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View Full Version : Corroded Aluminum and Tank Sealing on a Fire Truck



Joel
08-01-2008, 06:44 PM
Push has come to shove, and I need to get the aluminum water tank on our (new to us) brush truck fixed as soon as possible. Being a volunteer FD, the simpler the solution, the better.

Here is the problem:
http://img.photobucket.com/albums/v186/JoelinTX/TankInterior4.jpg

We need to cut access holes in the top of the tank and clean out all of the corrosion without further damage to the aluminum. The tank is T shaped, baffled into 3 sections, holds just over 350 gallons, and is .188 thick. Access will be somewhat limited as we canít easily cut a large hole for the rearmost section of the tank. It doesnít currently leak, but there are corrosion pits to the outside so it will as soon as it is cleaned out.
After the cleaning, it needs to be prepped for an undetermined coating.
The bond between the aluminum and coating worries me. The repair must hold up to the twisting and bouncing that this truck will be subjected to on a regular basis and the corrosion pits are pretty severe in places.


So the questions are:
1) How best to clean out the tank? Phosphoric, muriatic, lye, possibly electrolytic? Being 350 gallons, I canít just fill it up with something expensive, but after pressure washing the big chunks off then spraying down with the acid, a good soak seems like a pretty fair idea. Due to the extent of damage, I really want to avoid removing any of the parent metal. Due to several factors, sandblasting seems like a poor choice.
2) What other preparation or treatment needs to be made for the coating to properly bond? Alodine, Zinc Chromate?
3) What coating to use? I have investigated several possibilities, but nothing so far seems conclusively suitable for this application.
As the pits are heavily veed out, we need something heavy bodied or high build. A good bond to the aluminum and some degree of flexibility are mandatory.


For those interested, here is the truck as we bought it. We have done a considerable amount of work since:
http://img.photobucket.com/albums/v186/JoelinTX/SideBefore10.jpg


Any and all assistance is greatly appreciated.

Sorry about the OT, but this is an exceptional group of problem solvers and we could really use the expertise!

fasto
08-01-2008, 06:56 PM
You could consider the stuff used to "slosh" aircraft fuel tanks. Aircraft tanks are aluminum, and the stuff seals great and is resistant to water, flexing, gasoline (of course), etc. It can be painted on or "sloshed" on by rolling the tank.
Aircraft Spruce should have it...

DENedbalek
08-01-2008, 07:01 PM
What about a rubber bladder type liner?

Dwayne

SGW
08-01-2008, 07:37 PM
Aluminum (in general) has good resistance to acids, poor resistance to bases...so I wouldn't use lye for cleaning it out.

wierdscience
08-01-2008, 08:43 PM
Aluminum wheel brite,local truckstop,fillit up,let it work awhile,rinse and dry.

Next up would be aircraft etching primer or xylene based automotive primer,that's about all I know of that will stick to aluminum permanantly.

Final coat would be with truckbed liner,it's good for all sorts of stuff besides truck beds.Done a few really thin boat hulls with it and it works.

x39
08-01-2008, 09:14 PM
"Kreem" gas tank sealer. You can get a kit with everything from the prep chemicals to the coating. Works good.

Duffy
08-01-2008, 11:11 PM
I believe that Devcon makes a specially formulated tank coating for repairing fuel tanks and non-potable water tanks-try them. Duffy

Mike Burdick
08-01-2008, 11:22 PM
Joel,

I know you asked about repairing the old tanks but what about replacing them with tanks manufactured from medium- or high-density polyethylene with U.V. inhibitors? These come in all shapes and sizes and are not that expensive.

Here are some examples and I'm sure there are many others...

http://www.plastic-mart.com/?gclid=CLGf2ueY7pQCFSEbagodaHoMrQ

Joel
08-02-2008, 02:15 AM
Is this it fasto?
http://www.aircraftspruce.com/catalog/appages/jeffcotank.php

Dwayne, I don't think lining would be possible due to the tank being 3 sections with baffles between.

I had never heard of the Davies Wheel Bright Darin. I didn't find an MSDS on it, any idea what kind of acid it is?
I had asked locally about bed liner, but they said they don't have enough room for their gun. Cotel sells a spray/brush on version of Herculiner that doesn't contain the rubber granules - it is on my list of possibilities.

Still trying to find good application information for Kreem and a suitable product from Devcon.

Mike, a drop-in would have to be custom made and was really hoping to avoid the cost, complications and difficulty of cutting off the entire top of the tank.

Thanks for the input so far everyone.

wierdscience
08-02-2008, 08:27 AM
Phosphoric acid I believe is the bulk of the wheel brite.Trailer wash is another common name for it.It will remove the top oxide layer no doubt and that's what you need to do to get anything to bond.

The bed liner,the spray in is the best,but the roll on might work if you use enough coats to build up the thickness.

If it gets to hairy,would it be possible to cut the top off the tank,I mean all the way off leaving just enough left for a bolt flange all the way around?

x39
08-02-2008, 08:44 AM
Phosphoric acid I believe is the bulk of the wheel brite..

Same stuff that comes in the "Kreem" kit.

QSIMDO
08-02-2008, 09:18 AM
Anyone in your area do "soda blasting"?
Bi-carb soda is the media and allows for plain water washup.
Then the suggestions on truck bed spray/roll on liner are spot on as tank lining is where the technology came from.

fasto
08-02-2008, 11:41 AM
Is this it fasto?
http://www.aircraftspruce.com/catalog/appages/jeffcotank.php


That looks like it would work OK, but I'd call the manufacturer to ask first.
I've used a diferent style (it was tar-like) and brand, but I had a production airplane and was required to follow the manufacturer's requirements even if there were better substitutes available.
Regards,
Aaron

Gary Reif
08-02-2008, 05:59 PM
Joel, Have you considered having a tank custom built of fiberglass?
We've got several custom tank builders here in central Kansas that build all kinds of shapes and sizes, and most of the rural trucks aroud here have fiberglass tanks on them. They can build them with bosses to fasten rails and brackets to also. Just my two cents.
Gary

Swarf&Sparks
08-03-2008, 08:01 AM
Just a punt, but howbout electro "derusting"?

What do you have to lose?
Use the tank itself, fill with electrolyte and support the anode inside.
Apply volts and cross fingers.

Evan
08-03-2008, 10:36 AM
The white deposits are aluminum hydrate which is hydrated aluminum oxide. They will dissolve rapidly in a weak lye solution. The lye solution will also cause mild etching of the base metal but no more than 1 or 2 thousandths of an inch in the hour or so it will take to dissolve the oxidation. The etch will also provide an ideal treatment for adhesion of a coating. After etching wash thoroughly with clean warm water. I would use an alodine wash after the etch and then coat with a two part epoxy paint system. Of course the tank must be completely dry before applying the epoxy. Epoxy has two properties that make it superior for dealing with possible pinholes. First, before it sets it has exceptionally low surface tension even in very viscous grades. That allows it to creep into very small spaces such as pinholes. Second, it sets via polymerization rather than drying so it will harden inside those pinholes as fast as it does elsewhere.

This is the procedure we would use on corroded aircraft floats many years ago.

[added]

Further note: If you are going to paint it with anything it is imperative that the aluminum hydrate deposits be completely removed or they will cause coating failure.

Joel
08-04-2008, 03:03 AM
Thank you, everyone.

Had overlooked soda blasting QSIMDO. The comparative ease of a chemical solution is appealing, however.
Thanks for the input Gary. The tank and bed are one unit, so it might be a difficult route.

Electrolysis has been strongly suggested to me by another member. Using chemicals seems the easiest and leaves the tank ready to alodine and topcoat. The only concern is the removal of valuable metal in the process. If the loss is really only a thou or two, even in the heavily pitted areas, perhaps the difference is too small to be concerned about?


Does anyone have any other for/against arguments regarding electrolysis, lye, or phosphoric acid? With three possibilities, surely one is superior.

Some research on alodining shows that phosphoric acid based cleaners are typically used. Will similar or better results (or perhaps less metal removed?) occur with lye?

Evan, how much lye would be required for a 380 gallon tank?
The corrosion lumps are pretty big, although I will get what I can off with the pressure washer. Would I just leave the lye in until the corrosion is dissolved and the the remaining pits look clean? Does the tank need to be neutralized afterwards or just rinsed well? Is there anything problematic about disposing of the solution?

Sorry to be pesky with all of the questions, but no surprises or ‘practice’ would be nice - it is important to get this right the first time around.

Evan
08-04-2008, 05:58 AM
Electrolysis won't remove the aluminum hydrate crystals. They do not conduct electricity and are an excellent insulator. Any removal by that method will happen because of the etching properties of the electrolyte, if any.

Aluminum is highly resistant to acids, especially phosphoric acid. Coca cola contains a significant amount of phosphoric acid and is packaged in aluminum cans. Phosphoric acid will remove some of the hydrated oxide but may leave some of it in place converted to aluminum phosphate. It's worth trying on a small test patch. Alodine prep solutions usually contain phosphoric acid but it can't be used on most aircraft alloys because they contain copper.

Sodium hydroxide is guaranteed to dissolve the oxides and is used for that specific purpose to strip anodizing from aluminum. It dissolves the alumina much faster than the aluminum. In either case the resulting solution is safe to dispose down the drain. The solutions are weak and both lye and phosphoric acid are normally used in cleaning solutions and plumbing treatments. For a lye solution I would mix about 1 or 2 grams per liter of clean water. That works out to about 2 lbs of lye for a full tank of water.

You can also use the weak lye solution as a pre cleaner for the cleaner used prior to alodining. The alodine solution is acid and will neutralize any traces of lye left after rinsing.

If all the aluminum hydrate deposits can't be completely removed they can be converted to gamma aluminum oxide by heat. It's the same process used in anodizing and simply requires the tank to be filled with water and the water heated to boiling for 30 minutes, if that is possible. The gamma form is inert and won't convert back to the hydrate form.

About the alodine solution, it contains hexavalent chromium which is a known and powerful carcinogen. Every precaution must be taken to avoid direct contact with skin.

Ausserdog
08-04-2008, 02:10 PM
Aluminum beverage cans are not bare aluminum on the inside. One of the stages of production is to "inside spray coat". Many moons ago it was laquer based. When I was involved in the industry 20-25 years ago it had already been changed to a plastic / synthetic based coating.

The coating itself is very thin and relatively clear. If you cut the top off of a can and feel the inside and then the outside, you can detect a difference.

lathehand
08-04-2008, 03:01 PM
+1 on the truck bed liner. They use it at the local water treatment plant on surfaces that are wet 24/7/365 and they say it lasts for years.