Structure and Corrosion Resistance of a Composite γ-Amino Propyl Triethoxy Silane and γ-Glycidoxy Propyl Trimethoxy Silane Conversion Coating on Galvanized Steel

In an aqueous solution of the mixtures of γ-amino propyl triethoxy silane and γ-glycidoxy propyl trimethoxy silane, a composite silane conversion coating was developed on galvanized steel. FESEM (field emission scanning electron microscope), XPS (X-ray photoelectron spectroscopy), ATR-FTIR (attenuated total reflection Flourier transform infrared spectroscopy) and SST (salt spray test) were used to characterize the obtained composite silane conversion coating and also the coating formation process was studied. The result showed that the surface of the composite silane conversion coating was complete, consecutive and compact. The coating could endure a neutral salt spray test for 72 h without corrosion. The result of salt spray test indicated that the composite silane conversion coating can provide a better corrosion inhibition than the coating which was composed of the single silane. Based on observation and analysis, it was proposed that the formation process of the silane coating on zinc should consist of three steps: the hydrolysis of the silane molecules, silane chemical adsorption and silane crosslinking condensation. The crosslinking reactions took place between γ-amino propyl triethoxy and γ-glycidoxy propyl trimethoxy silane during the forming process of the coating, and a high crosslinked density interpenetrating structure network was obtained, so the composite silane conversion coating could keep the corrosive substances from the zinc more effectively.