Effect of Mg Content on the Corrosion Resistance of Zn-Mg Alloy Coatings Deposited by Vacuum Palting

Microstructure and corrosion resistance of vacuum-evaporated Zn-Mg alloy coating were studied by salt spray test and electrochemical methods. The results show that when the mass fraction of Mg was 4.5%, the Zn-Mg alloy coating had relatively good corrosion resistance. After 720 h of salt spray test, no red rust appeared on the surface of the coating, and the corrosion resistance of the coating was at least 9 times that of the pure Zn coating. XRD analysis showed that MgZn₂ phase and Mg₂Zn₁₁ phase which could improve the corrosion resistance appeared in the coating. After salt spray corrosion, the corrosion products on the coating surface were mainly zinc oxide (ZnO), basic zinc chloride Zn₅(OH)₈Cl₂·H₂O, basic zinc carbonate Zn₄CO₃(OH)₆·H₂O and Zn₅(CO₃)₂(OH)₆ etc. Zn₅(OH)₈Cl₂·H₂O was dense and had good insulating properties, which could inhibit the cathodic reaction and the formation of loose ZnO, so that the Zn-Mg alloy coating had excellent corrosion resistance.