Abstract
The effect of zinc addition to the primary water on Primary Water Stress Corrosion Cracking (PWSCC) susceptibility and crack growth rate of mill annealed alloy 600 (600MA), covered with oxide film which were formed beforehand in the simulated PWR primary chemistry for long time, were examined in the chemistry at 360°C, using Slow Strain Rate Technique (SSRT) and Double Cantilever Beam (DCB) tests. In order to clarify the effect of zinc addition on the characteristics of the oxide films formed on alloy 600MA, chemical compositions and the film structures were also examined using Auger Electron Spectroscopy (AES), X-Ray Photoelectron Spectroscopy (XPS), and X-Ray Diffractiometry (XRD) techniques. The SSRT tests results showed that the 10ppb zinc addition to the chemistry reduced the PWSCC susceptibility of the prefilmed alloy 600MA. And the DCB tests results showed that the PWSCC crack growth rate of prefilmed alloy 600MA was reduced to 1/10 by the 10ppb zinc addition to the chemistry. AES, XPS and XRD studies showed that even though a double layered prefilm, i.e., the outer layer composed of nickel ferrite and the inner one composed of nichromite, was formed on alloy 600MA, the outer inverse spinel oxide was diminished and then the oxide film was thinned by the addition of 10ppb zinc in the chemistry. Zinc was incorporated into the inner spinel oxide of nichromite, and the good relationship was observed between PWSCC susceptibility and chromium content in the oxide.
Based on the above observations, it was concluded that zinc addition to the simulated PWR primary coolant enhanced the formation of stable oxide film composed zinc-chromium phase, and then increased PWSCC resistance of alloy 600MA.
