White vitreous enamel for ferrous metals with preliminary thermal activation of frit

Abstract

Vitreous enamels represent a class of inorganic coatings with different properties. In this article, a white vitreous enamel of the Na2O–B2O3–TiO2–SiO2 system for enameling of cast iron is introduced. In contrast to the enamel for other metals, the enamel for cast iron must exhibit a high spreading and coefficient of thermal expansion (CTE) of 40–70 mm and 110–120·10−7 1/K, respectively. The objective was to obtain an enamel with high whiteness (coefficient of diffuse reflection, CDR not less 80%) and chemical resistance not lower than class A+. This enamel was obtained by correcting the basic composition of the enamel and changing the ratio of SiO2/B2O3 and Na2O/K2O oxides. The formulation of the frit contained oxides in the following quantities in wt.%: SiO2, 43; B2O3, 12; P2O5, 4; TiO2, 15; Al2O3, 2; MgO, 2; Na2O, 12; K2O, 6; and ZnO, 4. This enamel exhibited a set of specified properties. At a temperature of 850 °C, the spreading of enamel was 53 mm, CTE was 110·10−7 1/K, and chemical resistance was of class A+. The whiteness property (CDR 83%) was achieved only using an additional technological operation, i.e., the thermal activation of the frit. The thermal activation of the frit at 500 °C for 30 min induced the formation of a fine-crystalline homogeneous coating structure and TiO2 in the form of anatase. Thermal activation shifts the crystallization temperature to the field of lower temperatures, this is confirmed by the data of the differential thermal analyses. Electron microscopy findings indicated differences in the structure of the enamel after etching in hydrofluoric acid obtained from the frit with and without thermal activation. Preliminary thermal activation of the frit helps obtain a uniform fine-crystalline structure of the coating.