I think the effect is related to crevice corrosion.  See
http://en.wikipedia.org/wiki/Crevice_corrosion

>>> Robert MacDonald  8/10/2010 4:21 PM
>>>
Does anyone out there know of any good academic papers on the chemistry

of undercut at a Cr-Au interface? This is a classic problem with this
metalization scheme. I have seen lateral etch ratios in this material
system in excess of 1000:1. I wonder that it isn't a matter of the
electrochemisty of the Cr-Au system. I have not been able to find any
papers which study this issue, but I'm sure they are out there. If
anyone knows of a good source I'd appreciate it.

Rob MacDonald
Shearwater Scientific