[Ksharindam]

Although standard reduction potential of aluminium(1.66v) is larger than zinc(.76v) but aluminium does not give more potential than zinc in simple voltaic cell.why?

[UG]

Have you considered the thin layer of aluminium oxide which forms on the surface of the metal? It is this oxide barrier which gives aluminium its good corrosion resistance.

[Ksharindam]

Have you considered the thin layer of aluminium oxide which forms on the surface of the metal? It is this oxide barrier which gives aluminium its good corrosion resistance.

i rubbed the surface of aluminium thoroughly to remove Al₂O₃ layer and also washed with hot acid.as aluminium oxide is amphoteric it will react with acid forming soluble salt.so there is no chance of remaining oxide layer.

[DevaDevil]

what is the electrolyte in your electrochemical cell?

[Ksharindam]

what is the electrolyte in your electrochemical cell?

The electrolyte is dilute HCl

[Enthalpy]

Aluminium oxide always forms again, even in secondary vacuum. BUT it takes some special aluminium alloys to be protective (others corrode very quickly, faster than steel does). Especially against your Cl- ions, alumina is a weak protection.

By the way, it's also the oxide layer that protects Zinc from corrosion.

One possible explanation direction, but I'm very uneasy with electrochemistry:
The cell voltage could depend on latest oxidation (Al++/Al+++), which may be less favourable than for Zinc (Zn+/Zn++).
Sorry if I've put nonsense.

[DevaDevil]

I do not think Al²⁺ or Zn⁺ is stable in aqueous media.