EIS study of the influence of aluminium etching on the growth of chromium phosphate conversion layers

This paper reports an EIS investigation of the influence of aluminium etching on the growth of chemical conversion layers. The influence of the pretreatment of the Al surface before conversion is studied by varying the etching time in a caustic soda solution. Furthermore, the concentration of fluoride in the conversion bath is varied as it acts as an etching agent during the growth of the film. The electric equivalent circuit proposed for chromate phosphate converted Al, allows a study of the growth of the film as a function of the two considered etching parameters. In this paper, it is shown that there exists a relation between the influence of the pretreatment time in the NaOH solution and the NaF concentration in the conversion solution. Without fluorine the conversion layer is not formed. For a NaF concentration higher than 1.5 g/dm(-3), the EIS spectra of the resulting converted Al sheets show that the pretreatment time in caustic soda is not affecting the layer characteristics. On the contrary, for intermediate NaF concentrations, the duration of the pretreatment in the NaOH solution has an influence on the conversion layer impedance. (C) 1998 Elsevier Science Ltd. All rights reserved.