Chloride stress corrosion cracking in austenitic stainless steel is characterized by the multi-branched "lightning bolt" transgranular crack pattern. The micrograph below (X300) illustrates SCC in a 316 stainless steel chemical processing piping system. The micrograph above (X500) illustrates intergranular SCC of an Inconel heat exchanger tube with the crack following the grain boundaries. A disastrous failure may occur unexpectedly, with minimal overall material loss. Experimental SCC data is notorious for a wide range of scatter. SCC is classified as a catastrophic form of corrosion, as the detection of such fine cracks can be very difficult and the damage not easily predicted. Macroscopically, SCC fractures have a brittle appearance. In the microstructure, these cracks can have an intergranular or a transgranular morphology. Usually, most of the surface remains unattacked, but with fine cracks penetrating into the material. SCC in a 316 stainless steel chemical processing piping system photo courtesy The situation with buried pipelines is a good example of such complexity. The required tensile stresses may be in the form of directly applied stresses or in the form of residual stresses, see an example of SCC of an aircraft component. The impact of SCC on a material usually falls between dry cracking and the fatigue threshold of that material. Stress corrosion cracking (SCC) is the cracking induced from the combined influence of tensile stress and a corrosive environment. SCC usually occurs in certain specific alloy-environment-stress combinations. The build-up of corrosion products in confined spaces can also generate significant stresses and should not be overlooked. The residual stresses set up as a result of welding operations tend to approach the yield strength. The magnitude and importance of such stresses is often underestimated. The problem itself can be quite complex.Ĭold deformation and forming, welding, heat treatment, machining and grinding can introduce residual stresses. The required tensile stresses may be in the form of directly applied stresses or in the form of residual stresses. Intergranular SCC of an Inconel heat exchanger tube with the crack following the grain boundaries. Technical Communities of Interest (TCI).Since plastic strain of appropriate magnitude in the metal beyond the penetrating tip will prevent filming, in the extreme, or result in the rupture of films that do form, intergranular stress corrosion cracking (IGSCC), therefore, can be simply an extension of IG corrosion in the absence of stress. But in some circumstances, the films growing from those surfaces must extend over the tip, and penetration will cease even though a potentially active path exists beyond the tip. The restriction of lateral dissolution in most systems must result from the growth of inactivating films from the sides of the geometrical discontinuity produced by the penetration. Thus, the system must achieve a critical balance between activity at the penetrating tip of the localized corrosion path and inactivity upon the remaining exposed surfaces, including those formed by the penetration. Environments that cause such localized corrosion, among other things, will need to maintain the metal in the borderline regions between corrosion and passivity. However, it is well known that initial IGA can be stifled early by the penetrating tip becoming inactive because of filming. This fact is well known to metallographers in that different solutions can cause or avoid grain-boundary etching. One answer is that the activity of galvanic cells is variable, depending upon the environment. It is likely that almost all commercial alloys exhibit some chemical heterogeneity at grain boundaries, and this raises the question as to why intergranular attack (IGA) is not more prevalent. The lattice characteristics in the region of a grain boundary are such that equilibrium segregation of solutes or the nucleation and growth of precipitates are favored reactions, and so grain boundaries are sites of chemical heterogeneity.
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