GUIDE
Which grade for seawater?
Seawater rules out most of the stainless catalogue. The grades that remain are separated by criteria PREN does not contain — and by a question people often forget to ask: is the circuit flowing or shut down?
Why ordinary stainless fails
The chloride ion breaches the passive film locally and starts a pit, which grows in depth without degrading the surface around it. In seawater the chloride concentration is of the order of 19,000 ppm — two orders of magnitude above the threshold for a 304L.
The threshold of around 40 PREN for seawater is a widely used engineering marker, not a value from a standard. It serves to rule candidates out. It is not enough to validate one.
Three questions to ask before the grade
What temperature? The aggressiveness of seawater rises sharply with temperature. Water at 15 °C and the same water at 40 °C do not pose the same problem in the same grade. On a condenser it is the outlet temperature that governs, not the inlet.
What flow velocity? It cuts both ways. Too low or nil, and deposits form and crevice corrosion sets in beneath them. Too high, and the protective film is eroded — that is the specific limit of copper-nickels, around 2.5 m/s for 90/10.
Are there prolonged shutdowns? This is the most often forgotten and the most decisive question. A seawater circuit left full during a shutdown is far more aggressive than one in service: deposits settle, oxygen is consumed locally, and crevice corrosion starts under the deposits. Many seawater failures occur during shutdowns, not in operation.
The four routes
Super duplex 2507 — PREN 42.5, and a yield strength that allows reduced wall thickness: a decisive argument offshore, where carried weight is paid for twice. In exchange, a qualified welding procedure is mandatory and the 300 °C limit applies. Cost index 260.
Super austenitic 254 SMO — PREN 44, welds and forms like an austenitic with no phase balance to watch, and without the duplex temperature ceiling. Its yield strength remains that of an austenitic, so no wall reduction. Cost index 420. ⚠️ The filler metal must be over-alloyed: a filler matching the parent metal gives a molybdenum-depleted weld of far lower PREN — corrosion starts exactly where the best grade was thought to have been placed.
Titanium grade 2 — No PREN, because the formula does not apply: titanium forms an oxide that chlorides do not breach. Practical immunity, no velocity limit, and a density of 4.51 that substantially lightens large bundles. In exchange, strict gas-shielded welding — face, root and cooling zone — and no resistance at all to reducing acids. Cost index 550.
Copper-nickels 90/10 and 70/30 — The only route with an antifouling effect: copper prevents marine organisms from settling. A copper-nickel condenser keeps its efficiency where a stainless or titanium bundle is colonised and forces cleaning. In exchange, a flow velocity limit, sensitivity to sulphides, and total incompatibility with ammonia. Cost indices 210 and 290.
Decision table
| Criterion | 2507 | 254 SMO | Titanium Gr 2 | CuNi 90/10 | CuNi 70/30 |
|---|---|---|---|---|---|
| PREN | 42.5 | 44 | does not apply | does not apply | does not apply |
| Mechanical strength | very high | moderate | moderate | low | low |
| High velocity | yes | yes | yes | no, ~2.5 m/s | better |
| Warm seawater | with reservation | yes | yes | with reservation | yes |
| Antifouling | no | no | no | yes | yes |
| Welding | duplex procedure | over-alloyed filler | strict shielding | — | — |
| Cost index | 260 | 420 | 550 | 210 | 290 |
What the grade will not fix
Crevice corrosion depends more on design than on material. A flange crevice, a hydraulic dead leg, an undrainable low point, an uncleared deposit: each creates a confined environment whose local chemistry diverges from that of the circuit. The best grade, badly assembled, will crevice.
Likewise, coupling different materials on one seawater circuit creates galvanic cells. Connecting titanium to steel, or copper-nickel to stainless, is handled by isolation or by design, never by ignoring it.
These points belong to the engineering department. This guide stops at ruling out the grades that cannot work and naming those that can.
Frequently asked questions
- Will 316L hold seawater for a short period?
- Not predictably. It might last weeks or perforate in days depending on temperature and stagnation. That is not a compromise, it is a gamble.
- Can copper-nickel and stainless be mixed on one circuit?
- Technically yes, with galvanic isolation and attention to flow direction. That is a design decision, not a purchasing one.
- Titanium or 254 SMO for a marine condenser?
- Titanium if the water is warm or heavily loaded, or if weight matters. 254 SMO if the fabricator has no qualified titanium procedure, or if the connection to steel piping must stay simple.