Numerical and Experimental Investigation on the Corrosion Resistance of the Coupled 7025 Aluminum Alloy Structure with 304 Stainless Steel Fasteners for Marine Application

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Shuku, Mengistu W. Tinsay, Obsa Tamiru This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8212976/v1 This work is licensed under a CC BY 4.0 License Status: Under Revision Version 1 posted 10 You are reading this latest preprint version Abstract This study investigates the galvanic corrosion of AA7025 aluminum alloy coupled with 304L stainless steel (SS) fasteners in marine structures, using a multi-methodological approach involving analytical modeling, SolidWorks 3D modeling, COMSOL Multiphysics simulations, and experimental validation. The AA7025-304L SS couples were tested in tap water, 3.5% NaCl, and 1M H₂SO₄ for 30 days. Corrosion rates increased significantly due to galvanic coupling: 4.92× in tap water, 3.16× in NaCl, and 4.26× in H₂SO₄, with predictive models accurate within 5% error. H₂SO₄ caused the highest hardness loss (10.97%) and a corrosion depth of 100.4 µm. Microstructural analysis showed CaCO₃ scaling in tap water, severe pitting in NaCl, and generalized dissolution in H₂SO₄. COMSOL simulations identified an Al-5Zn sacrificial anode as an effective mitigation strategy, providing a protective current density of 0.84 A/m² and a polarization potential of -1.08 V, significantly reducing AA7025 corrosion in marine-relevant environments. Physical sciences/Engineering Physical sciences/Materials science aluminum alloy stainless steel galvanic corrosion COMSOL Multiphysics marine environment cathodic protection 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 1. Introduction Aluminum alloys are indispensable in modern engineering due to their high strength-to-weight ratio, excellent conductivity, and recyclability[ 1 , 2 ], making them crucial for the aerospace, automotive, and marine industries [3]. The 7xxx series alloys, primarily alloyed with zinc, offer the highest strength among aluminum alloys, with properties comparable to some steels, which is critical for demanding structural applications [ 4 , 5 ]. Among these, AA7025 is valued for its mechanical properties and weldability. However, its application, particularly in marine environments, is challenged by its susceptibility to localized corrosion, such as pitting and stress corrosion cracking (SCC) [ 6 , 7 ]. A critical and often overlooked aspect of structural design is the interaction between the primary alloy and its fasteners. In real-world applications, structures involve thousands of mechanical joints, which are preferential sites for corrosion, especially when dissimilar materials are used [ 8 ]. The coupling of AA7025 aluminum alloy with more noble materials like 304L stainless steel (SS) fasteners creates a galvanic cell, where the aluminum alloy acts as the anode and undergoes accelerated corrosion [ 9 ]. While research has focused on enhancing the intrinsic corrosion resistance of 7xxx series alloys through alloying, heat treatments, and coatings [ 10 , 11 ], a significant knowledge gap exists regarding the quantitative effects of galvanic coupling in specific service environments. This deficit poses a substantial risk to the durability and safety of critical structures. To address this gap, this study provides a comprehensive investigation into the corrosion mechanisms and mechanical degradation of AA7025 when coupled with 304L SS fasteners. We examine the performance of this galvanic couple in three distinct aqueous environments: high-TDS tap water, simulated seawater (3.5% NaCl), and simulated acid rain (1M H₂SO₄). By integrating analytical modeling, COMSOL Multiphysics simulations [ 12 ], and rigorous experimental validation—including weight loss, hardness testing, and advanced microstructural characterization—this work quantifies the acceleration of corrosion, elucidates the underlying failure mechanisms, and offers validated predictive models. The findings establish crucial guidelines for material selection and introduce effective mitigation strategies, such as the optimization of sacrificial anodes, to extend the service life of these critical engineering structures. 2. Materials and Methods 2.1. Materials and Corrosive Environments As shown in (Table 1 ) and (Table 2 ), the materials AA7025 coupons (40×40×4 mm) and 304L SS fasteners (Ø10×120 mm) were used. The three corrosive environments were: Sourced from Hora Harsade Lake, Ethiopia (TDS: 1110 mg/L, pH: 8.81, Conductivity: 2220 µS/cm), Saline Solution: 3.5% (w/v) NaCl in deionized water to simulate a marine environment, and Acidic Solution: 1M H₂SO₄ in deionized water to simulate acid rain were used. Table 1.Elemental composition of AA7025 aluminum alloy Si Cu Mg Fe Zn Others Al 0.13 0.57 0.90 0.21 3.18 0.31 94.7 Table 2 . Elemental composition of 304L stainless steel C Cu Mn Ni Cr Others Fe 0.054 0.72 1.81 8.75 20.27 1.096 67.3 2.2. Experimental Design and Characterization Samples were divided into six groups (G1-G6) for a 30-day immersion test at 25 ± 1°C, with measurements taken at 10, 20, and 30 days in both Control Groups (AA7025 alone) and Test Groups (AA7025-304L SS) configurations. Microstructural and compositional analyses were performed using OM, SEM/EDS, XRD, and FTIR. 2.3. Corrosion Rate and Mechanical Degradation Analysis Corrosion rates were determined by weight loss measurements according to ASTM G31. Samples were cleaned, dried, and weighed before and after immersion. The corrosion rate (CR) in mm/year was calculated using the formula: CR = (K × W) / (A × T × D), where K is a constant, W is mass loss, A is sample area, T is exposure time, and D is density. Vickers microhardness tests (1 kgf load, 10 s dwell time) were performed on samples before and after exposure to quantify mechanical degradation. 2.4. Analytical and Numerical Modeling An analytical model based on mixed potential theory and Tafel equations was developed to predict corrosion potential (Ecorr) and corrosion current density (icorr). Numerical simulations were performed using COMSOL Multiphysics® v5.6 (Fig. 2 ). A 3D model of the AA7025-SS304L assembly (Fig. 1 ) was created in SolidWorks and imported into COMSOL. The Secondary Current Distribution interface was used to simulate the electrochemical behavior, employing Tafel kinetics and experimental electrolyte conductivity data to solve for potential and current density distribution across the surfaces. The simulation results were validated against analytical and experimental data. Additionally, COMSOL was used to model and compare the efficacy of different sacrificial anodes (Al-0.1In, Zn, Al-5Zn) for cathodic protection of the AA7025 alloy. 3. Results 3.1. Corrosion Rate Analysis: Experimental and Numerical The COMSOL simulations successfully captured the corrosion behavior in both uncoupled (pitting) and coupled (galvanic) conditions. Figure 3 shows the simulated current density for the uncoupled AA7025 samples, representing baseline pitting corrosion driven by micro-galvanic cells at intermetallics. The simulation in (Fig. 3 ) correctly predicts the hierarchy of environmental aggressiveness, with the current density increasing from tap water (a) to NaCl (b) to H₂SO₄ (c), perfectly mirroring the experimental trend for groups G1, G2, and G3. When the 304L SS fastener is introduced, the simulation shows a massive amplification of the corrosion current (Fig. 4 ). Comparing the current density scales between Fig. 3 and Fig. 4 , there is an order-of-magnitude increase, visually confirming the powerful accelerating effect of the macro-galvanic couple. 3.1. Corrosion Rate Analysis: Experimental vs. Modeled The corrosion rates of AA7025 were significantly influenced by both the environment and galvanic coupling. In non-galvanic conditions (G1-G3), the experimental corrosion rates were 0.0534 mmpy in tap water, 0.0864 mmpy in NaCl, and 0.2888 mmpy in H₂SO₄, showing that the acidic environment was the most aggressive. The introduction of galvanic coupling with 304L SS (G4-G6) dramatically accelerated corrosion. The experimental corrosion rate surged to 0.2630 mmpy in tap water (a 4.92× increase), 0.2690 mmpy in NaCl (a 3.16× increase), and a catastrophic 1.1942 mmpy in H₂SO₄ (a 4.26× increase).The analytical and numerical models demonstrated excellent predictive accuracy. As shown in Fig. 5 , the results from all three methods (analytical, numerical, and experimental) were in strong agreement, with all predictive errors remaining below 5%. The highest corrosion rate of ~ 100 µm/30 days (~ 1.2 mmpy) was consistently observed in the galvanically coupled acidic environment (G6). 3.2. Composition of Corrosion Products XRD analysis in (Fig. 6 ) identified the bulk corrosion products as calcite (CaCO₃) in tap water, bayerite (Al(OH)₃) in NaCl, and aluminum sulfate hydrate (Al₂(SO₄)₃) in H₂SO₄. FTIR spectroscopy (Fig. 7 ) confirmed the chemical nature of these products through their characteristic absorption bands. The unexposed (G0) surface was clean, showing only faint native Al-O bonds below 900 cm⁻¹. Exposure to tap water (G1) resulted in water bands (~ 3400, ~ 1630 cm⁻¹) and carbonate peaks (~ 1420, ~ 875 cm⁻¹) from calcite. The 3.5% NaCl (G2) solution caused intense hydration signals (~ 3400, ~ 1630 cm⁻¹), indicating hydrated aluminum oxides. Finally, 1M H₂SO₄ (G3) produced dominant sulfate bands (~ 1100, ~ 610 cm⁻¹), confirming an aluminum sulfate product. 3.2. Microstructural Degradation and Hardness Loss The degradation of the alloy's microstructure was visually confirmed by optical microscopy (Fig. 8 ). The as-received sample (Fig. 8 a) exhibited a smooth surface. In contrast, exposure to tap water (Fig. 8 b) resulted in minor etching, exposure to 3.5% NaCl solution (Fig. 8 c) caused localized pitting, and 1M H₂SO₄ solution (Fig. 8 d) induced severe, widespread dissolution. This damage correlated directly with a loss in mechanical integrity, with Vickers hardness dropping from a baseline of 188.6 HV to as low as 167.9 HV (a 10.99% loss). SEM analysis revealed distinct corrosion morphologies. In 3.5% NaCl (Fig. 9 a), deep pits were observed to nucleate at the interface of the aluminum matrix and bright, cathodic intermetallic particles. In 1M H₂SO₄ (Fig. 9 b), corrosion was widespread, resulting in a porous, degraded matrix with remnant, Fe-rich IMPs acting as local cathodes. Table 3 Summary of corrosion rates and hardness reduction for all experimental groups after 30 days. Group Condition Corrosion Rate (mmpy) Hardness (HV) Hardness Loss (%) G0 As-received - 188.6 - G1 Tap Water 0.0534 185.5 1.63 G2 3.5% NaCl 0.0864 182.3 3.34 G3 1M H₂SO₄ 0.2888 179.0 5.11 G4 Tap Water + 304L SS 0.2630 178.3 5.47 G5 3.5% NaCl + 304L SS 0.2690 176.6 6.37 G6 1M H₂SO₄ + 304L SS 1.1942 167.9 10.99 3.3. Mechanical Degradation The severe corrosion translated directly to a loss of mechanical integrity, evidenced by the progressive decline in Vickers hardness across all experimental groups, with a maximum loss of 10.99% in the G6 sample (Fig. 10 ). 4. Discussion The results of this study unequivocally demonstrate that while environmental conditions dictate baseline corrosivity, galvanic coupling is a critical accelerator of degradation for AA7025 aluminum alloy. The 4- to 5-fold increase in corrosion rates observed in coupled samples highlights the electrochemical driving force imposed by the more noble 304L stainless steel, which acts as an efficient cathode for oxygen reduction (in neutral media) or hydrogen evolution (in acidic media) [ 8 , 13 ].The environmental chemistry dictated the specific mode of failure. In tap water, the high TDS content and presence of carbonate ions led to the precipitation of a partially protective CaCO₃ scale [ 14 , 15 ]. While this scale mitigated uniform corrosion, it was not sufficient to prevent localized galvanic effects. In the 3.5% NaCl solution, the aggressive chloride ions caused a breakdown of the passive Al₂O₃ film, leading to severe pitting corrosion, a mechanism well-documented for aluminum alloys in marine environments [ 16 – 18 ]. The most catastrophic degradation occurred in 1M H₂SO₄, where the synergistic effect of a highly acidic environment and a strong galvanic potential difference led to rapid, widespread dissolution of the aluminum matrix, a behavior consistent with studies on aluminum alloys in acidic media [ 19 ].The strong correlation between microstructural damage and hardness reduction underscores the direct link between corrosion and loss of mechanical integrity [ 20 ]. The 10.99% hardness loss in the G6 sample after only 30 days is alarming and indicates a drastically shortened service life for structures in such conditions. This finding emphasizes the need for effective corrosion mitigation strategies. 4.1. Integrated Analysis of Corrosion Mechanisms The corrosion of AA7025 is a multi-scale process initiated by micro-galvanic cells at intermetallic particles. These IMPs act as local cathodes, driving the dissolution of the surrounding aluminum matrix. The macro-galvanic couple with the 304L SS fastener acts as a powerful amplifier, dramatically increasing the rate of these IMP-driven processes. This is visually confirmed by the COMSOL simulations (Fig. 3 ), which show a clear increase in galvanic current density that mirrors the measured corrosion rates. The specific chemical pathway is dictated by the environment. The FTIR analysis (Fig. 7 ) provides the chemical fingerprint for each pathway: carbonate scale formation in tap water, extensive hydration in NaCl, and sulfate product formation in H₂SO₄. The combined impact of all factors is summarized in the 3D corrosion map (Fig. 11 ), which identifies the galvanically coupled, acidic condition as the most destructive. The COMSOL simulations provide a powerful tool for both predicting corrosion and designing protective systems. The high accuracy of the corrosion models, consistent with other simulation-based corrosion studies [ 12 , 21 ], validates their use for engineering design and lifetime assessment. 4.2. Mitigation Strategy: Cathodic Protection Simulation COMSOL simulations were used to evaluate the performance of three sacrificial anodes for protecting the AA7025 alloy. The results (Table 4 ) showed that the Al-5Zn anode provided superior protection compared to pure Zn and Al-0.1In. The Al-5Zn anode generated the highest protective current density (0.84 A/m²) and induced the most negative polarization potential (-1.08 V) on the AA7025 surface. This potential is well below the typical protection threshold of -0.85 V, indicating a robust safety margin against corrosion. Table 4 Comparative performance of simulated sacrificial anodes for AA7025 cathodic protection. Anode Material Current Density (A/m²) Polarization Potential (V) Protection Uniformity Al-5Zn 0.84 -1.08 Superior Zn 0.71 -1.00 Good Al-0.1In 0.64 -0.99 Moderate To counteract the severe galvanic corrosion, a cathodic protection strategy using a sacrificial anode was simulated as shown in Fig. 12 . The protection mechanism relies on coupling the AA7025-304L SS assembly to a more active metal, which preferentially corrodes (acts as the anode), thereby protecting the AA7025 structure by forcing it to become a cathode. COMSOL simulations compared the performance of three anode materials. The results (Table 4 ) identified Al-5Zn as the optimal choice. Figure 12 illustrates its superior performance: the Al-5Zn anode generated a robust protective current density of 0.84 A/m² and uniformly polarized the entire AA7025 surface to a potential of -1.08 V. This is significantly more negative than the potentials achieved by Zn (-1.00 V) or Al-0.1In (-0.99 V) and provides a substantial safety margin below the − 0.85 V protection threshold. 5. Conclusions This study comprehensively investigated the critical challenge of galvanic corrosion acceleration in AA7025 aluminum alloy when coupled with 304L stainless steel fasteners across three corrosive environments: Hora Harsade lake tap water, (3.5% NaCl), and (1M H₂SO₄). A multi-methodological framework was employed, combining analytical corrosion rate calculations, SolidWorks 3D modeling, and current density/corrosion potential simulations via COMSOL Multiphysics v5.6. Rigorous experimental validation included weight loss tests (ASTM G31) for corrosion rates, Vickers micro hardness (ASTM E384) for mechanical degradation, and MATLAB (R2019a) for environmental aggressiveness mapping. Advanced microstructural characterization using OM, SEM/EDS, and FTIR elucidated corrosion mechanisms after a 30-day immersion period. First, the study successfully validated predictive models, showing excellent agreement between experimental corrosion rates and both analytical/numerical (COMSOL) simulations. All predictive errors were below 5% (0.07–4.44% for analytical-numerical; 1.71–3.86% for analytical-experimental), firmly establishing the theoretical framework's reliability. In line with the second objective, the investigation characterized the mechanical degradation and corrosion rate of the standalone AA7025 alloy. Initially, all samples exhibited high hardness (188.6 HV) and a defect-free microstructure with coherent η-MgZn₂ and S Al₂CuMg phases, and an intact Al₂O₃ passive layer. Corrosion severity was environment dependent: minimal in tap water (4.458 µm/30 days, 185.53 HV), moderate pitting in 3.5% NaCl (7.187 µm/30 days, 182.3 HV), and severe general corrosion in 1M H₂SO₄ (23.582 µm/30 days, 178.96 HV). Building on this baseline, the study characterized the significantly more aggressive galvanic corrosion of the AA7025–304L couple. Mechanical degradation and corrosion rates were substantially higher across all environments, highlighting the galvanic cell's critical role in accelerating structural decay. The quantitative effect of galvanic coupling was precisely evaluated, showing 304L stainless steel fasteners dramatically enhanced AA7025 corrosion rates by 3 to 5 times. Rates surged to 21.953 µm/30 days in tap water (4.92x uncoupled), 22.691 µm/30 days in NaCl (3.16x uncoupled), and 100.4 µm/30 days in H₂SO₄ (4.26x uncoupled), leading to substantial hardness reductions (lowest 167.9 HV) and shortened service lifetime. Reducing the anodic area also slightly increased corrosion, notably 7.01% in sulfuric acid. Finally, to understand the underlying causes of this behavior, detailed microstructural (OM, SEM), compositional (EDS), and phase identification (XRD, FTIR) analyses were employed. This characterization revealed specific corrosion mechanisms and chemical byproducts: minimal surface alteration and CaCO₃ deposition in tap water; severe pitting corrosion with Cl detection and Al(OH)₃ formation in 3.5% NaCl; and widespread general corrosion with S detection and Al₂(SO₄)₃ as the primary product in 1M H₂SO₄. These degradation pathways were significantly exacerbated under galvanic coupling. Future work should focus on the long-term experimental validation of the Al-5Zn sacrificial anode system to confirm its real-world effectiveness in preserving the structural integrity of AA7025 components in marine applications. Declarations Conflicts of Interest : The authors declare no conflict of interest. Funding Declaration Funding:-Not applicable Author Contribution Conceptualization, M.W.T.; methodology, B.G.S.; software, B.G.S.; validation, B.G.S. and M.W.T.; formal analysis, B.G.S.; investigation, B.G.S.; resources, M.W.T.; data curation, B.G.S.; writing original draft preparation, B.G.S.; writing review and editing, M.W.T.; visualization, B.G.S.; supervision, M.W.T.; project administration, M.W.T. All authors have read and agreed to the published version of the manuscript **.** Acknowledgment The authors would like to express their sincere gratitude to the Ethiopian Defence University for providing the necessary resources and facilities to conduct this research. Special thanks go to the Department of metallurgical and materials Engineering for their technical support and guidance during the numerical and analytical work of the study. 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Electrochemical characteristics of aluminum alloys in sea water for marine environment. Acta Phys. Pol. A 2019, 135, 1005–1011. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Under Revision Version 1 posted Editorial decision: Revision requested 14 Jan, 2026 Reviews received at journal 13 Jan, 2026 Reviews received at journal 05 Jan, 2026 Reviewers agreed at journal 22 Dec, 2025 Reviewers agreed at journal 17 Dec, 2025 Reviewers agreed at journal 17 Dec, 2025 Reviewers invited by journal 17 Dec, 2025 Editor assigned by journal 17 Dec, 2025 Submission checks completed at journal 16 Dec, 2025 First submitted to journal 26 Nov, 2025 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-8212976","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":561736752,"identity":"8fb3303f-bead-44ab-a0c3-63172e2d5ef6","order_by":0,"name":"Bayisa G. 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12:24:56","extension":"png","order_by":59,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":12208,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage22.png","url":"https://assets-eu.researchsquare.com/files/rs-8212976/v1/f3c4640b77f1abd49f762260.png"},{"id":98600094,"identity":"5ccbf27c-2626-4e80-91dc-b45bc01392ac","added_by":"auto","created_at":"2025-12-19 12:24:55","extension":"png","order_by":60,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":9732,"visible":true,"origin":"","legend":"","description":"","filename":"Onlinefloatimage23.png","url":"https://assets-eu.researchsquare.com/files/rs-8212976/v1/131298bb28ec8c531141c88e.png"},{"id":98627795,"identity":"d1b13626-1b1d-44de-9ef9-6138771ac111","added_by":"auto","created_at":"2025-12-19 17:10:39","extension":"xml","order_by":61,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":86243,"visible":true,"origin":"","legend":"","description":"","filename":"a2a33c5167804aa296d11b5a253c658f1structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-8212976/v1/a73e0454f11e100c45235cf9.xml"},{"id":98628377,"identity":"e823f836-6915-4406-a4d5-261e34adb632","added_by":"auto","created_at":"2025-12-19 17:11:21","extension":"html","order_by":62,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":97800,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8212976/v1/e501f3948c9d653347952e0c.html"},{"id":98600152,"identity":"db0793d3-6e61-4e93-9ca4-cb1373e74062","added_by":"auto","created_at":"2025-12-19 12:25:02","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":66209,"visible":true,"origin":"","legend":"\u003cp\u003eGeometric model of the AA7025-304L SS assembly used for COMSOL Multiphysics simulations: (A) 3D view of the aluminum alloy structure (anode) and stainless steel fastener (cathode); (B) 2D plane showing the contact area.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8212976/v1/3fb9779ca62ed19fb9bb4e54.png"},{"id":98600103,"identity":"d5d138a6-661d-4b58-b3e5-0fbd3a422977","added_by":"auto","created_at":"2025-12-19 12:24:56","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":149378,"visible":true,"origin":"","legend":"\u003cp\u003eGeometric characteristics of the AA7025-304L SS in COMSOL Multiphysics showing (A).the Geometry in electrolyte (B). Imported model geometry to COMSOL Multiphysics,(C). Mesh For Galvanic Corrosion, and (D). Mesh For Pitting Corrosion.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8212976/v1/6cffab612429867ee10a420f.png"},{"id":98600082,"identity":"6fb9be14-fd96-4cc3-a698-d0861d3c0a42","added_by":"auto","created_at":"2025-12-19 12:24:52","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":373314,"visible":true,"origin":"","legend":"\u003cp\u003eCOMSOL simulation of electrolyte current density (A/m²) for uncoupled AA7025 (pitting corrosion) after 30 days in: (a) Tap Water, (b) 3.5% NaCl, and (c) 1M H₂SO₄. The simulation correctly predicts the hierarchy of environmental aggressiveness.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8212976/v1/1502cfc0e8d99a40efc48c2a.png"},{"id":98629491,"identity":"ddfeea4e-eaf4-4deb-9ce1-c449f46ffc8f","added_by":"auto","created_at":"2025-12-19 17:14:03","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":310723,"visible":true,"origin":"","legend":"\u003cp\u003eCOMSOL simulation of electrolyte current density (A/m²) for the galvanically coupled AA7025-304L SS system. The dramatic increase in current density compared to Figure 3 visually confirms the severe accelerating effect of the galvanic couple.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8212976/v1/18aeb1e48de3a2bfb96163d7.png"},{"id":98600086,"identity":"ebfdf3d7-e73a-4673-8ec2-8fd368ef2a8d","added_by":"auto","created_at":"2025-12-19 12:24:54","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":43697,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of analytical, numerical, and experimental corrosion rates for AA7025 alloy in six different corrosive environments after 30 days. Error bars represent standard deviation for experimental results.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-8212976/v1/7a6b28b209e09b5d59891fd7.png"},{"id":98600090,"identity":"45bc0338-dd54-431f-ad0c-89d98c40e0e0","added_by":"auto","created_at":"2025-12-19 12:24:55","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":44234,"visible":true,"origin":"","legend":"\u003cp\u003eXRD patterns of corrosion products on AA7025 after 30 days. Key phases identified include calcite (G1), bayerite (G2), and aluminum sulfate hydrate (G3), compared to the uncorroded control (G0).\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-8212976/v1/8d1d7a811f46f64f8bc16a51.png"},{"id":98600109,"identity":"86f2aaec-219c-445b-b550-017a915bf824","added_by":"auto","created_at":"2025-12-19 12:24:58","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":32100,"visible":true,"origin":"","legend":"\u003cp\u003eFTIR spectra of the AA7025 surface after 30 days, confirming carbonate (G1), hydrated oxide (G2), and sulfate (G3) corrosion products.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-8212976/v1/5572565c69db7821736f9f7d.png"},{"id":98600107,"identity":"2551cb3c-952d-4150-8075-2593c7212cf7","added_by":"auto","created_at":"2025-12-19 12:24:57","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":377942,"visible":true,"origin":"","legend":"\u003cp\u003eOptical microscopy images of the AA7025 alloy surface: (a) as-received; (b) after 30 days in tap water; (c) after 30 days in 3.5% NaCl, showing distinct pits; and (d) after 30 days in 1M H₂SO₄, showing severe general corrosion.\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-8212976/v1/0b803d2e555555df114b19e1.png"},{"id":98629482,"identity":"deb4bc0b-1a78-46e1-8092-fc0c37c9747f","added_by":"auto","created_at":"2025-12-19 17:14:02","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":311906,"visible":true,"origin":"","legend":"\u003cp\u003eSEM analysis of AA7025 corrosion mechanisms. (a) In 3.5% NaCl, a pit forms directly adjacent to a cathodic intermetallic particle (IMP), with EDS confirming Cl attack. (b) In 1M H₂SO₄, the matrix is severely dissolved around remnant Fe-rich IMPs, with EDS confirming sulfur-based corrosion products and the summary of corrosion rates and hardness reduction for all experimental groups after 30 days was shown in Table 3.\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-8212976/v1/5b7d7c285e12c4484050271b.png"},{"id":98600124,"identity":"23502f57-8d3a-40f9-a16a-ab006b8f019a","added_by":"auto","created_at":"2025-12-19 12:25:00","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":24324,"visible":true,"origin":"","legend":"\u003cp\u003eVickers hardness of AA7025 samples after 30 days of exposure, illustrating the progressive loss of mechanical integrity.\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-8212976/v1/ddc2f7aa281d5882ddabb9aa.png"},{"id":98600095,"identity":"e6efd65e-402a-46a9-a18c-efefdba04f30","added_by":"auto","created_at":"2025-12-19 12:24:55","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":39324,"visible":true,"origin":"","legend":"\u003cp\u003e3D surface plot visualizing the relationship between corrosion rate, pH, and corrosion potential.\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-8212976/v1/c46ca0fcf64fdda845733b82.png"},{"id":98628480,"identity":"8cc36bf0-86f5-4393-a872-e6a88e32c955","added_by":"auto","created_at":"2025-12-19 17:11:35","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":152837,"visible":true,"origin":"","legend":"\u003cp\u003eCOMSOL simulation of cathodic protection using an Al-5Zn anode. The results show (a) the electrolyte potential distribution and (b) the electrode surface potential, demonstrating uniform polarization of the AA7025 structure to a highly protective -1.08 V.\u003c/p\u003e","description":"","filename":"12.png","url":"https://assets-eu.researchsquare.com/files/rs-8212976/v1/9ac95f04686e628e1bb6e0b9.png"},{"id":98632203,"identity":"d2f2cb16-bcfc-445f-b17a-37e3bcea872b","added_by":"auto","created_at":"2025-12-19 17:21:33","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2609736,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8212976/v1/2db00f82-3960-49ce-a960-4579e91e28f2.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Numerical and Experimental Investigation on the Corrosion Resistance of the Coupled 7025 Aluminum Alloy Structure with 304 Stainless Steel Fasteners for Marine Application","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eAluminum alloys are indispensable in modern engineering due to their high strength-to-weight ratio, excellent conductivity, and recyclability[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e], making them crucial for the aerospace, automotive, and marine industries [3]. The 7xxx series alloys, primarily alloyed with zinc, offer the highest strength among aluminum alloys, with properties comparable to some steels, which is critical for demanding structural applications [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e4\u003c/span\u003e, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. Among these, AA7025 is valued for its mechanical properties and weldability. However, its application, particularly in marine environments, is challenged by its susceptibility to localized corrosion, such as pitting and stress corrosion cracking (SCC) [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e7\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eA critical and often overlooked aspect of structural design is the interaction between the primary alloy and its fasteners. In real-world applications, structures involve thousands of mechanical joints, which are preferential sites for corrosion, especially when dissimilar materials are used [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. The coupling of AA7025 aluminum alloy with more noble materials like 304L stainless steel (SS) fasteners creates a galvanic cell, where the aluminum alloy acts as the anode and undergoes accelerated corrosion [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e9\u003c/span\u003e]. While research has focused on enhancing the intrinsic corrosion resistance of 7xxx series alloys through alloying, heat treatments, and coatings [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e11\u003c/span\u003e], a significant knowledge gap exists regarding the quantitative effects of galvanic coupling in specific service environments. This deficit poses a substantial risk to the durability and safety of critical structures.\u003c/p\u003e \u003cp\u003eTo address this gap, this study provides a comprehensive investigation into the corrosion mechanisms and mechanical degradation of AA7025 when coupled with 304L SS fasteners. We examine the performance of this galvanic couple in three distinct aqueous environments: high-TDS tap water, simulated seawater (3.5% NaCl), and simulated acid rain (1M H₂SO₄). By integrating analytical modeling, COMSOL Multiphysics simulations [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e12\u003c/span\u003e], and rigorous experimental validation\u0026mdash;including weight loss, hardness testing, and advanced microstructural characterization\u0026mdash;this work quantifies the acceleration of corrosion, elucidates the underlying failure mechanisms, and offers validated predictive models. The findings establish crucial guidelines for material selection and introduce effective mitigation strategies, such as the optimization of sacrificial anodes, to extend the service life of these critical engineering structures.\u003c/p\u003e"},{"header":"2. Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n \u003ch2\u003e2.1. Materials and Corrosive Environments\u003c/h2\u003e\n \u003cp\u003eAs shown in (Table \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e) and (Table \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e), the materials AA7025 coupons (40\u0026times;40\u0026times;4 mm) and 304L SS fasteners (\u0026Oslash;10\u0026times;120 mm) were used. The three corrosive environments were: Sourced from Hora Harsade Lake, Ethiopia (TDS: 1110 mg/L, pH: 8.81, Conductivity: 2220 \u0026micro;S/cm), Saline Solution: 3.5% (w/v) NaCl in deionized water to simulate a marine environment, and Acidic Solution: 1M H₂SO₄ in deionized water to simulate acid rain were used.\u003c/p\u003e\n \u003cp\u003eTable 1.Elemental composition of AA7025 aluminum alloy\u003c/p\u003e\n \u003cdiv style='margin-top:0in;margin-right:0in;margin-bottom:10.0pt;margin-left:0in;line-height:115%;font-size:15px;font-family:\"Calibri\",sans-serif;'\u003e\n \u003ctable style=\"width:453.8pt;border-collapse:collapse;border:none;\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"border-top: 1pt solid rgb(127, 127, 127);border-left: none;border-bottom: 1pt solid rgb(127, 127, 127);border-right: none;padding: 0in 5.4pt;height: 28.55pt;vertical-align: top;\"\u003e\n \u003cp style='margin:0in;font-size:15px;font-family:\"Calibri\",sans-serif;text-align:center;'\u003e\u003cstrong\u003e\u003cspan style='font-size:16px;font-family: \"Times New Roman\",serif;'\u003e\u0026nbsp;\u003c/span\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"border-top: 1pt solid rgb(127, 127, 127);border-left: none;border-bottom: 1pt solid rgb(127, 127, 127);border-right: none;padding: 0in 5.4pt;height: 28.55pt;vertical-align: top;\"\u003e\n \u003cp style='margin:0in;font-size:15px;font-family:\"Calibri\",sans-serif;text-align:center;'\u003e\u003cstrong\u003e\u003cspan style='font-size:16px;font-family: \"Times New Roman\",serif;'\u003eSi\u003c/span\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"border-top: 1pt solid rgb(127, 127, 127);border-left: none;border-bottom: 1pt solid rgb(127, 127, 127);border-right: none;padding: 0in 5.4pt;height: 28.55pt;vertical-align: top;\"\u003e\n \u003cp style='margin:0in;font-size:15px;font-family:\"Calibri\",sans-serif;text-align:center;'\u003e\u003cstrong\u003e\u003cspan style='font-size:16px;font-family: \"Times New Roman\",serif;'\u003eCu \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;Mg \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; Fe \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; Zn \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; Others \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;Al\u003c/span\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"border-top: none;border-right: none;border-left: none;border-image: initial;border-bottom: 1pt solid rgb(127, 127, 127);padding: 0in 5.4pt;height: 27.8pt;vertical-align: top;\"\u003e\n \u003cp style='margin:0in;font-size:15px;font-family:\"Calibri\",sans-serif;text-align:center;'\u003e\u003cstrong\u003e\u003cspan style='font-size:16px;font-family: \"Times New Roman\",serif;'\u003e\u0026nbsp;\u003c/span\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"border-top: none;border-right: none;border-left: none;border-image: initial;border-bottom: 1pt solid rgb(127, 127, 127);padding: 0in 5.4pt;height: 27.8pt;vertical-align: top;\"\u003e\n \u003cp style='margin:0in;font-size:15px;font-family:\"Calibri\",sans-serif;text-align:center;'\u003e\u003cspan style='font-size:16px;font-family:\"Times New Roman\",serif;'\u003e0.13\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"border-top: none;border-right: none;border-left: none;border-image: initial;border-bottom: 1pt solid rgb(127, 127, 127);padding: 0in 5.4pt;height: 27.8pt;vertical-align: top;\"\u003e\n \u003cp style='margin:0in;font-size:15px;font-family:\"Calibri\",sans-serif;text-align:center;'\u003e\u003cspan style='font-size:16px;font-family:\"Times New Roman\",serif;'\u003e0.57 \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; 0.90 \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;0.21 \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; 3.18 \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;0.31 \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;94.7\u003c/span\u003e\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\u003eTable 2\u003cstrong\u003e.\u003c/strong\u003e Elemental composition of 304L stainless steel\u003c/p\u003e\n \u003ctable style=\"width:489.85pt;border-collapse:collapse;border:none;\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"border-top: 1pt solid rgb(127, 127, 127);border-left: none;border-bottom: 1pt solid rgb(127, 127, 127);border-right: none;padding: 0in 5.4pt;height: 28.55pt;vertical-align: top;\"\u003e\n \u003cp style='margin:0in;font-size:15px;font-family:\"Calibri\",sans-serif;text-align:center;'\u003e\u003cstrong\u003e\u003cspan style='font-size:16px;font-family: \"Times New Roman\",serif;'\u003e\u0026nbsp;\u003c/span\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"border-top: 1pt solid rgb(127, 127, 127);border-left: none;border-bottom: 1pt solid rgb(127, 127, 127);border-right: none;padding: 0in 5.4pt;height: 28.55pt;vertical-align: top;\"\u003e\n \u003cp style='margin:0in;font-size:15px;font-family:\"Calibri\",sans-serif;text-align:center;'\u003e\u003cstrong\u003e\u003cspan style='font-size:16px;font-family: \"Times New Roman\",serif;'\u003e\u0026nbsp;\u003c/span\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"border-top: 1pt solid rgb(127, 127, 127);border-left: none;border-bottom: 1pt solid rgb(127, 127, 127);border-right: none;padding: 0in 5.4pt;height: 28.55pt;vertical-align: top;\"\u003e\n \u003cp style='margin:0in;font-size:15px;font-family:\"Calibri\",sans-serif;text-align:center;'\u003e\u003cstrong\u003e\u003cspan style='font-size:16px;font-family: \"Times New Roman\",serif;'\u003eC\u003c/span\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"border-top: 1pt solid rgb(127, 127, 127);border-left: none;border-bottom: 1pt solid rgb(127, 127, 127);border-right: none;padding: 0in 5.4pt;height: 28.55pt;vertical-align: top;\"\u003e\n \u003cp style='margin:0in;font-size:15px;font-family:\"Calibri\",sans-serif;text-align:center;'\u003e\u003cstrong\u003e\u003cspan style='font-size:16px;font-family: \"Times New Roman\",serif;'\u003eCu \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; Mn \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; Ni \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;Cr \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; Others \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; Fe\u003c/span\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"border-top: none;border-right: none;border-left: none;border-image: initial;border-bottom: 1pt solid rgb(127, 127, 127);padding: 0in 5.4pt;height: 27.8pt;vertical-align: top;\"\u003e\n \u003cp style='margin:0in;font-size:15px;font-family:\"Calibri\",sans-serif;text-align:center;'\u003e\u003cstrong\u003e\u003cspan style='font-size:16px;font-family: \"Times New Roman\",serif;'\u003e\u0026nbsp;\u003c/span\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"border-top: none;border-right: none;border-left: none;border-image: initial;border-bottom: 1pt solid rgb(127, 127, 127);padding: 0in 5.4pt;height: 27.8pt;vertical-align: top;\"\u003e\n \u003cp style='margin:0in;font-size:15px;font-family:\"Calibri\",sans-serif;text-align:center;'\u003e\u003cspan style='font-size:16px;font-family:\"Times New Roman\",serif;'\u003e\u0026nbsp;\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"border-top: none;border-right: none;border-left: none;border-image: initial;border-bottom: 1pt solid rgb(127, 127, 127);padding: 0in 5.4pt;height: 27.8pt;vertical-align: top;\"\u003e\n \u003cp style='margin:0in;font-size:15px;font-family:\"Calibri\",sans-serif;text-align:center;'\u003e\u003cspan style='font-size:16px;font-family:\"Times New Roman\",serif;'\u003e0.054\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"border-top: none;border-right: none;border-left: none;border-image: initial;border-bottom: 1pt solid rgb(127, 127, 127);padding: 0in 5.4pt;height: 27.8pt;vertical-align: top;\"\u003e\n \u003cp style='margin:0in;font-size:15px;font-family:\"Calibri\",sans-serif;text-align:center;'\u003e\u003cspan style='font-size:16px;font-family:\"Times New Roman\",serif;'\u003e0.72 \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;1.81 \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;8.75 \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; 20.27 \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;1.096 \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; 67.3\u003c/span\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n \u003c/table\u003e\n \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n \u003ch2\u003e2.2. Experimental Design and Characterization\u003c/h2\u003e\n \u003cp\u003eSamples were divided into six groups (G1-G6) for a 30-day immersion test at 25\u0026thinsp;\u0026plusmn;\u0026thinsp;1\u0026deg;C, with measurements taken at 10, 20, and 30 days in both Control Groups (AA7025 alone) and Test Groups (AA7025-304L SS) configurations. Microstructural and compositional analyses were performed using OM, SEM/EDS, XRD, and FTIR.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n \u003ch2\u003e2.3. Corrosion Rate and Mechanical Degradation Analysis\u003c/h2\u003e\n \u003cp\u003eCorrosion rates were determined by weight loss measurements according to ASTM G31. Samples were cleaned, dried, and weighed before and after immersion. The corrosion rate (CR) in mm/year was calculated using the formula: CR = (K \u0026times; W) / (A \u0026times; T \u0026times; D), where K is a constant, W is mass loss, A is sample area, T is exposure time, and D is density. Vickers microhardness tests (1 kgf load, 10 s dwell time) were performed on samples before and after exposure to quantify mechanical degradation.\u003c/p\u003e\n \u003c/div\u003e\n \u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\n \u003ch2\u003e2.4. Analytical and Numerical Modeling\u003c/h2\u003e\n \u003cp\u003eAn analytical model based on mixed potential theory and Tafel equations was developed to predict corrosion potential (Ecorr) and corrosion current density (icorr). Numerical simulations were performed using COMSOL Multiphysics\u0026reg; v5.6 (Fig. \u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e). A 3D model of the AA7025-SS304L assembly (Fig. \u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e) was created in SolidWorks and imported into COMSOL.\u003c/p\u003e\n \u003cp\u003eThe Secondary Current Distribution interface was used to simulate the electrochemical behavior, employing Tafel kinetics and experimental electrolyte conductivity data to solve for potential and current density distribution across the surfaces. The simulation results were validated against analytical and experimental data. Additionally, COMSOL was used to model and compare the efficacy of different sacrificial anodes (Al-0.1In, Zn, Al-5Zn) for cathodic protection of the AA7025 alloy.\u003c/p\u003e\n \u003c/div\u003e\n\u003c/div\u003e"},{"header":"3. Results","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\n \u003ch2\u003e3.1. Corrosion Rate Analysis: Experimental and Numerical\u003c/h2\u003e\n \u003cp\u003eThe COMSOL simulations successfully captured the corrosion behavior in both uncoupled (pitting) and coupled (galvanic) conditions. Figure \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e shows the simulated current density for the uncoupled AA7025 samples, representing baseline pitting corrosion driven by micro-galvanic cells at intermetallics. The simulation in (Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e) correctly predicts the hierarchy of environmental aggressiveness, with the current density increasing from tap water (a) to NaCl (b) to H₂SO₄ (c), perfectly mirroring the experimental trend for groups G1, G2, and G3.\u003c/p\u003e\n \u003cp\u003eWhen the 304L SS fastener is introduced, the simulation shows a massive amplification of the corrosion current (Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e). Comparing the current density scales between Fig. \u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003e and Fig. \u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003e, there is an order-of-magnitude increase, visually confirming the powerful accelerating effect of the macro-galvanic couple.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\n \u003ch2\u003e3.1. Corrosion Rate Analysis: Experimental vs. Modeled\u003c/h2\u003e\n \u003cp\u003eThe corrosion rates of AA7025 were significantly influenced by both the environment and galvanic coupling. In non-galvanic conditions (G1-G3), the experimental corrosion rates were 0.0534 mmpy in tap water, 0.0864 mmpy in NaCl, and 0.2888 mmpy in H₂SO₄, showing that the acidic environment was the most aggressive. The introduction of galvanic coupling with 304L SS (G4-G6) dramatically accelerated corrosion. The experimental corrosion rate surged to 0.2630 mmpy in tap water (a 4.92\u0026times; increase), 0.2690 mmpy in NaCl (a 3.16\u0026times; increase), and a catastrophic 1.1942 mmpy in H₂SO₄ (a 4.26\u0026times; increase).The analytical and numerical models demonstrated excellent predictive accuracy. As shown in Fig. \u003cspan class=\"InternalRef\"\u003e5\u003c/span\u003e, the results from all three methods (analytical, numerical, and experimental) were in strong agreement, with all predictive errors remaining below 5%. The highest corrosion rate of ~\u0026thinsp;100 \u0026micro;m/30 days (~\u0026thinsp;1.2 mmpy) was consistently observed in the galvanically coupled acidic environment (G6).\u003c/p\u003e\n \u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\n \u003ch2\u003e3.2. Composition of Corrosion Products\u003c/h2\u003e\n \u003cp\u003eXRD analysis in (Fig. \u003cspan class=\"InternalRef\"\u003e6\u003c/span\u003e) identified the bulk corrosion products as calcite (CaCO₃) in tap water, bayerite (Al(OH)₃) in NaCl, and aluminum sulfate hydrate (Al₂(SO₄)₃) in H₂SO₄. FTIR spectroscopy (Fig. \u003cspan class=\"InternalRef\"\u003e7\u003c/span\u003e) confirmed the chemical nature of these products through their characteristic absorption bands.\u003c/p\u003e\n \u003cp\u003eThe unexposed (G0) surface was clean, showing only faint native Al-O bonds below 900 cm⁻\u0026sup1;. Exposure to tap water (G1) resulted in water bands (~\u0026thinsp;3400, ~\u0026thinsp;1630 cm⁻\u0026sup1;) and carbonate peaks (~\u0026thinsp;1420, ~\u0026thinsp;875 cm⁻\u0026sup1;) from calcite. The 3.5% NaCl (G2) solution caused intense hydration signals (~\u0026thinsp;3400, ~\u0026thinsp;1630 cm⁻\u0026sup1;), indicating hydrated aluminum oxides. Finally, 1M H₂SO₄ (G3) produced dominant sulfate bands (~\u0026thinsp;1100, ~\u0026thinsp;610 cm⁻\u0026sup1;), confirming an aluminum sulfate product.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\n \u003ch2\u003e3.2. Microstructural Degradation and Hardness Loss\u003c/h2\u003e\n \u003cp\u003eThe degradation of the alloy\u0026apos;s microstructure was visually confirmed by optical microscopy (Fig. \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003e). The as-received sample (Fig. \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003ea) exhibited a smooth surface. In contrast, exposure to tap water (Fig. \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003eb) resulted in minor etching, exposure to 3.5% NaCl solution (Fig. \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003ec) caused localized pitting, and 1M H₂SO₄ solution (Fig. \u003cspan class=\"InternalRef\"\u003e8\u003c/span\u003ed) induced severe, widespread dissolution. This damage correlated directly with a loss in mechanical integrity, with Vickers hardness dropping from a baseline of 188.6 HV to as low as 167.9 HV (a 10.99% loss).\u003c/p\u003e\n \u003cp\u003eSEM analysis revealed distinct corrosion morphologies. In 3.5% NaCl (Fig. \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003ea), deep pits were observed to nucleate at the interface of the aluminum matrix and bright, cathodic intermetallic particles. In 1M H₂SO₄ (Fig. \u003cspan class=\"InternalRef\"\u003e9\u003c/span\u003eb), corrosion was widespread, resulting in a porous, degraded matrix with remnant, Fe-rich IMPs acting as local cathodes.\u003c/p\u003e\n \u003cdiv class=\"gridtable\"\u003e\u0026nbsp;\u0026nbsp;\u003ctable 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\u003eSummary of corrosion rates and hardness reduction for all experimental groups after 30 days.\u003c/p\u003e\n \u003c/div\u003e\n \u003c/caption\u003e\n \u003cthead\u003e\n \u003ctr\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eGroup\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eCondition\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eCorrosion Rate (mmpy)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eHardness (HV)\u003c/p\u003e\n \u003c/th\u003e\n \u003cth align=\"left\"\u003e\n \u003cp\u003eHardness Loss (%)\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\"\u003e\n \u003cp\u003eG0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eAs-received\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e188.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e-\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eG1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eTap Water\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.0534\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e185.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.63\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eG2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.5% NaCl\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.0864\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e182.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.34\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eG3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1M H₂SO₄\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.2888\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e179.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e5.11\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eG4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eTap Water\u0026thinsp;+\u0026thinsp;304L SS\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.2630\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e178.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e5.47\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eG5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e3.5% NaCl\u0026thinsp;+\u0026thinsp;304L SS\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e0.2690\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e176.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e6.37\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003eG6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1M H₂SO₄ + 304L SS\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e1.1942\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"char\"\u003e\n \u003cp\u003e167.9\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd align=\"left\"\u003e\n \u003cp\u003e10.99\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\u003c/div\u003e\n\u003cdiv id=\"Sec12\" class=\"Section2\"\u003e\n \u003ch2\u003e3.3. Mechanical Degradation\u003c/h2\u003e\n \u003cp\u003eThe severe corrosion translated directly to a loss of mechanical integrity, evidenced by the progressive decline in Vickers hardness across all experimental groups, with a maximum loss of 10.99% in the G6 sample (Fig. \u003cspan class=\"InternalRef\"\u003e10\u003c/span\u003e).\u003c/p\u003e\n\u003c/div\u003e"},{"header":"4. Discussion","content":"\u003cp\u003eThe results of this study unequivocally demonstrate that while environmental conditions dictate baseline corrosivity, galvanic coupling is a critical accelerator of degradation for AA7025 aluminum alloy. The 4- to 5-fold increase in corrosion rates observed in coupled samples highlights the electrochemical driving force imposed by the more noble 304L stainless steel, which acts as an efficient cathode for oxygen reduction (in neutral media) or hydrogen evolution (in acidic media) [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e13\u003c/span\u003e].The environmental chemistry dictated the specific mode of failure. In tap water, the high TDS content and presence of carbonate ions led to the precipitation of a partially protective CaCO₃ scale [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. While this scale mitigated uniform corrosion, it was not sufficient to prevent localized galvanic effects. In the 3.5% NaCl solution, the aggressive chloride ions caused a breakdown of the passive Al₂O₃ film, leading to severe pitting corrosion, a mechanism well-documented for aluminum alloys in marine environments [\u003cspan additionalcitationids=\"CR17\" citationid=\"CR15\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. The most catastrophic degradation occurred in 1M H₂SO₄, where the synergistic effect of a highly acidic environment and a strong galvanic potential difference led to rapid, widespread dissolution of the aluminum matrix, a behavior consistent with studies on aluminum alloys in acidic media [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e19\u003c/span\u003e].The strong correlation between microstructural damage and hardness reduction underscores the direct link between corrosion and loss of mechanical integrity [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. The 10.99% hardness loss in the G6 sample after only 30 days is alarming and indicates a drastically shortened service life for structures in such conditions. This finding emphasizes the need for effective corrosion mitigation strategies.\u003c/p\u003e \u003cdiv id=\"Sec14\" class=\"Section2\"\u003e \u003ch2\u003e4.1. Integrated Analysis of Corrosion Mechanisms\u003c/h2\u003e \u003cp\u003eThe corrosion of AA7025 is a multi-scale process initiated by micro-galvanic cells at intermetallic particles. These IMPs act as local cathodes, driving the dissolution of the surrounding aluminum matrix. The macro-galvanic couple with the 304L SS fastener acts as a powerful amplifier, dramatically increasing the rate of these IMP-driven processes. This is visually confirmed by the COMSOL simulations (Fig.\u0026nbsp;\u003cspan refid=\"Fig10\" class=\"InternalRef\"\u003e3\u003c/span\u003e), which show a clear increase in galvanic current density that mirrors the measured corrosion rates. The specific chemical pathway is dictated by the environment. The FTIR analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e7\u003c/span\u003e) provides the chemical fingerprint for each pathway: carbonate scale formation in tap water, extensive hydration in NaCl, and sulfate product formation in H₂SO₄. The combined impact of all factors is summarized in the 3D corrosion map (Fig.\u0026nbsp;\u003cspan refid=\"Fig9\" class=\"InternalRef\"\u003e11\u003c/span\u003e), which identifies the galvanically coupled, acidic condition as the most destructive. The COMSOL simulations provide a powerful tool for both predicting corrosion and designing protective systems. The high accuracy of the corrosion models, consistent with other simulation-based corrosion studies [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e21\u003c/span\u003e], validates their use for engineering design and lifetime assessment.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section2\"\u003e \u003ch2\u003e4.2. Mitigation Strategy: Cathodic Protection Simulation\u003c/h2\u003e \u003cp\u003eCOMSOL simulations were used to evaluate the performance of three sacrificial anodes for protecting the AA7025 alloy. The results (Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e) showed that the Al-5Zn anode provided superior protection compared to pure Zn and Al-0.1In. The Al-5Zn anode generated the highest protective current density (0.84 A/m\u0026sup2;) and induced the most negative polarization potential (-1.08 V) on the AA7025 surface. This potential is well below the typical protection threshold of -0.85 V, indicating a robust safety margin against corrosion.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eComparative performance of simulated sacrificial anodes for AA7025 cathodic protection.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAnode Material\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eCurrent Density (A/m\u0026sup2;)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003ePolarization Potential (V)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eProtection Uniformity\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAl-5Zn\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.84\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e-1.08\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eSuperior\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eZn\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.71\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e-1.00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eGood\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAl-0.1In\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e0.64\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e-0.99\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003eModerate\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003eTo counteract the severe galvanic corrosion, a cathodic protection strategy using a sacrificial anode was simulated as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e. The protection mechanism relies on coupling the AA7025-304L SS assembly to a more active metal, which preferentially corrodes (acts as the anode), thereby protecting the AA7025 structure by forcing it to become a cathode. COMSOL simulations compared the performance of three anode materials. The results (Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e) identified Al-5Zn as the optimal choice. Figure\u0026nbsp;\u003cspan refid=\"Fig12\" class=\"InternalRef\"\u003e12\u003c/span\u003e illustrates its superior performance: the Al-5Zn anode generated a robust protective current density of 0.84 A/m\u0026sup2; and uniformly polarized the entire AA7025 surface to a potential of -1.08 V. This is significantly more negative than the potentials achieved by Zn (-1.00 V) or Al-0.1In (-0.99 V) and provides a substantial safety margin below the \u0026minus;\u0026thinsp;0.85 V protection threshold.\u003c/p\u003e \u003c/div\u003e"},{"header":"5. Conclusions","content":"\u003cp\u003eThis study comprehensively investigated the critical challenge of galvanic corrosion acceleration in AA7025 aluminum alloy when coupled with 304L stainless steel fasteners across three corrosive environments: Hora Harsade lake tap water, (3.5% NaCl), and (1M H₂SO₄). A multi-methodological framework was employed, combining analytical corrosion rate calculations, SolidWorks 3D modeling, and current density/corrosion potential simulations via COMSOL Multiphysics v5.6. Rigorous experimental validation included weight loss tests (ASTM G31) for corrosion rates, Vickers micro hardness (ASTM E384) for mechanical degradation, and MATLAB (R2019a) for environmental aggressiveness mapping. Advanced microstructural characterization using OM, SEM/EDS, and FTIR elucidated corrosion mechanisms after a 30-day immersion period.\u003c/p\u003e \u003cp\u003e First, the study successfully validated predictive models, showing excellent agreement between experimental corrosion rates and both analytical/numerical (COMSOL) simulations. All predictive errors were below 5% (0.07\u0026ndash;4.44% for analytical-numerical; 1.71\u0026ndash;3.86% for analytical-experimental), firmly establishing the theoretical framework's reliability.\u003c/p\u003e \u003cp\u003eIn line with the second objective, the investigation characterized the mechanical degradation and corrosion rate of the standalone AA7025 alloy. Initially, all samples exhibited high hardness (188.6 HV) and a defect-free microstructure with coherent η-MgZn₂ and S Al₂CuMg phases, and an intact Al₂O₃ passive layer. Corrosion severity was environment dependent: minimal in tap water (4.458 \u0026micro;m/30 days, 185.53 HV), moderate pitting in 3.5% NaCl (7.187 \u0026micro;m/30 days, 182.3 HV), and severe general corrosion in 1M H₂SO₄ (23.582 \u0026micro;m/30 days, 178.96 HV).\u003c/p\u003e \u003cp\u003eBuilding on this baseline, the study characterized the significantly more aggressive galvanic corrosion of the AA7025\u0026ndash;304L couple. Mechanical degradation and corrosion rates were substantially higher across all environments, highlighting the galvanic cell's critical role in accelerating structural decay.\u003c/p\u003e \u003cp\u003eThe quantitative effect of galvanic coupling was precisely evaluated, showing 304L stainless steel fasteners dramatically enhanced AA7025 corrosion rates by 3 to 5 times. Rates surged to 21.953 \u0026micro;m/30 days in tap water (4.92x uncoupled), 22.691 \u0026micro;m/30 days in NaCl (3.16x uncoupled), and 100.4 \u0026micro;m/30 days in H₂SO₄ (4.26x uncoupled), leading to substantial hardness reductions (lowest 167.9 HV) and shortened service lifetime.\u003c/p\u003e \u003cp\u003eReducing the anodic area also slightly increased corrosion, notably 7.01% in sulfuric acid. Finally, to understand the underlying causes of this behavior, detailed microstructural (OM, SEM), compositional (EDS), and phase identification (XRD, FTIR) analyses were employed. This characterization revealed specific corrosion mechanisms and chemical byproducts: minimal surface alteration and CaCO₃ deposition in tap water; severe pitting corrosion with Cl detection and Al(OH)₃ formation in 3.5% NaCl; and widespread general corrosion with S detection and Al₂(SO₄)₃ as the primary product in 1M H₂SO₄. These degradation pathways were significantly exacerbated under galvanic coupling. Future work should focus on the long-term experimental validation of the Al-5Zn sacrificial anode system to confirm its real-world effectiveness in preserving the structural integrity of AA7025 components in marine applications.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003e \u003cb\u003eConflicts of Interest\u003c/b\u003e:\u003c/h2\u003e \u003cp\u003eThe authors declare no conflict of interest.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eFunding\u003c/h2\u003e \u003cp\u003eDeclaration\u003c/p\u003e \u003cp\u003eFunding:-Not applicable\u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eConceptualization, M.W.T.; methodology, B.G.S.; software, B.G.S.; validation, B.G.S. and M.W.T.; formal analysis, B.G.S.; investigation, B.G.S.; resources, M.W.T.; data curation, B.G.S.; writing original draft preparation, B.G.S.; writing review and editing, M.W.T.; visualization, B.G.S.; supervision, M.W.T.; project administration, M.W.T. All authors have read and agreed to the published version of the manuscript **.**\u003c/p\u003e\u003ch2\u003eAcknowledgment\u003c/h2\u003e \u003cp\u003eThe authors would like to express their sincere gratitude to the Ethiopian Defence University for providing the necessary resources and facilities to conduct this research. Special thanks go to the Department of metallurgical and materials Engineering for their technical support and guidance during the numerical and analytical work of the study.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eThe experimental data and simulation files supporting the findings of this study will be made available upon reasonable request to the corresponding author and the collaborative author.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eTimelli, G. Aluminium Alloys: Design and Development of Innovative Alloys, Manufacturing Processes and Applications; IntechOpen, 2022.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLi, S.; Yue, X.; Li, Q.; Peng, H.; Dong, B.; Liu, T.; Yang, H.; Fan, J.; Shu, S.; Qiu, F. Development and applications of aluminum alloys for aerospace industry. J. Mater. Res. Technol. 2023, 27, 944\u0026ndash;983.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eCzerwinski, F. Aluminum alloys for electrical engineering: a review. J. Mater. Sci. 2024, 59, 14847\u0026ndash;14892.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eMiteva, A.; Bouzekova-Penkova, A. Advancements in Aerospace Alloys: Navigating the Future of Aviation and Space Exploration. Aerosp. Res. Bulg. 2025, 37, 223\u0026ndash;238.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eVenkatesh, S.; Kathiravan, A.A.; Kavipriyan, M.; Nandhini, S.S.G.; Avinasilingam, M. A review on aluminium 7075 alloy: Micro structure, mechanical properties and application. AIP Conf. Proc. 2024, 3221, 20001.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWang, X.; Zhang, D.; Li, A.; Yi, D.; Li, T. A review on traditional processes and laser powder bed fusion of aluminum alloy microstructures, mechanical properties, costs, and applications. Materials 2024, 17, 2553.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLima, J. S. de; Santos, O. C. dos; Silva, A. A.; Melo, R. H. F. de; Maciel, T. M. Influence of Welding Parameters on the Mechanical Properties and Microstructure of 7075-T651 Aluminum Alloys Welded Joints Performed by FSW Process. Mater. Res. 2022, 25, e20210629.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eStansbury, E.E.; Buchanan, R.A. Fundamentals of electrochemical corrosion; ASM international, 2000.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBrossia, C. S.; Cragnolino, G. A. Effect of environmental variables on localized corrosion of carbon steel. Corrosion 2000, 56, 505\u0026ndash;514.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWang, Q.; Ran, H.; Li, Y. Comparative study on stress corrosion resistance of 7150/7055/7A56 aluminum alloys. J. Phys. Conf. Ser. 2024, 2873, 12027. Fabris, R.; Masi, G.; Bignozzi, M. Corrosion Behavior of Aluminum Alloys in Different Alkaline Environments: Effect of Alloying Elements and Anodization Treatments. Coatings 2024, 14, 240.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAl-Shemmary, B. R.; Matloub, F. K.; Al-Fetlawi, H. Modelling and Simulation of Pitting Corrosion. J. Mech. Eng. Res. Dev. 2021, 44, 10\u0026ndash;17.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGupta, G.; Choudhury, D.; Maurya, R.; Sharma, S.; Neergat, M. 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A Review on Tribocorrosion Behavior of Aluminum Alloys: From Fundamental Mechanisms to Alloy Design Strategies. Corros. Mater. Degrad. 2023, 4, 594\u0026ndash;622.\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eYang, Y. J.; Kim, S. J. Electrochemical characteristics of aluminum alloys in sea water for marine environment. Acta Phys. Pol. A 2019, 135, 1005\u0026ndash;1011.\u003c/span\u003e\u003c/li\u003e\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":"npj-materials-degradation","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"npjmatdeg","sideBox":"Learn more about [npj Materials Degradation](http://www.nature.com/npjmatdeg/)","snPcode":"41529","submissionUrl":"https://submission.springernature.com/new-submission/41529/3","title":"npj Materials Degradation","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"NPJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"aluminum alloy, stainless steel, galvanic corrosion, COMSOL Multiphysics, marine environment, cathodic protection","lastPublishedDoi":"10.21203/rs.3.rs-8212976/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8212976/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis study investigates the galvanic corrosion of AA7025 aluminum alloy coupled with 304L stainless steel (SS) fasteners in marine structures, using a multi-methodological approach involving analytical modeling, SolidWorks 3D modeling, COMSOL Multiphysics simulations, and experimental validation. The AA7025-304L SS couples were tested in tap water, 3.5% NaCl, and 1M H₂SO₄ for 30 days. Corrosion rates increased significantly due to galvanic coupling: 4.92\u0026times; in tap water, 3.16\u0026times; in NaCl, and 4.26\u0026times; in H₂SO₄, with predictive models accurate within 5% error. H₂SO₄ caused the highest hardness loss (10.97%) and a corrosion depth of 100.4 \u0026micro;m. Microstructural analysis showed CaCO₃ scaling in tap water, severe pitting in NaCl, and generalized dissolution in H₂SO₄. COMSOL simulations identified an Al-5Zn sacrificial anode as an effective mitigation strategy, providing a protective current density of 0.84 A/m\u0026sup2; and a polarization potential of -1.08 V, significantly reducing AA7025 corrosion in marine-relevant environments.\u003c/p\u003e","manuscriptTitle":"Numerical and Experimental Investigation on the Corrosion Resistance of the Coupled 7025 Aluminum Alloy Structure with 304 Stainless Steel Fasteners for Marine Application","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-19 12:24:02","doi":"10.21203/rs.3.rs-8212976/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-01-15T01:57:53+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-13T10:34:46+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-01-05T20:30:04+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"174935194014395435144588524478201458113","date":"2025-12-23T04:57:01+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"227161028205467573295945459824172498848","date":"2025-12-17T11:16:30+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"190109823825858418566753422594701582444","date":"2025-12-17T10:14:25+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-12-17T10:02:24+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-12-17T06:17:54+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-12-16T15:10:06+00:00","index":"","fulltext":""},{"type":"submitted","content":"npj Materials Degradation","date":"2025-11-26T12:25:57+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"npj-materials-degradation","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"npjmatdeg","sideBox":"Learn more about [npj Materials Degradation](http://www.nature.com/npjmatdeg/)","snPcode":"41529","submissionUrl":"https://submission.springernature.com/new-submission/41529/3","title":"npj Materials Degradation","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"NPJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"ccc721b3-c20f-47d8-a124-768d1ec90cbe","owner":[],"postedDate":"December 19th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"in-revision","subjectAreas":[{"id":59812758,"name":"Physical sciences/Engineering"},{"id":59812759,"name":"Physical sciences/Materials science"}],"tags":[],"updatedAt":"2026-01-15T02:09:00+00:00","versionOfRecord":[],"versionCreatedAt":"2025-12-19 12:24:02","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8212976","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8212976","identity":"rs-8212976","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}