An Overview of Near-Neutral pH Stress Corrosion Cracking in Pipelines and Mitigation Strategies for Its Initiation and Growth

Stress corrosion cracking (SCC) of pipeline steels in near-neutral pH environments has remained a significant integrity risk for oil and gas pipelines. Although it has traditionally been termed stress corrosion cracking, crack growth has never been observed under a static loading condition. It was determined later that the cracking is driven by corrosion-fatigue mechanisms with some uniqueness. First, the loading frequencies typically vary over a wide range from 10(-1) Hz to 10(-6) Hz, which is usually beyond the scope of most fatigue investigations. Second, the rate of corrosion is typically well below 0.1 mm/y at which a premature failure solely by corrosion would occur much longer than that actually found in the field. Third, hydrogen, a by-product of corrosion, can be generated to a level at which hydrogen embrittlement may occur only under special conditions. Fourth, pipelines are operated under variable pressure fluctuations that may lead to enhanced crack growth resulting from load-interactions effects. All existing crack growth models were developed based on the results obtained from tests either under constant load for the case of SCC or under constant stress amplitude loading for the case of fatigue or corrosion fatigue. These models generally yield predictions that are deviated from the crack growth rates being measured because they fail to consider both the stress-dependent and the time-dependent load interactions during variable pressure fluctuations. This overview will discuss details on how these factors are synergistically interacted to cause failures of pipeline steels in the field. Based on the understanding of the cracking mechanisms, strategies to mitigate field crack initiation and propagation will be introduced.