The cladding alloy is only 1.5–3 mm of the pipe wall, but it determines whether the line survives 25 years or fails in two. Choose too low an alloy and you get pitting, crevice corrosion, or sour-service cracking. Choose too high and you pay 3–4× the material cost for protection you don't need. This guide matches the most common cladding alloys to real service environments.
1. Start with PREN, Not the Alloy Name
The Pitting Resistance Equivalent Number (PREN = %Cr + 3.3×%Mo + 16×%N) is the standard first-pass filter for chloride service. It correlates with resistance to pitting and crevice corrosion in chloride-bearing environments — the dominant failure mode in seawater, produced water, and many chemical services.
| Alloy | Type | Typical PREN | Relative Material Cost |
|---|---|---|---|
| 316L | Austenitic stainless | 25–28 | 1× (baseline) |
| Duplex 2205 | Duplex stainless | ~35 | 1.5–2× |
| Super Duplex 2507 | Super duplex | ≥ 40 | 2.5–3× |
| Alloy 825 | Ni-Fe-Cr-Mo-Cu | ~31–33 | 4–6× |
| Alloy 625 | Ni-Cr-Mo-Nb | ≥ 45 | 6–10× |
PREN is necessary but not sufficient. Alloy 825, for example, carries a moderate PREN but adds copper for sulfuric acid resistance and titanium for stabilization — properties the PREN formula does not capture. Always match the alloy to the specific corrosive species, not just the chloride number.
2. Alloy-by-Alloy: Where Each One Belongs
316L — sweet water, mild chemicals, budget-critical projects. The workhorse for non-chloride or low-chloride service: potable water, refined products, mild process fluids. Do not specify 316L for long-term seawater immersion or warm chloride service — field history shows pitting failures within months to a few years in warm seawater, with full pipeline replacement as the consequence.
Duplex 2205 — produced water, cooling water, moderate chlorides. Roughly double the yield strength of 316L and a meaningful PREN step up. Standard choice for oilfield produced water and closed-loop cooling systems where chlorides are present but controlled and temperatures stay moderate. Warm seawater at the upper end of its range risks crevice corrosion — apply a safety margin or move up.
Super Duplex 2507 — seawater, high chlorides, higher temperature. PREN ≥ 40 clears the threshold generally accepted for critical seawater service. This is the alloy class behind high-pressure reverse-osmosis desalination piping specifications. Watch welding control: 2507 demands tighter heat-input and interpass temperature limits than 2205, which adds fabrication cost that buyers should price in.
Alloy 825 — sour service, sulfuric acid, reducing environments. Copper plus molybdenum gives 825 its edge in sulfuric acid and mixed reducing/oxidizing media, and it meets NACE MR0175/ISO 15156 sour-service limits over a wide range. Common in oil and gas wells with H₂S and in chemical plant piping where stainless steels fall short. If your threat is H₂S rather than chloride, 825 often beats a higher-PREN duplex on both performance and cost.
Alloy 625 — the insurance policy. Highest PREN of the group, excellent resistance to pitting, crevice corrosion, and chloride stress corrosion cracking, and full sour-service compliance. Specify it where the environment is unpredictable, where shutdown is unthinkable (subsea, remote trunk lines), or where decades of chloride exposure at elevated temperature are certain. The cost is real; so is the failure cost it prevents.
3. Quick Selection Matrix
| Service Environment | Recommended Cladding | Rationale |
|---|---|---|
| Potable / fresh water, ambient | 316L | Adequate resistance at minimum cost |
| Produced water, moderate chlorides | 2205 | PREN ~35 with good strength and cost balance |
| Seawater intake / injection, critical service | 2507 or 625 | PREN ≥ 40 required; 625 for maximum margin |
| Sour service (H₂S present) | 825 or 625 | NACE MR0175 compliance; Ni alloys tolerate H₂S beyond duplex limits |
| Sulfuric acid / mixed chemicals | 825 | Copper content resists reducing acid attack |
| High-temperature chlorides, uncertain chemistry | 625 | Highest PREN and SCC resistance |
4. Cost Reality: Clad vs Solid
Whatever alloy you select, the economics of cladding work the same way: only 1.5–3 mm of expensive alloy is consumed instead of a full-wall solid pipe. Published project cases put clad pipe at 40–70% below solid CRA cost for large-diameter lines, and a documented North Sea subsea project rejected solid Alloy 825 in favor of super duplex precisely because of a multi-million-dollar cost difference. For NPS 4 and below, solid CRA is usually simpler; above that, cladding is where the value is.
This is where bimetallic composite pipe earns its place: ZONX PIPE bonds each of these alloys — 316L, 2205, 2507, 825, and 625 — onto carbon steel base pipe across the large diameters where solid CRA pricing breaks project budgets. The selection logic in this guide applies directly to those sizes, because it is at NPS 8 and above that the 40–70% cost gap between clad and solid becomes a line-item decision rather than a rounding error.
5. Common Selection Mistakes
- Upgrading to 625 "to be safe" everywhere. Over-specification burns budget that is better spent on inspection or corrosion allowance elsewhere. Match the alloy to the medium.
- Using PREN alone in sour service. Chloride resistance and H₂S resistance are different properties. Check NACE MR0175 limits for the actual H₂S partial pressure and pH.
- Ignoring welding constraints. 2507 and nickel alloys need qualified procedures and tighter heat control — confirm your fabricator's capability before finalizing the alloy.
- Specifying 316L for warm seawater. Repeatedly documented as a failure mode. It is the single most common alloy selection error in marine service.
6. Conclusion
Alloy selection is a lifecycle decision, not a unit-price decision. A pipe that fails from pitting in two years costs far more than the delta between 2205 and 625. Define the medium, temperature, chloride level, H₂S partial pressure, and design life first — the right alloy usually selects itself.
ZONX PIPE supplies bimetallic CRA clad pipe with 316L, 2205, 2507, 825, and 625 cladding layers under API 5LD, manufactured with RSP™ (Right Spinning Pressure) metallurgical bonding and verified by full-length ultrasonic bond inspection. We provide project-specific alloy recommendations based on your service envelope. For the bigger picture on when clad pipe beats both lined and solid CRA, see our clad vs lined vs solid selection guide. Contact us for a selection review.