ISO 15156-1:2020 Standard

The ISO 15156 Series: Three Parts and What Each Covers

The series splits into general principles, carbon and low-alloy steels, and corrosion-resistant alloys. Pick the wrong part and you will be reading limits for a material family you are not buying.

For stainless, duplex and nickel alloy tubulars, ISO 15156-3 is the operative document. Every nickel alloy pipe grade qualified for sour duty appears in an ISO 15156-3 Annex A table with its own metallurgical condition, hardness ceiling and environmental envelope.

Supporting test methods sit outside the series. NACE TM0177 covers laboratory SSC and SCC testing; NACE TM0284 covers HIC evaluation of pipeline and pressure vessel steels.

7 Cracking Mechanisms Covered by ISO 15156-1: SSC, SCC, HIC, SWC, SOHIC, SZC and GHSC

ISO 15156-1 addresses all mechanisms of cracking that H₂S can cause. Seven are named in the scope, and each attacks a different weakness. Five need tensile stress (residual or applied), and GHSC additionally requires a galvanic couple. Two, HIC and SWC, need no externally applied stress.

Sulfide Stress Cracking (SSC)

SSC is cracking of metal involving corrosion and tensile stress in the presence of water and H₂S. It is a form of hydrogen stress cracking. Acid corrosion on the metal surface produces atomic hydrogen, sulfides promote its uptake, and the hydrogen diffuses into the metal, cutting ductility. High-strength materials and hard weld zones are the first to go.

Stress Corrosion Cracking (SCC)

SCC involves anodic processes of localised corrosion plus tensile stress in the presence of water and H₂S. Chlorides, oxidants and elevated temperature all raise susceptibility. This is why the Part 3 tables carry chloride and temperature columns alongside H₂S partial pressure.

Hydrogen-Induced Cracking (HIC)

HIC is planar cracking in carbon and low-alloy steels when atomic hydrogen diffuses in and recombines as molecular hydrogen at trap sites. No externally applied stress is needed; pressurisation of the trap sites does the work. Trap sites cluster in steels with high impurity levels, dense planar inclusions and segregation banding.

Stepwise Cracking (SWC)

SWC describes the crack appearance when hydrogen-induced cracks on adjacent planes link up. The linking depends on local strain between the cracks and on embrittlement of the surrounding steel by dissolved hydrogen. HIC and SWC travel together, usually in low-strength plate steels used for pipe and vessels.

Stress-Oriented Hydrogen-Induced Cracking (SOHIC)

SOHIC produces staggered small cracks approximately perpendicular to the principal stress (residual or applied), forming a ladder-like array that links pre-existing HIC cracks. It shows up in the parent material of longitudinally welded pipe and in the heat-affected zone of pressure vessel welds. The standard calls it a relatively uncommon phenomenon.

Soft-Zone Cracking (SZC)

SZC is a form of SSC that occurs when a steel contains a local soft zone of low-yield-strength material. Under service loads that zone yields and accumulates plastic strain, raising cracking susceptibility in material that would otherwise pass. Soft zones are typically associated with welds in carbon steels.

Galvanically Induced Hydrogen Stress Cracking (GHSC)

GHSC is the newest definition in the 2020 edition. Cracking results from hydrogen induced in the cathode of a galvanic couple plus tensile stress. Mixed-metallurgy assemblies and cathodically protected CRA components are where this bites, and Part 3 carries dedicated qualification testing for it.

Equipment Covered and Excluded by ISO 15156-1

Table 1 of the standard lists equipment within scope alongside specific exclusions. The exclusions matter as much as the inclusions. The following table is a non-exhaustive list of equipment to which the standard applies, with exclusions. Several exclusions are conditional (see the table footnotes), so confirm each against the standard. For sucker rod pumps and rods, NACE MR0176 may be referenced.

ISO 15156-1 is not necessarily applicable to refining or downstream processes and equipment. For those, NACE MR0103 is the companion document. Wireline lubricators and lubricator connecting devices are not excluded, despite the general wireline exclusion.

The standard applies to equipment designed with load-controlled design methods. Where strain-based design is used, the equipment user and supplier must agree other test methods and acceptance criteria, because the ISO 15156 series does not address them.

Service Condition Factors You Must Document

Before selecting or qualifying any material, the equipment user shall define, evaluate, and document the service conditions for each application. The documented conditions must cover both intended exposures and unintended exposures that could follow the failure of primary containment or protection.

The equipment user shall quantify and document the following service condition factors, paying particular attention to their evaluation:

• H₂S partial pressure or H₂S concentration or chemical activity in the water phase
• In situ pH: the actual pH at operating service conditions, rather than a room-temperature bench measurement
• Concentration of dissolved chloride or other halide ions present in the aqueous phase
• Presence of elemental sulphur or other oxidants: that increase environmental severity
• Temperature: including maximum design temperature and the full operating temperature envelope
• Galvanic effects: resulting from dissimilar metal couples or cathodic protection
• Mechanical stress: taking into account both applied operational loads and residual manufacturing/welding stresses
• Time of exposure: sustained or intermittent contact duration with a liquid water phase

The 2020 edition expanded the guidance here. For systems with a gas phase, evaluations based on H₂S partial pressure imply a severity that equals or exceeds the field service and are acceptable. ISO 15156-2:2020 Annex C.1 allows H₂S fugacity to be used to reduce this excess severity. For gas-free liquid systems an alternative to partial pressure is needed, and Annex C.2 gives guidance for evaluating them. Note that the environmental severity regions in Part 2 and the prequalified limits in Part 3 are stated in H₂S partial pressure. The standard gives no equivalent limits in terms of fugacity or chemical activity.

The degree to which partial-pressure severity overshoots real service is set by non-ideal thermodynamic effects. Generally it increases with rising pressure and decreases with rising temperature, which is why Annex C recommends non-ideal thermodynamic rules for high-pressure oil wells.

3 Routes to Qualify a Material for H₂S Service: Existing Lists, Field Experience and Laboratory Testing

ISO 15156-1 gives three qualification routes. Most projects use the first, because it needs no testing. The other two exist for materials outside the Annex A tables or for service conditions beyond the listed envelopes.

Route 1: Selection From Existing Lists and Tables

SSC-resistant carbon and low-alloy steels may be selected from ISO 15156-2:2020 Annex A; SSC and SCC-resistant CRAs from ISO 15156-3:2020 Annex A. No additional laboratory testing is generally required. The listed materials have given acceptable performance under stated metallurgical, environmental and mechanical conditions based on field experience and laboratory testing.

The standard still nudges the user: where the potential consequences of failure justify it, specific testing should be considered even for listed materials. That recommendation carries the WARNING clause behind it.

Route 2: Qualification Based on Field Experience

Documented field experience must run at least two years and should preferably include a full examination of the equipment after field use. The material description must meet Clause 8.1 and the service-condition description must meet Clause 6.1. The severity of intended service shall not exceed that of the documented experience.

Route 3: Qualification Based on Laboratory Testing

Laboratory testing can only approximate field service; the standard says so directly. It is used to qualify materials up to the limits applying to similar listed types, to qualify them under other limits, to qualify steels for HIC, SOHIC or SZC resistance, to qualify CRAs for GHSC resistance, and to generate data for later inclusion in Annex A.

Sampling rules are strict. Test samples shall be representative of the commercial product, and where multiple batches exist, the metallurgical condition with the greatest susceptibility to cracking shall be used. Source, preparation method and surface condition all require documentation, and the sampling method needs the equipment user’s acceptance.

All applied test conditions shall be at least as severe as the defined field service with respect to the potential failure mode, and the applied pH shall represent the service in situ pH. Acceptance criteria come from the test methods in Part 2 and Part 3.

Sour Service CRA Grades Under ISO 15156-3

Part 3 qualifies corrosion-resistant alloys by family, each with its own metallurgical condition, hardness ceiling and environmental envelope. The families below cover the tubular grades most often specified for sour duty. Each alloy carries a different balance of chloride resistance, H₂S tolerance and cost.

316L Austenitic Stainless Steel

316L seamless pipe is the entry point to sour service and the most restricted. ISO 15156-3 Table A.2 caps hardness at 22 HRC (237 HBW) and requires solution-annealed-and-quenched material free of cold work applied to raise mechanical properties. The H₂S ceiling is low and chloride tolerance is modest, which is why 316L suits low-severity sour streams rather than deep sour wells.

904L and 254 SMO Highly Alloyed Austenitics

904L seamless pipe adds 4–5 % molybdenum and 1–2 % copper over 316L, moving the chloride pitting threshold up without changing the austenitic structure. It fills the gap between standard austenitics and duplex where reducing acids are present alongside H₂S.

254 SMO seamless pipe reaches 6 % molybdenum with 0.18–0.22 % nitrogen, giving a PREN above 42. The 6Mo super-austenitics hold their own against seawater injection chemistry, and their fully austenitic structure avoids the ferrite-balance checks that duplex grades demand.

Duplex 2205 (S31803 / S32205)

Duplex 2205 seamless pipe is the workhorse of sour flowline and topside piping. ISO 15156-3 Table A.24 requires annealed and quenched material with 35 % to 65 % ferrite and a hardness ceiling of 28 HRC. The ferrite window is a hard gate: fall outside it and the material is unqualified regardless of chemistry.

Duplex S31803 seamless pipe carries the wider original chemistry window, while S32205 tightens nitrogen and chromium to the upper half of that range. For sour service, S32205 is the safer buy: the tighter nitrogen floor pushes PREN up and improves the reliability of the phase balance after annealing.

Super Duplex 2507 and Zeron 100

Super duplex 2507 seamless pipe lifts chromium to 24–26 % and nitrogen to 0.24–0.32 %, producing a PREN above 40. Yield strength around 550 MPa (80 ksi) is more than double 316L, so wall thickness drops for the same pressure rating. The 28 HRC ceiling and 35–65 % ferrite requirement still apply.

Super duplex S32760 seamless pipe adds tungsten and copper to the 25Cr base. Both S32750 and S32760 appear in Table A.24 with the same metallurgical conditions, and the choice between them usually comes down to fabricator familiarity and mill availability rather than a sour-service performance gap.

Incoloy 825

Incoloy 825 seamless pipe brings 38–46 % nickel with 2.5–3.5 % molybdenum, 1.5–3.0 % copper and titanium stabilisation. The copper addition handles reducing acids that attack duplex grades, making 825 a standard pick for sour wells with CO₂ and organic acid loading. Hardness for annealed material is capped at 35 HRC.

Inconel 625

Inconel 625 seamless pipe combines 20–23 % chromium, 8–10 % molybdenum and 3.15–4.15 % niobium for a PREN above 50. Table A.13 covers annealed material at 35 HRC maximum. Where severity exceeds what duplex can take — high H₂S partial pressure, high chloride and elevated temperature together — 625 is the usual step up.

Hastelloy C-276 and C-22

Hastelloy C-276 seamless pipe runs 15–17 % molybdenum with 3–4.5 % tungsten on a nickel base, placing it at the top of the sour-service hierarchy. Solution-annealed material qualifies at 35 HRC. Few production environments need this much alloy, and the ones that do are usually high-temperature sour gas with free sulphur.

Hastelloy C-22 seamless pipe trades some molybdenum for higher chromium at 20–22.5 %, improving performance in oxidising conditions while holding comparable reducing-acid resistance. Both alloys sit in the Ni-Cr-Mo tables of ISO 15156-3 Annex A.

Monel 400

Monel 400 seamless pipe is a 63 % nickel, 28–34 % copper alloy with a long sour-service record in hydrofluoric and reducing environments. It contains no chromium, so it offers nothing against oxidising conditions; the trade-off is deliberate and the application envelope is correspondingly narrow.

Incoloy 925 Precipitation-Hardened Alloy

Incoloy 925 seamless pipe is the age-hardenable version of 825, reaching far higher strength through gamma-prime precipitation. Precipitation-hardened alloys carry their own Annex A tables with tighter condition requirements than solid-solution grades, because the ageing treatment itself governs cracking resistance.