Effect of the process parameters on the microstructure and corrosion resistance of ZnMg alloy coatings prepared via a combined PVD and reaction-diffusion process

A combined physical vapor deposition (PVD) and reaction-diffusion process was adopted to prepare ZnMg alloy coatings. First, a magnesium film with a thickness of a few hundred nanometers was deposited onto the galvanized iron (GI) sheet by DC magnetron sputtering. The effect of the PVD working pressure on the thickness and morphology of Mg was studied. Then, a 20 s reaction-diffusion step was performed at 350, 380 or 415 °C in atmosphere. The resulting alloy phase composition was analyzed by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy and X-ray diffraction (XRD). The morphology of the corroded samples was analyzed by SEM and XRD as well. The electrochemical test revealed the prepared alloy coatings exhibit a much smaller self-corrosion current density and a bigger impedance resistance than the GI coating. The corrosion mechanism and the reason why the ZnMg alloy coatings showed a superior corrosion resistance were analyzed. The optimal process parameters and the relationship between the remarkably high corrosion resistance and the alloy phase composition are discussed.