Preparation of Coating on Zircaloy-۴ 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-۴ 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 (۰, ۲.۵, ۵, ۷.۵, and ۱۰ 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 ۰.۵ M LiOH solution. The results show that, ZrO۲ coating causes an increase in corrosion resistance inner layer from ۰.۶۱ MΩ.cm-۲ for the bare sample to ۱۲۶ MΩ.cm-۲ for the sample which was coated in an electrolyte with the combination of ۱۰g/L Na۲SiO۳ and ۵g/L Na۴P۲O۷ (sample S۵). This corrosion behavior was related to the coating characteristics. So that in the electrolyte of ۵g/L Na۴P۲O۷ (S۵) has compact/uniform structure in comparison to other coatings. Also, this sample contains the monoclinic ZrO۲ phase of ۹۴% and the tetragonal ZrO۲ phase of ۶% respectively.

Keywords:

Zircaloy-۴ alloy , electrochemical impedance spectroscopy , plasma electrolytic oxidation , sodium pyrophosphate , آلیاژ زیرکونیوم zircaloy-۴ , طیف سنجی امپدانس الکتروشیمیایی , اکسیداسیون الکترولیتی پلاسمایی , تترا سدیم پیروفسفات , سدیم سیلیکات