Effect of permanganate concentration on the formation and properties of phosphate/permanganate conversion coating on AZ31 magnesium alloy

This study investigated the role of permanganate in phosphate solutions in the formation and corrosion resistance of phosphate/permanganate coatings on AZ31 magnesium alloys. Experimental results showed that permanganate was reduced to manganese (IV) oxide together with the dissolution of magnesium during the conversion coating treatment. Adding more permanganate to the phosphate solution resulted in a thinner coating with a compact magnesium oxide layer contacting the AZ31 plate. Moreover, the thinner coating had fewer cracks and displayed higher polarization resistance and corrosion resistance than the thicker counterpart formed in the solution with less permanganate.

► Phosphate/permanganate coating on an AZ31 displays a three-layered structure.
► Permanganate ions reduce the size of cracks on phosphated AZ31 Mg alloy.
► Cracks of phosphate/permanganate coating deteriorate its corrosion protection.
► Permanganate ions reduce the phosphate/permanganate coating growth rate.