Dear Naga

Can you tell me what type of equeous solution you are using for the potentiodynamic polarization test.

We used 3.5%Nacl solution

Dear Naga

One ought to know that the electrolysis of the aluminum/aluminum alloys is remained ambiguous because the existance of an inherent thin-oxide film,i.e., Al2O3, on the substrate of the aluminum/aluminum alloys in air. So, during the potentiodynamic polarization test of aluminium - chromium alloys in 3.5%Nacl solution, one ought to think of the electrolysis of the thin oxide film,i.e., Al2O3 as well. The electrolysis of the oxide film is dependent on several factors; thickness, stability, and adherebility of the film on the substrate.

I would assume that in your case, the potentiodynamic polarization test on the aluminum alloy in th 3.5% NaCl solution, reaches the oxygen overpotential because there is enough charge, high electron density, on the surface of the alloy to reduce the oxygen cation ,in the solution, to oxygen atom , then to oxygen molecule leading to the overpotential; namely the evolution of the oxygen molecules.