Duplex vs 13Cr vs Incoloy 825: CRA Selection for HPHT Sour Wells

CRA grade selection for HPHT sour wells comes down to one question that reservoir data has to answer before any material conversation begins: how much H₂S is in the system, and what happens to that number as the well depletes? The material that handles a pure CO2 condensate system is rarely the right answer once even trace H₂S appears — and once H₂S partial pressure climbs past the qualification envelope of the lower-alloy CRAs, the cost and qualification framework shift completely.

ZC Steel Pipe supplies duplex OCTG and L80-13Cr to gas condensate operators across the Middle East and Southeast Asia. The grade selection call comes up most often when reservoir chemistry shifts from pure CO2 to a mixed CO2/H₂S system — and that shift changes the material choice completely. What follows is the technical basis for that decision, grounded in NACE MR0175 / ISO 15156-3:2009 with Technical Circular 1 (2011) and API Specification 5CT, 11th Edition.

The Four Materials and What Governs Them

Four materials cover most of the HPHT CRA selection space in oil and gas OCTG:

L80-13Cr — a martensitic chromium-alloyed steel produced to API 5CT. It is the go-to material for CO2 corrosion in gas condensate wells when H₂S is absent or below trace levels. The chromium content (12–14% per API 5CT) creates a passive oxide film that resists carbonic acid attack in flowing wells. This is its strength and its limit: the passive film is not designed for sulfide stress cracking (SSC) resistance.

2205 Duplex (UNS S31803) — a two-phase austenitic-ferritic stainless steel with roughly equal proportions of each phase. The duplex microstructure gives it significantly higher strength than austenitic grades while retaining corrosion resistance. Under ISO 15156-3:2009/Cir.1:2011 Table A.24, it is qualified for limited sour service.

2507 Super Duplex (UNS S32750) — a higher-alloy duplex grade with elevated Cr, Mo, and N content. FPREN consistently falls in the 38–48 range, placing it in the higher-qualified band under Table A.24. Its H₂S ceiling is twice that of 2205 duplex.

Incoloy 825 (UNS N08825) — a nickel-iron-chromium austenitic alloy qualified under ISO 15156-3, Clause A.4. With 38–46% Ni (balance Fe), it is in a different alloy family entirely. It does not operate under the FPREN framework that governs duplex grades — it qualifies under the solid-solution nickel-based alloy clause.

L80-13Cr: CO2 Grade, Not Sour Service

The API 5CT mechanical property profile for L80-13Cr is:

The hardness limit of HRC 23.0 matches the maximum hardness required for sour service L80-1 under NACE MR0175 / ISO 15156-2. That coincidence trips up procurement teams every week.

The chemistry per API 5CT for L80-13Cr is: C 0.15–0.22%, Mn 0.25–1.0%, Cr 12–14%, Ni max 0.5%, Cu max 0.25%, Si max 1.0%, P max 0.02%, S max 0.01%. Mo is not restricted by API 5CT for this grade. The carbon content (0.15–0.22%) is what drives the martensitic transformation — it is also what creates susceptibility to hydrogen embrittlement in H₂S environments at the hardness levels that result from the Q+T cycle.

The FPREN Framework for Duplex Selection

NACE MR0175 / ISO 15156-3:2009 with Technical Circular 1 (2011) uses the Pitting Resistance Equivalent Number to sort duplex grades into qualification tiers:

FPREN = %Cr + 3.3×%Mo + 16×%N

The formula weights nitrogen heavily relative to its mass fraction because nitrogen is exceptionally effective at stabilizing the austenite phase and improving pitting resistance in the passive film. The 3.3× factor on molybdenum reflects Mo's well-documented role in suppressing pitting in chloride environments.

Worked Calculation — 2205 Duplex, Typical Heat

A 2205 duplex heat with nominal chemistry (22% Cr, 3% Mo, 0.14% N):

FPREN = 22 + (3.3 × 3) + (16 × 0.14) FPREN = 22 + 9.9 + 2.24 FPREN = 34.1

This heat falls squarely in the 30–40 band of ISO 15156-3 Table A.24. The qualification limit is: max temperature 232°C (450°F), max H₂S partial pressure 10 kPa (1.5 psi). Any combination of chloride concentration and in-situ pH occurring in production environments is acceptable at these limits, provided the material is solution-annealed and liquid-quenched with ferrite content 35–65 vol%.

Worked Calculation — 2507 Super Duplex, Typical Heat

A 2507 super duplex heat with nominal chemistry (25% Cr, 4% Mo, 0.28% N):

FPREN = 25 + (3.3 × 4) + (16 × 0.28) FPREN = 25 + 13.2 + 4.48 FPREN = 42.7

This heat lands in the FPREN 40–45 band of Table A.24. The qualification limit is: max temperature 232°C (450°F), max H₂S partial pressure 20 kPa (3 psi). The temperature ceiling is identical to 2205 duplex — the distinction is in H₂S tolerance, not temperature.

2205 Duplex Chemistry and Qualification

The chemical composition of S31803 per ISO 15156-3:2009/Cir.1:2011 Table D.7:

The low carbon maximum (0.03%) is a key differentiator from L80-13Cr's 0.15–0.22% C. Low carbon minimizes carbide precipitation at grain boundaries during welding or heat exposure, which would compromise corrosion resistance by depleting chromium from the passive film zone.

Qualification conditions per ISO 15156-3 Table A.24 (FPREN 30–40, Mo ≥ 1.5%): the material must be solution-annealed and liquid-quenched (or rapidly cooled equivalent), ferrite content 35–65 vol%, no ageing heat treatments permitted. The duplex microstructure is metastable — sigma phase and alpha-prime embrittlement can form above approximately 300°C during manufacture or field exposure. This is why Table A.24 caps temperature at 232°C (450°F).

2507 Super Duplex Chemistry and Qualification

The chemical composition of S32750 per ISO 15156-3 Table D.7:

The narrower Mn and Si limits relative to 2205 duplex reflect tighter manufacturing controls — 2507 is more sensitive to thermal history during production. The higher N content (0.24–0.32% vs 0.08–0.20%) is what drives the superior FPREN and the additional pitting resistance, but it also means 2507 is more prone to sigma-phase precipitation if cooling from solution anneal is too slow. Qualifying documentation should include the actual cooling rate record, not just the anneal temperature.

Qualification conditions per ISO 15156-3 Table A.24 (FPREN 40–45): max temperature 232°C (450°F), max H₂S partial pressure 20 kPa (3 psi). Elemental sulfur resistance: NDS (No Data Submitted). This NDS entry is not a footnote — it is a hard boundary. The ISO 15156-3 qualification framework only covers conditions for which data has been submitted and approved by the maintenance agency. NDS means the alloy has not been qualified for elemental sulfur environments.

Incoloy 825 (N08825): When You Leave the Duplex Framework

Incoloy 825 occupies a different part of the ISO 15156-3 structure. Its composition per Table D.3:

Incoloy 825 is a nickel-iron-chromium austenitic alloy. The high nickel content (38–46%) is what makes it fundamentally different from both 13Cr martensitic grades and duplex grades: at those nickel levels, the alloy is stable austenite across its entire thermal processing range, with no martensitic or ferritic transformation. This eliminates the sigma-phase and SSC susceptibility mechanisms that constrain duplex and 13Cr grades.

The FPREN formula does not apply to nickel-based alloys — it was developed specifically for duplex stainless steels where ferrite-austenite balance and pitting in the passive film are the dominant corrosion mechanisms. ISO 15156-3 qualifies Incoloy 825 under Clause A.4, which covers solid-solution nickel-based alloys and addresses their distinct failure modes.

The copper addition (1.5–3.0%) in Incoloy 825 provides additional resistance to reducing acid environments, including H₂S-containing fluids. The titanium addition (max 0.6%) stabilizes the alloy against sensitization during welding by tying up carbon in stable TiC precipitates rather than allowing Cr₂₃C₆ grain boundary precipitation.

For the full API 5CT grade specifications and chemistry limits, see the API 5CT specification tables →

For NACE MR0175 / ISO 15156 qualification limits and environmental thresholds, see the NACE MR0175 specification tables →

Four-Material Comparison Table

Read this table alongside the reservoir fluid analysis, not as a standalone decision matrix. The cost multipliers assume standard tubing sizes — at large casing diameters, the cost differential narrows in percentage terms but widens in absolute dollar impact per string.

To match a grade to your well conditions based on H₂S partial pressure, temperature, and chloride content, use the Sour Service Selector →

To generate a full OCTG grade recommendation from well parameters, use the AI Pipe Grade Selector →

When NOT to Use These Materials

The conditions below are where each material fails — either technically or commercially:

• L80-13Cr when H₂S is present above trace levels. API 5CT classifies it as a CO₂ corrosion grade. ISO 15156-2 requires specific qualification documentation for sour service that L80-13Cr does not carry. The threshold matters: even confirmed H₂S at 0.01 bar (1 kPa) partial pressure takes you out of the range where L80-13Cr can be used without separate qualification analysis.

• 2205 Duplex above 10 kPa (1.5 psi) H₂S partial pressure. Table A.24 is explicit. At FPREN 30–40 with Mo ≥ 1.5%, the limit is 10 kPa (1.5 psi). Reservoir conditions where H₂S sits at 15–20 kPa push into 2507 super duplex territory or above, regardless of what the well's operating temperature and chloride content look like.

• 2507 Super Duplex where elemental sulfur deposition is possible. NDS in Table A.24 is the standard's way of saying: no qualification data exists, therefore no approval can be inferred. Wells in the Middle East producing from sour carbonate reservoirs can deposit elemental sulfur during pressure drawdown. If elemental sulfur is on the list of potential produced fluids, 2507 super duplex has no Table A.24 standing for that condition.

• Incoloy 825 as a first-choice material for CO₂-only sweet wells. The alloy cost premium of 8–12× L80 carbon steel is justified in severe sour or mixed CO₂/H₂S/chloride environments where its corrosion resistance provides genuine integrity advantage. In a gas condensate well with CO₂ partial pressure as the primary corrosion driver and no H₂S, L80-13Cr at 3–5× or Super 13Cr at a moderate premium handles the corrosion requirement. Specifying Incoloy 825 in that environment adds 3–5× unnecessary cost per tonne of tubular.

• Any duplex grade without calculating FPREN from the actual mill heat chemistry. The specification range for S31803 alone spans FPREN 31 to 38. Using a generic "2205 is qualified for 10 kPa H₂S" statement without verifying that the specific heat produced by the specific mill lands above the FPREN minimum for Table A.24 qualification is an inspection gap. Review the MTC, calculate FPREN, and document it before accepting a shipment.

Named Failure Modes

Sulfide Stress Cracking (SSC) in L80-13Cr — The failure mechanism is hydrogen embrittlement driven by atomic hydrogen generated at the steel surface in H₂S environments. The martensitic microstructure of L80-13Cr, hardened by high carbon content and the Q+T cycle, is susceptible to hydrogen trapping at microstructural defects. The visible result is brittle fracture that looks nothing like mechanical overload — no ductile dimpling, no elongation. The crack typically initiates at the thread roots of connections or at surface imperfections, propagates under the residual stress from makeup, and can result in catastrophic string failure under loading that is well below the grade's rated minimum yield. This is the specific mechanism ISO 15156-2 controls through hardness limits and material qualification.

Sigma Phase Embrittlement in Duplex Grades — Both 2205 and 2507 duplex are susceptible to sigma-phase formation when held at temperatures between approximately 650°C and 1000°C during manufacture. Sigma phase is a hard, brittle intermetallic compound that forms at ferrite-austenite boundaries and depletes chromium from the surrounding matrix — simultaneously reducing ductility and corrosion resistance. It forms during slow cooling from solution anneal or during welding heat-affected zone exposure. The diagnostic on an MTC is the required ferrite count: a ferrite content outside 35–65 vol% at delivery is a red flag that thermal processing may not have been adequate. We request ferrite measurement records (by Fischer gauge or metallographic section) on critical OCTG strings.

Pitting from FPREN Underestimation — If the actual heat FPREN falls below the minimum for the intended Table A.24 row, the material is being used outside its qualification envelope. Pitting in duplex stainless steel initiates at the ferrite-austenite phase boundaries where the passive film is thinner. Once a pit initiates in a high-chloride, H₂S-bearing environment, it propagates quickly because the anodic dissolution inside the pit is accelerated by both chloride and the locally reduced pH from H₂S dissolution. The external surface of a pitted CRA tube looks unremarkable — the damage is localized and deep. It is not detectable by visual inspection or hydrostatic test until the pit has penetrated nearly through the wall.

Purchase Order Guidance — Getting the Specification Right

The Procurement Trap

Wrong PO language: "L80-13Cr casing, sour service, per NACE MR0175 / ISO 15156, OD 5.5", 17 lb/ft, Q+T, API 5CT 11th Edition."

What the mill ships: fully compliant API 5CT L80-13Cr, colour-banded per the standard, with a valid API 5CT MTC. Not a single thing wrong with the material as an API 5CT product. The problem is the sour service designation — L80-13Cr has sour_service: false in the API 5CT specification framework. The MTC will not carry ISO 15156-2 qualification because the grade does not have it.

What happens next: The well operator's third-party inspector reviews the MTC against the NACE MR0175 / ISO 15156-2 checklist, finds no SSC qualification, and puts the string on hold. The mill is fully compliant. The liability sits with the buyer for incorrect specification.

Correct PO language for a sour carbon steel alternative: "API 5CT L80 Type 1, minimum yield 552 MPa (80 ksi), maximum yield 655 MPa (95 ksi), max hardness HRC 23.0 (HBW 241), Q+T heat treatment, qualified per NACE MR0175 / ISO 15156-2 SSC Region 2, EN 10204 3.2 MTC required."

Correct PO language for a CRA in a sour well: "Duplex stainless steel OCTG, UNS S31803 (2205 duplex), solution-annealed and liquid-quenched, ferrite content 35–65 vol%, FPREN minimum [calculated value] to be confirmed from heat chemistry, qualified per NACE MR0175 / ISO 15156-3:2009/Cir.1:2011 Table A.24, max H₂S service 10 kPa (1.5 psi), EN 10204 3.2 MTC with actual heat chemistry and FPREN calculation required."

MTC Checklist for CRA OCTG

Before accepting a CRA shipment against a sour well specification, verify the following on the MTC:

1. UNS designation matches the specified grade — not a generic name.
2. Actual heat chemistry for Cr, Mo, and N reported as measured values, not specification minima.
3. FPREN calculated from actual chemistry and documented on MTC or supplementary report.
4. Heat treatment confirmation: solution anneal temperature, hold time, and quench method recorded.
5. Ferrite count reported for duplex grades — Fischer gauge reading or metallographic measurement.
6. For Incoloy 825: stabilisation anneal record (if applicable) and Ti content confirmed within the 0–0.6% range.
7. ISO 15156-3 reference and Table A.24 row cited on MTC or on a supplementary qualification document.
8. Third-party inspector witness stamp (EN 10204 3.2) for any sour HPHT application — we treat this as non-negotiable for critical strings regardless of what the project specification minimum requires.

The question of whether the heat chemistry actually qualifies for the row being claimed is an arithmetic check — FPREN = %Cr + 3.3×%Mo + 16×%N — that takes thirty seconds with the actual values from the MTC. We run it on every duplex heat we process. A heat that claims Table A.24 FPREN 40–45 qualification but whose actual chemistry calculates to FPREN 38.4 is being miscertified.