Technical guide
NACE MR0175 / ISO 15156: Sour Service Equipment Explained
Why H2S matters
Hydrogen sulfide lets atomic hydrogen enter steel. That can cause sulfide stress cracking (SSC) in hard, high-strength materials and hydrogen-induced cracking (HIC) in plate and pipe. Both can lead to sudden, brittle failure below the design stress. Because H2S is also toxic, a failure in sour service is a serious safety event.
The three parts of the standard
- Part 1: general principles for selecting cracking-resistant materials.
- Part 2: carbon and low-alloy steels. Hardness is limited to 22 HRC for most steels, with SSC regions defined by H2S partial pressure and pH.
- Part 3: corrosion-resistant alloys (CRAs), such as 13Cr, duplex, 6Mo and nickel alloys, with environmental limits for each.
What it means when you order equipment
| Item | Typical sour-service requirement |
|---|---|
| Line pipe | API 5L PSL2 Annex H, HIC test to NACE TM0284, SSC test to NACE TM0177 |
| Casing and tubing | API 5CT L80-1, C90, T95, C110 or proprietary SS grades |
| Wellhead equipment | API 6A material class DD, EE, FF or HH |
| Valves | NACE-compliant body, trim and bolting, with hardness records |
| Bolting | ASTM A193 B7M / A194 2HM (22 HRC max) where exposed |
For refinery sour service, the parallel standard is NACE MR0103 / ISO 17945.
Frequently asked questions
At what H2S level is equipment considered sour service?
Under NACE MR0175 / ISO 15156, special materials are required when the partial pressure of H2S in the gas phase is 0.05 psia (0.3 kPa) or greater. Partial pressure is the mole fraction of H2S multiplied by the total system pressure.
What is the difference between NACE MR0175 and MR0103?
MR0175 / ISO 15156 covers oil and gas production and processing, including wells, flowlines and gas plants. MR0103 / ISO 17945 covers sour petroleum refining and related process environments.
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