Stress corrosion cracking (SCC) is the growth of cracks through an alloy, typically along grain boundaries. It needs at least two conditions acting together:
- Tensile stress in the material, even below its yield strength. This normally comes from an external tensile load, but in larger sections or welded regions it can also arise from internal stresses. Poor design that introduces sharp edges or notches raises the internal stresses further.
- A corrosive medium, primarily chloride-containing or sour (hydrogen-sulphide containing). Pitting corrosion is often a precursor, both accelerating the corrosion and acting as a stress concentrator.
SCC has been a major concern in oil and gas applications, particularly in sour (H2S) environments. The NACE MR0175 standard was developed to identify alloys that can be confidently specified across a range of operating conditions, taking into account temperature, chloride concentration and the partial pressure of H2S.
Preventing stress corrosion cracking
SCC can be limited in several ways: specifying more corrosion-resistant alloys; introducing compressive surface stresses by shot-peening; reducing internal residual stresses through heat treatment; and controlling the mechanical properties of SCC-resistant alloys. The NACE MR0175 standard sets hardness limits for relevant alloys to help reduce the likelihood of cracking.
The mixed austenitic and ferritic microstructure of duplex and super duplex stainless steels, along with other corrosion-resistant alloys, gives some inherent resistance to SCC. Combined with their higher strength and excellent corrosion resistance, this makes them well suited to many demanding applications.