Laboratory Evaluation of Soil Stress Corrosion Cracking and Hydrogen Embrittlement of API Grade Steels

Stress corrosion cracking is one among several corrosion processes than can occur on buried pipelines. Efforts must be made in order to prevent this form of corrosion as long as pipeline collapse can cause accidents with high economic costs associated and environmental damage. In the present work, Stress Corrosion Cracking (SCC) and Hydrogen embrittlement (HE) of pipeline steels in contact with soil was investigated. Samples of different soils were obtained and prepared in order to determine its physical, chemical and bacteriological characteristics. Slow strain rate testing were carried out by using electrolytes obtained from soil samples extracted from different points located close to buried pipelines. Stress vs. strain curves of API X46, X60 and X80 steels were obtained at different electrode potentials (Ecorr, 100mV below Ecorr and 300mV below Ecorr). The results obtained demonstrated the conjoint incidence of SCC and HE, depending on the potential imposed. These results revealed the SCC susceptibility of the steels. It was also observed that the hydrogen embrittlement — HE — has an important contribution to cracking initiation and propagation. Cracking morphology was similar to the SCC reported on field condition where transgranular cracking were detected in a pipeline collapsed by land creeping. It was important to point out that even under cathodic potentials the material showed the incidence of secondary cracking and a significant decreasing of ductility. The mechanism was similar to the one described by Parkins and coworkers to explain the incidence of stress corrosion cracking of carbon steel in near neutral soil.