The effect of zinc additions on the oxide rupture strain and repassivation kinetics of Fe-base alloys in 288 degree C water

Zinc additions to the water chemistry of boiling water reactors (BWRs) decreases the incidence of intergranular stress corrosion cracking (IGSCC); of stainless steel and nickel-base alloys. Here, the effect of Zn water chemistry additions on the mechanism of intergranular stress corrosion cracking (IGSCC) of Fe-base alloys in water at 288 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 will improve the resistance to IGSCC. The oxide rupture strain of 304L stainless steel increases up to a factor of 2 in both deaerated and 200 ppb oxygenated, high purity water (<0.08 microS/cm outlet) after exposure to Zn concentrations up to 60 ppb and at 20 ppb Zn for >300 hours of exposure. Repassivation kinetics experiments show that Zn additions of ∼100 ppb increase the repassivation rate of an Fe 12Cr alloy up to a factor of 2 in various deaerated water environments at 288 C. Life prediction modeling reveals that the combination of a more ductile oxide film and faster repassivation kinetics results in a reduction in the overall crack growth rate by at least a factor of 4. This factor of improvement is consistent with data from compact tension experiments in similar environments where the crack growth rate (CGR) decreases as the Zn addition increases, with a greater decrease in CGR being realized at lower pre-Zn CGRs