Yes, Hastelloy C-276 is qualified for sour gas service under NACE MR0175 / ISO 15156. It is listed in ISO 15156-3 Annex A for plate, bar and tube in the solution-annealed condition, at a maximum hardness of 35 HRC, for any H₂S partial pressure, up to 232°C. That qualification is conditional, not automatic: the material must meet the standard’s heat-treatment and hardness requirements, and the certificate must prove it.
A wellhead materials engineer learns this the hard way. He specifies Hastelloy C-276 for a sour gas flowline, and the material arrives correctly marked “UNS N10276, solution annealed.” Then QA rejects it.
Why? The mill certificate never states an actual hardness value. It never cites the compliance region the standard requires. The alloy was right. The specification was incomplete.
If you specify Hastelloy for sour gas service, this guide gives you the same decision path the standards use. You’ll know how to calculate whether a stream is sour, which alloy and condition to choose, and exactly what belongs on the purchase order, so the material clears goods-in inspection the first time.
Key Takeaways
- Service is “sour” when the H₂S partial pressure reaches 0.05 psia (0.345 kPa). Partial pressure equals total absolute pressure multiplied by the H₂S mole fraction, so low ppm at high pressure can still be sour.
- Hastelloy C-276 (UNS N10276) is listed in ISO 15156-3 Annex A (Table A.26): solution annealed, ≤35 HRC, any H₂S partial pressure, up to 232°C.
- Elemental sulfur is the decisive test. Only annealed C-276 resisted cracking for 12 months up to 232°C in a sulfur-laden sour brine where Inconel 625, Incoloy 825, 904L, 718 and duplex grades failed at 177°C.
- Inconel 625 (UNS N06625) is qualified for the majority of sour wells and costs roughly 1.5 to 2 times less than C-276. Reserve C-276 for high-pressure, high-temperature wells with elemental sulfur and high chlorides.
- Above 232°C, no Annex A listing applies. The material leaves the standard’s qualified envelope and requires project-specific testing under Annex B.
- Compliance is proven by paperwork: actual measured hardness, heat-treatment records, an EN 10204 3.1/3.2 material test report, and an explicit statement citing NACE MR0175 / ISO 15156-3 and the Annex A table.
- NACE MR0175 (upstream and midstream) and NACE MR0103 (refinery) aren’t interchangeable. Compliance with one doesn’t guarantee the other.
Specifying sour-service alloy? Send Zhonggongte your H₂S partial pressure, temperature and chloride level, and our metallurgical engineers will confirm the grade, condition and certification within 24 hours. Request a quote →
What Is Sour Gas Service? The 0.05 psia Threshold Explained
Sour service is any oil and gas environment containing wet hydrogen sulfide (H₂S) at a partial pressure of 0.05 psia or higher. Below that threshold, the environment is treated as sweet and ordinary materials may apply. At or above it, NACE MR0175 / ISO 15156 governs material selection.
That single number, 0.05 psia, is the doorway to the whole standard. Everything else, the hardness limits, the temperature ceilings, the alloy tables, only matters once you have crossed it.
How to Calculate H₂S Partial Pressure
Partial pressure is what separates a real engineering decision from a guess. You don’t need H₂S concentration alone. You need concentration and pressure together.
The calculation is straightforward:
pH₂S = total absolute pressure × (ppm H₂S ÷ 1,000,000)
Two examples show why the result often surprises buyers:
- A 1,000 psi stream carrying 100 ppm H₂S: 1,000 × 0.0001 = 0.1 psi. That is double the threshold. The stream is sour.
- The same 1,000 psi stream carrying 0.1% H₂S: 1,000 × 0.001 = 1.0 psi. Deeply sour.
Use design pressure, never average operating pressure. A common and expensive mistake is treating a 50 to 100 ppm stream as sweet without running the numbers. At high enough pressure, a trace of H₂S is still sour.
One practical exclusion: crude oil and water-handling facilities below roughly 65 psia total pressure generally fall outside the scope. For everything else, calculate first, then select.
Why Free Water and In-Situ pH Decide the Outcome
H₂S only attacks metal when it dissolves. That requires free water. A dry gas stream at the same partial pressure behaves very differently from a wet one, which is why the standard is framed around “wet” H₂S service.
The severity then depends on more than the raw H₂S number. In-situ pH, chloride concentration and elemental sulfur all shift how aggressive the environment becomes. Sulfide stress cracking risk peaks near room temperature, around 25°C, and generally falls as temperature rises. High temperature brings different damage mechanisms into play, which is why the standard caps service temperature rather than treating higher heat as safer.
This is where specification errors get made. A buyer fixes on one number, usually H₂S percentage, when the controlling variable is often the one nobody measured: chlorides or free sulfur.
The Severity Regions (0 to 3)
ISO 15156 sorts sour environments into regions that reflect how much testing and how restrictive a material the service demands.
| Region | Condition | What It Means for Material Selection |
|---|---|---|
| Region 0 | Below 0.05 psia, no free water | No specific SSC requirements |
| Region 1 | Mild sour conditions | Qualified alloys from Annex A may apply |
| Region 2 | Moderate to severe sour conditions | Annex A alloys within their listed envelope |
| Region 3 | Severe, often low pH and high chloride | Project-specific qualification testing required |
Regions 1 and 2 cover the bulk of real projects and most qualified alloys. Region 3 always requires its own test program. Knowing your region early prevents the two most expensive outcomes: buying a premium alloy you did not need, or buying a standard alloy that fails the design review.
Why 316L and Duplex Stainless Steel Fail in Sour Service
Stainless steel earns its reputation in chloride service, so engineers reasonably reach for 316L or duplex 2205 in first-draft material selection. In sour service, that instinct fails. The presence of H₂S introduces cracking mechanisms stainless steel was never designed to resist.
Sulfide Stress Cracking (SSC)
Sulfide stress cracking is brittle failure of a stressed metal exposed to wet H₂S. Hydrogen from the corrosion reaction enters the steel and embrittles it. Under tensile stress, which includes residual stress from welding and cold work, the material cracks without the warning signs of ordinary corrosion.
This is why NACE MR0175 caps carbon and low-alloy steels at 22 HRC. Hardness is a proxy for the hard microstructures that make a steel vulnerable to SSC. Soften the steel, and you reduce the risk.
HIC, SOHIC, and Chloride SCC
Sulfide stress cracking isn’t the only mechanism. Hydrogen-induced cracking (HIC) and stress-oriented hydrogen-induced cracking (SOHIC) affect steels from the inside, often at inclusions and laminations. Chloride stress corrosion cracking was folded into ISO 15156’s scope in the 2003 revision, acknowledging that chlorides and H₂S often arrive together.
These mechanisms matter because compliance with one standard doesn’t cover all of them. NACE MR0103 addresses SSC in refinery service but does not address HIC or SOHIC at all.
Where Stainless Runs Out
316L has a narrow sour envelope. At 60°C, it is limited to roughly 15 psi H₂S partial pressure with no latitude on chlorides or pH. Duplex stainless steels extend the range but still fail once H₂S, chloride and temperature stack up.
A Caspian Sea development illustrates the shift well. When H₂S in the field reached 8 to 12 mol%, well above the roughly 0.14 MPa limit for the duplex grade in use, the operator moved to Inconel 625.
Need the full upgrade ladder? Our guide to Hastelloy vs stainless steel explains where standard grades stop and nickel alloys begin.
What NACE MR0175 and ISO 15156 Actually Require
NACE MR0175 and ISO 15156 are technically equivalent and published jointly. The standard, maintained today by AMPP (formerly NACE International), covers materials for use in H₂S-containing environments in oil and gas production: wellheads, flowlines, separators, gas processing plants and offshore facilities.
It is a materials standard, not a design code. It tells you which alloys are acceptable, in what condition and within what limits.
What “Listed in Annex A” Means
Materials listed in Annex A are acceptable for sour service without further testing, provided they meet the standard’s condition and hardness requirements for their product form. This is the qualified path that saves projects months of laboratory time.
The critical word is “conditional.” Annex A acceptance isn’t a blanket approval for the alloy name. It applies to a specific product form, in a specific heat-treatment condition, within a specific environmental envelope of temperature, pH and chloride. Hammering C-276 into a shape that exceeds the listed cold-work limit is enough to void the qualification.
Hardness Is the Gatekeeper
If you remember one practical point from this guide, remember this: composition never qualifies an alloy on its own. Delivery condition and hardness do.
Most rejected sour-service material isn’t the wrong grade. It’s the right grade at the wrong hardness. A solution-annealed C-276 that has been cold-formed past its listed limit may test above the ceiling. A fitting that was not properly annealed may sit at 40 HRC when 35 HRC is required.
This is why a mill certificate that states only “N10276, solution annealed” is incomplete. It doesn’t prove compliance. It states a grade. For a worked example of how hardness is sampled and reported, see TWI’s guidance on complying with NACE hardness requirements.
NACE MR0175 vs NACE MR0103
These two standards are routinely confused, and the confusion causes real procurement errors.
| NACE MR0175 / ISO 15156 | NACE MR0103 / ISO 17945 | |
|---|---|---|
| Scope | Oil and gas production, upstream and midstream | Petroleum refining, downstream |
| Material requirements | Composition, hardness, plus environmental limits (temperature, pH, chloride, pH₂S) | Composition and hardness only, no environmental restrictions |
| Cracking mechanisms | SSC and chloride SCC | SSC and polythionic acid SCC, not HIC or SOHIC |
| Welding rules | MR0175-specific | Governed by NACE SP0472 |
A refinery project and a wellhead project on the same site may need two different compliance statements. Meeting one standard doesn’t deliver the other. For how these grades behave in downstream plants, see our guide to Hastelloy for chemical processing.
The 232°C Temperature Ceiling
Nickel alloy listings in ISO 15156-3 top out at 232°C (450°F). This ceiling reflects the risk of environmentally assisted cracking at higher temperatures, not a limit on corrosion resistance.
Above 232°C, the alloy leaves the qualified Annex A envelope entirely. Service at that temperature requires project-specific testing under Annex B, or the component must be excluded from sour service scope. Projects that expect to operate above this line should budget for a qualification program, not a catalog purchase.
Sourcing question? If your service temperature approaches or exceeds 232°C, talk to our metallurgical team before you commit to a grade. We will tell you whether your case falls inside Annex A or needs Annex B testing. Speak with an engineer →
Hastelloy Sour Gas Service: Is C-276 the Right Choice?
Yes. Hastelloy C-276 is one of the most broadly qualified corrosion-resistant alloys for sour service, and its envelope is wide enough to cover the severe corner of the industry.
That width comes from chemistry. C-276 carries the highest molybdenum and tungsten content of the common Ni-Cr-Mo alloys, which drives its pitting resistance equivalence number (PREN) to roughly 67 to 75, compared with about 51 to 53 for Inconel 625. Higher PREN translates directly into better performance in high-chloride sour brines.
The ISO 15156-3 Annex A Listing (Table A.26)
C-276 (UNS N10276) is listed in ISO 15156-3 Annex A, Table A.26, for bar, tube and plate. The listed envelope is generous:
| Parameter | C-276 (N10276) | Inconel 625 (N06625) |
|---|---|---|
| Annex A table | Table A.26 | Table A.29 |
| Hardness, annealed | ≤35 HRC | ≤35 HRC / 287 HBW |
| Hardness, cold worked | ≤40 HRC (to 40% max cold work) | ≤40 HRC (to 35% max cold work) |
| H₂S partial pressure | Any | Any |
| Maximum temperature | 232°C (450°F) | 232°C (450°F) |
| Condition | Solution annealed | Solution annealed |
A practical rule used across the industry: if your service contains H₂S partial pressure above 0.05 psia, free water and chlorides, C-276 in the solution-annealed condition is a strong candidate. (Verify the exact limits for your product form against the current edition of the standard before you specify.)
Condition, Hardness, and Cold Work
C-276 must be supplied solution annealed, typically at approximately 1120°C or above followed by a quench. Annealed hardness lands around 85 to 95 HRB, roughly 15 to 20 HRC, comfortably under the 35 HRC ceiling.
Cold work is where compliance is lost. Forming beyond the listed cold-work limit raises both hardness and tensile residual stress, the two conditions that drive sulfide stress cracking. Any forming past that limit must be followed by a full solution anneal to restore compliance. Fittings are commonly cited at 100 HRB maximum, tighter than the base-metal ceiling.
Welding and HAZ Requirements
Welded C-276 fabrications for sour service have three requirements worth putting in writing.
First, use matching filler metal. ERNiCrMo-4 for C-276 (ERNiCrMo-3 for 625). Deposits typically land in the 85 to 95 HRB range, well inside the limit.
Second, verify heat-affected-zone hardness. Many sour-service welding procedure specifications set a HAZ limit around 250 HV10. Work with a fabricator who measures it rather than assumes it.
Third, don’t post-weld heat treat C-276. This surprises fabricators who are used to stress-relieving for sour service. C-276 is not prone to sensitization, so PWHT is unnecessary, and heat treatment in the 600 to 1150°C range embrittles the alloy. As-welded service is a genuine advantage for sour fabrications, because it removes an entire process step and its risk.
Hastelloy C-276 vs Inconel 625 for Sour Service
Both alloys are qualified under ISO 15156-3 at any H₂S partial pressure up to 232°C. Both are common, both are weldable, and both ship with full certification. For Hastelloy sour gas service, the selection question isn’t which alloy is “better.” It’s which alloy matches the severity of your well.
Getting this wrong is expensive in one of two directions. Over-specify C-276 across a whole project, and you spend money you did not need to spend. Under-specify 625 into a sulfur-laden, high-pressure well, and you risk a failure that costs far more than the material ever did.
When Inconel 625 Is Enough
Inconel 625 handles the majority of sour wells. It’s broadly qualified, widely stocked and easier to source in short lead times. It performs well in sour service with low to moderate chlorides below roughly 150°C, and remains a good choice with high chlorides up to about 190°C after verification.
One caution specific to 625: cold-worked material beyond roughly 15% reduction is often rejected for sour service even when the hardness test passes. Buyers who substitute cold-finished 625 bar for annealed material to save a step frequently find themselves explaining a rejection.
When You Must Upgrade to C-276
The upgrade decision usually turns on elemental sulfur. This is the most under-appreciated variable in sour-service selection, and C-276 is where the evidence is strongest.
In deaerated 25% NaCl brine with 1 g/L elemental sulfur at pH 4.5 under 0.7 MPa H₂S, only annealed C-276 showed no cracking within 12 months at temperatures up to 232°C. Inconel 625, Incoloy 825, 904L, Alloy 28, Alloy 925, alloy 718 and duplex grades all failed at 177°C within months.
The mechanism explains the result. Free sulfur deposits on the surface of 625 and degrades or eliminates its ability to repassivate, causing localized pitting and a dark nickel sulfide scale. C-276’s higher molybdenum and tungsten content resists that breakdown.
A Middle East HPHT sour gas well shows the practical boundary. When Inconel 625 was ruled out by MR0175 limits at low pH, the operator selected C-276 downhole tubing. It performed for seven years at 180°C, 15% H₂S and 120,000 ppm chloride.
Upgrade criteria, in short:
- Elemental sulfur present at elevated temperature: C-276
- HPHT with high chloride and elemental sulfur above about 190°C: C-276
- Low in-situ pH combined with high H₂S and high chloride: C-276
- Direct elemental sulfur contact in highly sour brine above 100°C: evaluate C-276 or G-3
The Cost Trade-Off
C-276 costs roughly 1.5 to 2 times more than Inconel 625. That premium is justified only when 625 would fail. For the bulk of sour wells, 625 is sufficient.
This is where a credible supplier should be honest rather than upsell. A buying team that over-specifies C-276 across an entire project pays a premium on every component for protection it only needed in one or two locations.
| Service Condition | Recommended Alloy | Notes |
|---|---|---|
| Sweet service, below threshold | 316L or duplex | Standard materials apply |
| Sour, low chloride, below ~150°C | Inconel 625 | Economic default |
| Sour, high chloride up to ~190°C | Inconel 625 (verify) | Confirm with testing |
| Sour with elemental sulfur | Hastelloy C-276 | Only annealed C-276 held to 232°C |
| HPHT, high chloride, elemental sulfur | Hastelloy C-276 | The severe corner |
| Refinery internal service | Per NACE MR0103 | Different standard, different rules |
One further warning: alloy 718 isn’t interchangeable with C-276 or 625 in sour service. It can be susceptible below about 150°C, and it should never be substituted on the assumption that a higher strength alloy is automatically safer.
Comparing grades for your well? Our Inconel 625 round bar and Hastelloy C-276 plate pages list available forms, conditions and certifications. Or send us your well conditions and we will recommend between them.
Specifying, Documenting, and Sourcing Sour-Service Alloy
Everything above determines the right alloy. This section determines whether the material you receive actually qualifies.
The Six-Step Compliance Workflow
- Calculate pH₂S from design pressure multiplied by the H₂S mole fraction.
- Determine your severity region (0 to 3) from pH₂S, in-situ pH and chloride.
- Check temperature against the 232°C Annex A ceiling.
- Assess elemental sulfur and chloride, the two variables most often missed.
- Select the alloy and its condition: grade plus solution-annealed heat treatment.
- Specify hardness and documentation: limit, test scale, test location and certificate requirements.
Steps one through four are engineering. Steps five and six are procurement. Skipping either half is how projects end up with the right alloy and a rejected certificate.
The Documentation That Prevents Goods-In Rejection
Sour-service documentation is where most delays originate. Ask for these items on the purchase order:
- Material test report to EN 10204 3.1 or 3.2, reporting actual measured hardness, not a “typical” range.
- Three hardness readings averaged per lot, tested per ASTM E18 or E10 as applicable.
- Heat-treatment records confirming the solution-annealed condition and quench.
- Chemistry and product form matching the applicable ASTM specification.
- An explicit compliance statement citing “NACE MR0175 / ISO 15156-3, Region X, Annex A qualified.”
- Positive material identification (PMI) and, for welded components, WPS and WPQR per ASME Section IX.
Specify by product form using the correct standard: ASTM B575 for plate, B622 for seamless tube and pipe, B619 and B626 for welded, B574 for bar and rod, B564 for forgings and A494 for castings. Adjacent frameworks frequently appear in project specifications, including ASME B31.3 and B31.8, ASME Section VIII Division 1, API 6A and 17D, NORSOK M-630 and DNV-RP-F112.
A certificate that reports a “typical hardness range” is the single most common cause of goods-in rejection in sour service. Insist on measured values.
Sourcing Certified Sour-Service Alloy from Zhonggongte
Zhonggongte manufactures and supplies sour-service alloy in the forms projects actually need. Our portfolio includes Hastelloy C-276, C-2000 and B-2, plus Inconel 625 and 718, Incoloy 825 and 800H, and Monel 400 and K-500, available as plate and sheet, bar and rod, seamless and welded pipe and tube, forgings, flanges, fasteners and wire.
We operate from Wuxi with vacuum induction melting and electroslag remelting, free forging hammers and hot and cold tube rolling lines. Our in-house laboratory runs direct-reading spectrometers, tensile and hardness testing, metallographic analysis and ultrasonic NDT, so every batch is verified before it ships. We are also an authorized distributor for Outokumpu, ThyssenKrupp, Nippon Steel, Taiyuan Steel and other first-tier mills.
Practically, that means every order ships with a material test report, spectral verification and third-party inspection support on request. For sour-service orders, tell us the service conditions and the standard, and we will confirm hardness and heat-treatment documentation that matches.
When you send an inquiry, include pH₂S or the pressure and H₂S ppm, operating temperature, chloride level, whether elemental sulfur is present, the equipment or product form, and the governing standard. That is enough for our metallurgical engineers to confirm the grade, condition and documentation package, typically within 24 hours.
FAQ: Hastelloy and NACE MR0175 Sour Gas
What is NACE MR0175 and when does it apply?
NACE MR0175 / ISO 15156 is the material selection standard for equipment exposed to wet hydrogen sulfide in oil and gas production. It applies when the H₂S partial pressure reaches 0.05 psia or higher, or when free water and H₂S are both present and the environment is classified sour by project specification. It sets acceptable alloys, heat-treatment conditions, hardness limits and environmental envelopes for wellheads, flowlines, separators and offshore facilities.
What is the H₂S limit for NACE MR0175?
The defining threshold is an H₂S partial pressure of 0.05 psia, about 0.345 kPa. It is a partial pressure limit, not a concentration limit. Calculate it as total absolute pressure multiplied by the H₂S mole fraction. A 1,000 psi stream with only 100 ppm H₂S already reaches 0.1 psi and is sour.
What hardness is required for Hastelloy C-276 in sour service?
C-276 must be solution annealed and held to a maximum of 35 HRC for the annealed condition. Cold-worked material is permitted to 40 HRC when cold work does not exceed 40%. Weld deposits typically land in the 85 to 95 HRB range. The purchase order should require actual measured hardness, not a nominal range.
Is Hastelloy C-276 better than Inconel 625 for sour gas?
Neither is universally better. Both are qualified to 232°C at any H₂S partial pressure. Inconel 625 is sufficient and more economical for most sour wells.
C-276 becomes the required choice when elemental sulfur is present, or when high pressure, high temperature and high chloride combine. It also carries roughly twice the pitting resistance, which matters in high-chloride brines.
What is the difference between NACE MR0175 and NACE MR0103?
MR0175 / ISO 15156 covers oil and gas production, upstream and midstream, and imposes environmental limits on temperature, pH and chloride in addition to hardness. MR0103 / ISO 17945 covers petroleum refining, downstream, and sets only composition and hardness requirements. Compliance with one does not guarantee compliance with the other.
Does Hastelloy C-276 need post-weld heat treatment?
No, and it should not receive one. C-276 resists sensitization, so as-welded service is permitted. Heat treatment in the 600 to 1150°C range embrittles the alloy and is prohibited. This makes C-276 practical for sour-service fabrications because it removes the PWHT step entirely.
What is the best alloy for sour gas with elemental sulfur?
Annealed Hastelloy C-276 is the benchmark. In controlled testing in deaerated brine with 1 g/L elemental sulfur at pH 4.5 under 0.7 MPa H₂S, C-276 was the only common alloy that showed no cracking within 12 months at temperatures up to 232°C. Inconel 625, Incoloy 825, 904L, 718 and duplex grades failed at 177°C.
What documentation proves NACE MR0175 compliance?
A material test report to EN 10204 3.1 or 3.2 showing actual measured hardness with three readings averaged per lot, heat-treatment records confirming the solution-annealed condition, chemistry and product form matching the applicable ASTM standard, and an explicit statement citing “NACE MR0175 / ISO 15156-3, Region X, Annex A qualified.” Add WPS and WPQR per ASME Section IX for welded components.
Conclusion: Get the Environment Right, Then the Alloy
In Hastelloy sour gas service, the alloy grade is only half the answer. The environment decides the other half.
Calculate pH₂S, identify your region, check temperature, chloride, and elemental sulfur, and then choose the grade and its condition together. C-276 is the widest-envelope choice, and the one to reach for when elemental sulfur, high pressure, or high chloride would defeat Inconel 625. For the majority of wells, 625 is the cost-effective qualified answer.
Whatever grade you select, prove it on paper. Solution-annealed condition, 35 HRC maximum, a material test report with actual measured hardness, and a compliance statement that cites the standard and the Annex A table.
Zhonggongte supplies certified Hastelloy C-276, Inconel 625, and the full sour-service alloy portfolio from our Wuxi facility, with full traceability and spectral verification on every order. Send us your pH₂S or pressure and H₂S ppm, your temperature, your chloride level, and whether elemental sulfur is present. Our metallurgical engineers will confirm the right grade, condition and certification within 24 hours.
