Effect of PEO Coating Microstructure on Corrosion of Al 2024

Abstract

Transient self-feedback control (TSFC) was employed to prepare plasma electrolytic oxidation (PEO) coatings on 2024 Al alloy. Compared with Al2O3 PEO coating by the custom pulse voltage (CPV) process, the PEO coating using the TSFC mode generated a thinner outer layer and more compact inner layer. In order to study the microstructure effect on the practical corrosion properties, three Al2O3 PEO coatings using the CPV and TSFC processes were studied during long term immersion in a corrosive environment by electrochemical impedance spectroscopy (EIS) and neutral salt spray test (NSST). Two groups of electrical equivalent circuits were employed based on different microstructures. During the initial immersion period a double-layer element (CPEdl Rt) appeared due to the CPV PEO coating having a porous outer layer and thinner inner layer. Thereafter, continuous corrosion reactions happened at the metal/PEO interface, which resulted in the appearance of diffusion elements. However, for specimens prepared by TSFC, corrosion reactions only happened in the later stages of the immersion. The compact and thick inner layer mainly contributed to excellent barrier properties, stable microstructure after 380 h immersion in 3.5(wt%) NaCl solution and excellent performance in the salt spray test.