Exploring to Predict Pipeline High-PH SCC Crack Growth Rates

A mathematical model was developed that enables the prediction of crack growth rate (CGR) of high pH stress corrosion cracking (SCC) of buried steel pipelines. The goal of developing the model is to use it to understand the relative CGRs of SCC-susceptible locations along a pipeline, so that a more reliable prioritization of these locations for direct examination becomes possible. The model, in its current status, was used to predict CGRs using two methods, with the anodic current density at the crack tip being expressed by: (1) the anodic polarization curve, and (2) the Butler-Volmer equation. The merits and shortcomings of these two methods were analyzed. The former method tends to yield large CGRs, while the latter has the flexibility to predict low CGRs. Even though the high pH chemistry in the disbonded region outside of a crack is often associated with a concentrated carbonate and bicarbonate solution buffer, the model predicted that when the CGRs are large, the predicted chemistry and potential at the crack tip can vary significantly with time, while they are somewhat stable when the CGRs are small.