Effects of zinc additions on the stress corrosion crack growth rate of sensitized stainless steel, Alloy 600 and Alloy 182 weld metal in 288 C water

ZnO additions to boiling water reactor (BWR) water have been the focus of recent interest, primarily because of their beneficial influence in reducing buildup of radioactive species such as Co60 in the oxide film of structural components, e.g., stainless steel piping. The effect of ZnO additions on stress corrosion crack growth rates were studied using 1T CT fracture mechanics specimens of sensitized type 304 stainless steel, sensitized Alloy 600, and Alloy 182 weld metal exposed to ∼288 C water containing various levels of dissolved oxygen and impurities. Zn levels of 5 to 100 ppb Zn2+ were evaluated and found to reduce crack growth rates for all materials and in all water chemistries. Many Zn tests involved long term exposure and were performed at somewhat reduced corrosion potential (e.g., from ∼+200 to 0 -+50 mVshe); variations in corrosion potential from +200, to +50, to -50 mVshe clearly had an important effect. The benefit of Zn appeared to be most pronounced when the growth rate was decreased (e.g., by corrosion potential). This was consistent with the findings of mechanistic studies, which showed that Zn decreased the repassivation response at times >104 s, which is associated with low crack tip strain rates, i.e., low growth rates. Reduced corrosion potentials are also expected to directly effect Zn, since high (crack mouth) corrosion potentials inhibit the transport of Zn2+ into the crack. Zn also increased the fracture strain of the oxide on stainless steel, and may also reduce crack growth rates by increasing the pH in the crack. Similar benefits are expected for other structural materials, such as nonsensitized or irradiated stainless steel, carbon steel, low alloy steel, and other nickel alloys