MrHomeScientist
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Acid Etching Metals
I have a little Valentine's Day project I'm working on that I had a question about, if anyone is familiar with acid etching the surface of metals.
I've seen videos and pictures of the "bismuth heart" and that's what I'm trying to recreate.
Video example here: http://www.youtube.com/watch?v=W7PpIVi-U6k
To reveal the crystal regions, the surface is etched with an acid for a short period. Does anyone have any recommendations for a good acid solution to
do such etching? Type of acid, strength, etc. I've tried hardware store hydrochloric on bismuth and tin metals with decent results, but I was
wondering if there was an acid better suited to this. I'm making my heart out of tin because of its lighter weight.
I did a little googling and a few places recommended a combination of HF and HNO3. I'd prefer not to work with HF if I can avoid it (I do have
ammonium bifluoride), so perhaps nitric alone would yield acceptable results?
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AJKOER
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HCl/H2O2 is an etching solution, which you may give a try.
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Poppy
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In crystallography, etch figures appears as result of slow dissolution of the crystal interface from Crystals and Crystal Growth, buy Alan Holden and
Phylis Morrison, pg. 261
Corrosion with acids is commonly accompanied by the release of hydrogen in the form of bubles, which can mess the shape of the striations. Fo that you
can use a mix of acid and sodium chlorate, whereas the chlorate acts as an accelerator preventing the release of hydrogen in the atack site, much as
the HCl/ H2O2 mix.
Use very weak concentrations.
[Edited on 1-16-2013 by Poppy]
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bfesser
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I unintentionally did this with a lump of Zn once. If memory serves, it was in aqueous ethylenediaminetetraacetic acid (EDTA). It was beautiful.
EDTA may work on Sn.
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MrHomeScientist
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Thanks for the tips, all! The acid/chlorate mix is an interesting suggestion - what's the equation for such a reaction (i.e. where does the hydrogen
go)? I have some potassium chlorate that I'll try out in combination with HCl.
EDTA is another possibility, but can that be had OTC? Googling EDTA returns mostly health and nutrition type products, which I wouldn't want to go
through the trouble of extracting if at all possible.
I think so far the two most promising experiments will be HCl/chlorate and nitric acid, all in dilute solution. I'll do some testing this weekend.
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mnick12
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I worked in a lab where we did a lot of etching for examination of micro-structures. We were mostly looking at carbon steels, and for this purpose we
used a common etchant called nitol. Which is composed of 3-5% nitric acid in reagent alcohol. This work wonderfully for exposing the carbide
distribution in steel, but it was not effective for the more corrosion resistant alloys.
For the super alloys (A286 mostly) we used a solution of copper sulfate in concentrated hydrochloric acid, this forms the really pretty chlorocuprate
ions. This stuff is really strong and can easily over etch many metals. I would suggest you start out with very dilute enchants, and work your way up
until you find something that works for you.
It is very easy to over-etch stuff, stronger is not always better.
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chemrox
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Also talk to a machine shop or two about "pickling" solutions.
"When you let the dumbasses vote you end up with populism followed by autocracy and getting back is a bitch." Plato (sort of)
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phlogiston
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In addition to the mentioned etchants, you might try ferric chloride or ammonium persulfate, both used for etching electronic circuit boards and
readily available OTC for this purpose.
Do be careful with chlorate+acid!
The ClO2 gas released is explosive and poisonous. Check Woelens page for some actual experiments with this gas.
Why is it actually that these crystal faces become so clearly visible upon etching while you don't really see them if you simply cut and polish a
metal?
[Edited on 18-1-2013 by phlogiston]
-----
"If a rocket goes up, who cares where it comes down, that's not my concern said Wernher von Braun" - Tom Lehrer |
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MrHomeScientist
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phlogiston:
EEk! You're right about ClO2: chlorate + acid + reducing agent = chlorine dioxide. The metal being etched acting as the reducing agent. If I keep
everything very dilute, though, I think things should go smoothly. Just in case, I'll try this one outside in a wide container to avoid confining any
gas produced.
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12AX7
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I've used HCl + CuCl2 + NaClO3 numerous times for etching circuit boards. The chlorate must be added incrementally, only as needed. The copper
chloride serves as a redox reservoir, dissolving metal and forming Cu(I), which, with the Cu(II) and excess of chloride, forms an intensely dark brown
complex. Chlorate is instantly decomposed to Cl2 under these conditions, which further reacts with the Cu(I), turning the solution transparent green
once again. Never add more than the amount required to clarify the solution at any given time, as an excess results in bubbles of Cl2. (A really
fast addition probably would result in ClO2, so don't do that, either!)
Etch rate isn't too rapid, I'd guess I'd spend half an hour etching a 1 oz. board (0.0014" / 35um copper) at 60-80C. Agitation and bubbling help
greatly.
Tim
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MrHomeScientist
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I did some experimenting this weekend, and here are my notes.
Acids:
9.5M HCl (hardware store grade)
16M HNO<sub>3</sub>
Metals: Sn, Bi - each melted and cast into a graphite heart-shaped mold & allowed to cool slowly
I tried a number of different etching baths; each starting with 18mL distilled water and adding 1 - 3mL of acid to each. These amounts were governed
by the size of the small Petri dishes I used. Thus,
Bath #1 = 18mL H2O + 1mL HCl
Bath #2 = 18mL H2O + 2mL HCl
Bath #3 = 18mL H2O + 3mL HCl
Bath #4 = 18mL H2O + 1mL HNO3
Bath #5 = 18mL H2O + 2mL HNO3
Bath #6 = 18mL H2O + 3mL HNO3
I placed each metal in each bath for roughly 20 minutes. I stopped after this long because of the comment in the video in my original post, where he
claims his etching took place after only a "few minutes". The only solutions that had any visible impact on the bismuth were #3 and #6, the most
concentrated. These only revealed a very faint crystal structure; nowhere near the sparkle of the picture I showed above and not good enough for my
purposes.
The more interesting result was tin. It showed no change at all in HCl, but even the very low concentration of nitric acid quickly formed a black film
on its surface. I left both metals in Bath #3 for about an hour, just to see what happened, and there was slight etching on the Bi and no change to
Sn. Adding just a few drops of one of the dilute nitric baths to this one caused the black coating to develop on the tin within only a few minutes. It
can be buffed off, but no etching has occurred.
I tried etching of bismuth with HCl a long time ago, and if I remember right I diluted the acid to 50% its original strength (equal parts water &
acid). This gave decent results, so maybe I'll try something closer to that in my next set of trials. What's interesting is it looks like nitric acid
of any concentration is unsuitable for etching tin in the way I want (it may yet work for Bi, if I increase the concentration).
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