Environmental factors influencing stress corrosion cracking in boiling water reactors

The mechanisms of intergranular stress corrosion cracking (I.G.S.C.C.) of sensitized stainless steels in boiling water reactor (BWR) primary coolant are reviewed, with emphasis on the role the environment plays on both the initiation and propagation processes. Environmental factors discussed include oxygen (corrosion potential), temperature and dissolved ions in the water, and also the range of strain rates at which I.G.S.C.C. occurs. Both crack propagation rates and the range of strain rates at which I.G.S.C.C. occurs decrease rapidly as temperature is increased above approximately 200°C, in essentially the same manner as the solubility of magnetite decreases in acidic solutions. A mechanism of crack propagation is presented based on this observation. To establish water chemistry guidelines for crack-free operation of BWRs containing sensitized stainless steel, more information is needed on the role of absorption of impurities in the surface and deposited oxides and on the interaction between the oxygen and impurity levels required to maintain an electrochemical potential in a range where I.G.S.C.C. is unlikely to occur. The relative effects of short bursts of impurities and longer term lower concentrations of these same impurities also need to be evaluated.