Study Of Electrochemical Impedance Spectroscopy On Anodized Aluminum Alloys

Because aluminum and alloys have samller density, higher intensity, cheaper price, easy machining and excellent surface decorative performance, they have been widely used in aviation, architecture, vehicles, ornamental and household applications et al. Corrosion resistance of aluminum is strongly increased by the anodizing process. But electrochemical behavior on anodized aluminum alloys need to be research further.In this paper, the effects of anodizing current density and anodic time on the anodic films have been studied. The results show the anodic films are composed of amorphous alumina, the thickness of anodic films increases with anodic current density (or anodic time). Under higher anodic current density (or longer anodic time), there are more defects of films and surface color is more dark. Micro-hardness (HV) of anodic films increases with anodic time prolonging. But the micro-hardness (HV) of anodic films has a maximum with current density increasing. When anodizing current density increased (or anodic time prolonged), the corrosion resistance of films has been improved in first, and then decreased. Under same conditions, anodized films on LY12 is thinner than on L3. The suitablepreparation process of anodic films in 200 g/L H2SO4 (98%) + 20 g/L C2O4H2-2H2O + 15 g/L C3H5(OH)3 solution was a current density of1.5A/dm2 for 40 min at 20°C.Electrochemical behavior of aluminum and its anodic films in NaCl and Na2SO4 solution was studied by means of electrochemical impedance spectroscopy (EIS) and polarization curves. The L3 and LY12 alloy matrix showed very high anodic current density in the solution, while the anodized specimens showed stable passivity and the anodizing process strongly increased corrosion resistance of aluminum. The anodic oxide film of aluminum alloys is composed of a thick porous layer and a thin barrier layer. The double layers improved the corrosion resistance, but the effect of the porous layer of unsealed anodic films is smaller. In NaCl solution, the electrochemical behavior of the anodi films on L3 and LY12 is similar, but the corrosion resistance of LY12 films is decreased because of alloying elements. In Na2SO4 solution, the electrochemical behavior of the anodi films on L3 have been unconspicuous influence by concentration of solution, thickness of films and anodizing current density, and the effect of sealing process is smaller. The passive current densities increased with the increase of chloride concentration of the solution. The higher the aggressive ion concentration in the solution,the more effectively the sealing process improves corrosion resistance of anodized films.The electrochemical behaviors of anodized aluminum in neutral NaCl and Na2SO4 solutions were studied with method of electrochemical impedance spectroscopy (EIS). The results reveal that there is a self-sealing process for unsealed anodic film in neutral NaCl and Na2SO4 solutions. The resistance of the porous layer (Rp) and the capacitance of the barrier layer (CPEb) obviously increase and the capacitance of the porous layer (CPEP)...