Materials for seawater piping

Last verified 19 August 2026. Every candidate below has a property page with sourced values; nothing appears here that is not sourced there.

What decides it

Three things, in this order.

Chloride resistance, screened by PREN. Seawater is roughly 19 000 mg/l chloride, warm in many services and aerated. PREN puts candidates in an order and the order is broadly right — more chromium, molybdenum and nitrogen resist pitting better. What PREN does not do is tell you whether a grade survives your seawater at your temperature, because it is a linear fit to composition with no term for temperature, flow, chlorination or geometry. What PREN can and cannot do is the whole argument, and the British Stainless Steel Association’s own words are that it is for ranking, not qualification.

Strength, because wall thickness is cost. A duplex at its 500 MPa EN 10088-2 cold rolled minimum lets you specify a thinner wall than an austenitic at 240 for the same pressure, and on a long run of large-bore pipe that difference pays for a more expensive alloy. This is the reason duplex displaced 316L in seawater service rather than any corrosion argument.

Fabrication, which is where the choice is usually lost. Duplex has a narrower heat input window and needs qualified procedures; the superaustenitics need overalloyed filler to avoid a weld metal that is less resistant than the parent. A grade the fabricator cannot weld correctly performs worse than a lower grade they can.

What is not on this list is thermal conductivity, which appears on every stainless datasheet and decides nothing in piping. It matters in a heat exchanger. In a pipe it is a number people compare because it is printed.

Candidates

Every figure carries its evidence level, and on this site that level is E3 throughout — real values from resolvable supplier documents, none of them verified against the standard they cite. What the levels mean, and what our data does not cover. These are screening figures. None of them is a design allowable.

What usually goes wrong

The failure mode that actually appears: crevice corrosion at flanges, gaskets and under deposits, in lines that sit stagnant.

Pitting is what gets screened and crevice corrosion is what fails. The mechanism is the same chemistry in a geometry that keeps the local environment from being flushed: a flange face, a gasket, a badly fitted support, a deposit on a horizontal run. The critical crevice temperature of a grade is always well below its critical pitting temperature, which means a material chosen on a pitting number is being selected against the wrong test.

Two consequences follow, and neither is about the alloy.

Stagnation is a design condition, not an accident. A line that is commissioned, hydrotested with untreated seawater and then left full over a shutdown is a common way to lose a system that was correctly specified for flowing service. If the design cannot avoid stagnant legs, the grade has to be chosen for stagnant service.

The joint is the material. A 2507 pipe with a 316L bolt, a carbon steel support clamped against it, or a gasket that traps a millimetre of still water — each of these is the actual operating condition, not the pipe wall. Galvanic pairing and crevice geometry defeat more seawater systems than the grade selection ever does.

The third failure is chlorination. Seawater systems are dosed to control fouling, and dosing raises the potential of the metal, which lowers the temperature at which crevice attack starts. A grade qualified in natural seawater is not automatically qualified in chlorinated seawater at the same temperature.

The engineering judgement on this page carries no source

The property values are sourced and the reasoning around them is not. Which failure mode dominates, which grade is forgiving to fabricate, which screening number misleads — those are domain knowledge, and this page states them without a citation because we have not found one worth citing. They are the author’s judgement, held to the same standard as everything else here: if one of them is wrong, it is wrong in public and we will say so in the changelog.

Treat the table as evidence and the prose as an opinion from someone who has read the datasheets.

What this page cannot tell you

• Critical pitting and crevice temperatures, which are the numbers this application actually turns on. We hold none of them. They are a named gap on every candidate page here, and PREN is standing in for data we do not have.
• Temperature. Every PREN comparison on this page is composition arithmetic with no temperature term. Seawater at 40 °C behaves differently from seawater at 10 °C — both figures illustrative rather than measured — and nothing on this page distinguishes the two cases.
• Chlorination, velocity, sand loading and biofilm — all of which move the answer, none of which we hold data for.
• Galvanic pairing and bolt, gasket and support material, which is where these systems actually fail and which no property table addresses.
• Non-stainless answers. Titanium, copper-nickel and GRP are all mainstream in seawater service. They are absent because we hold no sourced pages for them, not because they lost a comparison.
• Whether any of these grades suits your system. A ranking is not a qualification. That distinction is the entire point of the page.

Take this further

If you are choosing for a real part, the useful output is not a grade — it is a record of why you chose it that survives the review in eighteen months. Build a decision record, or start from the selection process if the requirements are not settled yet.

Generated from data/applications/seawater-piping.yaml. Regenerate rather than editing this file by hand.