Why the marine environment is so harsh
A part submerged in seawater faces a triple threat: corrosion from dissolved salts, biological growth that fouls every surface, and mechanical stress from pressure and the constant vibration of the vessel. Few engineering environments attack a component from so many directions at once.
Unlike a lab or factory, the ocean is variable and unforgiving. Temperature, salinity, and oxygen levels shift with depth and season, and a prototype that survives a calm tank test can still fail in a real tidal or deep-water deployment.
Corrosion mechanisms in seawater
Saltwater is a strong electrolyte, so dissimilar metals in contact suffer galvanic corrosion, with the less noble metal eaten away. Even a single alloy degrades through pitting and crevice corrosion where oxygen is trapped, such as under a bolt head or in a tight joint.
Designing against corrosion starts with material choice and continuity: avoid mixed-metal couples, break crevices with generous radii and drainage, and specify alloys with proven seawater resistance rather than hoping a coating will save a bad geometry.
Metal material selection
316L stainless is the common marine grade for brackets, housings, and fittings, but it can still pit in stagnant seawater, so it suits moving, aerated conditions better than buried crevices. Titanium and high-grade bronzes resist corrosion far longer in aggressive service.
For the most demanding subsea hardware, duplex stainless and nickel alloys such as 625 hold up where 316L would degrade. Aluminum is light and cheap but needs anodizing or coating, because unprotected aluminum corrodes quickly in a salt electrolyte.
Polymers and composites for underwater use
Engineering plastics such as PEEK, nylon, and acetal resist corrosion entirely and are easy to machine or print into housings, spacers, and cable guides. They avoid galvanic issues but can absorb water and creep under sustained load, so the application must respect those limits.
Glass- and carbon-fiber composites give high stiffness at low weight for buoyancy frames and fairings, though fibers and resin must be sealed at cut edges to stop wicking. For sealed enclosures, SLA and SLS in specialist resins produce pressure-rated prototypes quickly.
Manufacturing processes that work underwater
CNC machining of 316L, titanium, or bronze yields dense, pressure-capable bodies with tight tolerances, which is why it is the default for functional subsea prototypes and any part carrying load or sealing surfaces.
Additive methods extend the toolkit: metal powder-bed printing makes consolidated manifolds and brackets, while SLA and SLS in engineered resins build sealed housings, flanges, and test brackets fast. Choose the process by the required strength, sealing, and volume, not by habit.
Sealing design: O-rings, grooves and pressure walls
A reliable underwater joint depends on a correctly sized O-ring seated in a groove with the right squeeze and a clean, machined land. Get the groove depth and width wrong and the ring rolls, extrudes, or leaks under pressure.
Pressure-rated walls must be thick enough for the target depth and free of sharp internal corners that concentrate stress. Design the pressure boundary first, model it against the depth in meters of seawater, and only then shape the exterior for hydrodynamics or appearance.
Preventing biofouling
Biofouling ranges from slime to hard coral-like growth that adds drag, blocks inlets, and hides corrosion. The first defense is material and finish: smooth, hard, non-porous surfaces foul more slowly than rough or absorbent ones.
For long deployments, plan active or chemical measures such as antifouling coatings, copper alloys, or periodic cleaning, and design parts so fouling can be inspected and removed. Leave access and sight lines so a diver or ROV can assess growth without disassembly.
Pressure and depth-rating testing
Validate early with a pressure chamber that cycles the prototype between surface and its service depth, holding at the extreme to expose leaks and seal creep. A slow pressure decay test reveals micro-leaks that a quick dunk will miss.
Pair pressure testing with a saltwater immersion soak, then inspect for crevice corrosion and seal compression set. Testing against the real depth and temperature range, not just a nominal rating, is what separates a demo part from a deployable one.
Surface finishes and protective coatings
Finish drives both corrosion life and fouling rate. A polished or bead-blasted 316L surface resists pitting and growth better than a rough mill finish, so specify the surface where the part meets seawater.
Coatings such as anodizing on aluminum, PTFE or epoxy barriers, and marine paints add a sacrificial or isolating layer, but they only work if the substrate is clean and the geometry has no trapped edges. Treat coating as insurance on top of good material choice, not a substitute.
Typical applications we see
Common subsea prototypes include sensor housings, connector shells, propeller and thruster parts, ROV brackets, and sampling valves, each demanding a different balance of corrosion resistance, strength, and sealed reliability.
Surface-vessel work adds deck hardware, through-hull fittings, and cooling-water components where vibration and splash meet salt. The right material and process differ by whether the part lives above or below the waterline.
A validation path and how FabProto helps
A sensible path is: pick the alloy or polymer for the service, machine or print a first article, pressure- and soak-test it, then refine the sealing and wall design before any field deployment. Keep sealed reference samples to compare against later field returns.
FabProto machines corrosion-resistant metals and prints engineered resins in-house, and we will flag sealing and material risks specific to your depth and salinity. Send the operating envelope and we return a prototype built and tested for the real marine condition.