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Nickel Etching Questions

I am working on a job that requires a stack of metal coatings on a glass substrate. The metals are chromium, nickel and gold, with the gold being the outer layer.

I have a need to etch away a portion of each layer to reveal the underlying layers. I am able to do this, but I had some questions about etching the nickel layer which has been giving me some difficulty recently:

1. When I etch away the gold to reveal the nickel, the gold etchant attacks the nickel layer to some degree. Are there gold etchants available that will not attack the nickel when exposed for a very brief duration, say <30 seconds? I'm using a solution of (primarily) iodine as a gold etchant - I don't know the other chemical constituents off hand. I have also used aqua regia (3 parts HCl, 1 part nitric) with similar results.

2. I know that nickel oxidizes readily and the purpose of gold in this case is to protect the nickel layer from oxidation. What happens when the nickel is being etched is that it will turn very dark, almost black, and then eventually will clear up and the typical chromium color will show through, ensuring that the nickel is all gone. Can that black color be defined as tarnish? Is that typical of nickel? Is that different from oxidation? Oxidized nickel seems to appear more whitish or greyish to me. If I remove the sample when the color is black will that sample then turn to the more typical color of nickel oxide or will it stay black? (I guess I could perform this simple test myself).

3. If a nickel sample is heavily oxidized can it still be etched? I have had success with etching nickel after it is somewhat oxidized (I honestly don't know the full extent of oxidation) by heating the sample on a hot plate in nickel etchant.

Thanks in advance for any help. I (and my colleagues) have only limited experience in this area.

Matthais Lendrix
Engineer - Tampa, Florida, USA
May 25, 2011