Formation Mechanism and Corrosion Performance of Phosphate Conversion Coatings on AZ91 and Mg-Gd-Y-Zr Alloy

Abstract

The microstructure, phase composition and corrosion resistance of phosphate conversion coating (PCC) on AZ91D and Mg-Gd-Y-Zr alloy were investigated to understand the coating formation mechanism. It was found that the Volta potential difference between the second phase and α-Mg provides the main driving force for deposition of MgHPO4 inner layer. Mg dissolution rate is accelerated in Mg-RE (where RE represents rare earth elements) alloy at the beginning of the treatment, leading to formation of a dense and protective conversion coating on Mg-RE compared to AZ91D Mg. Electrochemical noise signals and H2 evolution rate (HER) obtained during coating formation process were used to investigate and predict the microstructure and corrosion performance of the conversion coatings. A dense and protective conversion layer is produced if the noise resistance is high and HER is relatively low in the final stage of coating growth.