Chris Schwarz's Blog

Strip Zinc with Citric Acid

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Sorry zinc, I am not attracted to you. Yes, you keep the elements at bay, but your non-stop shiny countenance is irritating. It looks out of place on my traditional projects. And you do not age well.

Zinc, it’s over. I’m going to go get me a stripper.

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There are lots of ways to strip zinc, but most of them (fire, hydrogen chloride) are toxic or have an element of danger. Not so with citric acid. This weak organic acid is safe enough to dunk your hands into (though you will feel a sting if you have any open cuts). It’s safe enough to pour down the drain. And it’s fairly cheap. (It also can strip rust from your old tools.)

It’s easy to use. Take a bucket and pour enough water in there to cover your hardware. Pour about 1/2 cup of the citric acid powder in there for starters and stir the hardware and powder up. Wait a few minutes to see if the reaction begins. After 10 minutes you should start to see significant bubbling. If not, add a little more acid.

It takes about 30 minutes to strip the zinc off fasteners, which have a thin coating. Hinges usually take a little longer. And stuff that is intended for heavy-duty outdoor use can take overnight.

When the zinc is gone, remove the hardware and dry it. I shoot it with a little WD-40 or some oil. Pour the citric acid solution down the drain and you are done.

I used this process on the hardware for the home-center tool chest I built for an upcoming DVD with Popular Woodworking Magazine. Read about that here. Want to read about my experiments with hydrogen chloride as a stripper? Click here.

— Christopher Schwarz

P.S. The music in the video is Chatham County Line. You can find it at FreeMusicArchive.org.

24 thoughts on “Strip Zinc with Citric Acid

  1. tedwriter

    It’ll go even faster if you toss in a penny. Copper catalyzes the reaction between the acid and zinc. In fact, cut two tiny nicks in the rim of the penny (post 1984) and the zinc core will dissolve leaving a very thin copper shell.

    Should work with other acids, including vinegar, but some will take longer (and be potentially safer due to the dispersal of the hydrogen gas).

  2. wilsonmv

    To add to the caution notes, please perform this operation in a well-ventilated environment. The bubbles you see forming are hydrogen gas…explosive in an enclosed area like a small basement shop. Unless you are stripping massive quantities of hardware, it’s probably not a problem, but people should be aware.

  3. micksandine

    I found the citric acid product as a rosin bed cleaner for water softners. It is dry and stores easily. You might have to experiment with the mixture. I used 1 part dry citric cleaner to 2 parts water.

  4. Ron 1

    Be sure to run a lot of water…if you must “pour it down the drain”. Especially if you have steel or cast iron pipes. I’ve made a lot of money from people who think they can just pour anything down the drain.

  5. jagriz

    I’d be inclined to neutralize the citric acid with a baking soda and water solution, then rinse with clear water, then WD 40. This was something I picked up from journeymen crafts people I worked with at Colonial Williamsburg. Prior to that time I didn’t use the soda water step after using things like citric acid or muriatic acid to clean metal. It was explained to me that acid has a funny way of hanging around and doing things long after you think it’s done. I know jewelry makers who had staining problems with their soldered silver pieces until they found out about using a soda water rinse to neutralize the flux. In the case of hinges, latches and other hardware with nooks and crannies there could be long term unintended consequences, particularly in high humidity conditions. I figure if works for those folks it probably makes sense to do it myself.

    1. Christopher Hawkins

      I believe that citric acid solutions have been neutralized with sodium bicarbonate solutions safely many thousands of times. However, as an organic chemist with several years of using citric acid on an at least weekly basis, you want to be careful if you neutralize citric acid.

      When using concentrated citric acid and neutralizing it with a sodium hydroxide or ammonium hydroxide, the heat generated can very easily be enough to make the water boil. If you want to neutralize citric acid with solutions of baking soda, dilute them so that the % solids of both the citric acid and the baking soda is <2%. Using baking soda would be problematic because of the CO2 generated. If you were to take a concentrated citric acid solution and add solid baking soda to it you could end up with a very hot, foamy, acidic mess. If your waste water is treated by a public or private waste treatment facility, just pour it down the drain and flush with water. They will remove the zinc from the waste water. If you are on a septic system and want to be super ecofriendly, I recommend you let the water slowly evaporate, put the residue in a empty plastic container, and put it in your regular trash.

      "Soda water" is not a solution of baking soda in water. Soda water is made by adding CO2 to water. Soda water is acidic and will not neutralize citric acid.

      The rate of corrosion of citric acid solutions on steel at room temperature is low enough to not be an issue in most cases. I don't know specifically about cast iron or copper pipes.

    1. Steve_OH

      It really depends on what “it” is. Citric acid is a simple organic acid, and like most organic acids, has a very short half-life in the environment, with no long-term effects. In particular, citric acid is a component of the basic metabolic processes in all cells. It is, for all intents and purposes, completely non-toxic; the only hazard is its acidity in high concentration, which can cause burns.

      Zinc is a little more problematic, being moderately toxic, but it’s also prevalent in nature, and unless you dump a whole lot of it, you quickly reach background levels. (Ionic zinc in solution is very rapidly converted to zinc sulfide, which is relatively inert.)

      The real risks arise from lumping “chemicals” together and not distinguishing the “okay” from the “bad” from the “really, really bad.” People get unnecessarily jumpy about stuff that they really shouldn’t worry about, and careless about stuff that can cause big problems.

      1. robert

        Steve:

        I realize that and agree that everything you said was accurate.

        However, having worked as an environmental toxicologist in industry, academia and on contract for various government entities, it is the “just dump it down the drain” philosophy that I take issue with.

        There was a time when we all thought it was ok to dump unused pharmaceuticals down the drain, now tetracycline, naproxen, prednisone, fluoxetine, bezafibrate, simvastatin, atenolol have turn up in drinking water. Sure some of these are excreted in an un-metabolised form and dose make the poison and they are present only in trace amounts, but still.

        The potential issues associated with dumping damn near anything down the drain should at least be something that we keep in mind lest we become careless with stuff that can cause big problems.

        1. Christopher SchwarzChristopher Schwarz Post author

          OK, not trying to be pissy. Promise.

          I used to torch the zinc and then pee on the hardware. Honest. Is that worse for the environment? Seriously. Because I’d be happy to go back to that.

          What I would prefer (and I’ve said this before) is that I could get hardware without the zinc. I usually buy from BlacksmithBolt.com, but sometimes I need the hardware store stuff quick.

          1. robert

            Cris:

            I get where you are coming from. Lots of trade offs in this life. Everybody should be comfortable with their choices. Just wanted to provide some grist for the mill.

  6. reedwards

    I suspect that you never took a Chemistry class. Hydrogen Chloride (HCl) is properly called Hydrochloric Acid. You cannot place your hand in it without severe skin damage. Sometimes called Muratic Acid…

  7. thatguy

    So close, but you really want:

    That’s the proper syntax for an empty element.

    If you’re going to clock your screws, then you really ought to get your pseudo-xml syntax right too ;)

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