Which Alloys Comply with NACE MR0175 for Sour Service?

What makes a well "sour" under NACE MR0175?

NACE MR0175, now published jointly as ISO 15156, defines sour service by the partial pressure of hydrogen sulphide in the produced fluid rather than by a simple concentration. A system is sour when the H2S partial pressure in the gas phase reaches 0.3 kPa (0.05 psi) or above, and from that point the material selection rules of the standard apply. Hydrogen sulphide causes sulphide stress cracking in high-strength steels, and the higher the hardness, the greater the risk.

• Sour threshold: H2S partial pressure ≥ 0.3 kPa (0.05 psi) in the gas phase
• NACE MR0175 / ISO 15156:2015 is the current joint document, in three parts
• Part 1 covers general principles, Part 2 carbon and low-alloy steels, Part 3 corrosion-resistant alloys
• The buyer normally states "NACE MR0175 compliant" as a line item on the purchase order

Which nickel alloys are pre-qualified for sour service?

ISO 15156-3 lists corrosion-resistant alloys that may be used without further testing when supplied in the specified condition. For nickel-based alloys this is one of the few areas where the standard is generous: solution-annealed grades have no hardness restriction at all, because they do not depend on hardness for strength.

The practical reading is straightforward: if you can solve the corrosion problem with a solution-annealed nickel alloy, sour service approval is simple. The moment you need precipitation-hardened strength — 718, X-750, Monel K-500 — hardness control becomes a first-order requirement and every batch needs verification.

Why does hardness control matter so much?

Sulphide stress cracking is a hydrogen embrittlement mechanism. The harder and stronger the steel, the more sensitive it becomes to hydrogen that has diffused into the lattice from the corrosion reaction. That is why the standard is written around hardness ceilings rather than around alloy chemistry alone.

• Carbon and low-alloy steels: 22 HRC (250 HV) maximum for the base metal and welds
• Martensitic stainless steels: 22 HRC maximum
• Duplex and super duplex stainless steels: 28 HRC (30 HRC for some grades) maximum
• Precipitation-hardening nickel alloys: 40 HRC maximum, with the stated ageing condition
• Solution-annealed austenitic and nickel alloys: no limit, since hardness is not the strengthening mechanism

A hardness test reported on the mill certificate is therefore not a formality — it is the acceptance criterion. If the certificate shows only tensile results, ask for the hardness values before the material sails.

What is the difference between NACE MR0175 and NACE MR0103?

Both documents address sulphide stress cracking, but they cover different parts of the industry. MR0175 / ISO 15156 applies to oil and gas production, including upstream and pipeline environments. NACE MR0103 applies to refinery and petrochemical service, and it is somewhat less restrictive on certain alloys and conditions. Specifying the wrong one is a common and expensive mistake, because a refinery order written to MR0175 can push you into a more expensive alloy than the service actually requires.

• Upstream production, wellhead, flowlines, pipelines: NACE MR0175 / ISO 15156
• Refinery and petrochemical process units: NACE MR0103
• Some projects require both, and the purchase order will say so

Which standards and documents apply to sour service materials?

The standards below are the ones referenced on a typical sour service purchase order, and they should be quoted exactly as written:

• NACE MR0175 / ISO 15156 — general principles and alloy listings for sour service
• ASTM B622 — seamless nickel and cobalt alloy pipe and tube for corrosive service
• ASTM B564 — nickel alloy forgings, including 625, 718 and 825
• ASTM B424 — nickel-iron-chromium alloy plate, sheet and strip (Incoloy 825)
• ASTM A240 / A240M — chromium and nickel stainless steel plate, sheet and strip
• ASTM G48 — pitting and crevice corrosion resistance test methods

How do I specify sour service material to a mill?

State the sour requirement explicitly and attach the hardness criterion to the order. A vague instruction such as "sour service" is not enough, because the mill cannot tell which edition or which part of the standard you mean.

• 1. Name the alloy and UNS number, for example Hastelloy C-276, UNS N10276
• 2. Name the product standard, for example ASTM B622 for seamless tube
• 3. State the condition, for example solution annealed
• 4. Add the sour service clause: NACE MR0175 / ISO 15156-3 compliant, hardness to be reported
• 5. Require the hardness test result on the EN 10204 3.1 mill test certificate
• 6. Consider third-party witness of the test if the project demands it

Frequently asked questions