Seawater is one of the most aggressive service environments an engineer will ever specify for. Chloride ions penetrate passive films, dissimilar metals form galvanic cells, and biofouling accelerates crevice attack in every gap. Marine metal parts have to survive all three at once, often with 20-year service life expectations and limited access for maintenance. This guide covers the alloys, coatings, and design rules that actually work offshore and on deck.
The Three Failure Modes You Are Designing Against
Pitting and crevice corrosion is the first. Chlorides break down the passive oxide layer at localised points, and once a pit initiates the local chemistry becomes self-sustaining. Crevices under gaskets, washers, and lap joints are the worst offenders because the trapped electrolyte cannot exchange with the bulk seawater.
Galvanic corrosion is the second, and it is entirely a design problem. Connecting stainless fasteners to an aluminium hull, or bronze fittings to a steel plate, sets up a cell where the less noble metal dissolves. The third is stress-corrosion cracking, where tensile stress plus chlorides plus temperature produce cracking in materials that would otherwise be fine. Austenitic stainless above about 60°C is particularly vulnerable, which is one of the main reasons duplex grades exist.
Alloys That Survive Seawater
316L stainless is the entry point for splash-zone and above-deck hardware but is not reliable in stagnant immersed seawater. Duplex 2205 and super duplex 2507 handle immersed service, with a pitting resistance equivalent number above 40 for super duplex. Our stainless steel fabrication guide covers the forming and welding of these grades.
Copper-nickel 90/10 and 70/30 are the traditional choice for seawater piping and heat exchangers because they resist biofouling as well as corrosion. Nickel-aluminium bronze dominates propellers, pump casings, and valve bodies. Marine-grade aluminium 5083 and 6082 serve hulls and superstructures where weight matters, and titanium is specified for the most critical heat exchanger and subsea hardware. Galvanized carbon steel remains the economic choice for above-water structural parts where a coating system can be maintained.
Manufacturing Routes for Marine Hardware
Propellers, impellers, and pump casings are typically cast, since the geometry does not lend itself to machining from solid. Investment casting gives the surface finish and dimensional accuracy that reduces post-machining on smaller marine components, while sand casting handles the larger, heavier housings economically. Our precision casting guide covers the tolerance and finish differences between these routes.
Machined hardware such as shafts, glands, fittings, and fasteners is cut on CNC turning and CNC milling centres, with the same duplex machining discipline described in our alloy guides. Fabricated structures such as gratings, brackets, and enclosures are cut, formed, and welded, then finished as an assembly rather than as individual pieces so that no unprotected weld remains.
Coatings and Cathodic Protection
No alloy is protected forever without help. Stainless and duplex parts are passivated after machining to remove embedded iron and restore the chromium oxide film, which is a cheap step that measurably delays pitting. Carbon steel structures receive hot-dip galvanizing, often followed by a duplex system of paint over galvanizing that outlasts either layer alone.
For above-water equipment, a marine-grade powder coating over a zinc-rich primer offers good durability and colour retention. Our galvanizing services and passivation services pages cover the specifications and thickness targets for each. Immersed steel structures additionally rely on sacrificial anodes or impressed current, and the coating system and cathodic protection must be designed together rather than separately.
Design Rules That Prevent Corrosion
Four rules prevent most marine failures. Avoid crevices by using continuous welds instead of intermittent, sealing lap joints, and choosing bonded washers where fasteners penetrate a surface. Isolate dissimilar metals with non-conductive bushings, washers, or gaskets, and where isolation is impossible, make the small part the more noble one. Design for drainage so water cannot pool in channels or blind pockets. Provide access for inspection and anode replacement, because a protection system nobody can reach will not be maintained.
Fastener selection deserves special attention. A 316 bolt into an aluminium bracket will accelerate corrosion of the bracket around every hole. Either isolate the fastener or select a compatible material. Our custom metal fasteners guide covers material pairing and the coatings available for marine hardware.
Testing and Documentation
Marine parts are commonly validated by ASTM B117 neutral salt spray for coated components, ASTM G48 ferric chloride pitting tests for stainless and duplex, coating thickness measurement, and holiday detection on immersed coatings. Classification society approval from bodies such as DNV, ABS, or Lloyd’s Register may be required, and that means material certification traceable to heat lot plus qualified welding procedures. Requesting this documentation at quotation avoids discovering a gap at final inspection.
Frequently Asked Questions
Is 316 stainless good enough for immersed seawater?
Not reliably. It performs well in the splash zone but pits in stagnant immersion, where duplex or super duplex is preferred.
How do I stop galvanic corrosion between steel and aluminium?
Isolate the two with non-conductive bushings, washers, or gaskets, and keep the more noble metal the smaller area.
What coating lasts longest on marine steel?
Hot-dip galvanizing with a compatible paint topcoat, known as a duplex system, outlasts either layer used alone.
Why is passivation needed on stainless marine parts?
Machining embeds free iron that initiates rust spots. Passivation removes it and restores the passive film.
Do you supply class-approved documentation?
Yes, including material certification traceable to heat lot and qualified welding procedures where required.
Which alloy is best for seawater piping?
Copper-nickel 90/10 for its combination of corrosion resistance and biofouling resistance.
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