FURTHER DATA ON ENAMEL ADHERENCE *

A bstract Experiments designed to test the theories of gripping and electrochemical displacement as factors in the adherence of enamels to metals were performed. Rough surfaces on chromium, the stainless steels, and nickel produced no better adherence than smooth surfaces. Rough and smooth surfaces on iron enameled in vacuo and in nitrogen gave extremely poor adherence. Gripping action apparently functions on enameled noble metals. Poor adherence on chromium, a metal higher than cobalt in the electrochemical series, and good adherence on copper, a metal below cobalt in the series, tend to show that electrolytic reactions, if they do occur, are not the fundamental cause of adherence. Enamels fired on iron in vacuo or in nitrogen are given the most advantageous conditions for electrolytic reactions to take place, yet the adherence in these cases is extremely poor. Gripping action or electrolytic reactions could not function properly on oxidizable alloys and metals due to oxide layers which formed under the glass layer in firing. The importance of oxygen in enameling metals is emphasized by these experiments. A brief review of the work of Pfeil, Castro and Portevin, and Mathewson, Spire, and Milligan on the constitution of iron scale is given. It is shown that metallic iron particles may exist in ferrous‐oxide scale as true primary dendrites, as rounded grains of residual α‐iron from the base, and as minute specks or thin layers of secondary α‐iron. Microscopic study of iron‐enamel contact zones showed no true dendrites of iron to be present. In underfired enamels was found the α‐iron phase from the initial scale, but in the scale underlying this enamel both residual α‐iron phase and secondary α‐iron were found. In normally fired enamels only detached surface iron produced by slow oxidation of the iron base surface was found. The iron surface and the detached surface iron particles after proper etching showed a film of ferrous phase approximately 3 × 10 ‐5 inch thick between the glass and metal. This film of ferrous phase is believed to be the medium which bonds the enamel and iron together. Microscopic study also showed the effect of an increased diffusion of oxygen as the enamel became richer in iron oxide. Cobalt oxide in the enamel is believed to promote solution of ferrous phase and also to act as an oxygen carrier through the molten glass layer. A description of the six stages in the complete firing of cobalt ground‐coat enamels on iron is given and also the reactions which occur in each stage.