What “Sour Service” Means
A gas or multiphase stream is considered sour when the H2S partial pressure is at least 0.05 psia (≈ 0.34 kPa) at the operating conditions defined in NACE MR0175 / ISO 15156. In sour environments, wet H2S drives sulfide stress cracking (SSC) and hydrogen-induced cracking (HIC) in hard or segregated microstructures — the failure mode is sudden and brittle, not a slow leak.
What Changes in the Steel
| Requirement | Normal Service | Sour Service (MR0175 / Annex H) |
|---|---|---|
| Hardness (carbon/low alloy) | No special cap | ≤ 22 HRC (≈ 250 HV) |
| Max carbon (API 5L PSL2) | 0.22% typical | 0.12% (TMCP/M) – 0.18% (Q grades) |
| Sulfur | Standard limits | ≤ 0.003% typical, calcium-treated |
| OCTG grades | J55–P110 freely | Restricted yield: L80, C90, T95, C110 |
| Testing | Standard | HIC per NACE TM0284, SSC per TM0177 |
Selecting Materials in Practice
For pipelines, specify API 5L PSL2 with Annex H (sour service) and request HIC/SSC test reports on production heats. For OCTG, choose restricted-yield grades and verify the mill’s MR0175 compliance scope. Where CO2 dominates and H2S is low, 13Cr may be economical; higher chlorides push selection toward 22Cr/25Cr duplex or nickel alloys 825/625. Always screen the complete environment — pH, chloride, temperature and partial pressures together, not H2S alone.
MR0175 compliance is per-material and per-condition. A mill certificate stating “MR0175 compliant” without environment, hardness and test data proves nothing — ask for the heat-specific report.
Three Cracking Mechanisms to Know
SSC (sulfide stress cracking) — atomic hydrogen from the corrosion reaction enters hard microstructures and cracks them under tensile stress; prevented by the 22 HRC limit. HIC (hydrogen-induced cracking) — hydrogen accumulates at inclusions (mainly MnS) and forms internal blisters/cracks without external stress; prevented by low sulfur plus calcium treatment and sound rolling. SOHIC/SSC-HIC combinations — stress-oriented stacking of HIC planes near welds; the reason PWHT and hardness surveys extend to weld areas.
What the Lab Tests Actually Do
NACE TM0284 (HIC): immerse a test coupon in the standard sour solution (NACE A, pH ≈ 3) for 96 hours, section it and measure Crack Length/Area/Thickness Ratios (CLR/CTR/CSR). Acceptance limits are set by the purchaser per application — typical pipeline specs ask CLR ≤ 15%, CTR ≤ 3–5%, CSR ≤ 2–4%. NACE TM0177 (SSC): load a tensile bar at a defined stress (e.g. 72–80% of actual yield) in the sour solution for 720 hours; no failure is the pass criterion.
Sour Pipe Buying Checklist
- API 5L PSL2 with Annex H explicitly named in the PO (not just “sour service” in the email subject)
- HIC/SSC test reports on production heats, with heat numbers matching the MTCs
- Hardness survey scope covering base metal, weld and HAZ (≤ 250 HV / 22 HRC)
- Calcium-treated sulfur ≤ 0.003% stated on the certificate
- Third-party witness of HIC testing for critical orders
Frequently Asked Questions
Our field is “slightly sour” — can we skip Annex H?
The 0.05 psia threshold is defined at operating conditions, not on an average assay. If the worst-case (shut-in, rich gas) partial pressure exceeds it, the well is sour for material selection purposes. Ask your reservoir engineer for the worst-case calculation.
Does Annex H cover the weld seam?
Yes — for welded pipe, HIC testing and hardness limits apply to the weld and HAZ as well as the base metal. That is one of the main cost differences versus standard PSL2.
Is 22 HRC the whole story?
It is the necessary baseline for carbon and low-alloy steels, but environment severity (pH, chloride, temperature) can push selection toward restricted compositions, cladding or corrosion inhibitors. MR0175 is a materials standard, not a design shortcut.
Sour service is decided by worst-case conditions, not average ones. Get the shut-in H2S partial pressure in writing before you buy standard PSL2.
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Standards & references: AMPP (NACE MR0175) · ISO 15156 · API 5L Annex H
