NACE MR0175 / ISO 15156 Standard

Where the Standard Came From

Sudden failures of metallic oil and gas field components exposed to H₂S-containing production fluids drove the first edition of NACE MR0175, published in 1975 by the National Association of Corrosion Engineers.

Those early editions established the H₂S partial pressure thresholds above which precautions against sulfide stress cracking were always considered necessary, and gave guidance on selecting SSC-resistant materials once those thresholds were exceeded. Later editions added application limits for corrosion-resistant alloys in terms of environmental composition, pH, temperature and H₂S partial pressure.

Europe developed in parallel. The European Federation of Corrosion issued EFC Publication 16 in 1995 and EFC Publication 17 in 1996, complementary to the NACE documents but differing in scope and detail. In 2003 the ISO 15156 series and NACE MR0175/ISO 15156 were completed for the first time, merging all three sources into one technically identical pair of documents.

Maintenance now runs through a dedicated agency at DIN, set up after ISO Technical Management Board approval in document 34/2007. It includes experts from NACE, EFC and ISO/TC 67. Interim changes are published as Technical Corrigenda or Technical Circulars, which can affect the validity of dated references, so it is worth checking before relying on an older copy.

3 Parts of NACE MR0175 / ISO 15156 and What Each Covers

The standard splits method from material family. Part 1 gives the selection process that applies to every material; Parts 2 and 3 carry the qualified alloy lists for ferrous and corrosion-resistant materials respectively.

You never use Part 2 or Part 3 alone. Part 1 is what tells you which service conditions to document and how to record the selection, and both material parts assume that work has been done first.

Part 1: The Selection Method

Part 1 describes general principles and gives requirements for selecting and qualifying metallic materials for oil and gas production and natural-gas sweetening plants in H₂S environments. It addresses seven cracking mechanisms: SSC, SCC, HIC, SWC, SOHIC, SZC and the newly defined GHSC.

It also sets the three qualification routes (selection from the Annex A lists, documented field experience of at least two years, or laboratory testing) and the rule that qualification transfers only to conditions equal to or less severe in all respects. The ISO 15156-1 standard page covers the equipment scope table, service condition factors and reporting requirements in full.

Part 2: Carbon and Low-Alloy Steels

Part 2 maps H₂S partial pressure against in situ pH into four SSC regions, and the region you land in decides which part of Annex A you may select from. Parent metal baseline caps at 22 HRC; welds baseline cap at 250 HV or 70.6 HR 15N. Exceptions include: up to 275 HV (73.0 HR 15N) for unexposed weld caps (with equipment user agreement and thickness > 9 mm), 187 HBW for ASTM A105 forgings, 197 HBW for ASTM A234 Grade WPB/WPC fittings, and up to 26 HRC or 30 HRC for quenched and tempered Cr-Mo low-alloy steel downhole tubulars and BOP rams.

It covers qualified line pipe, casing, tubing and bolting grades, the cold deformation and stress relief rules, and the separate duty to evaluate flat-rolled products for HIC and SWC at even trace H₂S levels. The ISO 15156-2 standard page sets out the region map, the hardness tables and the Annex B test matrix.

Part 3: Corrosion-Resistant Alloys

Part 3 is the part that matters for stainless, duplex and nickel alloy tubulars. Thirteen materials groups across forty-one Annex A tables give environmental limits for temperature, H₂S partial pressure, chloride concentration and pH, plus the metallurgical condition each alloy must be supplied in.

Duplex is grouped by PREN band rather than grade name, and the PREN limits are absolute values calculated from actual composition. The ISO 15156-3 standard page carries the real Annex A envelopes for 316L, the 6Mo austenitics, duplex, super duplex and the nickel alloy types.

7 Cracking Mechanisms the Standard Addresses

H₂S attacks metal in more than one way, and a material resistant to one mechanism can be vulnerable to another. Part 1 names seven, and the material parts qualify against them separately.

The 2020/2021 fourth edition introduced three new definitions in Part 1: GHSC (3.14), chemical activity (3.27), and fugacity (3.28). It also introduced normative provisions (Part 1 Clause 6.1 and Part 2 Annex C) permitting the use of H₂S gas fugacity and aqueous chemical activity to evaluate environmental severity in high-pressure systems (> 350 bar / 5,076 psi). GHSC covers cracking from hydrogen induced in the cathode of a galvanic couple plus tensile stress, which is the condition that arises in mixed-metallurgy subsea assemblies and cathodically protected CRA components.

Which Alloys Qualify for Sour Service?

Part 3 qualifies CRAs by materials group, each with its own metallurgical condition, hardness ceiling and environmental envelope. The table gives the headline limits for the families most often specified in sour tubular work.

The hierarchy is readable straight off that table. 316L covers mild sour at low temperature, duplex covers high chloride at low H₂S, super duplex doubles the H₂S allowance, and annealed nickel alloys carry no environmental limits at all.

Austenitic Stainless for Mild Sour Duty

316L seamless pipe must be solution-annealed and quenched, free of cold work intended to raise mechanical properties, and held at 22 HRC maximum. Its dedicated Annex A rows reach much further than the generic austenitic envelope, which is why 316L appears in sour specifications more often than its reputation suggests.

6Mo Super-Austenitics for High Chloride

254 SMO seamless pipe qualifies through the PREN above 40.0 route, reaching 171 °C at 100 kPa H₂S with 5,000 mg/l chloride.

904L seamless pipe qualifies as a Type 3a austenitic stainless steel based on composition ((%Ni + 2 × %Mo) > 30 with Mo ≥ 2 %). It is restricted to a maximum temperature of 60 °C (140 °F), a maximum H₂S partial pressure of 100 kPa (15 psi), and maximum chlorides of 50 mg/l under Table A.8 (or 350 kPa pH₂S for downhole tubulars under Table A.9). It does not qualify for the extended Type 3b envelopes (121 °C–171 °C), which strictly require PREN > 40.0.

Duplex and Super Duplex for Strength Plus Chloride

Duplex 2205 seamless pipe sits in the PREN 30 to 40.0 band at 10 kPa H₂S, with 450 MPa (65 ksi) minimum yield and a mandatory 35 % to 65 % ferrite window. Ferrite outside that range disqualifies the material regardless of chemistry.

Super duplex 2507 seamless pipe clears PREN 40.0 and doubles the H₂S allowance to 20 kPa while reaching 550 MPa (80 ksi) minimum yield. That doubled allowance is the clearest commercial reason to step up from standard duplex in sour duty.

Nickel Alloys for Severe and Sulphur-Bearing Service

Incoloy 825 seamless pipe qualifies as a solid-solution nickel alloy in the annealed condition, carrying no limits on temperature, H₂S partial pressure, chloride or pH. Its copper content handles the reducing acid chemistry that attacks duplex.

Inconel 625 seamless pipe sits in the same unrestricted group with 8–10 % molybdenum and 3.15–4.15 % niobium, commonly specified as solid wall for critical components and as clad or overlay on carbon steel for longer runs.

Elemental sulphur narrows the field sharply. Duplex and highly alloyed austenitic rows carry no sulphur-service data or an outright No. Hastelloy C-276 seamless pipe and the other annealed nickel alloys are the ones marked sulphur-resistant.