the brake discs are made of cast iron.
they are not made in stainless steel. wear and specific heat of steel is not good for brakes. the only option is ceramic, which is 15K+
your options are to paint the hub with high temp paint. if you're talking about the disc surface, where the pads contact the disc, there is nothing that can be done about that.
Past: 2004 S60R 6MT
Past: 2007 E90 335i 6MT
Past: 2010 335xi ZMP, MPPK, MPE
Past: 2013 335i 6MT ZMM/ZDH/ZTP/3AG/494/508
Present: 2016 F34 335i xDrive ZCW/ZDA/ZDH/ZTP/ZMP/2V5/5DL