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Old 03-22-2011, 08:32 PM   #1
CATPart OP
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Diluting Phosphoric or Muriatic acid for fuel tank cleaning

I know this is a common question, and I searched around, but I still need some advice. I went to home depot and bought a couple gallons of muriatic acid (Hydrochloric acid 31.45%) and also a couple gallons of concrete clean & etch which is Phosphoric acid (no concentration listed on the label).

In most threads I read people say to dilute the acid by various ratios. So my question for the Muriatic is, since it is already diluted to 31.45% in the bottle, do I want to dilute it more? If so how much?

For the phosphoric acid, it does not state a concentration on the label, so did I buy the right stuff? If so, do I want to dilute it more?

I have not decided which acid to use yet. People state many different soaking times. I guess that varies by the condition of the tank. For the phosphoric I have read anywhere from 8 to 24 hours. For the muriatic I have read anywhere from 1 hour to 8 hours. I am curious what you may have to say.

Please lets try to keep this on point, so it can be a good condensed source to answer these specific questions. Lets not talk about vinegar or cola or other products, just muriatic and phosphoric acid.
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Old 03-22-2011, 09:19 PM   #2
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Sorry, I don't know the correct mix, and such, but, I put some really rusty parts into Muriatic for 20-30 min., and they are rust free, totally. This is with the store bought dilution. Watch your eyes, this stuff has a real bite, and will put you on your ass, if your dumb like me, and inhale, with your head over the bucket hope that helps a bit!
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Old 03-22-2011, 09:21 PM   #3
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Use vinegar, full strength.
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Old 03-22-2011, 09:31 PM   #4
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Can't help much on the ratios except to say make a trial brew and test on sacrifical parts.

When diluting acid solutions add acid to the water.
If you add water to the acid you may experience an exothermic reaction (gives out heat).
Plastic containers can melt and the Hydrogen liberated by acid metal reactions can ignite.
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Old 03-22-2011, 10:07 PM   #5
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i use this Jasco prep & prime. it's a phosphoric acid mix, but is safer to use. it's great stuff. i get mine at lowes for about 6 bucks a bottle.

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Old 03-23-2011, 03:50 AM   #6
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I use the Concrete Etch phosporic right out of jug for tank cleaning and parts washing. It is diluted sufficiently. If you want to make a ligther mix for general use in a spray bottle, or for wetting parts that you can scrub with a wire brush, mix 50/50 with water.
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Old 03-26-2011, 02:15 PM   #7
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Lets see what the Muriatic does....

I decided to try the Muriatic (hydrochloric) acid. Below are some pictures of my setup and also some before pictures of the inside of my tank. Also is a picture from the label on the bottle of acid.

Process so far:
1. Cleaned tank with BBs and dish detergent, rinsed thoroughly with water.
2. Put rubber stoppers in the petcock holes. These were black rubber stoppers picked up at the Home Depot hardware section. Secured the stoppers with some duct tape so the water pressure does not push them out.
3. Placed the tank on the orange buckets as seen in the image. The petcock holes are over an open bucket. My intent is to drain one side at a time when the soak is over.
4. Put on nitrile chemical resistant gloves (from home depot) and safety glasses. Poured 2 gallons of the muriatic acid into the tank very carefully, then filled the tank the brim with water. You must be very careful to avoid splashing.
5. I will let it sit and check it in about 4 hours to see what it looks like. I will keep checking until it looks clean inside.













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Old 03-26-2011, 06:30 PM   #8
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Make sure the gas cap is loose, you don't want to bottle up acid reactions and create pressure.
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Old 03-26-2011, 08:26 PM   #9
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good point ruffasswan, luckily I left the cap open, it is just laying there, no latched shut.
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Old 03-26-2011, 08:55 PM   #10
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acid test

Looks like you're keeping H/D in business...

I used muratic acid daily for years at work to clear rust off fresh cut cast iron bathtubs.
With that, you could pretty much put it on the misc drill holes and by the time you got to the last one, you were ready to rinse back off again.

In your photos is the last pic after 4 hours?
Should be shiny clean!
Really, it's that quick. I never dilutted it.

There's that stuff you asked us not to talk about,
and you said please, but...

after a soak with that, my thick crust of cake left the surface as one unit!
Wish I had taken a pic of it.

Can't tell ya how many times I got the big choke from them acid fumes.
Or the holes in my jeans after a wash.

We were using muratic acid from the pool supply house...
I didn't know if it was, or if that stuff is dilutted.
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Old 03-27-2011, 03:35 AM   #11
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Its ~30%, so it should be ready-to-use. Diluting makes it weak, thats why your gastank is still rusty from the inside. I've only used phosphoric acid though, it makes an iron phosphate finish to the surface you're treating.
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Old 03-27-2011, 08:15 AM   #12
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17 hours later....

I let the tank soak for 17 hours! Basically it got dark and cold and rainy outside so I decided it could wait until morning. Tha tank was perfectly clean inside, fantastic. Now I need to decide if I should bother with Red-kote sealer. After lots of rinsing and neutralizing I decided to dump the phosphoric in the tank straight out of the bottle, only 1/2 gallon. I wanted to produce the blessed iron phosphate coating that some think is important. The straignt phosphoric had a very strong reaction in the tank. Anyway, the after pictures are below, but these were taken after the muriatic but before the phosphoric treatment. The tank didn't look any different after the phosphoric. Also is a picture of when I pulled the stoppers and let the tank drain into the catch bucket of baking soda, again a strong reaction that bubbled acid all over the ground. I will just continue the steps where I left off:

6. Returned 17 hours later and pulled the stoppers into a bucket with baking soda to neutralize the acid.
7. Rinsed the inside of the tank thoroughly with water, then sloshed some baking soda and water inside the tank to neutralize any remaining acid, drained and rinsed again.
8. Could have probably pypassed this step: dumped 1/2 gallon of phosphoric in the tank and sloshed it around with the cap shut and stoppers in. This did cause pressure inside the tank witch could have caused a dangerous spray of acid, be careful. Rinsed and neutralized the same way as I did for the muriatic acid.
9. Rinsed the inside of the tank with MEK in preparation for Red-kote tank sealer.


Draining the muriatic into a bucket with baking soda to neutralize the acid.
Used 2 box of baking soda which was not nearly enough to neutralize the
acid. I think I would have needed 10 boxes.



After the 17 hour muriatic treatment.


After the muriatic


After the muriatic


After the muriatic
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Old 03-27-2011, 10:11 AM   #13
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Tank looks beautiful.

You could have skipped 8. Phosphoric Acid treatment as you rinsed it off and you made sure any phosphate was surely gone when you 9. MEK'd it off.

To "phosphate treat" any iron containing metal is to apply some formula ( most of which are secret) and allow the phosphate to dry on the item being treated.

On my cast iron tub for example, after cleaning, a few light treat & dry cycles you could see a white build-up of phosphate. This is paintable on metal with no issues.

On the raw motor shafts we made at work, these were spray treated with a basic water, PHA and light oil mix.

Keep going, you're doing it great.
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Old 03-27-2011, 10:34 AM   #14
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Agreed, probably could have skipped the phosphoric acid.

Is it ok for me to leave the tank bare inside? I was told by a motorcycle restoration expert friend of the family that if the tank doesn't leak, just leave the metal bare, don't coat it. The fuel inside should keep it from rusting. Well this tank doesn't leak, the seams are prefect, so maybe I will just leave it bare metal.
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Old 03-27-2011, 10:42 AM   #15
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Nice work!

Oakite 32 will pickel the base metal after the rust is gone and eat no further.
McMaster&Carr and others have it.

Never used Oakite 33.
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