NACE MR0175: Sour Service Pipe Material Selection Guide

NACE MR0175 / ISO 15156 defines material limits for oil and gas equipment exposed to H2S to prevent sulfide stress cracking (SSC) and stress-oriented hydrogen-induced cracking (SOHIC). It applies when the service is sour — typically H2S present with water and a pH such that the environment is cracking-susceptible. The standard sets allowable hardness, heat treatment, and chemistry by material class (for example CRAs, carbon and low-alloy steels). TOKO TECH supplies sour-qualified 316L, 2205, 825, and 625 with documentation traceable to the standard's Annex tables.

For corrosion-resistant alloys in sour service, NACE MR0175 generally caps hardness at HRC 22 (or 250 HV) for solid CRAs and weld zones to avoid SSC. Hardness rises from cold work, improper heat treatment, or welding without post-weld heat treatment. Control it via solution anneal, qualified welding, and post-weld hardness checks. TOKO TECH performs or witnesses hardness testing and PMI, and supplies material with EN 10204 3.2 and hardness certs so buyers meet the HRC 22 ceiling for valves, flanges, and fittings.

In increasing sour and corrosion capability: 316L resists mild H2S but is chloride-limited; duplex 2205 suits mildly sour plus high chloride; alloy 825 (N08825) handles moderate H2S/CO2 and oxidizing acids; Inconel 625 (N06625) covers severe H2S, chlorides, and high temperature. All must be in the soft, solution-annealed condition and within NACE chemistry. TOKO TECH stocks these sour-qualified grades as pipe, tube, fittings, and flanges with full MTRs mapped to ISO 15156 Annex.

SSC (sulfide stress cracking) is hard, tensile-stress cracking of high-strength or martensitic materials under H2S; it is the primary NACE concern and is avoided via low hardness (HRC 22) and soft microstructures. SZC or SOHIC (stress-oriented hydrogen-induced cracking) occurs in softer steels along bands or laminations under applied and residual stress. CRAs mostly face SSC risk; carbon steels can see both. TOKO TECH specifies grade and heat treatment so the selected material's failure mode is controlled per NACE MR0175 for the actual H2S level.

Sour-service CRAs must be in the solution-annealed (soft) condition — done correctly, it dissolves carbides and precipitates and prevents sensitization that worsens SSC. Verify with PMI (positive material identification) to confirm chemistry (Cr, Mo, Ni, N) matches the cert, and with a hardness test to confirm HRC at or below 22. TOKO TECH supplies solution-annealed 316L, 2205, 825, and 625 and performs PMI plus hardness verification, giving buyers documented assurance the shipment is NACE-compliant before it reaches the yard.

NACE MR0175 uses H2S partial pressure (pH2S) and pH to define sour severity. A common threshold: pH2S at or above 0.0003 psi (about 0.002 kPa) with water present puts a system in sour regime for carbon and low-alloy steels; CRAs have their own Annex C limits based on pH2S, chloride, and temperature. Above the limit, only qualified CRAs (2205, 825, 625) in soft condition apply. TOKO TECH helps interpret your pH2S, pH, and chloride data against Annex tables to pick the right grade and heat treatment.