The Characterization of Coatings Formed on As-Cast Al, Al-Si, and Al-Ca Aluminum Substrates by Plasma Electrolytic Oxidation

In this study, ceramic coatings were grown on the surface of as-cast aluminum alloys via plasma electrolytic oxidation (PEO). The effect of the Si- and Ca-alloying elements in aluminum on the growth process, morphology, composition, mechanical, and corrosion properties of the PEO coatings was investigated. Uniform coatings with a minimum number of defects were formed on the surfaces of Al-Ca alloys. Increasing the Si content in Al led to an increase in the bulk and surface porosity of the coatings. The & alpha;-Al2O3 phase mainly formed in the coatings synthesized on pure Al and Al-Ca alloys, while an increased amount of Si in Al alloys hindered the formation of the & alpha;-Al2O3 phase. The coatings had a microhardness of 660-1180 HV, which was 20-30 times higher than that of the original as-cast alloy. Moreover, the coating on the Al-Ca alloys had the highest peak hardness, which was probably caused by the formation of a greater amount of the & alpha;-Al2O3 phase. Electrochemical studies in 3.5% NaCl have shown that PEO coatings reduce the corrosion current density. Of all PEO-treated alloys, Al and Al1Ca have the lowest corrosion current density and hence the highest corrosion resistance due to the composition and uniformity of the coating.