Maybe Ford might have changed the particular alloy of aluminum?
There are many different aluminum alloys. Some, like those used in Russian MiG 15's, 17's and 19's, etc., are designed to oxidize; the aluminum oxide protection eliminates the need for painting. Then there's early Porsches, where on a cold, quiet winter night, it has been said that you could actually hear the faint hissing of the aluminum reacting with the salty slush and water.