Question

aluminium sheets are said to be corrosion free'comment.

Answer 1

They do corrode. The thing is that the form of corrosion is a very thin, basically invisible, layer of Aluminum Oxide which forms almost instantly upon exposure to air during the casting process. This thin layer of corrosion is very resistant to further corrosion, especially the pitting and flaking corrosion you might be thinking of. This layer of corrosion is why Aluminum must be arc welded using AC current rather than DC, the alternating polarity of the current cleans the surface of the material of oxides, and then delivers heat to the joint when the current reverses.