On Mon, 05 Apr 2004 17:02:26 GMT, "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

Unfortunately I can't view this because the site requires a download
which I am not about to do.

,;
,;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?

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.
,;
,;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?

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.
,;
,;I feel like I'm on the right track here but need some good advice or
,;confirmation.

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.