It seems to me that the chassis and much of the under-structure of Model S (and probably Model X) is bare aluminum with no protective coating. As a design engineer I've always seen and specified chromate conversion coatings (either a dip or spray) to reduce the possibility of corrosion and oxidation. Alodine or Iridite are two well known brands. Can either be clear or a yellowish-straw color. It's not very expensive and I wonder why (if I am correct) Tesla doesn't do this?