Formation of corundum-rich alumina coatings on low-carbon steel by plasma electrolytic oxidation

Abstract

Abstract The plasma electrolytic oxidation (PEO) is an innovative method for providing light metals and their alloys with protective ceramic surfaces. However, for iron-based materials, the process requires very high current densities and results in the formation of coatings which consist of less stable iron compounds. Therefore, it was the aim of this study to design a PEO procedure on low-carbon steel at moderate current densities, which allows for the formation of ceramic coatings whose chemical composition is dominated by the electrolyte constituents. The electrolyte used was based on aluminate and preselected by systematic electrochemical passivation experiments. The PEO treatment was monitored by electrical and optical process diagnostics. As a result of this, it was possible to obtain alumina layers of 80 micrometers in thickness, with a high corundum content of approximately 50 to 90%, after 37 minutes of treatment time, at a current density below 25 A/dm2 on C8C-steel. However, the coating’s microstructure was inhomogeneous and showed poor substrate bonding. Based on the results of the experimental work, explanatory approaches were provided and a course of action is suggested for counteracting these problems.