Corrosion Protection of ZK60 Wrought Magnesium Alloys by Micro-Arc Oxidation

Abstract

The influences of the forging process and micro-arc oxidation (MAO) coating on the corrosion behavior of ZK60 wrought magnesium alloys exposed to salt spray and constant stress corrosion conditions were investigated. The microstructure of the ZK60 Mg alloy specimens forged under different temperatures (i.e., 250, 300, and 450 °C) was characterized using metallography, EBSD, and SEM. It was demonstrated that the ZK60 alloy forged at 300 °C (i.e., ZK60EF-300) had finer grain and uniformly distributed β-phase and, thus, better corrosion resistance than the ZK60 forged at 450 °C. At the lower forging temperature (250 °C) twins formed in the ZK60 alloy, which accelerated the corrosion of the ZK60E-250 specimen. The MAO coating provided robust corrosion protection for all the ZK60 wrought Mg alloy substrates. The salt spray corrosion test results showed that when the MAO coating broke down at certain weak sites, the corrosion performance of the coated Mg alloy was predominantly determined by the alloy substrate. The stress corrosion behaviors of the uncoated and MAO-coated ZK60 alloy specimens were also investigated under a constant load of 80 MPa in 3.5 wt.% NaCl solution. The MAO coating was found to improve the stress-corrosion resistance of the ZK60 alloy pronouncedly.