Wheel treatments usually are meant to remove black brake dust which we couldn't see years ago with solid steel wheels. That dust often takes a while to develop because there are layers of different composition making up the brake pads. As they wear down, they tend to make more dust. There could very likely have been some rust buildup earlier that gradually got covered in that black dust. Now that the brake dust is gone, the rust is left.
Something else to consider is if a chemical was used, that might be residue and not rust, although I have to agree it does look like rust. Also, a residue would coat everything in the area pretty much the same.
Next, a lot of brake parts have a protective coating on them that is applied at the factory that made them. This is particularly true of rotors, calipers, and anti-rattle brake hardware. That coating is only meant to protect the surfaces until the parts are put in service but often it lasts longer than that. Road salt will eat any remaining coating away.
What I see in the photos is nothing out of the ordinary. In the very worst case, excessive rust will build up on the inside surface of the rotors and will cause the metal to flake off in large chunks. That has happened to my '80 Volare that has sat outside in snow and rain all of its life. On the rare occasions I drive it, I can feel that roughness but it wears away after a few miles. THAT's when the rust is excessive. It doesn't happen to most cars that are driven regularly.
Being a 30 year old car also means the rotors were built with substantial extra thickness. They can be machined multiple times during multiple brake jobs. On newer cars the rotors are very thin to save weight and often will be worn to the legal minimum thickness and must be replaced as part of a normal brake job. For that reason rust has less effect on the rotors' life.
Keep in mind too you wouldn't even see this rust if you had the less expensive steel wheels. Cast wheels lets you see through them to the rusty parts. What concerns me more, I guess, is the surface rust on the lug nuts. Every inexpensive replacement I bought at car shows has rusted the same way although they still worked fine. Original lug nuts are very high quality and take a long time to develop rust. The use of air tools when removing the wheels to rotate them will chip any coating on the nuts. That's real hard to avoid. I wouldn't worry about the brake parts, but keep an eye on the lug nuts. They won't fail, but if you want to replace them, stick with BMW original parts.
Wednesday, November 3rd, 2010 AT 8:53 PM