A stress corrosion cracking mechanism based on surface mobility

Basic aspects of stress-corrosion cracking (SCC) in metallic materials are outlined, followed by a summary of the numerous mechanisms that have been proposed for SCC. The characteristics of transgranular and intergranular SCC in model systems, e.g. pure metal and single-phase alloy single crystals and bi-crystals under testing conditions that facilitate discrimination between mechanisms, are then described. The applicability of the various proposed mechanisms, such as those based on dissolution, hydrogen embrittlement, film-induced cleavage, and adsorption, are discussed in detail for these systems. Mechanisms of SCC in complex commercial alloys are then considered in the light of these studies on model systems.

Mechanistic and fractographic aspects of stress corrosion cracking

Some observations on the current status in the understanding of stress-corrosion cracking of stainless steels

Transmission electron microscopy (TEM) was utilized to characterize the deformation substructure of 304 stainless steel tested for transgranular stress corrosion cracking (TGSCC) in 45 wt pet MgCl2 at 155 °C. The TEM characterization was conducted in thin foils prepared from the fracture surface and from a series of known depths below the fracture surface. The results indicate that the stacking fault energy (SFE) of the material immediately ahead of the crack tip is lowered, with the deformation mode at small distances (a few microns) in front of the growing crack front being entirely coplanar while at larger distances homogeneous. The reduction in the SFE is attributed to absorbed hydrogen formed during the cathodic reaction. Based on this and previous observations of transgranular stress corrosion characteristics of aus-tenitic stainless steels in chloride environments, a “hydrogen-induced cleavage” model is proposed. This model is essentially a modification of a model based on enhanced structural reactivity associated with Lomer-Cottrell locks proposed by Robertson and Tetelmann in 1962.27

A mechanistic study of transgranular stress corrosion cracking of type 304 stainless steel

The five categories of stainless steels and key features essential for stress corrosion cracking are introduced. The most damaging forms of stress corrosion cracking in austenitic stainless steels are chloride and caustic stress corrosion cracking and cracking in high temperature, high pressure aqueous environments. The influence of chemical composition, stacking fault energy, microstructure and environmental variables on stress corrosion cracking are summarized. The developments related to grain boundary engineering of austenitic stainless steels and the beneficial role of a high fraction of low energy grain boundaries against sensitization and sensitization induced stress corrosion cracking is described. Various fabrication parameters including cold/warm working, welding and residual stresses and surface finish on stress corrosion cracking are shown to influence stress corrosion cracking. Mitigation measures against stress corrosion cracking are briefly described.

Stress corrosion cracking (SCC) in stainless steels

The similarities and differences in the stress corrosion cracking response of ferritic and austenitic stainless steels in chloride solutions will be examined. Both classes of materials exhibit a cracking potential: similar transient response (to loading) of the potential in open circuit tests or the current in potentiostatic tests and similar enrichment of chromium and depletion of iron in the film associated with localized corrosion processes. The ferritic steels are more resistant to localized corrosion than are the austenitic steels, which is responsible for the difference in the influence of prior thermal and mechanical history on cracking susceptibility of the two types of steel. Similarities in the fractography of stress corrosion cracks and those produced by brittle delayed failure during cathodic charging of the ferritic steels indicate that hydrogen embrittlement is involved in the failure process.

R. F. Hehemann

Papers

Stress corrosion cracking of stainless steels