Abstract
Effect of some pretreatments for direct-on porcelain enameling, especially anodic etching, on the adhesion of the enamel has been investigated on steel sheets. It was found that in anodic etching of steel in electrolytes flowing fast, direct-on enamel with excellent adhesion was obtained even in an etching weight loss of 0.1lg/dm2 of steel (In the conventional pretreatment, removal of a surface layer of 0.2g/dm2 or more in sulfuric acid is required for direct-on porcelain enameling of low carbon steel). Most of the experiments were conducted with rotating anodes. Suitable direct-on enameling was found to be made under the conditions: (1) lower H2SO4 concentration (10, 20g/l etc) and higher FeSO4 concentration (100-500g FeSO4⋅7H2O/l) (2) higher rotational speed of anodes (e. g. 400rpm for a cylindrical sheet of 100mm∅in diameter and with distance of 30mm between the anode and a cathode) (3) higher rate of flow of electrolyte and (4) higher etching temperature (e. g. 80°C)
Electron microscopic observations showed that under these experimental conditions, a great number of etch pits on electron microscopic scale appeared on the anode steel surface. It is suggested that the etch pits give an anchor effect on the adhesion of the enamel after firing.
