Best method for repair corrosion areas on aluminum hull?

[Mike Banks]

&Unless you have spilled an acid or an alkali on it, all corrosion in aluminium is electrolytic.

However, this may have different causes. Usually it happens when one is using shore power, and there is not an isolating transformer in use. Your vessel then becomes part of the power grid earthing system for the neutral and earth--which means that stray currents and induced extraneous currents become connected to your vessel, and reach earth wholly or in part through your hull into the ocean--which is a nice electrolyte, being an ionised saline solution.

Another source of corrosion is your propeller. I would not be using a naked bronze propeller on an alloy hull. At the very least I would have it coated with Prop Speed, as well as having zinc anodes on the shaft and either side of the rudder blade. A stainless or alloy prop might be a better option, and carry a spare, with nut, cotter and key which one should in any vessel.

The next thing to check is that your hull has not become part of the negative rail of your battery system. That is a precursor to "Differential Aeration" which is a long-term very slow cause of corrosion, greatly accelerated by some fool using the metal mast or hull plate as a return leg for lighting instead of running two wires entirely insulated from the mast and anything else by running them in a plastic conduit.

Lightning protection systems on aluminium, hulls need to be aluminium cables, not copper. Yes they will be thicker in diameter, but not by much--and your lightning earth plate in the water should also be aluminium plate. It will also act as an anode--but since it will not be painted, better that which is easily replaced than your hull.

The same goes for your radio earth. It should be an aluminium unpainted plate, insulated from the hull..

Now we come to one so obvious it should not need to be mentioned--incompatible anti-fouling. Copper based anti-fouling and alloy hulls is a surefire source of corrosion, giving the appearance you show in your pictures. It is essential that special anti-fouling is used specifically designed for aluminium alloy or pure aluminium hulls. Nothing else will do. It is folly to expect the barrier coat used, before anti-fouling is applied, to shield the hull. It will not do so. Ions are very small atomic particles and they pass through some paints like sand through a sieve.

Repairs have been covered by others--but the best cure--having fixed or filled the pits, is to eliminate their source.

[tkeithlu]

Cool tool, thinwater. The butt end of a vernier caliper would also do it, but not into as fine a hole and not as easy to read.

[rogue trader]

pls DONT attempt to weld fuel tanks of any nature unless you are a professonal there are residual explosive gasses in the tank that can also get ingrained in the metal.TAKE IT TO A PRO FOR THE ACTUAL WELDING.a very very dangerous practice for a non pro

[tkeithlu]

Rogue, thanks so much for posting that.

I watched my cousin, when he was a teenager, apply a torch to an "empty" fuel tank. Scared me. Infuriated his father, my uncle, who was an aeronautical engineer. The cousin now restores antique race cars.

My professional welder added braces to my diesel tanks while they were full. In that situation, the only concern he had was getting the temperature right with all that heat sink on the back side.