Note: Descriptions are shown in the official language in which they were submitted.
This invention is concerned with a method of
making plated coin blanks and coins and similar articles
such as medals. This invention is particularly concerned
with nickel plated coin blanks and coins but may also be
utilized to provide coins with a copper exterior.
Coins have been made from nickel plated on
steel, but there is a tendency for rust spots to develop
at pinhole locations where the plating does not totally
cover the steel. Pinholes may occur in the plated layer
lo as a result of surface phenomenon in the layer of nickel
plating or may be the result of micropores at the surface
of the steel.
During coining, the dies stretch the metal,
particularly at the edge of the coin. Pinholes may
extend to expose the steel as a result of this
stretching. Cracks in the plating may also develop at
the edges. Either of these occurrences will result in
rust.
If there are pores in the plating and these are
bridged during the electroplating process, the entrapped
air may blow during the coining procedure causing
blisters. This is a severe problem in coinage. Some
manufacturers of coins have been known to pound the
metal with small steel balls to try to minimize the
problem of blisters and pinholes.
Another problem that develops during coining is
known as 'starbursts'. During nickel plating the layer
of nickel would build up peaks. During coining these
spots will be cut off or flattened. This abrasive action
would score the surface of the dies.
The object of this invention is to provide
plated coin blanks and coins and a method of making such
coin blanks and coins in which there are significant
improvements in overcoming such problems as compared with
present practice.
In the preferred practice of this invention we
employ a multi-layer plating of steel with nickel-
copper - nickel. There is less tendency for iron to be
2~1~3~8
-- 2
oxidized as it is protected by a layer of copper which
has a positive potential in the EMF Series of + 0.34 as
compared with iron at - 0.44 and nickel at - 0.25. Also
with a three layer system any micropore in one layer is
unlikely to penetrate all three layers to expose the
iron. If there is a surface micropore in the steel it
will probably be covered by at least one of the layers.
Some of the advantages of this invention can however be
achieved by plating with nickel - copper to leave a
copper exterior surface.
Electroplating with multilayers including
copper and nickel has been known for purposes such as
the electroplating of car bumpers. For example, U.S.
Patent 4,418,125 dated November 29, 1983 describes steel
coated with successive layers of nickel, cadmium,
copper, nickel and chromium. Also Canadian Patent
369,046 dated August 15, 1936 discloses a layer of
nickel, then copper, then nickel so that the copper will
give a visual indication of improper buffing.
There are problems accompanying the use of
successive layers of nickel, copper and nickel in coins,
which are not encountered, at least to the same extent,
in making car bumpers. One of the most severe problems
is that of blistering. As previously noted, this results
from bridging over gas trapped in micropores followed by
the application of pressure during coining. This
bridging is particularly likely to occur with multilayer
plating. Another problem is that of severe mechanical
deformation and stretching during minting.
It is therefore a further object of this
invention to provide a method for applying a multilayer
plating which minimizes problems during coining.
:
~B~95~ ~
This invention preferably provides a process
for making a coin blank, coin or like article comprising:
a) cleaning a ferrous metal blank so that it is
essentially free of oxides, oils or dirt;
b) electroplating said blank with a strike of
nickel;
c) electroplating the strike of nickel with a
coating of copper at an initial low current density,
followed by electroplating the copper at full
current density to minimize or avoid bridging of
micropores;
d) preferably electroplating the copper with an
outer layer of nickel at an initial low current
density followed by electroplating the nickel at
full current density to minimize or avoid bridging
of micropores;
e) annealing the copper at a moderate temperature
to increase malleability without causing blistering,
either before or after application of the outer
layer of nickel;
f) where the final product is a coin, pressing in
a coining operation without the development of pores
or cracks which would expose the ferrous metal.
This invention also includes coins and coin
blanks resulting from this process.
In the drawings which illustrate a preferred
embodiment of this invention;
Figure 1 is a diagrammatic cross sectional view
showing the deposit of a molecular layer of copper at low
current density at the beginniny of copper plating
followed by plating at full current density;
Figure 2 is a comparative cross sectional view
showing what may occur where copper is plated using a
high current density from the start.
2 ~ 3 ~ 8
- 4 -
We will now discuss the preferred practice in
accordance with this invention in more detail.
The manufacture of coins in accordance with
this invention commences with the cleaning of the steel
or other ferrous metal blanks. These blanks are tokens
in the form of discs having a diameter about twelve times
their thickness.
Round blanks, or blanks of other geometric
shapes, are cut from low carbon steel strip, with a
10carbon content below .02%, preferably at a level of 0.01%
or less. They are then rimmed to obtain a smooth edge on
the perimeter to eliminate denting and scratching while
being plated and to help in forming a good coin edge flat
upon minting with a reasonable tonnage.
15The blanks are then annealed at 700~ - soooc.
in an oxygen free atmosphere and cooled slowly. Under
slow coolin~ conditions, we are able to get a hardness of
approximately 40 R3OT. (This and similar references below
indicates the Rockwell Superficial Scale using a 30 kg
1/16" ball~. Without annealing it is found that the steel
surface is oxidized easily upon pickling. The annealing
under a hydrogen atmosphere helps remove the steel
surface oxides.
The blanks are then loaded into a rotary
plating barrel. The number of blanks used in this
development may vary from 90 to 200, depending on their
sizes. All figures given in the process description
refer to an average load size of 100 blanks.
Normal cleaning practices prior to
electroplating are used to prepare the blanks for
plating. This may include any or all of the following
steps: washing the blanks with special alkaline
detergents, rinsing, solvent degreasing, electrolytic
cleaning, and rinsing in deionized water.
2 ~ 3
The traditional method is to clean with a basic
solution followed by an acid pickling which is supposed
to improve the adhesion to the nickel or other coating.
We have found it to be advantageous to reverse this
procedure. We first use an acid pickling followed
immediately by a quick wash with a dilute sodium
hydroxide solution to buffer the acid. We have found
that with the traditional cleaning procedure, there is
some oxidation even if only a short time elapses before
electroplating. We find that this oxidation is
significantly decreased by reversing the order.
The pickling solution may be a 10%
hydrochloric acid solution for 30 seconds at 55~C. The
solution is applied in the rotary barrel previously
referred to, which is rotated at a rate of 10 rpm during
cleaning, pickling and rinsing. The rinse is with a mild
basic wash to neutralize the acid. A suitable rinse
solution contains sufficient sodium hydroxide to provide
a solution of a pH of 9Ø
The second step is to apply a nickel strike to
deposit a coating of nickel which is about 0.8 to 1.2% of
the final weight of the coin. The nickel used to apply
the nickel strike should be low sulphur nickel; that is
to say, dull nickel and not what is known as bright
nickel. Suitable conditions for applying the nickel
strike are described below in Example I.
ExamPle I
The blanks are flash coated with a nickel
strike. A Watts nickel sulfate bath is preferred since
it is less corrosive to steel than the Woods nickel
strike bath. A suitable composition of the nickel strike
bath and the operating parameters are given in Table 1.
- 6 _ 2~5~ 8
Table 1
Nickel sulfate300 g/l
Nickel chloride 90 g/l
Boric acid45 g/l
pH = 1-2
Temperature - 60~C
Current density - 8 ASF
The wetting agent used was a commercial
product, Y-17, supplied by M ~ T. Chemicals. The
quantity used was 0.1% by volume. This plating step
produces a very porous deposit.
Typically, for a load of 5 cents blank size,
the time for the 1% nickel strike is approximately 30
minutes and the dull nickel deposit is about 0.005 mm
thick (see Table 5).
The barrel and plated pieces are then rinsed in
a cold drag-out water tank. It is further rinsed with
hot water and finally, it is rinsed with cold deionized
water.
The third step is to plate with a layer of
copper. Copper is coated to provide about 4 to 7% and
preferably about 6% of the final weight of the coin. The
gauge of the coating on each surface will be about 20-
30 microns.
We prefer to use an acid bath for applying the
copper. Although energy efficiency is better with a
cyanide bath, higher current density can be used with an
acid bath which gives a saving in time which more than
offsets the lower energy efficiency. Also cyanide baths
are hazardous to use and disposing of the waste may
create environmental problems, if done improperly.
2 ~ 8
-- 7
We have found that it is advantageous in
applying the copper to commence at a low current of about
1/6 to 1/4 of full power, and then increase to full
power. This is important to ~in; i ze or avoid bridging
with consequent blistering. The plating should therefore
start at 1.2 to 1.8 amps per square foot for an initial
period of about 15 to 20 minutes. Power is then
increased to about 6 to 7 amps per square foot to
complete the copper coating.
The copper coating should have a levelled
finish to give a good foundation for the final coating.
A limited amount of wetting agent and carrier and
brightener may therefore be included in the electrolyte
solution. Any of a variety of commercially available
reagents may be used, most of which are of a proprietary
nature and can only be identified by Trade designations.
Examples of substances that may be used as wetting
agent, carrier and brightener are Barrel CuBath B-76
leveler which may be used as brightener and Barrel
CuBath B-76 Carrier, both supplied by Sel-Rex Oxy Metal
Industries or Deca-Lume D-l-R, D-2-R and ~-3-R supplied
by M & T Chemicals. As previously noted, the wetting
agent may be Y-17 supplied by M & T Chemicals.
Further information as to plating time for a
given thickness may be derived on a theoretical basis
using the following relationship:
Electrochemical Equivalents
Element Valence g/f mg/c g/A.h
change
Copper 1 63.55 .6585 2.371
2 31.78 .3293 1.186
Nickel 2 29.36 .3042 1.095
- 8 - 2~ 8
(g/f: grams per Faraday; mg/c: milligram per
coulomb; g/a.h: gram per ampere hour)
The following Example II will further
illustrate the copper plating operations:
Example II
Copper plating is carried out next. This is
done by immersing and rotating the plating barrel in an
acidic electroplating bath. The plating composition of
the copper bath and the operating parameters are given in
Table 2, as being typical:
Table 2
Copper sulfate255 g/l
Copper (as metal) 56 g/l
Sulfuric acid57 g/l
Choride ion70 ppm
pH = 1.0
Temperature = 24~C
Current density = 6-7 ASF
Phosphorized copper anodes
This is a commercial proprietary electroplating
system sold by Sel-Rex Oxy Metal Industries. The company
recommends that the CuBath B-76 replenisher blend be
added on an Ahr basis, at the rate of 1 cc/Ahr. It
contains a ratio of 8:1 of carrier to leveler.
Other commercial acid copper plating systems
are available and could have been used.
Our copper plating process differs from normal
plating practices in the fact that the plating is
initiated at a low current density, e.g., at 1/5 the full
current density for about 15 minutes (1.2 to 1.4 ASF).
After that short time, full current density is applied to
the load, for approximately 4 hours to build a coating of
approximately 6% by weight (See Table 5).
2 ~
- 9
As illustrated in Figure 1, the low current
density at the beginning allows the copper coating to
follow the contour of the micropores of the steel or
nickel undercoating. This avoids bridging of the
micropores which, in turn, causes tiny blisters to
develop later on upon annealing. In Figure 1 the
diagonally hatched steel core is identified as 10, the
nickel strike 11 is vertically hatched and the copper
molecular layer 12 built up at initiation is stippled.
The initial thin, electrodeposited film thereby
minimizes crevices, pits and scratches and helps to level
the plating surface.
Our work has shown that when the initial low
current density step is omitted, there is a great
tendency for small blistering to occur.
In addition, the acid copper plating solution
contains wetting agents and levelers whose performances
are promoted and enhanced by the very slow plating cycle.
If full current density is applied at the
beginning, as illustrated in Figure 2, the edge of the
micropore would have higher current intensity which
favors quick build-up at the edge. Eventually, the pore
is closed at the top and a site for blistering has been
formed above the microcavity of the pore. This
blistering may be caused by hydrogen or solution
entrapment and made more significant upon annealing. In
Figure 2(a) (which shows core lOa and nickel strike lla
similar to 10 and 11 of Figure 1) we have
diagrammatically illustrated how the high current density
at the start of plating promotes dendritic growth of the
copper 12a at the edge of the pore. Figure 2(b) shows
the final stage where bridqing occurs due to the copper
plating 12b depositing faster at the edge of the pore to
cause bridging.
2 ~ $ 8
-- 10 --
Typically, for a load of 5 cents size blanks, the
time for a 6% copper deposit is approximately 4 hours
and the copper layer deposit is about 0.034 mm (see Table
5).
The plated barrel is then rinsed in a cold
drag-out water tank, It is further rinsed with hot
water, and finally, it is rinsed with cold deionized
water for about 30 seconds.
Some 'pumping action' is created when cold
water rinsing follows the hot water rinse. The
dimensional contraction change at the microstructure
level helps remove the plating solution from the pore to
prevent staining and spotting out.
Proper control of the amount of brightener,
carrier or leveler, and wetting agent is important as is
known to those skilled in the art. The bath is initially
charged with 40 ml of the 8:1 replenisher blend, such as
the CuBath B-76 previously referred to, per gallon of
plating electrolyte. Replenishment of the additives at a
rate of ~ cc per ampere hour maintains the additives
included in the replenisher blend such as levelers,
stress reducers, grain refiners and carrier agents in
balance, and at the proper levels in the bath.
We can now proceed to the final nickel plating
or we may interrupt the plating sequence with an
intermediate annealing step. This is done to relieve
plating stress in the relatively thick copper coating, to
remove entrapped hydrogen, and to remove surface organic
components which are additives in the copper
electroplating bath and which may cause blistering in
subsequent nickel plating. It refines the grain
structure prior to the coining procedure. It also tends
to close micropores.
If the copper coated blank is to be annealed,
annealing should be in a reducing atmosphere such as
ydrogen so as to inhibit or even reduce oxidation.
Annealing should be at a temperature of 500~C. - 600OC.
It has previously been the practice to heat to a higher
temperature to try to fuse the nickel with the steel.
These higher temperatures should be avoided as blistering
may result. The annealed copper plated blank should
have a hardness of about 35 to 40 R30T.
This extra intermediate annealing step serves
many purposes. First, it removes hydrogen entrapped in
the copper and nickel plated layers. Since the copper
layer deposit if fairly thick, any hydrogen entrapped
during plating ought to be removed before further
plating. Secondly, the copper deposit is also highly
stressed and the thermal treatment will help relax and
remove the stress to prevent cracking due to the severe
deformation upon minting. It is well known that
annealing also modifies the grain structure of copper and
makes it more ductile in cold work. Finally, it removes
the organics on the surface of the copper and improves
the bonding between copper and nickel, and helps to
eliminate blistering. The organic material in the copper
plating solution was needed to ensure good coverage and
reduced pitting during copper plating. However, at the
end of the copper plating those organics have outlived
their usefulness and ought to be removed before nickel
plating. The coining material thus obtained has proven
to be blister free.
As an alternative, annealing at a temperature
between 200~C and 400~C in the presence of a reducing
atmosphere followed by annealing at a temperature of at
least 530~C may follow the application of the finishing
coat of nickel.
The next step is to provide a finishing coat
of nickel. This coating should be l to 1~% by weight of
the coin or about 4 to 8 microns increase in gauge
2 ~
- 12 -
thickness on each surface. In the electroplating
process a brightener is preferably included to give a
smooth, bright final coating. Once again, the nickel is
initially coated at a low current density of about 0.5 io
0.7 amps per square foot, which is about 1/6 to ~ of the
full current density. The low current density is used for
an initial time of 15 to 20 minutes followed by 100 to
120 minutes at full current density of 3-4 amps per
square foot. It is believed that this procedure of using
a low current density, together with the initial low
temperature annealing previously described, contributes
to good adhesion of the plated layer to its substrate and
also contributes to minimizing or avoiding bridging for
the reasons previously explained. The nickel coated
blank has a hardness of about 45 to 50 R30T.
The conditions under which the nickel plating
layer is applied are exemplified by Example III.
Example III
If we chose to proceed immediately with the
outside nickel plating layer, we would first immerse and
rotate the plating barrel in a 10% solution of sulfuric
acid at room temperature for 30 seconds, then place the
plating barrel into the final sulfamate nickel bath.
The composition of the nickel bath and the~5 operating parameters are given in Table 3.
TABLE 3
Nickel sulfamate77 g/l
Nickel chloride6 g/l
Boric acid37.5 g/1
pH = 3.8
Temperature = 50~C
Current density = 3-4 ASF
T h i s i s a c o m m e r c i a 1 n i c k e 1
electroplating system supplied by M & T Chemicals.
2~r3~8
- 13 -
An antipit agent Y-17, also supplied by M & T
Chemicals may be added as required, at 0.15% by volume.
A leveller or brightener, commercially available, may be
added to obtain different degrees of brightness. We find
it satisfactory to add 0.125 ml of Niproteq W brightener
per Ahr and 0.03 ml of Niproteq carrier per Ahr. Other
commercially available brighteners and carriers may be
used.
Again, it is important that we start off the
final nickel process at a low current density at 1/5 the
full current density for about 15 minutes (0.6 to 0.8
ASF) before taking the electroplating solution to full
power at 3-4 ASF, for 2 hours to build a coating of
approximately 1.5% by weight.
This 2-step nickel plating process follows the
same reasoning as for the copper plating. Again, we have
found that this stepping current density is very
important in minimizing or eliminating blisters.
The plating barrel is then rinsed in a cold
water drag-out tank. It is further rinsed with hot
water and finally with deionized water containing
isopropyl alcohol.
Typically, for a load of 5 cents size blanks,
the time for a 1~% nickel topcoat is approximately two
hours and the nickel layer deposit is about 0.008 mm (see
Table 5).
If the blanks have not been treated by
annealing as an intermediate step between the application
of the coating of copper and nickel, then the blanks are
finally annealed in the presence of a reducing
atmosphere at a moderate temperature between 200~C to
400~C for 40 minutes immediately followed by annealing at
a minimum temperature of 530~C for 20 minutes. The low
temperature annealing promotes the removal of entrapped
2 ~
- 14 -
hydrogen, while the higher temperature annealing removes
the final plating stress, changes the grain structure of
the plated copper, and promotes some bonding between the
copper and the nickel. Finally, the plated blanks are
cleaned and minted or coined, that is to say, pressed in
coining dies under impact force of the order of 170,000
to 200,000 p.s.i. to impart a suitable design to the
surfaces and to shape the edges to provide a rim and
sometimes a serrated edge.
A very large proportion of coins made in
accordance with this invention are free from defects and
remain free even under normal conditions of use such as
exposure to salt water or acidic perspiration during
handling.
The procedures previously described were used
in the following Examples IV and V.
2 ~
- 15 -
EXAMPLE IV
Table 4
EXAMPLE IV(a) 5 CENTS SIZE
Blank Diameter Gauge Current Time
MM MM Density Min
ASF
Steel 20.9201.376
1% Nickel Strike 20.938 1.392 8 28
6% Copper 21.0081.431 1.2 15
6-7 240
1~% Nickel 21.0301.445 0.6 15
3-4 129
EXAMPLE IV(b) 10 CENTS SIZE
Blank Diameter Gauge Current Time
MM MM Density Min.
ASF
Steel 17.5300.960
1% Nickel Strike 17.548 0.976 8 20
6% Copper 17.6081,015 1.2 15
6-7 210
1~ Nickei 17.6201.020 0.6 15
3-4 102
EXAMPLE IV(c) 25 CENTS SIZE
Blank Diameter Gauge Current Time
MM MM Density Min.
ASF
Steel 23.4991.224
1% Nickel Strike 23.508 2.240 8 28
6% Copper 23.5731.279 1.2 15
6-7 225
1~% Nickel 23.5892.288 0.6 15
3-4 122
- 16 - 2 ~ ~~ g
EXAMPLE V
TABLE 5
Typical load =100 blanks
EXAMPLE V(a) 5 CENTS SIZE
Thickness Plating Thickness Plating Thickness Plating
(mm) time (mm) time (mm) time
for 1% for 6% for 1.5%
at 8 ASF Copper Nickel
Nickel Strike
0.005 28 m~n 0.038 3hr 48 m~n* 0.0084 2hr 9min*
EXPMPLE V(b) 10 CENTS SIZE
Thickness Plating Thickness Plating Thickness Plating
time (mm) time (mm) time
for 1% for 6% for 1.5%
(~n) at 8 ASF Copper Nickel
Nickel Strike
*
0.0036 20 m m 0.030 3hr* 0.0069 Ihr42min
EXAMPIE V(c) 25 CENTS SIZE
Thickness Plating Thickness Plating Thickness Plating
(mm) time (mm) time (mm) time
~or 1% for 6% for 1.5%
at 8 ASF Copper Nickel
Nickel Strike
* *
0.0048 28m~n 0.037 3hr 42min 0.0082 2hr.58m~n
All time values have been aril' ~ Ally ~Al~l~ted. Thickness is
given as an overall average value.
~ NCTE: Plating time for the 2 levels of current density has
been lu~ped together.
. .
2 ~4~ ~ 8
- 17 -
Comparative tests have been conducted on struck
tokens prepared using the process of this invention
identified as "Ni-CU-Ni coated" and a commercially
available nickel coated struck token marketed by Sherritt
Gordon Mines Limited and believed to be made in
accordance with Canadian Patents 1,105,210 dated July 21,
1981, and 1,198,073 dated December 27, 1985, identified
as "nickel coated".
1. HUMIDITY ~AMR~R TEST
Struck tokens were dipped in artificial sweat
solution, all excess moisture was removed from the token
surface and the tokens were left 72 hours in the humidity
chamber at 95% relative humidity, at room temperature.
In the rating system used, 1 is good, 5 is poor, as
indicated in detail in Table 6 which follows.
Table 6
NUMBER RATING CORRESPONDING TO DEGREE OF SURFACE
CORROSION
NUMBER RATING DEGREE OF CORROSION
1 None
2 Minimal (slight haze)
3 Mild (some cloudiness,
yellowing; pre-corrosion stage)
4 Moderate (large degree of
clouding and/or brownish
spots)
Severe (distinct brown, red
or black spots)
The results obtained are given in Table 7.
2 ~ 8
- 18 -
Table 7
Denomination Ni-Cu-Ni coated Nickel coated
5 cent size90 rated at 175 rated at 1
5 rated at 2 20 rated at 2
5 rated at 3 5 rated at 3
10 cent size95 rated at 180 rated at 1
2.5 rated at 2 10 rated at 2
2.5 rated at 3 5 rated at 3
5 rated at 4
25 cent size95 rated at 190 rated at 1
5 rated at 2 10 rated at 2
2. CORROSION PIT TEST
Struck tokens were immersed in 2% NaC1 for 4 hours,
with the tokens being turned over after 2 hours in
solution. We should note that no rust appears on
the Ni-Cu-Ni system.
The results are given in Table 8.
Table 8
Denomination Ni-Cu-Ni coated Nickel coated
% %
5 cent size80 rated at 1 90 rated at 1
18 rated at 2 10 rated at 4
2 rated at 3
10 cent size84 rated at 1 85 rated at 1
14 rated at 2 15 rated at 4
2 rated at 3
25 cent size82 rated at 1 75 rated at 1
13 rated at 2 5 rated at 3
5 rated at 3 20 rated at 4
At no time, did we see any orange color or red
rust spot on the Ni-Cu-Ni token. The rating 3 indicates
some yellowish stain at the rim. On the other hand,
reddish black or orange black spots could be seen on the
nickel coated token, particularly around the rim.
2~ ~ 3~ 3
-- 19 --
3. WEAR AND TEAR TEST
Standard wear and tear tests were done on the
tokens for a period of 8 hours. Visual
observations of the coins were made at the end of
the test period. The results are shown in Table 9.
The Ni-Cu-Ni coated token offers much greater
resistance to wear than the nickel coated token.
Since the Ni-Cu-Ni surface is less damaged, the
coins appear brighter while the nickel coated coins
appear dull.
Table 9
Denomination Ni-Cu-Ni coated Nickel coated
* Average Wear Average Wear
hardness Rating hardness Rating
R3OT R3OT
5 cent size 54.87 1 58.85 3
10 cent size 46.45 1 48.09 3
25 cent size 50.98 1 56.48 3
* Blanks were not annealed before coining
It is important to note that, the coins with the
Ni-Cu-Ni system of this invention are about 10
percent lower in hardness than the nickel coated
coins that were tested, yet its wear resistance is
far superior. Therefore, it is expected that in
circulation the Ni-Cu-Ni coated coin will resist
far better the abuse than the commercially
available nickel coated coin.
The theoretical explanation for this superior
characteristic is that the monolayer of nickel in
the nickel coated coin has a single metallic grain
structure which is dendritic, and is more prone to
denting than the multilayer composition of Ni-Cu
2 ~ 8
- 20 -
-Ni coated coin whereby the grain structures and
sizes of the different layers are dissimilar and
therefore offer more resistance to penetration
caused by abuse, wear and tear.
Advantages of the invention additional to those
apparent from the comparative tests outlined above include:
a) Blanks made of a low carbon steel core and plated with
nickel, copper and nickel successively, by commercially
available plating solutions produce white finish coins,
which offer excellent visual appearance, and excellent
resistance against tarnishing and corrosion.
b) This system produces a coin which has excellent wear
and tear resistance when compared to high grade nickel
(99% plus), cupro nickel, 430 stainless steel, and
other commercially plated nickel or laminated nickel
coins. This is due to the nature of the multilayer
undercoating.
c) The coins produced by this method typically have an
underlayer of 0.8% to 1.5% nickel, an intermediate
layer of 4% to 7% copper and a top layer of 1% to 2%
nickel. There is therefore a saving of the cost of
material. The Ni-Cu-Ni system costs about 60-66% of
the material cost of the nickel coated coins tested.
d) This system offers better corrosion and pitting
corrosion resistance than a single layer of metal.
e) This system is flexible since one may stop at the
copper layer to produce a copper color coin or one may
proceed further with the nickel topcoat to produce a
white color coin.
f) The all acid plating solutions are fully compatible and
eliminate the dangerous mix of acidic nickel plating
and cyanide base copper plating. The Ni-Cu-Ni system is
environmentally more friendly than the cyanide system
often used. The preferred acid copper plating can be
easily neutralized and discarded. A cyanide based bath
2 ~
- 21 -
requires decomposition of the cyanide to a less
dangerous form before discharging into a regular waste
water treatment facility.
g) A 2-step current density plating operation is applied
during the copper phase plating and the final nickel
phase plating. A very low density, i.e. 1/5 of the
full current density, is applied at the beginning of
plating for a short period of time, say 15 to 30
minutes followed by a high current, i.e., full current
density after the initiation period. This 2-step
operation produces interlayer bondings which are
excellent compared to the normal practice of plating at
full current from the very start.
This practice promotes intermolecular bonding between
dissimilar materials and thorough coating of the
micropores. It also minimizes bridging and blistering
as a consequence.
This initial slow plating provides an intimate and
excellent physical bonding without high temperature
thermal treatment which is sometimes used to induce
metallurgical diffusion of metal for bonding purposes.
This operation, thereby, gives rise to energy saving
and may further reduce the overall processing time and
cost for blank manufacturing by possibly eliminating
the need for thermal treatment in some instances.
h) The heat treatment temperature used in this procedure
for annealing is low and does not approach the
temperature of phase changes or crystal structure
change from body centered cubic ferrite to face
centered cubic austenite. This explains the ability of
this process to easily produce low hardness blanks at
40 R30T compared to other commercial processes which
~ 3
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rely on high temperature, thermal diffusion,
consequently high temperature gradient which makes it
difficult to have blank hardness below 45 - 50 R30T on
average.
i) The highly ductile and soft copper intermediate layer
facilitates material flow during coining. Relatively
lower force is required to mint coins resulting in
definitely higher die life. Ni-Cu-Ni offers better
flow characteristics during minting than Ni, thanks to
the copper layer, which is much more ductile than the
single nickel layer. In fact, laboratory tests have
shown that the thicker the nickel coating, that is, the
higher the percentage of nickel, the earli.er scoring
marks are seen on the dies, that is, the lower the die
life is.
j) The thin nickel top layer, approximately 0.005 mm
thick, is relatively more ductile than a nickel layer 6
to 8 times thicker, needed to provide good coverage and
protection of the steel, when there is no copper
underlayer. This fact, in turn, has proven to be
beneficial in ; ni i zing or eliminating a phenomenon
known as 'starbursting' in coining. The thicker nickel
layer has higher dendritic peaks which are quite
abrasive. The abrasiveness of nickel scores the
coining die surface and damages it. The thinner nickel
layer on the other hand, has shorter dendritic peaks
which are relatively less abrasive.
k) The Ni-Cu-Ni coating system is about twice as fast as
the Ni coating system. To obtain a coating thickness
of 1% nickel, 4-7% copper and 1-1.5% nickel. The
laboratory plating time is from 5~ to 6 hours excluding
rinsing times while it took about 11-12 hours to obtain
6% plating of nickel.
2 ~ 8
- 23 -
1) The nature of the multilayer coating makes the whole
system less active in terms of galvanic action and emf
potential between the nickel and the steel. The
results of tests have shown that the multilayer system
is less prone to corrosion than the single nickel layer
system.
m) The nickel-copper-nickel structure on steel is more
economical to produce and offers better protection
against corrosion than a nickel on steel structure
because the thinner, more expensive nickel is only
there to provide a white finish while the thicker, less
expensive copper performs the protective function
toward steel.
