A ID, finite-difference based model, General Electromigration Model (GEM), was used with modifications For electrochemical kinetics. As expected, the cathodic hydrogen evolution rate and anodic iron dissolution rates were both found to affect the pH inside the crevice. The model also predicted that formation of iron carbonate, observed extensively in some pipeline failures, occurs under a specific combination of iron dissolution rate and hydrogen evolution rate. GEM provides a unique modeling tool because it is flexible enough to test the effects of a variety of environmental conditions as input parameters and because its predictions of solid mineral formation in crevices can be tested against field experience. The changes in crevice pH and potential were measured experimentally using microelectrodes. The experimental results at different applied potentials indicated that the experimental results were qualitatively consistent with model predictions.
BACKGROUND
Corrosion has accounted for about 12percent of reportable incidents of failure in pipelines from 1970to 1993?. While cathodic protection and coating developments have reduced the magnitude of the corrosion problems, stress Corrosion cracking(SCC) and corrosion under disbonded coatings are still considered to be significant problems2.The first recorded SCC failure of transmission pipe occurred in Natchitoches, Louisiana, in 1965.This and many other cases of external SCC in pipelines have been attributed to an alkaline environment and laboratory test methods have been developed based on an environment of 1Mcarbonatr+l Mbicarbonatemixture3.However, since 1985,sevenpipeline ruptures attributed to SCC have occurred in one company, TransCanada Pipeline Limited (TCPL). The latest rupture, resulting in an explosion, occurred in July 1995 and was the second since the National Energy Board released the results of an inquiry into previous failures4.The initial Canadian pipeline failures prompted SCC testing in a more acidic environment, based on field analyses of solution extracted from failed coating5.However, the recurrence of failures suggests that the environmental conditions leading to SCC have been insufficiently understood. Since SCC failures invariably occur under disbonded coatings, rigorous modeling and real-time measurements of the environment under disbonded coating are necessary for developing quantitative understanding and predictive techniques.
As mentioned previously, two different modes of external SCC of pipeline steels have been observed (I) Intergranular SCC (IGSCC), which has been shown to occur in a relatively concentrated carbonate-bicarbonate environment at a pH of about 10,and (ii) Transgranular SCC (TGSCC), which has been shown to occur in more dilute solutions containing chlorides at a pH close to 6.A comprehensive, phenomenological description of the SCC can be found inotherreviews5?6.There is little doubt that the HCO~-/CO~2-environment causes IGSCC, but the issue is how such an environment evolves from the nominal soil environment under a disbonded coating. The laboratory environment in which IGSCC has been readily observed is an order of magnitude more concentrated in carbonate and bicarbonate contents than the environments observed near-field failures. It must be emphasized that the field measurements of the alkaline environment under disbonded coatings for the IGSCC are sparse and subject to considerable experimental uncertainty.
IGSCC in the HCO~-/C0,2- environment has been shown to occur at a critical potential of approximately -650 mVm, about 200 mV less negative than the typical cathodic protection potential. The distribution of potential inside disbonded coatings was examined by Fessler et a
