Formation of ultra-thin amorphous conversion films on zinc alloy coatings

Within the two parts of this contribution a detailed investigation of the formation of ultra-thin amorphous conversion coatings on hot dip galvanised steel is reported. The second part deals with the nucleation, growth and the barrier properties of these ultra-thin films on model substrates. The chemical composition as well as the film growth kinetics of the phosphoric acid and complexing organic macromolecules within the conversion layer system on a hot dip galvanised steel substrate have been determined by infra-red reflection absorption spectroscopy (IRRAS) and X-ray photoelectron spectroscopy (XPS). Film nucleation was studied by atomic force microscopy (AFM) that revealed the initial formation of individual nanoscopic islands, which depends strongly on the initial surface chemistry. Cyclovoltammetry (CV) and potentiostat measurements were performed to characterise the blocking of ion transfer and electron transfer reactions on the modified surface. An excellent correlation with the barrier properties of the film with the state of film growth and the composition of the inorganic/organic nanocomposite layer was observed. The investigations are a basis for the understanding and future development of thin conversion films on zinc alloy surfaces. Finally, Scanning Kelvin Probe measurements showed the inhibiting effect of the conversion treatment under the conditions of atmospheric corrosion