The three conditions
SCC requires three conditions at the same time:
- a susceptible material — susceptibility is specific to the alloy, its heat treatment and its microstructure;
- tensile stress — applied stress from pressure or loading, and very often residual stress from welding, cold forming or fit-up, which is present even in an unloaded component;
- a specific environment — not simply a corrosive one, but a particular species for that alloy family, frequently at elevated temperature or concentration.
Remove any one and cracking does not occur. That is the basis of every prevention strategy.
Classic combinations
| Material | Environment | Common name |
|---|---|---|
| Austenitic stainless steels (304, 316) | Chlorides, especially hot | Chloride SCC |
| Carbon steel | Hot concentrated caustic (NaOH) | Caustic SCC, caustic embrittlement |
| Carbon steel | Amine solutions, carbonate solutions | Amine SCC, carbonate SCC |
| Brass and copper alloys | Ammonia | Season cracking |
Chloride SCC of austenitic stainless is common under insulation and at evaporation points where chlorides concentrate. Cracking in hydrogen sulfide environments is closely related and is governed for oil and gas equipment by ISO 15156 (NACE MR0175).
How it looks and how it is found
SCC produces little or no general corrosion. Cracks are fine, often branched, and run roughly perpendicular to the tensile stress. Chloride SCC in austenitic stainless is typically transgranular; caustic SCC in carbon steel is typically intergranular. Cracks often start at a surface defect — a pit, a weld toe, a crevice — which concentrates stress and chemistry.
Detection relies on crack-sensitive methods: dye penetrant, wet fluorescent magnetic particle on carbon steel, and shear-wave or phased-array ultrasonics for depth. Thickness gauging will not find it. Once cracks are sized, their acceptability is a fracture mechanics question handled through a fitness-for-service assessment rather than by visual judgement.
What to do about it
- Material: select an alloy resistant in that environment. For chloride service, duplex and higher-alloy grades outperform standard austenitic grades; PREN screens pitting resistance, not SCC, so it is only part of the selection.
- Stress: reduce residual stress by post-weld heat treatment where the service calls for it, as is common practice for carbon steel in caustic and amine service.
- Environment: control the species — keep chlorides off stainless surfaces, prevent concentration by evaporation, and coat the metal where external exposure is the source, as under insulation.