Preparation of Coating on Zircaloy-4 Alloy by Plasma Electrolytic Oxidation and Investigation the Effects of Tetrasodium Pyrophosphate Concentration on the Structure and Corrosion Behavior of Coating

In this paper, zirconium oxide coating was produced on Zircaloy-4 alloy using plasma electrolytic oxidation (PEO). Sodium silicate and sodium pyrophosphate based electrolyte was selected in PEO process and the effects of concentration of sodium pyrophosphate (0, 2.5, 5, 7.5, and 10 g/L) on the microstructure and the behavior of electrochemical corrosion of formed coatings was studied. In order to examine the morphology and phase structure of coatings, scanning electron microscopy (SEM) and x-ray diffraction (XRD) were used respectively. The corrosion behavior of the coated samples was evaluated by electrochemical impedance spectroscopy (EIS) in 0.5 M LiOH solution. The results show that, ZrO2 coating causes an increase in corrosion resistance inner layer from 0.61 MΩ.cm-2 for the bare sample to 126 MΩ.cm-2 for the sample which was coated in an electrolyte with the combination of 10g/L Na2SiO3 and 5g/L Na4P2O7 (sample S5). This corrosion behavior was related to the coating characteristics. So that in the electrolyte of 5g/L Na4P2O7 (S5) has compact/uniform structure in comparison to other coatings. Also, this sample contains the monoclinic ZrO2 phase of 94% and the tetragonal ZrO2 phase of 6% respectively.