It is a common observation that a strip of aluminium metal in aqueous copper(II)Sulfate does not show any visible reaction. Aluminium is normally expected to displace copper in solution since it is higher than copper in the electrochemical series.
The reason for this is that aluminium forms an oxide film around its surface which prevents reaction with aqueous copper(II)Sulfate. This oxides film protects the aluminium surface such that it is now unable to react with the aqueous copper(II)Sulfate
