Effects of magnesium on the microstructure and corrosion resistance of Zn–55Al–1.6Si coating

Abstract

In this paper, the effects of Mg addition (0, 0.5, 1.5 and 2.5wt.%) on the microstructure and corrosion resistance of Galvalume coating (Zn–55Al–1.6Si) have been studied. The coating surface microstructure and the corrosion product morphology were investigated, and phase compositions of the different coatings and the corrosion products were analyzed. The corrosion resistance of Galvalume coatings alloyed with different contents of Mg was assessed by means of neutral salt spray test (NSS) and electrochemical impedance spectroscopy (EIS). The results showed that a new intermetallic phases (MgZn2) appears in the zinc-rich interdendritic area of the Mg-containing Galvalume coating, improving the corrosion resistance of the zinc-rich interdendritic area. During the neutral salt spray test, serious localized corrosion occurs in the zinc-rich interdendritic area of the Galvalume coating, while the corrosion type of Zn–55Al–1.6Si–1.5Mg coating can be considered as uniform corrosion. The addition of Mg improves the compactness and stability of the corrosion product, and that of Galvalume coating consists of an incomplete outer layer and a defective inner layer. By contrast, the corrosion product formed on Zn–55Al–1.6Si–1.5Mg coating displays densely packed microstructure, showing the best barrier protection characteristics. The corrosion rate data and the EIS-Nyquist plots indicate that Mg can improve the corrosion resistance of Galvalume coating, and the Zn–55Al–1.6Si–1.5Mg coating shows the best corrosion resistance.