HIC vs SSC vs SOHIC: Distinguishing Hydrogen Damage Mechanisms in Sour Service Pipe Fittings

The Common Cause: Atomic Hydrogen from H₂S

In all three mechanisms, the trigger is the same: H₂S in a wet environment acts as a hydrogen recombination poison. When steel corrodes in acidic aqueous solution, atomic hydrogen (H⁰) is normally produced at the surface and rapidly recombines to H₂ gas which escapes. H₂S inhibits this recombination, so a larger fraction of H⁰ absorbs into the steel lattice before it can escape. It is the absorbed atomic hydrogen — not molecular H₂ gas — that drives all three damage mechanisms.

Hydrogen Induced Cracking (HIC)

HIC occurs when atomic hydrogen diffuses to non-metallic inclusions (manganese sulphide stringers, silicate clusters) within the steel and recombines to H₂ at these internal traps. The molecular hydrogen cannot diffuse back out, so pressure builds until internal blisters form — typically parallel to the rolling direction of the plate or pipe blank. In buttweld fittings made from hot-formed plate, HIC blisters appear as step-like internal cracks that are parallel to the fitting wall. HIC does not require applied or residual stress — it is purely an internal pressure mechanism driven by inclusion density. The primary control is steel cleanliness: NACE TM0284 tests specifically for HIC resistance, requiring Ca-treatment of the melt to spheroidise MnS inclusions and limiting sulphur to ≤0.003% for HIC-resistant plate. Ultrasonic testing of the base material before forming is the inspection method.

Sulphide Stress Cracking (SSC)

SSC is a form of hydrogen embrittlement that requires the simultaneous presence of tensile stress (applied or residual), H₂S, and a susceptible microstructure — specifically hard phases above approximately 22 HRC (237 HBW). The absorbed hydrogen segregates to highly stressed regions at notches, weld toes, or heat-affected zones and reduces the local fracture energy until brittle cracking initiates. SSC is most aggressive at ambient and near-ambient temperatures (below ~80°C) — at higher temperatures, hydrogen mobility is sufficient that it does not concentrate at stress raisers. The primary control is hardness limitation: NACE MR0175 / ISO 15156-2 limits carbon and low-alloy steel to ≤22 HRC throughout, including the weld and HAZ. This is why PWHT is required for all sour service fittings regardless of wall thickness — to temper the HAZ martensite below the hardness limit.

Stress-Oriented Hydrogen Induced Cracking (SOHIC)

SOHIC is a hybrid mechanism combining elements of both HIC and SSC. It produces a staircase crack pattern running through the wall thickness — arrays of HIC blisters linked by SSC-type cracks running perpendicular to the applied stress. SOHIC requires both a susceptible inclusion population (like HIC) and tensile stress (like SSC), and it tends to occur in the HAZ near welds where residual stress is highest. SOHIC is considered the most dangerous of the three because it propagates rapidly through wall thickness and can cause through-wall cracking with little warning. It is most common in vessels and thick-wall piping components. Inspection by wet fluorescent magnetic particle testing (WFMT) at the weld toe and phased array UT of the adjacent base metal is the standard method.

Comparison Table

Procurement Implications

For sour service pipe fittings per NACE MR0175 / ISO 15156, the specification must explicitly state which mechanisms are addressed. A simple "sour service per ISO 15156" clause covers SSC hardness limits but does not automatically invoke HIC testing (NACE TM0284) or SOHIC-specific weld procedure requirements. Projects in wet H₂S service should specify all three controls separately: base material HIC test per NACE TM0284; PWHT with hardness verification ≤22 HRC throughout; and weld procedure qualification including SOHIC-prone zone inspection. Arshya Pipe Fittings can provide Ca-treated, low-sulphur heats with NACE TM0284 HIC test certificates and full PWHT records for sour service applications.