Electro-chemical Etching of Steel

[Fibergeek]

Since this process is esentially plating in reverse, hopefully someone can provide some info.

The object is to do selective etching on carbon steel, for the purpose of etching logos or lettering in the steel surface. Basically, a photoresist is applied to the steel, exposed to UV through a mask which has the image in positive, the photoresist surface on the steel is then developed and fixed. This leaves the resist with the image in negative on the steel. Etching is accomplished by an etchant and a DC current using a stainless steel cathode, the work is the anode. The etchant is prevented from comtacting any of the steel not exposed as the image.

I've been experimenting useing Dupont ImagOn photopolymer film as the photoresist. For an etchant, I tryied saltwater (which seems to attact the photoresist) and dilute acids. Acetic acid (white vinegar) and dilute hydrochloric acid have been tried. Since the photopolymer is fixed in acetic acid, acids seem to be the appropriate etchants.

Is there a better etchant/electrolyte to use? The object is a deep etch (0.01" or so deep) and a minimum of undercutting. Do to the scale of the image, this setup is in miniature; the exposed surface area is much less than 1" square, the volume of the etchant about 3 oz. and DC currents between 25 and 200 mA with 100% voltage compliance.

Any insight would be appreciated.

[tbloss001]

I have used sulphuric acid and hydrochloric acid for electro etching and both work well on copper and brass. I have not used it on steel (never had the need). Hydrochloric acid can be purchased at swimming pool supply stores or large hardware stores, though I've found the storing and diluting of this particular acid to be a pain. A simpler way to get it in a more usable form is to buy a pickle called "Sparex", available at jewelry supply stores or one-line. It comes as a powder you just mix with distilled water in the amounts you want to use.

If you can't find a good electrolyte suggestion on this forum, try the knifemakers forums. They're always talking about electro-etching their knives with makers marks. As most shop knives are made of carbon steel, whatever they are using would probably be perfect for you.

Here's a link:

http://www.knifeforums.com/ubbthreads/ubbthreads.php

[Fibergeek]

Thanks for the reply.

I'm now getting very good results with this process.

For these experiments, the etched metal is C-1018 cold rolled steel.
The image used is the same in all cases; 5 lines of text ranging in size from 0.05 to 0.15 inches in height. Total image area < 1sq. in.
The cathode is a piece of 40x40 mesh stainless steel cloth, somewhat larger than the image.
The power source is a lab type power supply, operated in constant current mode. I found out quickly that 25-200 mA was inadaquate. For these experiments 4 Amps was used in all cases. Etching time was 60 sec.

The photopolymer is designed to withstand acids, a mild base (sodium carbonate solution) is used to "develop" it and a stronger solution removes it easily at the end of the process. The photopolymer is "fixed" with a mild acid (white vinegar).

Deep etching is required here, at least 0.010". In actual application the work will be hot blued after etching the image, only etch depth is available to provide contrast.


The results:

10% hydrochloric acid solution; required voltage compliance for 4A was 6V. Probing the etch with a needle under strong light and magnification, it appears to be 10 mils deep or so. No sign of undercutting, image quality is excellent.

Marking Methods, Inc. MSC-4 etchant; the results are indistinguishable from HCL. Required voltage compliance was 6.75V.

Martronics (Etch O Matic) GP Electrolyte; depth of etch was a disappointing 3 mils. no undercutting. Required voltage compliance was 6V.

I intend to also try a sulfuric acid solution, the MSC-4 smells exactly like dilute sulfuric acid.
I will also try a higher current, Maybe I can get to 15 mils etch depth before the image quality suffers. The GP electrolyte will not be used again.

BTW, you can get the black fill in the etching by rubbing in some cold blue after etching. This is one application where cold blue works perfectly.

[tbloss001]

Another way to get a "black" fill on your etched design, is to go from a DC voltage to an AC voltage.

Etch normally for however long it takes for your normal etch, then, leaving the electrode/anode in place, attach the leads to an Alternating Current supply of the same voltage -NOT YOUR HOME AC LINE!-

For example, if you're etching with a 5 volt DC supply, when you're done etching and you want to "blacken" the etched lines, leave everything as is, except now attach your leads to a 5 volt AC supply for about one minute.

This is what an "Electro-Etch" unit does to blacken the etched design after etching. Do a Google search on "Electro-Etch" for more info.

[tgoode]

Caswell sells a product called Etch-o-matic that does this.

[Fibergeek]

Another way to get a "black" fill on your etched design, is to go from a DC voltage to an AC voltage.

I don't know this for a fact, but I would expect if you just reversed the leads on the DC source you would get the same result. i.e. You go from etching (the work is the anode) to plating (the work is the cathode). An AC current is positive for half a cylce, and then negative for the other half. AC means alternating current.

I blackened the etched image in these experiments only as an aid to accessing the etch quality. In my real application, the work will be hot blued after etching. This means that the surface, including the etched image, will be an even, matte black. Depth of etch will be the only thing supplying contrast.

As for the Etch-o-Matic:
An Etch-o-matic can't supply the mandatory 10 mils of etch depth I require, I would like a deeper etch than 10 mils if I can get it. If you look at the Martronics website (www.etch-o-matic.com) you will see that they consider a "deep etch" to be 3 mils deep. 3 mils would literally disappear on a blued surface. That includes their "heavy duty indusrtial unit".
Martronics' somewhat more industrial oriented competitor, Marking Methods, Inc., claims "up to 15 mils" etch depth. Whether they can do it or not, I don't know.
Both of them use a stencil approach, I'm using a mask approach. Since I'm only interested in a single image, the multiple image capability of a stencil is of no value to me.
The mask method has the advantage of a liquid tight seal between the image mask and the work. This means it will be capable of much higher image resolution with less undercutting than a stencil.

None of this is intended to bad mouth anyone. I just need a process that meets the requirements for the job I'm doing.