Oxides on zirconium-niobium alloys: An investigation of the different corrosion behaviours of alpha-zirconium, beta-zirconium and beta-niobium phases.

摘要

Three single phase alloys namely $lpha$-Zr, $eta$-Zr and $eta$-Nb (with compositions of 0.59wt% Nb, 18.2wt% Nb and 95.2wt% Nb respectively) were made using, an arc-furnace. The alloys were first homogenized by heating at 1000$spcirc$C for 24 h, and then subsequently subjected to corrosion tests in high temperature (300$spcirc$C) pressurized (8.6MPa) 0.2M and 0.05M LiOH solutions. In the 0.2M LiOH solution, for the $lpha$-Zr alloy, corrosion followed a cubic rate law in the pre-transition period, and then changed to linear kinetics after breakaway (which is simply defined as an abrupt change (increase) in the corrosion rate). For the $eta$-Nb alloy, corrosion followed a parabolic rate law in the pre-transition period, and then changed to linear kinetics after breakaway. For the $eta$-Zr alloy, however, there was no transition observed from parabolic (or cubic) to linear kinetics for the exposure times examined (up to 300 hours). From the X-ray diffraction, SEM and XPS examination of the oxides that formed on the three alloys, it was shown that in both 0.2M and 0.05M LiOH solutions, the $eta$-Zr alloy had a low corrosion rate due to the formation of a coherent dark blue oxide film which was retained for a relatively long exposure time. Tetragonal ZrO$sb2$ was detected as being present throughout the whole corrosion process of the $eta$-Zr alloy, as well as a lesser amount of monoclinic ZrO$sb2$. The oxides on the $lpha$-Zr and $eta$-Nb alloys. on the other hand, turned to white and grey and became powdery after breakaway, especially in the higher concentration (0.2M) LiOH solution. Any traces of tetragonal ZrO$sb2$ disappeared even before breakaway for the $lpha$-Zr alloy. Thick niobium oxides, Nb$sb2$O$sb5$ and NbO, were formed as a layered structure on the $eta$-Nb alloy. The rapid thickening, of the oxide and the topography changes in the oxide films, indicated that the $eta$-Nb alloy has a poorer corrosion resistance than the $eta$-Zr alloy. The effect of LiOH on the corrosion of $lpha$-Zr, $eta$-Zr and $eta$-Nb alloys is discussed. As we know, the current nuclear reactor tubing material Zr-2.5Nb has a two-phase structure of $lpha + eta$-Zr. The present experimental results showed that the $eta$-Zr phase has a better corrosion resistance than both $lpha$-Zr and $eta$-Nb phases. The $lpha$-Zr phase has a faster corrosion rate than the $eta$-Zr phase, and since the $eta$-Zr phase exists as filaments along the $lpha$-Zr grains, no preferential attack occurs. At certain locations in the reactor, e.g., where the fuel bundle contacts the pressure tube, crevices may exist. The Li$sp{+}$ concentration could be built up at these crevices which would increase the pH value locally. At these higher pHu27s, the corrosion rate differences between the three phases are amplified and preferential attack of $eta$-Nb phase may become more serious. (Abstract shortened by UMI.)Dept. of Mechanical, Automotive, and Materials Engineering. Paper copy at Leddy Library: Theses u26 Major Papers - Basement, West Bldg. / Call Number: Thesis1995 .W35. Source: Dissertation Abstracts International, Volume: 57-07, Section: B, page: 4646. Adviser: D. O. Northwood. Thesis (Ph.D.)--University of Windsor (Canada), 1995.