Corrosion on bottom of fuel tank

[pelican]

What you have is called crevice corrosion. Aluminum's resistance to corrosion comes from a thin layer of oxidation that forms on the surface of the metal and acts as a protective layer against further oxidation. But it has to be able to breathe to work. In a tight space where water can be trapped against the aluminum, you get the type of pitting you've got.

I wouldn't trash the bottom of the tank unless you've got large holes or the pitting is extensive. If you've got someone who can weld it for you, clean out all the pitting with a wire brush and let him weld over the holes and fill the pitting. Just make sure you get all of it. And before you start the process, make sure you clean the tank thoroughly to get all fuel and fuel vapor out. When I did mine, I emptied all the fuel, filled it to the top with water and a healthy dose of dawn dish washing liquid, and then filled it and drained it twice with clean water. By filling and emptying the tank three times, you can be sure you've purged all of the fuel vapor.

You will probably want to coat the tank before reinstalling it to protect against future corrosion. There are several options. I use epoxy resin and glass cloth, but there are more economical ways to do it.
Dave

every time i read advice like this,i shake my head in disbelief...

using a wire brush to clean ? you're never gonna get everything - as in salt and other contaminates out - the surface must be pristine,or the weld will fail
i've never seen,or heard of any reputable welding shop,willing to "repair" a leaking fuel tank - the liability is huge.if it's corroded,it's lost it's strength.

wrapping a fuel tank in fiberglass matting isn't mabey the best approach - the tank's going to expand and contract - the glass will fracture - allowing water to penetrate - i've seen a large sportfishermen,a "missguided" gentleman,repalced the tanks - he wrapped the tanks with matting - the tanks failed in less that 3 yrs...

tank coatings - before any coating is put on a tank,that aluminum needs to be cleaned,scuff sanded,cleaned again,followed by a chemical etch prime,then the coating - me,i prefer and use interprotect 2000e - make the coating at least 10m thick - use a match book cover as a guide - alternate colors - white and grey - this assures complete coverage.

set the tank on starboard strips - 1/4" - use star bond to adhere the strips to the hull - before the tank's dropped in,coat the strips in 3m 5200 - this will form a "gasket",to prevent water from becoming trapped between the starboard and the tank - do not foam ! use "chocks",to secure the tank...this is a proper long lasting tank installation.

do yourself a favor - quite screwing around,playing a game trying to save a lousy $400 - get a replacement tank made,and be done with it...forget that repair idea - it's trouble...