border:none;padding:0cm 5.4pt 0cm 5.4pt;height:9.1pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e12,5\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width:75.2pt;padding:0cm 5.4pt 0cm 5.4pt;height:14.2pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003ePotential (V)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:30.3pt;padding:0cm 5.4pt 0cm 5.4pt;height:14.2pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e10,0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:30.3pt;padding:0cm 5.4pt 0cm 5.4pt;height:14.2pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e\u003cspan style=\"color:red;background:lightgrey;\"\u003e10,0\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:30.3pt;padding:0cm 5.4pt 0cm 5.4pt;height:14.2pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e20,0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:30.3pt;padding:0cm 5.4pt 0cm 5.4pt;height:14.2pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e20,0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:32.2pt;padding:0cm 5.4pt 0cm 5.4pt;height:14.2pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e15,0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:32.2pt;padding:0cm 5.4pt 0cm 5.4pt;height:14.2pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e15,0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:32.2pt;padding:0cm 5.4pt 0cm 5.4pt;height:14.2pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e7,93\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:35.25pt;padding:0cm 5.4pt 0cm 5.4pt;height:14.2pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e22,1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:32.2pt;padding:0cm 5.4pt 0cm 5.4pt;height:14.2pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e15,0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:32.2pt;padding:0cm 5.4pt 0cm 5.4pt;height:14.2pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e15,0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:32.2pt;padding:0cm 5.4pt 0cm 5.4pt;height:14.2pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e15,0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:32.2pt;padding:0cm 5.4pt 0cm 5.4pt;height:14.2pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e15,0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:30.3pt;padding:0cm 5.4pt 0cm 5.4pt;height:14.2pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e15,0\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width:75.2pt;border:none;border-bottom:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;height:14.2pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e\u003cem\u003eJoint Strength\u003c/em\u003e\u003cem\u003e\u0026nbsp;\u003c/em\u003e\u003cem\u003e(Mpa)\u003c/em\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:30.3pt;border:none;border-bottom:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;height:14.2pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e11\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:30.3pt;border:none;border-bottom:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;height:14.2pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e\u003cspan style=\"color:red;background:lightgrey;\"\u003e14\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:30.3pt;border:none;border-bottom:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;height:14.2pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:30.3pt;border:none;border-bottom:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;height:14.2pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e11\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:32.2pt;border:none;border-bottom:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;height:14.2pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e11\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:32.2pt;border:none;border-bottom:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;height:14.2pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e11\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:32.2pt;border:none;border-bottom:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;height:14.2pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e13\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:35.25pt;border:none;border-bottom:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;height:14.2pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e10\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:32.2pt;border:none;border-bottom:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;height:14.2pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:32.2pt;border:none;border-bottom:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;height:14.2pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:32.2pt;border:none;border-bottom:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;height:14.2pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:32.2pt;border:none;border-bottom:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;height:14.2pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width:30.3pt;border:none;border-bottom:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;height:14.2pt;\"\u003e\n \u003cp style='margin:0cm;font-size:13px;font-family:\"Times New Roman\",serif;text-align:center;'\u003e12\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003cp\u003eBased on the minimum and maximum values of anodizing time and potential defined during the preliminary stage, a Central Composite Rotatable Design (CCRD) was developed using Design Expert\u0026reg; software version 6.0.6 to investigate the influence of anodizing parameters on the mechanical strength of the welded joints. The objective was to establish a statistical model capable of predicting joint performance under different anodizing conditions. Thirteen experiments were carried out, distributed among axial, central, and factorial points. The coded levels of the factors (time and voltage) were adjusted to allow the generation of a reliable response surface, enabling the identification of optimal anodizing conditions.\u003c/p\u003e\n \u003cp\u003eJoint strength, obtained from Lap Shear Strength (LSS) tests, was adopted as the response variable for model development. Analysis of variance (ANOVA) was applied to evaluate the statistical significance of the main effects, their interactions, and the adequacy of the proposed model. Model validation was performed based on the coefficient of determination (R\u0026sup2;), residual analysis, and the statistical significance of the model terms, ensuring the robustness of the experimental approach, as presented in Table \u003cspan refid=\"Tab6\" class=\"InternalRef\"\u003e6\u003c/span\u003e\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable float=\"Yes\" id=\"Tab6\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 6\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eAnalysis of Variance (ANOVA) for the Anodizing Process of AA2024 Alloy.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"6\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eSource\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003eSum of squares\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003eDF\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003eMean Square\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003eF-Value\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003eProb\u0026thinsp;\u0026gt;\u0026thinsp;F\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eModel\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e12,94456376\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e2,588912753\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003e8,498750262\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003e0,0069\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eA (time)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003e6,565497623\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003e0,0374\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eB (Voltage)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e8,492640687\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e8,492640687\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003e27,87920612\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003e0,0011\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eA\u0026sup2;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e1,331521739\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e1,331521739\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003e4,371051407\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003e0,0749\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eB\u0026sup2;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e0,244565217\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e0,244565217\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003e0,802846177\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003e0,4000\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eAB\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003e3,282748811\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003e0,1129\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eResidual\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e2,132359313\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e0,304622759\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eLack of Fit\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e2,132359313\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e0,710786438\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eErro\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eCor. total\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e15,07692308\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eR\u0026sup2;\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e0,858568005\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\u0026nbsp;\u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003ctfoot\u003e\n \u003ctr\u003e\n \u003ctd colspan=\"6\"\u003e* Significant value\u0026thinsp;\u0026lt;\u0026thinsp;0,05\u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tfoot\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n \u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n \u003cp\u003eThe statistical analysis derived from the CCRD allowed the evaluation of the influence of anodizing parameters\u0026mdash;time and voltage\u0026mdash;on the mechanical strength of joints between the AA2024 aluminum alloy and the PEI/glass fiber composite. The quadratic model fitted to the response variable exhibited a satisfactory coefficient of determination (R\u0026sup2; = 0.8586), indicating that 85.86% of the variability in joint strength could be explained by the model.\u003c/p\u003e\n \u003cp\u003eBoth anodizing time (A) and voltage (B) were identified as statistically significant terms, confirming their direct influence on joint strength. However, the quadratic terms associated with time and voltage were not statistically significant, suggesting a predominantly linear behavior within the investigated experimental range. According to the ANOVA results, p-values (Prob\u0026thinsp;\u0026lt;\u0026thinsp;F) lower than 0.0500 indicate statistically significant model terms. Therefore, in the conventional anodizing process, both anodizing time and voltage were confirmed as critical parameters affecting bonding strength.\u003c/p\u003e\n \u003cp\u003eThe response surface generated by the model indicated an optimal anodizing region near 12.5 minutes and 15 V, which is consistent with the favorable results observed in preliminary tests. Lower anodizing times and voltages resulted in reduced joint strength, likely due to the formation of thinner and less stable anodic oxide layers. Conversely, although higher voltages such as 30 V promote thicker oxide films, they may also induce surface microcracks or structural inhomogeneities that compromise joint integrity [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. The absence of a statistically significant lack of fit further confirms the reliability of the proposed model, which can be applied as a predictive tool for optimizing the anodizing process and improving the mechanical performance of hybrid joints.\u003c/p\u003e\n \u003cp\u003eLapShear\u0026thinsp;=\u0026thinsp;+\u0026thinsp;12.00\u0026thinsp;+\u0026thinsp;2.00*A-0.94*B-0.44*A2-0.19*B2-0.50*A*B-1.00*A3-0.061*B3 ......................... (1)\u003c/p\u003e\n \u003cp\u003eThe model coefficients were estimated using the Ordinary Least Squares (OLS) method, which minimizes the difference between experimental and predicted response values. Based on the Design-Expert\u0026reg; analysis, response surface plots were generated (Fig. \u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e7\u003c/span\u003e), illustrating the relationship between anodizing conditions and the resulting Lap Shear Strength (LSS). The model predicted a maximum joint strength of approximately 14 MPa at anodizing conditions of 10 V and 18 minutes, identifying this region as the most effective for enhancing adhesion between anodized aluminum and the PEI/glass fiber composite.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\n \u003ch2\u003e3.4 Contact Angle Analysis\u003c/h2\u003e\n \u003cp\u003eIn this analysis, a water droplet was deposited onto the surface of the anodized AA2024 alloy to observe its shape and the angle formed with the surface (contact angle). When the measured angle is greater than 90\u0026deg;, the material is considered hydrophobic; when it is less than 90\u0026deg;, it is classified as hydrophilic, indicating a greater tendency for water penetration into the surface. This test was applied to evaluate the hydrophilicity of anodized AA2024 under different anodizing conditions. A comparison was performed between the preliminary conditions (10 V / 5 min, 15 V / 12.5 min, 20 V / 25 min, and 30 V / 25 min) and the optimal condition identified (10 V / 20 min). The contact angle values obtained for each anodizing condition are presented in Table \u003cspan refid=\"Tab7\" class=\"InternalRef\"\u003e7\u003c/span\u003e.\u003c/p\u003e\n \u003cp\u003eUnder the optimal anodizing condition (10 V / 20 min), the lowest contact angle value (8.21\u0026deg;) was observed, indicating a highly hydrophilic surface. Previous studies have reported a significant improvement in wettability starting from anodizing potentials of 10 V, with contact angles around 13\u0026deg; observed at 15 V for 30 minutes [\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. All anodized samples exhibited hydrophilic behavior, which is a critical factor for promoting improved adhesion during the welding process between the composite and the aluminum alloy. Similar behavior has been reported in the literature, where anodized aluminum surfaces present high porosity and roughness, characterized by peaks and valleys that increase surface energy and hydrophilicity, with average contact angle values close to 15\u0026deg; [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e].\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable float=\"Yes\" id=\"Tab7\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 7\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eContact angle values under different anodization conditions\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"6\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eSamples\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e10 V/5min\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e15 V/12,5min\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e20 V/25min\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003e30 V/25min\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003e10 V/20min*\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e\u003cstrong\u003eContact Angle (\u0026ordm;)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e19,14\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e19,21\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e9,29\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003e10,34\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003e8,21\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e\u003cstrong\u003eStandard Deviation\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e1,44\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e3,65\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e0,82\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003e1,37\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003e0,90\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003ctfoot\u003e\n \u003ctr\u003e\n \u003ctd colspan=\"6\"\u003e*Optimal anodization condition\u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tfoot\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n \u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n \u003cp\u003eA noticeable difference was observed between the contact angle values of samples anodized at higher potentials and those treated under optimal conditions. This behavior is commonly associated with increased surface roughness, which leads to higher surface free energy and enhanced hydrophilicity. Surface free energy plays a fundamental role in intermolecular interactions and strongly influences adsorption and adhesion mechanisms with other materials [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. The droplet profile analysis reinforces the previous results and helps explain the performance of the welded joints, as the joining process between the composite and the anodized aluminum relies on the penetration of the PEI matrix into surface pores through capillary action. Therefore, higher surface hydrophilicity favors greater polymer infiltration into these regions. Figure \u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e8\u003c/span\u003e illustrates the interaction between the anodized surface and the water droplet.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\n \u003ch2\u003e3.5 Microscopic Analysis\u003c/h2\u003e\n \u003cp\u003eThe surfaces of the AA2024 alloy were examined after the Lap Shear Strength (LSS) tests using a stereomicroscope in order to evaluate the quality of the interfacial anchorage between the PEI/glass fiber composite and the anodized aluminum surface. The analysis focused on the fracture regions, aiming to identify residual polymer matrix adhered to the metallic substrate, which is indicative of effective mechanical interlocking.\u003c/p\u003e\n \u003cp\u003eFigure \u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e9\u003c/span\u003e presents the fractured area, highlighted in red, on the aluminum surface selected for assessing metal\u0026ndash;composite adhesion. For the sample anodized at 10 V for 5 minutes (Fig.\u0026nbsp;\u003cspan refid=\"Fig8\" class=\"InternalRef\"\u003e10\u003c/span\u003ea), only a limited amount of residual composite was observed. This condition, associated with low anodizing potential and short treatment time, resulted in reduced interaction between the composite and the alloy surface. Regions of exposed aluminum substrate (indicated by red arrows) coexist with isolated areas containing residual composite (blue arrows), revealing a non-uniform bonding mechanism. The left side of the sample, corresponding to the edge region, exhibited a larger exposed substrate area, while the central region showed greater composite impregnation. This behavior is attributed to the direction of flame application during the OFW process, which progressed from the edge toward the center, leading to higher thermal input in the central zone. At 10\u0026times; magnification, discontinuous fiber imprints were observed on the aluminum surface, indicating insufficient anchorage under this anodizing condition.\u003c/p\u003e\n \u003cp\u003eIn contrast, samples anodized at higher potentials and longer times exhibited a substantially greater amount of residual composite on the aluminum surface, reflecting improved adhesion. This enhancement is associated with the increased thickness and roughness of the anodic oxide layer formed under these conditions, which promotes more effective mechanical interlocking [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. In particular, samples anodized at 30 V for 25 minutes showed well-defined and continuous fiber imprints across the interface, suggesting strong and homogeneous anchorage.\u003c/p\u003e\n \u003cp\u003eThe most pronounced anchoring behavior was observed for the optimal anodizing condition (10 V for 20 minutes). In this case, uniform composite impregnation was evident over the entire aluminum surface, with continuous fiber imprints clearly transferred to the metallic substrate. This result indicates that, despite the lower anodizing potential, the extended anodizing time enabled the formation of a porous and well-adhered oxide layer, facilitating resin infiltration through capillary action and promoting the development of a cohesive metal\u0026ndash;composite interface [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e].\u003c/p\u003e\n \u003cp\u003eOverall, the microscopy results confirm that both anodizing potential and time play a decisive role in controlling the quality of the metal\u0026ndash;composite interface. The optimal condition identified through experimental design provided the most favorable balance between oxide morphology and interfacial adhesion, directly correlating with the highest joint strength values and reinforcing the importance of surface parameter optimization for hybrid structural applications.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\n \u003ch2\u003e3.6 Scanning Electron Microscopy (SEM)\u003c/h2\u003e\n \u003cp\u003eIn order to gain a deeper understanding of the factors influencing the bonding between the AA2024 aluminum alloy and the PEI/glass fiber composite, Scanning Electron Microscopy (SEM) analyses were performed on the anodized alloy surfaces under different voltage and time conditions (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e11\u003c/span\u003e). The micrographs acquired at 1000\u0026times; magnification allowed a detailed evaluation of the oxide layer morphology, which plays a critical role in the mechanical anchoring of the composite and, consequently, in the mechanical strength of the welded joints. Since adhesion between dissimilar materials occurs predominantly through capillary action, surface topography becomes a key parameter governing interfacial performance.\u003c/p\u003e\n \u003cp\u003eUnder the anodizing condition of 10 V for 5 minutes, the surface exhibited a relatively smooth and uniform appearance, with a low density of visible pores, indicating the formation of a denser and more compact oxide film. Such morphology limits mechanical interlocking with the polymer matrix, which is consistent with the reduced joint strength observed under this condition. Increasing the anodizing parameters to 15 V for 12.5 minutes resulted in a noticeable increase in surface roughness, characterized by well-defined grooves and cavities distributed across the surface. This morphology favors resin penetration and mechanical anchoring, in agreement with the improved mechanical resistance obtained. A further increase to 20 V for 25 minutes intensified this effect, producing an even more irregular surface with a higher density of deep cavities, suggesting enhanced electrolyte attack and increased oxide layer porosity.\u003c/p\u003e\n \u003cp\u003eHowever, under the most aggressive condition of 30 V for 25 minutes, an opposite trend was observed, with the surface appearing visually smoother and less porous. This behavior is attributed to partial degradation of the oxide layer, likely associated with the accumulation of reaction by-products, such as aluminum hydroxides, which can block pores and reduce surface roughness [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. The anodizing condition of 10 V for 20 minutes\u0026mdash;identified as optimal through experimental design\u0026mdash;exhibited a homogeneous and well-developed morphology, with uniform pore distribution and clearly defined surface features. Although less rough than surfaces formed at intermediate voltages, this condition provided effective mechanical interlocking, indicating that oxide layer homogeneity is as important as absolute roughness for achieving strong interfacial bonding. These observations reinforce the hypothesis that both surface roughness and oxide layer uniformity are critical parameters for successful bonding between thermoplastic composites and anodized aluminum substrates. Similar trends reported in the literature indicate that increased anodizing potential enhances surface roughness up to a critical limit, beyond which oxide degradation or saturation effects may occur [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e] [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e].\u003c/p\u003e\n \u003cp\u003eFor a more detailed evaluation of the surface microstructure, additional SEM analyses were conducted at 40,000\u0026times; magnification (Fig. \u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e12\u003c/span\u003e). At this scale, microstructural features associated with oxide formation under different anodizing conditions become more evident. For the sample anodized at 10 V for 5 minutes (Fig. \u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e12\u003c/span\u003ea), the surface displayed a granular morphology composed of relatively large and shallow oxide \u0026ldquo;islands,\u0026rdquo; widely spaced from one another. This configuration indicates a low-porosity surface with limited specific surface area, which may restrict polymer penetration during bonding.\u003c/p\u003e\n \u003cp\u003eWhen the anodizing parameters were increased to 15 V for 12.5 minutes (Fig. \u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e12\u003c/span\u003eb), the surface exhibited a more compact and interconnected structure, with a well-distributed network of pores. This morphology enhances mechanical anchoring and correlates well with the higher shear strength values obtained. The sample anodized at 20 V for 25 minutes (Fig. \u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e12\u003c/span\u003ec) showed a marked increase in porosity and surface roughness, with smaller and more densely packed oxide features, reflecting intensified electrolyte action. While such morphology may favor capillary-driven adhesion, excessive roughness can also compromise interfacial homogeneity.\u003c/p\u003e\n \u003cp\u003eAt 30 V for 25 minutes (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e12\u003c/span\u003ed), the surface consisted of extremely fine and uniformly distributed particles, but with a flatter and more compact appearance. This suggests pore saturation or blockage, likely caused by reaction products filling the pore structure, which may reduce the functional effectiveness of the oxide layer. Finally, the optimal condition of 10 V for 20 minutes (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e12\u003c/span\u003ee) exhibited a balanced morphology, characterized by a homogeneous distribution of small oxide features forming a continuous porous network. This combination of uniformity and adequate porosity provides an optimal balance between roughness and structural integrity, directly supporting the superior joint performance observed in the mechanical tests.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec13\" class=\"Section2\"\u003e\n \u003ch2\u003e3.7 Electrochemical Impedance Spectroscopy (EIS)\u003c/h2\u003e\n \u003cp\u003eThe electrochemical behavior of the AA2024 alloy\u0026mdash;including the mechanically polished, non-anodized sample used as a control and the samples anodized under different voltage and time conditions\u0026mdash;was evaluated by electrochemical impedance spectroscopy (EIS). The objective was to assess the protective efficiency of the anodic films formed under different anodizing parameters. The measurements were carried out in a 0.1 mol L⁻\u0026sup1; sodium sulfate (Na₂SO₄) solution, chosen as a neutral and non-aggressive electrolyte. The experimental impedance data, presented in Nyquist and Bode representations (Figs.\u0026nbsp;\u003cspan refid=\"Fig11\" class=\"InternalRef\"\u003e13\u003c/span\u003e and \u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e14\u003c/span\u003e), were fitted using equivalent electrical circuit models to describe the electrochemical response of the system. Several circuit configurations were tested based on established approaches reported in the literature [27,28]. Among them, the models shown in Figs.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e15\u003c/span\u003e(a) and 15(b) provided the best fitting quality and physical relevance. The circuit in Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e15\u003c/span\u003e(a) consists of the solution resistance (Rs) in series with a parallel combination of a polarization resistance (Rp) and a constant phase element (CPE). The use of a CPE instead of an ideal capacitor accounts for the non-ideal capacitive behavior arising from surface heterogeneity, roughness, and non-uniform current distribution typically observed on corroding metallic surfaces.\u003c/p\u003e\n \u003cp\u003eTo better represent the electrochemical response of anodized samples, a more complex circuit (Fig.\u0026nbsp;\u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e15\u003c/span\u003e(b)) was adopted, incorporating two time constants. This approach is widely used for metallic substrates covered by anodic or protective films [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e29\u003c/span\u003e]. In this configuration, two CPE\u0026ndash;Rp elements are arranged in series, representing the contribution of different layers within the anodic oxide film. The first branch (CPE₁\u0026ndash;Rp₁) is associated with the outer porous layer and the immediate electrolyte/oxide interface, reflecting the influence of surface roughness, pore distribution, and electrolyte penetration. The second branch (CPE₂\u0026ndash;Rp₂) corresponds to the inner barrier-type oxide layer, which is primarily responsible for corrosion protection at low frequencies. The fitting parameters are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab8\" class=\"InternalRef\"\u003e8\u003c/span\u003e. The solution resistance (Rs) remained approximately constant at ~\u0026thinsp;50 Ω for all samples, including the control, as expected since the electrolyte composition was identical in all experiments. In contrast, the anodized samples exhibited significantly higher polarization resistance values compared to the pickled control sample, confirming the formation of anodic oxide films with a barrier effect against charge transfer.\u003c/p\u003e\n \u003cp\u003eDespite this general improvement, no monotonic increase in polarization resistance with increasing anodizing voltage or time was observed. This behavior reflects the complex relationship between anodizing parameters and oxide film quality. Under mild conditions (low voltage and short time), oxide formation may be incomplete, resulting in thinner or discontinuous films. Conversely, more aggressive conditions, such as anodizing at 30 V for extended times, can lead to excessive pore growth, increased defect density, or partial collapse of the oxide structure, ultimately compromising its protective performance. This interpretation is consistent with the SEM observations discussed previously, which revealed surface smoothing and pore blockage under severe anodizing conditions.\u003c/p\u003e\n \u003cp\u003eThe incorporation of two constant phase elements was essential to describe the non-ideal capacitive behavior associated with film heterogeneity. The admittance parameter (Y₀) of the control sample was significantly higher (4.40 \u0026micro;S\u0026middot;sⁿ) than those of the anodized samples (0.12\u0026ndash;0.57 \u0026micro;S\u0026middot;sⁿ), indicating a decrease in effective interfacial capacitance due to the formation of thicker and more insulating oxide layers. The CPE exponent values (n₁ and n₂) ranged between 0.90 and 0.99, suggesting a response close to that of an ideal capacitor and indicating relatively homogeneous electrical behavior, despite the morphological variations observed by SEM.\u003c/p\u003e\n \u003cp\u003eAccording to the results presented in Table 9, the sample anodized at 10 V for 20 minutes exhibited the highest Rp₂ value among all anodized conditions, demonstrating superior barrier layer performance. This finding correlates well with SEM analyses, which showed a homogeneous and well-developed porous morphology under this condition, favoring both corrosion protection and mechanical anchoring. In contrast, the sample anodized at 30 V for 25 minutes showed lower Rp₂ and n₂ values, indicating reduced protection due to increased roughness, heterogeneity, and possible pore blockage.\u003c/p\u003e\n \u003cp\u003eOverall, the EIS results confirm that anodizing significantly enhances the corrosion resistance of the AA2024 alloy compared to the non-anodized condition. However, the protective efficiency of the anodic films strongly depends on the anodizing parameters, which directly affect oxide morphology, uniformity, and defect density. The condition of 10 V for 20 minutes provided the most favorable balance between oxide homogeneity and barrier properties, in excellent agreement with the SEM observations and mechanical performance results. Equivalent circuit models with two Rp\u0026ndash;CPE elements in series, similar to those proposed in previous studies [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e30\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e], were also evaluated. Nevertheless, under the present experimental conditions, the adopted models (Fig. \u003cspan refid=\"Fig13\" class=\"InternalRef\"\u003e15\u003c/span\u003e) yielded superior fitting quality, with very low \u0026chi;\u0026sup2; values (10⁻\u0026sup3;\u0026ndash;10⁻⁴), confirming their adequacy in describing the electrochemical behavior of the anodized AA2024 alloy.\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u003ctable float=\"Yes\" id=\"Tab8\" border=\"1\"\u003e\n \u003ccaption language=\"En\"\u003e\n \u003cdiv class=\"CaptionNumber\"\u003eTable 8\u003c/div\u003e\n \u003cdiv class=\"CaptionContent\"\u003e\n \u003cp\u003eEIS fitting parameters obtained for different AA2024 samples (control and anodized) measured in 0.1 mol L⁻\u0026sup1; sodium sulfate (Na₂SO₄) solution.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003ccolgroup cols=\"9\"\u003e\u003c/colgroup\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eSamples\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e\u003cem\u003eR\u003c/em\u003e\u003csub\u003es\u003c/sub\u003e\u003c/p\u003e\n \u003cp\u003e(\u0026Omega;)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e\u003cem\u003eR\u003c/em\u003e\u003csub\u003ep1\u003c/sub\u003e\u003c/p\u003e\n \u003cp\u003e(kΩ)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003eCPE\u003csub\u003e1\u003c/sub\u003e\u003c/p\u003e\n \u003cp\u003e(\u0026micro;Mho. S\u003csup\u003e\u003cem\u003en\u003c/em\u003e\u003c/sup\u003e)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003e\u003cem\u003en\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003e\u003cem\u003eR\u003c/em\u003e\u003csub\u003ep2\u003c/sub\u003e\u003c/p\u003e\n \u003cp\u003e(kΩ)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c7\"\u003e\n \u003cp\u003eCPE\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\n \u003cp\u003e(\u0026micro;Mho. S\u003csup\u003e\u003cem\u003en\u003c/em\u003e\u003c/sup\u003e)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c8\"\u003e\n \u003cp\u003e\u003cem\u003en\u003c/em\u003e\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\" colname=\"c9\"\u003e\n \u003cp\u003e\u0026chi;\u003csup\u003e2\u003c/sup\u003e\u003c/p\u003e\n \u003c/th\u003e\n \u003c/tr\u003e\n \u003c/thead\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003eControl\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e49,4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e354\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e4,40\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003e0,94\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003e---\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c7\"\u003e\n \u003cp\u003e---\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c8\"\u003e\n \u003cp\u003e---\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c9\"\u003e\n \u003cp\u003e0,03\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e10 V, 5 min\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e54,8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e220\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e0,57\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003e0,97\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003e736\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c7\"\u003e\n \u003cp\u003e0,14\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c8\"\u003e\n \u003cp\u003e0,99\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c9\"\u003e\n \u003cp\u003e0,006\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e15 V, 12,5 min\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e55,0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e707\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e0,64\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003e0,97\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003e225\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c7\"\u003e\n \u003cp\u003e0,15\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c8\"\u003e\n \u003cp\u003e0,86\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c9\"\u003e\n \u003cp\u003e0,02\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e20 V, 25 min\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e51,4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e581\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e0,30\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003e0,97\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003e840\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c7\"\u003e\n \u003cp\u003e0,22\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c8\"\u003e\n \u003cp\u003e0,90\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c9\"\u003e\n \u003cp\u003e0,01\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e30 V, 25 min\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e51,3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e630\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e0,31\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003e0,97\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003e163\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c7\"\u003e\n \u003cp\u003e0,45\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c8\"\u003e\n \u003cp\u003e0,65\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c9\"\u003e\n \u003cp\u003e0,0004\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\" colname=\"c1\"\u003e\n \u003cp\u003e10 V, 20 min\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c2\"\u003e\n \u003cp\u003e48,8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c3\"\u003e\n \u003cp\u003e155\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c4\"\u003e\n \u003cp\u003e0,12\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c5\"\u003e\n \u003cp\u003e0,95\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c6\"\u003e\n \u003cp\u003e921\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c7\"\u003e\n \u003cp\u003e0,15\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c8\"\u003e\n \u003cp\u003e0,95\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\" colname=\"c9\"\u003e\n \u003cp\u003e0,002\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003c/div\u003e\n \u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n \u003cp\u003ePrevious studies have demonstrated that the electrochemical performance of anodic films on the AA2024 alloy is strongly influenced by the electrolyte composition, oxide layer morphology, and the use of modifying agents or post-treatment sealing steps. Significant improvements in barrier layer resistance have been reported when mixed electrolytes and corrosion inhibitors are employed, particularly when combined with sealing treatments, which can increase impedance values by several orders of magnitude [\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e31\u003c/span\u003e]. Other investigations have shown that, although hard anodizing conditions promote thicker oxide layers, long-term electrochemical stability is more closely related to pore morphology, connectivity, and film homogeneity than to barrier thickness alone [\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]. Additionally, the incorporation of inhibitors or sealing agents after anodizing has been shown to substantially enhance impedance modulus values during prolonged immersion tests, reinforcing the role of pore blocking and chemical stabilization in corrosion protection [\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e].\u003c/p\u003e\n \u003cp\u003eIn the present work, no corrosion inhibitors, sealing steps, or additional surface modifications were applied after anodizing. Even under these simplified conditions, the EIS results revealed a clear improvement in corrosion resistance relative to the non-anodized substrate, with the electrochemical response being dominated by the overall behavior of the anodic film. This observation supports the use of a simplified equivalent electrical circuit, as the contributions of the porous outer layer and the inner barrier layer appear to be strongly coupled under the investigated anodizing conditions. This coupled response is consistent with the SEM observations, which showed continuous and relatively homogeneous oxide morphologies, particularly for the condition of 10 V for 20 minutes, suggesting that film uniformity and controlled porosity play a more decisive role in electrochemical performance than maximum oxide thickness alone.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eThe experimental design strategy proved to be effective for optimizing the anodizing parameters of the AA2024 alloy for joining with PEI/glass fiber composite by oxy-fuel welding (OFW), yielding a reliable statistical model with a coefficient of determination (R\u0026sup2;) of 85.25%. Among the investigated conditions, anodizing at 10 V for 20 min was identified as the most suitable, providing an average lap shear strength of approximately 14 MPa. This result was consistently supported by the response surface analysis, experimental validation, and Lap Shear Strength (LSS) testing.\u003c/p\u003e \u003cp\u003eMacroscopic examination of the fractured joints revealed a strong dependence of adhesion quality on the anodizing condition. Under less favorable parameters (10 V for 5 min), limited polymer matrix impregnation and extensive exposed aluminum surface were observed, indicating weak interfacial bonding. Conversely, increasing anodizing potential and time (20 V for 25 min and 30 V for 25 min) promoted greater matrix impregnation. The optimized condition (10 V for 20 min) exhibited the highest and most uniform adhesion, reflecting an effective balance between oxide layer development and interfacial compatibility. These observations were corroborated by SEM analyses, which showed that low potential and short anodizing time resulted in a compact and relatively smooth oxide layer, while excessively severe conditions led to oxide degradation and aluminum hydroxide deposition, reducing effective roughness. In contrast, the optimized condition produced a homogeneous anodic morphology with nodular features and controlled roughness, favoring mechanical anchoring and capillary-driven polymer infiltration.\u003c/p\u003e \u003cp\u003eElectrochemical characterization further confirmed the benefits of anodizing on the corrosion performance of the AA2024 alloy. Electrochemical impedance spectroscopy demonstrated a significant increase in polarization resistance and impedance modulus at low frequencies for all anodized samples compared to the control, evidencing the formation of a protective oxide layer with a barrier effect against charge transfer. The EIS spectra were satisfactorily fitted using equivalent electrical circuit models, indicating that the electrochemical response is dominated by the overall behavior of the anodic film and can be described by a single dominant capacitive process. Among the tested conditions, anodizing at 10 V for 20 min exhibited the highest barrier layer resistance and near-ideal capacitive behavior, indicating a more homogeneous and protective oxide film.\u003c/p\u003e \u003cp\u003eOverall, the mechanical, morphological, and electrochemical results are in strong agreement and demonstrate that precise control of anodizing parameters is essential to optimize both adhesion performance and corrosion resistance in hybrid joints produced by OFW. The anodizing condition of 10 V for 20 min represents an optimal compromise between oxide layer morphology, interfacial bonding efficiency, and electrochemical protection, highlighting its suitability for the fabrication of durable aluminum\u0026ndash;thermoplastic composite hybrid structures.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eFunding\u003c/h2\u003e \u003cp\u003eThe author are grateful to the Brazilian Funding Institution CAPES fundation (Coordena\u0026ccedil;\u0026atilde;o de Aperfei\u0026ccedil;oamento de Pessoal de N\u0026iacute;vel Superior) for financial support.\u003c/p\u003e \u003cp\u003eAll authors contributed to the conception and planning of the study. The preparation of materials, data collection, and data analysis were carried out by Marcos Paulo Soza Ribeiro, Rafael Resende Lucas, Luis Felipe Barbosa Marques, Heloisa Andrea Acciari, and Rita de Cassia Sales. Edson Coccheri Botelho and Ana Beatriz Moreira Abrah\u0026atilde;o contributed to project development and provided methodological support. The project and manuscript was written by Marcos Paulo Soza Ribeiro under the supervision and guidance of Roberto Zenhei Nakazato.\u003c/p\u003e \u003cp\u003eThe author declare that they have no conflicts of interest related to the research, authorship, and/or publication of this article.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003e\u003cspan\u003ePEREIRA MC et al (2000) Efeito do tratamento t\u0026eacute;rmico na corros\u0026atilde;o das ligas de alum\u0026iacute;nio 2024 e 7050 em meio salino. Revista Latinoamericana de Metalurgia y Materiales, v. 20, n. 1, p. 63\u0026ndash;66\u003c/span\u003e\u003c/li\u003e\n \u003cli\u003e\u003cspan\u003ePRAKASHAIAH BG et al (2018) Corrosion inhibition of 2024-T3 aluminum alloy in 3.5% NaCl by thiosemicarbazone derivatives. 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DOI: https://doi.org/10.3390/ma15186401\u003c/span\u003e\u003c/li\u003e\n \u003cli\u003e\u003cspan\u003eBONORA PL, DEFLORIAN F, FEDRIZZI L (1996) Electrochemical impedance spectroscopy as a tool for investigating underpaint corrosion. Electrochim Acta 41:7\u0026ndash;8. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/0013-4686(95)00440-8\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\n \u003cli\u003e\u003cspan\u003eYANG J-J et al (2024) Effect of voltage on structure and properties of 2024 aluminum alloy surface anodized aluminum oxide films. Surf Coat Technol v 479:130508. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1016/j.surfcoat.2024.130508\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\n \u003cli\u003e\u003cspan\u003eSAEEDIKHANI M, JAVIDI M, VAFAKHAH S (2017) Anodising of 2024-T3 aluminium alloy in sulphuric\u0026ndash;boric\u0026ndash;phosphoric mixed acid containing cerium salt as corrosion inhibitor. Trans Nonferrous Met Soc China v 27:711\u0026ndash;721. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003e10.1016/S1003-6326(17)60079-7\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"welding-in-the-world","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"witw","sideBox":"Learn more about [Welding in the World](https://www.springer.com/journal/40194)","snPcode":"40194","submissionUrl":"https://www.editorialmanager.com/witw/","title":"Welding in the World","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Anodizing, Oxy-Fuel Welding (OFW), AA2024 Aluminum Alloy, PEI/Glass Fiber Composite, Lap Shear Strength (LSS), Corrosion Resistance","lastPublishedDoi":"10.21203/rs.3.rs-8957869/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8957869/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis study investigates the influence of anodizing parameters on the AA2024 aluminum alloy for joining with a PEI/glass fiber composite using the oxy-fuel welding (OFW) method. AA2024 samples were anodized in a 10% phosphoric acid solution under different voltage (10–30 V) and time (5–25 min) conditions. The hybrid joints were produced by OFW and evaluated by lap shear strength (LSS) tests using a full factorial experimental design. The anodizing condition of 10 V for 20 min exhibited the best mechanical performance, reaching 14 MPa. Electrochemical impedance spectroscopy (EIS), combined with wettability, roughness, optical microscopy, and scanning electron microscopy analyses, showed that anodizing promotes the formation of a porous and protective oxide layer, suitable for anchoring the polymer matrix. EIS results revealed a significant increase in corrosion resistance for anodized samples compared to the control sample, with the electrochemical response dominated by the overall behavior of the anodic film. Equivalent electrical circuit modeling confirmed the effectiveness of the barrier layer formed under optimized conditions. Overall, the results demonstrate the potential of anodizing as an effective surface treatment to enhance metal–composite bonding and improve the performance of hybrid joints produced by the OFW process.\u003c/p\u003e","manuscriptTitle":"Optimization of AA2024 Anodization for Oxy-Fuel Welding with PEI/Glass Fiber Composite","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-04-28 13:29:33","doi":"10.21203/rs.3.rs-8957869/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewerAgreed","content":"","date":"2026-05-02T11:15:10+00:00","index":0,"fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-04-20T06:29:16+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"Welding in the World","date":"2026-03-07T06:44:44+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-02-27T06:03:19+00:00","index":"","fulltext":""},{"type":"submitted","content":"Welding in the World","date":"2026-02-26T08:40:57+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"welding-in-the-world","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"witw","sideBox":"Learn more about [Welding in the World](https://www.springer.com/journal/40194)","snPcode":"40194","submissionUrl":"https://www.editorialmanager.com/witw/","title":"Welding in the World","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"be1ce78f-7f26-4ab0-9e91-d6dcf82702bf","owner":[],"postedDate":"April 28th, 2026","published":true,"recentEditorialEvents":[{"type":"reviewerAgreed","content":"","date":"2026-05-02T11:15:10+00:00","index":0,"fulltext":""}],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[],"tags":[],"updatedAt":"2026-04-28T13:29:33+00:00","versionOfRecord":[],"versionCreatedAt":"2026-04-28 13:29:33","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8957869","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8957869","identity":"rs-8957869","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}