Stranger Dust

[nyko46]

My 2009 E320 Bluetec front ring wheels gets dirt in less that one week.
This dirt is a kind of a dark (brownish) dust impregnated allover of the front ring wheels.
Passing my finger over the dust area it seems to come from the brake pads and /or brake rotors. The rear wheels does not have this issue. Only the front on both sides. This dust will come out only washing the wheels with a brush and soap. Any idea what it can be?