Technical article: Nature of Adherence of Porcelain Enamels to Metals
by B. W. KING, H. P. TRIPP, and W. H. DUCKWORTH – Battelle Memorial Institute, Columbus, Ohio
An investigation of the fundamentals of the adherence of porcelain enamels to metals indicated that good adherence is the result of metal-to metal bonds between the atoms in the base metal and the proper metallic ions in the enamel.
Adherence Mechanism Theory
Researchers throughout the time have suggested many theories of Enamel Adherence and it has seemed that the problem was solved, only to find new inconsistencies later on. Before discussing these theories, it is desirable to review some of the facts linked with the adherence phenomenon.
At some time during firing, the following statements are true
- Iron oxide is necessary at the iron-enamel interface
- Iron oxide dissolves in the enamel glass
- Water in solution in the glass reacts with the iron to form iron oxide and hydrogen
- Dendrites of iron, cobalt, nickel, and their alloys form at the interface
- Hydrogen from the reduction of the water dissolved in the glass enters the iron
- Molecular hydrogen trapped at the interface, either during or after firing, causes reboiling and fishscaling
- Ionic or atomic hydrogen passes easily through iron or glass, but molecular hydrogen will not pass through either, except when transferred as bubbles in the glass or riffs in the iron
- Hydrogen is present as ions, atoms, molecules, hydrides, or water
- Gases reported to be evolved during firing are hydrogen, carbon monoxide, carbon dioxide, carbon hydrides, nitrogen, and water
- Water is in solution in all glasses
- Since the glass is an electrolyte, when molten, plating can take place
- An oxidizing atmosphere is necessary in the early stages of firing
The study by B. W. KING, H. P. TRIPP, and W. H. DUCKWORTH – Battelle Memorial Institute, Columbus, Ohio on the on Nature of Adherence of Porcelain Enamels to Metals published on the Journal Of The American Ceramic Society –Vol.42 No.11, 1959, is going throughout above points trying to develop a coherent adherence theory. The conclusion they reached were:
- The bond between enamels and metals can be chemical in nature.
- Good adherence requires that the enamel at the interface be saturated with an oxide of the base metal, and this oxide must be one, which when in solution in the enamel, will not be reduced by the metal.
- Good adherence of enamels to metals appears to be the result of metal-to-metal bonding between the atoms in the base metal and metallic ions in the enamel when certain chemical and thermodynamic requirements are met.
- The oxides commonly known as adherence promoters help to establish and maintain the conditions necessary for good adherence without contributing directly to the bond developed.
Posting this article on IEI’s website has been done not because we are blind believers of this theory but because we are convinced that it will help enamellers to better understand the background of enamel adherence to metallic supports.
The International Enamellers Institute (IEI) wishes to thanks the American Ceramic Society’s Journal of The American Ceramic Society and John Wiley & Sons, Inc. for the courtesy of being allowed to post the King et al. article on IEI’s website