An atomic emission spectroelectrochemical study of passive film formation and dissolution on galvanized steel treated with silicate conversion coatings

The effect of a silicate conversion coating, which was applied on electrogalvanized steel (EG) and hot dip galvanized steel (HDG), on zinc passivation and dissolution is investigated by atomic emission spectroelectrochemistry (AESEC) using anodic/cathodic cycles in a neutral borate buffer solution. We are able to decompose the total zinc oxidation rate into a soluble (dissolution) and insoluble (passivation) component. It is found that the silicate conversion coating reduces the soluble component with no measurable effect on the insoluble component. The results imply that zinc passive film formation occurs unhindered underneath the silicate film but the film effectively blocks zinc dissolution.

► Silicate conversion coating properties are quantitatively determined with AESEC.
► Zinc oxidation current transients are resolved into a soluble and insoluble component.
► The soluble component (zinc dissolution) is markedly attenuated by a uniform silicate conversion coating.
► The insoluble component (passivation) is not significantly affected by a silicate conversion coating.