Corrosion fatigue and near-neutral pH stress corrosion cracking of pipeline steel and the effect of hydrogen sulfide

Crack advance has been studied in an X-70 pipeline steel, using the compliance technique. The electrolyte used in the study was a very dilute brine bubbled with 10% carbon dioxide (CO2). Cracking rates were studied at a range of frequencies and R-values (R is stress ratio). The R-values simulated typical high-pressure gas service. but the frequency used was significantly higher than that found in gas pipelines. Similar testing was carried out with the addition of 1% hydrogen sulfide (H2S) to the electrolyte using the Shell Canada sour gas test facility. Tests also were run to determine the rate of corrosion fatigue in the same solutions. The crack advance observed in dilute brine bubbled with 10% CO2 could be explained using corrosion fatigue and a threshold, DeltaK, of 10 MParootm. There has been some previous work done at lower frequencies in a less dilute but related electrolyte, and these data have been compared to the present results. The effect of 1% H2S was a dramatic increase in the cracking rate at these frequencies.