Etching 304 Stainless Steel with Ferric Chloride Problems

Hello,
We use Ferric Chloride in a spray etching application to etch 304 Stainless
Steel. We are having problems and cannot determine what they are. We do not
have a scientific background and are just looking for a few simple answers.
When the bath is new it seems to work beautifully. Then at some point we
start to get peculiar undesirable results.
We use 45 Baume Ferric Chloride and etch at 45°. The Ferric is less than 3%
hyrdro-chloric acid when received.
The following website I put up clearly demonstrates the problems with
photos.
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1.) What is actually happening in these pictures? Is part of the steel
decomposing faster than other parts?
2.) What measuring equipment should we be using to monitor the Ferric
Chloride? We have been measuring it's Baume with a hydrometer but should we
also be measuring the pH or anything else? If so, what type of instruments
and what are the ranges that they need to be?
3.) I've heard of regeneration. Is regeneration of the Ferric Chloride safe
to do with HCL (Muriatic)? By safe I mean are there any harmful / flammable
fumes generated? Do I need to make sure that the HCL stays at 3%? And how
much tolerance is there with it?
I feel like I'm on the right track here but need some good advice or
confirmation.
Reply to
Jon Lorber
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What are you using for resist? Is there a possibility that some resist is left in the large areas to be etched, and affecting the surface?
I only use FeCl to etch copper, as on printed circuit boards. But, I have a good deal of experience doing that.
How old is your FeCl solution? Have you tried starting with new solution? You should know that stainless steel is NOT pure iron, therefore, you can't replenish the etchant with HCl (muriatic acid). You have a significant Nickel concentration building up in any spent solution.
To test, take a piece of totally clean scrap and try etching it. If you still get the knobby surface, then it is the etchant that must be the problem. it is probably full of Nickel, and is preferentially etching the material depending on small local variations in the nickel mixture, grain boundary orientation, or something like that.
If it comes out smooth, then your problem is the resist masking process is leaving a trace of something that blocks the etching action of the FeCl. I've seen a LOT of that, even water spots left after rinsing after the resist pattern is developed can do stuff like this. And, the smooth near an edge but rough in the wide open spaces is something I've seen before, too.
Jon
Reply to
Jon Elson
We use Hydro-coat which is a sprayed on resist most of the time but sometimes we use a dye cut vinyl mask. I've had the bumpy areas on un-treated test pieces as well. In fact the last piece with the really bad etch was done with vinyl masking.
Yes I have found that generally speaking new bath has etched alot better but I don't know why. I have imagined the exact situation that you mention:
"it is probably full of Nickel, and is preferentially etching the material depending on small local variations in the nickel mixture, grain boundary orientation"
I think I can almost see it happening looking at it with a 25x Microscope. It almost looks like shingling on a house where as its flaking off at different rates in the same areal.
Will the nickel settle out into the bottom? Can it be filtered out? The Ferric is 4 or 5 months old. I'm fine with buying new Ferric if I have to but I just wish that I could determine that it was no good through some sort of measurement. Thanks for your help!
Reply to
Jon Lorber
Unfortunately I can't view this because the site requires a download which I am not about to do.
A density measurement is useless on solutions which have been used as an etchant because you now have ferrous chloride, nickel chloride and probably chromium chloride. Note that the ferrous chloride now includes the iron you removed from the stainless plus the reduced ferric chloride.
The easiest way (for me) would be an iodometric titration.
Regenerate FeCl3 by bubbling air through the solution. You will build up nickel and probably chromium as you etch and of course the total iron in solution increases as you use the solution. The HCl is there to keep the FeCl3 in solution. You can't regenerate with HCl.
This solution isn't going to last forever as you are continually adding salts. Try regenerating by bubbling air through the solution and you probably will need to add some HCl but sooner or later you will need to start a fresh solution.
Reply to
Unknown
I've no experience with etching or the chemistry thereof, but you might want to check out Hach, which is a company that sells all manner of wet chemistry test kits. It looks like they have quite a bit of info on their web site:
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If you can pin down the problem to a particular metal or chemical, Hach might a a fairly low priced test method and many of them are dead simple to use.
You might also want to check out
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(I think) which is a web-based forum for metal treating professionals (and not so professional).
Reply to
Mike Henry
No, to both. The ferric chloride is depleted, creating metal X chloride and iron dust. That metal x chloride may then etch something else, producing metal Y chloride and metal x dust. But, eventually, it forms a chloride that doesn't react with any of the metal present, and the action slows down and stops. The
And, the specific gravity (that's what you are measuring with the Baume hydrometer) is not going to change much as these reactions progress. If you are etching one element only, like I do (copper) then you know that you are converting the etchant to copper chloride and iron dust. I can put some muriatic acid in there when etching a big batch, and the iron is quickly converted back to ferric chloride. But, eventually, the bath is "poisoned" by the saturation of copper, and has to be replaced.
Jon
Reply to
Jon Elson
Great thanks all I will check out these leads.
What downloads does it show are required? They pictures JPGs referenced in the HTML file.
Reply to
Jon Lorber
Thanks for the help. I found this on finishing.com:
"A strong possibility is that you sometimes get 304F instead of 304 or 304L. The F grade has a high sulfur content for improved machineability, and etching on any plane that is not parallel to the rolling direction from when the material was originally hot-rolled, will give the sort of finish you describe. The sulfur forms discrete particles of manganese sulphide in the steel, which elongate with the steel during rolling. On any plane of the rolled product that is not parallel to the rolling direction, the sulphide stringers break the surface as spots (cross-sections of the stringers), and these spots are particularly susceptible to corrosion." -B. Reynolds
Any comments on this? While I think that the chemistry of the bath is critical but I have been miffed by the fact that one plate will come out pitted and another fine in the same bath.
Another point of interest is that there are some many things that can go wrong that it's sometimes difficult to determine the exact reason. That is what I need to do however and the stainless grade seems to be the most plausible so far.
Reply to
Jon Lorber
I should finish up the metal etching FAQ that I'm working on. Since it's not my main business anymore, I can drop this hint at least. You can regenerate Ferric Chloride with air bubbling and muriatic acid for about 3 refresh cycles. After that, it will become way too saturated with whatever you're etching in it. For copper/brass, the copper chloride will eventually crystalize into rings of needle crystals at the bottom of the tank. I would reccomend chilling your spent etchant to attempt to force more of the contaminant out of the solution. After that, you can regenerate what's left with 40 volume hydrogen peroxide from a hair salon [try Sally Beauty Supply]. Clairol hair bleach has nothing in it but the peroxide [some other brands have cleaning/foaming agents]. Make sure the etchant is in a heatproof container, I use a 3.5 gallon bucket inside of a 5 gallon bucket in case the bottom drops out. This causes a pretty violent reaction [foaming], so make sure the container is less than half full and add it slowly, stirring constantly with something like an acrylic stick. I use about one 16 oz bottle per gallon of etchant. You /could/ use 100 volume peroxide if you can get it, but the reaction may be extremely violent at that concentration. This /does/ produce hydrogen gas, so you need to do this outdoors and be careful with it. Then let it air bubble overnight using an aquarium pump or something. This seems to force the FeCl2 back into FeCl3. I don't measure baume or ph, but it changes etchant that took six hours to etch through a sheet with no agitation into a 2 hr job, which is a little longer than 'fresh' etchant. This has only been tested with etchant that's used with brass, but you can try it with stainless [I have yet to find a resist that will stay on the steel so I can't do steel etching yet]. You can also buy dry ferric chloride to add in if it doesn't seem strong enough, but don't add as much as if you were mixing it with water. It's still a lot cheaper than buying new etchant. You will need to use peroxide for each future regeneration, while keeping the pH up with muriatic acid. If you have testing equipment, then you can probably do a better job at finding out what ratios of each chemical to use than I do [I'm just a hobbyist].
Reply to
Xane T.
Not to be a safety nag, but concentrated hydrogen peroxide can be seriously dangerous chemixal and anyone contemplating it's use would be well advised to spend a least a few minutes looking over safety data on it. I saw a safety film once that demonstrated the result of pouring 50-70% H2O2 on a leather boot - the boot burst into flame - and that sure made an impression on me.
Reply to
Mike Henry
Thanks for the info! Can you post here if you finish the FAQ? The liquid resist that we use on stainless is called Hydro-coat. It's super expensive but doesn't come off very easily at all. In fact we use either an aircraft stripper or sand it off.
Jon
Reply to
Jon Lorber
I get an "Install on demand" window with "Vector Graphic Rendering (VML)" shown to be installed on my computer.
It ain't gonna happen.
Reply to
Unknown
We use photoresist from Etchcut, India, its cheap and powerful like German brand, infact the problem of etching stated above does not comes in spray etching machine, we are a sign company in USA, using Etchcut dual head etcher and photoresist with no issues at all, it has a gravity meter so we know when to change the chemical
Reply to
Michel
We use photoresist from Etchcut, India, its cheap and powerful like German brand, infact the problem of etching stated above does not comes in spray etching machine, we are a sign company in USA, using Etchcut dual head etcher and photoresist with no issues at all, it has a gravity meter so we know when to change the chemical
Reply to
Michel

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