NACE MR0175-2021 / ISO 15156:2020 provides a framework for selecting and qualifying metallic materials against cracking in H2S-containing oil and gas production environments. The supplied 141-page PDF combines Parts 1, 2 and 3: general principles, carbon and low-alloy steels, and corrosion-resistant alloys (CRAs). This original guide explains how the parts work together for pipe, tube and equipment procurement. Use the complete standard and documented service conditions for material acceptance.
NACE MR0175-2021 / ISO 15156:2020 PDF — Download Standard
What Is NACE MR0175-2021 / ISO 15156:2020?
The series addresses cracking associated with wet H2S exposure in oil and natural gas production and natural-gas treatment service. It supplements the applicable design codes, regulations and product specifications. A material grade, hardness result or general statement of “NACE compliance” does not by itself establish suitability for every sour environment.
The supplied document identifies the three parts as fourth editions identical to ISO 15156-1:2020, ISO 15156-2:2020 and ISO 15156-3:2020 respectively. Its designation is ANSI/NACE MR0175-2021; the copy also carries 2022 copyright information and an American National Standard approval date of 21 September 2022. State the complete designation and required parts on the purchase order rather than treating these different dates as interchangeable edition labels.
Standard at a Glance
| Item | Supplied combined standard |
|---|---|
| Designation | ANSI/NACE MR0175-2021 / ISO 15156:2020 |
| Edition | Fourth edition of each included part |
| PDF contents | Parts 1, 2 and 3, combined in 141 pages |
| Part 1 | General principles for selecting cracking-resistant materials |
| Part 2 | Cracking-resistant carbon and low-alloy steels; use of cast irons |
| Part 3 | Cracking-resistant CRAs and other alloys |
| Primary application | H2S-containing oil and gas production and natural-gas treatment equipment |
Part 1 vs Part 2 vs Part 3
| Part | Purpose | How to use it |
|---|---|---|
| Part 1 | Common principles, service definition and qualification framework | Start with the application, exposure conditions and documented selection basis |
| Part 2 | Carbon and low-alloy steel requirements | Assess SSC, HIC/SWC, SOHIC and SZC as relevant to the product and fabrication |
| Part 3 | CRA and other alloy requirements | Match the alloy, metallurgical condition and applicable environmental limits |
Use Part 1 together with the relevant material part. Duplex stainless steel and nickel alloys require the Part 3 provisions; carbon and low-alloy steels require Part 2. An assembly containing several material families may need both material parts.
Scope and Cracking Mechanisms
The scope includes equipment such as wells and wellheads, flowlines, gathering systems, field processing facilities, water injection and disposal equipment, and natural-gas treatment plants. The equipment tables also contain exclusions, so applicability must be checked for the actual component and exposure.
The series addresses cracking resistance. It does not provide a general guarantee against metal loss, pitting or other localized corrosion. Refining and downstream service require a separate assessment of the governing requirements. Strain-based designs also need specific consideration under Part 1.
| Mechanism | Practical consideration |
|---|---|
| SSC | Evaluate hydrogen exposure, tensile stress, material strength and hard zones |
| HIC and SWC | Assess hydrogen damage in carbon-steel products separately from SSC acceptance |
| SOHIC and SZC | Consider plate, welded equipment and susceptible heat-affected regions |
| SCC | Check CRA environment limits, including temperature and water chemistry |
| GHSC | Consider galvanic coupling and hydrogen effects for the relevant alloys |
Service Conditions and Material Selection
Part 1 requires the service conditions to be evaluated, defined and documented before material selection. Relevant factors include H2S exposure, temperature, in situ pH, water-phase chlorides, sulfur and other oxidants, applied and residual tensile stress, and material condition. Include credible shutdown, upset and chemical-treatment exposures.
For a gas-containing system, H2S partial pressure is related to its gas-phase fraction and absolute pressure. Part 2 Annex C also discusses alternative parameters such as fugacity and the treatment of gas-free liquid systems. Gas concentration alone is insufficient to describe every application.
Selection can use the existing provisions in the relevant material part, or an appropriate qualification supported by laboratory testing or documented field experience. Keep the qualified metallurgy, product condition, cracking mechanism and environmental range explicit. Qualification for one failure mechanism does not automatically prove resistance to another.
Carbon and Low-Alloy Steel Requirements
Part 2 addresses carbon and low-alloy steels, including hardness, heat treatment, welding and fabrication conditions. Under its Option 1, H2S partial pressure of at least 0.3 kPa (0.05 psi) calls for SSC-resistant steel selection using Annex A.2. Below that threshold, highly susceptible steels can still crack; the threshold must not be used as a universal exemption from every hydrogen-damage assessment.
The familiar 22 HRC limit is a conditional general parent-steel provision with composition and heat-treatment restrictions. Specific products, weld locations and applications can have different requirements. Hardness alone cannot establish complete acceptance.
HIC/SWC assessment is distinct from SSC selection. Plate and welded products may also require SOHIC or SZC consideration. Define any required tests, sampling and acceptance criteria before manufacture, and assess the final fabricated condition.
Duplex, Stainless Steel and Nickel Alloys
Part 3 covers stainless steel groups, duplex stainless steels, nickel-based alloys and other listed alloy families. Annex A supplies environmental and metallurgical provisions; Annex B supports laboratory qualification. Temperature, H2S partial pressure, chlorides, pH and sulfur-related provisions must be interpreted together with each table’s remarks and equipment restrictions.
Duplex steels are addressed in Annex A.7. Check the applicable equipment table, actual composition, heat treatment, ferrite balance, cold work and welding condition. A hardness limit for a specified downhole application cannot be generalized to every duplex pipe order. Nickel-alloy acceptance similarly depends on the particular alloy and metallurgical state.
Part 3 calculates FPREN as Cr + 3.3(Mo + 0.5W) + 16N using actual element mass percentages. PREN is useful for classification but does not independently establish sour-service cracking resistance. Apply the stated limits directly rather than rounding a failing result into acceptance.
Welding, Qualification and Traceability
Welding can create hard zones or change the microstructure and cracking susceptibility of a material. Keep welding procedure specifications and qualification records available, and use the hardness survey or microstructural examination required for the selected material group. Parent-metal evidence cannot automatically establish weld acceptance.
Laboratory qualification must represent the relevant material condition and intended environment. Documented field experience must also satisfy the Part 1 qualification framework. Record the selection or qualification method and retain supporting evidence.
Maintain traceability between the delivered material, heat or batch, test records and qualified service limits. For a special application, the associated environmental conditions form part of the traceability basis. Reassess suitability when operating conditions or fabrication details change.
Purchasing and Documentation Checklist
- Specify the full standard designation, editions and applicable parts.
- Identify alloy grade, product form, component use and governing manufacturing specification.
- Document H2S exposure, absolute pressure, temperature, minimum in situ pH and water chemistry.
- Include shutdown, upset and chemical-treatment conditions where relevant.
- State the accepted listed-material or application-specific qualification route.
- Specify delivery condition, heat treatment, strength, hardness and microstructure evidence as applicable.
- Agree WPS/PQR, weld surveys and any additional cracking tests and acceptance criteria.
- Require material certificates, heat traceability and the documented environmental limits of qualification.
For duplex pipe and tube, use the relevant product standard alongside Part 3. The sour-service standard adds material selection and qualification requirements to the manufacturing specification.
Frequently Asked Questions
Is MR0175-2021 the same edition year as ISO 15156:2020?
The designations use different years. The supplied fourth-edition parts state that they are identical to the respective ISO 15156:2020 parts. Record the full designation and applicable part.
Does this PDF contain all three parts?
Yes. The supplied 141-page file combines the general principles, carbon and low-alloy steel provisions, and CRA and other alloy provisions.
Does NACE compliance mean a grade is suitable for any sour service?
No. Suitability depends on defined service conditions, material condition, fabrication and the documented selection or qualification basis.
Does 22 HRC apply to duplex stainless steel?
Do not transfer the general Part 2 carbon-steel provision to duplex. Use the applicable Part 3 material and equipment requirements.
Does MR0175 replace ASTM A790 or ASTM A789?
No. Manufacturing specifications and sour-service selection requirements serve different purposes and must be applied together where specified.
Is cracking resistance a guarantee against all corrosion?
No. General and localized corrosion require separate assessment. The standard establishes cracking resistance within defined conditions.