Abstract
Anodic film formation on aluminum and AA1050 aluminum alloy has been examined during anodic polarization at constant current density and by cyclic potentiodynamic polarization in nitric acid. The resultant filmed substrates have been observed by scanning and transmission electron microscopies. Film formation under potentiodynamic conditions results in nonuniform film thickening that is limited by the onset of pitting and oxygen evolution at appropriate potential. For anodic polarization at constant current density, transformation of the nonuniform film to a porous anodic film with increase of polarization time is revealed. The relatively high chemical reactivity of nitric acid to the alumina film material is displayed by potentiodynamic polarization at low sweep rates, when the chemical dissolution rate, , is similar to the film growth rate. Addition of cathodic activity to aluminum and the AA1050 alloy and enhanced localized aluminum dissolution above the pitting potential do not significantly influence the kinetics of alumina growth and dissolution. Consistently high cathodic activity of the alloy during cyclic extended polarization is associated with the exposure of the second-phase particles within the pits at the surface of the AA1050 alloy that contrasts with the reducing cathodic activity of 99.99% aluminum.
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