📘 1. Executive Summary

This report provides a comprehensive comparative analysis and practical guidance on two critical corrosion standards in the oil, gas, and refining industries:

• NACE MR0175 / ISO 15156: Governs the selection and qualification of materials exposed to H₂S-containing environments in upstream oil and gas production.
• NACE MR0103: Applies to materials used in refinery environments, where H₂S-induced stress corrosion cracking (SCC) may occur under lower pressures and different conditions.

Understanding and implementing these standards is essential for ensuring safety, asset integrity, and regulatory compliance across all stages of hydrocarbon processing.

🔍 2. Background: The Corrosive Threat of Hydrogen Sulfide

Hydrogen Sulfide (H₂S) is a highly toxic, corrosive gas found in hydrocarbon production. Its interaction with steels can lead to severe cracking mechanisms, including:

• Sulfide Stress Cracking (SSC)
• Hydrogen Induced Cracking (HIC)
• Stress-Oriented HIC (SOHIC)
• Stepwise Cracking (SWC)
• Galvanically Induced HSC (GHSC)

While MR0175 / ISO 15156 applies to production systems under pressure, MR0103 is tailored to atmospheric or low-pressure systems typical of downstream refining and chemical environments. Both standards define material properties, environmental limits, and qualification protocols for cracking resistance.

🧾 3. Standard Origins and Evolution

3.1 NACE MR0175 / ISO 15156

• First published in 1975 by the National Association of Corrosion Engineers (NACE).
• Adopted as ISO 15156 in 2003 for global harmonization.
• Structured in three parts:
  • Part 1: General principles
  • Part 2: Carbon and low alloy steels
  • Part 3: Corrosion-resistant alloys (CRAs)
• Emphasizes high-pressure, sour gas production environments.
• Maintained by ISO/TC 67 and the Maintenance Panel (MP).

3.2 NACE MR0103

• First published in 2003 as a separate standard.
• Targets refining and petrochemical equipment operating at or near atmospheric pressure.
• Simpler structure focused on:
  • SSC-resistant materials
  • Equipment with tensile stress (welded, bolted, cold-worked)
  • Hardness limits and inspection requirements
• Does not include CRA pitting resistance or chloride effects.

🏗️ 4. Scope and Application

🧪 5. Failure Mechanisms and Material Behavior

5.1 Sulfide Stress Cracking (SSC)

Initiated by H₂S dissociation and hydrogen diffusion into the steel's crystal lattice. It is exacerbated by:

• High tensile stress (applied or residual)
• High hardness (typically > 22 HRC)
• Residual stresses from welding or cold work

5.2 Hydrogen Induced Cracking (HIC)

Results from atomic hydrogen recombining to form molecular hydrogen gas at internal interfaces, creating high-pressure pockets or blisters. This is particularly critical in carbon steels with non-metallic inclusions or laminations.

5.3 SOHIC / SWC / GHSC

These are more complex cracking mechanisms that occur in specific stress states and microstructures, often found in pressure vessels and heat-affected zones (HAZ) of welds. Their occurrence depends on a combination of microstructure, stress state, and environmental severity.

🛠️ 6. Materials, Hardness, and Metallurgical Requirements

6.1 NACE MR0175 / ISO 15156

Carbon and Low Alloy Steels:

• Acceptable if hardness is ≤ 22 HRC (or equivalent).
• Weld procedures must control hardness in the Heat-Affected Zone (HAZ).
• Must be free of cold work that imparts stresses, particularly in components like threads or press fits.

CRAs (Stainless/Nickel Alloys):

• Listed in tables by their UNS designation.
• Subject to strict limits on H₂S partial pressure, chloride content, temperature, and pH.
• Example: UNS S31603 (316L SS) is acceptable up to 200°F, 0.3 bar H₂S, and 100,000 ppm chloride.

6.2 NACE MR0103

• Acceptable alloys are listed in Table 1 of the standard.
• Hardness is strictly limited to 22 HRC or 248 HV maximum for most materials.
• Some forgings and castings have a limit of 200 HBW.
• Requires Post-Weld Heat Treatment (PWHT) if the HAZ hardness exceeds the specified limit.
• Cold work should be minimized or the component must be subsequently annealed.

🧪 7. Qualification and Testing Procedures

7.1 MR0175 / ISO 15156

Provides three distinct paths for material qualification:

1. Pre-qualified (Annex A list): Materials listed in the standard are acceptable if they meet the specified metallurgical condition and environmental limits. Traceability is key.
2. Field Qualification: Requires a minimum of two years of documented, uncracked service in the specific operating conditions (T, P, pH₂S).
3. Laboratory Testing: Involves rigorous testing such as SSC tests per NACE TM0177 and SCC tests in simulated environments. Acceptance criteria require no cracks at specified stress levels.

Documentation Required: Comprehensive records including Material Test Reports (MTRs), welding procedures, test records, and a detailed definition of the service environment (Cl⁻, H₂S, CO₂, pH, temp, pressure).

7.2 MR0103

Focuses primarily on preventing SSC:

• Only requires SSC testing (NACE TM0177, Method A or B).
• Does not mandate tests for SCC, GHSC, or SOHIC.
• Documentation is simpler, focusing on HRC/Vickers test reports, heat treatment records, and visual weld inspection for defects.

📄 8. Compliance and Marking

Both standards mandate strict traceability and documentation:

• Materials must be traceable to their lot/heat number and manufacturer.
• Purchase documentation must explicitly specify the standard (e.g., MR0175-2015, MR0103-2012), metallurgical condition, and hardness or SSC test results.
• API 6A Class ZZ for wellhead equipment specifically incorporates MR0175 requirements.
• Field Failures: If cracking occurs in a component despite initial compliance, the standard mandates requalification and replacement, often with a more resistant material.

🧰 9. Field Application & Case Studies

📦 10. Appendices and Worksheets

ISO 15156

• Appendix A-C: Detailed environmental tables for CRAs.
• Appendix D-E: Forms and flowcharts to guide the qualification process.

MR0103

• Tables: Lists of acceptable materials and their corresponding hardness limits.
• Definitions: Clarifications on terms and potential sources of stress.

💡 11. Key Implementation Challenges

• Hardness Testing: Variations between Rockwell and Vickers methods can cause disputes. Accessing HAZs for testing in complex geometries is often difficult.
• Supplier Compliance: Many vendors are not fully aware of the crucial differences between MR0103 and MR0175, leading to incorrect material supply.
• Engineering Design: The feasibility of performing PWHT on large or field-assembled components can be a major constraint. The risk of inducing cold work in fasteners or during machining is often overlooked.
• Documentation Gaps: Field experience reports often lack precise data on pH₂S or pH, making qualification by this method unreliable. Many older specifications still reference obsolete versions of MR0175.

📌 12. Recommendations for Practitioners

• Use MR0175 for all upstream systems where H₂S partial pressure exceeds 0.05 psia.
• Use MR0103 in refining/chemical equipment exposed to aqueous H₂S environments, even at low pressures.
• Specify the exact standard version and relevant annex/table on all Purchase Orders (POs).
• Verify post-weld hardness with calibrated, certified testers before putting equipment into service.
• Conduct regular inspections and hardness verification during planned shutdowns.
• Utilize the flowcharts in the standards to guide material selection for new vs. existing facilities.
• Incorporate qualification steps directly into project QA/QC plans.
• Train supply chain partners on the distinction between the two standards to prevent costly errors.
• Retest materials if processing conditions change (e.g., introduction of cold work, new weld procedure).
• Stay updated on interpretations and revisions from the NACE/ISO Maintenance Panel.

✅ 13. Conclusion

Together, NACE MR0175/ISO 15156 and NACE MR0103 provide a robust framework for ensuring material integrity in H₂S-laden environments. While MR0175 is the definitive standard for the high-pressure production domain, MR0103 is specifically tailored to the unique conditions of the refining sector. Understanding their differences, scope, and technical requirements is vital to preventing catastrophic cracking failures, maintaining operational uptime, and ensuring worker safety.

By rigorously applying the guidelines, verifying metallurgical conditions, and documenting every decision, engineers and asset managers can confidently protect critical infrastructure and meet global regulatory expectations.