“Is this sour service?” is one of the first questions that should get asked on any wellhead or valve job — because the answer changes what materials are allowed, and getting it wrong isn’t a cosmetic mistake. It’s the difference between equipment that lasts its design life and equipment that cracks in service.
What makes a service “sour”
A well stream is considered sour when it contains hydrogen sulfide (H2S) above a threshold partial pressure. In the presence of H2S, moisture, and tensile stress together, high-strength steels can suffer sulfide stress cracking (SSC) — a form of hydrogen embrittlement that causes sudden, brittle failure at stress levels well below the material’s normal rating, often with little or no warning. This isn’t a slow corrosion process; it can crack a component that otherwise looks fine.
What NACE MR0175 / ISO 15156 actually specifies
NACE MR0175 (now jointly published as ISO 15156) is the standard that defines which materials are qualified for sour service and under what conditions. It doesn’t ban high-strength steel outright — instead it sets hardness limits (commonly a maximum of 22 HRC for carbon and low-alloy steels), specifies acceptable heat treatment conditions, and defines the environmental limits (H2S partial pressure, pH, chloride content, temperature) within which a given material is considered qualified.
In practice this means: the same API 6A wellhead component might be readily available in a standard carbon steel grade for sweet service, but require a specific NACE-compliant heat treatment, a lower-strength grade, or a corrosion-resistant alloy (CRA) for a sour well — even though both look identical on the outside.
Where this shows up in equipment selection
- Wellhead and tree components — body material, hardness, and heat treatment must be verified against the actual (not assumed) H2S partial pressure of the well.
- Bolting — standard ASTM A193 B7 studs are not automatically sour-service qualified at full strength; sour applications often require B7M or equivalent lower-hardness alternatives.
- Valve trim and seals — elastomer and trim material compatibility with H2S needs separate verification from the base metal’s SSC qualification.
The practical takeaway
Sour service qualification isn’t a single checkbox — it’s a combination of material selection, hardness control, and verification against the specific environmental severity of the well, done at the design stage rather than retrofitted after a failure. If a well’s H2S content is uncertain or changes over field life, that uncertainty needs to be designed around, not assumed away.
Need help specifying materials for sour or corrosive service? See our Design & Engineering services, or talk to an engineer. See also our Glossary entry on sour service and NACE MR0175.