1. Standard Overview

NACE MR0175/ISO 15156 is the globally recognized standard for the selection of metallic materials for use in H₂S-containing (sour) oil and gas production environments. Jointly published by NACE International (now AMPP) and the International Organization for Standardization (ISO), the current effective revision is ANSI/NACE MR0175-2021/ISO 15156:2020.

The establishment of this standard stems from the industry's push into deeper formations and more remote offshore resources. As development of sour oil and gas fields increases, corrosion caused by hydrogen sulfide has become increasingly critical—particularly sulfide stress cracking (SSC), which can cause brittle failure in high-strength carbon and low-alloy steels, leading to catastrophic incidents.

2. Standard Structure and Scope

The standard is divided into three parts, combined into a single PDF file, and includes multiple Technical Circulars published in 2021 and 2022:

Part	Scope	Key Content
Part 1	General Principles	Defines sour environment, basic material selection principles, qualification methods
Part 2	Carbon & Low-Alloy Steels	Hardness limits (≤22 HRC), welding requirements, chemistry control
Part 3	CRAs & Other Alloys	Environmental limits per alloy family (temperature, pH₂S, chloride concentration)

The standard applies to equipment used in oil and gas production and natural gas sweetening plants where failure could pose risks to public and personnel health and safety or to the environment. It addresses all cracking mechanisms caused by H₂S, including SSC, SCC, HIC, SWC, and others.

3. Definition of Sour Environment

According to the standard, when the H₂S partial pressure in the gas phase exceeds 0.3 kPa (0.05 psia), the environment is classified as sour, requiring material selection per the standard's requirements.

Applicable equipment includes wellheads, Christmas trees, piping systems, separators, flowlines in upstream production facilities, and pipeline systems transporting sour hydrocarbons.

4. Key Material Requirements

Carbon and Low-Alloy Steels (Part 2)

Base metal, weld metal, and HAZ hardness must not exceed 22 HRC (248 HV10)

Post-weld heat treatment (PWHT) is mandatory

Strict chemistry control required

Corrosion-Resistant Alloys (Part 3)

Commonly qualified materials include: 316 SS, 6-moly alloys, 2507 duplex SS, Alloy 825, Alloy 625, Alloy C-276, and Alloy 400. Each has specific environmental limits for temperature, H₂S partial pressure, and chloride concentration.

5. Key Updates and Future Outlook

The 2021/2020 edition has remained structurally consistent with the 2003 edition, with incremental revisions through Technical Circulars. In 2018, the governing committees agreed that a major rewrite was necessary to address ease of use and incorporate industry advancements.

A completely rewritten version is currently undergoing ISO balloting and publication, restructured into three new parts:

Part 1: Material and processing requirements

Part 2: Service environment assessment and material selection

Part 3: Verification, qualification, and balloting requirements

 The new version provides significantly more detail on H₂S testing procedures, including guidance on sampling, test method selection, and the extent of testing required for product qualification.

6. Practical Significance

NACE MR0175/ISO 15156 serves as the fundamental basis for material selection in sour oil and gas field development. The standard emphasizes that no stainless steel is completely immune to chloride-induced stress corrosion cracking—thorough understanding of alloy limitations is essential for material selection in demanding sour environments. Equipment particularly susceptible to corrosion, such as valves, fittings, and instrumentation, requires strict compliance with the standard. All sour service materials must maintain full traceability, with mill test certificates confirming hardness testing, chemical compliance, and heat treatment condition.

In sour oil and gas field environments, material selection primarily follows the NACE MR0175 / ISO 15156 standard. This standard is the "golden rule" for ensuring the safe operation of equipment in hydrogen sulfide (H₂S)‑containing environments. It covers all pressure‑containing equipment and components, from wellhead facilities to downstream processing units.

Wellhead and downhole equipment: wellhead equipment, tubing and casing strings (production tubing, casing).

Piping and connecting components: line pipe used for long‑distance oil and gas transmission, for example, API 5L grade X65 line pipe, etc.

Surface processing and transfer equipment: pressure vessels, separators – pressure‑containing equipment used for oil/gas separation and processing.

In addition to the above products, the standard also covers various material types, including carbon steel, low‑alloy steel, stainless steel, and nickel‑based alloys.