Effect of cerium ion on the microstructure and properties of permanganate conversion coating on LZ91 magnesium alloy

LZ91 magnesium alloys have excellent mechanical properties, but resistance to corrosion is reduced because of the presence of lithium and the dual-phase induced galvanic corrosion. This study develops a new permanganate/cerium conversion coating on LZ91, which results in the formation of a compact coating with improved corrosion resistance. This study also determines the effect of the coated LZ91 alloy in the presence of cerium ions in the permanganate bath on the microstructure and corrosion resistance in comparison with the chromate conversion coating. The morphology, crystal structure, and composition of the coating are characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and x-ray photoelectron spectroscopy (XPS). The corrosion resistance of the coating is characterized by polarization, electrochemical impedance spectroscopy (EIS) analysis and salt spray WA (SST). The permanganate/cerium coating that is formed after 30 s of immersion has few cracks and a compact overlay of approximately 660 nm in thickness. The permanganate/cerium conversion coating exhibits better corrosion resistance than permanganate coating and gives LZ91 Mg alloy much better corrosion protection than a traditional chromate-based coating.