Effect of Initial Oxide Film on the Formation and Performance of Plasma Electrolytic Oxidation Coating on 7075 Aluminum Alloy

In the early stage of plasma electrolytic oxidation (PEO), the quick formation of the initial oxide film on the surface of the aluminum substrate is necessary for the subsequent discharge spark process. Furthermore, the compactness of the initial oxide film greatly affects the quality of the PEO coating, but the related mechanisms are not investigated in detail. In this paper, the status of the initial oxide film was adjusted by adding the (NaPO3)6 into the based electrolyte, and then the effect of initial oxide film on the formation of the PEO coating was compared. Microstructure and chemical composition were analyzed by scanning electron microscope, transmission electron microscopy and X-ray photoelectron spectrometer. The compactness of the initial oxide film was characterized by Mott–Schottky plots. The corrosion resistance was measured by electrochemical impedance spectroscopy. The results show that the addition of (NaPO3)6 can promote the co-deposition of phosphide compound into the initial oxide film and improve the film compactness, which is beneficial for the more uniform spark discharge in the later stage. As a result, the addition of (NaPO3)6 is helpful to obtain PEO coating with better performance.