Oil & Gas Corrosion at a Glance
- Sour (H2S) flow path, downhole tubing, wellheads, flowlines and exchangers run on Incoloy 825 for sour service, a Ni-Fe-Cr alloy that won't crack under chloride stress corrosion.
- Sour pressure boundary, separators, knock-out drums, storage and vessels use A516 Gr 70 NACE-HIC plate. It's HIC-tested per NACE TM0284 and normalized to hold its toughness at low temperature.
- High-chloride produced water and acid stimulation call for something tougher. 904L super-austenitic stainless, with a PREN near 34, beats 316L against chloride SCC and shrugs off sulfuric acid.
- For mild, low-chloride duty, instrumentation, trays and sweet piping rely on 316L stainless (PREN ~24). It's the cost-effective baseline, carrying 2–3% molybdenum.
- Every sour service falls under NACE MR0175 / ISO 15156. All material ships ex-stock with an EN 10204 3.1 mill test certificate.
The Oil & Gas Corrosion Environment
Oilfield corrosion isn't one problem but several, and each pushes grade selection a rung up the ladder. Reading the service correctly matters: H2S partial pressure, chloride level, CO2, temperature. Get that right, and you separate a material that lasts from one that cracks. The full taxonomy is covered under oilfield corrosion types.
Sour Service (H2S → Sulfide Stress Cracking and HIC)
Where hydrogen sulfide is present, dissolved atomic hydrogen embrittles the metal. In high-strength or high-hardness material, this drives sulfide stress cracking (SSC); in rolled carbon-steel plate, it drives hydrogen-induced cracking (HIC) instead. That's the reason sour service (H2S) gets its own dedicated standard, and why hardness, grade and heat-treat condition are all controlled on the order.
Chlorides, Pitting and Crevice Attack
Produced water and seawater carry chlorides that break down the passive film, starting pitting and crevice corrosion and, in warm austenitic stainless, chloride stress-corrosion cracking. Resistance scales with molybdenum and nickel, the basis of the PREN ladder below.
CO2 ("Sweet") Corrosion
Dissolved carbon dioxide forms carbonic acid, and that acid wears down carbon steel through uniform and mesa attack in sweet wells and flowlines. When CO2 gets severe, or when it teams up with chlorides, the job shifts away from plain carbon steel to a corrosion-resistant alloy (CRA).
Acids (Well Stimulation and Regeneration)
Acid stimulation and regeneration fluids put wetted parts in contact with reducing acids. The usual fix is copper-bearing super-austenitic 904L, or Incoloy 825 where you need resistance to both reducing and oxidizing acids. For more on this, see acid resistance.
NACE MR0175 / ISO 15156: The Governing Standard for Sour Service
NACE MR0175 / ISO 15156 is the metallic-materials standard for equipment used in H2S-containing oil and gas production. It does not simply name "approved" grades, it sets the limits of H2S partial pressure, chloride and temperature within which a material is qualified, together with the controls that make the qualification valid: maximum hardness, acceptable metallurgical condition (for example solution-annealed CRA), and required testing. For carbon-steel plate in wet-H2S vessels, resistance to hydrogen-induced cracking is demonstrated by HIC testing to NACE TM0284. Cite MR0175/ISO 15156 on the purchase order so the heat is supplied in the qualified condition and the documentation matches the duty.
Grade × Environment Map for Oil & Gas
Material selection comes down to one thing: match the alloy to the oilfield environment, not to a brochure. Nandini maps each service condition to the lowest grade that still holds up, using a proven ladder, and then hands off to the grade page that closes the sale.
| Environment / component | Service driver | Recommended grade | Metallurgical fit (why) | Grade page |
|---|---|---|---|---|
| Sour gas/oil, tubing, wellhead, heat exchangers, flowlines | H2S (SSC) + chloride | Incoloy 825 (UNS N08825) | Ni 38–46% gives immunity to chloride SCC; Mo + Cu and Ti stabilization resist acids; NACE MR0175 qualified | Incoloy 825 |
| Sour pressure boundary, separators, KO drums, storage, vessels | H2S (HIC) + pressure | A516 Gr 70 (NACE-HIC) | Killed, fine-grain PVQ plate, HIC-tested per NACE TM0284, normalized for low-temperature toughness | A516 Gr 70 NACE-HIC plate |
| High-chloride produced water, heat exchanger, acid stimulation | Chloride pitting + acid | 904L | PREN ~34, ~25% Ni raises the chloride-SCC threshold, copper-bearing for acids; outperforms 316L | 904L super-austenitic |
| General / low-chloride piping, instrumentation, trays | Mild / sweet duty | 316 / 316L | Mo 2–3%, PREN ~24, cost-effective baseline; upgrade when chlorides or H2S exceed its limit | 316L stainless |
A machine-readable copy of this map sits alongside the standards list, and you can pull both through the grade selector.
Incoloy 825: The Sour-Service Corrosion-Resistant Alloy
Why 825 for H2S
Incoloy 825 isn't a stainless steel, it's a Ni-Fe-Cr alloy, and that 38–46% nickel content is exactly why engineers reach for it when H2S and chlorides show up together. High nickel gives it immunity to chloride stress-corrosion cracking that no austenitic stainless can touch. Add 2.5–3.5% molybdenum, 1.5–3% copper, and titanium stabilization, and you get resistance to both reducing and oxidizing acids. Supplied in the NACE MR0175 / ISO 15156 qualified condition, this is the workhorse CRA of the sour flow path. For background on this family, see nickel alloys and the primer what is Incoloy 825.
Components
- Downhole and production tubing
- Wellhead and christmas-tree components
- Shell-and-tube heat exchangers in sour duty
- Flowlines and manifold trim
A common receiving-inspection dispute is the designation itself: buyers routinely write "Inconel 825" on the PO, but it is Incoloy / Alloy 825 (UNS N08825, WNr 2.4858). Get the designation and the NACE condition right on the order, cross-check it against the UNS / Werkstoff cross-reference, and you'll avoid disputes against the mill test certificate. For coil-form duty, 825 is available as Incoloy 825 coil, though only on a limited basis.
A516 Gr 70 (NACE-HIC): The Sour Pressure Boundary
The vessel holding the sour fluid matters just as much as the tubing feeding it. A516 Gr 70 NACE-HIC plate is the killed, fine-grain pressure-vessel-quality carbon steel used for separators, KO drums, storage tanks, and the pressure boundary on sour-service vessels. For wet-H2S duty, though, you need the HIC-tested variant per NACE TM0284, not plain Gr 70. It's also normalized, which brings back the low-temperature impact toughness.
Buyers under-specify two things here. First, impact testing: a low-temperature vessel needs normalized plate with Charpy values verified at the design temperature, not as-rolled material. Second, the HIC call-out itself. State the plate thickness up front, because the mechanical minimums and the normalizing requirement change with section. For the metallurgy behind this class, see pressure-vessel (PVQ) steel and boiler-quality plate explained.
904L: High-Chloride and Acid Service
904L super-austenitic stainless is the step above 316L for produced water, acid stimulation returns and heat exchangers where chlorides are high but a full nickel alloy is not warranted. Its ~4.5% molybdenum lifts the PREN to ~34, ~25% nickel raises the chloride-SCC threshold well above standard austenitics, and ~1.5% copper gives genuine sulfuric-acid resistance.
The super-austenitic family, built around sulfuric acid resistance, is tricky to spec correctly, and the error cuts both ways. Teams often reach for Alloy 20, Inconel 625 or titanium for sulfuric-acid and high-chloride duty that the copper-bearing 904L chemistry already handles at a lower price. Before paying for that upgrade, check the acid concentration and temperature against the 904L iso-corrosion limits. On the flip side, don't under-spec 904L in cases where a 6-Mo grade or super-duplex is truly needed. For more on this, see super-austenitic stainless and what is 904L. Coil is available as 904L coil.
316 / 316L: The Baseline for Mild / Sweet Duty
316L stainless is the baseline for low-chloride, sweet oil & gas duty: instrumentation, tubing bundles, trays and general piping. Its 2–3% molybdenum is what you are buying over 304, PREN ~24, with meaningful pitting resistance. But 316 is austenitic and remains susceptible to chloride stress-corrosion cracking, so it has a hard ceiling. When hot chlorides, produced water or H2S exceed that limit, the correct move is up to 904L or Incoloy 825, not more 316. Use 316L rather than 316 for welded fabrications so the weld heat-affected zone stays corrosion-resistant.
Selecting the Grade: The Oil & Gas PREN & Severity Ladder
Pitting Resistance Equivalent Number (PREN = Cr + 3.3·Mo + 16·N) ranks how well a grade stands up to chloride pitting, and as you climb that same ladder, nickel-driven resistance to chloride SCC climbs right along with it. A516 doesn't sit on the CRA ladder at all; it's the carbon-steel pressure boundary, and its selection runs through HIC testing rather than a PREN rank. Full theory is under PREN and PREN explained.
| Grade | PREN (approx.) | Family | Choose when |
|---|---|---|---|
| 316 / 316L | ~24 | Austenitic SS | Low chloride, no H2S, mild temperature, the sweet-service baseline |
| 904L | ~34 | Super-austenitic SS | High-chloride produced water and reducing acids where 316L pits or cracks |
| Incoloy 825 | ~31–40 | Ni-Fe-Cr alloy | H2S present (sour) and/or chloride SCC risk, mixed reducing/oxidizing acids |
| A516 Gr 70 (NACE-HIC) | Off-ladder | PVQ carbon steel | Sour pressure boundary, ranked by HIC test (NACE TM0284) and toughness, not PREN |
The composition drivers behind the ladder sit here; full specs for each grade live on that grade's own page.
| Grade | Ni | Cr | Mo | Cu | Driver |
|---|---|---|---|---|---|
| Incoloy 825 | 38–46% | 19.5–23.5% | 2.5–3.5% | 1.5–3% | High Ni → chloride-SCC immunity; Mo + Cu + Ti → acid and pitting resistance |
| 904L | ~25% | 19–23% | ~4.5% | ~1.5% | Mo → PREN ~34; Cu → sulfuric-acid resistance; Ni → raised SCC threshold |
| 316 / 316L | 10–14% | 16–18% | 2–3% | – | Mo gives the pitting resistance 304 lacks |
Narrow the shortlist with the grade selector, or walk through the method in how to select a stainless grade.
Forms & Standards Supplied for Oil & Gas
Nandini keeps each grade ex-stock in whatever form the oil and gas work demands, and every batch is certified to the governing specification. The firm operates as an ISO 9001:2015-certified stockist and supplier, sourcing genuine mill plate from Aperam, Outokumpu, Jindal, POSCO, Nippon Steel and JFE. Its real strength lies in specialty plates: 904L, Alloy 28, Incoloy 800 and Hastelloy, all backed by third-party certification.
| Grade | Forms | Governing standard |
|---|---|---|
| Incoloy 825 | Sheet, plate (coil limited) | ASTM B424 / ASME SB-424; NACE MR0175 |
| 904L | Sheet, plate, coil | ASTM A240 / ASME SA-240 |
| 316 / 316L | Sheet, plate, coil | ASTM A240 / ASME SA-240 |
| A516 Gr 70 (NACE-HIC) | Plate (primary) | ASTM A516 / ASME SA-516 + ASTM A20; NACE MR0175 / TM0284 |
All documentation comes with an EN 10204 3.1 mill test certificate, the MTC. A 3.2 third-party certification is available if you request it.
Testing, NACE Compliance & Certification
A sour-service QA package is as much about paperwork as it is about metal. On oil and gas orders, Nandini provides a NACE MR0175 / ISO 15156 conformance statement alongside the hardness report. For A516 Gr 70 NACE-HIC plate, HIC test results per NACE TM0284 come with it too. Positive material identification (PMI), ultrasonic (UT) and radiographic (RT) examination are all available when the specification calls for them. Every heat ships with full cast and heat traceability, and an EN 10204 3.1/3.2 mill test certificate goes out with every order. That's what lets the material on site map straight back to the mill certificate.
For more detail, see the en 10204 3.1 mill test certificate page, the mill test certificate reference, the firm's quality assurance process and third-party certifications. You can also read more about the company at about Nandini Steel.
How to Order Corrosion-Resistant Steel for Your Oil & Gas Project
- Define the environment. State the H2S partial pressure, chloride level, CO2, temperature and pressure, these are the inputs that determine which rung of the ladder you need.
- Pick the grade and form. Consult the grade × environment map above, or the grade selector, to choose Incoloy 825, A516 Gr 70 NACE-HIC, 904L or 316L, along with the form it ships in.
- Specify the standard, NACE level and MTC. Name the ASTM/ASME specification, NACE MR0175 / ISO 15156 (and TM0284 HIC for plate), and put the EN 10204 3.1 or 3.2 certificate level on the purchase order.
- Estimate quantity and send the RFQ. Size the order with the weight calculator, and contact us for live pricing. All pricing runs through RFQ, and every stocked grade dispatches immediately from Mumbai.
Nandini's been in the trade since 2003, over 20 years now, and has handled more than 307 projects for 260-plus clients. Her work ships to 36-plus countries, oil and gas regions included.
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FAQ
What steel is used for sour service in oil and gas?
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When should I upgrade from 316L to 904L or Incoloy 825?
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Request a Quote
Set your environment, choose the grade and form, then send over the spec: Incoloy 825, A516 Gr 70 NACE-HIC, 904L or 316L, naming the standard, NACE level and MTC level. Every grade sits ex-stock, ready for immediate dispatch, and pricing runs by RFQ. Contact Nandini Steel or reach us on WhatsApp at +91 99202 09184. You can also call +91 22 43404600 or email reachus@nandinisteel.com.