In watch making, an old method to remove stubborn or broken screws from watch bodies is to dissolve them out using Alum solution (Potassium Aluminium Sulphate). I've just successfully applied this technique to a watch with rusted in screws and can confirm that it does indeed work and that the brass and nickel parts of the plate were unaffected.
I'd like to know what the chemical reaction involved is as it doesn't seem to be simple and I can find no explanation on the web.
The process uses a warmed, saturated aqueous solution of Alum. The steel parts are corroded away and produce a stream of gas bubbles in the process. There is some deposit of dark precipitate that can easily be wiped away afterwards. A screw is removed in an hour or three depending on size (its pocket watch parts so 0.6mm diameter screws or thereabouts)
What is the chemistry involved and why does it not attack the brass and nickel parts at all (there is no visible effect on the rest of the plate) ? Apparently this also works to remove steel parts from Aluminium.