How to Spot and Prevent Corrosion on an Aluminum Boat

• White, powdery or crusty deposits that build up and come back after cleaning
• Pitting: small craters or pinholes in the metal, often with white powder around them
• Paint that bubbles, blisters or lifts around fittings, fasteners, seams or anodes
• Weeping or pinhole leaks in the hull or a pontoon tube
• Anodes that disappear in weeks, or anodes that never wear at all

With the boat on the trailer or lift, go over the bottom and sides with a flashlight. Look hardest at:

• Through-hull fittings for livewells, bilge pump outlets and drains
• Transducer and trolling motor mounts, ladders and any bolted-on bracket
• Rivet lines, the keel, chines and the transom around the motor brackets
• Spots where carpeted bunks or rollers rest against the hull
• On pontoons, the tube brackets, nose cones and weld seams

Mark anything suspicious and photograph it, so you can tell next time whether it has grown.

Lift hatches and floor panels where you can. Look in the bilge, under the fuel tank, behind livewells and under wet carpet or plywood. Boating Magazine notes that fuel tanks should not sit on moisture-absorbing material like carpet, and that water should drain off them. Anything that stays wet against aluminum is a corrosion spot in the making.

Boating Magazine's list of corrosion prevention tips includes keeping hooks, sinkers, bottle caps, coins and other metal debris out of the bilge. A lead sinker or a copper penny sitting in salty bilge water against an aluminum hull can eat a pit right through it. Vacuum or sponge the bilge clean and dry.

Most fasteners and fittings on an aluminum boat are stainless steel, which is more noble than aluminum. Boating Magazine recommends 300-series stainless isolated from the hull with plastic or nylon washers or pads, and bedded with a marine bedding compound to keep water out. It also advises against graphite-based lubricants on aluminum boats and against brass fittings screwed straight into aluminum, such as on fuel tanks. When you add accessories, follow the same rules. Mercury's owner FAQ adds that whenever you add stainless steel underwater, such as a stainless propeller or trim tabs, it is a good idea to have the corrosion protection checked again to make sure it is still adequate.

Copper is one of aluminum's worst enemies.

• Bottom paint: International Yacht Paint (Interlux) says antifouling for aluminum should not contain cuprous oxide, because its high copper content causes corrosion. Use only an antifouling made and labeled for aluminum, over the correct primer system. See our bottom paint guide.
• If a previous owner used the wrong paint, the usual fix is to strip it back to bare aluminum and start over with the right system. That is a big job; get advice from the paint maker or a yard first.
• Copper wire and fittings: keep them from touching the hull, and seal any connections in the bilge.

Anodes are the metal you want to dissolve. Your outboard or sterndrive has its own; some aluminum boats and pontoons also have hull or tube anodes.

• Fresh water: magnesium is the common choice. Martyr, an anode maker, says magnesium has the highest driving voltage for fresh water but is not suitable for salt water because it corrodes too quickly. Many engine makers also approve aluminum anodes in fresh water.
• Brackish and salt water: aluminum alloy anodes (or zinc in salt water). Never leave magnesium on a boat that stays in salt or brackish water.
• Always follow your engine and boat makers' instructions for which alloy to use, and do not mix alloys unless they say it is OK. Martyr explains why: if magnesium sits on the same boat as zinc or aluminum anodes, the magnesium does all the work and wears away fast while the others look new and protect nothing.
• A zinc anode that never wears is not a good sign in fresh or brackish water. Martyr notes zinc can grow a hard surface film there and stop working while still looking intact.

Replace anodes when about half gone, keep the mounting surface bare and clean so they make good contact, and never paint them. Boating Magazine notes that aluminum can be damaged by too much protection as well: softening or blistering bottom paint near anodes is an early warning. See our anodes guide for details.

• Keep all DC wiring, connections and battery cables up out of the bilge water. BoatUS describes boats sunk in days because a battery cable was left dangling in the bilge.
• Seal any connection that has to be low in a weatherproof junction box or heat-shrink.
• Use a marine battery charger, not an automotive one; Boating Magazine specifically warns against automotive chargers on aluminum boats.
• If anodes are eaten in a few weeks, or new pitting appears quickly, have a marine electrician or ABYC-certified corrosion technician test the boat. They use a reference electrode and meter to measure the hull's protection level.

Shore power connects your boat's underwater metals to every other boat on the same dock through the green ground wire. BoatUS recommends a galvanic isolator, which blocks small galvanic currents while still letting dangerous fault current flow to ground; an isolation transformer is a more expensive option that is often used on larger and aluminum boats. Have the shore power system and dock wiring checked by a qualified marine electrician. A boat lift with a shore-powered motor deserves the same check.

Boating Magazine recommends washing an aluminum boat down with fresh water after every use to remove salt. Flush the bilge, rinse under the floor where you can and around fittings and brackets, pull the drain plug on the trailer so water can run out, and let carpet and storage dry before you cover the boat.

Boating Magazine advises repairing paint chips that expose bare aluminum as soon as possible, using only primers and coatings made for aluminum. The general process:

1. Wash the area and remove wax and grease with the cleaner the paint maker specifies.
2. Scuff the bare spot and the paint edge with sandpaper or a non-metal abrasive pad. Avoid steel wool and steel wire brushes, which can leave steel particles behind.
   • Owners often ask how to get white corrosion out of the bottom of pits. Use a stainless or nylon brush kept only for aluminum, and stay away from strong caustic (lye or sodium hydroxide) cleaners and strong acids, which attack aluminum themselves. If a pit is deep enough that you cannot reach clean metal, have a welder look at it instead of filling it with paint.
3. Wipe clean, then apply the aluminum primer the paint maker calls for, within the time window on the label.
4. Topcoat with a paint from the same system.

Always follow the product label for temperature, humidity, thinning and recoat times. Below the waterline, use the full primer and antifouling system the maker specifies for aluminum.

• Pinholes or pits that go through the metal: a temporary epoxy patch made for aluminum can get you home, but the proper repair is welding or a welded patch plate by an aluminum welding shop.
• Cracks at the transom, keel, brackets, tube welds or around fittings need welding.
• Large areas of thin, crumbling metal may need a new section of plate or a new pontoon tube.
• Leaking rivets on a riveted hull are a different repair; see our rivet leak guide.

Welding a hull or tube is not a DIY job. Tanks and tubes must be drained, dried and vented, fuel and fuel tanks removed from the area, and the shop should be experienced with marine aluminum alloys. If the boat is under warranty, call the dealer first.

Write down anode replacement dates and how worn the old ones were, plus any pits or paint damage you found. Anodes wearing much faster or slower than last season is one of the best early warnings that something has changed.