NACE MR0175 Sour Service: H₂S, Hardness Limits & Material Requirements

In 1952, a catastrophic pipeline failure in Alberta, Canada released sour gas — a refinery explosion the following year in Texas killed 35 workers. The cause in both cases was sulphide stress cracking (SSC): H₂S from the process fluid permeated into high-strength carbon steel and caused sudden brittle fracture with no warning. NACE Standard MR0175 (now also published as ISO 15156 in three parts) was developed specifically to prevent these failures by defining the metallurgical requirements for all metals used in H₂S-containing oilfield environments.

Sour Service Definition — H₂S Threshold

NACE MR0175 / ISO 15156 applies when the partial pressure of H₂S in the gas phase equals or exceeds 0.05 psia (0.0003 MPa, 345 Pa). In a wet gas stream, H₂S dissolves in free water to form a mildly acidic solution — any system where free water is present with H₂S is considered sour. The partial pressure is calculated as: pH₂S = total system pressure × mole fraction of H₂S in gas phase. A typical sour gas well producing at 500 psia with 0.1 mol% H₂S has pH₂S = 0.5 psia — well above the 0.05 psia threshold. If the produced gas is recompressed to 5000 psia, pH₂S = 5 psia — requiring more stringent NACE compliance.

Cracking Mechanisms in Sour Service

Two distinct cracking mechanisms must be addressed. Sulphide Stress Cracking (SSC) is a form of hydrogen embrittlement: H₂S in acidic solution promotes hydrogen absorption into steel; if the steel has high strength (high hardness), the absorbed hydrogen causes brittle cracking under applied or residual stress. SSC is most severe in high-strength, high-hardness steels — welds and heat-affected zones are particularly susceptible. Hydrogen-Induced Cracking (HIC) is stepwise cracking caused by hydrogen accumulation at internal inclusions in plate/pipe steel — HIC occurs without applied stress and is particularly problematic in rolled steel plates and large-bore pipe seam welds. NACE MR0175 Part 1 addresses carbon and low-alloy steels (SSC and HIC); Part 2 addresses Corrosion Resistant Alloys (CRAs); Part 3 covers cracking-resistant CRAs for specific environments.

Hardness Limits for Carbon Steel — The 22 HRC Rule

NACE MR0175 limits the maximum hardness of carbon steel and low-alloy steel (Groups 1 and 2 in Part 1) to 22 HRC (Rockwell Hardness C scale), equivalent to approximately 248 HV or 237 HB. This limit applies to the base metal, weld metal, and heat-affected zone (HAZ). For valve bodies (ASTM A216 WCB), the standard normalise-and-temper heat treatment produces hardness well within the 22 HRC limit. For weld repairs and attachment welds, post-weld heat treatment (PWHT) is required to relieve residual stresses and temper the HAZ martensite below 22 HRC. High-strength alloy steel bolting (ASTM A193 B7) is not acceptable for sour service in many environments — ASTM A193 B7M (tempered to lower hardness, ≤ 22 HRC per NACE) is required.

CRA Material Requirements for Sour Service

Corrosion Resistant Alloys (CRAs) are governed by NACE MR0175 Part 2, which defines three regions of application based on H₂S partial pressure and temperature: Region 1 (low H₂S, ambient temperature) allows many standard austenitic stainless steels; Region 2 (moderate H₂S) restricts hardness and heat treatment; Region 3 (high H₂S, elevated temperature) requires qualification testing. Key CRA requirements: SS 316/316L (UNS S31600/S31603) is generally acceptable per Part 2 in Region 1 conditions (low H₂S, < 60°C for some concentrations); Duplex 2205 (S31803/S32205) is acceptable to hardness ≤ 36 HRC in annealed condition; Super Duplex 2507 (S32750) is acceptable with Charpy impact test results > 40J at -46°C; Inconel 625 (N06625) is acceptable for most sour service environments without temperature restriction.

NACE-Compliant Valve Specification Requirements

To specify a NACE MR0175-compliant valve, the procurement document (MR or datasheet) must include: 'NACE MR0175 / ISO 15156 compliance required' designation; maximum H₂S partial pressure at operating conditions (to determine applicable region and hardness limit); requirement for EN 10204 3.1 MTCs with hardness test results; requirement for PWHT on all weld repairs to valve bodies and trim; Charpy impact test at design temperature for all bolting; PMI (positive material identification) on all wetted CRA components; and the mill certificate must confirm heat treatment condition (normalised+tempered for WCB; solution annealed for austenitic stainless; solution annealed for duplex stainless).

HIC Testing — NACE TM0284

NACE TM0284 (Evaluation of Pipeline and Pressure Vessel Steels for Resistance to Hydrogen-Induced Cracking) is the standard test method for HIC resistance in plate and pipe steel used in sour service. Test samples are immersed in NACE TM0177 Solution A (0.5% acetic acid + 5% NaCl, saturated with H₂S at 1 atm, pH 2.7–3.3) for 96 hours, then sectioned and examined for internal cracking. Acceptance criteria per NACE TM0284: CLR (Crack Length Ratio) ≤ 15%, CTR (Crack Thickness Ratio) ≤ 5%, CSR (Crack Sensitivity Ratio) ≤ 2%. Most linepipe steels (X52 through X70) meeting NACE TM0284 are pre-qualified; valve bodies (castings) are generally exempt from TM0284 HIC testing as the casting morphology is different from rolled plate — SSC testing per NACE TM0177 typically applies to valve bodies instead.

Sour Service Valve Procurement Checklist

• Confirm H₂S partial pressure at operating conditions — triggers NACE MR0175 at ≥ 0.05 psia
• Specify 'NACE MR0175 / ISO 15156 compliant' on all valve datasheets and purchase orders
• Specify maximum 22 HRC hardness for carbon steel body, trim, bolting (use B7M not B7 bolting)
• Require PWHT on all weld repairs — document PWHT parameters (temperature, hold time, cooling rate) in MDR
• Specify EN 10204 3.1 MTCs with hardness test results for all pressure-containing components
• For CRA valves: specify NACE MR0175 Part 2 region applicability based on H₂S pp and temperature
• For trim materials: specify Inconel 625, SS 316L, or other Part 2 compliant alloys — no carbon steel trim
• Require 100% PMI on all CRA (SS, Duplex, Inconel) wetted parts before despatch