How it works
When steel corrodes in water containing hydrogen sulfide, the corrosion reaction releases atomic hydrogen at the surface. Normally those atoms combine into hydrogen gas and escape, but sulfide poisons that recombination, so a large share of the hydrogen enters the metal instead. The atoms diffuse through the steel until they reach internal discontinuities — elongated manganese sulfide inclusions, laminations, segregated bands — where they recombine into molecular hydrogen that can no longer diffuse out. Pressure builds in the trap.
Near the surface, that pressure lifts the metal into hydrogen blisters. Deeper in the wall, cracks form along the inclusion planes and link between adjacent planes, giving the characteristic stepwise pattern of HIC. No applied or residual stress is needed; the driving force is the trapped gas.
HIC, SOHIC and SSC
These three wet H2S damage mechanisms are often confused but differ in what drives them:
| Mechanism | Driving force | Typical appearance |
|---|---|---|
| HIC | Internal hydrogen pressure at inclusions | Stepwise cracks parallel to the plate surface, blisters |
| SOHIC (stress-oriented HIC) | Hydrogen damage aligned by tensile stress | Stacked small cracks running through the thickness, often near welds |
| SSC (sulfide stress cracking) | Hydrogen embrittlement under tensile stress | Through-thickness cracking in hard, high-strength zones such as weld deposits and heat-affected zones |
How resistance is tested and specified
NACE TM0284 is the standard laboratory test for HIC resistance of pipeline and pressure vessel steels: specimens are exposed to an H2S-saturated solution, then sectioned and the cracks measured as crack length, thickness and sensitivity ratios. Acceptance limits are set by the purchase specification. For oil and gas production equipment, ISO 15156 (NACE MR0175) sets material requirements for H2S-containing environments, including HIC and SSC resistance of carbon and low-alloy steels.
What to do about it
- Material: HIC-resistant plate made with low sulfur and inclusion shape control, qualified by TM0284 testing.
- Barrier: linings, coatings or corrosion-resistant cladding that keep wet H2S off the steel.
- Process: reduce the aggressiveness of the water phase with inhibitors or by controlling contaminants that promote hydrogen charging.
- Inspection: HIC is internal and does not reduce wall thickness at the surface, so a corrosion allowance offers no protection and spot thickness readings miss it. Ultrasonic scanning detects laminar indications; found damage is evaluated by a fitness-for-service assessment under API 579-1/ASME FFS-1. For the crack family it belongs to, see stress corrosion cracking.