Stress corrosion cracking occurs when the environment, material condition and mechanical loading achieve certain critical values. Additionally, intergranular stress corrosion cracking occurs when the grain boundary microstructure and microchemistry are altered in a specific fashion. This alteration may be the result of equilibrium impurity segregation, precipitation of active phases or depletion of passive film forming elements. Examples of each type include P segregation in rotor steels, AlsMg2 precipitation in 5xxx series Al alloys and chromium depletion in thermally sensitized austenitic stainless steel, respectively. There are other possible grain boundary factors that could affect IGSCC such as impurity or precipitate effects on grain boundary sliding but these are less well understood and will therefore not be covered extensively in this review.
Intergranular stress corrosion cracking (IGSCC) occurs in a large number of materials, components and industries. Some classic examples include natural gas pipelines, steam turbine rotors, discs and shrouds, nuclear piping, steam generator tubing and core components, sulfide stress corrosion cracking in oil/ gas well components and aluminum alloys for aerospace and future automotive applications. Intergranular stress corrosion cracking is the predominant crack path when the grain boundary microchemistry or microstructure has certain characteristics. These characteristics include segregation of an active species such as S or P, precipitation of an active phase at the grain boundary such as AbMg2 in AI-Mg alloys or chromium depletion from crZ3c6 precipitation or irradiation. There are examples where stress corrosion is predominantly transgranular or mixed intergranular and transgranular. However, the purpose of this paper is to summarize examples where the crack path is predominantly intergranular and to describe the microchemical and microstructural causes for this path dependence. A comprehensive review of these phenomena are beyond the scope of this paper but examples have been chosen to illustrate trends.
Segregation Induced IGSCC
Steam turbine rotors, discs and shrouds and sulfide stress corrosion cracking of oil/gas well components are clear examples where impurity segregation to grain boundaries plays a dominant role in IGSCC. The crack growth mechanism may be anodic dissolution or hydrogen induced or some combination but the central factor is the role of impurity segregation. These components are made predominantly of iron and nickel based alloys so the emphasis in this section will be on these materials. However, there are alloys where segregation may play a secondary role such as Mg segregation in AI-Mg alloys or impurity segregation in IGSCC of austenitic stainless steels. These segregation effects will be discussed in the sections on active precipitation and chromium depletion induced IGSCC, respectively.
Grain-boundary segregation of impurity elements such as P, S, Sn, and Sb in Fe and Ni have been shown (1) to affect dissolution and hydrogen-induced subcritical crack growth. Likewise, nitrate solutions have long been recognized as causing intergranular stress corrosion cracking (IGSCC) of Fe and Fe alloys where C was identified as the impurity responsible for the intergranular crack growth mode. Beca
