One of the most expensive mistakes in tubular procurement is assuming the grade stamped on a joint proves it is safe for sour service. It does not. An API 5CT designation describes mechanical properties and a manufacturing route. It says nothing about whether that joint will survive a sour gas well.
Sour service OCTG covers casing, tubing, couplings, pup joints, and related downhole components exposed to produced fluid. When those fluids contain H₂S, the governing standard is NACE MR0175 / ISO 15156 (now maintained by AMPP), currently the 4th edition, NACE MR0175-2021 / ISO 15156:2020.
It covers several cracking mechanisms: sulfide stress cracking (SSC), hydrogen-induced cracking (HIC), stress-oriented hydrogen-induced cracking (SOHIC), and stress corrosion cracking (SCC). This article focuses on SSC, the dominant concern for high-strength OCTG.
At Imex Canada, we have spent more than 30 years sourcing casing and tubing for operators, including sour wells where the paperwork matters as much as the pipe. Here is how to get from well-fluid data to a defensible purchase order.
Key Takeaways
- An API 5CT grade stamp does not prove NACE MR0175 / ISO 15156 compliance.
- Sour service starts at 0.05 psi H₂S partial pressure, but that is only a screening trigger.
- A ppmv figure means nothing without total pressure.
- Select from the full environment: temperature, pH, chlorides, CO₂, and sulfur.
- Hardness, heat treatment, and traceable mill reports matter as much as the grade.
- Every wetted component needs the same review, and Imex Canada can supply the documentation.
What Makes an OCTG Application “Sour”?
The Cracking Mechanism
In a water phase, H₂S drives a corrosion reaction producing atomic hydrogen at the steel surface. Sulfide ions block its recombination into harmless gas, so hydrogen atoms diffuse into the steel and collect at grain boundaries, inclusions, and high-stress areas.
In a susceptible high-strength steel, that trapped hydrogen lowers the load the material can carry before cracking.
Four conditions have to converge for sulfide stress cracking:
- An H₂S-containing aqueous environment (liquid water must be present)
- Tensile stress, whether applied by well loads or left as residual stress
- Susceptible metallurgy and microstructure
- Excessive hardness or an unfavorable heat treatment condition
That makes this a safety problem, not a maintenance one. SSC does not announce itself the way general corrosion does. There is no wall-loss trend on an inspection log. A tubular can look sound, then fail suddenly at a load it carried yesterday.
How SSC Differs From HIC, SOHIC, and SCC
SSC is hydrogen embrittlement under tensile stress, targeting hard, high-strength steels. HIC needs no applied stress: hydrogen collects at elongated inclusions and forms internal blisters, making it a low-strength plate and pipe problem more than an OCTG one.
SOHIC is HIC linked into a stacked crack array under stress. SCC covers chloride and other environmental cracking in corrosion-resistant alloys.
The 0.05 PSI Threshold and Why It Isn’t the Whole Answer
The usual screening threshold for sour service is an H₂S partial pressure at or above 0.05 psi (0.3 kPa) in the gas phase. Cross it, and the well is treated as sour.
The threshold is a trigger, not a design basis. Selection weighs the complete environment: temperature, in-situ pH, chlorides, CO₂ partial pressure, and elemental sulfur all shift how a material behaves.
A ppmv figure alone is meaningless, since partial pressure depends on gas composition and total pressure. Two wells can report the same concentration and present very different material problems.
API Grade ≠ Automatically NACE-Compliant
This is the most important specifying principle: L80, P110, or T95 on a designation line does not prove MR0175 / ISO 15156 compliance.
What the Material Must Actually Satisfy
Compliance is a property of a specific batch produced a specific way, not of a grade name. What has to line up: chemical composition, metallurgical condition, heat treatment, yield and tensile strength, hardness, cold working and straightening history, manufacturing route, SSC resistance where required, and environmental limits.
Two joints with the same grade marking, from two mills, can behave very differently in one well.
How ISO 15156-2:2020 Is Structured
ISO 15156-2:2020 sets requirements and environmental limits for carbon and low-alloy steel OCTG, pairing a material with conditions it has been shown to tolerate. It issues no blanket endorsement of any API grade for H₂S service. No page says a grade is universally acceptable.
Common OCTG Materials and Their Sour-Service Considerations
| OCTG material | Sour-service consideration |
| L80 Type 1 | Common starting point for sour casing and tubing, provided the actual material and environment satisfy ISO 15156-2:2020 |
| L80 13Cr | Corrosion-resistant alloy (CRA) option for appropriate CO₂/H₂S environments; environmental limits require careful checking |
| T95 | Higher strength; requires verified sour qualification plus hardness and microstructure control |
| P110 | High strength does not confer sour suitability; conventional P110 should not be assumed to be compliant |
| Q125 and higher | Particularly careful qualification is needed; strength is not a selection criterion |
| Proprietary sour grades | May offer strength above conventional sour grades, but compliance must be demonstrated against applicable ISO 15156-2:2020 requirements |
Conditional Acceptance, Not Blanket Approval
Read the table as starting points, not conclusions. ISO 15156-2:2020 recognizes certain casing and tubing materials subject to conditions on temperature, strength, hardness, and qualification testing.
Provisions for Cr-Mo tubulars carry hardness and strength limits and sometimes an SSC test requirement. “L80 Type 1 is the sour grade” starts a conversation with a mill; it does not end one.
The Design Inputs You Need Before Choosing a Grade
You cannot select a material before describing the environment. Before anyone names a grade, document the following:
- H₂S partial pressure
- Total system pressure
- Temperature, including the full operating range
- In-situ pH
- Chloride concentration
- CO₂ partial pressure
- Water and brine chemistry
- Elemental sulfur, if present
- Expected pressure and temperature changes over the life of production
- Mechanical stresses and applied loads
ISO 15156 is an environment-versus-material assessment. Part 2 covers assessing the environment and selecting carbon and low-alloy steels against it.
Worked Example: Why PPMV Misleads
| Case | H₂S concentration | Total pressure | H₂S partial pressure |
| A | 1,000 ppmv | 5,000 psi | 0.001 × 5,000 = 5 psi |
| B | 1,000 ppmv | 100 psi | 0.001 × 100 = 0.1 psi |
Identical concentrations, two very different problems. Case A sits far above the sour threshold and demands serious qualification work. Case B is barely over the line. Specifying ppm hides that.
Hardness and Heat Treatment Are Critical
Microstructure and hardness matter as much as the grade designation, and in many failures more. A quenched and tempered steel with a uniform fine-grained structure behaves nothing like the same chemistry carrying untempered martensite in a hard band.
What the Manufacturing Specification Should Control
Purchase documents should reach into the mill process, not stop at the grade: quench and tempering practice, minimum tempering temperature, final hardness, tensile and yield properties, grain structure, limits on segregation and inclusions, cold work, straightening, and threading and machining effects on hardness.
Threads are a frequent trouble spot, since machining and cold forming can push hardness above the pipe body limit.
Beware the “≤22 HRC” Shortcut
Many specifications carry a blanket 22 HRC ceiling because someone recalled it from a training course. NACE and ISO limits vary by material family, service region, and qualification route.
A generic ceiling should never be applied across all OCTG grades without checking the relevant ISO 15156-2:2020 material table and its conditions. The wrong number is not conservative; it can be too strict for one material, too loose for another.
SSC Testing: What It Proves and What It Doesn’t
Qualification testing becomes necessary where the selected material is not directly covered by the standard’s material provisions, which is common with proprietary grades and materials pushed to the edges of their listed limits.
NACE TM0177 Methods
NACE TM0177, currently the 2024 edition, gives laboratory methods for evaluating resistance to sulfide stress cracking and stress corrosion cracking in H₂S environments.
It contains four methods: uniaxial tensile (Method A), three-point bent-beam (Method B), C-ring (Method C), and double-cantilever-beam (DCB) (Method D). Four-point bend testing is covered separately in TM0316. Proof rings are a loading fixture used within Method A, not a method in their own right.
The Distinction That Gets Confused
TM0177 is a test method. MR0175 / ISO 15156 is the material selection standard. A certificate referencing one does not satisfy the other.
A passed SSC test also does not make a material universally NACE compliant. Test conditions, applied stress, solution, and acceptance criteria must match the qualification requirements for your service. A test at mild conditions proves little about a hostile well.
What to Put on a Sour-Service OCTG Purchase Order
| Category | What to state |
| Material | API 5CT / ISO 11960 / ISO 13680 basis, exact grade, product specification level (PSL) requirements, proprietary grade designation if applicable |
| Sour-service requirement | “NACE MR0175 / ISO 15156-2:2020 compliant,” applicable to the Part 2 material and environment category, qualification route if the material is not directly listed |
| Mechanical and metallurgical | Hardness limits, yield and tensile requirements, heat-treatment condition, chemical composition restrictions, cold-work limitations |
| Testing | SSC qualification where required, TM0177 method and acceptance criteria, hardness and tensile testing, metallographic examination, and non-destructive examination (NDE) requirements |
| Traceability | Heat number, heat-treatment batch, test batch, mill test report (MTR) or certified mill test report (CMTR), SSC test reports |
Traceability turns everything above it into evidence. Without heat and batch identification tied to test reports, you have a claim, not a record.
Specify the Entire Completion String, Not Just the Tubing
A recurring failure mode has nothing to do with tubing. Someone specifies sour-service tubing carefully, then a coupling or valve in the same sour gas stream gets ordered to a standard catalog spec. The well does not care which line item got reviewed. Every wetted component needs review:
- Production tubing
- Casing exposed to sour fluids
- Couplings and pup joints
- Nipples and landing nipples
- Packer components
- Subsurface safety valves
- Mandrels and polished components
- Completion accessories
- Downhole gauges and their connections
The standard’s scope supports this. It covers equipment exposed to H₂S where failure could threaten personnel, the public, the environment, or the equipment itself. A cracked coupling meets that description as readily as a cracked joint of pipe.
A Practical Compliance Workflow
- Gather well-fluid data: gas composition, water chemistry, pressure, and temperature.
- Calculate H₂S partial pressure from composition and total pressure.
- Determine the ISO 15156-2:2020 environment that applies.
- Select candidate OCTG materials for that environment.
- Check the grade against material limits and any attached conditions.
- Verify hardness and heat-treatment requirements for that material.
- Verify SSC qualification, including test conditions and acceptance criteria.
- Review manufacturing and NDE documentation from the mill.
- Approve the MTR or CMTR against what you actually specified.
- Control field installation: makeup torque, handling, and any field machining.
Compare that chain of evidence with the alternative: writing “NACE compliant L80” and hoping the supplier reads it as you meant it. The first produces documents you can hand a regulator. The second produces a phrase.
How Imex Canada Can Help
Building that evidence chain is easier with a supplier set up to work this way. We supply OCTG casing and tubing in grades from J55 through Q125, including L80-9Cr, L80-13Cr, and mill proprietary grades engineered for H₂S service to NACE MR0175 / ISO 15156.
Our Calgary team supports grade and connection selection against your well conditions, and shipments arrive with mill test reports covering chemistry and mechanical properties, plus third-party inspection on request.
Getting Sour Service OCTG Right the First Time
“API 5CT grade” and “NACE MR0175 compliant” are not interchangeable, and treating them as if they were causes most sour-service tubular problems.
Correct selection flows from the environment: H₂S partial pressure, temperature, pH, chlorides, and the other parameters defining the service. Compliance is then verified with the material, heat treatment, hardness, strength, and manufacturing condition inside the applicable ISO 15156-2:2020 requirements.
Get that sequence backwards, picking a grade first and looking for justification afterward, and you are asking a stamp to do work it was never designed to do.
If you have questions about matching an OCTG grade to your H₂S partial pressure or what hardness, heat treatment, and SSC documentation to require from the mill, contact us. We are glad to help before the purchase order goes out.