Chemical Conversion Layer Research Articles

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

Abstract 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.

Corrosion behaviour of fluotitanate pretreated and painted aluminium sheets

“Chromium free” aluminium conversion treatments usually produce a very thin chemical conversion layer; moreover they do not possess the well known inhibition properties of chromates. Hence, the corrosion protection of the substrate is, in general, less effective than that offered by the thicker chromate layers. However, the aluminium/paint adhesion is noticeably improved by the use of fluotitanate pretreatment baths. Our previous works also showed that the behaviour of fluotitanate pretreatments can be similar to that of chromatized aluminium surfaces when the aluminium is painted with a protective thermoplastic polyester resin. The aim of this work is to study new bath formulations and treatment procedures in order to further improve the effectiveness of this kind of environmentally friendly aluminium pretreatment. Some production parameters such as surface degreasing and pickling, pH and salt concentration of the bath and time of immersion were varied in order to obtain more efficient conversion layers. Aluminium alloy sheets were pretreated in fluotitanate baths and for comparison, in traditional chromate baths or they were only degreased and pickled; these samples were then studied with and without the application of organic coatings by the use of electrochemical impedance spectroscopy. The painted samples were coated with epoxy or vinyl resin types. The coatings were kept sufficiently thin (10–20 μm) in order to speed up undercoating corrosion thus highlighting the pretreatment action. By the use of proper testing solutions and of suitable fitting procedures of the impedance data, it was possible to identify the contribution of the pretreatment to the corrosion resistance of the painted aluminium alloys.

Characterisation of conversion layers on aluminium by means of electrochemical impedance spectroscopy

This paper reports the characterisation of chemical conversion layers on aluminium by means of Electrochemical Impedance Spectroscopy. To fit the EIS spectra non-trivial dispersion phenomena, which are usually described by means of a constant phase element taking into account fractal dimensions or porosity effects, have to be considered. By investigating the influence of the nature and the conductivity of the EIS electrolyte on the impedance behaviour, the observed dispersion can be attributed to a ladder-network of electrolyte resistances and double layer capacitors in the pores. The proposed electric equivalent circuit allows to study the growth of the film as a function of the conversion parameters. In this paper the influence of the conversion time on the film thickness is worked out. In agreement with AES, SE and FTIRS the layer growth is in proportion to the conversion time.