NACE MR0175 / ISO 15156:2015
Full Title: Petroleum and natural gas industries — Materials for use in H₂S-containing environments in oil and gas production
This is an international standard jointly issued by NACE (National Association of Corrosion Engineers) and the International Organization for Standardization (ISO). It provides guidance on the selection and use of metallic materials in environments containing hydrogen sulfide (H₂S), with the goal of preventing corrosion and cracking in oil and gas production systems.
Why Does the NACE MR0175 / ISO 15156 Standard Exist?
The purpose of NACE MR0175 / ISO 15156 is to prevent “invisible brittle failures” in metallic materials exposed to H₂S environments that can lead to catastrophic accidents.
In oil and gas production, many reservoirs contain hydrogen sulfide (H₂S) — such environments are referred to as “sour service.” H₂S is extremely corrosive and can cause several types of material failures:
| Corrosion Type | Full Name | Primary Failure Mechanism |
| SSC | Sulfide Stress Cracking | Brittle fracture of metal under tensile stress in the presence of H₂S (typical for carbon steel) |
| HIC | Hydrogen Induced Cracking | Formation of hydrogen bubbles within steel, leading to laminar cracks |
| SOHIC | Stress Oriented Hydrogen Induced Cracking | Multi-layer cracks oriented along principal stress directions |
| SCC | Stress Corrosion Cracking | Cracking in the presence of chlorides or other aggressive media |
These failures often occur without warning, leading to sudden rupture of wellheads, pipelines, valves, and pressure vessels, causing major safety, environmental, and financial losses.
Such incidents drove the National Association of Corrosion Engineers (NACE) to publish the first NACE MR0175 standard in 1975, providing technical guidance for selecting and qualifying metallic materials suitable for H₂S environments.
With increasing global collaboration among oil companies (e.g., Shell, BP, Total, Petrobras, CNPC, Saudi Aramco), the lack of unified standards (EN, ASTM, API, etc.) created inconsistencies.
Therefore:
- 2003 – NACE MR0175 was harmonized with ISO 15156;
- 2009 and 2015 – Major revisions were issued;
- Today’s version: NACE MR0175 / ISO 15156:2015 is jointly maintained by ISO/TC 67 and the NACE/ISO Working Group 7 (WG7).
Typical Application Scenarios
| Application | Description |
| Sour Gas Field Development | Wellhead assemblies, Christmas trees, tubing, casing, valves |
| Gas Gathering Pipelines | High-pressure H₂S-containing flowlines |
| Refinery Units | Catalytic cracking, desulfurization, hydroprocessing units |
| Offshore Platforms | Combined H₂S + chloride corrosion environments |
Detailed Material Requirements under NACE MR0175 / ISO 15156:2015
The standard is divided into three parts:、
Part 1 – General Principles
Defines scope, terminology, corrosion mechanisms, environmental parameters (temperature, pH, H₂S partial pressure), and establishes material selection and qualification procedures.
Part 2 – Carbon and Low Alloy Steels (Cr ≤ 13%)
Specifies requirements for hardness, heat treatment, weldment condition, and SSC resistance.
- Typical maximum hardness: ≤ 22 HRC (250 HV)
- Requires control of S and P impurities
- Avoids hardened microstructures in high-stress regions
Main Requirements
| Parameter | Requirement |
| Hardness | ≤ 22 HRC (≈ 250 HV, Rockwell B 95) |
| Heat Treatment | Normalized (N), Annealed (A), or Quenched & Tempered (Q&T) — no untempered martensite |
| Weld Zone | Post-weld heat treatment required; hardness ≤ 22 HRC |
| Sulfur / Phosphorus | S ≤ 0.010%, P ≤ 0.020% (recommended) |
| Environment | Applicable when H₂S pₚ ≥ 0.0003 MPa; pH > 3.5; T < 250°C |
| Stress Control | ≤ 75% of yield strength |
| Welding Material | Must meet equivalent hardness and compatibility requirements |
Part 3 – Corrosion Resistant Alloys (CRAs)
Covers stainless steels, nickel-based alloys, and titanium alloys, categorized into six metallurgical classes (A–F).
A. Martensitic Stainless Steels
| Grade | UNS | Heat Treatment & Hardness | Service Limitation |
| Type 410, 420 | S41000, S42000 | Tempered; ≤ 23 HRC | pH > 3.5; low Cl⁻, low H₂S |
| Type 431 | S43100 | Tempered; ≤ 28 HRC | Same as above |
| Modified 13Cr (e.g., S41425) | Q&T | ≤ 26 HRC | Mild sour conditions only |
B. Ferritic Stainless Steels
| Grade | UNS | Requirement | Limitation |
| 444 / 446 | S44400 / S44600 | Solution annealed; ≤ 250 HV | ≤ 60°C, low Cl⁻ |
C. Austenitic Stainless Steels
| 304L / 316L | S30403 / S31603 | Solution annealed, no strain-induced martensite | Cl⁻ ≤ 100,000 mg/L, ≤ 60°C |
| 317L / 904L | S31703 / N08904 | Same | Improved Cl⁻ and H₂S resistance |
| 254SMO | S31254 | Solution annealed | Up to 100°C, high Cl⁻ |
| Alloy 20 | N08020 | Annealed | Moderate Cl⁻, ≤ 60°C |
D. Duplex Stainless Steels
| Grade | UNS | Phase Balance | Hardness | Service Limitation |
| 2205 | S31803 / S32205 | 35–65% ferrite–austenite | ≤ 32 HRC | T ≤ 232°C, Cl⁻ ≤ 100,000 mg/L |
| 2507 | S32750 / S32760 | Same | ≤ 32 HRC | T ≤ 250°C |
| Lean Duplex 2101 / 2304 | S32101 / S32304 | Same | ≤ 28 HRC | Low H₂S environments only |
E. Nickel-Based Alloys
| Alloy | UNS | Condition | Hardness | Typical Use |
| Alloy 625 | N06625 | Annealed | ≤ 40 HRC | High H₂S and high temperature |
| Alloy 718 | N07718 | Aged | ≤ 40 HRC | High-temp wellhead, fasteners |
| Alloy C-276 | N10276 | Annealed | ≤ 40 HRC | Strong acid, high Cl⁻ |
| Alloy 825 | N08825 | Annealed | ≤ 35 HRC | Moderate sour service |
| Alloy 400 (Monel) | N04400 | Annealed | ≤ 35 HRC | CO₂ + H₂S mixed environments |
F. Titanium and Titanium Alloys
| Grade | UNS | Condition | Remarks |
| Gr.2 (CP Ti) | R50400 | Annealed | Excellent H₂S and SSC resistance |
| Gr.5 (Ti-6Al-4V) | R56400 | Annealed | Control hydrogen uptake; avoid high Cl⁻ |
| Gr.12 (Ti-0.3Mo-0.8Ni) | R53400 | Annealed | Improved high-temperature stability |
Typical Environmental Limits
| Parameter | Carbon Steel | Duplex SS | Austenitic SS | Nickel Alloys | Titanium |
| H₂S Partial Pressure (MPa) | ≤ 0.003 | ≤ 1.0 | ≤ 0.1 | up to ≥ 10 | Virtually unlimited |
| Cl⁻ Concentration (mg/L) | — | ≤ 100,000 | ≤ 100,000 | ≤ 300,000 | ≤ 10,000 |
| Temperature (°C) | ≤ 250 | ≤ 250 | ≤ 100 | ≤ 300 | ≤ 315 |
| pH | > 3.5 | > 3.0 | > 3.0 | > 2.0 | No limitation |
Qualification and Compliance
- Pre-qualified materials: Those listed in ISO 15156 tables.
- Unlisted materials: Must be validated per ISO 15156-1 Annex B (Qualification Testing).
- Certificate of Compliance should state:
- UNS material designation
- Hardness and heat treatment condition
- Conformance declaration (e.g., Conforms to NACE MR0175 / ISO 15156:2015 Part X, Clause Y)
Examples of MR0175-Compliant Materials
| Category | UNS | Material Name | Typical Standard | MR0175 Status |
| Carbon Steel | — | A105 / A106 Gr.B | ASME B16.5 | ✅ N, ≤22 HRC |
| Low Alloy | K21590 | AISI 4130 | ASTM A182 F22 | ✅ Q&T, ≤22 HRC |
| Austenitic SS | S31603 | 316L | ASTM A182 F316L | ✅ SA |
| Duplex SS | S32205 | 2205 | ASTM A182 F51 | ✅ SA, ≤32 HRC |
| Super Duplex | S32750 | 2507 | ASTM A182 F53 | ✅ |
| Nickel Alloy | N06625 | Alloy 625 | ASTM B446 | ✅ |
| Titanium | R50400 | Grade 2 | ASTM B348 | ✅ |
FAQ
- Carbon steels and low alloy steels (e.g., A105N, A106, A333)
- Stainless steels (austenitic and duplex grades)
- Nickel-based, titanium-based, and other corrosion-resistant alloys (CRA)
Each category must meet specific requirements for chemical composition, hardness, and resistance to cracking in H₂S environments.
The Mill Test Certificate (MTC) should clearly state:
- “Material meets the requirements of NACE MR0175 / ISO 15156.”
And include:
- Hardness test results
- Chemical composition report
- HIC and SSC test results (if applicable)
- Heat treatment record
No.
NACE MR0175 applies only to sour (H₂S-containing) environments.
If the system operates in “sweet service” (no H₂S), standard materials such as ASTM A105 or A106 can be used without NACE compliance.
Only materials used in sour service conditions are required to meet NACE MR0175 / ISO 15156.
Summary
The essence of NACE MR0175 / ISO 15156:
A global framework for safe material selection in sour (H₂S) environments — defining the limits, mechanical conditions, and qualification requirements for metallic materials used in oil and gas production.
It specifies:
- Maximum allowable hardness
- Required heat treatment and microstructure
- Environmental boundaries (T / pH / Cl⁻ / H₂S pressure)
- Qualification and certification rules
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