What Is NACE MR0175 / ISO 15156?
NACE MR0175 / ISO 15156 is the internationally recognised standard for selecting metallic materials in oil and gas environments containing hydrogen sulphide (H₂S). It defines which alloys and grades are acceptable to prevent sulfide stress cracking (SSC), hydrogen induced cracking (HIC), and related failure mechanisms.
The standard is structured in three parts:
- ISO 15156-1 — General principles for selecting cracking-resistant materials.
- ISO 15156-2 — Requirements for carbon and low-alloy steels.
- ISO 15156-3 — Requirements for corrosion-resistant alloys (CRAs): stainless steels, duplex SS, and nickel alloys.
Most purchase orders reference both designations — e.g.
NACE MR0175 / ISO 15156 Part 3 — to avoid ambiguity. Major operators and EPC contractors worldwide mandate compliance for all sour service equipment.
Sour Service vs Sweet Service
Sweet service contains no H₂S above threshold levels. Standard materials like ASTM A105 or A106 are acceptable without NACE MR0175 compliance.
Sour service exceeds those thresholds. H₂S partial pressure, in-situ pH, temperature, and chloride content all determine whether NACE MR0175 requirements apply. A common field reference point: H₂S partial pressure above 0.0003 MPa (0.05 psia) with pH below 3.5 is sour — but always confirm against the governing project specification. Operating conditions, not general rules, determine compliance scope.
Three Failure Mechanisms NACE MR0175 Prevents
Sulfide Stress Cracking (SSC)
The most common H₂S failure mode. Hydrogen enters the metal lattice under tensile stress, causing brittle fracture. NACE MR0175 controls SSC through hardness limits — maximum
22 HRC (248 HBW) for carbon and low-alloy steels — applying to base metal, weld metal, and heat-affected zone (HAZ).
Hydrogen Induced Cracking (HIC)
Stress-independent failure: hydrogen accumulates at internal inclusions, causing blistering in plate and sour service pipe. Controlled through steel cleanliness requirements and mandatory HIC testing for plate and seam-welded pipe in sour service.
Stress Oriented Hydrogen Induced Cracking (SOHIC)
HIC blisters link stepwise under stress, particularly in weld HAZs. Requires careful post-weld heat treatment (PWHT) and welding procedure validation.
NACE MR0175 Compliant Materials
The following covers the most commonly specified sour service materials, their applicable ISO 15156 part, SSC resistance, and key compliance requirements. Always verify against the applicable standard part and Annex A tables for your actual operating conditions.
Carbon Steel (Low Alloy) — ISO 15156 Part 2
SSC resistance acceptable within limits. Maximum 22 HRC / 248 HBW. PWHT required on welds. HIC testing mandatory for plate and seam-welded pipe.
316L Stainless Steel (UNS S31603) — ISO 15156 Part 3
Limited SSC resistance. Acceptable at low H₂S partial pressure only. Not suitable for aggressive sour service.
Duplex SS 2205 (UNS S32205) — ISO 15156 Part 3
Good SSC resistance within limits. Maximum 36 HRC. Temperature and H₂S limits apply per Annex A tables.
Super Duplex SS 2507 (UNS S32750) — ISO 15156 Part 3
Good SSC resistance within stricter limits than 2205. Operating conditions must be confirmed against Annex A before specification.
Alloy 825 / Incoloy 825 (UNS N08825) — ISO 15156 Part 3
Excellent SSC resistance. Cost-effective CRA widely used in GCC sour pipelines.
Alloy 625 / Inconel 625 (UNS N06625) — ISO 15156 Part 3
Excellent SSC resistance. High-performance CRA for aggressive sour service and downhole applications.
Monel K500 (UNS N05500) — ISO 15156 Part 3
Excellent SSC resistance in the age-hardened (H) condition. Standard for downhole shafts and fasteners in UAE and GCC wells.
How to Specify NACE MR0175 in a Purchase Order
A properly specified sour service PO must include:
Standard reference: State
NACE MR0175 / ISO 15156 with the applicable part number. Referencing both designations prevents ambiguity between supplier and buyer QA teams.
Environmental conditions: H₂S partial pressure, in-situ pH, chloride content, and maximum operating temperature. These are required for CRA grade confirmation against ISO 15156-3 Annex A — without them, a supplier cannot confirm the grade is acceptable for your specific service.
Hardness limits: State the maximum HRC, HBW, or HV10 explicitly. The limit applies to base metal, weld metal, and HAZ — not base metal alone.
PWHT requirement: State whether post-weld heat treatment is required and to which specification. For carbon and low-alloy steels in sour service, PWHT is typically mandatory on welded joints.
Certificate type: EN 10204 3.1 MTC as minimum, confirming NACE MR0175 compliance. EN 10204 3.2 with independent third-party verification is required for critical offshore, subsea, and major operator project specifications.
For a detailed breakdown of EN 10204 certificate types and when to specify each, refer to our
EN 10204 2.2 vs 3.1 vs 3.2 certification guide.
Frequently Asked Questions
What are the NACE MR0175 hardness requirements for carbon steel?
Maximum 22 HRC (248 HBW / 250 HV10) for base metal, weld metal, and HAZ. PWHT is typically required on weld joints to bring HAZ hardness within this limit.
Which nickel alloys comply with NACE MR0175?
Alloy 625, Alloy 825, and Monel K500 are all accepted under ISO 15156-3. Alloy 625 and 825 are used for aggressive sour CRA applications; Monel K500 in the age-hardened condition is standard for downhole components in UAE and GCC wells.
Can duplex stainless steel be used in sour service?
UNS S32205 and S32750 are acceptable within defined H₂S partial pressure, temperature, and pH limits under ISO 15156-3 Annex A. Confirm your actual operating conditions before specifying duplex for sour service pipe or fittings — the Annex A tables are the governing reference.
Is NACE MR0175 the same as ISO 15156?
Yes — technically identical in content. ISO 15156 is the international adoption of NACE MR0175. The current edition is harmonised as ISO 15156:2015. Project specifications typically reference both to avoid ambiguity at the supply chain level.
Which EN 10204 certificate should I specify for NACE MR0175 materials?
A 3.1 MTC confirming NACE MR0175 compliance is the standard minimum for sour service projects. EN 10204 3.2 with independent third-party countersignature may be required by the operator or project engineer for critical components. State the certificate type explicitly in the PO line item.
Source NACE MR0175 Certified Materials from Dubai
Nifty Alloys LLC is a Dubai-based specialist metals stockist supplying NACE MR0175 / ISO 15156 compliant materials to oil and gas, petrochemical, and marine projects across the UAE, GCC, and global markets.
We supply Monel K500, Alloy 625, Alloy 825, duplex UNS S32205, super duplex UNS S32750, 316L stainless, and low-alloy carbon steels — with EN 10204 3.1 MTCs confirming NACE compliance as standard. EN 10204 3.2 with independent third-party certification is available for critical offshore and major operator project requirements. All certificates are reviewed by our in-house QA/QC team before dispatch.
Contact Nifty Alloys for material availability, certified supply, and sourcing support:
niftyalloys.com
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