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 (alpha-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 (7.1x 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.