I'll guess that the Ni and Au set up a galvanic cell.
Au is more electronegative (or more "noble") than Ni, so it will tend to
remove electrons from the Ni when the two are in contact. This aids in
forming positive nickel ions for the etch reaction.

I've seen this type of situation with other pairs of metals, such as Cr
touching Au and Al touching W.

   --Kirt Williams

-----Original Message-----
From: mems-talk-bounces@memsnet.org [mailto:mems-talk-bounces@memsnet.org]
On Behalf Of Ismail Erkin Gonenli
Sent: Friday, May 15, 2009 11:32 AM
To: mems-talk@memsnet.org
Subject: [mems-talk] Gold etching with Ni on top

Hello,

Does anyone have a reference explaining why Ni is etched in KI:I2 based gold
etchant when it is in contact with gold? Does it have anything to do with
the miscibility gap between Ni and Au at low temperatures?

Thanks,
Erkin