Carbon steel line pipe for wet sour gas or sour crude has to resist two hydrogen-related failure modes: hydrogen-induced cracking (HIC) and sulfide stress cracking (SSC). Neither can be confirmed by chemistry or tensile tests alone, so sour-service pipe is qualified with laboratory exposure tests in H2S-saturated solutions. This article explains what the HIC and SSC tests actually measure, the acceptance criteria in API 5L Annex H, and what a buyer needs to write into the purchase order.
HIC and SSC: two different mechanisms
HIC happens without any applied stress. Atomic hydrogen produced by corrosion in H2S diffuses into the steel, collects at elongated manganese sulfide inclusions and centreline segregation bands, recombines into molecular hydrogen and builds enough pressure to open internal cracks parallel to the surface. Cracks can link through the wall in a stepwise pattern. Blistering is the near-surface form of the same mechanism. HIC is mainly a problem for plate and coil used in longitudinally welded (SAW and HFW) pipe.
SSC needs tensile stress, either applied or residual. It is a form of hydrogen embrittlement that cracks hard microstructures, typically weld zones, local hard spots and high-strength steels. Hardness control is the main defence, which is why ISO 15156-2 and API 5L Annex H cap hardness at 250 HV10.
The HIC test: NACE TM0284
NACE TM0284 (now published by AMPP) is the standard HIC test. In outline:
- Three specimens, usually 100 mm long and 20 mm wide, are cut from each test pipe or plate, in the through-thickness positions the standard defines, with weld seam specimens where required.
- Specimens are exposed unstressed for 96 hours in an H2S-saturated solution at ambient temperature.
- Each specimen is sectioned at three faces, polished and examined for cracks. Crack lengths and thicknesses are measured to calculate three ratios.
The ratios are CLR (crack length ratio, total crack length over section width), CTR (crack thickness ratio, total crack thickness over wall thickness) and CSR (crack sensitivity ratio, the sum of crack length × thickness over section area). Blisters are recorded separately.
Test solutions A and B
Solution A is 5% NaCl and 0.5% acetic acid in distilled water, saturated with H2S, with an initial pH of about 2.7. It is the most severe condition and the default in API 5L Annex H. Solution B is synthetic seawater to ASTM D1141 saturated with H2S, with pH around 4.8–5.4. It is milder and is sometimes accepted for less severe service by agreement. Results from one solution cannot be compared directly with the other, so the solution must be stated on the order.
API 5L Annex H acceptance criteria
API 5L Annex H covers PSL 2 pipe for sour service, identified by the "S" suffix on the grade (for example L360QS / X52QS or L450MS / X65MS). Its HIC acceptance criteria, applied to the average of the three sections of each specimen, are commonly quoted as follows:
| Parameter | API 5L Annex H limit | Typical tighter project limit |
|---|---|---|
| CSR (crack sensitivity ratio) | ≤ 2% | ≤ 1% or 1.5% |
| CLR (crack length ratio) | ≤ 15% | ≤ 10% |
| CTR (crack thickness ratio) | ≤ 5% | ≤ 3% |
| Test solution | NACE TM0284 Solution A unless agreed | Solution A |
| Hardness | 250 HV10 maximum | 250 HV10, sometimes 248 HV10 or lower in the root |
The tighter values in the right-hand column are examples seen in operator specifications, not universal figures. Check the edition of API 5L referenced in your contract, since clause numbering and details change between editions.
The SSC test: NACE TM0177 and four-point bend
NACE TM0177 defines four SSC test methods:
- Method A: uniaxial tensile specimen under constant load, usually 720 hours, pass or fail on failure.
- Method B: bent-beam specimen, used mainly for ranking materials.
- Method C: C-ring specimen, useful for tubular products and transverse stressing.
- Method D: double cantilever beam (DCB), which measures a threshold stress intensity, KISSC, used heavily for OCTG.
For line pipe, the usual test is the four-point bend method to ISO 7539-2 or ASTM G39, now also standardized for sour testing as NACE TM0316. Specimens taken from the base metal and across the weld are loaded to a defined percentage of yield strength, commonly 72% of SMYS in API 5L Annex H, and exposed for 720 hours in TM0177 Solution A saturated with H2S. Acceptance is no surface-breaking cracks when examined at 10× magnification. Some projects specify higher stress levels referenced to actual yield strength, or fit-for-purpose tests at the actual H2S partial pressure and pH following ISO 15156-2 Annex B or EFC Publication 16.
Mill qualification and test frequency
For welded pipe, HIC resistance is mostly decided at the steel mill: clean steelmaking with very low sulfur (Annex H Table H.1 limits sulfur to around 0.002–0.003%), calcium treatment for inclusion shape control, low phosphorus, and controlled centreline segregation during continuous casting. The pipe mill then needs a qualified procedure covering forming and welding.
API 5L Annex H requires HIC testing on the first heats of an order and then at a reduced frequency per group of heats, while SSC testing is normally part of the manufacturing procedure qualification (Annex B). Many operators tighten this to one HIC test per heat, plus HIC testing of the plate or coil before pipe making. Whatever frequency you choose, it must be written into the order.
Purchase order checklist for sour line pipe
- Product specification, PSL 2, grade with S suffix and delivery condition, for example API 5L X65MS SAWL.
- Annex H compliance and manufacturing procedure qualification to Annex B.
- HIC test to NACE TM0284, Solution A, with CLR/CTR/CSR limits and test frequency stated.
- SSC test method (four-point bend), stress level, solution and specimen locations.
- Chemistry limits including S, P, CE and Ca/S ratio, and hardness limits for body, weld and HAZ.
- Ultrasonic testing of plate or coil for laminations and of the weld seam.
- Inspection by an independent third party and certification to EN 10204 3.2 where required.
Apply the same thinking to butt-weld fittings and flanges on the same line. A fitting made from plate that has not been HIC tested undermines a fully qualified pipeline. For the overall framework, see our guide to NACE MR0175 / ISO 15156.
Sending an RFQ for sour-service line pipe
Send us the grade, size, wall thickness, length, quantity, manufacturing route and your project specification, together with the HIC and SSC requirements listed above and the certification level. If you already have test criteria from your licensor or operator, attach them. You can submit your RFQ through our contact page and we will respond with offers that state compliance clause by clause.
Frequently asked questions
Does seamless line pipe need HIC testing?
HIC is mainly a risk in plate and coil for welded pipe, but API 5L Annex H applies its HIC testing requirements to sour-service pipe generally, and many project specifications require HIC tests on seamless pipe too. Confirm what your specification demands.
What is the difference between NACE TM0284 Solution A and Solution B?
Solution A is 5% NaCl plus 0.5% acetic acid saturated with H2S at a pH of about 2.7, the most severe condition. Solution B is H2S-saturated synthetic seawater at pH about 4.8–5.4, which is milder. Results are not interchangeable.
How long do HIC and SSC tests take?
The HIC exposure is 96 hours and SSC four-point bend tests run 720 hours, plus specimen preparation, sectioning and reporting. Build these weeks into the delivery schedule, especially when tests are needed on each heat.
Can a mill use previous HIC test results?
Historical results can support a mill's capability, but they do not replace testing of the heats supplied under your order unless your specification says so. Ask for test reports traceable to the actual heats and pipe numbers.