Electrochemical response of MgO/Co3O4 oxide layers produced by plasma electrolytic oxidation and post treatment using cobalt nitrate

Abstract

This work looked into the influence of the sealing treatment on the structural feature and electrochemical response of AZ31 Mg alloy coated via plasma electrolytic oxidation (PEO). Here, the inorganic layers produced by PEO in an alkaline-phosphate electrolyte were subsequently immersed for different periods in cold (60 °C) and hot (100 °C) aqueous solutions containing either 1 or 3 gr of cobalt nitrate hexahydrate in the presence of hydrogen peroxide as an initiator. The results showed that the sealing treatments in the hot solutions could trigger the hydration reactions of PEO coating which would largely assist the surface incorporation of Co3O4 into the coating. In contrast, the sealing in cold solutions led to less compact coatings, which was attributed to the fact the hydration reactions would be restricted at 60 °C. A nearly fully sealed coating with a porosity of ∼ 0.5% was successfully formed on the sample immersed in the hot solution containing 1 gr of cobalt nitrate hexahydrate. Thus, the electrochemical stability of this fully sealed coating was superior to the other samples as it had the lowest corrosion current density (4.71 × 10−10 A·cm−2) and the highest outer layer resistance (3.81 × 107 Ω·cm2). The composite coatings developed in this study are ideal for applications requiring high electrochemical stability.

Keywords

• AZ31 Mg alloy
• Plasma electrolytic oxidation
• Co3O4
• Hydration
• Corrosion