Why Can Cobalt Oxide Improve Adhesion of Enamel?

In order to improve the adhesion effect between the enamel ground coat and the steel plate, the enamel factory and the enamel factory usually use the blue ground coat containing 0.3~0.6% of cobalt oxide, and add 1.5% of manganese oxide and 0.4% of nickel oxide to make the adhesion stronger, but if these oxides completely replace cobalt oxide, it will reduce the adhesion of the enamel.

Why can a certain amount of cobalt oxide promote the adhesion between the ground coat and the steel plate? The reasons may be as follows:

1. The addition of cobalt oxide frit can erode the iron surface, resulting in rough surface, thereby strengthening the adhesion between the enamel and the steel plate;

2. During firing, a small amount of iron oxide melts and enters the ground coat. If the ground coat contains cobalt oxide, its melting concentration can be controlled, so as to create the best conditions for the ground coat to adhere to the steel plate.

The enamel ground coat softens and flows at the firing temperature of 820~870°C to infiltrate the surface of the steel plate. In order to prevent large distortion in the ground coat layer when it is cooled after firing, the expansion coefficient of the ground coat should be slightly smaller than that of the steel plate. When the temperature is above 400°C, the expansion coefficient of the ground coat is larger than that of the steel plate, and after the average expansion coefficient at low temperature, the two are about equal.

To obtain the above-mentioned ground coat, its composition is limited to a certain range, and the weight composition can refer to the following ratio:

SiO2 40~50% Al2O3 10~20% Ba2O3 15~25% KNaO 5~10%

CaO 5~10% BaCO3, CoO, CuO, FeO, P2O5 3~5%