What Is Stress Corrosion Cracking (SCC)?
Quick Answer: Stress corrosion cracking (SCC) is a failure mechanism that causes brittle fracture of a normally ductile metal under the combined action of tensile stress and a specific corrosive environment. SCC requires all three conditions simultaneously: a susceptible material, a critical environment, and sufficient tensile stress (applied or residual). Remove any one factor and SCC cannot occur.
The Three Requirements for SCC
| Condition | Description | Examples |
|---|---|---|
| Susceptible material | Specific alloy-environment combinations are vulnerable; not all metals crack in all environments | Austenitic stainless steel in chlorides; carbon steel in caustic (NaOH); brass in ammonia |
| Corrosive environment | A specific chemical species must be present above a threshold concentration and temperature | Chlorides >50 ppm + temperature >60°C for 304/316; H2S + water for carbon steel (SSC) |
| Tensile stress | Applied load, pressure stress, thermal stress, or residual stress from welding or cold work | Residual welding stress can reach yield strength without any applied load |
The insidious nature of SCC is that failure can occur at stress levels well below the material’s yield strength and with negligible general corrosion. Cracking propagates rapidly once initiated, often with no visible warning.
Common Material-Environment SCC Pairs
| Material | Cracking Environment | Crack Morphology | Relevant Standard |
|---|---|---|---|
| 304/316 austenitic SS | Chlorides (Cl-) at >60°C | Transgranular, branching | NACE MR0175 Part 3 |
| 304/316 austenitic SS | Polythionic acid (H2SxO6) | Intergranular | NACE SP0170 |
| Carbon steel | Caustic soda (NaOH) at >50°C | Intergranular | API RP 571 |
| Carbon steel | H2S + water (SSC mode) | Transgranular, brittle | NACE MR0175/ISO 15156 |
| Carbon steel | Carbonates/bicarbonates | Intergranular | NACE SP0472 |
| Duplex stainless steel | Chlorides at high temperature | Mixed mode | NACE MR0175 Part 3 |
| Copper alloys (brass) | Ammonia (NH3), amines | Intergranular | - |
| Nickel alloy 600 | High-temperature caustic, chlorides | Intergranular | ASME Section III |
Chloride SCC in Stainless Steel
Chloride SCC is the most common form in oil and gas and chemical processing piping. Austenitic grades 304L and 316L are highly susceptible above 60°C (140°F) when chloride ions are present; failures have been reported at concentrations as low as 10 ppm in evaporative conditions. Dissolved oxygen and neutral-to-acidic pH accelerate crack initiation.
Duplex stainless steels (2205, 2507) offer significantly higher chloride SCC resistance due to their ferritic-austenitic microstructure.
Prevention Methods
| Strategy | Application |
|---|---|
| Material selection | Use duplex, super duplex, or nickel alloys instead of austenitic SS in chloride + high-temperature service |
| Stress relief | Post-weld heat treatment (PWHT) reduces residual stresses below the SCC threshold |
| Temperature control | Keep operating temperature below the SCC threshold for the material-environment pair |
| Environmental control | Remove or reduce the aggressive species (e.g., chloride stripping, caustic concentration control) |
| Coatings/linings | Barrier coatings (pipe cladding) isolate the susceptible metal from the corrosive environment |
| Cathodic protection | Shifts the metal potential away from the SCC-susceptible range (limited applicability) |
| Shot peening | Introduces compressive residual stress on the surface, counteracting tensile stress |
NACE MR0175/ISO 15156 for Sour Service
For piping in wet H2S, NACE MR0175/ISO 15156 defines material requirements to prevent sulfide stress cracking (SSC), a specific form of SCC. Requirements include hardness limits (22 HRC max for carbon steel), acceptable heat treatment conditions, and cold work restrictions. All materials (base metal, weld, and HAZ) must be verified through hardness testing and mill test certificate review.
Leave a Comment
Have a question or feedback? Send us a message.