A phenomenological model is developed for environmentally assisted cracking of pipeline steels in the near-neutral pH soil environments, based on the hypothesis that the cracking is dominated by the corrosion fatigue mechanism promoted by hydrogen embrittlement. The comparison with the data of laboratory indicates that the new model can provide reasonable predictions for the dependence of the crack growth rates on the stress intensity factor, stress ratio, loading frequency, solution pH and electrochemical potential.
Laboratory investigations and field observations have indicated that cracking can occur on the external surface of underground pipelines when exposed to dilute groundwater with a pH in the range of 6-8.1 Laboratory investigations and field observations have revealed that the crack growth is dominated by the hydrogen-assisted cracking (HAC) mechanism,1-4 although anodic dissolution may also play a role.2,5-9 Experiments have shown that anodic dissolution under constant load could lead to dormancy of cracks 4,8, while others suggested that it might assist cracking.10 In this work, the role of anodic dissolution in the crack propagation is not considered. The crack growth is assumed to be fully governed by HAC mechanisms. Laboratory tests show that cracks of pipeline steel cannot propagate in the near-neutral pH groundwater unless the cyclic load is applied,1-2,8 although a few researchers claimed that they observed crack initiation11 and propagation12 under constant load conditions. Recently, Chen et al.13 observed that cracks that arrested under constant load would restart propagating when cyclic loads were applied. In operation of pipelines, cyclic stresses are unavoidable, owing to internal pressure fluctuations, and thus, cyclic stresses are likely to play a key role the crack growth. The critical role of cyclic loading implies that the cracking is likely to be dominated by a mechanism of corrosion fatigue.
