Calcium Doped Flash-PEO Coatings for Corrosion Protection of Mg Alloy

Abstract

This study demonstrates a significant improvement of the corrosion resistance of an AZ31B magnesium alloy achieved by the application of 1 um-thin coatings generated by an environmentally friendly flash plasma electrolytic oxidation (FPEO) process in Ca-containing electrolytes. Two compounds with different solubility, calcium oxide (CaO) or calcium glycerophosphate (CaGlyP), were used as sources of Ca in the electrolyte. Very short durations (20–45 s) of the FPEO process were employed with the aim of limiting the energy consumption. The corrosion performance of the developed coatings was compared with that of a commercial conversion coating (CC) of similar thickness. The viability of the coatings in a full system protection approach, consisting of FPEO combined with an inhibitor-free epoxy primer, was verified in neutral salt spray and paint adhesion tests. The superior corrosion performance of the FPEO_CaGlyP coating, both as a stand-alone coating and as a full system, was attributed to the formation of a greater complexity of Ca2+ bonds with SiO2 and PO4 3- species within the MgO ceramic network during the in situ incorporation of Ca into the coating from a double chelated electrolyte and the resultant difficulties with the hydrolysis of such a network. The deterioration of the FPEO_CaGlyP coating during immersion was found over ten times slower compared with Ca-free flash-PEO coating.