TOPIC: Electrolysis?

This is definitely not about a "Glassic", but we are puzzled. We have a 1989 Weeres Pontoon boat. It has always been in fresh water and pulled out every fall with no dents or scrapes on the pontoons. This fall when we pulled it, we heard water sloshing in one pontoon. The plug was tight, but when we took it out, there was about 3 gallons of water in it. With closer inspection we noticed a lot of small spots of corrosion on the pontoons. When we scratched off the corrosion there were pits in the aluminum under all of them, with some being rather deep. We haven't had a chance to pressurize the pontoon and check it for leaks yet, but we were wondering what caused the pits. It is the first time we noticed them. There is no power going to the dock that it is moored to other than the battery in the boat. Could it be electrolysis? And how do we go about fixing it. Is there a paint or coating we could put on it after we clean it good that will seal it ? Thanks for any input.
Bill

For starters no part of the boats electrical system should be attached to the boat. No common groundings using the aluminum of the boat. Stray currents at marinas can be a serious problem.

I don't know where you moor the boat but if there are other boats around you that are leaking dc current into the water, it can affect your boat as well.

As VinTinFan said, your electrical system should not be connected in any way to the hull. This causes what you called electrolysis but is more correctly known as stray current corrosion.

Check your system. Make sure your don't have any accidental connections to the hull. If this boat has outboards look at where they are mounted. There should be something between the outboard and the hull to act as an insulator. The outboard is the boats ground. if that ground is connected to the hull it could be leaking current, but if it is it also means there is something wrong somewhere in the system. Some where the positive side may be making contact with ground. This is probably a very high resistance connection or else your battery would be dead very quickly. Check every connection for corrosion and for a firm good connection. Check wires to make sure they aren't frayed, cracked or abraded and making slight contact with either a ground wire or some part of the boat. Make sure none of the wires are hanging in an area that collects water so that the wires are submerged. Old insulation on wires will leak tiny current through water.

A totally different thing. I have corrosion on my sterndrive that looks just like stray current corrosion. But it is caused by chemicals in the water from pollution, primarily fertilizers for lawns and gardens. Ask around about your lake's water quality. Local marine mechanics may have encountered this before. Mine did and clued me into what was happenign.

Last but most important make sure you have anodes to protect the hull.

Thanks for the comments. The boat is moored at our private all wood dock on the Mississippi. There is no outside power on the dock. There is an anode on the bottom of the compartment for the outboard and is in good shape yet. There is another dock about 20 feet away that has power on only when being used, otherwise it is disconnected. We haven't had any battery problems, but we will check all of the wiring and connections in the spring. Thanks again for all of the good information.
Bill

I would ask the owner of the other dock to have the water tested for stray current when the power is on. It is very simple to do and any electrician should be able to do it. 20 feet is not that far away when you are using 120 Volt AC. If it is leaking it is not only a hazard to your boat, it is a shock hazard to anyone in the water. See Electric Shock Drowning: www.electricshockdrowning.org/

I know the man who founded this and the death of his son from ESD was horrifying. He is now an employee of the American Boat And Yacht Council ( www.abycinc.org ), and his main campaign is preventing ESD.

Get the dock checked.

Thanks Ike,
We will get this checked out. This could be a serious problem that needs to get taken care of. This one of the many reasons I love this site. The experience and knowledge is unending.
Bill