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National Advisory Committee for Aeronautics, Technical Notes - Migration of Cobalt During Firing of Ground Coat Enamels on Iron

The utility of a coating depends to a considerable extent upon
its ability to adhere to the surface to which it is applied. When the
adherence is destroyed, the coating flakes off, and the underlying
surface is exposed.

Good adherence is particularly important for ceramic coatings in
high-temperature service, because the service conditions usually tend
to accentuate stresses between the coatings and the metal. Conven-
tional porcelain-enamel ground coats are similar in many ways to high-
temperature ceramic coatings but generally consist of a single glassy
phase with no solids present. For this reason, they lend themseIVes
more readiky to studies of the mechanisms of adherence.

Cdbalt oxide has been used to promote adherence of porcelain
enamels to iron since the first commercial development of these coatings
a half century or more ago. The earLy enamelers recognized the neces-
sity for this material in the ground coat, even though they did not
understand the mechanism by which it promotes adherence. If used alone,
cobalt oxide is most effective in amounts of about 0.5 to 1.2 percent
by weight of the enamel frit. Because of the high cost of cobalt oxide,
however, it is usually supplemented with the less-expensive oxides of
nickel and manganese, both of which appear to promote adherence in the
presence of cobalt oxide, but neither of which is very effective by
itself.

Conventional porcelain—enamel ground coats are complex alkali
borosilicates, usually containing at least 8 and sometimes 10 or 12 com—
ponents. Because of the similarity of the physical and chemical prop-
erties of iron, cobalt, and nickel, the identification and quantitative
estimation of small amounts of one of these materials in the presence
of the others in such a complex system are difficult by ordinary chemical
and physical methods. The use of radioactive tracers greatly simplifies
this problem, since the metal can be positively identified and the amount
quantitatively estimated by this method in concentrations smaller by
many orders of magnitude than is possible by any other method, including
spectrography. A discussion of the properties of radioactive isotopes
is given in appendix A.