Effect of Hf Element on the Hot Salt Corrosion Resistance of NiCoCrAlY Coatings

Effect of Hf Element on the Hot Salt Corrosion Resistance of NiCoCrAlY Coatings | 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 Effect of Hf Element on the Hot Salt Corrosion Resistance of NiCoCrAlY Coatings Huaixu Guo, Peixuan Ouyang, Fei Xiao, Zhaoran Zheng, Fukang Yue, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8783198/v1 This work is licensed under a CC BY 4.0 License Status: Under Revision Version 1 posted 9 You are reading this latest preprint version Abstract To improve the hot corrosion resistance of NiCoCrAlY coatings under high‑temperature salt‑laden conditions, two free‑standing coatings—NiCoCrAlY and NiCoCrAlYHfSi—were prepared by high‑velocity oxy‑fuel spraying. Their corrosion behavior was systematically investigated at 900 °C in a mixed salt environment of 75 wt.% Na 2 SO 4 + 25 wt.% NaCl. The results show that the addition of Hf and Si significantly enhances coating densification and structural integrity, effectively suppressing TGO thickening, crack propagation, and internal oxidation. Corrosion kinetics analysis indicates a lower corrosion rate for the NiCoCrAlYHfSi coating. Microstructural characterization confirms that Hf forms a dense HfO 2 layer, which strengthens TGO adhesion through “pinning effects” and blocks corrosive ion penetration. Additionally, Hf effectively traps and immobilizes sulfur, inhibiting internal sulfidation. This study provides experimental and theoretical support for the application of Hf‑doped NiCoCrAlY coatings in severe high‑temperature corrosive environments. Hot salt corrosion NiCoCrAlY coating High-velocity oxygen fuel (HVOF) spraying Hf Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Introduction Hot-end components of gas turbine engines, such as combustion chambers and turbine blades, are not only exposed to high-temperature oxidizing media like O 2 and H 2 O during service but also suffer corrosion from hot corrosive gases such as sulfur oxides (SO x ) and hydrogen chloride (HCl). To meet the strength and corrosion resistance requirements of nickel-based alloys used in turbine blades and other hot-end components under high-temperature conditions, MCrAlY (M = Ni, Co, or a combination thereof) coatings are typically applied for protection[1–5]。MCrAlY coatings are widely used as bond coats in thermal barrier coating systems or as standalone overlay coatings, offering excellent resistance to high-temperature oxidation and hot corrosion[6]. When exposed to oxidizing or corrosive environments, the abundant Al and Cr within MCrAlY coatings form a thermally grown oxide (TGO) layer on the surface, which acts as a diffusion barrier to slow further oxidation or corrosion of the coating. The failure of MCrAlY coatings is associated with TGO growth stresses and thermal mismatch stresses. During service, with prolonged exposure, the protective Al 2 O 3 scale is rapidly consumed, and the volume fraction of the β-NiAl phase in the coating gradually decreases. This leads to the formation of complex spinel-type oxides within the TGO layer, which reduce the density and adhesion of the oxide scale, thereby compromising the high-temperature performance of the coating[7–12].Furthermore, hot corrosion induced by high-temperature corrosive media like sulfur oxides (SO x ) and hydrogen chloride (HCl) damages the protective oxide film on the coating surface, thereby accelerating the overall failure process[2,13–15].Therefore, improving the comprehensive high-temperature oxidation and hot corrosion resistance of MCrAlY coatings remains a current research priority. Studies have shown that the addition of trace elements to MCrAlY coatings can effectively enhance their high-temperature performance.，Li[16] investigated the effect of the trace element Ta on the oxidation resistance of NiCrAlY alloy at 1050°C by adding Ta to the alloy. The study shows that the addition of the refractory element Ta not only promotes the precipitation of Cr 2 O 3 on the alloy surface but also increases the thickness of the oxide film as the Ta content increases. The film structure becomes denser, effectively blocking the penetration of oxygen into the substrate, thereby enhancing the oxidation resistance of the alloy.Salam[17] investigated the effect of the trace element Re on the oxidation resistance of CoNiCrAlY alloy at 1100°C by adding Re to the alloy. The study demonstrates that the addition of Re promotes the transformation of θ-Al 2 O 3 to α-Al 2 O 3 , thereby slowing down the oxidation rate. Moreover, Re can inhibit the interdiffusion of refractory elements, particularly Mo, between the coating and the substrate, consequently enhancing the oxidation resistance of the coating.Ebach-Stahl[18] investigated the effect of doping 0.15–0.60 at.% Hf on the oxidation resistance of NiCoCrAlY bond coats at 1100°C and found that the reactive element Hf enhances the adhesion of the Al 2 O 3 oxide scale. This improvement in oxidation resistance of the bond coat extends the service life of the thermal barrier coating.Furthermore, the addition of Hf element can significantly enhance the adhesion between the coating and the substrate, improving the interfacial bonding performance.Zheng[19]prepared a Re-modified NiCoCrAlY coating using arc ion plating technology and investigated its hot salt corrosion behavior in an environment of 75 wt.% Na 2 SO 4 + 25 wt.% NaCl at 900°C. A Re-rich α-Cr phase precipitated at the interface between the coating and the oxide scale. Due to the similar thermal expansion coefficients of α-Cr and α-Al 2 O 4 , the α-Cr phase helps reduce the thermal stress in the oxide scale and enhances its spallation resistance. Therefore, the addition of Re can improve the hot salt corrosion resistance of the coating.In summary, the aforementioned literature primarily focuses on the performance of coatings doped with trace elements in resisting high-temperature oxidizing media. However, research on the influence of Hf addition on the hot salt corrosion resistance of coatings, particularly the interaction mechanism between Hf and sulfur elements in corrosive environments, remains scarce and inadequately reported. In summary, this study employed the high-velocity oxygen fuel (HVOF) spraying method to prepare NiCoCrAlY and NiCoCrAlYHfSi coatings, and comparatively investigated their hot salt corrosion resistance. The phase composition, surface morphology, cross-sectional morphology, and elemental distribution of the oxide scales on the coatings were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Furthermore, the mechanism by which Hf inhibits the penetration of sulfur/chlorine elements and maintains the structural integrity of NiCoCrAlY-based coatings was explored. This research is expected to provide data references for the development and application of high-performance MCrAlY series coatings. Experimental Materials and Methods 1.1 Coating Preparation In this study, NiCoCrAlY and NiCoCrAlYHfSi alloy powders were used as feedstocks for coating deposition. The nominal chemical composition of NiCoCrAlY was 47.5Ni-23Co-17Cr-12Al-0.5Y (wt.%), and that of NiCoCrAlYHfSi was 47.6Ni-22Co-17Cr-12Al-0.5Y-0.5Hf-0.4Si (wt.%). Free‑standing thick coatings (unsupported standalone coatings) were prepared using a JP8000 high‑velocity oxy‑fuel (HVOF) spraying system equipped with a propane/oxygen combustion system and a high‑precision dual‑hopper powder feeder. Low‑carbon steel plates, cleaned with acetone and polished with fine‑grade sandpaper, were used as the deposition substrates.To ensure consistent melting of the powders in the flame and stable coating deposition, the NiCoCrAlY and NiCoCrAlYHfSi powders were separately preheated in a drying oven at 100 °C for 20 h to remove surface‑adsorbed moisture. After coating deposition, the well‑adhered thick coatings were completely separated from the steel substrates by mechanical means, utilizing the difference in thermal expansion coefficients between the coatings and the substrate, thereby obtaining free‑standing coating plates.Subsequently, the free‑standing coatings were cut into specimens with dimensions of 10 × 20 × 2 mm using a precision diamond cutter under continuous water cooling. After machining, the specimens were ultrasonically cleaned in deionized water (300 W, 40 kHz, 15 min) and their initial mass ( m₀ ) was measured using an electronic balance with an accuracy of 0.01 mg. 1.2 Hot Salt Corrosion Experiment at 900°C The salt corrosion experiment was conducted in a muffle furnace set at 900°C. The mixed salt solution was prepared according to the GB/T10125-2021 standard, consisting of 75 wt.% Na 2 SO 4 + 25 wt.% NaCl. The test specimens included two types of coatings after wire cutting: NiCoCrAlY coating and NiCoCrAlYHfSi coating. Three parallel samples of each coating type were prepared to ensure the reliability of the experimental results. The mixed salt solution was evenly applied to the surface of the coated specimens, with the salt loading controlled at approximately 3 mg/cm². The specimens were then placed in a preheated muffle furnace at 900°C for an isothermal corrosion duration of 100 hours. Each 20-hour interval constituted one cycle. At the end of each cycle, the specimens were removed, cooled to room temperature, rinsed with deionized water to remove residual salts, and dried. The mass was then measured using a precision electronic balance (accuracy: 0.01 mg), after which the salt was reapplied for the next cycle. The total experimental duration was 100 hours. The mass change for each cycle (recorded as mᵢ ) was tracked, and the corrosion kinetic constant (K p ) was determined based on the relationship between mass gain and corrosion time. Additionally, a corrosion kinetic curve (mass change per unit area versus time curve) was plotted: Where m i : coating mass gain per cycle, m o: initial mass; A : Coating surface area; K P: parabolic rate constant; t : exposure time; n : time exponent[20]。 1.3 Microstructure Characterization and Analysis First, scanning electron microscopy (SEM) was employed to characterize the surface and cross-sectional morphology of both coatings. After various intervals of the hot salt corrosion tests, cross-sectional samples of the coating specimens were cold-mounted in conductive resin, sequentially ground using #180 to #2000 SiC abrasive paper, and polished with a 2.5 µm polishing paste. High-resolution scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) were used to observe the microstructure of the coating surfaces and cross-sections, including surface corrosion pits, crack morphology and distribution, as well as phenomena such as coating spallation and delamination in the cross-section. Energy dispersive spectroscopy (EDS) was combined for qualitative and quantitative analysis of micro-region compositions to determine the elemental composition of the corrosion products. X-ray diffraction (XRD) was utilized for phase analysis of the coatings, accurately identifying the types and content changes of phases before and after corrosion. This analysis enabled the investigation of chemical reactions and phase transformations occurring during the corrosion process, providing deeper insights into the corrosion mechanisms of the coatings[21]. Results and Discussion 2.1 Phase Analysis of Coating Surfaces Figure 1 shows the XRD patterns of the as-sprayed NiCoCrAlY and NiCoCrAlYHfSi coatings. In the NiCoCrAlY coating, the characteristic diffraction peaks of the γ-phase exhibit high intensity, while those of the β-phase are relatively weak, indicating that in the absence of Hf addition, the γ-phase in the coating is well-crystallized and accounts for a larger proportion of the phase composition. In contrast, with the addition of Hf and Si elements, the diffraction peak intensity of the γ-phase in the coating decreases compared to that of the NiCoCrAlY coating. This may be attributed to the higher activity of Hf, which more readily participates in the crystal growth process during coating formation, thereby interfering with the crystallization of the γ-phase and potentially inhibiting its crystallization or reducing its content[4]. 2.2 Hot Salt Corrosion Test of Coatings at 900°C 2.2.1 Analysis of Corrosion Kinetic Curves Figure 2(a) shows the curves of mass gain per unit area versus corrosion time for NiCoCrAlY and NiCoCrAlYHfSi coatings during the corrosion process. As the corrosion time increases, the mass gain per unit area of both coatings exhibits a parabolic growth trend. After 100 hours of hot salt corrosion, the mass gain per unit area of the NiCoCrAlY coating is 12.1 mg/cm², while that of the NiCoCrAlYHfSi coating is 7.8 mg/cm², representing a reduction of 35.5% in the latter compared to the former. At the same corrosion time, the mass gain per unit area of the NiCoCrAlY coating is higher than that of the NiCoCrAlYHfSi coating, indicating that the addition of Hf elements can effectively mitigate the mass increase caused by corrosion to some extent and plays a positive role in inhibiting the corrosion process.Figure 2(b) shows the linear fitting curves of mass gain per unit area versus the square root of corrosion time for the two coatings, where Kp represents the corrosion rate and R² is the coefficient of determination of the linear fitting. The slope Kp for the NiCoCrAlY and NiCoCrAlYHfSi coatings is 1.1901 and 0.8037, respectively, with coefficients of determination R 2 of 0.9997 and 0.9993, both close to 1. This indicates that the mass gain per unit area of both coatings has a strong linear relationship with the square root of corrosion time. Based on the slope values, the corrosion-induced mass gain rate of the NiCoCrAlY coating is faster than that of the NiCoCrAlYHfSi coating, further confirming that the addition of Hf and Si elements can effectively reduce the corrosion rate of the coating. delay the corrosion process in hot salt environments, and enhance its corrosion resistance in such conditions. 2.2.2 Analysis of Coating Surface Morphology and Phases Figure 3 shows the surface XRD patterns of the NiCoCrAlY and NiCoCrAlYHfSi coatings after 100 hours of hot salt corrosion. The results indicate that both coatings contain Ni 3 Al, Al 2 O 3 , spinel phases, and yttrium aluminum garnet (YAG). Additionally, the HfO₂ phase is detected in the NiCoCrAlYHfSi coating. The presence of the HfO 2 phase promotes the formation of Al 2 O 3 while inhibiting the retention of Ni 3 Al, enabling more effective participation of Al in the formation of the oxide scale. Therefore, as observed in the XRD patterns, the diffraction peak intensity of the Ni₃Al phase in the NiCoCrAlYHfSi coating is significantly lower than that in the NiCoCrAlY coating, while the diffraction peak intensity of the Al 2 O 3 phase is higher. Furthermore, the diffraction peak intensity of the spinel phase in the NiCoCrAlYHfSi coating is notably reduced, indicating that the addition of Hf effectively suppresses the formation of the spinel phase. In conclusion, Hf significantly enhances the corrosion resistance of the coating in hot salt environments by promoting the formation of the protective Al₂O₃ scale and inhibiting the generation of detrimental spinel phases. Figure 4 shows the surface SEM images of the NiCoCrAlY and NiCoCrAlYHfSi coatings before and after 100 hours of hot salt corrosion. Prior to corrosion, both coatings exhibited similar surface morphologies, characterized by a molten-like structure with a small number of unmelted particles. However, after 100 hours of hot salt corrosion, the oxide scales on both coatings aggregated into nodule-like features. The NiCoCrAlY coating surface displayed multiple visible cracks and significant roughness, as shown in Figure 4(c). In contrast, the NiCoCrAlYHfSi coating surface maintained relatively good densification with no obvious defects and appeared smoother overall compared to the NiCoCrAlY coating, as seen in Figure 4(d). This dense and uniform surface structure can effectively enhance the barrier capability against the inward penetration of O 2 , S 2- , and other corrosive species. Therefore, the introduction of Hf can significantly inhibit crack initiation during hot salt corrosion and improve the coating's resistance to hot salt corrosion. 2.2.3 Microstructural Cross-Sectional Analysis and Elemental Analysis of Coatings Figure 5 shows the cross-sectional SEM images and EDS analysis of the NiCoCrAlY and NiCoCrAlYHfSi coatings after 20 hours of hot salt corrosion. From Figures 5 (a) and (b), it can be observed that the NiCoCrAlY coating exhibits pores and internal oxidation, which provide pathways for the ingress of corrosive media and accelerate coating degradation. In contrast, compared to the NiCoCrAlY coating, the NiCoCrAlYHfSi coating (Figures (c) and (d)) shows a more intact cross-sectional structure with fewer pores and milder internal oxidation. In terms of the morphology and continuity of the thermally grown oxide (TGO) layer, the TGO layer of the NiCoCrAlY coating is less dense and exhibits poorer continuity, while the TGO layer of the NiCoCrAlYHfSi coating is denser and more continuous. After 20 hours of hot salt corrosion, EDS analysis revealed no significant presence of sulfur (S) elements in either coating, indicating that both coatings demonstrate effective resistance to sulfidation corrosion in the short term. Figure 6 shows the cross-sectional SEM images and EDS analysis of the NiCoCrAlY and NiCoCrAlYHfSi coatings after 60 hours of hot salt corrosion. From Figures (a) and (b) and the elemental distribution, it can be observed that the TGO layer on the surface of the NiCoCrAlY coating exhibits significant separation and contains numerous voids, with internal oxidation further aggravated. In contrast, from Figures (c) and (d) and the elemental distribution, the cross-section of the NiCoCrAlYHfSi coating appears relatively flat and dense, with no obvious separation in the TGO layer. Although internal oxidation is also observed at this stage, its extent is milder compared to that in the NiCoCrAlY coating.Regarding the thickness and continuity of the TGO layer, both coatings show thicker TGO layers after 60 hours of hot salt corrosion compared to those after 20 hours. The TGO layer of the NiCoCrAlY coating exhibits cracks and numerous wrinkles, indicating poor continuity, while the TGO layer of the NiCoCrAlYHfSi coating remains relatively smooth and continuous. Based on the EDS mapping, sulfur (S) penetration into the interior of the NiCoCrAlY coating is observed after 60 hours, reaching a depth of approximately 30 μm, indicating that the hot salt has caused further corrosion of the coating. In contrast, no sulfur penetration is detected inside the NiCoCrAlYHfSi coating, with S elements observed only within the TGO layer. This suggests that the dense TGO layer and coating structure effectively block the penetration of sulfur, preventing the spread of sulfidation corrosion into the coating interior. Figure 7 presents the cross-sectional SEM images and EDS analysis of the NiCoCrAlY and NiCoCrAlYHfSi coatings after 100 hours of hot salt corrosion. The corrosion condition of the NiCoCrAlY coating (Figures 7(a) and (b)) further deteriorated. The TGO layer on the coating surface not only exhibited more pronounced separation but also showed a significant increase in the number and size of voids. Internal oxidation intensified, and the formation of diffusion channels within the coating provided preferential pathways for the deep penetration of sulfur (S) elements. This accelerated the diffusion process, leading to premature coating failure. Sulfur penetrated into the coating interior along these diffusion channels, reaching localized depths of up to 180 µm.For the NiCoCrAlYHfSi coating (Figures 7(c) and (d)), although internal oxidation was also observed, no significant voids were detected. While sulfur was found within the coating, the corrosion depth was limited to approximately 60 µm. In terms of TGO thickness and continuity, the TGO layer of the NiCoCrAlY coating exhibited poor continuity and uneven thickness, whereas the TGO layer of the NiCoCrAlYHfSi coating demonstrated better continuity and uniform thickness. Figure 8 shows the cross-sectional EPMA and EDS images of the NiCoCrAlY and NiCoCrAlYHfSi coatings after 100 hours of hot salt corrosion. A comparison of the elemental distribution maps of Cr, S, and Hf in the two coatings reveals that after 100 hours of hot salt corrosion, the distribution of Cr in the NiCoCrAlY coating exhibits significant non-uniformity. Cr is a key element in the formation of the protective Cr 2 O 3 scale in the coating and also works synergistically with Al to promote the growth of a dense Al 2 O 3 scale. If Cr distribution is uneven, localized regions of the coating may experience Cr enrichment while others suffer from Cr deficiency. Areas deficient in Cr struggle to form a continuous Cr 2 O 3 protective scale, which also weakens the stability of the Al 2 O 3 scale, making these regions vulnerable weak points for the ingress of corrosive media. Conversely, regions with Cr enrichment tend to form brittle spinel phases, increasing the risk of cracking and spalling of the oxide scale and accelerating the corrosion process of the coating[4,13,22].In contrast, within the NiCoCrAlYHfSi coating, Hf and S elements exhibit distinct co-localization and are present both in the Al 2 O 3 scale and inside the coating. Hf is dispersed throughout the coating matrix and interfacial regions, showing clear co-distribution with S, as well as with O, primarily within the Al 2 O 3 scale. On one hand, Hf promotes the selective oxidation of Cr, facilitating the formation of a continuous and dense protective oxide scale[23].On the other hand, Hf combines with O to form a dense HfO₂ phase within the Al 2 O 3 scale, which effectively blocks grain boundary pathways. Concurrently, it seals the diffusion channels for S, structurally hindering the penetration path of S into the coating interior[24].From the aggregation regions of S and Hf elements, it can be clearly observed that Hf exhibits excellent "sulfur-trapping" capability, efficiently capturing S and significantly inhibiting its penetration and sulfidation corrosion. This results in a notably lower distribution of S in the NiCoCrAlYHfSi coating compared to the NiCoCrAlY coating. Simultaneously, Cr is more uniformly distributed in the NiCoCrAlYHfSi coating, indicating that the presence of Hf effectively suppresses the sulfidation reaction of Cr. This contributes to enhancing the coating's corrosion resistance at lower temperatures, thereby extending its service life. 2.3 Discussion on the Mechanism of Hf Element Effects Based on the experimental results presented above, the mechanism by which Hf enhances the hot salt corrosion resistance of NiCoCrAlY coatings at 900°C can be systematically explained. The strengthening effect primarily stems from the following aspects: 1.Effect of Hf Pinning on the Oxide Scale Hf exhibits strong oxygen affinity and can synergize with Al to promote the formation of a continuous and dense Al 2 O 3 scale. This is corroborated by the stronger Al 2 O 3 diffraction peaks observed in the NiCoCrAlYHfSi coating compared to those in the NiCoCrAlY coating in Figure 3. Additionally, Hf suppresses the excessive formation of non-protective, porous spinel phases, as evidenced by the reduced intensity of spinel phase peaks in the NiCoCrAlYHfSi coating shown in Figure 3. Meanwhile, Hf incorporates into the TGO layer, forming a dense HfO 2 phase. The results from Figure 8 in this experiment also indicate that the TGO layer of the NiCoCrAlYHfSi coating is denser, and Figure 8 further reveals a stronger bond between the TGO layer and the NiCoCrAlYHfSi coating. Therefore, HfO 2 improves the TGO/coating matrix bonding via the "pinning effect" (Figure. 9), inhibiting TGO delamination and cracking in the NiCoCrAlY coating. Furthermore, the high melting point of HfO 2 contributes to refining the TGO grain structure, reducing internal stress accumulation, and maintaining a smooth, continuous, and dense TGO structure[2,25].As shown in Figure 7, the TGO layer of the NiCoCrAlYHfSi coating is more continuous and dense compared to that of the NiCoCrAlY coating, which significantly enhances the physical barrier effect of the TGO layer against corrosive media. 2.The Impact of Hf’s "Preferential Sulfur Trapping" Effect on Coating Corrosion Resistance During the mid-to-late stages of hot corrosion, after molten salts penetrate the initial oxide scale, the infiltration of sulfur (S) and internal sulfidation become key factors leading to accelerated coating failure. The EDS analysis results from this study (Figure 8) clearly demonstrate that, after hot salt corrosion of the NiCoCrAlYHfSi coating, Hf and the infiltrated S exhibit significant co-distribution. This co-localization is primarily concentrated in the coating's surface layer and at the oxide/coating interface. These microstructural features directly confirm the notable interaction between Hf and S during the hot corrosion process[26,27]. Goebel[28]和Bornstein[29]'s models of hot corrosion through salt dissolution of oxide scales propose that Na₂SO₄ exhibits the following thermodynamic equilibrium at high temperatures: Na 2 SO 4 →Na 2 O+SO 3 In hot salt corrosion, sulfur primarily originates from SO₃. From a thermodynamic perspective, the binding affinity between elements can be evaluated based on the standard Gibbs free energy of formation (ΔG θ ) of their sulfides. A larger absolute value of ΔGθ indicates higher compound stability and a greater priority for element bonding[24].In this study, thermodynamic software (HSC Chemistry) was employed to calculate the standard Gibbs free energy of formation (ΔG θ ) for sulfides generated by the reactions of Hf, Cr, Al, and Y with SO₃ under hot salt corrosion conditions at 900°C (1173 K), as summarized below:： 4Hf(s)+2SO 3 (g) = HfS₂(s)+3HfO 2 (s) ΔG θ ≈ -520kJ/mol 8Y(s)+3SO 3 (g) → Y 2 S 3 (s)+3Y 2 O 3 (s) ΔG θ ≈ -480kJ/mol 8Al(s)+3SO 3 (g) →Al 2 S 3 (s)+3Al 2 O 3 (s) ΔG θ ≈ -420kJ/mol 8Cr(s)+3SO 3 (g) → Cr 2 S 3 (s)+3Cr 2 O 3 (s) ΔG θ ≈ -350kJ/mol At 900°C, the order of Gibbs free energy for the reaction of each element with S from high to low is: Cr > Al > Y > Hf, meaning Hf has the strongest binding ability with S, far exceeding the core protective elements Al and Cr in the coating. This thermodynamic advantage enables Hf to preferentially combine with S infiltrating into the coating, forming stable HfS 2 phases. This process effectively acts as a "capture and fixation" mechanism for S by Hf, significantly reducing the probability of contact and reaction between S and critical elements such as Al and Cr. Consequently, the formation of harmful sulfides like Al₂S₃ and Cr 2 S 3 is suppressed, preventing the degradation of the protective oxide layer due to the depletion of active elements.On the other hand, the extremely low Gibbs free energy of formation for hafnium sulfide means that once formed, it is difficult to decompose and does not release S again, thus avoiding secondary corrosion. This blocks the diffusion and erosion chain of S within the coating at its source. Therefore, through the preferential binding and stable fixation of S, Hf effectively inhibits internal sulfidation during hot salt corrosion, significantly enhancing the hot salt corrosion resistance of NiCoCrAlY coatings[30].Figure 10(a) shows that in the early stage of the reaction at hot corrosion temperatures, elements such as Hf and Al in the NiCoCrAlYHfSi coating diffuse toward the salt/TGO interface to form a protective oxide layer. Simultaneously, the β-phase in the coating acts as an aluminum-rich source, continuously supplying Al for diffusion to the TGO interface. As the corrosion reaction progresses to the stage shown in Figure 10(b), large amounts of oxygen and corrosive media diffuse into the TGO through channels. The β-phase is consumed to replenish the reacted Al 2 O 3 , while Hf preferentially captures S to form hafnium sulfides[26].As the corrosion reaction progresses to the severe stage depicted in Figure 10(c), sulfur (S) penetrates through the TGO layer and infiltrates into the interior of the coating. Hf demonstrates exceptional sulfur-trapping capability, efficiently capturing S to form stable hafnium sulfides. This process significantly inhibits the penetration of S and mitigates sulfidation corrosion. In summary, through the dual synergistic mechanisms of "physical barrier (macro/micro)" and "chemical capture (elemental level)", Hf collectively constructs a robust defense line against high-temperature molten salt corrosion. The NiCoCrAlYHfSi coating demonstrates significantly enhanced effectiveness in all these mechanisms compared to the NiCoCrAlY coating, which is fully consistent with the experimental results showing a lower corrosion rate for the NiCoCrAlYHfSi coating. Conclusion This study successfully prepared Hf- and Si-modified NiCoCrAlY coatings using high-velocity oxygen fuel (HVOF) spraying technology. Through systematic investigation of their hot corrosion behavior in a Na 2 SO 4 -NaCl mixed salt environment at 900°C, the following main conclusions were drawn: 1. Corrosion kinetics analysis indicates that the addition of Hf effectively reduces the corrosion rate of the coating. After 100 hours of hot salt corrosion, the mass gain per unit area of the NiCoCrAlY coating was 12.1 mg/cm², while that of the NiCoCrAlYHfSi coating was 7.8 mg/cm², representing a reduction of 35.5% in the latter compared to the former. 2. Before hot salt corrosion, the surface morphologies of the two coatings were similar. However, after 100 hours of hot salt corrosion, the surface oxide of the NiCoCrAlY coating agglomerated into nodular structures with obvious cracks and significant roughness. In contrast, the NiCoCrAlYHfSi coating remained dense and smooth without noticeable defects. Such a structure effectively enhances its barrier capability against O 2 and S 2- .From cross-sectional morphology analysis, after 100 hours of corrosion, sulfur (S) penetrated into the interior of the NiCoCrAlY coating through diffusion channels, reaching localized depths of up to 180 µm. Although sulfur was also detected inside the NiCoCrAlYHfSi coating, the corrosion depth was only about 60 µm. This demonstrates the superior corrosion resistance of the NiCoCrAlYHfSi coating. 3. XRD and EDS analyses confirmed that Hf promotes the formation of a protective Al₂O₃ scale while suppressing the generation of harmful spinel phases. More importantly, Hf incorporates into the TGO layer to form a dense HfO 2 phase, which enhances the adhesion between the TGO layer and the coating substrate through a "pinning effect," thereby preventing separation and cracking of the TGO layer observed in the NiCoCrAlY coating. Additionally, Hf effectively captures and immobilizes infiltrated sulfur, fundamentally inhibiting internal sulfidation and preserving the structural integrity of the coating. This study not only clarifies the effectiveness and mechanism of Hf in enhancing the hot corrosion resistance of NiCoCrAlY coatings, but also provides important theoretical foundations and technical references for the development of a new generation of high-performance, long-life high-temperature protective coatings. Declarations Conflict of interest The authors declare no conflicts of interest relevant to this work. Acknowlegements The authors gratefully acknowledge Beijing Mining New Materials Technology Co., Ltd. for providing the coating materials used in this thesis.The authors would like to thank Xiao Fei from the Hydrogen Combustion Protection Technology Research Institute of Liaoning Materials Laboratory and Zheng Zhaoran from BGRIMM Technology Group for their valuable guidance in this thesis. Author Contributions Huaixu Guo:Propose and design the research plan, organize and analyze the data, draft the manuscript, and conduct rigorous revisions on the core academic content of the manuscript.Peixuan Ouyang:To provide theoretical guidance for the mechanism of hot corrosion and the evolution of the TGO.Fei Xiao：Participated in the revision of the experimental details section in the manuscript.Zhaoran Zheng:Carried out work on the preparation of supersonic flame‑sprayed coatings and high‑temperature corrosion testing.Fukang Yue:Assisted in sample preparation and testing operations, and verified the experimental data and its reproducibility.Yingjie Han:Participated in data interpretation and discussion, and collected and analyzed the experimental data.Shuting Zhang:Secure research funding support, oversee project implementation and progress, and revise the manuscript from the perspective of academic accuracy.All authors have read and approved the initial draft for journal publication. Funding This work was supported by the National Natural Science Foundation of China (Grant No. 52305328) Data availability All data associated with this work will be made available upon a reasonable request to the corresponding author. References Negami M, Morihashi R, Yoshino T, Sahara R, Yamabe-Mitarai Y. Effect of reactive elements in MCrAlX bond coat for durability improvement of thermal barrier coatings. Corrosion Science . 2024;237:112329. Lv X, Li D, Duan W, Li C, Huang B, Qiang W. Comprehensive study of the effect of Hf and Ta co-doped MCrAlY bond coat on the high-temperature properties of thermal barrier coating. Surfaces and Interfaces . 2024;54:105277. Yang G, Han C, Chen Y, et al. Interfacial Stability between High-Entropy (La0.2Yb0.2Sm0.2Eu0.2Gd0.2)2Zr2O7 and Yttria-Stabilized Zirconia for Advanced Thermal Barrier Coating Applications. Coatings . 2024;14:269. Duan W, Li Y, Qiang W. 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Giese S, Neumeier S, Bergholz J, et al. Influence of Different Annealing Atmospheres on the Mechanical Properties of Freestanding MCrAlY Bond Coats Investigated by Micro-Tensile Creep Tests. Metals . 2019;9:692. Gupta AK, Immarigeon JP, Patnaik PC. A review of factors controlling the gas turbine hot section environment and their influence on hot salt corrosion test methods. High Temperature Technology . 1989;7:173–186. Mehta A, Vasudev H. A Review on the Thermal Barrier Coatings in Hot Corrosion Protection: Innovations and Future Directions for High-Temperature Alloys. J Fail. Anal. and Preven. 2025;25:1003–1039. Yang Z, Zhang J, Luo C, Yu C, Li M, Han W. Effect of pre-oxidation and sea salt on the hot corrosion behavior of MCrAlY coatings and Al Si coatings. Surface and Coatings Technology . 2024;477:130354. Li C, Yuan X, Li D, et al. Test atmospheres affecting voids distribution on MCrAlY-bond coats for TBCs at 1050 °C. Corrosion Science . 2022;195:109967. Razmgir V. Investigation of hot corrosion behavior of NiCoCrAlY coatings in molten Na2SO4 - V2O5 at 900°C. Chemical Review and Letters . 2025;8:856–866. Li Q, Liu F, Wang Z, Zhao Y, Tan L, Huang L. Corrosion behavior of three nickel-based single-crystal superalloys in mixed Na2SO4 and NaCl molten salts at 700 °C. J. Cent. South Univ. 2025;32:3220–3236. Salam S, Yuduo Z, Haifeng W, Zhigang Y, Chi Z. Influence of Re on High-Temperature Oxidation Behavior of MCrAlY Type Coating Alloys. Rare Metal Materials and Engineering . 2015; Ebach-Stahl A, Schulz U, Swadźba R. Lifetime improvement of EB-PVD 7YSZ TBCs by doping of Hf or Zr in NiCoCrAlY bond coats. Corrosion Science . 2021;181:109205. Zheng Z, Du K, Pi Z, et al. Hot Salt Corrosion Behavior of MCrAlY Coatings at 900 °C. Non-Ferrous Metals . 2025;15:1702–1707. Unocic KA, Bergholz J, Huang T, et al. High-temperature behavior of oxide dispersion strengthening CoNiCrAlY. Materials at High Temperatures . 2018;35:108–119. Okada M, Vassen R, Karger M, et al. Deposition and Oxidation of Oxide-Dispersed CoNiCrAlY Bondcoats. J Therm Spray Tech . 2014;23:147–153. Zhao SC, Jiang X, Zhang XD, et al. Tuning bias voltage to enhance oxidation resistance of AlCoCrNi high-entropy alloy coatings at 1000 ℃. Journal of Alloys and Compounds . 2024;1003:175525. Yang H, Zou J, Shi Q, et al. Growth stress and interdiffusion analysis of NiCoCrAlYTa coating during oxidation. Surface Engineering . 2021;37:808–817. Duan W, Huang B, Li Y, Huang X, Zhou M, Qiang W. Hf and Ta co-doping MCrAlY alloy to improve the lifetime of coatings. Surface and Coatings Technology . 2023;468:129781. Song Y, Zhou C, Xu H. Corrosion behavior of thermal barrier coatings exposed to NaCl plus water vapor at 1050 °C. Thin Solid Films . 2008;516:5686–5689. Zhang WL, Li W, Fu LB, et al. Hot Corrosion Behavior of Hf-Doped NiAl Coating in the Mixed Salt of Na2SO4 + K2SO4 at 900 °C. Acta Metall. Sin. (Engl. Lett.) . 2023;36:1409–1420. Rapp RA, Zhang Y-S. Hot corrosion of materials: Fundamental studies. JOM . 1994;46:47–55. Goebel JA, Pettit FS, Goward GW. Mechanisms for the hot corrosion of nickel-base alloys. Metall Trans . 1973;4:261–278. Bornstein NS, DeCrescente MA. The role of sodium in the accelerated oxidation phenomenon termed sulfidation. Metall Trans . 1971;2:2875–2883. Ren SX, Ren P, Wang QW, et al. Hot corrosion behavior of a Hf+Cr co-modified (Ni,Pt)Al coating in the mixed salt of Na2SO4–NaCl at 900 °C. J Mater Sci . 2025;60:22899–22916. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Under Revision Version 1 posted Editorial decision: Revision requested 10 Feb, 2026 Reviews received at journal 08 Feb, 2026 Reviews received at journal 08 Feb, 2026 Reviewers agreed at journal 07 Feb, 2026 Reviewers agreed at journal 07 Feb, 2026 Reviewers invited by journal 07 Feb, 2026 Editor assigned by journal 05 Feb, 2026 Submission checks completed at journal 05 Feb, 2026 First submitted to journal 04 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. 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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-8783198","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":588869565,"identity":"9beb23f6-7c15-49aa-960a-7061a051004e","order_by":0,"name":"Huaixu Guo","email":"","orcid":"","institution":"North China University of Technology","correspondingAuthor":false,"prefix":"","firstName":"Huaixu","middleName":"","lastName":"Guo","suffix":""},{"id":588869568,"identity":"5352bbb2-41a5-4b3a-9f69-0d521b9ffdb9","order_by":1,"name":"Peixuan Ouyang","email":"","orcid":"","institution":"North China University of 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University of Technology","correspondingAuthor":true,"prefix":"","firstName":"Shuting","middleName":"","lastName":"Zhang","suffix":""}],"badges":[],"createdAt":"2026-02-04 07:39:42","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8783198/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8783198/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":102616521,"identity":"d2fb467d-75c3-436c-b55b-88b4d226c2d6","added_by":"auto","created_at":"2026-02-13 15:50:03","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":73410,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eXRD patterns of NiCoCrAlY coating and NiCoCrAlYHfSi coating\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-8783198/v1/bac0498f3b2446dc34887623.png"},{"id":102616524,"identity":"ced9224f-d59a-411a-960c-6d64c6286797","added_by":"auto","created_at":"2026-02-13 15:50:03","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":83248,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eHot salt corrosion mass gain curves and fitted straight lines of NiCoCrAlY and NiCoCrAlYHfSi coatings\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e(a) Hot salt corrosion mass gain kinetic curves (b) Fitted straight lines\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-8783198/v1/7fc70fe90d4296236734abbf.png"},{"id":102747014,"identity":"2a6bcd42-f8bc-4d5c-9685-f262b4002c24","added_by":"auto","created_at":"2026-02-16 09:03:34","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":72039,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eXRD patterns of NiCoCrAlY coating and NiCoCrAlYHfSi coating after 100 hours of hot salt corrosion\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-8783198/v1/3fcc0f97559855c5ede02477.png"},{"id":102616522,"identity":"8271f443-fc45-4ea8-b6bb-74f064dafc8b","added_by":"auto","created_at":"2026-02-13 15:50:03","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":911049,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSEM images of coating surfaces before and after 100 h of hot salt corrosion\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e(a) NiCoCrAlY coating before corrosion(b) NiCoCrAlYHfSi coating before corrosion\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e(c) NiCoCrAlY coating after 100 h corrosion(d) NiCoCrAlYHfSi coating after 100 h corrosion\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-8783198/v1/f00f4d50d42022d8610e4ecf.png"},{"id":102616525,"identity":"cd55089b-1de6-4160-9fe3-fa61a82d5024","added_by":"auto","created_at":"2026-02-13 15:50:03","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":461054,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCross-sectional SEM images and EDS mapping of main elements for NiCoCrAlY coating and NiCoCrAlYHfSi coating after 20 hours of hot salt corrosion\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e(a) NiCoCrAlY coating (b) NiCoCrAlYHfSi coating\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-8783198/v1/676e8eec684dccc597df7a1d.png"},{"id":102746933,"identity":"ec7e3168-5972-4b67-95e8-9c46587e6dac","added_by":"auto","created_at":"2026-02-16 09:02:59","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":570895,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCross-sectional SEM images and EDS mapping of main elements for NiCoCrAlY coating and NiCoCrAlYHfSi coating after 60 hours of hot salt corrosion\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e(a) NiCoCrAlY coating (b) NiCoCrAlYHfSi coating\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-8783198/v1/267e6b8eb671fc8ecebe3a9d.png"},{"id":102748462,"identity":"fe2fddb2-98f4-4cd0-af9d-8966b1b02b23","added_by":"auto","created_at":"2026-02-16 09:11:00","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":602352,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCross-sectional SEM images and EDS mapping of main elements for NiCoCrAlY coating and NiCoCrAlYHfSi coating after 100 hours of hot salt corrosion\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e(a) NiCoCrAlY coating(b) NiCoCrAlYHfSi coating\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-8783198/v1/c9c4b5644f6b0f40c3108d0f.png"},{"id":102616528,"identity":"948473b0-4819-48b3-b01e-055b2af8f02e","added_by":"auto","created_at":"2026-02-13 15:50:03","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":1103174,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eCross-sectional EPMA and EDS mapping of main elements for NiCoCrAlY coating and NiCoCrAlYHfSi coating after 100 hours of hot salt corrosion\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e(a) NiCoCrAlY coating (b) NiCoCrAlYHfSi coating\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"8.png","url":"https://assets-eu.researchsquare.com/files/rs-8783198/v1/4a632d7d583ba40076a728aa.png"},{"id":102616529,"identity":"ebad4c38-8a83-40fe-8b0f-6b3cf2207983","added_by":"auto","created_at":"2026-02-13 15:50:03","extension":"png","order_by":9,"title":"Figure 9","display":"","copyAsset":false,"role":"figure","size":88743,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSchematic diagram of the \"pinning effect\" mechanism of reactive element Hf in the oxide layer\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-8783198/v1/48bbc0f38031fb96a2cc78aa.png"},{"id":102616530,"identity":"172c6f48-2d34-43b5-998d-38b9d87e0c60","added_by":"auto","created_at":"2026-02-13 15:50:03","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":94656,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eSchematic diagram illustrating the hot corrosion behavior of the NiCoCrAlYHfSi coating\u003c/strong\u003e\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-8783198/v1/a0a0f6c57b98e7743a1e12f3.png"},{"id":102751213,"identity":"e958a8cf-971b-4c0a-9f09-88caaa4e8d06","added_by":"auto","created_at":"2026-02-16 09:24:19","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":5002600,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8783198/v1/df645fff-711e-4828-a473-97086570717c.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Effect of Hf Element on the Hot Salt Corrosion Resistance of NiCoCrAlY Coatings","fulltext":[{"header":"Introduction","content":"\u003cp\u003eHot-end components of gas turbine engines, such as combustion chambers and turbine blades, are not only exposed to high-temperature oxidizing media like O\u003csub\u003e2\u003c/sub\u003e and H\u003csub\u003e2\u003c/sub\u003eO during service but also suffer corrosion from hot corrosive gases such as sulfur oxides (SO\u003csub\u003ex\u003c/sub\u003e) and hydrogen chloride (HCl). To meet the strength and corrosion resistance requirements of nickel-based alloys used in turbine blades and other hot-end components under high-temperature conditions, MCrAlY (M = Ni, Co, or a combination thereof) coatings are typically applied for protection[1\u0026ndash;5]。MCrAlY coatings are widely used as bond coats in thermal barrier coating systems or as standalone overlay coatings, offering excellent resistance to high-temperature oxidation and hot corrosion[6]. When exposed to oxidizing or corrosive environments, the abundant Al and Cr within MCrAlY coatings form a thermally grown oxide (TGO) layer on the surface, which acts as a diffusion barrier to slow further oxidation or corrosion of the coating. The failure of MCrAlY coatings is associated with TGO growth stresses and thermal mismatch stresses. During service, with prolonged exposure, the protective Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e scale is rapidly consumed, and the volume fraction of the \u0026beta;-NiAl phase in the coating gradually decreases. This leads to the formation of complex spinel-type oxides within the TGO layer, which reduce the density and adhesion of the oxide scale, thereby compromising the high-temperature performance of the coating[7\u0026ndash;12].Furthermore, hot corrosion induced by high-temperature corrosive media like sulfur oxides (SO\u003csub\u003ex\u003c/sub\u003e) and hydrogen chloride (HCl) damages the protective oxide film on the coating surface, thereby accelerating the overall failure process[2,13\u0026ndash;15].Therefore, improving the comprehensive high-temperature oxidation and hot corrosion resistance of MCrAlY coatings remains a current research priority. Studies have shown that the addition of trace elements to MCrAlY coatings can effectively enhance their high-temperature performance.，Li[16]\u0026nbsp;investigated the effect of the trace element Ta on the oxidation resistance of NiCrAlY alloy at 1050\u0026deg;C by adding Ta to the alloy. The study shows that the addition of the refractory element Ta not only promotes the precipitation of Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e on the alloy surface but also increases the thickness of the oxide film as the Ta content increases. The film structure becomes denser, effectively blocking the penetration of oxygen into the substrate, thereby enhancing the oxidation resistance of the alloy.Salam[17]\u0026nbsp;investigated the effect of the trace element Re on the oxidation resistance of CoNiCrAlY alloy at 1100\u0026deg;C by adding Re to the alloy. The study demonstrates that the addition of Re promotes the transformation of \u0026theta;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e to \u0026alpha;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, thereby slowing down the oxidation rate. Moreover, Re can inhibit the interdiffusion of refractory elements, particularly Mo, between the coating and the substrate, consequently enhancing the oxidation resistance of the coating.Ebach-Stahl[18]\u0026nbsp;investigated the effect of doping 0.15\u0026ndash;0.60 at.% Hf on the oxidation resistance of NiCoCrAlY bond coats at 1100\u0026deg;C and found that the reactive element Hf enhances the adhesion of the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e oxide scale. This improvement in oxidation resistance of the bond coat extends the service life of the thermal barrier coating.Furthermore, the addition of Hf element can significantly enhance the adhesion between the coating and the substrate, improving the interfacial bonding performance.Zheng[19]prepared a Re-modified NiCoCrAlY coating using arc ion plating technology and investigated its hot salt corrosion behavior in an environment of 75 wt.% Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e + 25 wt.% NaCl at 900\u0026deg;C. A Re-rich \u0026alpha;-Cr phase precipitated at the interface between the coating and the oxide scale. Due to the similar thermal expansion coefficients of \u0026alpha;-Cr and \u0026alpha;-Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e4\u003c/sub\u003e, the \u0026alpha;-Cr phase helps reduce the thermal stress in the oxide scale and enhances its spallation resistance. Therefore, the addition of Re can improve the hot salt corrosion resistance of the coating.In summary, the aforementioned literature primarily focuses on the performance of coatings doped with trace elements in resisting high-temperature oxidizing media. However, research on the influence of Hf addition on the hot salt corrosion resistance of coatings, particularly the interaction mechanism between Hf and sulfur elements in corrosive environments, remains scarce and inadequately reported.\u003c/p\u003e\n\u003cp\u003eIn summary, this study employed the high-velocity oxygen fuel (HVOF) spraying method to prepare NiCoCrAlY and NiCoCrAlYHfSi coatings, and comparatively investigated their hot salt corrosion resistance. The phase composition, surface morphology, cross-sectional morphology, and elemental distribution of the oxide scales on the coatings were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe microanalysis (EPMA). Furthermore, the mechanism by which Hf inhibits the penetration of sulfur/chlorine elements and maintains the structural integrity of NiCoCrAlY-based coatings was explored. This research is expected to provide data references for the development and application of high-performance MCrAlY series coatings.\u003c/p\u003e"},{"header":"Experimental Materials and Methods","content":"\u003cp\u003e\u003cstrong\u003e1.1 Coating Preparation\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eIn this study, NiCoCrAlY and NiCoCrAlYHfSi alloy powders were used as feedstocks for coating deposition. The nominal chemical composition of NiCoCrAlY was 47.5Ni-23Co-17Cr-12Al-0.5Y (wt.%), and that of NiCoCrAlYHfSi was 47.6Ni-22Co-17Cr-12Al-0.5Y-0.5Hf-0.4Si (wt.%). Free‑standing thick coatings (unsupported standalone coatings) were prepared using a JP8000 high‑velocity oxy‑fuel (HVOF) spraying system equipped with a propane/oxygen combustion system and a high‑precision dual‑hopper powder feeder. Low‑carbon steel plates, cleaned with acetone and polished with fine‑grade sandpaper, were used as the deposition substrates.To ensure consistent melting of the powders in the flame and stable coating deposition, the NiCoCrAlY and NiCoCrAlYHfSi powders were separately preheated in a drying oven at 100 \u0026deg;C for 20 h to remove surface‑adsorbed moisture. After coating deposition, the well‑adhered thick coatings were completely separated from the steel substrates by mechanical means, utilizing the difference in thermal expansion coefficients between the coatings and the substrate, thereby obtaining free‑standing coating plates.Subsequently, the free‑standing coatings were cut into specimens with dimensions of 10 \u0026times; 20 \u0026times; 2 mm using a precision diamond cutter under continuous water cooling. After machining, the specimens were ultrasonically cleaned in deionized water (300 W, 40 kHz, 15 min) and their initial mass (\u003cem\u003em₀\u003c/em\u003e) was measured using an electronic balance with an accuracy of 0.01 mg.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e1.2 Hot Salt Corrosion Experiment at 900\u0026deg;C\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe salt corrosion experiment was conducted in a muffle furnace set at 900\u0026deg;C. The mixed salt solution was prepared according to the GB/T10125-2021 standard, consisting of 75 wt.% Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e + 25 wt.% NaCl. The test specimens included two types of coatings after wire cutting: NiCoCrAlY coating and NiCoCrAlYHfSi coating. Three parallel samples of each coating type were prepared to ensure the reliability of the experimental results.\u003c/p\u003e\n\u003cp\u003eThe mixed salt solution was evenly applied to the surface of the coated specimens, with the salt loading controlled at approximately 3 mg/cm\u0026sup2;. The specimens were then placed in a preheated muffle furnace at 900\u0026deg;C for an isothermal corrosion duration of 100 hours. Each 20-hour interval constituted one cycle. At the end of each cycle, the specimens were removed, cooled to room temperature, rinsed with deionized water to remove residual salts, and dried. The mass was then measured using a precision electronic balance (accuracy: 0.01 mg), after which the salt was reapplied for the next cycle. The total experimental duration was 100 hours. The mass change for each cycle (recorded as \u003cem\u003emᵢ\u003c/em\u003e) was tracked, and the corrosion kinetic constant (K\u003csub\u003ep\u003c/sub\u003e) was determined based on the relationship between mass gain and corrosion time. Additionally, a corrosion kinetic curve (mass change per unit area versus time curve) was plotted:\u003c/p\u003e\n\u003cp\u003e\u003cimg src=\"data:image/png;base64,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\"\u003e\u003c/p\u003e\n\u003cp\u003eWhere \u003cem\u003em\u003csub\u003ei\u003c/sub\u003e\u003c/em\u003e\u003csub\u003e:\u003cem\u003e\u0026nbsp;\u003c/em\u003e\u003c/sub\u003ecoating mass gain per cycle,\u003cem\u003em\u003csub\u003eo:\u003c/sub\u003e\u003c/em\u003einitial mass;\u003cem\u003eA\u003c/em\u003e: Coating surface area;\u003cem\u003eK\u003csub\u003eP:\u0026nbsp;\u003c/sub\u003e\u003c/em\u003eparabolic rate constant;\u003cem\u003et\u003c/em\u003e: exposure time;\u003cem\u003en\u003c/em\u003e: time exponent[20]。\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e1.3 Microstructure Characterization and Analysis\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFirst, scanning electron microscopy (SEM) was employed to characterize the surface and cross-sectional morphology of both coatings. After various intervals of the hot salt corrosion tests, cross-sectional samples of the coating specimens were cold-mounted in conductive resin, sequentially ground using #180 to #2000 SiC abrasive paper, and polished with a 2.5 \u0026micro;m polishing paste. High-resolution scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) were used to observe the microstructure of the coating surfaces and cross-sections, including surface corrosion pits, crack morphology and distribution, as well as phenomena such as coating spallation and delamination in the cross-section. Energy dispersive spectroscopy (EDS) was combined for qualitative and quantitative analysis of micro-region compositions to determine the elemental composition of the corrosion products. X-ray diffraction (XRD) was utilized for phase analysis of the coatings, accurately identifying the types and content changes of phases before and after corrosion. This analysis enabled the investigation of chemical reactions and phase transformations occurring during the corrosion process, providing deeper insights into the corrosion mechanisms of the coatings[21].\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cp\u003e\u003cstrong\u003e2.1 Phase Analysis of Coating Surfaces\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFigure 1 shows the XRD patterns of the as-sprayed NiCoCrAlY and NiCoCrAlYHfSi coatings. In the NiCoCrAlY coating, the characteristic diffraction peaks of the \u0026gamma;-phase exhibit high intensity, while those of the \u0026beta;-phase are relatively weak, indicating that in the absence of Hf addition, the \u0026gamma;-phase in the coating is well-crystallized and accounts for a larger proportion of the phase composition. In contrast, with the addition of Hf and Si elements, the diffraction peak intensity of the \u0026gamma;-phase in the coating decreases compared to that of the NiCoCrAlY coating. This may be attributed to the higher activity of Hf, which more readily participates in the crystal growth process during coating formation, thereby interfering with the crystallization of the \u0026gamma;-phase and potentially inhibiting its crystallization or reducing its content[4].\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.2 Hot Salt Corrosion Test of Coatings at 900\u0026deg;C\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.2.1 Analysis of Corrosion Kinetic Curves\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFigure 2(a) shows the curves of mass gain per unit area versus corrosion time for NiCoCrAlY and NiCoCrAlYHfSi coatings during the corrosion process. As the corrosion time increases, the mass gain per unit area of both coatings exhibits a parabolic growth trend. After 100 hours of hot salt corrosion, the mass gain per unit area of the NiCoCrAlY coating is 12.1 mg/cm\u0026sup2;, while that of the NiCoCrAlYHfSi coating is 7.8 mg/cm\u0026sup2;, representing a reduction of 35.5% in the latter compared to the former. At the same corrosion time, the mass gain per unit area of the NiCoCrAlY coating is higher than that of the NiCoCrAlYHfSi coating, indicating that the addition of Hf elements can effectively mitigate the mass increase caused by corrosion to some extent and plays a positive role in inhibiting the corrosion process.Figure 2(b) shows the linear fitting curves of mass gain per unit area versus the square root of corrosion time for the two coatings, where Kp represents the corrosion rate and R\u0026sup2; is the coefficient of determination of the linear fitting. The slope Kp for the NiCoCrAlY and NiCoCrAlYHfSi coatings is 1.1901 and 0.8037, respectively, with coefficients of determination R\u003csub\u003e2\u003c/sub\u003e of 0.9997 and 0.9993, both close to 1. This indicates that the mass gain per unit area of both coatings has a strong linear relationship with the square root of corrosion time. Based on the slope values, the corrosion-induced mass gain rate of the NiCoCrAlY coating is faster than that of the NiCoCrAlYHfSi coating, further confirming that the addition of Hf and Si elements can effectively reduce the corrosion rate of the coating. delay the corrosion process in hot salt environments, and enhance its corrosion resistance in such conditions.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.2.2 Analysis of Coating Surface Morphology and Phases\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFigure 3 shows the surface XRD patterns of the NiCoCrAlY and NiCoCrAlYHfSi coatings after 100 hours of hot salt corrosion. The results indicate that both coatings contain Ni\u003csub\u003e3\u003c/sub\u003eAl, Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, spinel phases, and yttrium aluminum garnet (YAG). Additionally, the HfO₂ phase is detected in the NiCoCrAlYHfSi coating. The presence of the HfO\u003csub\u003e2\u003c/sub\u003e phase promotes the formation of Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e while inhibiting the retention of Ni\u003csub\u003e3\u003c/sub\u003eAl, enabling more effective participation of Al in the formation of the oxide scale. Therefore, as observed in the XRD patterns, the diffraction peak intensity of the Ni₃Al phase in the NiCoCrAlYHfSi coating is significantly lower than that in the NiCoCrAlY coating, while the diffraction peak intensity of the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e phase is higher. Furthermore, the diffraction peak intensity of the spinel phase in the NiCoCrAlYHfSi coating is notably reduced, indicating that the addition of Hf effectively suppresses the formation of the spinel phase. In conclusion, Hf significantly enhances the corrosion resistance of the coating in hot salt environments by promoting the formation of the protective Al₂O₃ scale and inhibiting the generation of detrimental spinel phases.\u003c/p\u003e\n\u003cp\u003eFigure 4 shows the surface SEM images of the NiCoCrAlY and NiCoCrAlYHfSi coatings before and after 100 hours of hot salt corrosion. Prior to corrosion, both coatings exhibited similar surface morphologies, characterized by a molten-like structure with a small number of unmelted particles. However, after 100 hours of hot salt corrosion, the oxide scales on both coatings aggregated into nodule-like features. The NiCoCrAlY coating surface displayed multiple visible cracks and significant roughness, as shown in Figure 4(c). In contrast, the NiCoCrAlYHfSi coating surface maintained relatively good densification with no obvious defects and appeared smoother overall compared to the NiCoCrAlY coating, as seen in Figure 4(d). This dense and uniform surface structure can effectively enhance the barrier capability against the inward penetration of O\u003csub\u003e2\u003c/sub\u003e, S\u003csup\u003e2-\u003c/sup\u003e, and other corrosive species. Therefore, the introduction of Hf can significantly inhibit crack initiation during hot salt corrosion and improve the coating\u0026apos;s resistance to hot salt corrosion.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.2.3 Microstructural Cross-Sectional Analysis and Elemental Analysis of Coatings\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFigure 5 shows the cross-sectional SEM images and EDS analysis of the NiCoCrAlY and NiCoCrAlYHfSi coatings after 20 hours of hot salt corrosion. From Figures 5 (a) and (b), it can be observed that the NiCoCrAlY coating exhibits pores and internal oxidation, which provide pathways for the ingress of corrosive media and accelerate coating degradation. In contrast, compared to the NiCoCrAlY coating, the NiCoCrAlYHfSi coating (Figures (c) and (d)) shows a more intact cross-sectional structure with fewer pores and milder internal oxidation. In terms of the morphology and continuity of the thermally grown oxide (TGO) layer, the TGO layer of the NiCoCrAlY coating is less dense and exhibits poorer continuity, while the TGO layer of the NiCoCrAlYHfSi coating is denser and more continuous. After 20 hours of hot salt corrosion, EDS analysis revealed no significant presence of sulfur (S) elements in either coating, indicating that both coatings demonstrate effective resistance to sulfidation corrosion in the short term.\u003c/p\u003e\n\u003cp\u003eFigure 6 shows the cross-sectional SEM images and EDS analysis of the NiCoCrAlY and NiCoCrAlYHfSi coatings after 60 hours of hot salt corrosion. From Figures (a) and (b) and the elemental distribution, it can be observed that the TGO layer on the surface of the NiCoCrAlY coating exhibits significant separation and contains numerous voids, with internal oxidation further aggravated. In contrast, from Figures (c) and (d) and the elemental distribution, the cross-section of the NiCoCrAlYHfSi coating appears relatively flat and dense, with no obvious separation in the TGO layer. Although internal oxidation is also observed at this stage, its extent is milder compared to that in the NiCoCrAlY coating.Regarding the thickness and continuity of the TGO layer, both coatings show thicker TGO layers after 60 hours of hot salt corrosion compared to those after 20 hours. The TGO layer of the NiCoCrAlY coating exhibits cracks and numerous wrinkles, indicating poor continuity, while the TGO layer of the NiCoCrAlYHfSi coating remains relatively smooth and continuous. Based on the EDS mapping, sulfur (S) penetration into the interior of the NiCoCrAlY coating is observed after 60 hours, reaching a depth of approximately 30 \u0026mu;m, indicating that the hot salt has caused further corrosion of the coating. In contrast, no sulfur penetration is detected inside the NiCoCrAlYHfSi coating, with S elements observed only within the TGO layer. This suggests that the dense TGO layer and coating structure effectively block the penetration of sulfur, preventing the spread of sulfidation corrosion into the coating interior.\u003c/p\u003e\n\u003cp\u003eFigure 7 presents the cross-sectional SEM images and EDS analysis of the NiCoCrAlY and NiCoCrAlYHfSi coatings after 100 hours of hot salt corrosion. The corrosion condition of the NiCoCrAlY coating (Figures 7(a) and (b)) further deteriorated. The TGO layer on the coating surface not only exhibited more pronounced separation but also showed a significant increase in the number and size of voids. Internal oxidation intensified, and the formation of diffusion channels within the coating provided preferential pathways for the deep penetration of sulfur (S) elements. This accelerated the diffusion process, leading to premature coating failure. Sulfur penetrated into the coating interior along these diffusion channels, reaching localized depths of up to 180 \u0026micro;m.For the NiCoCrAlYHfSi coating (Figures 7(c) and (d)), although internal oxidation was also observed, no significant voids were detected. While sulfur was found within the coating, the corrosion depth was limited to approximately 60 \u0026micro;m. In terms of TGO thickness and continuity, the TGO layer of the NiCoCrAlY coating exhibited poor continuity and uneven thickness, whereas the TGO layer of the NiCoCrAlYHfSi coating demonstrated better continuity and uniform thickness.\u003c/p\u003e\n\u003cp\u003eFigure 8 shows the cross-sectional EPMA and EDS images of the NiCoCrAlY and NiCoCrAlYHfSi coatings after 100 hours of hot salt corrosion. A comparison of the elemental distribution maps of Cr, S, and Hf in the two coatings reveals that after 100 hours of hot salt corrosion, the distribution of Cr in the NiCoCrAlY coating exhibits significant non-uniformity. Cr is a key element in the formation of the protective Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e scale in the coating and also works synergistically with Al to promote the growth of a dense Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e scale. If Cr distribution is uneven, localized regions of the coating may experience Cr enrichment while others suffer from Cr deficiency. Areas deficient in Cr struggle to form a continuous Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e protective scale, which also weakens the stability of the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u0026nbsp;\u003c/sub\u003escale, making these regions vulnerable weak points for the ingress of corrosive media. Conversely, regions with Cr enrichment tend to form brittle spinel phases, increasing the risk of cracking and spalling of the oxide scale and accelerating the corrosion process of the coating[4,13,22].In contrast, within the NiCoCrAlYHfSi coating, Hf and S elements exhibit distinct co-localization and are present both in the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e scale and inside the coating. Hf is dispersed throughout the coating matrix and interfacial regions, showing clear co-distribution with S, as well as with O, primarily within the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e scale. On one hand, Hf promotes the selective oxidation of Cr, facilitating the formation of a continuous and dense protective oxide scale[23].On the other hand, Hf combines with O to form a dense HfO₂ phase within the Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e scale, which effectively blocks grain boundary pathways. Concurrently, it seals the diffusion channels for S, structurally hindering the penetration path of S into the coating interior[24].From the aggregation regions of S and Hf elements, it can be clearly observed that Hf exhibits excellent \u0026quot;sulfur-trapping\u0026quot; capability, efficiently capturing S and significantly inhibiting its penetration and sulfidation corrosion. This results in a notably lower distribution of S in the NiCoCrAlYHfSi coating compared to the NiCoCrAlY coating. Simultaneously, Cr is more uniformly distributed in the NiCoCrAlYHfSi coating, indicating that the presence of Hf effectively suppresses the sulfidation reaction of Cr. This contributes to enhancing the coating\u0026apos;s corrosion resistance at lower temperatures, thereby extending its service life.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e2.3 Discussion on the Mechanism of Hf Element Effects\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eBased on the experimental results presented above, the mechanism by which Hf enhances the hot salt corrosion resistance of NiCoCrAlY coatings at 900\u0026deg;C can be systematically explained. The strengthening effect primarily stems from the following aspects:\u003c/p\u003e\n\u003cp\u003e1.Effect of Hf Pinning on the Oxide Scale\u003c/p\u003e\n\u003cp\u003eHf exhibits strong oxygen affinity and can synergize with Al to promote the formation of a continuous and dense Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e scale. This is corroborated by the stronger Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e diffraction peaks observed in the NiCoCrAlYHfSi coating compared to those in the NiCoCrAlY coating in Figure 3. Additionally, Hf suppresses the excessive formation of non-protective, porous spinel phases, as evidenced by the reduced intensity of spinel phase peaks in the NiCoCrAlYHfSi coating shown in Figure 3. Meanwhile, Hf incorporates into the TGO layer, forming a dense HfO\u003csub\u003e2\u003c/sub\u003e phase. The results from Figure 8 in this experiment also indicate that the TGO layer of the NiCoCrAlYHfSi coating is denser, and Figure 8 further reveals a stronger bond between the TGO layer and the NiCoCrAlYHfSi coating.\u0026nbsp;Therefore, HfO\u003csub\u003e2\u003c/sub\u003e improves the TGO/coating matrix bonding via the \u0026quot;pinning effect\u0026quot; (Figure. 9), inhibiting TGO delamination and cracking in the NiCoCrAlY coating.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;Furthermore, the high melting point of HfO\u003csub\u003e2\u003c/sub\u003e contributes to refining the TGO grain structure, reducing internal stress accumulation, and maintaining a smooth, continuous, and dense TGO structure[2,25].As shown in Figure 7, the TGO layer of the NiCoCrAlYHfSi coating is more continuous and dense compared to that of the NiCoCrAlY coating, which significantly enhances the physical barrier effect of the TGO layer against corrosive media.\u003c/p\u003e\n\u003cp\u003e2.The Impact of Hf\u0026rsquo;s \u0026quot;Preferential Sulfur Trapping\u0026quot; Effect on Coating Corrosion Resistance\u003c/p\u003e\n\u003cp\u003eDuring the mid-to-late stages of hot corrosion, after molten salts penetrate the initial oxide scale, the infiltration of sulfur (S) and internal sulfidation become key factors leading to accelerated coating failure. The EDS analysis results from this study (Figure 8) clearly demonstrate that, after hot salt corrosion of the NiCoCrAlYHfSi coating, Hf and the infiltrated S exhibit significant co-distribution. This co-localization is primarily concentrated in the coating\u0026apos;s surface layer and at the oxide/coating interface. These microstructural features directly confirm the notable interaction between Hf and S during the hot corrosion process[26,27].\u003c/p\u003e\n\u003cp\u003eGoebel[28]和Bornstein[29]\u0026apos;s models of hot corrosion through salt dissolution of oxide scales propose that Na₂SO₄ exhibits the following thermodynamic equilibrium at high temperatures:\u003c/p\u003e\n\u003cp\u003eNa\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e\u0026rarr;Na\u003csub\u003e2\u003c/sub\u003eO+SO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\n\u003cp\u003eIn hot salt corrosion, sulfur primarily originates from SO₃. From a thermodynamic perspective, the binding affinity between elements can be evaluated based on the standard Gibbs free energy of formation (\u0026Delta;G\u003csup\u003e\u0026theta;\u003c/sup\u003e) of their sulfides. A larger absolute value of \u0026Delta;G\u0026theta; indicates higher compound stability and a greater priority for element bonding[24].In this study, thermodynamic software (HSC Chemistry) was employed to calculate the standard Gibbs free energy of formation (\u0026Delta;G\u003csup\u003e\u0026theta;\u003c/sup\u003e) for sulfides generated by the reactions of Hf, Cr, Al, and Y with SO₃ under hot salt corrosion conditions at 900\u0026deg;C (1173 K), as summarized below:：\u003c/p\u003e\n\u003cp\u003e4Hf(s)+2SO\u003csub\u003e3\u003c/sub\u003e(g) = HfS₂(s)+3HfO\u003csub\u003e2\u003c/sub\u003e(s) \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026Delta;G\u003csup\u003e\u0026theta;\u0026nbsp;\u003c/sup\u003e\u0026asymp; -520kJ/mol\u003c/p\u003e\n\u003cp\u003e8Y(s)+3SO\u003csub\u003e3\u003c/sub\u003e(g) \u0026rarr; Y\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e3\u003c/sub\u003e(s)+3Y\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e(s) \u0026nbsp; \u0026nbsp; \u0026nbsp;\u0026Delta;G\u003csup\u003e\u0026theta;\u0026nbsp;\u003c/sup\u003e\u0026asymp; -480kJ/mol\u003c/p\u003e\n\u003cp\u003e8Al(s)+3SO\u003csub\u003e3\u003c/sub\u003e(g) \u0026rarr;Al\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e3\u003c/sub\u003e(s)+3Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e(s) \u0026nbsp; \u0026nbsp; \u0026Delta;G\u003csup\u003e\u0026theta;\u0026nbsp;\u003c/sup\u003e\u0026asymp; -420kJ/mol\u003c/p\u003e\n\u003cp\u003e8Cr(s)+3SO\u003csub\u003e3\u003c/sub\u003e(g) \u0026rarr; Cr\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e3\u003c/sub\u003e(s)+3Cr\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e(s) \u0026nbsp; \u0026nbsp; \u0026Delta;G\u003csup\u003e\u0026theta;\u003c/sup\u003e\u003csup\u003e\u0026nbsp;\u003c/sup\u003e\u0026asymp; -350kJ/mol\u003c/p\u003e\n\u003cp\u003eAt 900\u0026deg;C, the order of Gibbs free energy for the reaction of each element with S from high to low is: Cr \u0026gt; Al \u0026gt; Y \u0026gt; Hf, meaning Hf has the strongest binding ability with S, far exceeding the core protective elements Al and Cr in the coating. This thermodynamic advantage enables Hf to preferentially combine with S infiltrating into the coating, forming stable HfS\u003csub\u003e2\u003c/sub\u003e phases. This process effectively acts as a \u0026quot;capture and fixation\u0026quot; mechanism for S by Hf, significantly reducing the probability of contact and reaction between S and critical elements such as Al and Cr. Consequently, the formation of harmful sulfides like Al₂S₃ and Cr\u003csub\u003e2\u003c/sub\u003eS\u003csub\u003e3\u003c/sub\u003e is suppressed, preventing the degradation of the protective oxide layer due to the depletion of active elements.On the other hand, the extremely low Gibbs free energy of formation for hafnium sulfide means that once formed, it is difficult to decompose and does not release S again, thus avoiding secondary corrosion. This blocks the diffusion and erosion chain of S within the coating at its source. Therefore, through the preferential binding and stable fixation of S, Hf effectively inhibits internal sulfidation during hot salt corrosion, significantly enhancing the hot salt corrosion resistance of NiCoCrAlY coatings[30].Figure 10(a) shows that in the early stage of the reaction at hot corrosion temperatures, elements such as Hf and Al in the NiCoCrAlYHfSi coating diffuse toward the salt/TGO interface to form a protective oxide layer. Simultaneously, the \u0026beta;-phase in the coating acts as an aluminum-rich source, continuously supplying Al for diffusion to the TGO interface. As the corrosion reaction progresses to the stage shown in Figure 10(b), large amounts of oxygen and corrosive media diffuse into the TGO through channels. The \u0026beta;-phase is consumed to replenish the reacted Al\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, while Hf preferentially captures S to form hafnium sulfides[26].As the corrosion reaction progresses to the severe stage depicted in Figure 10(c), sulfur (S) penetrates through the TGO layer and infiltrates into the interior of the coating. Hf demonstrates exceptional sulfur-trapping capability, efficiently capturing S to form stable hafnium sulfides. This process significantly inhibits the penetration of S and mitigates sulfidation corrosion.\u003c/p\u003e\n\u003cp\u003eIn summary, through the dual synergistic mechanisms of \u0026quot;physical barrier (macro/micro)\u0026quot; and \u0026quot;chemical capture (elemental level)\u0026quot;, Hf collectively constructs a robust defense line against high-temperature molten salt corrosion. The NiCoCrAlYHfSi coating demonstrates significantly enhanced effectiveness in all these mechanisms compared to the NiCoCrAlY coating, which is fully consistent with the experimental results showing a lower corrosion rate for the NiCoCrAlYHfSi coating.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eThis study successfully prepared Hf- and Si-modified NiCoCrAlY coatings using high-velocity oxygen fuel (HVOF) spraying technology. Through systematic investigation of their hot corrosion behavior in a Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e-NaCl mixed salt environment at 900\u0026deg;C, the following main conclusions were drawn:\u003c/p\u003e\n\u003cp\u003e1. Corrosion kinetics analysis indicates that the addition of Hf effectively reduces the corrosion rate of the coating. After 100 hours of hot salt corrosion, the mass gain per unit area of the NiCoCrAlY coating was 12.1 mg/cm\u0026sup2;, while that of the NiCoCrAlYHfSi coating was 7.8 mg/cm\u0026sup2;, representing a reduction of 35.5% in the latter compared to the former.\u003c/p\u003e\n\u003cp\u003e2. Before hot salt corrosion, the surface morphologies of the two coatings were similar. However, after 100 hours of hot salt corrosion, the surface oxide of the NiCoCrAlY coating agglomerated into nodular structures with obvious cracks and significant roughness. In contrast, the NiCoCrAlYHfSi coating remained dense and smooth without noticeable defects. Such a structure effectively enhances its barrier capability against O\u003csub\u003e2\u003c/sub\u003e and S\u003csup\u003e2-\u003c/sup\u003e.From cross-sectional morphology analysis, after 100 hours of corrosion, sulfur (S) penetrated into the interior of the NiCoCrAlY coating through diffusion channels, reaching localized depths of up to 180 \u0026micro;m. Although sulfur was also detected inside the NiCoCrAlYHfSi coating, the corrosion depth was only about 60 \u0026micro;m. This demonstrates the superior corrosion resistance of the NiCoCrAlYHfSi coating.\u003c/p\u003e\n\u003cp\u003e3. XRD and EDS analyses confirmed that Hf promotes the formation of a protective Al₂O₃ scale while suppressing the generation of harmful spinel phases. More importantly, Hf incorporates into the TGO layer to form a dense HfO\u003csub\u003e2\u003c/sub\u003e phase, which enhances the adhesion between the TGO layer and the coating substrate through a \u0026quot;pinning effect,\u0026quot; thereby preventing separation and cracking of the TGO layer observed in the NiCoCrAlY coating. Additionally, Hf effectively captures and immobilizes infiltrated sulfur, fundamentally inhibiting internal sulfidation and preserving the structural integrity of the coating.\u003c/p\u003e\n\u003cp\u003eThis study not only clarifies the effectiveness and mechanism of Hf in enhancing the hot corrosion resistance of NiCoCrAlY coatings, but also provides important theoretical foundations and technical references for the development of a new generation of high-performance, long-life high-temperature protective coatings.\u003cstrong\u003e\u003cbr\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003eConflict of interest The authors declare no conflicts of interest relevant to this work.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowlegements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors gratefully acknowledge Beijing Mining New Materials Technology Co., Ltd. for providing the coating materials used in this thesis.The authors would like to thank Xiao Fei from the Hydrogen Combustion Protection Technology Research Institute of Liaoning Materials Laboratory and Zheng Zhaoran from \u0026nbsp;BGRIMM Technology Group for their valuable guidance in this thesis.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eHuaixu Guo:Propose and design the research plan, organize and analyze the data, draft the manuscript, and conduct rigorous revisions on the core academic content of the manuscript.Peixuan Ouyang:To provide theoretical guidance for the mechanism of hot corrosion and the evolution of the TGO.Fei Xiao：Participated in the revision of the experimental details section in the manuscript.Zhaoran Zheng:Carried out work on the preparation of supersonic flame‑sprayed coatings and high‑temperature corrosion testing.Fukang Yue:Assisted in sample preparation and testing operations, and verified the experimental data and its reproducibility.Yingjie Han:Participated in data interpretation and discussion, and collected and analyzed the experimental data.Shuting Zhang:Secure research funding support, oversee project implementation and progress, and revise the manuscript from the perspective of academic accuracy.All authors have read and approved the initial draft for journal publication.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by the National Natural Science Foundation of China (Grant No. 52305328)\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll data associated with this work will be made available upon a reasonable request to the corresponding author.\u003c/p\u003e\n"},{"header":"References","content":"\u003col\u003e\n \u003cli\u003eNegami M, Morihashi R, Yoshino T, Sahara R, Yamabe-Mitarai Y. 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Hot corrosion of materials: Fundamental studies. \u003cem\u003eJOM\u003c/em\u003e. 1994;46:47\u0026ndash;55.\u003c/li\u003e\n \u003cli\u003eGoebel JA, Pettit FS, Goward GW. Mechanisms for the hot corrosion of nickel-base alloys. \u003cem\u003eMetall Trans\u003c/em\u003e. 1973;4:261\u0026ndash;278.\u003c/li\u003e\n \u003cli\u003eBornstein NS, DeCrescente MA. The role of sodium in the accelerated oxidation phenomenon termed sulfidation. \u003cem\u003eMetall Trans\u003c/em\u003e. 1971;2:2875\u0026ndash;2883.\u003c/li\u003e\n \u003cli\u003eRen SX, Ren P, Wang QW, et al. Hot corrosion behavior of a Hf+Cr co-modified (Ni,Pt)Al coating in the mixed salt of Na2SO4\u0026ndash;NaCl at 900 \u0026deg;C. \u003cem\u003eJ Mater Sci\u003c/em\u003e. 2025;60:22899\u0026ndash;22916.\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":"high-temperature-corrosion-of-materials","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"Learn more about [High Temperature Corrosion of Materials](https://www.springer.com/journal/11085)","snPcode":"11085","submissionUrl":"https://submission.nature.com/new-submission/11085/3","title":"High Temperature Corrosion of Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Hot salt corrosion, NiCoCrAlY coating, High-velocity oxygen fuel (HVOF) spraying, Hf","lastPublishedDoi":"10.21203/rs.3.rs-8783198/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8783198/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eTo improve the hot corrosion resistance of NiCoCrAlY coatings under high‑temperature salt‑laden conditions, two free‑standing coatings—NiCoCrAlY and NiCoCrAlYHfSi—were prepared by high‑velocity oxy‑fuel spraying. Their corrosion behavior was systematically investigated at 900 °C in a mixed salt environment of 75 wt.% Na\u003csub\u003e2\u003c/sub\u003eSO\u003csub\u003e4\u003c/sub\u003e + 25 wt.% NaCl. The results show that the addition of Hf and Si significantly enhances coating densification and structural integrity, effectively suppressing TGO thickening, crack propagation, and internal oxidation. Corrosion kinetics analysis indicates a lower corrosion rate for the NiCoCrAlYHfSi coating. Microstructural characterization confirms that Hf forms a dense HfO\u003csub\u003e2\u003c/sub\u003e layer, which strengthens TGO adhesion through “pinning effects” and blocks corrosive ion penetration. Additionally, Hf effectively traps and immobilizes sulfur, inhibiting internal sulfidation. This study provides experimental and theoretical support for the application of Hf‑doped NiCoCrAlY coatings in severe high‑temperature corrosive environments.\u003c/p\u003e","manuscriptTitle":"Effect of Hf Element on the Hot Salt Corrosion Resistance of NiCoCrAlY Coatings","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-02-13 15:49:58","doi":"10.21203/rs.3.rs-8783198/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-02-10T09:56:26+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-02-09T04:30:45+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-02-08T11:46:24+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"161809981376708876169830030832981708898","date":"2026-02-08T04:33:08+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"69003376327156992889214368519990597160","date":"2026-02-08T04:30:35+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-02-08T04:27:44+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-02-05T15:47:43+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-02-05T15:47:24+00:00","index":"","fulltext":""},{"type":"submitted","content":"High Temperature Corrosion of Materials","date":"2026-02-04T07:06:05+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"high-temperature-corrosion-of-materials","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"Learn more about [High Temperature Corrosion of Materials](https://www.springer.com/journal/11085)","snPcode":"11085","submissionUrl":"https://submission.nature.com/new-submission/11085/3","title":"High Temperature Corrosion of Materials","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"d7817276-4dff-429f-9b29-83b9f98609be","owner":[],"postedDate":"February 13th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"in-revision","subjectAreas":[],"tags":[],"updatedAt":"2026-02-13T15:49:58+00:00","versionOfRecord":[],"versionCreatedAt":"2026-02-13 15:49:58","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8783198","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8783198","identity":"rs-8783198","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}