Mn oxide conversion coating on AZ31B Mg alloy with hexafluorozirconate as an inhibitor in the permanganate conversion bath
Journal
Applied Surface Science
Journal Volume
660
Date Issued
2024-07-01
Author(s)
Hsiao, Ting Hsuan
Abstract
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.
Subjects
Corrosion inhibitor | Electrochemical test | Galvanic coupling | Magnesium | Passive films
SDGs
Type
journal article