Effect of Pre-Oxidation in Oxygen on the Steam Corrosion Behavior of Zircaloy-2

The effect of pre-oxidation i dry oxygen gas on the corrosion behavior of Zircaloy-2 and unalloyed zirconium in steam was investigated. In the case of Zircaloy-2, pre-oxidation has no effect on corrosion behavior, the kinetics being the same whether all or part of the total exposure is in oxygen. A transi-tion in kinetics from quasi-cubic to l inear occurs in the corrosion of Zircaloy-2 in either medium, indicating that the transition is not associated with corrosion product hydrogen. On the other hand, unalloyed Zr shows neither transition nor breakaway (spalling) when exposed to oxygen, and exhibits breakaway on exposure to steam whether pre-oxidized or not, suggesting that breakaway is associated with corrosion product hydrogen. The phenomena of breakaway and transition are shown to be distinctly different. The corrosion properties of Zircaloy-2 sponge Zr base with 1.5 % Sn, 0.12 % Fe, 0.10 % Cr, and 0.05% Ni have been studied as a funct ion of temperature and t ime (1, 2). The corrosion behavior in 750~ (400 ~ steam at 1500 psi, Fig. 1, exhibits a change in slope ( " t rans i t ion") at a weight gain of 40 mg/dm ~ after 40 days (3). It has been shown that Zr and Zircaloy-2 pick up H dur ing exposure to high tem-perature water or steam, and it has been suggested that the transit ion ormal ly results when the pick-up of H dur ing corrosion causes hydr ide to form at the meta l /ox ide interface or at some other site in the reaction zone. If hydr ide format ion is a factor in causing t rans i-tion, specimens which have been exposed to dry oxygen for various periods of t ime and subsequent ly t ransferred to steam should show transi t ion at total weight gains and total t imes greater than those for specimens not previously oxidized in dry oxygen. Furthermore, specimens which have been exposed only to dry oxygen should not show transit ion. The purpose of the work described here was to deter