Corrosion protection performance of single and dual Plasma Electrolytic Oxidation (PEO) coating for aerospace applications

• Influence of dual PEO coating on AZ31 Mg alloy substrate was evaluated.
• XRD confirmed formation of thin MgO inner, Mg3(PO4)2 and Mg2SiO4 outer layer.
• SEM results showed uniform coating with no cracks and relatively less micro pores.
• Micro hardness of dual PEO coatings is higher than single PEO coatings.
• Dual coating provides superior corrosion performance due to its synergistic effect.

Plasma Electrolytic Oxidation (PEO) coatings are known to be one of the most appropriate method for corrosion protection of magnesium (Mg) alloy. The improvement of PEO coatings and the optimization of their surface aspects are of major importance. In this current work, the influence of dual PEO coating on strip-cast AZ31 Mg alloy substrate has been evaluated with the aim of improving the surface and corrosion protection aspects. For this purpose, AZ31 Mg substrates are subjected to single and dual PEO processing in silicate and phosphate electrolyte under similar condition. Scanning electron microscopy (SEM) analysis confirmed that the number of pores in PEO coating processed in silicate electrolyte is higher than others. X-ray diffraction analysis of PEO coatings showed that the surface coating is mainly comprised of Mg2SiO4, Mg3(PO4)2 and MgO with different quantity based on PEO processing. Compared with the AZ31 Mg, the corrosion potential (Ecorr) of both type PEO coatings was positively shifted about 250–400 mV and the corrosion current density (icorr) was lowered by 3-4 orders of magnitude as result of adequate corrosion protection to the Mg alloy in 3.5% NaCl solution. All of the observation obviously showed that the dual PEO coating provides better corrosion protection performance than their respective single due to its synergistic beneficial effect.