Question

Why aluminium sheet is said to be corrosion free

Answer 1

because it reacts with atmospheric oxygen to form a thin layer of aluminium oxide over it . This layer prevents further corrosin of aluminium.
Hope this helps....

Answer 2

They do corrode. The thing is that the form of corrosion is very thin, basically invisible, larger of aluminium 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 .