View Single Post
  #6   Report Post  
Jon Elson
 
Posts: n/a
Default Etching 304 Stainless Steel with Ferric Chloride Problems



Jon Lorber wrote:
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?

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
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!

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