Localized corrosion of pressure vessel steel in a boiling water reactor cladding flaw: Modeling of electrochemical conditions and dedicated experiments

Stress corrosion cracking (SCC) tests performed in oxygenated high-temperature water with compact tension (CT) specimens have indicated that very small concentrations of chloride accelerate significantly the rate of crack growth in low-alloyed steels (LAS). In the present work, the electrochemical conditions in a CT specimen are simulated and compared to those in a cladding flaw. The Cl- and Fe2+ concentrations and the corrosion potential are predicted to be significantly higher in the crevice of a CT specimen than in the cladding flaw, whereas pH is significantly lower. Thus, a much more aggressive environment is established in the CT specimen. General corrosion rates of LAS in the presence of chloride, estimated by impedance spectroscopy, weight loss and thickness of oxide films, increase significantly at potentials more positive than the corrosion potential. In addition, slow strain rate tests showed susceptibility to SCC at E > -0.3 V vs. SHE, the tendency increasing with potential. It can be concluded that due to the higher potential in the crevice of CT specimen, determined by a more aggressive chemistry, the conditions within it are not representative for a realistic cladding flaw.