Alumina-Silica Composite Coatings on Aluminum by Plasma Electrolytic Oxidation: The Effect of Coating Time on Microstructure, Phase, and Corrosion Behavior

In this study, alumina-silica composite coating was coated on an aluminum substrate by the plasma electrolytic oxidation technique at 30 °C and current density of 20 A dm−2 for 30, 60, and 90 min. Microstructure of the coatings was evaluated by an atomic force microscope and a scanning electron microscope. X-ray diffraction analysis was employed to evaluate the phase analysis of the coatings. In addition, corrosion behavior of the coated and uncoated samples was investigated by using potentiodynamic polarization and electrochemical impedance spectroscopy tests in a 3.5 wt.% NaCl solution at 25 °C. Results showed that, by increasing the coating time, the coating thickness and porosity size were increased. However, the number of porosities was decreased by increasing the coating time. Phase analysis results revealed that the coating was mainly composed of mullite, alumina, and silica. Moreover, the results of the corrosion tests indicated that the alumina-silica composite coating formed by the plasma electrolytic oxidation technique considerably improved the corrosion resistance. The sample coated for 60 min showed a better corrosion resistance in comparison with other samples coated for 30 and 90 min.