[BMathison]

As Nail24 stated, it's ferrous iron and that is natural. The material was chosen mainly for it's friction properties (among a few other things) not corrosion.
Just use a little steel wool and don't fret it.

Even if you never took care to clean it, they would probably need replaced from normal wear before the corrosion ever got bad enough to worry about.

Edit: I was assuming they were iron, after looking again, I'm not sure they are.