Calculating the Temperature-Maximum and the Lower Potential Limit for the Crack Growth Rate in Type 304 SS Using the CEFM

Abstract The coupled-environment fracture model (CEFM) has been used extensively to model intergranular stress corrosion cracking (IGSCC) of Type 304 (UNS S30400) stainless steel (SS) in high temperature, relatively pure water. Recently, it has been extended to incorporate the effects of sulfuric acid additions to the water and to include thermal activation of the crack tip strain rate and an approximate analytical version of the model has also been developed and published. The latter is particularly valuable for high-speed computation of the crack growth rate when, for example, employing the model to calculate the integral damage over a specified evolutionary path. The extension of the CEFM to dilute sulfuric acid has allowed comparisons to be made between theoretically estimated and experimentally determined crack growth rates over a considerable temperature range after calibration of the model at only one temperature and the assumption of an appropriate value for the activation energy of the crack tip strain rate. The model predicts, in agreement with experiment, that the crack growth rate passes through a maximum with increasing temperature at about 180 °C. The temperature dependence of the crack growth rate is attributed to the competing effects of temperature on the thermally activated processes that occur at the crack tip and the properties (including Ecorr and conductivity) of the external environment. The CEFM also defines a lower limit to the external electrochemical potential (EIGSCC), below which the crack grows only by creep. In the present work, we have adopted the creep model of Wilkinson and Vitek, which attributes crack advance to the coalescence of micro voids that nucleate ahead of the crack tip. With increasing crack length, EIGSCC increases and when it surpasses (becomes more positive than) the corrosion potential in the external environment no further stress corrosion crack growth can occur and fracture results only from creep.