[Moybin]

2 cent opinion, nothing more...

Salt air in SFO can't be good. Like JonyRR says, there's no such thing as stainless steel, it's 'corrosion resistant' steel. That said, I'd be surprised to learn that the rotors were 'stainless'. Series 300 SS is high in nickel and won't temper or take an edge, so what we are looking at is not 300 series (look at the machining marks). I bet if you hold a magnet to the rotor it sticks. Only the 400 series SS will do that; no nickel just chrome and carbon, and that WILL tarnish. And magnets stick to non-stainless alloy steels all of which rust.

Then you have surface contamination in stainless steel, something called 'free iron'. If iron and/or low alloy steel particles get embedded into the surface, such as during manufacturing, and usually due to cross contamination from grinding and other metal working equipment, then the 'stainless' steel will start to corrode, very quickly, from the contamination sites.

Most of the bolts on the R11S series were carbon steel. Hence all the SS screw kits out there. BMW probably can't justify the cost of SS fasteners to the bean counters, so the R12S hardware is probably also carbon steel.