Materials and NACE MR0175 for Sour Service: What to Specify

"NACE compliant" on a quotation is not a specification. NACE MR0175 / ISO 15156 is a standard for selecting and qualifying metallic materials for H2S-containing service, and the correct material depends on the actual process conditions.

The four inputs that decide the material

H2S partial pressure — from the gas composition and total pressure; the standard's severity regions are defined on this basis.
In-situ pH — acidic service pushes selection towards higher-alloy materials.
Chloride concentration — drives chloride stress corrosion cracking risk in stainless steels.
Temperature — both maximum and minimum, including any upset condition, and whether low-temperature toughness is required.

Give us these four and the material selection is a calculation, not a guess. Without them, the honest answer is a range of options with different prices.

Materials we commonly quote for sour service

Service	Typical body / trim	Notes
Sweet, moderate pressure	Carbon steel (A105 / LF2), 316 SS trim	Standard offering, economy option
Sour, carbon steel route	A105 / LF2 to NACE with hardness control, F6a or F51 trim	Hardness verification is part of the certificate
Sour with chlorides	Duplex 22Cr / 25Cr, or super duplex	Chloride level and temperature decide the grade
Severe sour / high chloride	Inconel 625 overlay or solid, nickel alloys	Highest cost, longest lead time — plan early
Instrument lines	316 SS with NACE-qualified bar stock	316 is not automatically acceptable in all sour regimes

What an offer must state

The material specification itself (e.g. A105 + ENP, F6a, F51, UNS S31803), not just "NACE".
The hardness limit applied and how it is verified.
Test standard and whether the test is witnessed.
Any restriction: NACE and low-temperature impact testing are not always compatible choices — the offer should say which governs.

Related requirements that travel together

Sour service rarely arrives alone. Fire-safe testing (API 607 / ISO 10497), fugitive emission limits (ISO 15848) and third-party inspection are frequently specified on the same order — see TPI. Quoting them together avoids surprises after the PO.

How to get a firm answer

Send the process conditions, datasheet or line list to websale@sawye.com. We quote the material against your conditions and mark clearly where an assumption was made, so your engineer can check it rather than discover it.