Effect of zinc additions on oxide rupture strain and repassivation kinetics of iron-based alloys in 288 degrees C water

The effect of Zn water chemistry additions on the mechanism of intergranular stress corrosion cracking (IGSCC) of Fe-based alloys in water at 288 degrees C was evaluated in terms of the slip-dissolution model. In this model an increase in the oxide film rupture strain or surface film repassivation kinetics improved resistance to IGSCC. The oxide rupture strain of type 304L (UNS S30403) stainless steel (SS) increased up to a factor of two in deaerated and 200 ppb oxygenated, high-purity water (< 0.08 mu S/cm [0.20 mu S/in.] outlet) after exposure to Zn concentrations up to 60 ppb for 30 h to 170 h and at 20 ppb Zn for > 300 h of exposure. Repassivation kinetics experiments showed Zn additions of similar to 100 ppb increased the repassivation rate of an Fe-12% Cr alloy up to a factor of two in various deaerated water environments at 288 degrees C. Life prediction modeling revealed that the combination of a more ductile oxide film and faster repassivation kinetics resulted in a reduction in the overall crack growth rate (CGR) by at least a factor of four. This factor of improvement was consistent with data from compact tension experiments in similar environments where CGR decreased as the Zn addition increased, with a greater decrease in CGR realized at lower pre-Zn CGR.