Originally Posted by bd3500

There was a service bulletin out for this:

E70 (X5) produced up to June 8, 2009, and equipped with Running Boards (SA328)

SI B 51 30 09 - Permanent Discoloration on the Running Boards

SITUATION
A permanent discoloration is present on the aluminum surface of the running boards. This corrosion has the appearance of white discolored patches. Buffing and/or polishing the affected areas will not remove the corrosion.

CAUSE
Discoloration on the aluminum surface caused by environmental effects

CORRECTION
Replace the affected running boards with updated parts. Updated parts are manufactured using a more environmentally-resistant aluminum.

WARRANTY INFORMATION
Covered under the terms of the BMW New Vehicle Limited Warranty.