Mn oxide conversion coating on AZ31B Mg alloy with hexafluorozirconate as an inhibitor in the permanganate conversion bath

Surface modification is essential for Mg alloys, including conversion coating, plasma electrolytic oxidation, organic coating, and plasma spraying. Permanganate conversion coating is a potential alternative to chromate coating for Mg alloys. A continuous MnO2 coating on commercial Mg alloys, which generally contain micrometric secondary phase particles, is still challenging. A thin, continuous MnO2 conversion coating layer on AZ31B Mg alloy was successfully prepared by adding hexafluorozirconate anions (ZrF62−) to an acidic permanganate conversion bath. AZ31B Mg alloy comprised α-Mg matrix and micrometric Al8Mn5 particles. In addition to MnO2 and MgO/Mg(OH)2, MgF2 was present in the coating formed in the presence of ZrF62−, but the amount of ZrO2 was scarce. The average thickness of the coating was reduced from 665 to 66 nm when 0.02 M ZrF62− was added. As a result, the contact resistance was reduced from 367 to 8.59 mΩ. Meanwhile, the corrosion resistance was improved. The uniformity of MnO2 coating was discussed, focusing on the Al8Mn5/Mg galvanic coupling effect.