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 alpha-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 H-2 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. (C) 2018 The Electrochemical Society.