The effect of anodization and subsequent treatments on corrosion resistance of aluminium

Abstract

Aluminium samples were chemically prepared by following operations: degreasing, etching I, etching II, and brightening), prior to anodizing in sulfuric acid. Aluminium surface area was 0.2 dm2. The composition of used aluminium samples was determined by the energy dispersive X-ray spectroscopy. Chemically prepared aluminium samples were electrochemically anodized for 45 minutes in a solution of 190 gdm-3 H2SO4 at room temperature, at a current density of 1.7 Adm-2. In the anodizing process, the aluminium sample served as the anode, with lead cathodes. After anodizing, the aluminium samples underwent a colouring process in five pairs of solutions (systems), where each system consisted of two solutions of inorganic salts RxA + RxB (x = 1-5, numbers of solutions). Colouring of the anodized aluminium was carried out at room temperature by immersing the samples in each solution for 7 minutes (e.g., R1A + R1B, τ =7 min + 7 min). Each used colouring system provides a different colour: green-yellow, brown, light-grey, blue, and orange-gold. After colouring, the samples were treated in a special solution to improve corrosion resistance and silication, resulting in a change in the obtained colour shade. All obtained colours were stable with very nice appearance, allowing such coloured aluminium to be used for decorative purposes. The corrosion resistance of the coloured anodized aluminium samples was investigated by determining the corrosion potential, corrosion current and polarization resistance using potentiodynamic polarization method, as well as by electrochemical impedance spectroscopy. A common feature of all tested samples is a significant improvement in the corrosion resistance of the anodized aluminium after colouring and subsequent treatment in the corrosion resistance improvement solution, particularly after the additional silane treatment.