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

• Published in: Applied surface science Vol. 509; p. 144767
• Main Authors: Jian, Shun-Yi, Chang, Ko-Lun
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.04.2020
• Subjects: Chromate conversion coating; EIS; LZ91 magnesium alloy; Microstructure; Salt spray test; Chromate conversion coating; Salt spray test; Microstructure; EIS; LZ91 magnesium alloy
• ISSN: 0169-4332; 1873-5584
• DOI: 10.1016/j.apsusc.2019.144767

[Display omitted] •The addition of cerium in permanganate bath to produce a relatively-compact coating.•Cracking is reduced in permanganate/cerium coating and it has a thickness of 660 nm.•Permanganate/cerium coating on LZ91 Mg alloy exhibits a two-layered structure.•Permanganate/cerium coating is more anticorrosive than its chromate counterpart.•The corrosion area of permanganate/cerium coating less than 3% after 72 h SST. 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 test (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.