Anti-rust for inside of air compressor

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I have a small Taiwan-made home-use air compressor for my tires, engine cleaning, etc. I empty the water that collects in the tank every so often but the water that comes out is always rusty. I'm afraid rust will eat a hole through the iron wall of the tank sooner or later. What would you suggest I use to prevent this? I'm thinking to put in a small amount of phosphoric acid into the drain hole and slosh it around but make sure none enter the inlet tube. Then drain it out. Is this a good idea?
 
How long in between drains? I bought a used 27 gallon compressor on Kijiji and the guy never drained it. When I got the drain plug removed it must have drained 2 liters of red rusty water that looked like gravy.

I installed a ball valve and I usually leave it open after I use the compressor. It's been fine ever since and that's been over 8 years ago.
 
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They're 🤥 supposed to be drained daily! The best thing I found was that Auto drain they sell at Harbor Freight. Plumb it in and toss a small evaporation tray under and forget about it. I didn't think mine worked and let it go for a year. I finally felt guilty and opened it up thinking that there would be a gallon in there or more. Nuthin. I guess that's what made the brown stain on the floor🤗.
The directions aren't too swift like all things made of Chinesium but it's simple enough to figure out..
 
Originally Posted by berniedd
I'm thinking to put in a small amount of phosphoric acid into the drain hole and slosh it around but make sure none enter the inlet tube. Then drain it out. Is this a good idea?


I'd guess not. I'd guess it might dissolve the rust, but then you'll just get more, and if any of it gets back to the pump it might cause damage.

Vegetable (my standard anti-rust stuff) oil is likely to grow mould here, so my guess would be de-water it with ethanol, and then maybe just keep it open and drained when not in use.

I dunno if contamination of the air with mineral oil or lanolin based rust inhibitors is likely to be acceptable.

Also seems like there COULD be a slight risk of dieseling, which could lead to shrapnel.

You don't want shrapnel.

I have a shrapnel scar on the palm of my right hand (admittedly from a 105mm shell splinter, which might be a bit sharper than a HF air tank fragment) and it took ages to heal.
 
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ASME certified tanks last many many many years with regular draining

Few options here rather than contaminating your air:


2 piece drain. Put a piece of pipe underneath the dank in place of the stock drain to allow water to drain out of the tank into holding

Rig up an air drier on the intake of the compressor
 
There's nothing you can do that I know of, tanks are consumables. There are a lot of neglected air compressors out there with lots of water in the tanks that will live long lives. Auto drain on anything less that a 60g tank is overkill, especially if it is used infrequently. It just won't condense enough water to be worth while. Tanks are pretty cheap, you can get used and sometimes brand new vertical 60-80gs on kijiji here for $100-$300.

It's going to take longer to rust out than you think it will, trust me. You think a carpenter or roofer is going to remember to drain the tank on his air compressor regularly?
 
I wouldn't worry about it.. The compressor or the motor will probably conk out first.
 
I probably wouldn't worry too much about it (and I don't on my steel tank compressor), but I have a smaller compressor for indoor nailing use that has an aluminum tank. I opened the valve for the first time in about 2 months (I heard water sloshing inside), the water came out perfectly clear. So that might be an option for your next compressor.
 
Ducked, when treated with phosphoric acid, the rust turns to iron phosphate, which does not rust again. Your idea of zinc sacrificial metal is a good idea.
 
I drain my compressor after every use, or if I'm busy with a project, every night. I took out the crummy, hard to reach valve on the bottom and put in an elbow, a short length of pipe and a 1/4 turn ball valve. The pipe places the valve just beyond the edge of the tank, so it's easy to reach. When I'm ready to drain the water, a quick turn of the valve and the water is drained. IF the compressor is going to be turned off, I just leave the valve open.

Note that the air rushing out of the ippe and valve really loud! I actually make certain to have ear muffs on...

Also, if I just leave it open, towards the end of the draining process, the air rushing out stops, hisses, and then goes again, and stops again. Took me a while to figure out why.

As the air expands from 150 PSI in the tank to the outside, it cools, and as it's rushing out and cooling, it actually forms ice in the drain pipe. So, as it spits and pops, bits of ice are forming, impeding air flow, then being forced out. I never get much water, perhaps a couple of ounces from the bottom of the tank blasts out on the initial opening of the valve, but the remaining vapor condenses combines with the tiny bit of water left and freezes...darndest thing...

By the way, I bought the compressor in 2002. It gets used several times a month.
 
Thanks for the replies, guys. I guess I will keep the plug off the drain hole when I'm not using the compressor and drain the water out after each use. Now if I could find some zinc...
 
Originally Posted by berniedd
Thanks for the replies, guys. I guess I will keep the plug off the drain hole when I'm not using the compressor and drain the water out after each use. Now if I could find some zinc...


Aluminium should work, though not as well. Magnesium too, if you have any old expensive alloy wheels in the neighbourhood.

Careful with that stuff though. Burns pretty hot.
 
Originally Posted by berniedd
Just got a eureka moment. Ordinary household primary batteries are carbon-ZINC type!!!


That might work, though you'd want to clean the inside goo off pretty thoroughly.

School science classes...er....when ah were a lad, (so back when the dinosaurs were making the oil), used to demonstrate magnesium ribbon burning, so if they still do (or if they've been told to stop but havn't cleaned the store cupboard out) you might be able to score some there.

OTOH that might put you on a Homeland Security no-fly list.

Dunno.
 
You can eBay chunks of magnesium fairly easily I bought a bunchto shave pieces off to use as fire starter when camping. Definitely burns hot!
 
Like said above, for a seldom used unit like this, just leave the belly drain open when it's not being used. That will keep it as dry as possible.

For compressors that see lots of use, the tank gets warm enough to push moisture out the air line, plus the heavy use transfers crankcase oil into the tank. The interior of the tank gets an oil film on it.
 
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