Development of fast algorithms for estimating stress corrosion crack growth rate

A simplified method is proposed for modeling the chemistry and potential distribution in a stress corrosion crack in sensitized stainless steel in boiling water reactor (BWR) coolant environments. The model is based on an assumption that only those species that are present in the largest concentrations in the crevice determine the potential distribution down the crack. The advantage of this method is that it permits simplification of the mathematics and allows predictions to be made of the potential and concentration distributions without knowing various parameters, such as the equilibrium constants for homogeneous chemical reactions in the cavity and diffusion coefficients of species that are present at relatively low concentrations near the crack tip. In some important cases, analytical expressions can be obtained for the pH, potential near the crack tip, and crack propagation rate. The conditions for which the potential on the crack flanks and that in the vicinity of the crack tip coincide with the free corrosion potential in the local environment, and hence, for which there exists a balance between rates of the local anodic and cathodic partial processes, are determined. The impact of the potential drop in the external environment on the potential and concentration distributions down the crack and on the crack propagation rate is also investigated. Excellent agreement is obtained between calculated and measured crack growth rates.