NACE MR0175 / ISO 15156: Material Selection for Sour Service Pipe Fittings

What Makes a Service "Sour"

NACE MR0175 / ISO 15156-1 defines sour conditions by the partial pressure of H₂S in the gas phase. For gas systems: any H₂S partial pressure ≥0.0003 MPa (0.05 psia) constitutes sour service. For multiphase or liquid systems, the threshold depends on pH and chloride concentration — see ISO 15156-2 Fig. 1 for the pH / H₂S partial pressure envelope. Once a system is classified as sour, all pressure-retaining metallic materials — including pipe fittings — must comply.

Sulphide Stress Cracking (SSC)

SSC is hydrogen embrittlement driven by atomic hydrogen generated by the H₂S corrosion reaction. High-strength, hard, or martensitic microstructures are most susceptible. Carbon and low-alloy steels must be in the normalised or PWHT condition with hardness ≤22 HRC (≤248 HBW) — this is the key number cited in NACE MR0175. ASTM A234 WPB fittings supplied in the as-rolled condition frequently exceed this limit at weld seams and must be normalised before use in sour service.

Grade Qualification by Material Family

• Carbon steel (WPB): Must be normalised or normalised-and-tempered. Maximum hardness 22 HRC / 248 HBW. Sulphur ≤0.010% (low-sulphur steel required in aggressive environments).
• Alloy steel CrMo (P11, P22): Post-weld heat treated, hardness ≤22 HRC. P91 and P92 have specific provisions in ISO 15156-2 Table A.3 — consult clause by clause.
• Austenitic stainless steel (316L, 304L, 321): Generally acceptable with limitations. Cold-work must not raise hardness above 22 HRC. No σ phase or sensitisation. Susceptible to SSC at yield strengths above 760 MPa — not an issue for standard A403 fittings in solution-annealed condition.
• Duplex 2205: Permitted under ISO 15156-3 in the solution-annealed condition. Ferrite number FN 30–70. Maximum hardness 28 HRC (higher than carbon steel because austenite phase provides resistance).
• Inconel 625 (N06625): Qualified under ISO 15156-3. No PWHT required. One of the most robust choices for combined sour + chloride environments.

What Must Appear on the Certificate

For sour service procurement, the EN 10204 3.1 (or 3.2) certificate must show: heat chemical analysis confirming sulphur ≤0.010% for low-sulphur steel, hardness test results (Brinell or Vickers converted to Rockwell C equivalent, per heat or per fitting for critical applications), heat treatment condition, and a statement of NACE MR0175 / ISO 15156 compliance. Simply marking "sour service" on the purchase order without verifying the certificate content is a common audit failure.

PMI for Sour Service

All alloy steel and stainless/nickel alloy fittings for sour service should be subject to 100% PMI (positive material identification) by XRF at goods receipt. Substituted or mislabelled materials in sour service are a safety-critical non-conformance — the consequences of an SSC failure in a live sour gas system are severe. Arshya Pipe Fittings provides PMI reports as standard for all alloy, stainless, and nickel alloy fittings.