Page 62

DISCUSSION.
little extent upon the thickness of the copper plate, since the
diffusion rate will be materially lowered as the surface of the
zinc metal under the coating becomes richer in the constituent
which is being absorbed. Thus we noted from our experiments
that our heaviest plated coatings were not absorbed after heating
at 1000 C. for over one month. As would be expected, the rate of
absorption is greatly influenced by the temperature. At a higher
temperature, absorption of the thinner coatings was completed
in some instances within a few minutes.
From our examinations of actual plated articles which had
previously been flashed with a coating of copper, we could find no
indication that peeling was due to any absorption of the under-
lying copper plate. In many instances, where the plated coating
could be peeled off, the underlying copper flashing was found to
have peeled from the zinc base and to be tightly adherent to the
outer coating.
It is apparent, also, that trouble resulting from diffusion of
plated coatings can be overcome to a considerable extent by
increasing the thickness of the copper flashing which is usually
deposited preparatory to the finished plating.
The writer has recently examined microscopically a number of
nickel plated zinc sheets of German origin, which were several
years old, in which the "flash" coat of copper is still clearly
visible.
This material carried a copper coating of 0.024 g./sq. dm. over
which had been deposited a nickel coating of 0.204 g./sq. drn.
62