Controlled adsorption of titanium(IV) oxide particles on electroplated zinc coatings to improve the corrosion resistance of chromium(VI)‐free conversion layers

Abstract

Adsorption of nano‐scaled titanium(IV) oxide particles on electroplated zinc is performed by a simple dip‐coating technique in an aqueous titanium(IV) oxide suspension prepared with a stirred media mill. X‐ray photoelectron spectroscopy, scanning electron microscopy and X‐ray fluorescence spectroscopy are carried out to investigate the composition of the zinc surface and the thickness and porosity of the adsorbed titania films. The zinc surface formed during the electrodeposition process is of oxyhydroxide nature and the thickness of the adsorbed titania particle layer is controlled by the pH value and the solid concentration of the suspension. In the range of 10 wt.%–30 wt.% titanium(IV) oxide, a linear dependence between the titania film thickness and the solid content of titania particles in the suspension is found. Highest film thicknesses are obtained in alkaline media (pH≥9). At 13.5 wt.% titania particles and pH values below pH = 2.4, the titania particle film is not closely packed and the zinc layer underneath is still visible in electron microscopy, which is a prerequisite for imbedding these particles by a thin second zinc layer for formation of a robust chromium(VI)‐free passivation layer containing the titania particles.