When a clad pipe (bi-metal pipe) specification says "CRA layer: Alloy 625," the engineer who wrote it usually tried duplex first — and got told no. Alloy 625 (Inconel 625, UNS N06625) is the default cladding alloy for the harshest sour-service duty on earth: deep, hot, chloride-rich, H₂S-heavy wells. This guide covers what 625 is, exactly when it is required (and when it is over-specified), and how to buy it without overpaying.
1. What Is Inconel 625 Clad Pipe?
It is a bimetallic pipe: a carbon or low-alloy steel backing — typically API 5L X52 through X65 — carries the pressure, while a 2–4 mm layer of Alloy 625, metallurgically bonded per API 5LD, faces the medium. Nickel alloy content is the dominant cost in CRA pipe, so replacing a solid 625 wall with a 2.5 mm layer delivers the same corrosion envelope at 30–50% lower cost. ZONX supplies 625-clad line pipe and spools under the DURACORE™ line.
2. The Alloy in Numbers
| Property | Value | Why It Matters |
|---|---|---|
| UNS designation | N06625 | Material traceability on MTRs |
| Ni content | ≥ 58% | Immunity to chloride stress corrosion cracking |
| Cr / Mo | 20–23% / 8–10% | Pitting + crevice resistance in sour brines |
| Nb + Ta | 3.15–4.15% | Stabilization; resists sensitization in HAZ |
| PREN | ≥ 45 | Suitable for > 5,000 ppm chloride at temperature |
| Max service temp (sour) | ~ 450 °C | Covers nearly all oil & gas duty |
| NACE MR0175 | Compliant, all regions | No sulfide stress cracking limit in practice |
3. 625 vs the Alternatives: Where Each One Stops
| Limit Condition | 316L | Duplex 2205 | Alloy 825 | Alloy 625 |
|---|---|---|---|---|
| Chlorides, ppm | ~ 1,000 | ~ 5,000 | ~ 10,000 | No practical limit |
| H₂S partial pressure | Low only | MR0175 limits | High | Any, all regions |
| Temperature ceiling | 60 °C | ~ 150 °C | ~ 200 °C | ~ 450 °C |
| Sulfur / polythionic acid | ❌ | ❌ | ⚠️ Marginal | ✅ |
| Relative clad cost | 1.0× | 1.5× | 2.2× | 2.7× |
The pattern: 625 is not "better stainless" — it is the alloy you step up to when the cheaper options hit a hard environmental wall. If your medium sits inside duplex 2205's envelope, paying for 625 is waste; our alloy selection guide walks the decision path.
4. When 625 Is (and Is Not) the Answer
Specify 625 cladding when any of these is true:
- NACE MR0175 places the service in a region where duplex or 825 is disqualified (high pH₂S + temperature combination)
- Chlorides exceed ~5,000 ppm with temperature above 100 °C
- Free sulfur, oxygen ingress, or polythionic acid exposure is possible during operation or shutdowns
- Design life exceeds 25 years in sour service with no intervention access (subsea, buried gathering)
- The operator's corrosion allowance philosophy forbids any pitting initiation in the design case
Do not specify 625 when: the service is sweet or mildly sour below 60 °C — 316L or duplex at a third of the price does the job. Over-specification is the most common procurement error in CRA pipe, and it is expensive at nickel-alloy rates.
5. Welding 625 Clad Pipe
Field welding follows the standard clad pipe procedure: ERNiCrMo-3 (Alloy 625) buttering deposited on the carbon-steel side of the bevel, then a CRA-to-CRA closure weld. Because the bond is metallurgical, the weld behaves like solid CRA pipe — no liner end-seal welds, no collapse risk, standard transition joint procedures per ASME IX with PWHT per qualified WPS. Post-weld HAZ hardness is verified ≤ 22 HRC for NACE compliance. This weldability is the practical reason 625 clad wins over mechanically lined pipe in sour duty.
6. Sourcing Checklist for 625 Clad Pipe
- CRA layer certified UNS N06625 per ASTM B443, with PMI traceability to heat number
- Bond shear strength ≥ 140 MPa, verified per API 5LD Annex (full-length UT)
- NACE MR0175/ISO 15156 compliance documentation for the actual service envelope
- HAZ hardness mapping (≤ 22 HRC) on weld procedure qualification records
- Clad thickness 2–4 mm with tolerance stated; corrosion allowance calculation from your corrosion engineer
- Third-party inspection scope (BV / SGS / TÜV) defined at RFQ, not after
- Nickel-index escalation clause for orders beyond ~90 days of firm pricing
7. Conclusion
Alloy 625 earns its premium only inside its envelope — severe sour, hot, chloride-rich, long-life service. Below that envelope, duplex and 825 deliver the same reliability for half the alloy cost. The engineering value of a qualified 625 clad supplier is knowing both sides of that line. Send ZONX your line list with H₂S, CO₂, chloride, and temperature data, and the answer comes back as a specified alloy — not a guess.