An increasing volume of evidence1 gathered over the last decade in the USA has indicated that stress corrosion cracking can occur from the soil side in buried pipelines. The characteristics of the failures are such that similar experiences were expected in transmission lines elsewhere when these reached a similar age to that of existing American systems and such failures have now occurred. The visible manifestations of the failures are groups of cracks near the fracture oriented essentially longitudinally along the pipelines and they typically penetrate about half the wall thickness. Failure occurs when the cracks propagate longitudinally to reach a critical size, resulting in fast mechanical fractures that propagate for considerable distances beyond the ends of the stress corrosion cracks, or when a crack penetrates the entire wall thickness to produce a leak. The cracks initiate from the soil side and follow an essentially intergranular path in the steel, the stress corrosion fracture surfaces acquiring a black deposit, mostly of Fe3O4 but sometimes with FeCO3 also detectable. Small quantities of liquids found in voids between the coating and the pipe near to stress corrosion cracks are essentially dilute sodium carbonate-sodium bicarbonate solutions, and in addition crystals of sodium bicarbonate have been found on pipe surfaces near stress corrosion cracks, suggesting that the environment causing the cracks in many, if not all instances, was a carbonate-bicarbonate solution.
