The influence of metallurgical state of substrate on the efficiency of plasma electrolytic oxidation (PEO) process on magnesium alloy

Abstract

A Gd,Y rare-earth containing magnesium alloy (content in weight %, 10 Gd, 3 Y, 0.4 Zr and Mg as balance) is PEO processed for three different metallurgical states: (i) as-extruded, (ii) T4 solution (2 h at 500 °C) and (iii) T6 peak-aged precipitation (2 h at 500 °C followed by 14 h at 225 °C). The thickest coating combined with the lowest porosity is achieved for the softest solid solution T4 treated substrate resulting in the development of a harder oxide coating. The presence of Mg(GdY)-like precipitates for the precipitating T6 condition is correlated with the appearance of cathodic micro-discharges that are known to damage the growing PEO oxide layer on magnesium. Particularly, results evidence that local segregated bands of precipitates are facing large discharge channels through the PEO coating suggesting the local ignition of strong cathodic micro-discharges. The proposed explanation consists in considering the relationship between the presence of precipitates and the isoelectric point IEP of the oxide surface with respect to the electrolyte pH. This study proves that the successful development of protective PEO coatings requires the right management of the process parameters (electrolyte chemistry, electrical conditions) together with an adequate attention to the substrate pre-treatment (mainly the metallurgical heat treatment).