Dissolving Iron with Alum?

[Ian]

Hi all you chemistry buffs out there!

Someone asked me about the fumes given off when they tried to dissolve a broken off steel screw out of a brass fitting. I have no idea what the fumes are, but I don't even understand the reaction. Alum is Potassium Aluminium Sulfate. Brass is an alloy of Copper and Zinc. Aluminium and Potassium are both more reactive than Iron, so why would the iron dissolve? Why wouldn't the zinc dissolve, ruining the brass?

When I "Googled" it, it seems that you can use alum to remove iron from aluminium fittings too.

Can anybody explain what is happening here? What would the "fumes" be?

Cheers
Ian