Mechanistic studies on stress corrosion cracking of pipeline steels in near-neutral pH environments

Experimental and theoretical approaches towards the study on mechanism of stress corrosion cracking (SCC) of pipeline steels in near-neutral pH environment have been performed. The subject of the investigations included SCC susceptibility at various testing conditions, hydrogen distribution around SCC crack tip and the role of hydrogen in cracking process. SCC tests were mainly conducted using slow strain rate testing (SSRT). The hydrogen distribution around a stress corrosion crack tip was measured using secondary ion mass spectrometry (SIMS), and modelled using an elastic-plastic analysis. A thermodynamic model was proposed and used to calculate the effect that the presence of hydrogen and stress has on the SCC growth rate.; SSRTs showed that transgranular stress corrosion cracking (TGSCC) of pipeline steels could occur in a dilute bicarbonate solution with a near-neutral pH value. SCC susceptibility increased as the applied electrochemical potential, the pH value of solution and applied strain rates during tension testing decreased. Hydrogen precharging or addition of carbon dioxide (CO₂) facilitated the process of SCC, suggesting that dissolution and ingress of hydrogen are both involved in the cracking process.; SIMS measurement indicated that low pH values and applying cathodic potential facilitate the generation, evolution and enrichment of hydrogen around the SCC crack tip. Hydrogen plays an important role in the SCC of pipeline steels by promoting anodic dissolution and SCC susceptibility.; Thermodynamic analysis showed that hydrogen interacted with the stress field and changed the internal energy and entropy of the steel. These changes could result in an increase of the anodic dissolution rate of the steel and enhancing the SCC growth rate. The hydrogen-facilitated SCC growth rate obtained from the proposed model was in agreement with SSRT measurements.; The mechanism of SCC for pipeline steels in near-neutral pH solutions may be that at near free corrosion potential (E_Corr), hydrogen was evolved from environment and accumulated in a highly stressed region of the steel, which enhanced local dissolution or pitting process. The combination of preferential dissolution with stresses induced SCC cracks initiation. Once crack initiated it propagated by hydrogen-facilitated dissolution. At cathodic potentials, when hydrogen concentration reached a critical value, the cracking process is controlled by hydrogen-induced cracking (HIC).