Stress corrosion cracking of high-pressure gas transmission pipelines

Abstract

Stress corrosion cracks that have been found on the outside surfaces of gas pipelines in the USA have been characterized by a branched, intergranular path and a black Fe₃O₄ deposit on the crack surfaces. An ongoing research programme sponsored by the Pipeline Research Committee of the American Gas Association has revealed much about the causes of the cracking and about possible preventive measures.

It is believed that most, if not all, of the cracks have formed in a sodium carbonate—sodium bicarbonate environment in which a critical balance of activity and passivity has been achieved. Laboratory experiments have shown that the narrow range of potentials in which cracking is possible and the rate of crack growth can be correlated with the anodic polarization behaviour. Temperature has a strong effect on the rate of crack growth and on the probability of establishing appropriate electrochemical conditions for cracking. The occurrence of cracking has not been related to any unusual or specific composition of steel but modification of steel processing might eventually offer a practical way of preventing the problem. The function of stress is primarily to produce a rate of straining sufficient to continually expose fresh metal at the tip of the crack.

A large number of conceivable ways of avoiding stress corrosion cracking are being investigated. For existing pipelines, lowering the discharge temperature at compressor stations and hydrostatic retesting are being considered. Additional alternatives for future pipelines include modification of the steel and its surface condition, improvement of the coating, and the use of inhibitors.