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

Plasma electrolytic oxidation (PEO) process is a suitable method for producing hard, thick anddense ceramic coatings on light weight metals like Al and its alloys. This method can improvethe 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 toevaluate the effect of these parameters on properties of coatings formed on Al ۷۰۷۵ alloy. Themorphology, microstructure and phase composition of coatings were analyzed by scanningelectron microscope (SEM) and X-ray diffraction technique (XRD). Thickness and roughness ofcoatings surfaces were measured by Eddy current gauge and surface roughness tester. Thecorrosion properties of coatings and substrate were evaluated by potentiostatic polarization testsin ۳.۵% NaCl solution. X-ray diffraction patterns of coatings demonstrate that all of coatings aremainly consist of γ-Al۲O۳. SEM images showed that coating produced at duty cycle ۸۰ % hada lot of bigger pores on its surface compared with others. Maximum coating thickness of ۱۰.۴ μmwas achieved at condition of ۰.۱۴ A/cm۲ current density, ۵۰ % duty cycle and ۳۰ min oftreatment time. Also, the coating was formed in this condition showed the best corrosionresistance with corrosion current density about three orders of magnitude lower than substratethat could be because of its higher thickness.

Keywords:

plasma electrolytic oxidation , corrosion , Al alloy