Paint adhesion on titanium and zirconium oxide conversion coated galvanised steel

Abstract

The effect of cleaning and rinsing on the formation of titanium and zirconium oxide based conversion coatings was investigated. The conversion coatings were applied by coil coating on hot-dip galvanised steel (Z), electrolytic galvanised steel (ZE) and the novel zinc–magnesium–aluminium alloy coated steel (ZM). Consequences on paint adhesion were also studied. Two different conversion coating solutions were used which differ in pH as well as in their chemical composition. Both solutions are titanium oxide based where as one contains a zirconium compound in addition. Glow discharge optical emission spectroscopy (GDOES) confirmed that the addition of rinsing steps during the coating process removes significant amounts of coating components from all surfaces. The coating weight varies depending on the different cleaning and substrate and is highest on ZM when using the titanium based conversion coating. The adhesion test results of the full-coat system show a good correlation between the conversion coating wetting behaviour, the effect of inclusion of a rinsing step into the coating process and paint adhesion as tested qualitatively by a water-storage test for a duration of 1008 h. Independent of cleaning and pretreatment, the traditional ZE showed excellent performance. Paint adhesion on ZM was best after acidic cleaning, pretreatment without zirconium oxide, and rinsing. For Z, the zirconium-oxide free coating also showed strongest adhesion, independent of cleaning. Industrial substrates were compared with model substrates consisting of vapour deposited zinc, magnesium or aluminium. Clear differences in the arising conversion layer build-up structures were observed by scanning electron microscopy (SEM); no typical conversion layer structure was found on aluminium. Consequently, the aluminium component of the mixed surface oxide is hypothesised to slow down conversion coating formation during coil coating.