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Vacuum metallization is a process for coating objects, most often engineering plastics, with an extremely thin (~0.1 μm) metallic coating. It’s done, essentially, by condensing vaporized metal fumes on the surface to be treated inside a vacuum chamber, and is a common means of “chrome plating” plastic parts used, for example, in toys.

I have been hacking on some cheap R/C cars, lately, and wanted to etch these metal films off of a few of the bits, A) to prevent it from shorting across exposed electrical connections and B) for aesthetic purposes. It looks gaudy, IMHO, and does not take paint very well. I knew that the usual strong acid and base suspects would remove it, and at first I was etching parts in disposable plastic cups containing about 12 oz of tapwater and 1/8 tsp Red Devil lye, which of course is sodium hydroxide. Hydrochloric (aka muriatic) acid will also do the trick.

I am not especially intimidated by these substances, but many people don’t keep them around, for whatever reasons. And unless you’re in a hurry, there’s no reason to use them if something milder will serve. I got curious, and did a simple test with some household chemicals. I broke a metalized “roll cage” from an R/C truck into five pieces and soaked each in a different solution overnight. I used vinegar, an arbitrary mixture of vinegar and hydrogen peroxide (the strong hair-bleaching kind), straight hydrogen peroxide, diet coke, and an arbitrary mixture of diet coke and hydrogen peroxide.

It was not a very scientific process, but vinegar seems to work well. Specifically, I used “extra strength” distilled white vinegar labelled as “9% acidity.” Hydrogen peroxide seems to have no useful effect. Diet coke works pretty well, but the carbonation causes bubbles that displace etchant and leave shiny spots on the surface. Your mileage may vary, depending on just how your parts were metalized (and with what metal), but if you have need of this process, vinegar may be a good place to start experimenting. Also, though I wanted to completely remove the metal films from my parts, it should be fairly straightforward to apply masking agents and etch decorative and/or functional patterns in them, if you should need or want to.

Sean Michael Ragan

I am descended from 5,000 generations of tool-using primates. Also, I went to college and stuff. I write for MAKE, serve as Technical Editor for MAKE magazine, and develop original DIY content for Make: Projects.


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Comments

  1. Wilson! says:

    Another option is automotive brake fluid. It’s often used by scale model builders to remove “chrome” plating.

  2. Oven cleaner sprayed in a ZipLock with the parts to be dechromed works very quickly. Just have to be careful with the fumes!

  3. adcurtin says:

    If the carbonation of the diet coke was the only problem, I bet flat diet coke would work great.

  4. Johnny Kaw says:

    When I was building plastic models (1960s), I noticed that the acetone-based glue didn’t stick well to the ‘chrome’ plated surfaces. The recommendation from the hobby shop was to sand or scratch off the plating. I discovered that a regular pencil eraser could also take off the plating easily — I just ‘erased’ the plating!

  5. Alex Swavely says:

    Purple degreaser fluid (“Super Clean”, “Purple Power”, etc.) works pretty well, too.

  6. Jim Hunter says:

    Is electrostripping an option? This seems better suited to someone who doesn’t care for caustic hydroxides, and I think it could be done with just baking soda/water.

  7. rick says:

    Did you try urine?

    1. Sean Ragan says:

      Are you recommending it?

  8. Paul Snith says:

    The trick to getting better results from vinegar is to add plain salt. This is why ketchup or brown sauce works so much better than vinegar alone in cleaning pennies.

    1. Sean Ragan says:

      Actually, that may only work with copper, though the chemistry gets complicated.

      We wrote about the penny-cleaning phenomenon back in 2010:

      http://blog.makezine.com/2010/10/06/why-hot-sauce-cleans-pennies/

      Many people think salt + vinegar makes some small quantity of hydrochloric acid, but that’s so energetically unfavorable it just does not happen. Here’s how I explained it back then:

      “The pKa of acetic acid is 4.76; the pKa of HCl is generally given as -7. That means, essentially, that HCl is 12 orders of magnitude stronger, as an acid, than vinegar. The position of equilibrium always favors formation of the weaker acid, meaning that, essentially, the reaction you’ve written is statistically unfavorable, literally by a factor of 1 trillion.”

      And here’s the abstract of a paper a chemist explaining what’s really going on in the pennies/copper situation, and why it’s unusual and interesting:

      http://pubs.acs.org/doi/abs/10.1021/ed078p513

      And this page includes a plain-language explanation from the same chemist for those who can’t get to the “serious” journal article:

      http://www.cruftbox.com/cruft/docs/cleaningcopper.html

      So, would it work on other metals besides copper? I dunno. My impression is the adding-salt trick works so well on pennies because of the particular redox properties of copper, and that other metals may not necessarily benefit from the addition of salt. But experiment would be more productive than speculation.

  9. [...] response to Etching Metallized Plastic, Johnny Kaw says: When I was building plastic models (1960s), I noticed that the acetone-based glue [...]

  10. Rob says:

    The metallized coating is coated with a topcoat, usually a lacquer, that keeps the aluminum, which is the metal that was evaporated, from oxidizing so you won’t just be etching the aluminum. To learn more about vacuum metallizing, go to http://www.tungsten.com. If you click on TIPs, we offer some insights into the industry.

    1. Sean Ragan says:

      Interesting. Why would it matter if the aluminum oxidizes?

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