Sulfide Stress Cracking
Sulfide stress cracking (SSC) is a hydrogen-embrittlement-related cracking mode in which susceptible high-strength steels crack under tensile stress while exposed to water and hydrogen sulfide (H₂S). It is a progressive brittle failure driven by corrosion-generated hydrogen, applied or residual stress, and a hard microstructure, making it a major concern for sour-service oilfield, valve, and pipeline components.
On the shop floor, SSC matters whenever a machined or welded part is destined for sour service—oilfield hardware, valve trim, BOP parts, or pipelines in wet H₂S. The shop controls risk through material selection, hardness, and residual stress. Parts must comply with ISO 15156/NACE MR0175 hardness limits; welds and heat-affected zones are the first places to check because hydrogen concentrates in hard spots. Fabrication processes such as welding, aggressive forming, and heavy cold work can leave tensile residual stress, so stress relief, PWHT, controlled heat input, and low-hydrogen consumables are common safeguards. On the CNC side, finishing practice matters: excessive heat, worn tooling, or tearing leaves a stressed surface layer that reduces tolerance for hydrogen-assisted cracking. Inspection combines portable hardness testing with wet fluorescent magnetic particle testing to catch surface cracks before release. The goal is a clean, low-stress surface with base material and heat-affected zones within sour-service limits.
Is sulfide stress cracking the same as stress corrosion cracking?
No. SSC is commonly treated as a hydrogen embrittlement/cathodic cracking mechanism tied to wet H₂S service, while stress corrosion cracking is a broader term covering other corrosive cracking systems.
What environmental condition is required for SSC?
The classic condition is wet H₂S exposure; water enables corrosion reactions, H₂S promotes hydrogen entry, and tensile stress allows cracking to progress.
Why do high-hardness areas crack first?
Hard microstructures are less tolerant of hydrogen-assisted plastic deformation, so atomic hydrogen accumulates and embrittles the region, making crack initiation and propagation easier.