Aluminum does not rust. That is the reason people buy it, and it is true — but it leads owners to assume the hull needs no attention at all, which is not true. Aluminum corrodes by a different mechanism than steel, and the one that does real damage is largely invisible until it is advanced.
Why aluminum resists corrosion at all
Bare aluminum reacts with air almost instantly to form a thin, hard oxide layer. That layer is what protects the metal underneath, and it is self-healing — scratch it and it reforms in moments.
Everything that damages an aluminum hull is something that prevents that oxide layer from doing its job: another metal in contact with it in water, a crevice where oxygen cannot reach, or a coating that traps moisture against the surface.
Galvanic corrosion: the one that matters
Put two dissimilar metals in contact in an electrolyte — and water, especially salt water, is an excellent electrolyte — and you have built a battery. Current flows, and the less noble metal corrodes away to protect the other.
Aluminum is quite low on that scale, which means it loses to almost everything: stainless steel, bronze, copper, brass. A bronze fitting bolted directly to an aluminum hull in salt water will eat a halo into the plate around it.
What to do about it:
- Fit sacrificial anodes and replace them. An anode is a lump of metal even less noble than aluminum — zinc traditionally, though aluminum anodes are now preferred in fresh and brackish water, and magnesium in pure fresh water. It corrodes so the hull does not. An anode that looks pristine after a season is not doing its job; it is probably not making good electrical contact. Replace anodes at around half consumed.
- Isolate dissimilar metals. Where stainless fasteners go into aluminum, use an isolating washer, a nylon bushing, or a barrier compound. This matters far more than the fastener material itself.
- Be careful in marinas. Stray current from shore power in a marina can corrode an aluminum hull remarkably quickly. If you keep the boat in a slip with shore power nearby, a galvanic isolator is worth investigating.
- Never use copper-based antifouling on aluminum. This is the classic expensive mistake. Copper and aluminum in salt water is close to the worst pairing available. Use a paint specifically formulated for aluminum hulls.
Crevice and poultice corrosion
The oxide layer needs oxygen to maintain itself. Anywhere water sits in a tight gap with no oxygen exchange, protection breaks down and corrosion proceeds quietly.
The usual suspects are under bunk carpet that stays wet, beneath foam flotation that has absorbed water, under fittings bedded without sealant, and inside seams where debris traps moisture. This is called poultice corrosion when it happens under a wet material held against the hull, and it is the reason a boat can look immaculate outside and be pitted underneath a carpeted bunk.
What to do: let the boat drain and dry rather than storing it wet with the plug in. Lift and inspect under anything that sits against the hull. Bed fittings properly so water cannot wick underneath.
Salt water specifically
Aluminum handles salt water well — 5052 is the marine-grade alloy precisely because it tolerates chloride exposure better than the cheaper alloys used in bargain boats. But salt accelerates every mechanism above, and salt left on the boat keeps working while the boat sits.
- Rinse thoroughly with fresh water after every salt outing. Hull, fittings, trailer, everything.
- Flush the outboard as the manufacturer specifies.
- Pay attention to places salt collects and does not rinse away easily: under trim, inside the transom, in the bilge, around the bow eye.
- Check anodes more often than you would in fresh water.
Fresh water is easier, not exempt
Fresh water is far less aggressive, but galvanic corrosion still occurs, and poultice corrosion under a wet bunk does not care about salinity. Fresh-water boats also tend to be stored outdoors uncovered for longer, which is its own slow problem.
Paint, or leave it bare?
Bare aluminum is genuinely low-maintenance — the oxide layer handles it, and the hull goes dull rather than deteriorating. Paint improves appearance and adds a barrier, but a paint film that is damaged and then traps water underneath is worse than no paint at all. If you paint, use a system formulated for aluminum with the correct etch primer, and repair chips promptly rather than leaving them.
Storage
- Drain completely and store with the plug out and the bow raised so water runs out rather than pooling.
- Cover it, but use a breathable cover. A sealed tarp traps condensation against the hull and creates exactly the oxygen-starved damp that causes crevice corrosion.
- Get it off wet bunks for long storage where you can, or at least ensure the carpet dries.
- Disconnect batteries.
The short version
Rinse after salt. Replace your anodes. Do not bolt dissimilar metals straight to the hull. Do not put copper antifouling on aluminum. Let it dry before you store it.
Do that and an aluminum hull will outlast most of the things around it. Our boats are built from 5052 marine-grade aluminum with a 2 mm plate for exactly this reason — the alloy is chosen for chloride tolerance and repeated flexing, not just for weight.
