Inter-diffusion of Mg and Zn in vacuum-evaporated Mg films on Zn coated steel substrates

The Zn-Mg alloy coatings were grown on the Zn-coated steel substrates by vacuum evaporation of Mg and rapid annealing. The impacts of the alloy film growth conditions on the corrosion resistance were evaluated. The Zn-Mg coatings were characterized with X-ray diffraction, secondary ion mass spectroscopy and scanning electron microscopy. The results show that the inter-diffusion of Zn and Mg strongly affects the microstructures and corrosion resistance of the alloy coating. In the annealing, Mg rapidly diffused into Zn matrix, accompanied with the formation of the Mg-rich layer inside the Zn coating, and a slow diffusion of Zn into Mg layer. Moreover, the diffusion of Fe into Zn coating resulted in Zn-Fe alloy formation. The Mg-rich layer blocked the diffusion of Mg into Zn and the surface segregation of Fe. The annealing at 370°C for 60~120 s significantly increased the corrosion resistance of Zn-Mg alloy coating.