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

Bill Brueck b2 at chooka.net
Mon Jul 3 08:04:05 PDT 2006


Thanks for a thoughtful and complete report on this.  

Better than my technique of half-assed cleaning out the tank and then
coating with quick-poly to bond the rust and coat the insides.  Although
with a sheet metal tank this sure is a good way to seal up any little leaks.

Will file for reference. 

B²
 
Confusion is a higher state of knowledge than ignorance.


-----Original Message-----
From: sel-bounces at lists.stationary-engine.com
[mailto:sel-bounces at lists.stationary-engine.com] On Behalf Of Curt
Sent: Monday, July 03, 2006 9:45 AM
To: SEL; SEL (Oldengine.org); OFES
Subject: [SEL] Fun with acid and old engines (cast iron fuel tank cleaning).

 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












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