Tetragonal ZrO₂ phase stabilization in coating layers prepared on Zr–2.5%Nb alloy during plasma electrolytic oxidation in sodium aluminate electrolytes

Plasma Electrolytic Oxidation Treatment of Zr-2.5%Nb Alloy for Corrosion Resistance Enhancement

“…From table 2, it can be seen a dominant monoclinic ZrO 2 phase (~97%) associated with an inhibited tetragonal phase of zirconia (~3%), totally in contrast to our previous results reported in [12] where either NaAlO 2 (pH = 13.0) or NaAlO 2 and NaOH (pH = 13.2) aqueous solutions were used in PEO process. Although identical potentiostatic operating regime was used, crystallographic phase modifications occur and could be related to the substrate surface chemistry or the chemical nature of electrolytic solution employed.…”

“…Also, the volumic fraction of t-ZrO 2 phase was estimated from the equation: (2) For this purpose, the diffraction lines (-111) and (111) of monoclinic ZrO 2 and diffraction line (101) of tetragonal ZrO 2 , respectively, were successfully indexed and the calculated integrated intensities were refined using the pseudo-Voigt analytical function. The obtained integrated intensities have been used to estimate m-ZrO 2 and t-ZrO 2 phase volume fractions involving equations (1) and (2) [12].…”

“…In the case of nuclear pressure tubes made from Zr-2.5%Nb alloy, the black oxide layer formed on this commercial alloy protects the surface against oxidation and corrosion at high temperatures during in-reactor operation [10]. However, plasma electrolytic oxidation (PEO) coatings attracted attention in the last decade regarding the improvement of wear and corrosion resistance of zirconium nuclear alloys by growing ZrO 2 protective layers during electrochemical oxidation of metal surfaces [11,12]. Additionally, taking into account the limited available literature on PEO-treated Zr alloys, including Zy-2 [13,14], Zy-4 [15,16], Zr-1%Nb [17,18] and Zr-2.5%Nb [19,20], further work will be required to optimize and improve the coating layers in the approach of material-coolant interaction.…”

“…Plasma electrolytic oxidation process was performed on samples cut out of a commercial Zr-2.5%Nb CANDU pressure tube without mechanically removing the initial black oxide coating, in order to probe another deposition approach as compared to our previous work [12].…”

“…A pulsed unipolar power supply with 150 Hz frequency was employed in a constant voltage regime [21]. A full description of the preparation method was described elsewhere [12]. Table 1 shows the sample codes, applied voltage (U) and PEO process time (τ) in E1 and E2 electrolytes.…”