Interdiffusion-induced devitrification of porcelain enamel on steel

Devitrification which occurred in the ground coat (fired at 830°C for 0–30 min) due to interdiffusion near the steel-enamel interface was characterized by electron microscopy (SEM and TEM) coupled with energy-dispersive X-ray analysis. Spinel oxide, Fe3O4, which was alloyed with Ni if nickel-dip pre-treatment was adopted and was compatible with α-Fe, crystallized in the ground coat near the glass-metal interface. Independent of nickel-dip pre-treatment the spinel crystals were extensively developed at the edges of the specimen slab, but were fewer and smaller in the centre of the flat faces. The quantity and size of crystals increased with the firing time. Within the titania-opacified cover coat (fired subsequently at 800°C for 3 min) equiaxed TiO2 crystals (anatase with a minor amount of rutile) were distributed uniformly. The TiO2 crystals grew inwards to some extent into the ground coat and became needle-like with their long axes aligned diagonally to the interface. The devitrification of enamel was affected significantly by diffusion across the interfaces which resulted in a saturation with oxide components.