Inhibitory effect of zinc addition to high-temperature hydrogenated water on mill-annealed and prefilmed Alloy 600

The effect of zinc addition to pressurized water reactor (PWR) primary water on primary water stress corrosion cracking (PWSCC) susceptibility of mill-annealed Alloy 600 (UNS N06600, 600MA) was examined at 360degreesC using slow strain rate technique (SSRT). In addition, the influence of oxide films, which were formed beforehand over a long exposure period in the simulated PWR primary chemistry, was examined. To clarify the effect of zinc addition on the characteristics of the oxide fllms formed on Alloy 600MA and prefilmed Alloy 600MA, oxide film structures and chemical compositions also were examined using Auger electron spectroscopy (AES), x-ray photoetectron spectroscopy (XPS), and x-ray diffractometry (XRD) techniques. An alternating current (AC) impedance study was conducted to examine the stability of the oxide film. SSRT and AC impedance test results showed that > 10 ppb Zn addition to the simulated PWR primary water reduced PWSCC susceptibility and increased the polarization resistance of the oxide film formed on Alloy 600MA and prefilmed Alloy 600MA. AES, XPS, and XRD studies showed that a double-layered oxidefilm was formed on Alloy 600MA in the simulated primary water. The outer layer was composed of nickel ferrite (NLFe2O4), and the inner layer of nickel chromite (NiCr2O4) spinet. With the addition of zinc at approximately the 10-ppb level in solution, the outer inverse spinet structure was diminished, and the film was thinner. Even though a double-layered prefilm was formed on Alloy 600MA, the outer inverse spinet oxide was reduced, and zinc was incorporated into the inner spinet oxide of NiCr2O4 by the addition of 10 ppb Zn in the simulated PWR chemistry. A good correlation was observed between PWSCC susceptibility and chromium content in the oxide, even when an oxide prefilm was formed on Alloy 600MA. Based on these observations, it appeared that zinc addition to the simulated primary coolant enhanced the formation of a stable oxidefilm composed of a zinc-chromium phase. It thereby improved PWSCC resistance of Alloy 600MA, even when a surface oxide prefilm was formed before zinc addition.