Mechanisms of Stress Corrosion Cracking

Stress corrosion cracking (SCC) even nowadays is the cause of significant service failures, it can occur in almost any type of components of the chemical industry such as steam generator tubes, pressurizer instrument penetrations and heater sleeves, control rod drive mechanism (CRDM) nozzles, heat exchangers [1]-[6]. One of the most recent major accident happened in 2009, where a 50-foot-tall highpressure vessel ruptured, resulting in one fatality and one injury of the public. Based on the review of the National Institute of Standards and Technology the failure was caused by the combination of stress corrosion cracking and a reduction in material toughness during service [6]. In case of existing nuclear power plants the stress corrosion cracking counts as one of the important ageing degradations. For stress corrosion to occur three indispensable factors have to be present, which are the tensile stress, the environment and the susceptible material. If changes are made in any of these factors then the susceptibility to SCC often can be eliminated or reduced. The tensile stress can originate from residual stresses, operational loadings, the structural and metallurgical factors includes the degree of grain size, thermal treatment, cold work, and finally the environment factors are resulted from the water chemistry, the operating temperature [1]. However this type of failure is sudden and difficult to predict and also material properties may change with time or due to material processing. Therefore the right way of prediction and modelling of SCC is still a vital research area. In this article the different type of SCC mechanisms are presented, the ones used in the nuclear industry in more details.