Wet H₂S Cracking in Carbon Steel Pipe Fittings: SSC, HIC, SOHIC, and NACE MR0103 Requirements

Hydrogen Charging in Wet H₂S Environments

The common root cause of all three wet H₂S cracking mechanisms is hydrogen charging of the steel. In aqueous H₂S environments, the cathodic reaction at the steel surface produces atomic hydrogen: H⁺ + e⁻ → H(ads). Normally, atomic hydrogen recombines at the surface to form molecular H₂ gas and escapes. However, H₂S acts as a "hydrogen recombination poison" — it adsorbs to the steel surface and inhibits the recombination reaction, increasing the fraction of atomic hydrogen that absorbs into the steel lattice rather than escaping as gas. Once absorbed, atomic hydrogen diffuses through the steel microstructure and accumulates at trapping sites (grain boundaries, carbide interfaces, inclusion interfaces, and lattice defects). The hydrogen concentration in trapping sites can far exceed the bulk hydrogen concentration, and when local hydrogen concentration exceeds the threshold for the relevant cracking mechanism, cracking initiates.

Sulfide Stress Cracking (SSC)

SSC is a form of hydrogen embrittlement that occurs at high-stress, high-hardness regions of the fitting — particularly in weld heat-affected zones where martensite or upper bainite microstructures may be present from rapid cooling. The mechanism: absorbed hydrogen accumulates at high-stress sites (stress concentrations at weld toes, notches, or transitions), reduces the cohesive strength of the steel lattice, and causes brittle fracture at applied stresses well below the normal yield strength. SSC is controlled by hardness — higher hardness (harder martensite from rapid cooling) corresponds to higher hydrogen trapping and lower fracture toughness in H₂S. NACE MR0103 limits hardness to: 22 HRC maximum (approximately 237 HV or 222 HB) for base metal and weld metal of carbon steel fittings in sour service. HAZ hardness is also limited — this is the critical region where martensite forms during welding. PWHT (Post-Weld Heat Treatment) is required by NACE MR0103 for all carbon steel fittings in sour service: PWHT at 620–680°C for minimum 1 hour per 25 mm of thickness tempers any martensite in the HAZ, reduces residual welding stress, and lowers hardness to below the 22 HRC limit. PWHT is one of the most critical requirements for sour service fittings — purchase orders must explicitly require PWHT with time-temperature records and post-PWHT hardness testing certification.

Hydrogen-Induced Cracking (HIC)

HIC is distinct from SSC — it does not require applied stress and occurs in the base metal of the fitting rather than at welds. The mechanism: atomic hydrogen absorbed from the H₂S environment accumulates at the interface between the steel matrix and elongated MnS (manganese sulfide) inclusions. MnS inclusions are a normal feature of carbon steel microstructure — they form during solidification and are rolled into flat, elongated plates during hot rolling. Hydrogen molecules (H₂) form at inclusion-matrix interfaces, building up internal pressure that exceeds the local fracture toughness and separates the inclusion from the matrix. Adjacent cracks from neighbouring inclusions link up to form the characteristic "stepwise" crack morphology of HIC — cracks run parallel to the plate surface (along the rolling direction) and are connected by short transverse steps. HIC is controlled by: MnS inclusion content and morphology — fittings manufactured from plate with low sulfur (S ≤ 0.002%, versus standard ≤ 0.030% for ASTM A234) have fewer and smaller inclusions and dramatically reduced HIC susceptibility. HIC-resistant plate (tested per ASTM G39 / NACE TM0284) is specified for fittings in sour service; calcium treatment — calcium injection during steelmaking modifies MnS inclusions from elongated plates to small, near-spherical CaS particles that are far less susceptible to HIC; and carbon equivalent — lower CE steels have simpler microstructures with fewer hydrogen trapping sites.

Stress-Oriented HIC (SOHIC)

SOHIC is a combination of SSC and HIC mechanisms: HIC crack arrays form in the base metal (from inclusion-associated hydrogen) and are then linked by SSC-type cracking driven by applied or residual stress. SOHIC creates through-wall cracks by connecting HIC stepwise cracks with transverse fracture segments — the result is a crack path that can penetrate the full wall thickness of the fitting even where individual HIC cracks would be limited to a shallow band. SOHIC is most common near welds, where residual stress combines with the base metal's HIC susceptibility. Prevention requires both HIC-resistant plate (low S, Ca-treated) and PWHT (to reduce residual stress and HAZ hardness). NACE TM0103 and EFC Publication 16 provide guidance on SOHIC testing and assessment. For carbon steel fittings in refinery sour service — wet H₂S above 50 ppm H₂S in the gas phase and free water present — the complete specification should include: ASTM A234 WPB with S ≤ 0.002% and Ca treatment; NACE TM0284 HIC test on the plate heat (acceptance criteria: CLR ≤ 15%, CTR ≤ 5%, CSR ≤ 2%); PWHT at 620–680°C; post-PWHT hardness ≤ 22 HRC; and NACE MR0103 compliance statement on the material test certificate.