Corrosion on bottom of fuel tank

Max_Florida · #1 09-22-2011, 06:40 PM

I pulled the fuel tank today and was cleaning off the bottom of it. There were three strips of some type of rubber that had either adhered on their own or were glued to the bottom of the tank. When I started pulling them off I noticed that there was corrosion under them in some places. One spot in particular was bad, so I kind of dug at it with a screwdriver since it was pretty soft. Sure enough I ended up with a little hole.

$405 for a brand new custom tank. I'm already way overbudget for the current round of projects, so I'm looking to get this fixed somewhat cheaply.

I called around and found a guy who will weld on a patch.

What I was thinking of doing was cutting off the bottom of the tank(leaving a 2" lip for him to weld to), cleaning the tank out and then having him weld on a new bottom.

Is there any reason not to go this route? Is the aluminum inherently corrosion resistant or does it need to be treated/coated? Is there a particular type of aluminum I need to use or will any "good" 1/8" aluminum do?

Blue_Heron · #2 09-22-2011, 09:28 PM

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

abl1111 · #3 09-22-2011, 09:51 PM

Use closed cell foam strips or strips of lexan glued on the new tank. Water combined with little-to-no oxygen will allow aluminum ( most metals ) to corrode like that. Even if you keep water out, condensation over time will do it too. The strips are just to keep tank from sitting in any water - so Lexan will do the trick - BUT, the seal between the strips and the tank must be water proof !

I'd use a barrier coat of some sort, maybe InterProtect or similar on the new tank so aluminum is isolated even further.

I pulled my OEM tank, a '75, years ago and had the same thing you had. Cleaned it out, had it welded, epoxy coated it used closed cell foam strips - no problem. Looks like new !

Good luck

pelican · #4 09-23-2011, 10:03 AM

Quote:

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 · #5 09-23-2011, 05:37 PM

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 · #6 09-23-2011, 08:33 PM

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 · #7 09-23-2011, 08:44 PM

Quote:

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 · #8 09-23-2011, 09:42 PM

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.

gofastsandman · #9 09-22-2011, 09:56 PM

Quote:

I pulled the fuel tank today and was cleaning off the bottom of it. There were three strips of some type of rubber that had either adhered on their own or were glued to the bottom of the tank. When I started pulling them off I noticed that there was corrosion under them in some places. One spot in particular was bad, so I kind of dug at it with a screwdriver since it was pretty soft. Sure enough I ended up with a little hole.

$405 for a brand new custom tank. I'm already way overbudget for the current round of projects, so I'm looking to get this fixed somewhat cheaply.

I called around and found a guy who will weld on a patch.

What I was thinking of doing was cutting off the bottom of the tank(leaving a 2" lip for him to weld to), cleaning the tank out and then having him weld on a new bottom.

Is there any reason not to go this route? Is the aluminum inherently corrosion resistant or does it need to be treated/coated? Is there a particular type of aluminum I need to use or will any "good" 1/8" aluminum do?

Cheap equals boom. If it is leaking, shut her down.

You have corrosion inside as well. You just can`t see it.

I push and squeeze food to look inside, but stiff things do not allow me that advantage.

Boom is not a good sound, and I know someone will say that I will never hear the boom. Too fast.

Me, not so much.
Cheers,
GFS

Bushwacker · #10 09-23-2011, 12:12 AM

Good thing you pulled the tank - I think you would have been smelling fuel in the bilge before long! Check out Coal Tar Epoxy for a coating - it's supposed to be one of the best coatings. And put the plastic strips on the bottom when you're done to keep it from sitting on a damp surface. Might want to have tank pressure tested after welding to make sure you got all the leaks.