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Key takeaways
- NACE MR0175 / ISO 15156 sets material requirements for equipment in H2S-containing environments in oil and gas production.
- For carbon steel, sulfide stress cracking (SSC) requirements apply when the H2S partial pressure is 0.3 kPa (0.05 psia) or more.
- The best-known rule is a hardness limit: carbon and low-alloy steel base metal and welds generally at or below 22 HRC (about 250 HV).
What “sour” means
Hydrogen sulfide (H2S) in the presence of water causes atomic hydrogen to enter steel. In hard or highly stressed steel this leads to sulfide stress cracking (SSC), a brittle cracking mechanism that can fail a component quickly and without visible corrosion. Other mechanisms include hydrogen-induced cracking (HIC) in plate and pipe, and stress-oriented HIC.
The standard
NACE MR0175 / ISO 15156 applies to oil and gas production and processing equipment: wells, flowlines, gathering systems, gas plants and offshore facilities. It has three parts:
- Part 1: general principles for selecting cracking-resistant materials.
- Part 2: carbon and low-alloy steels, including the H2S threshold and hardness limits.
- Part 3: corrosion-resistant alloys (CRAs) such as stainless, duplex and nickel alloys, with environmental limits for each.
Refinery and downstream units use a related standard, NACE MR0103 / ISO 17945.
When does a line become sour?
For carbon and low-alloy steels, Part 2 requires SSC-resistant materials when the partial pressure of H2S in the gas phase is 0.3 kPa (0.05 psia) or more. Partial pressure is the mole fraction of H2S times the total absolute pressure. A small concentration at high pressure can easily cross the line.
Example: 100 ppm H2S (0.0001 mole fraction) at 50 bar absolute gives a partial pressure of 0.005 bar, which is 0.5 kPa. That is sour.
What changes in the piping class
- Hardness limits on base metal, welds and heat-affected zones, commonly 22 HRC (250 HV) for carbon steel, verified by hardness testing of the welding procedure.
- Welding procedures qualified with hardness surveys, and sometimes PWHT to bring hardness down.
- HIC-tested pipe and plate for wet sour service, such as API 5L Annex H pipe.
- Bolting and small parts in compliant materials, which is often forgotten for instrument fittings and valve trim.
- Material certificates stating compliance with the standard.
Code reference
NACE MR0175 / ISO 15156 Materials for use in H2S-containing environments in oil and gas production.
NACE MR0103 / ISO 17945 Metallic materials resistant to SSC in corrosive petroleum refining environments.
API 5L Annex H PSL2 line pipe for sour service.
What to take back to your desk
Ask process for the H2S content and the operating pressure range, including upset cases, and calculate the partial pressure yourself. Then check that the line class, welding procedures and valve datasheets all say “sour service” consistently.
Educational content. Always design to your project specification.
