Soft Soldering Iron Issues

I am having a bizarre issue with soft soldering and have been having
it for a long time. Every time I guy a soldering iron, the tip
disintegrates VERY fast, like within 5 or 10 pieces I have made. I
am using flux that is made for the solder which is nonlead general
use solder. I am soldering together raw brass settings for Austrian
rhinestones. I have tried many different kinds of soldering irons
and tips, but every one of them basically pits and disintegrates. I
use both an iron and a torch to make the solder flow very smooth and
thin.

Can anyone suggest either a solder that will not do this, preferably
not Tix, or an iron that would not do this?

Thank you in advance.
Chris
http://www.christopherjewels.com