Stress Corrosion Cracking (SCC)

Stress corrosion cracking is to bear the stress of the alloy in the corrosive environment because of strong expansion of the alternate failure pattern of a generic term. Stress corrosion cracking of brittle fracture surface, but it may also occur in high toughness materials. Stress corrosion cracking in the necessary condition is to have tensile strength (residual stress, whether or external stress, or both) and the existence of a specific corrosive medium. Type pattern formation and expansion of roughly perpendicular to the tensile strength. This led to the stress value of stress corrosion cracking, corrosion medium than there is no need of material stress fracture is much smaller.

At the micro level, through the grain as transgranular cracks crack extension along the grain boundaries as intergranular cracks map crack, when the stress corrosion cracking extended to one depth (here, bear the load of the material cross section The stress of it in the air in the fracture stress), then the normal material on the crack (in ductile materials, usually through microscopic defects in the aggregate) and disconnect.

Therefore, due to stress corrosion cracking and failure of parts of the section, will contain the characteristics of stress corrosion cracking of the region as well as micro-defects have been linked to aggregate the "dimple" area. Stress corrosion cracking (SCC) is the cracking induced from the combined influence of tensile stress and a corrosive environment. The impact of SCC on a material usually falls between dry cracking and the fatigue threshold of that material. 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 problem itself can be quite complex. The situation with buried pipelines is a good example of such complexity. The impact is most commonly catastrophic but rarely as it was for the historical failure of the UK Flixborough chemical reactor in 1974.

Cold deformation and forming, welding, heat treatment, machining and grinding can introduce residual stresses. The magnitude and importance of such stresses is often underestimated. The residual stresses set up as a result of welding operations tend to approach the yield strength. The build-up of corrosion products in confined spaces can also generate significant stresses and should not be overlooked. SCC usually occurs in certain specific alloy-environment-stress combinations.

The micrograph on the left (X300) illustrates SCC in a 316 stainless steel chemical processing piping system.Chloride stress corrosion cracking in austenitic stainless steel is characterized by the multi-branched "lightning bolt" transgranular crack pattern.

The most effective means of preventing SCC are: 1) properly with the right materials; 2) reduce stresses; 3) remove critical environmental species such as hydroxides, chlorides, and oxygen; 4) and avoid stagnant areas and crevices in heat exchangers where chloride and hydroxide might become concentrated. Low alloy steels are less susceptible than high alloy steels, but they are subject to SCC in water containing chloride ions.

Chloride SCC

One of the most important forms of stress corrosion that concerns the nuclear industry is chloride stress corrosion. Chloride stress corrosion is a type of intergranular corrosion and occurs in austenitic stainless steel under tensile stress in the presence of oxygen, chloride ions, and high temperature. It is thought to start with chromium carbide deposits along grain boundaries that leave the metal open to corrosion. This form of corrosion is controlled by maintaining low chloride ion and oxygen content in the environment and use of low carbon steels.

Caustic SCC

the extensive qualification of Inconel for specific applications, a number of corrosion problems have arisen with Inconel tubing. Improved resistance to caustic stress corrosion cracking can be given to Inconel by heat treating it at 620°C to 705°C, depending upon prior solution treating temperature. Other problems that have been observed with Inconel include wastage, tube denting, pitting, and intergranular attack.