Assessment of duplex coating combining plasma electrolytic oxidation and polymer layer on AZ31 magnesium alloy

Abstract

Mechanical and corrosion tests were performed on a polymer-coated AZ31 magnesium alloy pre-treated by plasma electrolytic oxidation (PEO). Results were compared with a fluorotitanate–zirconate conversion coating pre-treatment. Mechanical performance was assessed by standardized adhesion (ISO 2409:2007), impact (ISO 6272‐1:2004) and impact+ adhesion (ISO 6272/ASTMD2794) pass/fail tests. Corrosion behavior was monitored using electrochemical impedance spectroscopy (EIS), ac/dc/ac measurements and continuous exposure to salt fog per ASTM B117 and cyclic exposure per VDA 621‐415 (VDA). The PEO+polymer coating revealed lower impact resistance but better corrosion resistance than the Ti/Zr+polymer coating. The ac/dc/ac procedure demonstrated to be faster than EIS measurements for evaluation of the corrosion performance of studied coatings.