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...

[Max_Florida]

Cut the bottom of the tank off today. Surprisingly the inside looked nearly pristine. There were a few small areas(about the size of a quarter) that were white and appeared to be corroded slightly. Those were all on the part of the tank I cut off.

Custom fabrication shop here is going to weld on a new bottom, clean it out and pressure test it for $170. The rest of the tank is in great shape. The inside looks pristine and the rest of the exterior surfaces only have very minor corrosion. I'm thinking that this tank might not be the original, or if it is previous owners took really good care of the fuel system because there was pretty much no gunk stuck in the bottom(I did flush it with simple green and a hose 2x before opening it, but nothing other than whitish water came out). The only areas that had anything more than very slight corrosion going on were under those rubber strips.

I know some of you are going to criticize me for not spending the extra $250(after taxes) to get a new tank, but that's $250 I could put towards something else that would have a much greater impact on the safety/performance of the boat. This repair will last for a good while. The rest of the tank is solid, the weld job is going to be solid... I don't see any real benefit from buying a new tank.

[pelican]

bottom's off the tank - funny - tanks have baffles - these are welded to both the top,and the bottom of the tank - that means,the sides would need to be cut out,to weld the baffles to the replacement bottom...

dropping $250,to repair an old tank,versus $400 to replace,that's a no brainer...glad i'm not docked next to you...

[DonV]

but that's $250 I could put towards something else that would have a much greater impact on the safety/performance of the boat.

No Sir. A new tank is about as big as you can get for safety!!

[Max_Florida]

What's the reasoning behind buying a new tank if the tank that is being repaired is 99.9% corrosion free and the job is being done by professionals?

As far as the baffles, they are going to slot weld them.

If there is a reason then I'm not opposed to dropping the extra money, I just don't see one.

[77SceptreOB]

If there is a reason then I'm not opposed to dropping the extra money, I just don't see one.

Read above Comments^^^^^^^^ Your family and crew will thank you.

[abl1111]

Respectfully,

I think the concensus is obvious. You have heard from some of the most experienced guys on the site - what they would do.

If you ask enough, I'm sure you'll find someone to agree with what you want to hear - I've done that myself at times - bucking the majority. Sometimes it works out but usually I realize I have wasted a lot of time and end up doing what the concensus originally recommended

In the end, you weigh-out all the advice you receive - and do what you think.

Good luck !