Structure and Properties of Coatings Formed on Cast Al–Ca Alloys by Plasma Electrolytic Oxidation

The structure and properties of coatings formed by plasma electrolytic oxidation (PEO) on as-cast Al–Ca alloy (7 wt.% Ca) and pure Al (99.99 wt.% Al) are studied. It is shown that substrates have a significant impact on the quality, structure and properties of a coating formed. According to SEM analysis results, presence of Ca within a substrate leads to an increase in coating thickness, an increase in inner layer porosity, as well as formation of a wide calcium-containing region within a coating outer layer. Calcium contributes to a significant change in coating phase composition (α-Al₂O₃/g-Al₂O₃), preventing a-Al₂O₃ formation. A change in phase composition leads to a reduction in coating average microhardness from 912 HV on pure aluminum to 714 HV for a coating upon Al–Ca alloy. The electrochemical behavior of as-cast Al7Ca alloy and PEO-treated alloys in 3.5% NaCl solution is studied. As a result of electrochemical studies, it is found that a PEO coating significantly increases corrosion resistance, demonstrating a decrease in corrosion current density (0.410–3 mA/cm²) compared with an uncoated specimen (7.1× 10^–3 mA/cm²). A coating formed upon pure aluminum due to greater uniformity shows a lower corrosion current density value (0.0710–3 mA/cm²) and improved corrosion properties.