What is stress corrosion cracking? Corrosion
Stress corrosion cracking (SCC) is identified by cracks propagating through alloys, typically along grain boundaries. It requires at least two conditions to be acting in combination:
1/ Tensile stresses in the material, although below the actual yield strength. The stress seen by the metal is normally from an external tensile load, but in extreme circumstances can also be associated with internal stresses i.e. in larger sections or welded regions. The poor design of components to include sharp edges or notches will also raise the internal stresses experienced.
2/ A corrosive medium, primarily chloride-containing or sour i.e. hydrogen-sulphide containing. Pitting corrosion can often be a precursor, both accelerating the corrosion witnessed by the metal and acting as a stress-concentrator in the component.
This failure mechanism was a major concern for oil and gas applications, particularly in sour (H2S) environments. The NACE MR-1075 standard was developed to identify alloys that could be confidently specified in a range of operating conditions. As well as the temperature and chloride concentration, it also considers the partial pressure of H2S observed.
Stress corrosion cracking can be prevented by using more corrosion resistant alloys; introducing compressive stresses to a component, through shot-peening the surface; reducing internal residual stresses through heat treatment; using SCC-resistant alloys and controlling their mechanical properties. The NACE MR-1075 standard suggests hardness limits for relevant alloys that should help to limit the likelihood of stress corrosion cracking.
The combination of the mixed austenitic and ferritic grains within duplex and super duplex stainless steels infers some resistance to stress corrosion cracking. When combined with their higher strength and excellent initial corrosion resistance it makes them well-suited to many applications.
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