Aluminium and Ferric Chloride

I had to try it to see for myself. Actually the aim of the experiment was three-fold: 1) To see how well FeCl3 etches aluminum
2) To assess the properties of my new home-made resist 3) To assess the properties of the new back-spray method
A small 2' x 2.5" x 1/8" aluminum plate was sanded down on one side to 400 grit, 220 on the opposite side. Cleaned with acetone. Resist applied and some Roman numerals carved in as a pattern. The plate was then back sprayed with two coats and allowed to dry overnight.
A small amount of commercially available FeCl3 solution, used may times previously on brass and steel, was placed in a Pyrex dish and this in turn placed on a dinner warming plate dedicated to that purpose (I usually try to get the solution to about 35 degC).
Results.
1) Nothing much happened at first. After 20 minutes the solution was, however, boiling lustily. Adding water made no difference, so the plate was removed and rinsed in water. This stopped the boil and the solution was immediately neutralized to pH>9.
2) The (rather soft in the first place) resist all but melted and what was left was easily brushed away.
3) The back spray partially peeled off, the rest was bubbly as one would expect if significant heat was applied to paint.
4) The etching result is here:
http://www.flickr.com/photos/27683124@N07/5225357650/in/set-72157622484352534 / http://www.flickr.com/photos/27683124@N07/5225357726/in/set-72157622484352534 /
The interesting effect which I have seen in someone else's work but never in mine is the deeper etch at the edges of the etched area where the bare metal meets the resist. I have confirmed with a magnifying glass that the etched lines are deeper at the edges with a slight ridge in the middle. On the back it is even more pronounced with a trench running along the border where the back spray paint peeled off. Anyone knows what this is due to? I suspect that gas production is in some ways to blame but I cannot guess at the mechanism.
Conclusions: 1) The new resist is useless (actually I knew that before the etch - it is too soft). 2) No conclusion can be made about the back spray method - not many will withstand boiling temperatures. 3) FeCl3 does not need much help to etch aluminum, unlike copper, brass or steel.
Michael Koblic, Campbell River, BC
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On Wed, 01 Dec 2010 22:13:33 -0800, snipped-for-privacy@gmail.com wrote:

I like the aged look of that piece a lot. The right side looks as if it were micro-peened.

My WAG: I think you're right. Maybe the gas is building up against the resist (taller barrier?), trapping it a bit longer than it does in a more open area.

It's perfect as an antiquing resist, oui?

Intriguing experiment, Mikey. Thanks for sharing.
-- Do not taunt Happy Fun Ball!
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On Thu, 02 Dec 2010 04:45:56 -0800, Larry Jaques
[...]

That's the only reason I kept the bottle. I quite liked the effect, but just at this moment I don't know what to do with it.

Note to self: Next time pay more attention to the heating part.
Michael Koblic, Campbell River, BC
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snipped-for-privacy@gmail.com wrote:

I ran across this reaction years ago when doing some circuit boards and put a pencil eraser in the solution and it rapidly ate the aluminium ferrule. I mentioned it to my chemistry teacher and she said it was the same class of reaction as thermite but FeCl is in solution in this case, still exothermic though.
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On Thu, 02 Dec 2010 16:44:07 -0800, snipped-for-privacy@gmail.com wrote:

Practice your hieroglyphics and sell januwine Egyptian aluminium stela to the BC tourists?

Roger that.
-- "Human nature itself is evermore an advocate for liberty. There is also in human nature a resentment of injury, and indignation against wrong. A love of truth and a veneration of virtue. These amiable passions, are the latent spark. If the people are capable of understanding, seeing and feeling the differences between true and false, right and wrong, virtue and vice, to what better principle can the friends of mankind apply than to the sense of this difference?" --John Adams
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Mine would be that in open areas being etched there is a greater localized depletion of the etchant, towards an edge, especially a significant one the solution is not as depleted, and so etches faster.
jk
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I thought it bubbled as it etched and self-refilled the depleted area as a result of the bubble leaving a vacuum underneath/behind it.
-- "Human nature itself is evermore an advocate for liberty. There is also in human nature a resentment of injury, and indignation against wrong. A love of truth and a veneration of virtue. These amiable passions, are the latent spark. If the people are capable of understanding, seeing and feeling the differences between true and false, right and wrong, virtue and vice, to what better principle can the friends of mankind apply than to the sense of this difference?" --John Adams
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On Fri, 03 Dec 2010 19:49:37 -0800, Larry Jaques

Right. Commenting on someone else's work another person postulated "cavitation" as somehow related to this phenomenon. If I read you correctly that would be very similar. I still find it hard to visualize why this happens in such a well defined area of the etching. Also I thought cavitation is essentially a high pressure difference thing.
Michael Koblic, Campbell River, BC
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On Fri, 03 Dec 2010 23:52:48 -0800, snipped-for-privacy@gmail.com wrote:

Ahhh, I'll bet that's it. Churning at the edge would be under a bit higher pressure due to the barrier of the resist, causing higher collapse and reformation of bubbles. That would bring in fresher etch solution more frequently, increasing depth. Some definitions agree.
http://www.google.com/search?l&q fine%3A+cavitation
(I love Google's "define:" function to clarify my thoughts & memories about a word.)
-- "Human nature itself is evermore an advocate for liberty. There is also in human nature a resentment of injury, and indignation against wrong. A love of truth and a veneration of virtue. These amiable passions, are the latent spark. If the people are capable of understanding, seeing and feeling the differences between true and false, right and wrong, virtue and vice, to what better principle can the friends of mankind apply than to the sense of this difference?" --John Adams
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On Sat, 04 Dec 2010 06:15:51 -0800, Larry Jaques

I shall just have to accept it even if I do not understand it. Like black magic.
Michael Koblic, Campbell River, BC
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Yes, but as the bubble leaves, in the center it is refilled with a more depleted solution, than the solution at the edge jk
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To make things even more confusing:
I tried a different electro-etching method today. I made myself a hand-held electrode (cathode) and applied it through a coffee filter folded several times and soaked with electrolyte and through a stencil made of acetate sheet to the work-piece (anode). The electrolyte was salt with some vinegar.
Results:
1) The current declined rather quickly and within 30 seconds was less than 10 mA. 2) There was a green and orange discoloration at the site of the stencil on the filter paper indicating formation of iron hydroxides. I was able to prevent this with the vinegar acidification previously. 2) There was a visible etch of undetermined depth both on mild steel and on stainless. The etch had well defined edges. However, when I tried it on brass just to see what happens there was a "halo" around the pattern. 3) Examining the etch with 10x magnification it was clear that a similar phenomenon to that described previously was present, i.e. deeper etch around the periphery of the etched area in a form of a well defined "trench".
I found the last bit puzzling. Yes, there is gas produced during this process (hydrogen - I had previously ruled out any significant production of chlorine with these electrodes) but this gas should be produced on the *cathode*, not on the anode (the work piece). The way the electrolyte is applied here should provide an additional barrier to the gas penetrating near the anode. So I wonder: Is it really the gas bubbles that are causing this phenomenon?
I assume that the current drop off seen with this set-up is due to the electrolyte depletion at the site of the etching. The appearance of the hydroxides would support it. I did not see such current decline when I used a salt bath (with or without vinegar) where there is presumably free recirculation of the electrolyte.
I am also puzzled why one gets the halo on brass and not on iron. I tried the experiment with a copper electroplating solution obtained commercially (I believe it contains copper sulphate and sulphuric acid but in what proportions I do not know). The halo was still present.
I am not sure that I am any wiser now than when I started.
Michael Koblic, Campbell River, BC
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snipped-for-privacy@gmail.com wrote:

See what happens with reverse polarity.
Cheers! Rich
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On Wed, 08 Dec 2010 19:56:01 -0800, Rich Grise

Black deposit in the middle of the stencil pattern and the anode (previously cathode) starts getting eating away. Current pattern about the same.
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On Wed, 08 Dec 2010 18:26:16 -0800, snipped-for-privacy@gmail.com wrote:

Limited refreshment of electrolyte under the pad, perhaps?

Do you get a green/orange scum collecting on top of the solution in all cases? Perhaps the paper is trapping what otherwise would be washed away by the "scrubbing bubbles" of electrolysis.

Difference in conduction rates causing a highly-charged, stratified layer above the plate, perhaps?

So electrolysis has its own sort of HAZ area. Question: Could it be that a more open surface is "scrubbed" more by bubbles, leaving more oxides at the edge? Is it a "dirt in the corners" kind of thing?

Hmm, are the bubbles a secondary-process or merely a byproduct of the electrolysis? (I have no college-level chem or science class exp.)

SWAG: It's probably some interaction between fluid flow and conduction rate for certain materials, electrolytes, and electrodes.

But you're having fun, so who cares? Note the results and use those specific processes you like where you like. As with many other things, there's some magic in patinas and etching.
-- Invest in America: Buy a CONgresscritter today!
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On Thu, 09 Dec 2010 07:24:38 -0800, Larry Jaques
[...]

With pure salt solution, yes. The green scum starts appearing almost immediately and later it progresses to orange. The whole bowl looks disgusting - like curry gone bad.
When I add vinegar to the salt hardly any precipitate forms.
My rudimentary chemistry tells me that the former case is formation of insoluble hydroxides (ferrous=green, ferric=orange). In the latter case ferrous acetate is formed which is soluble. However, soaked in the paper there is no circulation of the electrolyte so the acidification is swamped. Or I just did not put enough vinegar in...
[...]

Fun is one thing, wasting time is another. I was hoping to have a settled process to build on. Never mind, I have got a couple of further experiments lined up.
OTOH my wife is relieved that I no longer boil asphaltum in the garage. I do not think she particularly cares for the smell and it scared her on arrival with the shopping.
Michael Koblic, Campbell River, BC
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On Thu, 09 Dec 2010 21:47:32 -0800, snipped-for-privacy@gmail.com wrote:

Six of one...

Well, you-know-who said "Results! Why, man, I have gotten a lot of results. I know several thousand things that won't work.", so don't fret about it. (Has the light bulb come on yet as to who said that? ;)

She doesn't appreciate the "tarred and feathered" scents, eh? ROTFL!
-- Invest in America: Buy a CONgresscritter today!
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On Fri, 10 Dec 2010 07:36:38 -0800, Larry Jaques
[...]

Jara (da) Cimrman

One can see that a smell of boiling asphalt coming out of your house without an immediately obvious explanation might be of some concern.
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On Fri, 10 Dec 2010 19:10:20 -0800, snipped-for-privacy@gmail.com wrote:

Thomas Edison, inventor of the working light bulb after thousands of tries with damnear every element and alloy known to man at the time.

Yabbut, only once. Then again, she has an inquisitive and creative husband, so she should know better. After all, this isn't the first time you've pulled this cra^H^H^H^H^H^H^H^H^H^H^H^H^H^H^Hexperimented, is it?
-- That is what learning is. You suddenly understand something you've understood all your life, but in a new way. -- Doris Lessing
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Larry Jaques wrote:
(...)

Er. Slight misspelling of "Joseph Wilson Swan" there, Larry.
--Winston
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