Corrosion Resistance of Silane Films on Hot-Dip Galvanized Steel by Molybdate Post-Sealing

To improve the corrosion resistance of silane films further, silanized hot-dip galvanized (HDG) steel was post-sealed with molybdate solution. The surface morphology and corrosion resistance of the composite films were investigated by scanning electron microscopy (SEM), a neutral salt spray (NSS) test, a salt water test, and electrochemical measurements. Results showed that pores in the silane films were filled after post-sealing the silanized HDG steel with molybdate solution. This was accompanied by the formation of a continuous, complete, and compact composite film that was composed of a silane film and a molybdate conversion film on the surface of the HDG steel. The composite coatings have much better corrosion resistance than a single silane film and this is related to the sealing time. Silanized HDG steel that underwent post-sealing for 60 s in molybdate solution showed the best corrosion resistance. Results of the electrochemical measurements in 5%(w, mass fraction) NaCl solution reveal that both the anodic and cathodic processes of zinc corrosion on the samples are conspicuously suppressed resulting in a large decrease in corrosion current density. A synergistic protection effect for the single silane film and the single molybdate conversion film is evident. The corrosion protection efficiency value was as much as 99.1%. Furthermore, the low frequency inductive loop usually observed in electrochemical impedance spectroscopy (EIS) disappeared and the low frequency impedance values for the composite films initially increased and then decreased with increasing immersion time. This indicates self-healing activity for the composite films. The corrosion resistance of these composite films is superior to some chromate passivation films.