Surface protective coating systems constructed via thermal spraying technology can overcome the limitations of traditional coating processes in material selection and thickness control, effectively enhancing the mechanical properties and corrosion resistance of magnesium alloy surfaces. Among these coatings, 316 stainless steel coatings provide effective protection for magnesium alloys due to their superior mechanical performance and corrosion resistance. However, despite the significant improvements in wear resistance, corrosion resistance, and impact resistance imparted by metallic coatings, the extremely low standard electrode potential of magnesium alloys (-2.36 V) may induce galvanic corrosion between the coating and substrate. Therefore, investigating the correlation between the microstructure, corrosion resistance, and galvanic corrosion behavior of 316 stainless steel coatings on magnesium alloys is critical for expanding their application fields and service conditions.
The 316 stainless steel coating was fabricated on AZ31B magnesium alloy substrates through a two-pass HVOF process, achieving a coating thickness of approximately 200 μm. Prior to spraying, the substrate surfaces were meticulously prepared by cleaning with acetone and grit blasting to enhance the adhesion of the sprayed particles. The HVOF process parameters, such as propane flow rate of 68 m3/min, oxygen flow rate of 240 m3/min, compressed air flow rate of 375 m3/min, powder feed rate of 38 g/min, spray distance of 240 mm, and scanning velocity of 300 mm/s, were optimized to ensure coating quality. The microstructure and phase composition of the coating were characterized through scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The corrosion behavior was evaluated through electrochemical testing, including potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and zero-resistance ammetry (ZRA) measurements.
The 316 stainless steel coating significantly reduced the corrosion current density of the AZ31B magnesium alloy by two orders of magnitude, demonstrating its initial effectiveness as a physical barrier against uniform corrosion. However, interfacial oxidation during the HVOF deposition process and the precipitation of trace sigma (σ) phases led to localized chromium (Cr) depletion in adjacent regions, forming Cr-depleted zones that acted as preferential sites for pitting initiation. The synergistic interaction between surface pores and interlamellar interfaces within the coating promoted the formation of localized corrosion-aggregated zones, leading to the development of penetrative pores that allowed corrosive media to establish direct electrical contact with the substrate. Zero-resistance ammetry measurements revealed a galvanic current density of 1.142× 10-4 A/cm2, accompanied by a negative shift in corrosion potential (Ecorr) to -1.218 V, which approached the intrinsic corrosion potential of the AZ31B substrate (-1.521 V). This indicated that the coating failure led to accelerated corrosion of the magnesium alloy and promoted coating delamination. The accumulation of galvanic corrosion products at the coating-substrate interface generated mechanical stresses, leading to coating cracking and delamination, which further accelerated the failure of the protective system.
The 316 stainless steel coating initially enhances the corrosion resistance of AZ31B magnesium alloy by acting as a physical barrier. However, the coupling of surface pores and interlamellar interfaces facilitates the formation of penetrative pores, which establishes galvanic coupling pathways between the coating and substrate. This accelerates the corrosion rate of the magnesium alloy and promotes coating delamination. The degradation is driven by defect-mediated localized corrosion and sustained galvanic interactions. Furthermore, the accumulation of galvanic corrosion products at the coating-substrate interface generates mechanical stresses, leading to coating cracking and delamination, which accelerates the failure of the protective system. Optimizing HVOF parameters to suppress interfacial oxidation, minimize σ-phase formation, and reduce porosity is critical for improving the durability of magnesium alloy coatings in chloride-containing environments.
Key words
316 stainless steel coating /
magnesium alloy /
HVOF /
electrochemistry /
corrosion mechanism
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Funding
National Natural Science Foundation of China (W2412069); Programme of Introducing Talents of Discipline to Universities (D21032)
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