Strain-shock-induced early stage stress corrosion crack initiation and growth

Corrosion is a common but predictable and preventable problem in the engineering field and can be defined as a pervasive process of deterioration of metals and alloys. It can be costly problem. Because of it, buildings and bridges may collapse or oil pipelines break. Corrosion is caused by several factors, including the properties of the materials involved, the environment of operation and the mode of operation. Many engineering components, like buried pipeline steels, are operated either under displacement control or stress/load control or both. As such, the stress corrosion cracking (SCC) observed on most buried pipeline steels are oriented in either the circumferential or along the pipe length directions.

SCC is generally a consequence of the synergistic interactions between the tensile stresses and the vulnerable materials when subjected to corrosive environments. Typically, pipeline steels exhibit two types of SCC cracking: high pH SCC and near-neutral pH SCC. The former involves the formation of corrosion-induced passivating films and the rupture of these films when the strain rate reaches a critical value. In contrast, near-neutral pH SCC is characterized by the interaction of pipeline steels with atomic hydrogen under variable stresses.

Stress-strain curves is one of the best and most commonly used approaches to visualize the stress/strain rate variations and responses for steels. These curves can be divided into two: those with discontinuous yielding and those without discontinuous yielding. In pipeline steels, discontinuous yielding is attributed to two main factors: the dislocation locking following the exhaustion of mobile dislocations and the segregation of interstitial species.

For most stress-strain curves, the stress rate is usually linearly proportional to the strain rate in the elastic phase. However, higher stress beyond the upper yield point is often characterized by strain-shock, which involves much higher strain rates. Although the phenomenon of discontinuous yielding of pipeline steels is extensively studied in the literature, the impact of strain-shock in pipeline steels with discontinuous yielding on SCC is still not explored. This is mainly attributed to the limitations of the existing methods being used.

Herein, the University of Alberta researchers: Dr. Shidong Wang, Dr. Hamid Niazi and Professor Weixing Chen in collaboration with Dr. Lyndon Lamborn from Enbridge Pipelines Inc. investigated the occurrence of strain-shock associated with pipeline steels with discontinuous yielding and their corresponding effects on the early stage high pH SCC crack initiation and growth. Specifically, the study focused on hoop stress-driven longitudinal cracks in pipeline steels under load control. The characteristics of the strain rates in the pipeline steels with discontinuous yielding was also explored. Their work is currently published in the peer-reviewed journal, Corrosion Science.

The research team revealed that the cracking was directly related to the strain rate attributed to the stress variation in the hoop direction of the pipeline. In the early crack growth stages, microcrack initiation in the plastic zone and microcrack coalescence to the main crack tip were identified as the primary growth mechanism. These observations cannot be explained by the traditional stochastic initiation and coalescence mechanism assumed to bridge the crack growth from the early stages to the subsequent stages.

According to the authors, temperatures exceeding 40 °C caused strain ageing, thus increasing the risk of high pH SCC in pipeline steels. Under load-control, the discontinuous yielding-induced strain-shock effect enhanced the early-stage crack initiation and growth. The findings demonstrated the limitation of the slow strain rate testing (SSRT) approach, a typical displacement control, in predicting the susceptibility of a material to SCC when the effects of slow hardening rates and discontinuous yielding are considered, suggesting the viability of load-control.

In summary, this is the first study to investigate strain-shock-induced early-stage SCC crack initiation and growth in pipeline steels. The current research work is ground-breaking, and the theory proposed herein is particularly original and innovative. Given the strain-shock phenomenon is prevalent in engineering components under load control, many significant findings on the cracking of structural materials under various service conditions will no doubt follow. The findings provided new insights into material design requirements for enhanced resistance to SCC. In a statement to Advances in Engineering, the authors stated that insights provided in their study would contribute to the development of effective strategies that would enhance the safety and service lifetime of steels, especially those operated under load control.