The effects of a nickel oxide precoat on the gas bubble structures and fish-scaling resistance in vitreous enamels

The effects of a NiO precoat on the interfacial microchemistry and the structure of gas bubbles at the steel–enamel interface were investigated using scanning electron microscopy (SEM), electron probe microanalysis (EPMA), thermogravimetric analysis (TGA) and image analysis techniques. The experimental evidence demonstrates that nickel oxide applied to the steel substrate as a precoat accelerates the diffusion of Fe from the steel into the enamel and promotes decarburisation of the steel substrate, this latter reaction generating CO and/or CO2 as gases. The resulting increase in FeO concentration reduces the viscosity of enamel. The apparent decrease in the viscosity of liquid enamel, along with formation of substantial quantities of CO and/or CO2 gases control the distribution of gas bubbles in the enamel layer. The investigation also explains the reason for the association of large gas bubbles with Fe–Ni metal-rich particles with a dendritic appearance (termed ‘dendrites in the following text) at the enamel–steel interface. The role of these Fe–Ni metal-rich ‘dendrites’ in reducing the tendency for hydrogen flaking or cracking of the enamel layer, generally referred to as fish-scaling, is also elucidated.