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National Advisory Committee for Aeronautics, Technical Notes - Role of Nickel Dip in Enameling of Sheet Steel

An investigation was made of the effects of (a) the firing time and
(b) the weight of nickel deposited from the nickel-dip solution on the
adherence developed by a cobalt-free and a cobalt-bearing ground—coat
enamel on both enameling iron and a titanium-bearing low-carbon steel.
At 15500 F it was found that the maximum benefit from the nickel on the
measured adherence index occurred at 2-minute firing. The effect of the
nickel, however, dropped markedly with longer firing times. It was also
found that there was an optimum nickel deposit for maximum adherence.
This optimum.varied from 50 to about 120 milligrams per square foot,
depending on the type of cleaning used and the type of enamel applied.

It was found that the nickel dip reduced the tendency toward fish—
scaling. Furthermore, there appeared to be a relation between fishscaling
resistance and good adherence; no specimen with an adherence index of 70
or over showed fishscaling tendencies.

Metallographic studies of the interface of coated specimens showed
that nickel dipping of the steel prior to enameling brought about a sur—
face roughening during the firing operation. A relation was noted between
the degree of roughening and the measured adherence index. The roughening
of the steel surface was ascribed to galvanic corrosion.

The nickel dip, or nickel flash, is used extensively in the
porcelain-enameling industry to improve coating quality. The process
consists of immersing the part to be coated for about 5 minutes in a
weak aqueous solution of nickel salts at 1500 to 1700 F and at a pH
adjusted usually to 5.0. The part is immersed in the nickel—dip solu-
tion following the cleaning operations but prior to the application of
the coating. During the process a thin film of metallic nickel is plated
on the steel by a galvanic replacement reaction.