Hard TiO₂-SiO₂ sol-gel coatings for enamel against chemical corrosion

Sol-gel derived coatings composed of TiO₂-SiO₂ were deposited on enamelled stainless steel. In order to study the influence of the thickness on the morphological parameters, achieve continuous enamel surfacing and improve the corrosion-resistance and mechanical strength of silicate enamel for stainless steel were used. Sol-gel coatings with different compositions, heat treatment and numbers of layers were prepared in this work. The structure and morphology of the films were demonstrated by the means of X-ray diffraction analysis (XRD), scanning electron (SEM) and atomic force (AFM) microscopies. Thin triple layer crack-free films with a thickness of 470nm were obtained. The mechanical strength (HV) of the three-layer coating was 10.4GPa and was achieved at a temperature of 700°C. Thin SiO₂-TiO₂ sol-gel coatings were applied in order to enhance the chemical durability of these coatings in both acidic and basic solutions in accordance with ISO 28706 standards. Corrosion tests with the uncoated and sol-gel coated enamel were performed in 3wt.% boiling oxalic acid at 95-98°C and in 1N NaOH solution at 80°C. Crack-free 470nm 3-layer silica-titania coatings significantly improved chemical resistance against the basic medium (up to 80%) and stability against the acid medium (up to 28%) for the S80 sample fired at 700°C.