like most pe-sets these days, this is simply plated brass.
I don't know exactly why they do that, but Eduard (and thus HGW, same company) has been using that technique for many years. If you scratch the surface slightly, you will see brass underneath a very thin steel-like caoting.
I actually prefer this coating, if you airbrush a matt coat on it, it turns a slightly darker, dull metal colour that looks a lot like wheatered metal and I use some parts unpainted in that fashion ( I am a huge Eduard fan, use it on most of my 1/48 builds).
You don't need to remove the plating or anything or clean anything, you can use tha parts straight from the fret as they are,