NACE MR0175 — now published jointly as ISO 15156 — is the reason a perfectly good 316 fitting can be wrong for the same fluid the moment H2S enters it. Sour service does not change what the alloy is; it changes what condition the alloy is allowed to be in. Hardness limits, heat-treatment state and cold-work history all join the material list, and a certificate that says only “316” answers none of the questions the standard asks. For oil-and-gas buyers this is the gate every wetted part walks through.
Key takeaways
- The rules bite when H2S partial pressure passes about 0.3 kPa (0.05 psi) in the gas phase — below that, MR0175 generally does not apply.
- Austenitics like 316L serve under limits: solution-annealed condition, with procurement practice capping hardness at about 22 HRC; the standard’s own austenitic table centers on the solution-annealed state near 253 HV.
- Duplex 2205 is conditionally qualified inside a published envelope — chloride, pH, temperature and H2S partial pressure all draw its boundary; nickel alloys carry the severe sour frontier.
- Hardness is the compliance lever, and it collides with cold forming: cold-drawn tube and heavily worked fittings must be verified, not assumed.
- The RFQ carries four numbers — H2S partial pressure, chloride, pH, temperature — and they, not the alloy name, decide the material list.
What sour service means
Sour service is oil and gas production with hydrogen sulfide in the stream, and MR0175/ISO 15156 is the materials rulebook for it. The applicability gate is quantitative: once H2S partial pressure in the gas phase passes roughly 0.3 kPa — 0.05 psi — at operating pressure, the standard’s material restrictions apply; below that line, its sour provisions generally do not. Above it, the standard maps severity through the four environmental inputs — H2S partial pressure, in-situ pH, chloride and temperature — and each material family carries its own envelope inside that map. The failure mode the whole apparatus exists to prevent is sulfide stress cracking: hydrogen absorbed at the corroding surface embrittles high-strength microstructures, and the crack needs no visible corrosion to run. Hence the obsession with one material property — hardness.
The material list
Each family enters sour service on its own terms, and the honest summary is conditional for all of them:
- Austenitic 316L — the default CRA for moderate sour duty, acceptable under ISO 15156-3 when supplied in the solution-annealed, quenched condition. Procurement practice caps hardness at about 22 HRC, and the published austenitic table centers on the solution-annealed state — roughly 253 HV — rather than a blanket HRC number. Its envelope is the modest corner of the map: low H2S partial pressure, moderate temperature.
- Duplex 2205 — conditionally qualified, with a published envelope that bounds chloride, pH, temperature and H2S partial pressure together; one sourced envelope puts the 22Cr duplex near 3% chloride, pH 3.5 minimum, 150 °C maximum and H2S around 0.02–0.1 bar depending on chloride. Hardness practice caps near 28 HRC in the common editions, with later editions allowing more for some duplex grades, and the steel must be solution-annealed and water-quenched with ferrite balance verified. Verify against the current edition — the limits move between revisions.
- 6-Mo and nickel alloys — the step above. Nickel alloys carry the severe-sour frontier where duplex envelopes end; 6-Mo alloys resist H2S better than 316 but are not blanket-approved across sour conditions, and the references steer combined sour-plus-seawater duty toward super duplex or nickel alloys precisely because those carry full qualification. The chloride side of sour duty overlaps the territory of chloride stress corrosion cracking, and the family selection logic sits in the grades field guide.
Hardness is the lever
Every line of MR0175 lands on the same property: hardness. The carbon-steel clause is the template — a hard ceiling of 22 HRC, applied to base metal, weld metal and heat-affected zone alike, with single over-limit readings counting as non-conformance. Austenitic and duplex families carry their own ceilings and conditions, and nickel alloys rate higher still. The practical collision is with cold forming: cold-drawn tube and heavily worked fittings gain strength and hardness exactly where the standard draws its line, so sour-service procurement verifies hardness on the finished condition of the part — not the mill’s annealed promise. A drawing that specifies hard-drawn tube and an MR0175 duty on the same line is a specification argument waiting to happen.
What changes on the RFQ
Four numbers carry the material decision — write them and the alloy follows:
H2S partial pressure— the applicability gate and the severity axis.chloride content— the second axis of every envelope.in-situ pH— the third; severity regions are drawn in pH–H2S space.temperature— the fourth envelope boundary.
Add solution-annealed condition, hardness certified for the austenitic or duplex lines, and NACE MR0175 / ISO 15156-3 compliant with certificate where the duty demands it. The material name alone buys nothing in sour service; the condition and the certificate buy everything.
FAQ
Is 316 always OK in sour service?
No — 316 and 316L are acceptable only inside their limits: solution-annealed condition, hardness at or below the cap, and an environmental envelope of low H2S partial pressure and moderate temperature. Outside the envelope, the standard points to duplex or nickel alloys. “316” on a datasheet without condition and envelope is a start of a conversation, not a compliance claim.
What actually triggers NACE requirements?
H2S partial pressure is the gate: about 0.3 kPa (0.05 psi) in the gas phase, with an aqueous phase present and operating pressure above the low threshold — pass all three and the sour provisions apply. Below the gate, ordinary material rules stand. The same three-question test is worth re-running after any process change that adds H2S or raises pressure.
Can I use duplex 2205 fittings in sour service?
Conditionally. 2205 carries a published sour envelope — bounds on chloride, pH, temperature and H2S partial pressure that move together — and it must arrive solution-annealed, water-quenched and hardness-verified. Inside the envelope it is the workhorse; outside it, the references route to nickel alloys. State the four environmental numbers and let the envelope decide.
Why does cold-drawn tube fail NACE review?
Because cold drawing raises strength and hardness — the very property the standard caps — and the review reads the finished condition, not the starting bar. Solution-annealed product meets the austenitic and duplex conditions; heavily cold-worked product must be hardness-verified part by part, and often is not eligible at all. The forming method and the sour duty belong in the same RFQ paragraph.
What numbers decide my material for sour duty?
Four: H2S partial pressure, chloride content, in-situ pH and temperature. The standard’s envelopes are drawn in exactly those axes, and every compliant material list is a point inside a family envelope. Send the four numbers with the size list and the material answer follows from the standard rather than from habit.
