Corrosion resistance and electrical contact resistance of a thin permanganate conversion coating on dual-phase LZ91 Mg–Li alloy
Sung-Mao Hung, Han Lin, Huanwen Chen, Siao-Ying Chen, Chao-Sung Lin
Abstract
A strongly acidic permanganate conversion coating solution containing 0.1 M KMnO4 at pH 1.5 adjusted with H2SO4 was employed for LZ91 magnesium-lithium alloy, realizing the formation of a thin protective coating with a low electrical contact resistance. The corrosion resistance of LZ91 alloy was markedly improved by the permanganate conversion coating, which consisted of MgO, MnO2, and Mg(OH)2. Coating formation proceeded on both the α-Mg phase and β-Li phase in this strongly acidic permanganate solution, achieving a full coverage of permanganate conversion coating within a short treatment time. When the immersion was prolonged to 30 s, cracks were present on the permanganate conversion coating, which deteriorated the corrosion resistance substantially. Moreover, the increase in the thickness of the coating elevated the electrical contact resistance. However, the relatively thin areas on the thin permanganate conversion coating became active sites where corrosion commenced. As a result, a thin uniform permanganate conversion coating is crucial for both enhanced corrosion resistance and reduced electrical contact resistance.