(607e) Plasma Interface Engineered Coating Systems for Magnesium Alloys

As metallic materials, magnesium (Mg) and its alloys possess many advantageous properties, including high thermal conductivity, good dimensional stability, excellent electromagnetic shielding characteristics, and especially their lower weight and excellent mechanical properties. However, Mg is highly chemically reactive when exposed to air or water and forms oxide/hydroxide layer, which then make it very susceptible to corrosion. There are a large number of coating technologies available for protecting Mg and its alloys. Among these coating technologies, chromate conversion coatings have been used extensively as pretreatment methods for various Mg alloys for achieving good corrosion resistance and paint adhesion performance. Due to their toxic and carcinogenic nature, however, chromate conversion coatings are stringently limited and will soon be banned by environmental regulations for continuous use in coating process. Therefore, research and development of new coating methods become extremely important in successful fabrication of effective and environmentally benign coating systems for corrosion protection of various Mg alloys.

Low-temperature plasma polymerization is a promising alternative technique in creation of environmentally friendly coating systems for metallic corrosion protection. In this study, a DC low-temperature plasma technique, including plasma treatment and plasma polymerization, was used to create interface engineered coating systems with a structure of Mg/plasma interlayer/cathodic electrocoating (E-coat) for a magnesium (Mg) alloy AZ31B. The plasma interlayer deposited from trimethylsilane (TMS) precursor had a nano-scale thickness of ~ 65 nm and its surface properties was well-controlled by subsequent plasma treatments in order to achieve different level of interfacial adhesion between the E-coat and the Mg substrates. The surface wettability of the plasma interlayer was monitored by surface contact angle measurement. The interface adhesion of the coating system was evaluated using N-methylpyrrolidinone (NMP) paint removal test and ASTM tape test conducted under dry and wet conditions. Electrochemical impedance spectroscopy (EIS) was employed to investigate the effects of plasma interlayer properties including surface wettability and adhesion enhancement on corrosion protection properties of the coating systems to AZ31B Mg alloy. It was found that a more wettable interface enhanced the electrolyte penetration through the coating and thus reduced the corrosion resistance of the coating system. The detailed results obtained through this study will be presented and discussed.