My answer to How do you weld an aluminum boat?
Answer by Desmond Last:
Aluminum is used to reduce weight. However for marine use it does have disadvantages.
Aluminium in salt water acts like a big battery. This accelerates the salt-water corrosion and can cause the aluminum to rapidly deteriorate.
Note: Boats that are kept on berths will require the anodes to be checked more regularly than boats on moorings as the berthed boats picks up stray electrical currents from the shore power system.
Sacrificial anodes are mounted on the hull under the water which attract stray electrical currents. The anodes are corroded rather than the hull.
The temper of aluminum also changes over time and it work hardens. This makes it less malleable and more brittle or harder. As boats flex this causes cracks to occur in load and stress areas. This is also common on aluminium bodied trucks.
That is why you must never buy a boat unless you have the complete boat professionally inspected.
Welding an aluminum boat requires detailed preparation. The area to be welded should be sandblasted to remove all salt and corrosion and should be cleaned with a light acid.
The positive connection must be disconnected to the Boat and if fitted Generator alternator(s) and the earth connection for the TIG or MIG welder must be tested to ensure conductivity.
If the weld is structural you can have it x-rayed to check for weld penetration and oxidization.
If the area welded has to be repainted then make sure the surface is prepared and etch primer is used as the undercoat.