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. It is this absorbed atomic hydrogen that drives all three mechanisms. Once absorbed, it diffuses through the steel and accumulates at trapping sites (grain boundaries, carbide interfaces, inclusion interfaces and lattice defects). The hydrogen concentration at trapping sites can far exceed the bulk concentration, and when the local concentration exceeds the threshold for the relevant mechanism, cracking initiates.

Sulfide Stress Cracking (SSC)

SSC is a form of hydrogen embrittlement that needs the simultaneous presence of tensile stress (applied or residual), H₂S and a susceptible, hard microstructure — particularly in weld heat-affected zones where martensite or upper bainite may form from rapid cooling. Absorbed hydrogen accumulates at high-stress sites (weld toes, notches, transitions), reduces the cohesive strength of the steel lattice, and causes brittle fracture at stresses well below the normal yield strength. SSC is most severe at ambient and near-ambient temperatures and becomes less likely as temperature rises, because hydrogen is then less able to concentrate at stress raisers.

SSC is controlled by hardness. For carbon steel, NACE MR0175/ISO 15156-2 and NACE MR0103 limit base metal hardness to 22 HRC (about 237 HBW), and they control weld and HAZ hardness through hardness limits and weld procedure qualification. PWHT is one way of meeting these limits — it tempers any martensite in the HAZ, reduces residual welding stress and lowers hardness — and many refinery and project specifications require it for welded sour-service components. But the standards do not require PWHT for every carbon steel fitting regardless of thickness; what they require is that the hardness limits are met and demonstrated. Where PWHT is specified, it is carried out in the code temperature range for P-No. 1 carbon steel with a hold time based on thickness, and the purchase order should call for time-temperature records and post-PWHT hardness test results on the certificate.

Hydrogen-Induced Cracking (HIC)

HIC is distinct from SSC — it does not require applied or residual stress and occurs in the base metal of the fitting rather than at welds. Atomic hydrogen accumulates at the interfaces between the steel matrix and elongated non-metallic inclusions, mainly manganese sulphide (MnS) stringers. MnS inclusions form during solidification and are rolled into flat, elongated plates during hot rolling. Hydrogen recombines to molecular H₂ at these internal interfaces; the molecular hydrogen cannot diffuse back out, so pressure builds until it separates the inclusion from the matrix, sometimes forming blisters near the surface. Cracks from neighbouring inclusions link up to form the characteristic "stepwise" HIC morphology — cracks running parallel to the plate surface (along the rolling direction) connected by short transverse steps. In buttweld fittings made from hot-formed plate, HIC appears as step-like internal cracks parallel to the fitting wall.

HIC is controlled by steel cleanliness and chemistry:

• Very low sulphur: HIC-resistant steel is made with sulphur far below the limit permitted by the standard fitting specification (project specifications typically set a maximum of a few thousandths of a percent), giving fewer and smaller inclusions.
• Calcium treatment: calcium injection during steelmaking modifies MnS inclusions from elongated plates to small, near-spherical particles that are far less susceptible to HIC.
• Low carbon equivalent and controlled segregation: lower-CE steels with less centreline segregation have fewer hard bands and hydrogen trapping sites.
• HIC testing: resistance is verified by NACE TM0284 testing of the plate or pipe heat, reported as crack length, thickness and sensitivity ratios (CLR, CTR, CSR). Ultrasonic testing of the starting material for laminar indications is the corresponding inspection method.

Stress-Oriented HIC (SOHIC)

SOHIC is a hybrid of HIC and SSC: arrays of small HIC cracks in the base metal are stacked and linked by SSC-type cracking driven by applied or residual stress, producing a staircase crack pattern through the wall thickness, oriented perpendicular to the stress. The result is a crack path that can penetrate the full wall of the fitting even where individual HIC cracks would be limited to a shallow band. SOHIC is most common in the base metal next to welds, where residual stress combines with the base metal's HIC susceptibility, and it is considered the most dangerous of the three because it can propagate through-wall with little warning. It is most often found in vessels and thick-wall piping components. Prevention requires both HIC-resistant material (low sulphur, Ca-treated) and PWHT to reduce residual stress and HAZ hardness. NACE TM0103 and EFC Publication 16 provide guidance on SOHIC testing and assessment. Inspection in service uses wet fluorescent magnetic particle testing (WFMT) at the weld toe and phased array UT of the adjacent base metal.

HIC vs SSC vs SOHIC — Comparison

Procurement Implications

A simple "sour service per NACE MR0103" or "per ISO 15156" clause addresses the SSC hardness limits, but it does not automatically invoke HIC testing to NACE TM0284 or SOHIC-specific requirements. NACE MR0103 defines when a refinery environment counts as wet H₂S service (based on H₂S in the free water, pH, cyanides and H₂S partial pressure); once a line is in that category, the specification should address all three mechanisms separately. For carbon steel buttweld fittings in wet H₂S service, a complete specification typically includes:

• ASTM A234 WPB (or WPC) with the project's low sulphur limit and calcium treatment for HIC resistance
• NACE TM0284 HIC test on the plate or pipe heat, with stated acceptance criteria (CLR ≤ 15%, CTR ≤ 5%, CSR ≤ 2% are commonly used)
• Hardness limits for base metal, weld and HAZ (22 HRC maximum for base metal) with hardness test results reported
• PWHT where the project specification requires it, with time-temperature records
• Weld procedure qualification with HAZ hardness surveys
• NACE MR0103 or MR0175/ISO 15156 compliance statement on the EN 10204 3.1 (or 3.2) certificate

State these requirements on every line item so that the supplier can confirm the material source and testing before order acceptance.