Formation of Phosphate/Permanganate Conversion Coating on AZ31 Magnesium Alloy

The properties of conversion coating on magnesium can be closely related to its microstructure and composition. This study investigated the evolution and microstructure of phosphate/permanganate conversion coating on AZ31 magnesium alloy. Results show that increasing the solution temperature reduced the growth rate of the coating. The temperature effect lies in the different structure associated with the coating layer contacting the substrate: a porous layer mainly composed of magnesium, aluminum hydroxides/phosphate formed in the solution at 40°C, while a compact magnesium oxide layer formed in the solution at 80°C. The formation mechanism of phosphate/permanganate coating was discussed in detail, with emphasis on the effect of the solution temperature.