Material qualification for H2S, CO2, and chloride service in oil and gas production
NACE MR0175 (now harmonised with ISO 15156-3) is the controlling document for material selection in upstream oil and gas service where the produced fluid contains hydrogen sulphide (H2S). The standard establishes acceptance limits for cracking-resistant materials based on environmental severity (H2S partial pressure, in-situ temperature, chloride concentration, free-sulphur presence, and pH). Ferralium 255 (UNS S32550) is listed in ISO 15156-3 Table A.10 as a qualified super duplex grade with the following environmental envelope: maximum H2S partial pressure 100 kPa (15 psi), maximum in-situ temperature 232 °C, maximum chloride at 100 °C of 167 g/L, conditional acceptance to higher chloride at lower temperature per the table. The metallurgical acceptance criteria are: hardness ≤ 32 HRC measured at the bolt mid-radius location per NACE TM0177, ferrite content within 35 to 55% per ASTM E562, supplied in the solution-annealed condition (no precipitation or transformation hardening permitted).
| Parameter | Limit | Notes |
|---|---|---|
| H2S partial pressure, max | 100 kPa (15 psi) | Above this limit, switch to Inconel 718 or Hastelloy C-276 |
| In-situ temperature, max | 232 °C | Above this limit super duplex risks 475 °C embrittlement on long exposure |
| Chloride at 100 °C, max | 167 g/L | De-rate at higher temperature per Table A.10 footnote |
| Free elemental sulphur | NOT permitted | Material is disqualified for sulphur-bearing service |
| pH (in-situ) | ≥ 3.5 typical | Combined with H2S limit per Table 1 of ISO 15156-2 |
| Hardness, max | 32 HRC | Per NACE TM0177 (mid-radius location) |
| Ferrite content | 35 to 55% | Per ASTM E562 point count, transverse and longitudinal sections |
Per-heat MTC (EN 10204 type 3.2) documents the chemistry per ASTM A479, the mechanical properties per ASTM E8 / A370, the hardness traverse at three locations per piece, the ferrite-content point count per ASTM E562 with the grid sketch, the Charpy V-notch energy at the qualifying temperature (typically 45 J at -46 °C for North Sea / Aramco service), and the explicit NACE MR0175 conformance statement with the ISO 15156-3 Table A.10 reference. For sour service in NORSOK-governed projects the supplementary requirement S60 (NACE TM0177 method A: tensile-stress sulphide stress cracking test) is often added; this is a 720-hour test on a stressed specimen in a saturated-H2S aqueous environment, with the specimen showing no cracking at 80% yield-strength applied stress.
Above 100 kPa H2S partial pressure, above 232 °C in-situ temperature, in free-elemental-sulphur service, or in any combination of those conditions plus chloride above the Table A.10 limit, Ferralium 255 should not be specified. The escalation route is to nickel-base alloys: Inconel 718 (UNS N07718) for high-strength bolting, Inconel 625 (UNS N06625) for piping and pressure shells, Hastelloy C-276 (UNS N10276) for severe acid + chloride service. For service strictly within the envelope but where the higher PREN of 2507 (≥ 41) or Zeron 100 (≥ 40 with W-bearing benefit) gives extra margin, see Ferralium vs 2507 and Ferralium vs Zeron 100 comparison pages.
