Corrosion Resistance of Chromate-Free Passivation Coating for Galvanized Steel

In order to enhance the corrosion resistance of galvanized steel and replace toxic chromate,a chromate - free passivation bath for galvanized steel was formulated by physical mixing, using waterborne acrylic resin latexand colloidal silica sol as the film - forming agent, molybdate as the corrosion inhibitor, and phytic acid as the additive. The optimal composition of the bath was determined based on neutral salt spray tests (NSS). The corrosion resistance and anticorrosion mechanism of the passivation film were studied making use of polarizationcurve tests, while the appearance and element compositionof the passivation film were analyzed using a scanning electronmicroscope. Results showed that the corrosion resistance of the galvanizing coating was considerably increased after being passivated with the passivation fluid, and the percentage of corrodedarea of the passivated galvanizing coating was only 5% after uninterrupted 60 h of NSS. A protective passivation layer of a certain thickness was formed on the galvanizing coating after passivationtreatment, and the passivation layer was capable of retardingthe cathode process of the galvanizing coating, leading to a positive shift of the open - circuit potential from-1098 mV (vs SCE) of the naked galvanized steel to-1054 mV ( vs SCE) of the passivated galvanbizing coating.