Abstract
A metallurgical investigation of multiple, externally-initiated, in-service leaks on an aboveground, mineral wool insulated, oil emulsion pipeline concluded that the likely crack-initiating mechanism was stress corrosion cracking (SCC). However, the leaks occurred on an above-ground pipeline and as such, it was not clear to what extent the classic mechanistic principles of SCC identified on buried carbon steel pipelines were applicable to the current instance of SCC, if at all. A root cause analysis (RCA) was used to identify the likely contributors to the SCC, including stress, environment, and the carbon steel material. As part of the RCA, laboratory testing and chemical analyses were performed. The purposes of the laboratory testing were to (1) reproduce the external cracking found in the pipeline, and (2) evaluate the effects of insulation source, electrolyte concentration, and temperature on the SCC behavior of pipe steel in laboratory-produced leachates. Testing consisted of cyclic potentiodynamic polarization (CPP) tests, slow-strain rate (SSR) testing using leachates solutions made from mineral wool insulation, and chemical analysis of those solutions. This paper outlines key findings from the laboratory testing and a current understanding of the primary environmental contributors given that SCC on above-ground insulated pipelines is not commonly observed.
