[SEL] Fun with acid and old engines (cast iron fuel tank cleaning).

[Curt]

 From time to time the subject of cleaning cast ron fuel tanks that are 
built into engine bases comes up. I've been working on this Alamo engine 
that has a tank cast into the base. Once I got the base turned over and 
the 3" bungs out of the bottom it was a mess inside. This engine had had 
lots of water in it as the first thing I did was to vacuum at least 5 
pounds of rust dust out of it! As you can imagine there was still huge 
amounts of rust flakes remaining and it would be an impossible job to 
get your arm in a 3" opening and wire brush the flakes loose. Even then 
the results would have been marginal.
Sometimes it's good to work in a chemical plant. Some estimates on the 
scale thickness, the surface area in the tank, and the pounds of iron 
oxide to dissolve were made. Then a few calculation for the required 
amount of acid needed to dissolve that rust were made. Also a few 
experiments were run in the lab, with different concentration of 
sulphuric acid. Sulphuric acid was chosen because it has a much lower 
vapor pressure that the other commonly available acid, hydrochloric / 
muriatic. (An important consideration if you want to breath anywhere 
near the tank being treated!)
Based on the lab tests it looked like 15% (by volume) sulphuric acid 
would remove the rust. For ease of mixing I decided to make it 25%.
Here at the plant we buy bulk 99% pure sulphuric acid so I brought home 
a 5 gallon pail at the whopping high price of 57 cents per gallon :-)
Like I said, sometimes it's good to work in a chemical plant.

The plan was to fill the tank about 3/4 full while upside down and run a 
batch of acid. Then turn the base right side up, fill again 3/4 full and 
run the second batch thru. This would hopefully get the entire inside 
clean. When working will 99% sulphuric there are some rules you have to 
follow as this is some dangerous stuff! A full body suit, coveralls, 
Tyvec or something to cover all the skin, long rubber gloves, safety 
glasses or even better a full face shield. Most important is ALWAY add 
the acid to the water, not the other way around. Simple dilution of the 
acid is an exothermic reaction, instantly boiling the water at the point 
of mixing. If one were to add the water to the acid I suspect the water 
being added would instantly turn to steam flashing one with acid. The 
volume of my tank is 13 gallons. 9 gallon of water was added to the tank 
first. Then 3 gallons of the 99% sulphuric acid was poured in a bucket. 
For added good measure the correct quantity of Tri Ethyl Amine was added 
to act as a corrosion inhibitor in case all the acid was not washed 
clean after. But the results would have been the same with or without 
the TEA. The TEA has to be added to the acid as it is not soluble in 
water. Once the TEA was added the 3 gallons of acid was poured into the 
9 gallons of water in the tank. I was pouring into the 3" bung opening 
in the bottom of the tank, and that was damned interesting (and scary) 
watching the violent reaction of the acid and water mixing. Just the 
dilution of the acid in the water instantly took the temperature up to 
about 140°F. As the reaction cooked and worked on the rust, additional 
heat continued to build. This is good, because heat makes reactions work 
better.

Admittedly I was a little concerned about eating up the rust and then 
having the acid attack the good base metal. So I only let the reaction 
run for 1 hour. I drained the acid into about 50# of soda ash to 
neutralize it. This produces lots of carbon dioxide gas an is excellent 
entertainment as it bubbles and puffs. The tank was washed out very well 
to remove any traces of acid. Then a shop vac hose was stuck in the tank 
to blow a large volume of air in the tank to quickly dry it before rust 
could start again. Once it was all dry the results were pretty decent. I 
would say it was 90% flake free with no damage to the base metal. This 
told me that on the next batch I would run it for 2 hours instead of 
just the 1 hour.

The next day I turned the base over and ran another 25% concentration. I 
filled the tank within a gallon of being full to clean the rest of what 
I did not get the day before. This time the results were much better. 
The inside of the tank has a dry rust color patina, but there are no 
flakes of rust left. All in all I am very pleased with the results.

In summary it looks like 25% sulphuric acid can be added to a cast iron 
gas tank and reacted for about 2 hours (at 90°F outside temperature), 
emptied and washed out well, and the results should be a well cleaned 
fuel tank ready for service.

Hope this is useful information. Has anyone else tried this method? What 
were your results?
Curt Holland
Gastonia, NC