High Pressure Gas Pipelines - Their Ageing and Propensity Towards Intergranular Stress Corrosion Cracking Failure

Natural gas is usually transmitted from its source to regions of consumption in steel pipelines operating at pressures typically in the region of 7000 kPa. The lines are buried under 1 m or more of soil and, to counteract their tendency to corrode in such an environment, they have coatings applied over the outer surface and are subjected to cathodic protection. While those approaches to preventing corrosion are usually effective they are not invariably so, because coatings deteriorate with time, developing defects that allow the ingress of ground waters to the steel. Disbonding of the coating from the steel extends the area over which water has access to the pipe and cathodic current may not penetrate sufficiently into the crevice between pipe and coating to provide protection. The consequence is usually that pits develop and if they penetrate the pipe wall a gas leak will occur. However, in some situations stress corrosion cracks may be generated, sometimes without penetrating the wall, and if they reach an adequate size the fracture toughness of the steel will be exceeded, obviously with much more dire consequences than those for a simple leak. Ground waters are usually dilute solutions containing HCO₃-, C1- and SO₄2-, and often free CO₂, with balancing cations of Na, K, Ca and Mg, and, while capable of promoting pitting, such solutions do not promote intergranular stress corrosion cracking. The latter requires the generation of a relatively concentrated bicarbonate solution, as considered later.