Galvanic corrosion behavior of micro-arc oxidation coated Al-Zn-Mg-Cu alloy coupled with 316 L stainless steel in the salt-spray atmosphere

Abstract

Micro-arc oxidation (MAO) coating with thickness, potential, current density, and impedance of 30 μm, −0.62 V, 7.2×10−8 A/cm2 and 88,730 Ω·cm2 was prepared on the surface of Al–Zn–Mg–Cu alloy to inhibit galvanic corrosion caused by its coupling with 316 L stainless steel. It was found that the MAO coating increased the potential of the galvanic corrosion zone (GCZ) from −0.67 V to −0.43 V, and decreased its current density from 3×10−3 A/cm2 to 4.4×10−7 A/cm2, resulted in a much lower corrosion rate and longer failure time. Salt-spray erosion can cause the MAO coating to detach gradually and formed the stepped corrosion pits at the GCZ. The galvanic corrosion process of the MAO coated 7085 alloy could be divided into three stages: (1) the electrolyte passes through the interconnected defects in the coating, leading to pitting corrosion beneath the coating; (2) the corrosion products block the interconnected defect channels in the coating, reducing the rate of galvanic corrosion; (3) the Al alloy substrate becomes fully exposed and contacts with 316 L stainless steel through the electrolyte, accelerating the galvanic corrosion and corrosion detachment. These findings confirm the effectiveness of MAO coatings in inhibiting galvanic corrosion and clarify the complex corrosion mechanisms involved, providing valuable insights for enhancing the durability of Al–Zn–Mg–Cu alloy in coupled environments.