Aluminum Corrosion in hull floor in Engine Room

After cleaning my blige I found a couple of blisters on the aluminum hull floor under engines. When I removed the blister about half of the depth of the aluminum plate is gone in the area about 1 inch by 2 inches. Structurally I am not worried about it but want to stop this corrosion and possibly repair it. Problem is Aluminum boat is in the water and its kind of large to take out. I am thinking I might be able to low temp braze/solder some zinc or aluminum/zinc alloy into the hole even though I won't be able to bring hull up to welding temperature. Another thought would be some kind of zinc chromate and then some epoxy, but I am afraid corrosion would continue around it.

 

Before you do anything, you need to establish the cause of the corrosion.

 

I think it is a good idea to have sacrificial anodes in the bilges.

 

We always paint the bilges of our hulls with white/grey epoxy paint.
Shows up dirt/grease and prevents the basic forms of corrosion starting too...

 

I have all my bilges painted and treated except this one. I think what happened is some rust from something on the engine fell there and was covered up by oil.

 

A low temperature braze will handle the pitting or divot, but John is correct, find out what caused it. My recommendation is have a plastic drip pan made or make one yourself. A catch pan can solve a number of problems and make life easier in your bilge. Yeah, it's an area that absolutely sucks to work in, but a lot of stuff can be unbolted a swung to the side. The last one I made I used 1" foam, glued together in pieces for removal, shaped to custom fit the area. It had rolled lips, a drain sump and flange for a fitting, ribs to stand it off the bilge, etc. Once the pieces were removed, I used them as a male mold to layup some bias and mat. The fitting was tapped, edges smoothed and I was able to slide it back in place as a two piece assembly, which was bonded together with some goo and tape. I can now change oil by letting it run into the pan and draining it off afterward, though I haven't done this, I could in a pinch.