Permanganate conversion coating on AZ31 magnesium alloys with enhanced corrosion resistance

• Nearly crack-free conversion coating on AZ31 is formed in acidic permanganate solution.
• Relatively-thin permanganate coating improves the corrosion resistance of AZ31.
• Permanganate coating has corrosion resistance inferior to chromate counterpart.
• Permanganate coating has sufficient electrical conductivity and corrosion resistance.

Necessity for developing alternatives to chromate conversion coatings has increased in the last decade. A chromate-free conversion coating treatment, mainly in permanganate solution, was developed in this study for AZ31 magnesium alloys. The coating formed after 90 s of immersion consisted of a continuous nearly crack-free layer with an average thickness of 230 nm. The results of the electrochemical measurements and the salt spray tests demonstrated that the corrosion resistance of the AZ31 alloy was markedly improved by the permanganate conversion treatment. Moreover, this thin permanganate conversion coating had sufficient electrical conductivity, ensuring the magnetic shielding properties of the AZ31.