What H₂S level makes a service "sour"?

For gas systems, ISO 15156 / NACE MR0175 treats an H₂S partial pressure at or above 0.05 psia (about 0.3 kPa, 0.0035 bar) as sour, triggering the material requirements. Multiphase and liquid systems have their own rules, but that partial-pressure threshold is the usual trigger.

What hardness is allowed for carbon steel in sour service?

Carbon and low-alloy steels are generally limited to 22 HRC maximum, because hardness tracks susceptibility to sulfide stress cracking. Weld and HAZ hardness must meet the same limit, which is why preheat, PWHT and consumable control matter so much in sour fabrication.

Is SSC the only cracking mechanism to worry about?

No. Sulfide stress cracking (SSC) is hardness-driven and worst at low temperature, but wet H₂S also drives hydrogen-induced cracking (HIC), stepwise cracking and SOHIC, which depend on steel cleanliness and microstructure rather than hardness. A full assessment covers all of them.

Corrosion · guide

Sour service: NACE MR0175 / ISO 15156

Hydrogen sulfide turns ordinary material selection into a cracking problem. NACE MR0175 / ISO 15156 is the governing standard for metals in H₂S-bearing oil-and-gas service. Here is how to decide if you are in scope and what the standard is actually controlling.

Are you in sour service at all?

The first question is binary: does the environment contain enough H₂S to matter? The standard sets that trigger by partial pressure rather than concentration, because partial pressure is what drives hydrogen into the steel:

p_H₂S = (mole fraction H₂S) × (total pressure) ≥ 0.05 psia → sour (gas)

Below the threshold the standard's restrictions generally do not apply; at or above it, materials must be qualified for the severity of the environment.

Severity: pH₂S and in-situ pH together

Sulfide stress cracking risk is not set by H₂S alone — it depends on the combination of H₂S partial pressure and the in-situ pH of the water phase. ISO 15156-2 maps that pair onto SSC severity regions (commonly labelled 0 through 3): low pH₂S and high pH is mild, high pH₂S and low pH is severe. The region you fall in dictates which carbon-steel conditions are acceptable without further qualification.

Computing in-situ pH needs the bicarbonate/organic-acid chemistry and the CO₂/H₂S partial pressures — it is not the pH of a sampled liquid at the surface. Get this wrong and the severity region (and the whole material decision) can shift.

What the standard controls

For carbon and low-alloy steels the dominant lever is hardness: a 22 HRC ceiling on base metal, weld and HAZ, because hard martensitic microstructures are the most SSC-susceptible. For corrosion-resistant alloys the standard instead gives environmental limits (temperature, pH₂S, chloride, pH) for each alloy family and product form. Either way, the goal is to keep the material below its cracking threshold for the specific environment.

Open the calculatorSour-service screening →Enter H₂S/CO₂ partial pressures and water chemistry to get the in-situ pH, the SSC severity region and a carbon-steel acceptability flag.

Beyond hardness: HIC and SOHIC

Even a soft, low-hardness steel can fail in wet H₂S by hydrogen-induced cracking, where atomic hydrogen recombines at inclusions and elongated manganese sulfides to form blisters and stepwise cracks. Controlling it is a steelmaking problem — low sulfur, calcium treatment for inclusion shape, clean microstructure — and is verified by HIC testing rather than a hardness check. Specify both where the service warrants it.

Frequently asked

What H₂S level makes a service "sour"?: For gas systems, ISO 15156 / NACE MR0175 treats an H₂S partial pressure at or above 0.05 psia (about 0.3 kPa, 0.0035 bar) as sour, triggering the material requirements. Multiphase and liquid systems have their own rules, but that partial-pressure threshold is the usual trigger.
What hardness is allowed for carbon steel in sour service?: Carbon and low-alloy steels are generally limited to 22 HRC maximum, because hardness tracks susceptibility to sulfide stress cracking. Weld and HAZ hardness must meet the same limit, which is why preheat, PWHT and consumable control matter so much in sour fabrication.
Is SSC the only cracking mechanism to worry about?: No. Sulfide stress cracking (SSC) is hardness-driven and worst at low temperature, but wet H₂S also drives hydrogen-induced cracking (HIC), stepwise cracking and SOHIC, which depend on steel cleanliness and microstructure rather than hardness. A full assessment covers all of them.

References

ANSI/NACE MR0175 / ISO 15156, "Petroleum and natural gas industries — Materials for use in H₂S-containing environments in oil and gas production."
NACE TM0284, "Evaluation of Pipeline and Pressure Vessel Steels for Resistance to Hydrogen-Induced Cracking."
NACE TM0177, "Laboratory Testing of Metals for Resistance to Sulfide Stress Cracking and Stress Corrosion Cracking in H₂S Environments."
R.D. Kane, M.S. Cayard, "Roles of H₂S in the Behavior of Engineering Alloys," NACE.

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