NACE MR0175 / ISO 15156: Sour Service Materials Explained

Quick Answer

NACE MR0175 / ISO 15156 is the international standard governing metallic material selection for sour service — environments containing hydrogen sulphide (H₂S) — in oil and gas production, pipelines, and processing. It exists because H₂S causes sulphide stress cracking (SSC): sudden, brittle fracture with no prior deformation and no warning. The standard applies when the partial pressure of H₂S in the gas phase reaches or exceeds 0.05 psia (0.3 kPa, 345 Pa), calculated as total system pressure × H₂S mole fraction; systems operating below roughly 65 psia (448 kPaa) total pressure in crude and water handling are generally excluded. The standard has three parts: Part 1 covers general principles for selecting cracking-resistant materials, Part 2 covers carbon and low-alloy steels, and Part 3 covers corrosion-resistant alloys (CRAs) including stainless, duplex, and nickel alloys. The single most quoted requirement is the hardness limit for carbon and low-alloy steels: maximum 22 HRC (250 HV10) in the parent metal, weld metal, and heat-affected zone — all three, not just the base material. Selection depends on the full environment, not H₂S alone: partial pressure, in-situ pH, temperature, and chloride concentration together determine which materials qualify. Compliance is proven by hardness data on the material test certificate, not by a statement.

Sulphide stress cracking does not announce itself. A carbon steel component in sour service can operate for years within its pressure rating, show no wall loss, pass thickness inspection, and then fracture in a brittle manner with no measurable deformation beforehand. That failure mode — sudden, complete, and unpredictable — is why NACE MR0175 exists, and why compliance is not treated as optional anywhere in the oil and gas industry.

The mechanism is hydrogen. When H₂S dissolves in free water it forms a mildly acidic solution that corrodes steel, and the corrosion reaction releases atomic hydrogen at the metal surface. The sulphide present prevents that hydrogen from recombining into harmless H₂ gas, so instead it diffuses into the steel. Once inside, it collects at inclusions, laminations, and regions of high hardness and residual stress, embrittling the material and initiating cracks under tensile stress.

That is why the standard's central control is hardness. Hard microstructures — untempered martensite in particular — are far more susceptible to hydrogen embrittlement than softer, tempered structures. Controlling hardness controls susceptibility. And because welding creates exactly the hard, stressed microstructures that crack, the limit applies to the weld and heat-affected zone as strictly as to the parent metal.

For piping engineers, materials specialists, valve specifiers, and procurement teams working in oil and gas — this guide explains NACE MR0175 / ISO 15156: when it applies, the cracking mechanisms, hardness and material requirements, CRA selection, and how to specify and verify compliance.

For related materials, see Carbon Steel Pipe, API 5L Line Pipe, and Duplex & Super Duplex Pipe.

When Does Sour Service Apply?

The H₂S Threshold

NACE MR0175 / ISO 15156 applies when the partial pressure of H₂S in the gas phase equals or exceeds 0.05 psia (0.0003 MPa, 345 Pa, or 0.3 kPa).

Partial pressure is calculated simply:

pH₂S = total system pressure × mole fraction of H₂S in the gas phase

So a stream with a very low H₂S concentration can still be sour if the total pressure is high enough — a point that catches people out. 100 ppm H₂S at 500 psia gives a partial pressure of 0.05 psia and is therefore sour.

The Free Water Requirement

H₂S alone is not the problem. Free water must be present for H₂S to dissolve and form the acidic solution that drives the corrosion reaction. Dry H₂S is not corrosive in this sense.

In practice, any system where free water is present with H₂S is treated as sour.

Exclusions

Crude oil storage and handling facilities and water handling facilities operating below approximately 65 psia (448 kPaa, 4.3 bara) total pressure are generally excluded from the requirements of ISO 15156-1.

The Full Environment Matters

The H₂S threshold determines whether the standard applies. It does not by itself determine which materials qualify. Selection requires knowledge of:

• H₂S partial pressure (kPa or psi)
• In-situ pH — the acidity at the metal surface
• Operating temperature — susceptibility varies with temperature, and for some materials the risk peaks at moderate temperatures
• Chloride concentration — critical for corrosion-resistant alloys
• Presence of elemental sulphur

The standard provides tables of qualified materials mapped to defined environmental regions. A material acceptable in one region may be unqualified in another.

The Cracking Mechanisms

MR0175 addresses several related hydrogen damage mechanisms:

SSC — Sulphide Stress Cracking. The primary concern. Brittle cracking under the combined action of tensile stress and hydrogen absorbed from the sour environment. Hard microstructures are most susceptible. Failure is sudden with no prior deformation.

HIC — Hydrogen Induced Cracking. Atomic hydrogen collects at internal discontinuities — typically elongated manganese sulphide inclusions in rolled plate and pipe — forming internal blisters and cracks that link up in a stepwise pattern. HIC does not require applied stress. It is controlled through clean steelmaking practice (low sulphur, inclusion shape control) rather than hardness, which is why HIC-tested plate and pipe are specified separately.

SOHIC — Stress Oriented Hydrogen Induced Cracking. A combination: HIC-type damage that aligns perpendicular to applied stress, typically near welds. Particularly relevant in pressure vessels and pipe adjacent to welds.

SZC — Soft Zone Cracking and galvanic effects are also addressed in the standard.

Note that HIC resistance and SSC resistance are different requirements. A material meeting the 22 HRC hardness limit is not automatically HIC-resistant. Where HIC is a concern, specify HIC testing (commonly per NACE TM0284) explicitly.

The Standard's Structure

ISO 15156-1 / MR0175 Part 1 — General principles. Defines sour service, the H₂S thresholds, the environmental parameters, and the framework for selecting cracking-resistant materials. Also covers qualification routes for materials not pre-qualified in Parts 2 and 3.

ISO 15156-2 / Part 2 — Carbon and low-alloy steels (including cast irons). Identifies pre-qualified carbon and low-alloy steels providing SSC resistance, and sets the hardness and metallurgical requirements.

ISO 15156-3 / Part 3 — Corrosion-resistant alloys (CRAs). Covers stainless steels, duplex stainless steels, nickel alloys, and other specialty alloys, with environmental limits for each.

Purchase orders should reference both designations — for example "NACE MR0175 / ISO 15156 Part 3" — to avoid ambiguity about which document version and which part applies.

Carbon and Low-Alloy Steels (Part 2)

The 22 HRC Rule

The core requirement: carbon and low-alloy steels must not exceed 22 HRC (250 HV10) hardness.

Critically, this applies to:

• Parent metal
• Weld metal
• Heat affected zone (HAZ)

All three. The HAZ is often the hardest region in a fabricated component and is where SSC failures concentrate, so weld procedure qualification for sour service must include hardness surveys across the weld and HAZ.

Metallurgical Requirements

Beyond hardness, Part 2 addresses:

• Heat treatment condition — normalised, normalised and tempered, or quenched and tempered as appropriate; as-rolled material is often unacceptable
• Chemistry restrictions on certain elements
• Avoidance of untempered martensite
• Cold work limits — cold-formed components may require stress relief, because cold work raises hardness and residual stress
• Weld procedure qualification including hardness testing

Common Compliant Materials

Materials routinely supplied in NACE-compliant condition include:

• ASTM A106 Gr B seamless carbon steel pipe
• API 5L PSL2 line pipe grades with sour service supplementary requirements
• ASTM A105 forgings for flanges and fittings (with hardness control)
• ASTM A350 LF2 low-temperature forgings
• ASTM A234 WPB fittings
• ASTM A193 B7M / A194 2HM bolting — note the "M" grades, which are the hardness-controlled versions specifically for sour service

The B7M/2HM point is worth emphasising: standard B7 studs and 2H nuts exceed the sour service hardness limit. Specifying B7 on a NACE line is a common and consequential error.

Corrosion-Resistant Alloys (Part 3)

Where carbon steel cannot provide adequate corrosion resistance — or where the environment is too severe — CRAs are used. Part 3 sets environmental limits for each.

Austenitic stainless (316/316L) — acceptable within defined limits of H₂S partial pressure, chloride, temperature, and pH. Limits are relatively restrictive.

Duplex 2205 (S32205) — significantly wider envelope than austenitic stainless, widely used in sour service where chlorides are also present.

Super duplex 2507 (S32750) — acceptable with defined conditions; Charpy impact requirements apply (commonly cited as greater than 40 J at −46°C for qualification).

Nickel alloys — Inconel 625 (N06625) and similar are acceptable across most sour service environments with few temperature restrictions, which is why they are chosen for the most severe wells and completions.

For duplex material detail, see Duplex & Super Duplex Pipe.

Important: CRA qualification is environment-specific. A duplex grade qualified at one H₂S partial pressure, chloride level, and temperature is not automatically qualified at another. The tables in Part 3 must be checked against the actual service conditions.

How to Specify NACE Compliance

A purchase order or datasheet that simply says "NACE" is inadequate and produces disputes. A complete specification includes:

1. The standard and part: "NACE MR0175 / ISO 15156 Part 2" (or Part 3 for CRAs).

2. The service environment:

• H₂S partial pressure
• Total system pressure
• In-situ pH
• Operating and design temperature
• Chloride concentration
• Presence of free water and elemental sulphur

3. Material specification and grade, including the sour-service variant where one exists (B7M rather than B7, PSL2 with sour supplements).

4. Hardness requirements — maximum 22 HRC / 250 HV10 on parent metal, weld, and HAZ, with hardness test results required on the MTC.

5. Additional testing where applicable:

• HIC testing (NACE TM0284) for plate and pipe where HIC is credible
• SSC testing (NACE TM0177) where qualification testing is required
• Charpy impact testing with temperature for CRAs
• Ferrite count for duplex

6. Certification: EN 10204 Type 3.1 or 3.2 material test certificates carrying the hardness data and NACE compliance statement. See Material Test Certificates.

7. Welding requirements — WPS/PQR qualified for sour service including HAZ hardness surveys.

8. Marking — components should carry NACE identification.

Verification: Paper Is Not Proof

The most common compliance failure is accepting a certificate that states NACE compliance without supporting data.

What to check:

• Hardness results are present on the MTC, with actual values — parent metal, and weld/HAZ for welded items
• Values are within 22 HRC / 250 HV10
• Heat treatment condition is recorded
• Heat number traceability links the certificate to the physical material
• HIC test reports where specified, with acceptance criteria
• For CRAs, the environmental limits used for qualification are consistent with your service
• Bolting is the M grade (B7M/2HM), not standard B7/2H

PMI on receipt verifies alloy composition independently of the paperwork and is standard practice for CRA materials.

Common Specification Mistakes

After 15+ years supplying materials to oil and gas projects:

Mistake 1: Specifying "NACE" With No Environment Data

Purchase order states "NACE MR0175 compliant" with no H₂S partial pressure, pH, temperature, or chloride data. The supplier cannot confirm which material qualifies, and disputes follow.

Prevention: Provide the full service environment. Material qualification is environment-specific, not generic.

Mistake 2: Standard B7 Bolting on a Sour Line

A193 B7 studs and A194 2H nuts specified on a NACE-compliant flanged joint. Both exceed the sour service hardness limit.

Prevention: Specify B7M studs and 2HM nuts — the hardness-controlled variants — for sour service.

Mistake 3: Ignoring Weld and HAZ Hardness

Parent material certified to 22 HRC, but the weld procedure was never qualified for sour service. The HAZ exceeds the limit and cracks in service.

Prevention: Require WPS/PQR qualified for sour service with hardness surveys across weld and HAZ. Specify HAZ hardness testing on production welds where warranted.

Mistake 4: Assuming Hardness Compliance Means HIC Resistance

Material meets 22 HRC and is assumed suitable. HIC then occurs, because HIC is controlled by steel cleanliness and inclusion shape, not hardness.

Prevention: Where HIC is credible — particularly in plate, welded pipe, and pressure vessels — specify HIC testing per NACE TM0284 separately.

Mistake 5: Accepting a Compliance Statement Without Hardness Data

Certificate says "NACE MR0175 compliant" but contains no hardness results. The claim cannot be verified and will fail audit.

Prevention: Require actual hardness values on the MTC as a condition of acceptance.

Mistake 6: Cold-Formed Components Without Stress Relief

Cold-bent pipe or cold-formed fittings supplied without stress relief. Cold work raises hardness and residual stress, both of which drive SSC.

Prevention: Apply the standard's cold work limits and specify stress relief where required.

Mistake 7: Treating Low H₂S Concentration as Sweet

Stream with only 50–100 ppm H₂S assumed to be sweet service, without calculating partial pressure at operating pressure.

Prevention: Calculate pH₂S = total pressure × mole fraction. At high pressures, low concentrations still exceed the 0.05 psia threshold.

Mistake 8: Applying a CRA Beyond Its Qualified Envelope

Duplex 2205 used at an H₂S partial pressure, chloride level, or temperature outside its Part 3 qualification.

Prevention: Check the actual service conditions against the Part 3 tables for the specific alloy. Upgrade to super duplex or nickel alloy where the envelope is exceeded.

Supply from Kasko Makine

Kasko Makine supplies NACE MR0175 / ISO 15156-compliant materials for oil and gas, refining, and petrochemical projects:

Piping and components:

• Carbon steel pipe: ASTM A106 Gr B, A333 low temperature
• Line pipe: API 5L PSL2 with sour service supplementary requirements
• Fittings: ASTM A234 WPB and alloy grades
• Flanges and forgings: ASTM A105, A350 LF2, A182 F-grades
• Stainless, duplex (S32205), and super duplex (S32750) per Part 3
• Nickel alloys for severe service

Bolting:

• ASTM A193 B7M studs and A194 2HM nuts — hardness-controlled for sour service
• B8M stainless bolting where specified
• Coated options with sour service compatibility

Valves and equipment:

• NACE-compliant valves (gate, globe, ball, check, control) with compliant body, trim, and bolting
• Pressure vessels and heat exchangers with sour service materials

Testing and certification supplied:

• Hardness test results on the MTC — parent metal, weld, and HAZ as applicable
• EN 10204 Type 3.1 / 3.2 certificates
• HIC testing per NACE TM0284 where specified
• SSC testing per NACE TM0177 where qualification is required
• Charpy impact testing with stated temperature for CRAs
• Ferrite count for duplex materials
• Heat treatment records
• PMI reports
• Full heat number traceability
• Third-party inspection (Bureau Veritas, SGS, TÜV, Lloyd's) available

Engineering support:

• Confirming whether your service is sour (partial pressure calculation)
• Material selection against the actual environment (pH₂S, pH, temperature, chloride)
• CRA qualification envelope checks against Part 3
• Bolting grade verification (B7M/2HM)
• Welding and hardness requirement guidance
• Certificate review before shipment

Logistics: NACE-compliant materials shipped from Istanbul to projects across Africa, the Middle East, Central Asia, and beyond. Standard grades 6-10 weeks; HIC-tested and CRA materials 10-20 weeks depending on testing requirements.

Need NACE MR0175-compliant materials? Send us your service environment (H₂S partial pressure, total pressure, pH, temperature, chloride concentration), material specification and grade, quantities, and required testing (HIC, SSC, impact) to info@kaskomakine.com or WhatsApp +90 (537) 521 1399. We'll confirm material qualification for your environment, specify the correct bolting grades, and provide a quotation with full certification within 48 hours.

Continue Reading: Materials & Specification Guides

• Carbon Steel Pipe — Grades and sour service variants
• API 5L Line Pipe — PSL2 and sour supplements
• Duplex & Super Duplex Pipe — CRA selection for sour and chloride service
• Material Test Certificates: EN 10204 — Certifying NACE compliance
• Industrial Valves Guide — NACE-compliant valve specification
• Pipe Flanges — Flange and bolting for sour service

Frequently Asked Questions

Q: What is NACE MR0175?
A: NACE MR0175, now published jointly as ISO 15156, is the international standard governing the selection and qualification of metallic materials for sour service — environments containing hydrogen sulphide — in oil and gas production, pipelines, and processing. First published in 1975 in response to catastrophic failures from sulphide stress cracking, it establishes hardness limits, metallurgical requirements, and environmental limits within which approved materials may be safely used. It is structured in three parts: Part 1 covers general principles for selecting cracking-resistant materials, Part 2 covers carbon and low-alloy steels including cast irons, and Part 3 covers corrosion-resistant alloys including stainless steels, duplex, and nickel alloys. Compliance is mandatory for upstream sour service equipment and is specified by operators and EPC contractors worldwide.

Q: When does sour service apply under NACE MR0175?
A: NACE MR0175 / ISO 15156 applies when the partial pressure of H₂S in the gas phase equals or exceeds 0.05 psia (0.0003 MPa, 345 Pa). Partial pressure is calculated as total system pressure multiplied by the mole fraction of H₂S in the gas phase, which means a stream with low H₂S concentration can still be sour at high total pressure — a common oversight. Free water must also be present for H₂S to dissolve and form the acidic solution that drives the corrosion reaction. Crude oil and water handling facilities operating below approximately 65 psia (448 kPaa) total pressure are generally excluded. Beyond the threshold, material qualification depends on the full environment: partial pressure, in-situ pH, temperature, and chloride concentration.

Q: What is the hardness limit for NACE MR0175?
A: For carbon and low-alloy steels, the maximum hardness is 22 HRC (250 HV10). Critically, this limit applies to the parent metal, the weld metal, and the heat-affected zone — all three, not just the base material. The heat-affected zone is often the hardest region in a fabricated component and is where sulphide stress cracking failures concentrate, so welding procedures for sour service must be qualified with hardness surveys across the weld and HAZ. Hardness is the primary control because hard microstructures, particularly untempered martensite, are far more susceptible to hydrogen embrittlement. The standard also restricts cold work, which raises hardness and residual stress, and specifies acceptable heat treatment conditions.

Q: What is the difference between SSC and HIC?
A: Sulphide stress cracking (SSC) is brittle cracking under the combined action of applied tensile stress and hydrogen absorbed from the sour environment. It requires stress, occurs suddenly with no prior deformation, and is controlled primarily through hardness limits — hence the 22 HRC rule. Hydrogen induced cracking (HIC) is different: atomic hydrogen collects at internal discontinuities, typically elongated manganese sulphide inclusions in rolled plate and pipe, forming internal blisters and stepwise cracks. HIC does not require applied stress and is controlled through clean steelmaking practice — low sulphur and inclusion shape control — rather than hardness. A material meeting the 22 HRC limit is therefore not automatically HIC-resistant, and HIC testing per NACE TM0284 must be specified separately where relevant.

Q: What bolting is required for NACE sour service?
A: Standard ASTM A193 Grade B7 studs and A194 Grade 2H nuts exceed the NACE MR0175 hardness limit and are not acceptable for sour service. The correct specification is ASTM A193 Grade B7M studs with ASTM A194 Grade 2HM nuts — the "M" designating the hardness-controlled variants produced and tested specifically for sour service, with hardness within the 22 HRC / 250 HV10 limit. B8M stainless bolting is used where specified. Specifying standard B7/2H on a NACE-compliant line is a common and consequential error, because the bolting becomes the weakest link in an otherwise compliant joint. Hardness results for the bolting should appear on the material test certificate.

Q: How do you verify NACE MR0175 compliance?
A: Verification requires data, not statements. The material test certificate must contain actual hardness test results with values within 22 HRC / 250 HV10 — a bare "NACE compliant" statement without supporting hardness data is unverifiable and will fail audit. Check that hardness covers weld and heat-affected zone for welded items, that heat treatment condition is recorded, that heat numbers on the certificate match the physical material markings, and that HIC test reports are present where specified. For corrosion-resistant alloys, confirm that the environmental limits used for qualification match your actual service conditions. Positive Material Identification on receipt provides independent physical verification of alloy composition regardless of the paperwork.