-- "Timo" wrote in message
...
The KA flange in the drain, visible when looking down at the sink, is
still in very good condition. It's the mounting flange under the sink that
is all rusty. Upon closer inspection, when looking up at the sink from
below, it seems the sink itself, all around the circumference of the
drain, may be corroded. We'll have to remove the disposal to see for sure.
Timo

Sounds like a cast iron sink that has lost the coating. Paint may help.