The properties of direct-on enamels obtained by replacing TiO2totally or partially, by ZnO have been studied. The first experiments were made with a frit containing 20 wt% ZnO In place of TiO2, the other constituents being the usual ones. The optimal fusion temperature of the frit was 1150°C: this shows that ZnO acts as a flux (the normal fusion temperature is about 1200°C). Decarburised steel sheets were enameled using this ZnO-frit, for various firing times at 800°C. The binding strength, estimated with an adherence-meter, was satisfactory, but the coats were translucents. The incorporation of several colouring agents has been tested. Opaque enamels of good adherence have been obtained by replacing only a fraction (up to 70%) of TiO2by ZnO. The interface reactions have been studied, in each case, by means of the electron microprobe, the scanning electron microscope and X-ray diffraction. The irregular, corroded-like appearance of the interface may be attributed to the formation of an FeO layer on the support, during the heating period, and to the subsequent dissolution of this layer in the fused coat. A compound is formed at the interface, which contains essentially iron, silicon and zinc oxides. It has been identified by X-ray diffraction as a willemite-type solid solution. The fact that willemite only appears at the interface led us to study the influence of FeO on the formation and crystallization of this compound. One may reasonably think that this willimite formation promotes adherence. © 1976.
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