Guide to NACE MR0175 / ISO 15156 | Materials for H₂S in Oil & Gas

Comprehensive Report on NACE MR0175 / ISO 15156

Materials for Use in H₂S-Containing Environments in Oil and Gas Production

Based on the Canadian Association of Petroleum Producers (CAPP) Guide, December 2005

1. Introduction and Context

Hydrogen sulfide (H₂S), a naturally occurring contaminant in oil and gas production, is highly toxic and corrosive. It poses significant risks to personnel, equipment, and the environment. To mitigate these risks, the industry relies on standards like NACE MR0175 / ISO 15156 to guide the selection of materials resistant to H₂S-induced cracking. This global standard harmonizes material requirements and testing protocols to ensure safety and reliability in sour service environments. This report summarizes the 2005 guide from the Canadian Association of Petroleum Producers (CAPP), designed to help stakeholders correctly interpret and apply this critical standard.

2. Objectives of the Standard

Prevent Failures

Prevent sulfide stress cracking (SSC) and other hydrogen-induced failures in H₂S environments.

Provide Criteria

Offer clear criteria for pre-qualified, field-tested, or laboratory-tested materials to ensure suitability.

Establish Documentation

Mandate robust documentation and traceability procedures for all materials to ensure compliance and accountability.

3. Historical Development and Standard Evolution

1975: Original NACE MR0175

Focused primarily on preventing Sulfide Stress Cracking (SSC) by establishing H₂S partial pressure limits and providing lists of SSC-resistant materials.

2003: Integration with ISO 15156

NACE collaborated with the European Federation of Corrosion (EFC) and ISO to create a harmonized global standard. This first joint edition broadened the scope significantly.

Expanded Failure Mechanisms

The modern standard now addresses a comprehensive range of H₂S-related cracking mechanisms, including:

SSC
HIC
SOHIC
SWC
SZC
GHSC
SCC

4. Organizational Structure and Maintenance

Maintenance Panel (MP)

Composed of 15 members with 4-year terms.
Handles interpretations and revisions of the standard.
Decisions are passed to ISO/TC 67/WG 7 if consensus is reached.

Oversight Committee (TG299)

Consists of 30-50 members with 5-year terms.
Ballots on proposals from the Maintenance Panel.
Requires a 2/3 consensus for proposals to pass.

5. Standard Composition: A Three-Part Breakdown

Part 1: General Principles

Applies only to upstream oil and gas operations (not refining).
Excludes equipment loaded only in compression.
Focuses solely on cracking mechanisms, not general corrosion.

Part 2: Carbon & Low Alloy Steels

SSC-resistance is required if H₂S partial pressure (P_H₂S) ≥ 0.05 psi.
Classifies materials by environmental severity (Regions 0 to 3).
Strict hardness requirements (Vickers, Rockwell, HRC).

Part 3: Corrosion Resistant Alloys (CRA)

Covers SSC, SCC, and Galvanically-Induced Hydrogen Stress Cracking (GHSC).
Selection tables define acceptable alloys and environmental limits.
Uses PREN (Pitting Resistance Equivalent Number) for chloride environments.

6. Qualification Pathways for Materials

Pre-Qualified Materials

These materials are listed in Annex A of the standard, based on established field performance or prior lab results. They require full traceability and proper marking for use.

Qualification by Field Experience

Requires a minimum of 2 years of successful, crack-free operation. This pathway demands detailed documentation of the material's chemical composition, heat treatment, service environment, and inspection records.

Qualification by Laboratory Testing

Used when a material is not pre-qualified and field data is unavailable. This involves a rigorous, documented process of selecting test procedures, defining the test environment, and establishing clear acceptance criteria.

7. Application Guide for End Users

Decision Flow for New vs. Existing Facilities

The end-user is responsible for defining service conditions and ensuring materials are qualified. The process differs for existing assets versus new projects.

Existing Facility

Inspect Components: Assess current equipment for any signs of cracking.
If Cracked: The material is unsuitable. Replace with an alternative, qualified material.
If Uncracked: Validate the material against the pre-qualified lists in Annex A. If it matches, it can continue to be used with proper documentation.

New Project

Assess Service Conditions: Determine if the new service is identical to a proven, existing application.
If Identical: The same qualified material can be used.
If Not Identical: The material must be fully qualified for the new conditions via field experience or laboratory testing.

8. Summary of Key Concepts

Section	Key Concept
Evolution	Transitioned from an SSC-only focus to a comprehensive risk model for all H₂S-related cracking mechanisms.
Structure	Organized into a 3-part ISO standard tailored to general principles, carbon/low alloy steels, and corrosion-resistant alloys (CRAs).
Responsibility	End-users hold the ultimate responsibility to define service conditions, qualify materials, and maintain documentation.
Qualification	Materials not on the pre-qualified list must be qualified through documented field experience or rigorous laboratory testing.
Traceability	Marking, material IDs, and purchase records are mandatory to ensure a clear link between a component and its qualification documentation.

9. Conclusion

NACE MR0175 / ISO 15156 is the international benchmark for selecting materials for sour gas environments. It ensures safety and long-term reliability by mandating rigorous evaluation, documentation, and qualification procedures. The CAPP guide provides essential, practical support for interpreting and applying this standard. Adherence not only ensures regulatory compliance but also significantly reduces the risk of catastrophic equipment failure due to environmental cracking, protecting people, assets, and the environment.