Effect of Pigging Damage on Sulfide Stress Corrosion Cracking of Type 316L Stainless Steel Cladding

Abstract

Scouring pigging of oil and gas pipelines is used to maintain flow efficiency by abrading the inner surface to remove precipitates such as wax and scale. There has been uncertainty over whether pigging of corrosion resistant alloy pipelines used in sour service could adversely affect their resistance to localized corrosion and stress corrosion cracking (SCC). In this work, a novel environmental chamber was developed to enable scouring pigging to be simulated on pipeline steels while under tensile stress and exposed to deoxygenated brine. Using this configuration, representative pigging damage was reproduced on Type 316L stainless steel cladding with both ground and as-received finish prior to four-point bend testing in a range of environments. Under conditions in which the alloy is considered to be resistant to SCC, simulated pigging had no adverse effect on the resistance of the alloy to localized corrosion or cracking. Under more severe test conditions, in which the alloy is considered to be susceptible to SCC, specimens with an as-received surface experienced preferential pitting and cracking located at pigging scratches, whereas no such effect was observed for ground specimens. Electrochemical impedance spectroscopy measurements demonstrated that the abraded surface had a thinner passive film and a lower corrosion resistance than the as-received surface. However, this detrimental effect of surface abrasion, which is associated with the removal of the pickled and passivated surface, is taken into account by the conservatism of SCC test programs, in which ground specimens are typically included. These findings support the perspective that under normal operating conditions scouring pigging is not expected to increase the risk of pitting or SCC.