Once a specification says "sour service", the subject is no longer only pressure and temperature. Hydrogen sulphide changes how steel behaves, and a material that is otherwise safe can turn brittle.
What H2S does
In a wet environment hydrogen sulphide starts a corrosion reaction at the steel surface, and the hydrogen atoms released enter the metal. In hard zones under stress that hydrogen accumulates and initiates cracks. This is sulphide stress cracking, and it develops suddenly, without the warning of thinning or weeping.
Three things set the risk: H2S partial pressure, the pH of the environment and the hardness of the material.
What NACE MR0175 defines
- Hardness limits. The typical limit for carbon and low alloy steels is 22 HRC.
- Heat treatment condition. The standard defines the condition in which the material must be supplied.
- Welding and heat treatment. Weld zone hardness must also stay within limits, with post weld heat treatment where required.
- Coating is not an exemption. A coating does not substitute for the material meeting the hardness requirement.
Which items are covered
Not only pipe but every item in contact with the fluid: flanges, fittings, valve bodies and trim, gaskets and fasteners. Studs and nuts must be checked for grade and hardness, because a high strength bolt grade may not be acceptable in sour service.
Ordering and documentation
Writing "NACE MR0175 / ISO 15156 compliant" on the enquiry is not enough; hardness values should be reported on the certificate. When an equivalent is offered, compliance must be confirmed separately.
Send your list with material grade and service details: enquiry form. See our refinery and petrochemical page for our approach in that sector.
FAQ
How is the sour service limit determined?
H2S partial pressure and the pH of the environment are assessed together; the standard defines regions from those two values.
Does the 22 HRC limit apply to every material?
It is the typical limit for carbon and low alloy steels. Stainless and nickel alloys have their own acceptance conditions.
Does coating satisfy the hardness requirement?
No. A coating slows corrosion but the material itself must meet the hardness and heat treatment requirements.
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