Effect of Mg content on microstructure and corrosion behavior of hot dipped Zn–Al–Mg coatings

• Two-layer structured Zn–Al–Mg coatings were prepared by hot dipping method.
• Mg exists in the outer layer while Al exists in the inner layer of Zn–Al–Mg coating.
• Zn–Al–Mg coating has better protective ability than Zn and Zn–Al coatings.
• The Mg-modified simonkolleite is the reason of the enhanced corrosion resistance.

In this article, Zn–Al–Mg coatings were prepared by hot dipping method. The surface morphology, cross–section microstructure, microhardness, composition, corrosion behaviour of ZAM coatings were investigated by using X–ray diffraction (XRD), Optical microscope, Environmental scanning electron microscopy equipped with EDS (FESEM–EDS), Microhardness tester and Electrochemical analysis respectively. Corrosion test was also performed in a standard salt fog spray chamber. Microstructure studies indicates that Zn grain size was refined and eutectic areas at Zn grain boundary areas increased with increasing Mg content. ZA5M1.5 and ZA5M2 coatings have two distinct layers. Mg tends to exist in the outer layer while Al is in the inner layer. The inner layer is composed of Al5Fe2Zn0.4 intermetallic, which may to contribute to the microhardness. The outer layer is Zn grains surrounded by Zn–Mg etutectics, which may improve the corrosion resistance. The microhardness is more than 700 HV50g for Al-rich layer and around 151 HV25g for Mg-rich layer. The improved corrosion resistance of Zn–5%Al-1.5%Mg coating comes from the corrosion product of flocculent type simonkolleite, which prolongs the micro-path and impedes the movement of O2 and H2O, ultimately retards the overall corrosion process.