CORROSION FATIGUE AND NEAR-NEUTRAL pH STRESS CORROSION CRACKING OF PIPELINE STEEL IN VERY DILUTE CARBONATE/BICARBONATE WITH AND WITHOUT THE PRESENCE OF HYDROGEN SULFIDE USING THE COMPLIANCE TECHNIQUE

摘要

Crack advance has been studied in a X-70 pipeline steel, using the compliance technique. The electrolyte was very dilute brine bubbled with 10 percent carbon dioxide. Cracking rates were studied at a range of frequencies and R-values. The R-values simulated typical high-pressure gas service, but the frequency used was significantly higher. Similar testing was carried out in the presence of a 1 percent hydrogen sulfide using the Shell Canada sour gas test facility. The crack advance observed in dilute brine bubbled with 10 percent carbon dioxide could be explained using the rates of corrosion fatigue and a threshold DELTA K of 10MPa√m. If we extrapolate the results to much lower frequency then crack advance in the pipelines that is attributed to near-neutral pH stress corrosion cracking could also be explained as corrosion fatigue provided the DELTA K at about R=0.65 exceeds this threshold value. Since the extrapolation is over at least two orders of magnitude in frequency this requires further experimental confirmation. The effect of 1 percent hydrogen sulfide was to dramatically increase the cracking rate at the tested frequencies.