Fabrication of zirconia tetragonal phase by plasma electrolytic oxidation on Zircaloy-4 alloy and its electrochemical corrosion behavior

Zirconium has suitable characteristics for the fabrication of the fuel cladding tubes and coolant channel materials in water-cooled power reactors, such as low neutron absorption coefficient, high flexural strength, good corrosion resistance and high temperature mechanical properties. The native oxide film (zirconia) formed on the surface of zirconium and its alloys provides an effective corrosion resistance. The primary coolant in pressurized water reactors (PWR) contains lithium hydroxide (LiOH), which is used for pH control since it has been recognized that a more constant pH is desirable to reduce radiation field and fuel depositions. However it has been known that the native film may be destroyed in LiOH environment [1, 2]. A technique for surface treatment of zirconium and its alloys is plasma electrolyte oxidation (PEO) which enables the production of a uniform, hard and dense coating results in improvement of corrosion and wear resistance [3]. In this paper, zirconium oxide coating was produced on Zircaloy-4 alloy using plasma electrolytic oxidation (PEO). Sodium silicate and sodium aluminate based electrolytes were selected in PEO process and the effects of the concentration of sodium aluminate (2.5, 5, 7.5, and 10 g/L) on the phase structure and the corrosion behavior of the formed coatings was studied. The phase structure of coatings was investigated by x-ray diffraction (XRD). Corrosion properties of the Zr-1%Nb alloys were assessed by electrochemical impedance spectroscopy (EIS) test in 1 M LiOH solution at 293 K in order to evaluate the corrosion behavior. EIS test was carried out at a frequency range of 1mHz to 100 kHz at the open circuit potential and the perturbation amplitude 14th Annual Electrochemistry Seminar of Iran Materials and Energy Research Center (MERC), 12- 13 Dec, 2018 237 was 10 mV. Fig.1 shows XRD pattern of the PEO coatings formed in different electrolytes. Sodium aluminate leads to inhibition of formation of low-temperature monoclinic Zirconia phase in PEO coatings. By increasing the concentration of NaAlO2 to up to 10g/L the content of t-ZrO2 phase increases (Fig.1 and table1). After the addition of 10 g/L Sodium aluminate (s10) in the electrolyte %20 tetragonal zirconia was formed on the surface. The improvement in corrosion properties mainly depends on the phase composition of the produced coatings. The corrosion resistance values of coatings are in the decreasing order of Zr s2.5 s5 s7.5 s10 (Fig.2). The addition of NaAlO2 into the electrolyte decreased the pore density of the produced coatings as well as increasing the corrosion resistance. So the electrolyte of 10g/L NaAlO2 (S10) had more compact/uniform structure in comparison with the other coatings.