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:

www.hach.com

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 www.finishing.com (I think) which is a
web-based forum for metal treating professionals (and not so professional).

"Jon Lorber" wrote in message
et...
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!





"Jon Elson" wrote in message
...


Jon Lorber wrote:
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.

http://www.precisionsigns.com/badetchingindex.htm

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.



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