After Two Decades of Predicting Crack Growth Rate-What Have We Learned?

Abstract A review is presented of models that have been developed by the authors for estimating crack growth rate in sensitized Type 304 SS in Boiling Water Reactor (BWR) heat transport circuit (HTC) structural components, with emphasis on the electrochemistry of the process and on determinism (i.e., models whose prediction are constrained explicitly by the natural laws). Intergranular Stress Corrosion Cracking (IGSCC) under normal BWR operating conditions (T = 288°C, pure water) is primarily an electrochemical process that occurs at potentials that are more positive than a critical value of EIGSCC = - 0.23 Vshe. However, the crack growth rate (CGR) at E > EIGSCC is also a function of potential, conductivity, degree of sensitization of the steel, flow velocity, mechanical load, and crack length. The dominance of electrochemical, solution, and hydrodynamic factors in controlling CGR has led to the development of various techniques for mitigating IGSCC in sensitized Type 304 SS by modifying the environment, such that the corrosion potential (ECP) is displaced to a value that is more negative than EIGSCC. However, even in those regions of the HTC where the corrosion potential cannot be displaced sufficiently in the negative direction to satisfy the condition ECP < EIGSCC, considerable benefit is obtained because of the roughly exponential dependence of CGR on potential. The most important parameters in affecting electrochemical control over the ECP and CGR in an operating reactor are the kinetic parameters (exchange current densities and Tafel constants) for the redox reactions involving the principal radiolysis products of water (O2, H2, and H2O2), and feed water composition and conductivity. The kinetic parameters for the redox reactions essemially determine the charge transfer impedance of the steel surface, which is known to be instrumental in determining the magnitude of the coupling current that flows through the solution from the crack to the external surface and hence in determining the CGR. The exchange current densities, in particular, are amenable to control by catalysis or inhibition of the redox reactions occurring on the surfaces external to the crack enclave, with the result that surface modification techniques have proven to be highly effective in mitigating and controlling IGSCC in reactor heat transport circuits.