Effect of current density, duty cycle and duration time on corrosion behavior of Al alloy treated by plasma electrolytic oxidation

Plasma electrolytic oxidation (PEO) process is a suitable method for producing hard, thick and dense ceramic coatings on light weight metals like Al and its alloys. This method can improve the corrosion properties of Al alloys that are tightly dependent to process parameters. Therefore, in this study we used different combinations of current density, duty cycle and treatment time to evaluate the effect of these parameters on properties of coatings formed on Al 7075 alloy. The morphology, microstructure and phase composition of coatings were analyzed by scanning electron microscope (SEM) and X-ray diffraction technique (XRD). Thickness and roughness of coatings surfaces were measured by Eddy current gauge and surface roughness tester. The corrosion properties of coatings and substrate were evaluated by potentiostatic polarization tests in 3.5% NaCl solution. X-ray diffraction patterns of coatings demonstrate that all of coatings are mainly consist of γ-Al2O3. SEM images showed that coating produced at duty cycle 80 % had a lot of bigger pores on its surface compared with others. Maximum coating thickness of 10.4 µm was achieved at condition of 0.14 A/cm2 current density, 50 % duty cycle and 30 min of treatment time. Also, the coating was formed in this condition showed the best corrosion resistance with corrosion current density about three orders of magnitude lower than substrate that could be because of its higher thickness.