Electrodeposition of Zn-TiO₂ dispersion coatings: Study of particle incorporation in chloride and sulfate baths

Zn and Zn-TiO₂ coatings were galvanostatically electrodeposited on steel substrates from chloride- and sulfate-plating baths. The influence of the electrolyte type on the dispersion behavior of TiO₂ nano-particles as well as on the amount of TiO₂ incorporation was studied. The electrokinetic behavior of titania in diluted electrolytes is strongly influenced by the specific adsorption of SO₄^2- anions. Chemical depth profile analysis using glow discharge optical emission spectroscopy confirmed the incorporation of particles throughout the layers (up to 0.53 wt% and 0.78 wt% from chloride- and sulfate-bath respectively). Focused-ion-beam assisted cross-sectional analysis showed that TiO₂ was incorporated in zinc as small agglomerates (up to ∼200 nm). The TiO₂ incorporation was favorable at slightly acid pH values (below pH 5.5) for both baths. The pH zone for maximum incorporation of particles is related with the adsorption of ions at the particle surface. The chloride electrolyte led to a higher co-deposition of TiO₂ over a broader pH range (2-5.5) as well as a wider range of current density (2-20 A/dm²) in contrast to sulfate electrolytes. A common feature is that the (102), (103) and (112) preferred orientations were promoted as a result of particle incorporation.