Characterization of the protective effect of aluminium surface treatments by d.c. and a.c. measurements

Steady-state and electrochemical impedance spectroscopy measurements have b een made on anodic layers on 1050 and 2024T3 aluminium alloys prepared from solutions of phosphoric acid, boric acid and sodium tetraborate, before an d after impregnation treatment with zinc. Corrosion characteristics of the anodic layers were dependent on the aluminium substrate and the electrolyte . Aluminium alloy composition was found to be the most important factor for corrosion resistance; alloying elements of 2024T3 alloy (like copper) had a harmful influence on this layer property. Steady-state measurements allow ed the oxide layer behaviour to be studied in the anodic range by the deter mination of an anodic passivity domain. This domain was characterized by a weak aluminium oxidation through the oxide layer. The zinc impregnation tre atment had a marked protective effect on each studied anodic layer. This tr eatment can be used as an alternative to hot sealing in water or chromic ac id solution. (C) 2001 Kluwer Academic Publishers.