Your crank is probably aluminum (or aluminium where you live) so it won't rust. It will form a white oxide layer where the metal is exposed, but this isn't a problem with aluminium because the oxide prevents further oxidation. If you want to paint it black, get some fine (400) sandpaper, sand the area that was chipped lightly, prime it with some automotive primer, then spray some black on top. This really isn't neccessary for anything other than cosmetic reasons.