Electrochemical impedance spectroscopy characteristic of Al₁₈B₄O₃₃w/AA6061 and the evolution of growth morphology of cerium conversion coating on its surface

Because of their good properties and low density, whiskers reinforced aluminium composites are used in the aircraft, automobile industries and marine environments. Alumina borate whisker reinforced aluminium composites attracted attention for the lower price of the whisker and excellent properties of the composites. However, they are susceptible to the corrosive medium, which limits their extensive application. Coating is an effective method to improve corrosion resistance of the composites. In this manuscript, electrochemical impedance spectroscopy measurements were performed on the Al₁₈B₄O₃₃w/AA6061 composites with and without hydrofluoric acid pre-treatment. A similar electrochemical characterisation was exhibited on the composites with and without pre-treatment during exposure in a 3.5 wt-% NaCl solution. However, their corrosion mechanisms in the corrosion media were completely different. During the immersion process, it was noted that the corrosion resistance gradually increased for the composite with the pre-treatment and continuously decreased for the no pre-treatment one. The difference in the surface states of the composites was responsible to the change of the corrosion mechanism. Cerium conversion coating was deposited on the composites with and without pre-treatment by a conventional dip method. The formation, growth and morphology of the coatings were affected by the pre-treatment prior to the coatings.