Investigation of the anodic oxide layer on 1050 and 2024T3 aluminium alloys by electron microscopy and electrochemical impedance spectroscopy

Transmission and scanning electron microscopy and electrochemical impedance spectroscopy measurements were made on anodic layers on 1050 and 2024T3 al uminium alloys, prepared from solutions of phosphoric acid, boric acid and sodium tetraborate. The microstructural parameters were dependent on the al uminium substrate and the type of electrolyte. A structural model for the o xide layer was generated from the data in the case of 1050 alloy. We showed that a similar model could not be made for the 2024 alloy, because of the disorganised morphology of the porous layer. The a.c. impedance behaviour o f the layers was investigated. According to the characteristics of the equi valent circuit, we showed that electrochemical impedance spectroscopy provi ded a method for the evaluation of the thickness of the barrier layer, what ever the morphology of the oxide layer: well-organised (1050 alloy), or dis organised (2024T3 alloy). This method gave the same results as those obtain ed using transmission electron microscopy. (C) 2000 Elsevier Science S.A. A ll rights reserved.