What “sour” actually puts on the order line

Sour service is shorthand for produced fluid carrying enough hydrogen sulfide to change how steel behaves. The industry draws the line at an H2S partial pressure above roughly 0.05 psi. Above that threshold the usual assumptions about carbon steel stop holding: atomic hydrogen from the corrosion reaction diffuses into the steel instead of recombining at the surface, and what it does there depends on whether the steel is under load.

The confusion starts with the name. NACE MR0175 — now issued jointly as ISO 15156 — is a materials standard. It lists which alloys and which hardness conditions are acceptable in sour environments. It does not produce a test report. The testing lives in separate documents, and it is worth knowing which is which before you write an enquiry. On our own pipe pages, the additional-testing line for ASTM A53 carbon steel pipe and ASTM A335 P91 seamless alloy pipe lists NACE MR0175, NACE TM0177, NACE TM0284, HIC test, SSC test, H2 service and IBR side by side, which is a fair reflection of how buyers actually order. But they are four different questions: TM0177 is the method for sulfide stress cracking, TM0284 is the method for hydrogen-induced cracking, MR0175 is the materials list, and H2 service is the environment claim.

HIC and SSC are not the same failure

Hydrogen-induced cracking needs no applied stress. Hydrogen collects at inclusions and at the mid-thickness segregation band in rolled plate, recombines into molecular hydrogen, and the pressure splits the steel open in stepwise cracks parallel to the rolling plane — blisters and internal steps in a pipe that has never been pressurised. TM0284 exposes unstressed coupons to a saturated H2S solution and reports crack length, thickness and sensitivity ratios. Clean numbers tell you the steelmaking was clean; nothing more.

Sulfide stress cracking is different because it needs tensile stress — applied, residual, or locked in by welding. TM0177 covers that, with the familiar Method A tensile, Method B bent beam and Method C C-ring geometries. This is why a hardness cap keeps appearing in sour specifications: for wrought carbon steel the industry convention sits at 22 HRC, and weld heat-affected zones are the place where that cap is most often breached. A pipe can pass every mill test and still crack in a hard HAZ.

Which is why base chemistry matters before anything is welded. Under ASTM A106 seamless pressure pipe, carbon runs 0.25 / 0.30 / 0.35 per cent maximum across Grades A, B and C, manganese 0.27–0.93 and 0.29–1.06, phosphorus capped at 0.035. Those numbers are not cosmetic: manganese and phosphorus segregation is where hydrogen collects. The API 5L ladder makes the same point more sharply, because the PSL1 to PSL2 gap is largely a cleanliness and toughness gap — phosphorus 0.030 to 0.025, sulfur 0.030 to 0.015, plus carbon-equivalent limits and Charpy testing.

Where the grade choice starts to matter

The X-number in seamless API 5L line pipe is the specified minimum yield strength in ksi: X42 reads 42 ksi yield and 60 ksi tensile, X52 gives 52 and 66, X65 gives 65 and 77, X70 gives 70 and 82, all with a yield-to-tensile ratio held at or below 0.93. Higher strength is not free here. The higher the yield, the more sensitive the steel tends to be to hydrogen, and the tighter the hardness control has to be. Plenty of sour lines are deliberately specified at X52 or below for that reason rather than for pressure.

Seamless versus welded is the other fork. In sour duty the concern is not weld quality in the ordinary sense but the fact that an ERW seam and its heat-treated zone carry a different metallurgical history from the parent metal, and some codes restrict ERW in hydrogen service for that reason. For alloy duty the chromium-molybdenum grades take over: ASTM A335 P22 at 1.90–2.60 chromium and 0.87–1.13 molybdenum, 415 MPa tensile and 205 MPa yield, and P91 at 8.0–9.5 chromium with vanadium and niobium, 585 and 415 MPa, hardness capped at 250 HB.

When carbon steel stops being the answer

Past a certain H2S and chloride concentration, hardness control and corrosion allowance stop being economic and the specification moves up the alloy ladder. The stainless options start with 317LMN, at 4.0–5.0 molybdenum and 0.10–0.20 nitrogen on 0.035 carbon maximum, with the highest aqueous corrosion resistance of the standard stainless grades. Above that, 904L brings 19–21 chromium, 24–26 nickel, 4–5 molybdenum and 1.2–2.0 copper at 0.02 carbon and 530–730 MPa tensile; usefully for welded fabrication, its intergranular corrosion resistance is guaranteed in the welded condition. Duplex 2205 gives 550 MPa yield and 800 MPa tensile at 22–23 chromium and 3.0–3.5 molybdenum; 2507 goes to 25 chromium, 4 molybdenum and 7 nickel at 116 ksi tensile and 80 ksi yield, held below 316 °C.

The one grade on our list with an explicit sour-service listing is super duplex S32760 / F55. Its data sheet states it is listed in NACE MR 01-75 for sour service and carries ASME approval for pressure vessel work, with PREN 40, 550 MPa proof stress, 750 MPa tensile, 25 per cent elongation, and 80 J Charpy at ambient falling to 45 J average at −46 °C. It ships with 3.1 certification; 3.2 is available at surcharge. Beyond stainless, the nickel alloys take over: Alloy 825 is titanium-stabilised to resist intergranular attack after exposure in the range that would sensitise an unstabilised stainless, and sour gas components appear in its own application list; Alloy 625 is niobium-stabilised against sensitisation during welding at 992.9 MPa tensile and 579.2 MPa yield; Hastelloy C-276 keeps carbon low so that grain-boundary carbide precipitation during welding does not spoil the heat-affected zone. Full range sits under Steel Pipe.

What to write on the enquiry

Four things decide whether the order arrives testable. First, name the edition of MR0175 or ISO 15156 you are working to. Second, name each test separately — TM0177 for SSC with the method letter and stress level, TM0284 for HIC with the solution and acceptance ratios — rather than writing “NACE compliant” and hoping. Third, state whether tests run per heat or per lot and per pipe or per batch; that line is usually the difference between two quotations. Fourth, ask where hardness is measured, including weld and HAZ.

Documentation closes it. Mill certificates under EN 10204 3.1B are standard on our A53 range; S32760 ships 3.1 with 3.2 on request. Third-party inspection — SGS, BV, TUV, ABS, LR — is a normal line item on our S31803 duplex and S32205 duplex ranges, and A106 is hydrostatic tested per length or full-body NDE as the alternative.

Where the money actually is

A TM0284 coupon set costs very little against a line that has to be cut out and replaced. The expensive outcome is a pipe that arrived with the right grade stamp, the right mill certificate and the wrong assumption about which mechanism was being guarded against. Decide whether the risk is blistering or cracking, name the test that measures it, and the rest of the order tends to resolve itself.