How to use hydrochloric acid to remove rust from metal

Wednesday, April 17, 2019
washer with rust removed with HCL


Rust never sleeps but hydrochloric acid sure can put it to bed!


Sometimes it happens because your tools get wet or rust slowly creeps along and then bam!

Rust attack!!

It's suddenly covered all your drill bits, Allen keys, and screwdrivers.

I was tidying the shed one morning and I found two things.

A massive jar of rusting drill bits and screwdriver heads and also some hydrochloric acid which I had stored for 8 years since I moved into our house and never used.

It seemed like the time had arrived.

A sample of what the screws looked like before the HCL

I placed all the bits and items in a plastic container, put on my safety glasses and added some of the hydrochloric acid to it. I then added the same amount of water.

Immediately small bubbles began to rise from the bits, the reaction was obvious.

I then placed the stuff high on a shelf in the shed so the kids couldn't get at it. I went and had a look about 6 hours later and could some pretty shiny items in the container!

screws soaking in acid hcl


Well, I then promptly forgot all about my experiment until I got home from work the next day - so everything had 24 hours to soak in the solution!

I carefully drained the solution and placed the bits and pieces on some an old cloth. 

As I did this, I was wearing long sleeves and protective eyewear. A more seasoned pro may have worn plastic gloves (and not your good chainsawing gloves!). 

I then sprayed a thick coat of CRC on the items to form a protective barrier for the short term and to dilute the acid. I don't know if this was the best move, just something that I did as I'd read acids can strip away protective layers on tools. 

So how did they come out?

Black!

screws and drill bits after hydrochloric acid bath


Many of the drill bits came out coated black, some worse than others. They were originally coated silver!

The acid also ate through this bit extender tool so the lesson is maybe don't buy cheap parts and don't leave the metal in the acid too long!




The rusty screws came out silver, so all good there - so what is this black residue?

I've done some research and there appears to be two chemical reactions at play here. The first is the HCL reacting with the rust, and the second is the HCL with the iron of the bit itself:

Fe (s) + 2 HCl (aq) ---> FeCl2 (aq) + H2 (g)

So the bubbles we see are hydrogen and the black seems to be iron chloride.

So what I should have done is not used such a strong dilution (50:50) and not left it to react for 24 hours.

My also research tells me that while I may have cleaned off all the rust, I have also potentially opened up my items to suffering a complete secondary rust action. This is because there is probably not going to be an oxide covering the metal surface (an oxide layer can prevent rust from occurring).

I suspect some of the higher quality components were a bit more impervious to the acid.

So without doing any real research, I cleaned the bits with a towel and then sprayed them with CRC!

That's definitely a short-term solution.

It sounds like what I should have done is used a different acid instead of HCL, such as phosphoric acid (found in Cocoa Cola!).

Phosphoric acid is readily available from your hardware store (or try Amazon) and is apparently a proven rust remover.

Well. kinda it seems.

It will turn the rust (iron oxide) into iron phosphate which can be a black substance that forms on the items you are removing the rust from. This then needs to be scrubbed off, often with a wire brush.

If you are looking for a more commercial rust remover then there are many products that you can try which is Rust911 is a concentrated rust remover. When mixed with water (necessary) 16 oz makes 2-gallons which you can immerse your rusty item in.

Free All Deep Penetrating Oil by Gasoila Chemicals features a rust eating agent that not only penetrates rust but helps to loosen and dissolve it away meaning it's great for helping unscrew tough nuts and screws.

free all rust remover

As a spray on, this Free All will help to free stuck threaded pipe connections, rusted machine screws, rusted bolts, rusted or frozen nuts, automotive clamps, pillow blocks, universal joints, locks and the like. Check out the price on Amazon.



Evapo-Rust Gel combines the rust removing power of Evapo-Rust with the ability to cling to vertical, rusted surfaces. Simply brush on and allow an hour for the gel to work its magic, then rinse away the rust with water. 

This gel appears to be safer than other gelled rust removers. Evapo-Rust get does not harm copper, brass, or aluminum so has a variety of applications. 

The only complaint about this product is that the gel can be tricky to apply and for that reason, many users tend to prefer Naval Jelly.

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