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Electrolytic cleaning of air compressor tank?

Joined
Jun 9, 2008
Location
Burbank, CA
I have a 31 year old 60 gallon vertical asme air compessor tank that has rust on the inside, especially the bottom 1/6.

Would there be any downside to electrolytically removing this rust?

I would fill the tank about 1/4 with a water/washing soda solution, then fabricate an anode that would fit inside through the inspection port, insulate the anode from the tank, then use the body of the tank as the cathode.

I'm worried about causing harm to the steel tank. I have electrolytically cleaned a lot of small items and noticed that rust does come back in short order unless you somehow protect the steel after the process. I would imagine that this would be a problem in a humid environment such as the inside of a compressor tank. Any thoughts on how to protect the newly derusted interior, assuming of course this is a sound application of the electrolytic process?


Chuck
Burbank, CA
 
I'd hesitate to do it, for just the reasons you noted.

Seems to me you may not get the tank perfectly clear of every last bit of rust around fittings, seams, oil covered patches, etc. It's almost sure that the fresh surface will be far more likely to aggressively rust than the existing coat of rust, if you don't perfectly coat it ASAP. And, I'm not aware of any great way to get that perfect coating inside, given the hidden spot of rust or poorly prepared spot here here or there that may have been missed. All that seems to point to a greater likelihood of a catastrophic rather than gradual failure?? Most rusted tanks fail gracefully, with pinhole leaks. Not sure that will remain the case.

Someone may chime in with a scientifically proven (e.g. more than "it seemed to work for me") way to extend the life of your tank. I doubt the manufacturers will have much interest in researching how to extend the life of their tanks. But perhaps some company that has scores of tanks in service may have done their homework?
 
What about putting a few gallons of sand in it and spin the tank for a hour or so. Can a tank be painted on the inside by filling it with paint and spining it?
 
Forrest wrote: "Hydro it first. No point in putting a lot of work in a dud tank."

I intend to do a hydro test but at the moment, with the inspection ports open it would be easy to do the electrolytic cleaning now.

My concern right now is weather the process itself would have a negative long term effect.


Thanks for all the replies so far.

Chuck
Burbank, CA
 
i've got a fifty year old 60 gallon "antique" air compressor that ended up having, all in all, about two gallons of scale in it. i used liberal amounts of water and a soft mallet to flush and pound the sides of the tank. i couldnt get the access plugs out so i had to "finger" the 1 inch bung that was three inches from the floor to finagle the scale out. I literally rubbed away half the skin from my index finger.
i just run it cleaned out. it'll outlive me, at least thats the way i put in my wager. a lot of scale is indicative, to me, of a lack of draining the tank, which eventually plugs the bottom bung, which ends up making it worse, a vicious circle. Keeping it drained is the key, imo. My tank is still fairly clean as far as i am concerned, although i have no real proof and i'm too lazy to do an inspection.
If you really need to see a clean inner tank i'd do the pounding treatment first to remove all the scale, as the rust removal technique you mention wount go through substancial rust, what is de-rusted is a function of its proximity and access to a clear "view" of the rod you put in the tank. I've read something about hydrogen embrittlement with this technique, and although i don't know what that is or whether there is any evidence that that is what happens, i wouldn't take any chances on a air compressor tank with anything that could possibly place a term with the word brittle in it into the equation...
If hell froze over and i decided to do something of that sort, i would put a few gallons of phosphoric acid into it and roll it around for a few days. From what i've read it puts down a anti rusting finish automatically. Theres your paint job!
You running the original compressor and motor? I've got a period correct motor in the wrong hp size running mine (Running a smaller pulley). Its a big cast case, 100 pound 2.5 hp motor and its cool as hell. I'll run down the tank sometimes until the motor get nice and warm and start taking drags of the warm ozone coming off the motor. Best smell i've ever smelled in my life, no joke... strange...
Regards.
mp
 
Unless you have a good air dryer between the compressor and tank or live in Arizona, the tank is at 100% humidity because the water vapor compresses along with the air, i. e. if the outside air is at 25% humidity and you compress it 4:1, which is only about 45 PSI, you will start to get condensation. After electrolytically cleaning and rinsing the tank, I would dry it thoroughly by turning a heat gun or hair dryer into it for a good long time, then coat it with slushing compound, which is specifically designed for tank coatings. You just dump it in, insert all the plugs, and turn the tank very which way to make it run over all the surface, then drain the excess, which you can save for another day. Slushing compound can be found at aircraft supply stores.

Bill
 
if you want to double the life of your tank, you should consider cutting the legs off, and the mounts, and turning the tank upside down.You should be able to get another 31 years out of it that way. And it probably wouldn't hurt to stick some zincs inside of it, so they rest on the bottom.

Good luck, Tom
 
Can it be done - yes.
Has it been done before - yes.
Is there any pressure holding quality to rust on the inside of a pressure vessel - probably.
If it were my pressure vessel and I was going to be living near it for the future would I do it - HELL NO.
The rust has probably eroded 25 to 50% of the steel vessel.

OK, so you sort of know how to do the derusting process, personally I'd use a screne electrode I could collapse to get into the vessel and open in place for maximum surface area.

Run the electrical process to a complete conclusion, dump the electrolyte and pressure wash the inside of the vessel.

Allow a layer of surface rust to develope and then put a few gallons of 5% by volume Phosphoric in the vessel.
Slowly rotate the vessel to insure the phosphoric FULLY WETS the inside of the vessel. probably 24 hours on inverted skates.

Drain the phosphoric and completely dry the inside of the vessel.

Put a gallon of Rustoleum primer in the vessel and repeat inside coating process.

Drain primer and let primer dry

Repeat painting process with Rustoleum enamel.

Hydro the vessel
 
Completely "out there", but I'll ask anyway. Is there any chance of causing hydrogen embrittlement in this process? I've wondered. I think a hardened part might be more susceptible, which this is not, but this IS a pressure vessel.....

Richard
 
... if the outside air is at 25% humidity and you compress it 4:1, which is only about 45 PSI, you will start to get condensation.


Standard atmospheric pressure is 14.696 PSI.
Four times that is 58.784 PSI.

You must live on a mountain or lost a finger and "four" now looks like "three". :D
 
If you use phosphoric acid and water to dissolve the rust, it will leave a protective coating of iron phosphate on the metal. It may be expensive unless you only partly fill it and roll it over periodically to treat a fresh patch.
 
Standard atmospheric pressure is 14.696 PSI.
Four times that is 58.784 PSI.

You must live on a mountain or lost a finger and "four" now looks like "three". :D

I should have said 45 PSIG. I assumed that an air compressor would have a typical gauge, which says 0 at one atmosphere. If you compress atmospheric air to 1/4 its volume the gauge will say about 45 after the air cools. Of course, if we want to split hairs further, it should actually say 44.088, but most such gauges are difficult to read to 1 PSI, hence the approximation.

Bill
 
Why not just buy a new one. 60 gal tanks aren't that much. You can have the tank powder coated inside and out for a few bucks more. That should last a while, and while your at it put an auto drain on the tank so you don't have to mess with it.
 
I think Franz is being overly security conscious on this. About the worst that can happen, is that somewhere down the line you get pinholes in the rusty part, which is down at the bottom. so you could come back from vacation and realize that your air compressor was running the whole time you were gone. I wouldn't worry about hydrogen inbrittlement , any hydrogen is not likely to go very far into the base metal, only a few molecular layers deep. And all that stuff about washing it in phosphoric, and painting it, well, by the time you get through doing all of that you might as well go buy another tank. Any coatings or painting that you put on it will probably protect it for a year or two, but we're talking about trying to squeeze another 31 out of it.

Remember Occam's razor, the simplest solution holds the most probability of success.

Good luck, Tom
 
If the rust was tight, meaning not loose and scaly, I'd try to get some POR -15 paint in there and slosh it over all the rusty area. It bonds to rust better than regular paint bonds to clean steel and prevents future rust. The cleaning and painting done all in one process.
 
Any thoughts on how to protect the newly derusted interior

I de-rusted a 120 gal tank with pool acid followed by phosphoric acid. First remove
whatever you can with wood scrapers and flush the tank with something like lacquer
thinner. The muriatic acid removes the rust and the phosphoric acid rinse removes the
flash rust from the muriatic acid. Poured a little over a quart of industrial primer into the
tank and rolled it all around. Before the paint dried I tilted the tank slightly so that the
bottom of the tank had a buildup of paint which sloped to the drain hole. Use a hard drying
tough two part primer, not a one part paint like Rustoleum. Since 4 years ago the drain
condensation water has been crystal clear when released onto a white towel.
 
Overly cautious, perhaps, but I've been near an air compressor receiver that initially exhibited a small pinhole leak. It wasn't long after the initial leak that the receiver nearly reversed its shape. In that metalforming process the receiver shaprd itself aeound the compressor flywheel and bent the compressor shaft. Until then I never thought shuch a dramatic failure was possible.

POR 15 is a complete mismarketing of polymeric coating designed for temporary fixes in salt water environments. Overpriced CRAP!

Hydrogen embrittlement, my thinking is that the entire theory is bogus. The vast majority of objects exposed to electroplating rust to another electrode are already weakened by the rust itself. Deposition of hydrogen to the object is unlikely due to the direction of current flow. 20 years of developing the process have exposed me to a boatload of theorys and ideas, most of which have been proven wrong.

I long ago left the University experts behind in developing the process, I do not have their budget or unlimited time. I just took the process ahead as I could, and my tank runs around the clock efficiently without electrode clraning or a layer of scum on top. I know what works and what doesn't.
 








 
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