Sour service materials: NACE MR0175 selection for offshore parts

In an environment with hydrogen sulphide a material that serves perfectly in sweet service can crack without warning - that is sulphide stress cracking, quiet and sudden. NACE MR0175 together with ISO 15156 is the standard that says what material may be used there, and it follows the part through its documentation. Here is what fit for sour service actually means and what to demand before installation.

Sour means hydrogen sulphide, not a smell

A sour environment is one where hydrogen sulphide is present in an amount able to cause cracking. The threshold is set by the H2S partial pressure together with the pH and chloride content - not a single number but an envelope of conditions in which the standard permits a given material. So a sour-rated part without the stated environment is incomplete: the same alloy can be allowed in a milder environment and disallowed in a harsher one.

Hardness is a limit, not a preference

For carbon and low-alloy steels the standard sets a hard hardness limit - commonly at 22 HRC - because it is hardness, not the grade alone, that governs susceptibility to sulphide cracking. That includes the heat-affected zones of welds: a badly chosen or untempered weld can be the hardest, and so the weakest, spot on a part that is otherwise fine on material. Hence the weight on heat treatment and hardness control, not just alloy composition.

Metal, elastomer and decompression

• Metallic parts: steels qualified for the environment, and for harsher conditions corrosion-resistant alloys - duplex, Inconel 625 and 825 and relatives.
• Elastomers: seals in HNBR, FKM or FFKM chosen not only for the medium but for rapid decompression - on a sudden pressure drop the gas expanding in the rubber tears it from inside (RGD/AED).
• The sour-service requirement covers the whole part, not just the body - stems, springs, fasteners and seals have to qualify too.

The traceability that proves it

Sour-service compliance is worth exactly as much as the documentation that proves it. The part should carry a material certificate with heat and batch numbers, hardness results, confirmation of heat treatment and a statement of the standard and the environment it is qualified for. A declaration without traceability is not compliance, only a promise - and with sulphide cracking the difference can be catastrophic. The document travels with the part, to be checked before installation, not after.

How this works with us

The platform category is sourced to order precisely because these parts are bought against the environment and the documentation, not off a shelf. Give us the standard and the environment conditions - H2S, pH, chlorides, temperature - and the part comes in a material qualified for your environment, with hardness, heat treatment and certificate in the set, and elastomers chosen for decompression as well. What you approve is a part fit to install, with the paperwork already in hand.