Note: Descriptions are shown in the official language in which they were submitted.
12~Z~7
The invention relates to a galvanic element with negative zinc
electrode provided with a take-off conductor.
Because of the thermodynamic instability of zinc, particularly in
alkaline media, zinc electrodes in galvanic cells are subject to a corrosion
process which shortens the life of the cells. The oxidation and dissolution
of the zinc is further accelerated by foreign metal impurities on its
surface. This is because these impurities, which consist mainly of more
noble metals, form galvanic pairs with the zinc such that the zinc corrodes
at the contact points with the foreign metal and hydrogen evolves at the
impurities, so that a bulging of the cells takes place see for example
H. Rem, Lehrbuch don Anorganischen Chemise II (English translation: Text-
book of Inorganic Chemistry II) Akademische ~erlagsgesellschaft Guest
Porting KEG., Leipzig 1961, Pages 512 and 513).
Although the refined zinc which is generally used for battery
purposes is derived through distillation of the zinc ore, its content of
impurities, particularly of the heavy metals Pub, Cud, Sun, Cut Co, hi, Sub,
and Fe, of which Fe, Co and Nix are particularly deleterious, still amounts
collectively to a few tenths of a percent.
To reduce the harmful effects of these impurities, it is
customary to amalgamate the zinc (mercury content. 3 to 8% by weight).
The amalgamated zinc has a substantially higher hydrogen over-po~ential
than mercury-free zinc. Therefore, the formation of the previously
mentioned galvanic pairs or local elements is then no longer possible upon
the uniformly amalgamated electrode surface.
For reasons of health, however, one desires, to replace
the mercury with other substances having inhibiting
fact, and yet in so doirlg to provide zinc electrodes ox it
least equal electro-chemical properties to the amalgamated
ones. Known proposals include among others alloy additives, in
which case it is necessary to select metals which, themselves
have a high hydrogen over-potential and also additives to the
electrolyte such as Nays or K2Cr2O7, which becomes
transformer into stable cover layers of Ins or Cry upon
the zinc surface. when using organic inhibitors in alkaline
electrolytes, e.g. p-dicyclohexylbenzol in accordance Ruth U.S.
Patent 3,281,276 there are probably also formed monomolecular
cover layers upon the metal which are attributable to the
adsorption or commiseration of the organic compound upon the
metal surface.
It is not unusual or this to lead to a blocking of
the electrode activity.
Apart from the fact that many organic inhibitors are
not without physiological danger/ the literature also provides
indications that their effectiveness is limited. For example,
from German Patent 2,246,753, ethylene oxide polymers can be
derived as inhibitors, whose use concentration of 0.1 to 1% by
weight relative to the zinc is obviously not sufficient to
permit dispensing completely with amalgamation. The mercury
content is reduced only from 8 to I by weight.
indeed, investigation of the corrosion properties of
non amalgamated zinc in an alkaline medium, with determination
of the developer hydrogen annotate, yielded a corrosion rate
that was definitely below some literature data which are valid
for amalgamated or otherwise inhibited zinc (e.g. F. Mans~eld
and 5. Oilman: J. Electrochem. Sock 117 (1970) 58~, or R.N.
Snyder and Jo Lander: Electrochem. Tuitional. 3, 1/2 (1965)
Jo
page 161). Thus, the corrosion rate of 0.0065 my Zn/cm2 . pa
h determined for the unprotected zinc itself contrasts Whitney
widely dispersed literature values from 0.14~ to 4,200 my
Zn/cm . 24 h for protected zinc samples.
In battery practice, however, until now there has
always been used amalgamated or inhibited zinc, because other
techniques did not make it possible to prevent gassing in the
cell. The unsatisfactory experiences which were generally
connected with prior attempts to achieve a good and long
persistent corrosion stability of zinc electrodes both in
alkaline and also in weak acid electrolytes, gave rise to the
objective of providing the most practical possible technique
of corrosion inhibition that would also take into account the
increased concern for environmental protection.
In accordance with the invention, there is
provided a galvanic element with a negative zinc electrode
and a take-off conductor, wherein the zinc electrode is made
of mercury-free zinc powder or of zinc powder with a mercury
content of at most 1% by weight, and wherein the surface of
the metal which constitutes the take-of-f conductor has been
amalgamated separately from the zinc electrode.
The anode zinc which is preferably used is
mercury-free and can if desired, contain 0.05 to 0.5% by
weight of mercury.
For further details, reference is made to the
discussion which follows, in light of the accompanying drawing,
wherein the single figure shows the results of performing
various tests relative to the invention.
I, ;"
I
It has been found that zinc corrodes especially
strongly when in contact with take-off conductors,
independently of its metallurgical preparation and purity
and whether amalgamated or mercury-free, because at the
contact point between zinc and take-off conductor the hydrogen
I pa -
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over-potential is reduced through formation o-f local elements. By
the technique embodying the invention there is evidently achieved
a quasi-homogeneity of the electro-chemical potentials of take-off
conductor and anode zinc, so that the cell becomes less susceptible
to hydrogen gassing. Presumably, this is because the hydrogen over-
potential of the take-off conductor metal, which is low relative to
the anode zinc, is raised through the amalgam formation.
Indeed, by experimentation, it can be specifically shown
how strong the zinc corrosion is under the influence of different
lo take-off conductor materials. This shows a dependence of the zinc
corrosion rate, in the sense that it increases with the sequence of
test materials gold, copper, nickel, brass.
To perform these tests there are introduced into a 300
ml Erlenmeyer flask 50 g of New Jersey zinc powder of grain size
200-500,u free of mercury under argon as protective atmosphere.
These are overlaid with 300 ml of 8 n KOCH, and then are contaminated
over a period of several weeks with a metal sheet of one of the
above-mentioned materials yin case of gold a gold-plated nickel sheet)
of 1 cm size. By running in parallel a series of tests with
respective amalgamated sheet sample sin completion of the technique
embodying the invention substantial reduction of the anodic zinc
dissolution is observed evidenced by greatly diminished or almost
stopped gas evolution. The quantity of zinc which has gone into
solution from the respective zinc powder used is determined by
quantitative analysis with 1/10 molar disodium salt of ethylene
Damon - tetraacetic acid EDIT) in the presence of eriochrome black
T as indicator.
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37
The table which follows gives the results of tests
using different takeoff conductor materials. Zen it tune pure
zinc powder Jithout contact with a foreign metal (reference
material) and the additive thug indicates a respective
amalgamated metal sheet. The amalgamation is carried out in
known manner by permitting an aqueous mercury (II) chloride
solution (27.1 grams HgC12 in 1 liter HO) to operate upon
the sheets which 'nave previously been etched with 0.2 normal
Focal As the amalgamating solution there cyan also be used a 1
to 10~ mercury (II) nitrate solution which is acidulated with
nitric acid to prevent hydrolysis.
Tall
Influence of various takeoff conductor metals upon the
corrosion rate of the zinc powder in 8 normal KOCH.
Take-Offmg of dissolved zinc/50 g weight
Experiment Conductor Duration of experiment (hours)
' No. Material 24 48 168 240 484
.
1 Zen .,..........45.149.7 68.7 79 102
2 Owe 113.8148.~225.6 305 360
3 Cut OWE 68.785.7 143.2 175 210
4 , Cut (Hug) ....... 47.149.7 68.7 79 102
- Brass .... I. 98.1 150.3 229.5 325 399
6 Brass (Hug) . 45.149.7 68.7 85 115
7 A ............ 78.5 87.0 107.9 120 153
8 A (Hug) ....... 45.1 49.7 68.7 79 102
The drawing illustrates the increase in corrosion
(dissolved zinc (in mg/50 y Zen) as a function of time (in
hours) following the number sequence of the foregoing Table, in
,
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r~7
series of graphs with the various take-off conductor metals-
as the variable. The numbers of the curves correspond Jo the
experiment numbers used on the Table.
As the significant result, it becomes evident that tune
zinc corrosion upon contamination with the amalgamated sheets
(solid curves) is strongly reduced, compared with the
non amalgamated ones (dotted curves) and remains, over the
entire duration of the experiment, close to the uniformly loo
level of the reference test sample (without sheet).
Separately performed investigations further warrant
the conclusion that a zinc powder which is amalgamated with 6
by weight of mercury does not differ in its corrosion
characteristics from the mercury-free Jew Jersey zinc of the
reference sample This -finding is of significance for the
technique embodying the present invention in that amalgamation
of the zinc electrode itself proves to be unnecessary, since it
does not contribute further to the corrosion inhibition which
it is the present objective to achieve. wherefore, the
quantity of mercury which is conventionally used fur that
purpose can be saved.
Because the mercury in the zinc does not take part in
the discharge, and therefore does not itself make any
contribution to capacity, the zinc electrode becomes lighter
through the absence of mercury and does not create the risk of
short circuiting during discharge due to running drops o-f
mercury. In contrast/ amalgamation or inhibition through
"exotic" additives makes the zinc needlessly expensive.
If one starts with the 102 my of Zen dissolved after
~84 test hours from 50 g of non-contaminated zinc powder
(drawing, curve 1), then the zinc quantity dissolved in the
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Lo
tame time period using a gold-plated takeoff cond~lctof al~.oll~t~
to 1.5 times as much (curve I using a Cut take-off kinkier
it amounts to 2 times as much (curve 3), using a joy takeoff
conductor it amounts to approximately 3.5 times as much (curve
2), and using a brass take-off conductor 4 times as much (curve
5).
For takeoff conductors used in electro-chemical cells
one usually deals with nails (dry cells), wires (simultaneously
providing the cell connectors in multi-cell dry batteries), or
housing elements of button cells (ego lids. In large
quantities their amalgamation takes place, with the use of a
transport mechanism, or example an endless welt prorated to
be studded with nails, which passes by various treatment
stations during its travel, but in such a manner that the heads
of the nails remain excluded from amalgamation.
The overall treatment includes the following steps:
1. Decreasing
2. Etching
3. Amalgamating
4. Washing
5. Drying
The preparatory steps of decreasing and etching
contribute to the success of the amalgamation in accordance
with the invention in that the uniformity of the amalgam layer
is enhanced. Jon decreased surfaces of take-off conductors,
among which there can also be counted woven metal mesh, woven
metal baskets, expanded metal and perorated sheet metal, are
only incompletely wetted by Hug - salt solutions or coated by
Hug Likewise, metal surfaces with thin oxide coatings can not
be amalgamated, which is why etching treatment has -the purpose
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of removing the oxide layers. Uporl incompletely precleaned
metal surfaces there appears a spotty deposition of the
mercury; the resulting Sirius non-homogeneities can lead to
the formation of local elements resulting in undesired, Ho -
evolution and Zen corrosion. If, for example, an amalgamated Cut
take-off conductor (nail) is pressed into the Zen mass without
having been properly pretreated or having been completely
untreated, the on reduces the Cut or Queue which is still
present in some locations, so that bare Cut metal is freed-up-in
fine subdivision and with an undesirably low Ho
over-potential 7
The takeoff conductors used should also exhibit
crack-free surfaces because it has been found that cracks
enlarge under the influence of the amalgamating solution.
The following details are provided for the
pretreatment
The decreasing takes place with the aid o-E inorganic
or organic fat solvents depending upon the properties of the
take-off conductor. If it consists of copper, brass or bronze,
the liquid of the bath should not exceed a pi value of 10. A
more strongly alkaline medium is particularly suitable when tin
is present as an alloy component in the take-of-F conductor
metal due to possible dissolution (formation of alkali
stagnates). The cleaning process can be promoted through
increased temperature, movement of the parts, or ultrasound.
The inorganic fat solvents are aqueous solutions with additives
of surface active materials, the organic ones include solvents
such as alcohol, ether, kitten, chlorinated carbohydrates as,
for example, trichloroethylene. Particularly desirable in this
case is steam decreasing because it inherently includes the
I
concluding step of every solvent decreasing, namely, the
evaporation of the solvent, and renewed contamination by the
dissolved impurities is avoided. After decreasing with aqueous
solutions there always takes place washing and drying
The etching treatment with acids, such as dilute Hal
or H2S04, can advantageously take place with the use of
etching buffers (e.g. Thor, triethanolamine) in order to
avoid needless metal removal. For take-off conductors of
copper, brass or bronze, mixed acids ox ~N03, Hal, H2S04 ,H3P~4
are recommended. To obtain particularly smooth surfaces, a
polishing etch can be added after the prewash.
An alternative etching treatment is represented by
oxide removal with the use of strong reducing agents such as
hydrazine, Burnett or hydrogen. -
The conclusion of each etching consists of threshing with flowing water and drying, if desired with methanol
or isopropanol.
or the subsequent amalgamation step there can be used
the media which are described below as examples a to h. It is
important that only a short immersion (cay. 3 seconds of the
objects into the liquids suffices. The immersion time,
however, should be doubled to 6 seconds if only half of the
mercury salt quantities indicated further below can be used in
each case. This amalgamation takes place without the
application o-f current but, in certain cases, it can also be
carried-out under short cathodic current load of the
components, as in electrolytic depositor. If only partial
amalgamation is desired, but a correspondingly limited
immersion it not possible or could only be carried-out by
complicated suspension means, then the portions of the tough
conductor which are not to be treated calm be masked by
.
.. . .. . . . . ..
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intentional surface oxidation or by coating ~rlith locator rJr
paraffin,
If appropriate, aqueous amalgamating liquids are
adjusted through acid addition to the acidity of an
approximately 1 normal strong acid in order to prevent
hydrolysis of the mercury salt (see examples a and b).
Examples for amalgamating baths: -
a) log Hg(NO3)2 x HO in 11 HOWE HNO3 -
additive
b) 7.5g HgC12 in 11 HO, Hal additive
c) 7-5g HgC12 -I I NH4Cl in 11 HO
d) 6g Ilg(CN)2 5g KIN in 11 H20
(Formation of complex mercury alkali cyanide o-E
type MlHg~C~)3 or Ml Hg(C~4)
e) HNO3 acidic HgSO~ solutions
) Liquid mercury
g) Liquid alkali amalgam
h) Mercury vapor in a vacuum
Because of the light sensitivity of the mercury salts,
their solutions should ye stored in the dark.
After amalgamation, thorough washing and drying are a
matter of course. Prompt processing of the takeoff conductors
is then recommended. During storage, direct sunlight, moisture
and heat above 70C is to be excluded
As an alternative to such a wet process, one can also
use vapor deposition in a vacuum, if desired, -first with zinc
and then with mercury.
The advantage of amalgamation, in accordance with the
invention, of the takeoff conductor for mercur~-Eree zinc
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electrodes which was initially demonstrate with prototype
tests was also confirmed by tests with cells from a plot
production run For twenty cells at a time prom a particular
production lot, there was measured as the indicator for storage
capability at 45 C the so-called growth rate, which defines
the magnitude, or rather the rate, of increase of the internal
cell pressure due to zinc corrosion. Because, within limits,
this pressure correlates with the overall height hi of the
cell, and a change in cell height of 0.10 mm corresponds to a
change in internal cell pressure of about 2.1 bar, the growth
rate hut Howe t is characterized by two measured
values It amounted to 0.00~9 mm/day 1.89 . 10 2 bar/day
for cells for mercury-poor (0.1% Hug) pasted zinc electrodes in
the presence of etched and amalgamated brass take-off
conductors (embodying the invention), in contrast to 0.0117
mm/day 24.6 . 10 2 Howard for cells with bare brass
takeoff conductors.
Accordingly, it is possible to strongly inhibit the
corrosion through use of previously amalgamated take-off
conductors, which further indicates that inhibition of the zinc
is achieved even with minimal mercury quantities. In addition
to the take-off conductor materials previously named there can
also be used nickel, eopper-plated iron, nickel-plated iron,
gold-plated iron, alloy steel, or a tri-metal, at least as the
material for the base layer to be amalgamated. For a tri-metal
with the layer sequence Cu-Fe-Ni, of which buttonhole lids,
for example, are made, only the Cut side is amalgamated and
oriented toward the zinc. In accordance with the invention,
all of the above-mentioned takeoff conductors can also be
amalgamated on a base of a previously performed zinc plating.
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f
n a particularly advantageous en odilrlent ox trio
invention there is used an anode zinc which has been
silver-plated for purpose of corrosion inhibition, either in
powder form or as paste. The silver plating of a mercury-free,
pure zinc powder can be performed as follows:
In the aqueous solution of SO g Nazi .
5 H20 in 100 cm3 cooled to 8-10 C, there is dissolved
0.230 g Ago 100 g zinc powder is added and shaken for about 3
minutes. The silver-plated zinc is washed without residue, and
is tempered for at least 90 hours at 60 C in 30~ KOCH
containing 3% Zoo. Thereafter, there takes place the washing
process with distilled water, and the fastest possible drying
in a vacuum at room temperature.
After prolonged operation of an alkaline zinc cell
which is provided with non-amalgamated negative current
takeoff conductors i.e. which does not embody the invention,
one observes a decrease in corrosion rate with simultaneous
precipitation of zinc onto the take-off conductor in the form
of a very smooth layer. However, during intermittent discharge
of the cell, and also in cyclical operation this zinc layer is
again dissolved away, so that there again occurs increased
gassing. In addition, there exists the danger that an oxide
layer which is initially present upon the take-off conductor,
e.g. Cut, will dissolve into the electrolyte lye, and then
later deposit again upon the zinc, thereby creating the
possibility that local elements may form.
Furthermore, if amalgamated anode zinc is used in the
cell in non-inventive manner, reaction processes are likely to
take place in the neighborhood of the negative electrode
according to the following formulation
- 13 -
I Znx Ivy 2x KOCH-- xK2Zrl02 zoo yo-yo
I yelp zCu Cut Hey
This would indicate that, before the formation of the
protective amalgam layer upon the take-off conductor, an
equivalent portion of the zinc goes into solution with hydrogen
evolution. This means first a zinc loss (cell discharge) and
also there is the danger that rapid closure of the cell will be
accompanied by an increase in the build-up of pressure and with
it a bulging of the cello
The disadvantages described are eliminated by the
technique embodying the invention. First, through take-off
conductor amalgamation the operational capability ox an
alkaline zinc cell with mercury-free anode zinc is virtually
assure, even as a secondary cell. The mercury quantity to be
used for that purpose amounts to tractions of the mercury
contained in conventional zinc electrodes. A galvanic element
embodying the invention is therefore environmental preferable
and simultaneously has a more favorable cost structure.
.
