Enhancing corrosion resistance in Mg–Li alloy through plasma electrolytic oxidation coatings — Exploring the impact of ionic liquids (BmimBF₄)

The wide application of Mg–Li alloys is limited by the poor corrosion resistance, which needs to be improved by surface treatment. Plasma electrolytic oxidation (PEO) method is one of the most promising methods for the formation of oxide coatings on Mg–Li alloys. However, the conventional PEO technique with high energy consumption causes poor coating compactness and hampers efforts to enhance corrosion resistance. In this study, various concentrations of ionic liquids (1-butyl-3-methylimidazolium tetrafluoroborate, BmimBF₄) are introduced into the concentrated alkaline electrolyte to facilitate the formation of a PEO coating on the surface of LA91 magnesium‑lithium (Mg–Li) alloy. The influence of BmimBF₄ on PEO coatings has been investigated. The results indicate that a concentrated electrolyte significantly lowers the working voltage of PEO, thereby enhancing energy efficiency. Uneven adsorption on the substrate surface, resulting from the low concentration of BmimBF₄ (≤20 mL/L) in the electrolyte, induces partial discharge, leading to high surface roughness and the formation of large pores. However, when an excessive amount of BmimBF₄ (>20 mL/L) is present, it uniformly adsorbed onto the substrate surface during the initial stage of PEO process, forming a corrosion inhibitor layer. This leads to a consistent discharge on the surface of substrate throughout the entire PEO process, resulting in a reduction in both surface roughness and micropore size. Among these coatings, the one prepared in an electrolyte containing 20 mL/L BmimBF₄ exhibits the highest contact angle (66.8° ± 3.8°) and the best long-term corrosion resistance (hydrogen evolution rates <0.02 mL·cm⁻²·h⁻¹). These properties can be attributed to the physical and chemical adsorption of BmimBF₄, which contains hydrophobic long alkyl chains, as well as the maximum thickness with extended discharging channels. This study offers a guiding strategy for further reducing energy consumption and enhancing the corrosion resistance of PEO coatings.