Pores in aluminum trap and release gasses below the chrome. Corrosion on aluminum takes place more so when gasses and weather are trapped versus on just bare aluminum. Unprotected aluminum in a salt atmosphere will also show these white spots but are easily removed during routine maintaince. Anodizing is one of the best ways of protecting aluminum as far as coatings go. It is commonly used in the marine industry but you sacrifice some of the mirror finish.
I hope this has been helpful and thank
you 
James