Stress Corrosion cracking (SCC)
this phenomenon can be defined as material cracking due to the combined effect of static stress and corrosion. The stress involved can be residual stress and actual stress from an external load. The corrosion that occurs is generally very localized. Localized corrosion on the surface of the material usually acts as a stress raiser or stress concentrator. This corrosive cracking can be either transgranular or intergranular cracking depending on the type of alloy, metallurgical and environmental conditions.
there are 3 main causes that determine the occurrence of Stress Corrosion Cracking are:
- Tensile stress. This tensile stress can be a residual stress that occurs due to welding or metal forming and can be a real stress due to the workload received by the material. For example, a pipeline or pressure vessel that is operated with internal pressure.
- Temperature. In general, cases of Stress Corrosion Cracking occur at temperatures > 60 C
- The availability of certain ions, such as Chloride (Cl) ions. Until recently, engineers still have difficulty determining definite limits on the minimum limits for Stress Corrosion Cracking triggers
In general, Stress corrosion cracking resistant materials are stainless steel duplex or super-ferritic type.
avoiding residual stress to minimize stress corrosion cracking
Residual stress is defined as the stress acting on a material after the outer stress acting on the material is removed. In the case of welding, this outer stress is a thermal stress due to expansion during heating and shrinkage during cooling or temperature differences. The residual stress follows static equilibrium rules where the residual tension stress on the welding structure is equal to the residual compression stress so that the resultant stress is equal to zero. the highest residual stress location is in welding fusion and HAZ (heat affected zone). A welding engineer is responsible to design the welding that having minimum residual stress.