CA1214207A - Galvanic element - Google Patents

Abstract

. GALVANIC ELEMENT

Abstract of the Disclosure The zinc corrosion which prevails in galvanic cells having negative zinc electrodes, and which is accompanied by H2 gas evolution, can be effectively prevented by the use of an anode zinc which is either free of mercury or at most contains 1% by weight of mercury, and in which only the electrode take-off conductor is amalgamated. It generally takes the form of a nail, wire or the housing lid of button cells and can consist, among other things, of copper, brass, bronze, copper plated iron, alloy steel, or a Cu-Fe-Ni trimetal. Zinc plating may be performed prior to amalgamation of the take-off conductor.

Description

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.

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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 -'I

~23 4;~7 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.

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 , 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 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 ~Z~4~
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 .

.. . .. . . . . ..

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 1~4~(~7 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.

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 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.

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Claims (7)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. 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-off conductor has been amalgamated separately from the zinc electrode.

2. The galvanic element according to claim 1, wherein the zinc electrode contains from 0.05 to less than 0.5% by weight of mercury.

3. The galvanic element according to claim 1, wherein the mercury-free zinc is doped with silver.

4. The galvanic element according to claim 1, wherein the amalgamated base metal of the take-off conductor is selected from the group consisting of copper, brass, nickel, copper-plated iron, nickel-plated iron, gold-plated iron, alloy steel, or a tri-metal.

5. The galvanic element according to claim 2, wherein the amalgamated base metal of the take-off conductor is selected from the group consisting of copper, brass, nickel, copper-plated iron, nickel-plated iron, gold-plated iron, alloy steel, or a tri-metal.

6. The galvanic element of claim 4, wherein the base material has been zinc-plated prior to amalgamation.

7. The galvanic element according to claim 4, 5 or 6, wherein the mercury-free zinc is doped with silver.