NACE MR0175 / ISO 15156 governs metallic materials for H2S sour service in oil and gas. Operators and EPCs must comply when H2S partial pressure hits defined limits. Core rule: carbon and low-alloy steels must stay ≤22 HRC (250 HV10) in parent metal, weld, and HAZ, with hardness records on the MTC.
Overview
NACE MR0175 / ISO 15156 is the international standard governing the selection and qualification of metallic materials for use in oil and gas production environments containing hydrogen sulfide (H2S), commonly referred to as "sour service." H2S is highly corrosive to steel and can cause catastrophic sulfide stress cracking (SSC) if materials are not properly controlled. The standard establishes hardness limits (typically 250 HV10 or 22 HRC maximum for carbon and low-alloy steels), metallurgical requirements, and environmental limits (H2S partial pressure, pH, chloride concentration) within which approved materials may be safely used. NACE MR0175 is divided into three parts: Part 1 covers general principles for all equipment; Part 2 addresses cracking-resistant carbon and low-alloy steels; Part 3 covers corrosion-resistant alloys (CRAs) including stainless steels and nickel alloys. Compliance with NACE MR0175 is mandatory for upstream oil and gas equipment in sour fields, including wellhead components, pipelines, and process piping.
Understanding sour service and H2S cracking mechanisms
Sour service is defined as exposure to H2S at concentrations and pressures sufficient to cause sulfide stress cracking (SSC) or stress corrosion cracking (SCC). SSC is a brittle fracture mechanism affecting high-strength steels under tensile stress in H2S environments.
The severity of sour service is classified by ISO 15156 using H2S partial pressure (pH2S) and in-situ pH. Region 0 (no H2S) requires no special measures. Regions 1–3 define progressively severe conditions requiring increasingly stringent material controls.
Hardness is the primary control parameter: for carbon and low-alloy steels, hardness must not exceed 250 HV10 (or 22 HRC, or 237 HBW) to prevent SSC. Higher hardness increases susceptibility exponentially.
Hardness limits and metallurgical controls
- Carbon and low-alloy steels: 250 HV10 max (22 HRC, 237 HBW) — mandatory for sour service
- Cold-worked surfaces: Must limit cold deformation to 5% max (measured by dimensional change)
- Welds: Parent metal, HAZ, and weld metal must all meet hardness limits; PWHT often required
- Stainless steels (austenitic): Annealed condition required; 22 HRC max for some grades
- Duplex stainless: 28–32 HRC max depending on grade and specific sour service region
- CRAs (Nickel alloys, Titanium): Generally resistant but must be qualified for specific conditions
Material selection criteria for H2S service
The standard provides tables of acceptable materials qualified for specific sour service regions. Selection requires knowledge of the operating environment: H2S partial pressure (in kPa or psi), operating temperature, chloride concentration, and pH.
For carbon steel line pipe (API 5L), PSL2 grades are acceptable when ordered with NACE compliance and hardness testing. The manufacturing route must control chemistry and heat treatment to guarantee hardness below 250 HV10.
For downhole tubing and casing, low-alloy steels (L80, C90, T95, P110) have specific hardness and chemistry requirements. The "L" in L80 stands for "restricted yield" — a special chemistry and processing route for sour service.
Testing and qualification requirements
- Hardness testing: Vickers (HV10) is the reference method; Rockwell C (HRC) or Brinell (HBW) may be used with conversion
- Test locations: Parent metal, weld metal, heat-affected zone (HAZ) — three readings per location minimum
- SSC testing: Required for new alloys or when operating outside qualified envelopes (Method A per NACE TM0177)
- Documentation: Material must carry NACE compliance certificate with hardness test results
- Traceability: Heat number and testing records must link to the specific sour service qualification