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Author: Subject: Best voltage to etch metals in an electrolytic cell.
Tacho
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[*] posted on 10-11-2008 at 03:20
Best voltage to etch metals in an electrolytic cell.


I am considering an electrolytic cell to etch (corrode) metal.

I assume that I should work with the lowest voltage that ionizes the metal. Formation of gas destroys the mask and seems like a waste of energy. But higher currents speed up the process.

My question is: As I raise the voltage to increase the current, how will the energy be split between the formation of gases (O2, H2, Cl2 etc.) and the dissolution of the metal?

Is 5V much more efficient than 12V to etch metals?

edit:
P.S. Brine is cheap, but evolves chlorine. Any suggestion for a good cheap, universal electrolite?

[Edited on 10-11-2008 by Tacho]
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bquirky
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[*] posted on 10-11-2008 at 04:45


Hello,
Ive done some etching recently at work with stainless steel I haven't tried using a mask so i cant comment about how to not remove a mask.

not sure about the theoretics of the gas production. my only guess would be that any energy pumped in over 2 volts would be used for gas.


I had a requirement to etch holes about 0.4mm wide in 22gauge stainless steel Hyperdermic needles, so as not to leave microscopic metal shavings in the bore of the needle and not to alter its temper to significantly.

The needle to be etched was laid in the electrolyte with a very small copper cathode suspended over the area to be removed.

I tryd a number of electrolytes and voltages and found the following.

NaCl: Removed the steel fine with moderate selectivity however the surface quality of the remaining metal was poor.
The bath allso quickly fowled up with brown and green solids (presumably iron hydroxide or chloride)

NaOH: removed the metal poorly with poor selectivity and poor surface quality. made a big mess quick ! the bath filled with solids (presumably iron hydroxide)

Phosphoric acid: very slow at all voltage levels experment aborted

FeSO4: Removed the metal well with good initial selectivity and acceptable surface quality however formation of iron sponge quickly destroyed the selectivity by enlarging the cathode. and fowled the bath

H2SO4: Removed the metal extreemly well had good selectivity and good surface quality


As far as voltage go's With all of the above electrolytes the higher the voltage the more selective the removal of material from the annode. This may be due to large voltage gradients produced or its posible the large amounts of gass bubbles generated pasivated parts of the anode.
Interestingly the concentration of the above electrolytes appeard to make no differance to the etching quality


Ultimatly the best results I achived where using good old H2SO4 at about 10% concentration running at 20 volts.


Using that setup I found i could punch a fairly round hole through the side of a 22gauge needle in about 45 secconds.


Id try some sulfuric acid.. ive only done the experments using stainless steel. other metals are likley to produce different results :)

Good Luck !
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watson.fawkes
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[*] posted on 10-11-2008 at 06:36


Quote:
Originally posted by Tacho
I assume that I should work with the lowest voltage that ionizes the metal. Formation of gas destroys the mask and seems like a waste of energy. But higher currents speed up the process.

My question is: As I raise the voltage to increase the current, how will the energy be split between the formation of gases (O2, H2, Cl2 etc.) and the dissolution of the metal?

Is 5V much more efficient than 12V to etch metals?

edit:
P.S. Brine is cheap, but evolves chlorine. Any suggestion for a good cheap, universal electrolite?
There's more than the electrochemical reactions going on with your voltage. There's resistance in the electrolyte and in the electrode, each with their own voltage drops. The word used in electroplating to describe the effect of overvoltage used to increase rate is "throw". They typically don't run at the minimum voltages.

As for rate determination, this is affected by a huge number of variable: cell geometry, pH (both overall and local), temperature, electrode and electrolyte composition, power source. This is certainly an area where its easier to do a little experimentation rather than develop a theoretical model first.

As for an electrolyte, I'd suggest a hydroxide or a carbonate. In neither case is any noxious gas evolved.
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jokull
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[*] posted on 10-11-2008 at 06:57


Why don't you use proceedings specific for the metal you're interested in?

You can take a look at:

ASM Handbook, Volume 9: Metallography And Microstructures

or

CRC Handbook of metal etchants
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Tacho
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[*] posted on 11-11-2008 at 08:21


Thank you all.

@bquirky: Very interesting post. You sure have a hands-on experience on the subject. Sulfuric acid sounds perfect for stainless steel. I wonder if a sulfate like MgSO4 would be as good.

@watson and jokull: I am aware of the complexities of electrochemistry. However, in a workshop (as oposed to a lab) voltages are usually limited to 5, 6,9,12,15,24 volts, and the metals are limited to copper, brass, steel and aluminum.

Let me rephrase my question: If I spent 10Wh etching a metal in a bucket in my garage, does more energy goes to gas formation at 12 V than at 5V? Or metal dissolution and gas formation share the energy at a fixed rate?

Or is it a very complex matter that has no simple answer?

One more thing: I'm afraid carbonates and hydroxides quickly create an insuluble insulating coating on metal anodes and are not very useful as electrolytes.

[Edited on 11-11-2008 by Tacho]
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watson.fawkes
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[*] posted on 11-11-2008 at 10:16


@Tacho: Do you have a specific project in mind, or is this theoretical? What's the purpose of the etch: marking, removal, cutting, electropolishing, cleaning, surface finish? What material are you actually using, if any? What power supply do already have, if any? Are you after a one-shot project or an ongoing capability? Is their a specific reason you can't just use an acid or salt etch?
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chief
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[*] posted on 11-11-2008 at 15:19


In industrial electroshaping of very hard alloys etc. they use an electrolyte which contains NaNO3, because this gives a good 1:1 shaping. Basically with this method even the hardest metal-alloys can be worked, and that's what it's used for.
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bquirky
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[*] posted on 11-11-2008 at 18:50


You may be able to do a simple experiment to work it out.

get two pieces of metal of the same size shape and weight



etch one at 5v and say it draws 1 amp etch it for 2 hours to use 10wh of energy

and keeping the same electrode distance

etch the other at 12 volts and say it draws 2 amps run it for 25 minutes to use up the same 10wh


Then weigh each piece of metal to find out what voltage setting is more efficient.
I suspect that our favorite compromise between speed and efficiency will raise its head again.
Increasing the conductivity of the electrolyte will help reduce ohmic losses. you might be able to mess about with the size and shape of the cathode to get better results too.

im curious, are you etching words/patterns into the metal or are you doing some bulk removal of material to shape a part ?
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Tacho
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[*] posted on 12-11-2008 at 03:18


I think that electroetching can be an affordable and simple substitute for laser cutting.
Last weekend I picked a 20x20cm sheet of brass, about 0,5mm thick and made a mask using insulating tape and plastic film leaving a slit (where metal is exposed) about 0,3mm wide.
I have found out by previous experiments that when you try to electrolyse the brass in brine, some insulating coat grows on the brass, reducing the current, so, instead of dipping the whole thing in the electrolyte, I placed a piece of aluminum foil (as cathode) really close to the slit and kept brushing a saturated solution of MgSO4 over the foil/slit for about 30 minutes. It turns out that MgSO4 also creates the insulating layer, hence the need for constant brushing.
I used 5V from a computer power supply. The current averaged 0,1A.
I repeated the procedure on the other side, and ended up with a groove on each side, deep enough to allow a perfect cut with the help of some careful repetitive bending.

Of course, to brush it for one hour is a pain, so I figured that using more voltage I could do it quicker (more current) and also compensate for the loss in conductivity created by the misterious layer. But I know, also by previous experience, that bubbles can remove a varnish mask, and to make curves and shapes I have to use some ink or varnish.

I intent to learn about electroetching to have it as another tool to work with. A poor man's laser cutting. But since high currents are good but bubbles are bad, it would be useful to know how energy is used by the electrolysis. I mean, if all the energy goes to the bubbles, there is no point in raising the voltage, but if we have a 50-50 distribution, maybe its worthy to find a better mask and use, say, 24V.

Chief: indeed, I was thinking of using a nitrate based eletrolyte. That insulating layer is probably some insoluble salt mixed with bubbles and I can't think of an insoluble nitrate.

And yes, bquirky, one or two experiments would clarify it, but I was hoping someone had a quick answer. I'm not lazy, but I don't have much spare time.

[Edited on 12-11-2008 by Tacho]

[Edited on 12-11-2008 by Tacho]
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not_important
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[*] posted on 12-11-2008 at 06:04


You seem to be missing several important points.

First, when you dissolve some metal you're creating cations. Those need balancing anions, if not plated out immediately onto the other electrode you need a constant supply of fresh anions. Generally this means using an acidic electrolyte, or one that can supply anions in another way. Failure to do so will result in the formation of hydroxides or oxides of the metal being worked; the exact salt used as electrolyte isn't going to matter much as it has its own cation - Fe & 3 Na(+)NO3(-) =??=> Fe(+3)(NO3(-))3 & 3 Na(+) ain't gonna happen, the sodium nitrate stays sodium nitrate (in solution) and the iron ends up as hydroxide formed from water by releasing hydrogen. The hydroxides formed are usually soft and easy to remove, as you did; this is sometimes done with brushing, eletrolyte flow, or scrubbing by bubbles formed.

You could use sodium bisulfate as the electrolyte salt, or include free acids including some of the organic ones such as citric.

Alternatively you could use a salt where the cation is in a high oxidation state, reducing it frees anions. Printed circuit board etch based on ferric chloride is an example, copper metal becomes oxidised to cupric chloride as ferric chloride is reduced to ferrous chloride. The metal being removed reduces the existing cation, releasing the anions needed to balance the new cation of the dissolving metal.

Or you might use an electrolyte that complexes with the metal being dissolved, for copper a mixture of strong aqueous ammonia and an ammonium salt which will dissolve copper hydroxide.

Even if you do plate out the metal being worked you'll likely want a slightly acidic electrolyte to hold the metal in solution until it gets plated out on the other electrode.

And yes, once you're past the fairly voltage needed to drive whatever electrochemical reactions you're using to remove metal, overcoming cell resistance, plus the overvoltage potentials for the electrolyte and metal used; excess voltage goes into making gases and heat. Cell resistance depends on your electrolyte and electrode spacing. Gas formation can remove the hydroxides formed with a non-acidic electrolyte, replacing mechanical scrubbing. With very close electrode spacing electric discharge processes appear as the voltage is increased, the process transitions into spark based machining where the metal is removed by being vapourised.


There are references on electromachining and electroetching. And you might want to look at

http://www.electroetch.com/
http://chestofbooks.com/reference/Henley-s-20th-Century-Form...


[Edited on 12-11-2008 by not_important]
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Tacho
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[*] posted on 12-11-2008 at 06:47


not_important, I was really missing some of those points. Very informative post and links, thank you! That is all I wanted to know.
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phlogiston
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[*] posted on 12-11-2008 at 14:52


What you are trying to do is very similar to a technique called 'electrochemical machining' or ECM. Google will turn up quite a few hits.

Briefly, a cathode is maintained in close proximity to the workpiece/anode. The gap between them is kept constant, resulting in constant current and metal removal rate. The electrolyte is then pumped into this gap at a high rate to remove any insoluble stuff that will form (as you found out). The electrolyte is filtered and returned to the tank. The main advantages of this technique are that you can use it to cut arbitrarily hard metals without any tool wear, and that it is possible to achieve very high accuracy.

I can't speak from experience, but supposedly sodium nitrate is a good electrolyte, but NaCl is also used. Low concentrations are actually better than high, because the gap between anode/cathode will be smaller for a given current/metal removal rate (due to lower conductivity), resulting in higher accuracy.

I believe a typical voltage is around 10V.

I didn't get whether you were actually trying to cut metal in this way or merely etch an image into it. In the last case you should definately visit this guys amazing page:

http://steampunkworkshop.com/electroetch.shtml

edit: In analytical electrochemistry class I was taught that the rate of each chemical reaction taking place at the electrodes is eventually limited by the rate of diffusion of the ions to the electrodes if you increase the voltage enough. Typically, as you increase the voltage and monitor the current you will see a series of jumps, each of which represents another electrochemical reaction 'kicking in'. I suspect that at the voltages being used here, the ratio of the rate of evolution of gas/metal dissolvation is more or less constant. Increasing the pressure could reduce gas evolution (as it wil take more energy (which translates into a higher electrode potential) to overcome the pressure).

[Edited on 12-11-2008 by phlogiston]

[Edited on 12-11-2008 by phlogiston]




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bquirky
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[*] posted on 12-11-2008 at 18:13


Hmm very interesting, I have tried something similar with aluminium to make little knick knacks to give to some friends.

If you draw up an image in your favorite graphics package invert the black and white and print it out on a laser printer. its possible to heat the piece of aluminum up and gently lay the laser print on the warm aluminum and give the paper a bit of a rub. some of the toner will stick to the aluminium.

You can then put the aluminum in a week (so as not to have bubbles destroy the mask) NaOH solution and leave it to etch without any electrochemical action.

The problem with this approach though is that you cant get a very deep etch because the etchant gets around the mask and etches under it from the side.

Prehaps you chould get good results by using both a mask and a localised cathode.

the posibility allso exists that you chould put a mask on.. and then electroplate some material onto it !!
then you chould have diferent colour designs on the metal. prehaps iron with a copper plate ontop.

or perhaps you chould get a nickle salt to plate onto your brass ?


But i love the idea of bulk electroforming it should preserve the temper of the metal and prehaps more importantly not make a loud noise so you can make parts in your worksop of an evening / night time without geting complaints from the neigbors. :)

regards.
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