In Situ Monitoring of Environmental Conditions for Stress Corrosion Cracking

TransCanada Pipelines have been involved with monitoring environmental conditions at pipe depth for almost 10 years. The purpose of this monitoring is to understand the mechanism of SCC as it occurs in the field and to assist in the development of site-selection models for identifying locations where SCC might be occurring. Monitoring can either be done at many locations along the right-of-way at a given time or continuously at discrete locations in order to observe seasonal variations. A range of environmental parameters can be monitored. Early work focussed on parameters relevant to corrosion, such as soil resistivity, redox potential, temperature, pH, and on- and off-potentials. More recently, parameters relevant to SCC have also been monitored, such as soil CO₂ and permeable hydrogen concentrations. In addition, the extent to which these parameters change seasonally has been monitored to determine if the environment is conducive to SCC continuously or whether cracking might only occur at certain times of the year. In terms of implementation, the results of in situ monitoring can be used as part of a larger integrity management program to decide where and when to mitigate SCC. Correlations between soil parameters measured using the portable probe and known SCC sites can be used to identify other susceptible locations or to prioritize different lines for inspection. Seasonal variations at a particular location can be used to derive effective crack growth rates from accelerated laboratory testing in order to determine re-inspection intervals. Examples are provided of both portable and permanent NOVAProbe measurements for low-pH and high-pH SCC.