Effects of process parameters on structure and corrosion behavior of PEO coated A356 alloy

Abstract

Plasma electrolytic oxidation (PEO) coatings were formed on cast A356 aluminum alloy by applying various process parameters (current density, duty cycle and processing time) in silicate based electrolyte. The phase composition of coatings was analyzed by x-ray diffraction (XRD), surface and cross section morphology and microstructure of coatings were characterized by scanning electron microscope (SEM). The Results of x-ray diffraction showed that all coatings are composed of gamma alumina. Coating produced in higher current density, duty cycle and time was more compact with fewer amounts of defects. Thickness and roughness of coatings surfaces were measured by Eddy current gauge and surface roughness tester, respectively. The thickness of the Coatings was in the range of 3–7 μm and it increased by increasing current density and time, but duty cycle did not have a noticeable effect on it. Corrosion resistance of bare and coated samples was evaluated by polarization test in 3.5 wt% NaCl solution. Sample S176 showed the best corrosion resistance with corrosion current density of 4.17 × 10−8 A.cm−2 and polarization resistance of 6.31 × 105 Ω.cm2 that attributed to its more dense structure.