A study of undercoatings for nickel in the plating of zinc base die castings

49 Pages·01.353 MB·English

by  Fishbach, Adolph

A STUDY OF UNDERCOATIMGS FOR NICKEL IN THE
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PLATING OF ZINC BASE DIE CASTINGS
Adolph Fischbach
A Thesis submitted to the Faculty of the
Graduate School of Indiana University
in partial fulfillment of the
requirements for the degree
Doctor of Philosophy
January 1942
ProQuest Number: 10295099
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The writer wishes to express his sincere gratitude
and appreciation for the advice and invaluable assistance
rendered by Dr. Frank C. Mathers in the study of this
problem.
TABLE OF CONTENT S
Introduction ........ . . . . . . . . 1
Review of the Literature.................. 5
Apparatus ........................ 14
Procedure........................... 16
Experiments............................... 18
Nickel Undercoating . . * • • • ........... 19
Copper Undercoating ........................ 21
Copper-Lead Alloy Undercoating ........... 25
Zinc-Cadmium Alloy Undercoating • • • • • • 27
Lead Undercoating .........................30
Tin Undercoat i n g .................. • . . 34
Silver Undercoating ........................ 39
Cadmium Undercoating ...................41
Conclusion to Experiments.......................43
INTRODUCTION
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With the wide use of zinc base die castings in the auto­
mobile industry in recent years, nickel plating of these alloys
had become general. Here again the chromium finish was used
for appearance purposes** Door handles, radiator grills, car­
buretor bodies, fuel pumps and filters, accelerator parts,
and a host of others were conspicuous examples of the use of
zinc base die castings. Besides the automotive industry, the
zinc base die castings had become important in the production
of domestic oil burners and radios, in scientific instruments,
and in toy manufacture.
The zinc base die castings possess exceptional utility
for a wide range of uses, some of the chief reasons being:
(1) Adaptability to rapid production at moderate
cost and within closer dimensional limits as
compared to stampings and forgings,
(2) Adaptability to a wide range of shapes and
sizes, including complex parts with difficult
core work. Irregular contours and structural
elements, which though not so strong as wrought
parts, have unusual toughness as compared to
most low cost castings,
(3) The die castings in some instances may be pro­
duced with surfaces so smooth as to require
little or no polishing or even buffing prior
to plating or other finishings.
(4) The die castings may be produced in remark­
ably thin sections, yet with a stiffness not
attained with stamped parts.
(5) The relative ease of machining the die castings,
when machining is required, and the minimum
waste in metal removed are important from an
expense outlook.
(6) Comparative resistance to corrosion under most
conditions of service. Any corrosion that does
occur is confined to the surface and rarely af­
fects the strength or serviceability of the
part.
There are, however, limitations to the use of zinc
alloys, which involve temperature changes during service.
Prolonged use at temperatures above 300°F has not been recom­
mended. Although the impact strength is high at normal tem­
peratures, it decreases rapidly at very low temperatures}
but with a return to normal temperatures, the impact strength
is restored. Despite this fact, however, breakage attributed
to temperature in service has rarely been reported.
Most finishings on automotive parts are of the plated
type. In recent years practically the entire automotive
industry has standardized on certain definite specifications
for plated coatings on zinc base die castings. In general
they all require heavy coatings, the minimum varying from
-4-
•0008” to .001". This may be composed of a series of copper,
nickel, and chromium, or of nickel and chromium, or the option
being allowed in most cases* “Where the sequence is copper,
nickel, and chromium, the minimum copper thickness is specified
as .0002" or .0003". The nickel deposit is specified as a
minimum of .0003" to .0005", and the chromium in most cases
is required to be a minimum of .00001". It cannot be too
strongly emphasized that the satisfaction being obtained with
plated die castings in the automotive industry depends upon
the conscientious fulfillment of these specifications.
The big difficulty with the plating of zinc base die
castings is the subsequent blistering of the electrodeposit,
when the plated object is coated with organic finishes and
baked at 150°C. Microscopic and X-ray examinations have shown
that the zinc and copper form a diffusion layer, which con­
tains a brittle film of the alloy. The brittleness of this
film is responsible for the blistering of the deposit. The
diffusion of copper and zinc takes place slowly at ordinary
temperatures, but the effect is greatly increased at the ele­
vated temperatures. This blistering is a tremendous expense
to the industry.
In the litere.ture, copper and, only recently, brass were
found to be used as an undercoating for nickel. Thus, this
work is concerned mainly with undercoatings other than copper
and brass in the nickel plating of zinc base die castings.
REVIEW OF THE LITERATURE
-6-
1
R. J. Wirshing from a study of the corrosion of electro-
^Monthly Rev. Am, Electroplaters' Soc. 19, No. 8, (1932)
Chemical Abstracts 26, 5503 (1932)
plated steel drew the following conclusions! (l) The total
deposits should be at least .001" thick; (2) no thin or
"flash" deposits should be used with the exception of the
final chromium coating; and (3) at least two dissimilar me­
tals should be plated exclusive of the chromium "flash."
Mien these conclusions were applied to the plating of zinc
base die castings, using a thickness of .0005" copper and
.0005" nickel, followed by the usual chromium flash, extreme­
ly good corrosion-resisting deposits were obtained. The use
of the thick copper deposit enabled the low pH solution to
be used for the nickel plating in place of the usual high pH
nickel plating solution required when only a thin copper de­
posit was used, or when the nickel was plated directly on the
zinc die castings.
2
Carl Hussner reviewed the standards which should be
^ Monthly Rqv. Am. Electroplaters1 Soc. 20, No. 4,(1933)
Chemical Abstracts 2-8, 1930 (1934)
maintained to obtain satisfactory products. He discussed the
preliminary finishing and machining as well as the cleaning
and plating of the zinc die castings. Results were given to