Re: Holes in aluminum gas tank. (pics)
There are at least two main causes of corrosion on aluminum tanks.
First is just plain old corrosion caused by moisture. Aluminum has a natural coating of aluminum oxide. This occurs naturally by exposure to oxygen. The coating is invisible to the eye but it is there. This protects the aluminum and prevents corrosion. water can dissolve the oxide. However, if the oxide is removed one of two things will happen. If the aluminum dries then the oxide reforms. If it stays wet the oxide does not reform and the aluminum corrodes.
So if you encase an aluminum tank in foam the foam needs to be bonded closely to the tank. To do this correctly you need to do the same thing you do when painting aluminum. You need to etch it with an acid to remove the oxide and then encase it in the foam before the oxide reforms. Unfortunately this is rarely done and the foam does not adhere well to the tank. After lots of vibrations, and repeated slamming and all the other gyrations boats go through, the bond between tank and foam breaks. Now you have a gap. If moisture gets in there, it does not evaporate. It stays against the surface of the tank and eventually corrodes it.
The second kind of corrosion is even more insidious than the first and can happen to any aluminum tank, whether foamed or not. That is, phase separation of ethanol (ethyl alcohol) in the fuel. Without going into a long explanation of the chemistry behind it, gasoline and alcohol will separate out of solution if allowed to stand for a long period of time, say three months. When that occurs you get alcohol in the bottom of the tank and alcohol is hydroscopic, which means it loves water. So water, mainly water vapor from the air in the tank, is attracted to the alcohol. But it isn't the water that causes the problem, It is acids formed at the boundary layer between the different layers of water, alcohol, and gas. These acids literally eat your tank from the inside out.
From looking at the photo above of the two holes I would say, that is what happened. You can't see what is going on because it is inside the tank, and you get no warning until the fuel suddenly starts leaking.
The preventive measures are simpl. Aluminum tanks should be mounted so that all surfaces are exposed to air, and kept dry. You never want it in the bilge. Don't foam in a tank or put anything under it that traps moisture. Obviously mounting it on a flat wood surface would trap moisture. Some neoprene will also trap moisture under the tank. Don't paint it. To paint it correctly requires removal of the oxide and then it has to be redone periodically. Just leave it bare.
The best way to mount an aluminum tank is to have flanges welded to the tank that can be bolted to stringers so the tank is either hung between the stringers or mounted above them. Moisture may get under the flanges where they are bolted but that won't affect the integrity of the tank.
Gas: use ethanol free gas if you can find it. But that is not always an option. Use stabilizers to prevent phase separation. There are two schools of thought on how to store the boat for the winter. Some say fill the tank to the top, others say empty the tank. Both work to prevent phase separation and water in the fuel. But lately the fill it crowd seems to be winning the argument. If you do fill it add stabilizers and biocides to the fuel.
When the boat is not in storage, use it!. Cars and trucks don't have these problems because the fuel doesn't sit in them for long periods. Most of us fill the tank on our cars once a week, or more often. The stuff never has time to separate and work it's evil magic.
PS: There is another solution. replace the aluminum tank with a plastic one. Plastic tanks (actually roto-molded cross-linked polyethylene) do not have any of these problems. The meet all of the USCG and EPA requirements just like aluminum tanks and will last the life of the boat. About half of the gasoline powered boat built in the USA have plastic tanks. The are widely accepted.
