Insight into effect of rust layer evolution on corrosion resistance of 55AlZnMg coating

Abstract

The relationship between rust layer evolution and the corresponding corrosion resistance of 55AlZnMg coatings in NaCl solution were investigated by Scanning electron microscope, micro-Raman spectroscopy and electrochemical impedance spectroscopy. Alloy phases composing the coating were not corroded synchronically, and thus the coating would not transfer into a rust layer integrally along the depth direction. The rapid thickening of the rust layer was triggered with an outward migration of such cations as Zn2+ and Al3+, which were generated from the preferential corrosion of interdendritic MgZn2/Zn-rich phases. During this process, benefiting from the compact rust layer consisting of protective corrosion products (e.g., simonkolleite, hydrozincite and layered double hydroxide), the corrosion resistance of the coating was sharply enhanced until the immersion for 28 days. Later, because of transformations of simonkolleite and hydrozincite, the stratification of the rust layer and micro-cracks caused by the formation of Zn(OH)2 in large quantities, which led to a formation of fast diffusion tunnel and thus impaired the corrosion resistance of the coating. It could be concluded that the corrosion resistance of 55AlZnMg coatings in NaCl solution were not only determined by the properties of its corrosion products, but also, on the overall properties of rust layer.