Cell corrosion reduction
This is a continuation-in-part of application U.S. Ser. No. 629,011 filed July 9, 1984, now abandoned. 
    
    
     This invention relates to methods utilized in the reduction of corrosion and gassing in aqueous electrochemical cells particularly in alkaline type cells having zinc anodes. 
     A problem in aqueous electrochemical cells has been the evolution of hydrogen gas in the sealed cell container. Such gassing has resulted in corrosion, leakage of the electrolyte from the cell, cell container deformation and rupture, and a possible hazard when the cell is disposed of in a fire. Various expedients have been utilized in preventing, minimizing and controlling such hydrogen gas evolution and its consequences. Such expedients have included mechanical means such as vents and additional volume for storing the hydrogen without excessive pressure build up. Chemical expedients have included: corrosion and gassing inhibitors such as lead, indium, tin, cadmium, bismuth, thallium, and gallium; hydrogen getters such as rare metals supported on oxides such as platinum supported on aluminum oxide or palladium or finely dispersed nickel mixed with polytetrafluoroethylene and manganese dioxide, and hydrides such as LaNiH; hydrogen recombination with oxygen particularly utilized in nickel cadmium cells; and removal of chemicals such as chlorides from the surface of the anode metal which tend to accelerate corrosion. The most common, most effective and the oldest expedient (particularly in alkaline electrolyte cells) has been the utilization of mercury to amalgamate the anode metal such as zinc to increase the normally high hydrogen overpotential and to provide for a uniform equipotential surface on the anode metal. Recently, with the increase of environmental concerns, reduction or elimination of mercury without substantial concomitant increase in cell corrosion or gassing has been vigorously pursued. For example, in U.S. Pat. No. 4,487,651 assigned to the same assignee as the present invention, the amount of mercury utilized for protective amalgamation of zinc anodes is disclosed as being reduced from about 7% to about 4% by the use of single crystal zinc particles without ill effect, i.e. without increase in gassing. 
     It is an object of the present invention to provide a means for reduction or elimination of mercury in cell anodes without loss of corrosion protection and increase in cell gassing. 
     This and other objects, features and advantages of the present invention will become more evident from the following discussion. 
     Generally the present invention comprises a method for making an electrochemical cell subject to reduced gassing by means of utilization of specific materials in specific states; such materials and the cell itself. The method is particularly applicable to a cell having an anode comprised of a mercury amalgamated powdered metal such as zinc. In the method of the present invention the powdered metal is substantially formed into individual discrete single crystal particles and a small amount of one or more of indium, cadmium, gallium, thallium, bismuth, tin, and lead is added to the anodic material i.e. the powdered metal (amalgamated or unamalgamated) or to mercury which is then amalgamated with the powdered metal. The mercury and the additive, in this latter procedure, generally form a surface alloy on each of the particles. Alternatively, the powdered anode material, such as zinc, is prealloyed with a small amount of one or more of indium, cadmium, gallium, thallium, bismuth, tin, and lead and then formed into the individual discrete single crystal particles which are thereafter amalgamated with mercury. 
     Though the use of indium and/or other additives has been known to effectively permit some reduction of mercury content in polycrystalline metal anodes without detrimental increase in gassing, the effect of utilizing single crystal metal anodes with contained indium or other additives is significantly greater. The utilization of an indium additive with polycrystalline zinc amalgam anodes permits the reduction of mercury from about 6-7% to about 3.5%. However, in accordance with the present invention, the use of a single crystal zinc amalgam with an indium additive or a single crystal alloy of the zinc with indium and/or other additives unexpectedly permits the effective reduction of the mercury to about 1.5%. 
     The single crystals of zinc or of the zinc alloy are preferably prepared by the method disclosed in the aforementioned U.S. Pat. No. 4,487,651 (the disclosure of which is incorporated herein by reference thereto) and comprises initially forming a thin skin crucible on each of the zinc or zinc alloy particles by oxidation in air at a temperature just below the melting point (419° C.) of the zinc or zinc alloy (somewhat different oxidation temperature depending upon the alloy formed and its melting point), heating of the oxide-skin enclosed zinc or zinc alloy particles in an inert atmosphere above the melting point of the zinc or zinc alloy and slow cooling thereafter with removal of the oxide skins. Anode metal particle sizes generally range between 80 and 600 microns for utility in electrochemical cells and such method provides an effective means for making single crystal particles of such small dimensions. Such particles are discrete individual single crystals which is to be differentiated both physically and electrochemically from the electrodeposited needles and fibers described in U.S. Pat. Nos. 3,853,625 and 3,844,838 wherein each of the needles or fibers is comprised of one or more single crystals as a spine and polycrystalline poly-directional side growths. 
     Generally the amounts of indium or other additive added to the anode metal or anode metal amalgam or originally alloyed with the anode metal prior to formation of the single crystals may range between 25-5000 ppm and preferably between 100-1000 ppm. Such material may be directly added to the mercury itself. For example indium is highly soluble in mercury and can be directly added thereto in the form of powder or granules. Alternatively, the additive may be plated on the surface of the anode metal from salts of the additive prior to amalgamation with mercury. Such salts include the halides, particularly chlorides, oxides and acetates of the materials such as indium. It has been discovered that the additives such as indium whether by addition to the mercury or by plating on the single crystal anode metal particles do not in fact disrupt the single crystal nature thereof to any detrimental extent as may have been expected. In fact it has been discovered that the additives may form part of the single crystal as a prealloyed anode metal-additive material. The single crystal components of the fibers or needles of said U.S. Pat. Nos. 3,853,625 and 3,844,838 cannot, by their described method of formation by electrodeposition, be an alloy, since only one metal is electrodeposited and such metal is described as being exceedingly pure. 
     The amount of mercury in the anode amalgam may range from 0-4% depending upon the cell utilization and the degree of gassing to be tolerated. 
     The amalgamated single crystal metal particles with additives or prealloyed inclusions of materials such as indium are then formed into anodes for electrochemical cells particularly alkaline electrochemical cells. Such cells generally have anodes of zinc and cathodes of materials such as manganese dioxide, silver oxide, mercuric oxide and the like. Electrolytes in such cells are generally alkaline and usually comprise hydroxide solutions such as of sodium or potassium hydroxide. Other anode metals capable of being formed into single crystal powders and which are useful in electrochemical cells include Al, Cd, Ca, Cu, Pb, Mg, Ni, and Sn. It is understood that with anodes of these metals the additive is not the same as the anode active material but is less electrochemically active. 
    
    
     The effects of the present invention can be more clearly evidenced by consideration of comparative gassing rates and discharge capacities as shown in the following examples. It is understood that such examples are for illustrative purposes and are not to be construed as a limitation on the present invention. In the examples as well as throughout this discussion all parts are parts by weight unless otherwise indicated. 
     EXAMPLE 1 
     Zinc powder amalgams containing 1.5% mercury are made with polycrystalline zinc alone, polycrystalline zinc with 0.1% indium as an additive element with the mercury, single crystal zinc, and single crystal zinc with 0.1% indium as an additive element with the mercury. Equal amounts of the amalgam powders are then placed in equal amounts of 37% KOH alkaline solution (typical electrolyte solution of alkaline cells) and tested for gassing at a temperature of 71° C. The amount of gassing, measured in microliters/gram per day (uL/g-day) and the rate reduction factors (with the polycrystalline zinc control being 1) are set forth in Table 1: 
     
                       TABLE 1                                                     
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                            RATE                                          
                  GASSING   REDUCTION                                     
ANODE MATERIAL    RATE      FACTOR                                        
______________________________________                                    
Polycrystalline zinc, 1.5% Hg                                             
                  295       1                                             
Polycrystalline zinc, 1.5% Hg                                             
                  105       2.8                                           
0.1% indium                                                               
Single crystal zinc, 1.5% Hg                                              
                  140       2.1                                           
Single crystal zinc, 1.5% Hg                                              
                   30       9.8                                           
0.1% indium                                                               
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     A rate reduction factor (if any) would at most have been expected to be about 5.9 (2.8×2.1) for a combined utilization of single crystal zinc and indium with a gassing rate reduction to about 50 uL/g-day. The combination however synergistically reduces the gassing to about double the expected reduction. 
     EXAMPLE 2 
     Fifteen amalgams of single crystal zinc particles with various combinations of additive materials of indium, thallium, gallium and lead are prepared and tested for corrosion. The additive materials are plated on the zinc particles from salts thereof. All the amalgams contain 1.5% by weight of mercury. The amounts of each of the additive materials (designated by &#34;+&#34; and &#34;-&#34; if not present) are 0.1% indium (In), 0.05% thallium (Tl ), 0.01% gallium (Ga), and 0.04% lead (Pb). Two grams of each of the amalgams are placed in a 37% KOH electrolyte solution with gassing at the end of 24 and 48 hours being measured at 90° C. as being representative of corrosion. As controls two amalgams are made with one containing no additive material but with 1.5% mercury and the other containing polycrystalline zinc with 7% mercury similar to that commonly used in alkaline type cells. Results of such tests are given in Table 2. 
     
                       TABLE 2                                                     
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                         VOLUME OF GAS                                    
ADDITIVE ELEMENT         (mL), 90&#39;C.                                      
In   Tl     Ga     Pb            24 Hours                                 
                                         48 Hours                         
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+    -      -      -             0.17    0.51                             
-    +      -      -             0.13    0.49                             
-    -      +      -             0.42    1.35                             
-    -      -      +             0.40    1.15                             
+    +      -      -             0.13    0.36                             
+    -      +      -             0.17    0.47                             
+    -      -      +             0.15    0.42                             
-    +      +      -             0.12    0.40                             
-    +      -      +             0.12    0.56                             
-    -      +      +             0.40    1.13                             
+    +      +      -             0.13    0.42                             
+    +      -      +             0.11    0.32                             
-    +      +      +             0.15    0.60                             
+    -      +      +             0.14    0.44                             
+    +      +      +             0.10    0.28                             
-    -      -      -     (control)                                        
                                 0.41    1.39                             
Control 7% Hg--polycrystalline Zn                                         
                     0.16      0.43                                       
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     Indium and/or thallium as shown in the above table provide the most efficacious reduction of gassing and are thus preferred embodiments of the present invention. However increase in percentage of gallium or lead is expected to provide similar effects. Other materials similar in effect to indium such as cadmium, tin and bismuth may be similarly expected to provide the enhanced effect of the present invention. 
     EXAMPLE 3 
     Seven zinc amalgams containing 1.5% Hg are prepared with three being comprised of single crystal zinc (made as described above) and four being comprised of polycrystalline zinc. The amalgams containing single crystal zinc comprise two having 0.1% indium (one added to the mercury prior to amalgamation and the other being plated on the zinc particle surfaces prior to amalgamation) and one having no indium. The polycrystalline zinc amalgams include three without lead for direct comparison with the single crystal zinc which contains no lead and one polycrystalline amalgam with lead as commonly utilized in electrochemical cells. The lead free polycrystalline amalgams are directly analagous to the three single crystal zinc amalgams. The polycrystalline leaded zinc amalgam contains 0.02% indium. A control of polycrystalline zinc with lead and 7% mercury (as commonly utilized in alkaline cells) is also prepared. Two gram samples of each of the above amalgams are tested for gassing at elevated temperatures (71° C. and 90° C.) for varying time periods with total gas volume and gassing rates being comparatively determined as set forth in Table 3 below. 
     EXAMPLE  4 
     Cells are made with the amalgams described in Example 3 of single crystal zinc with 0.1% indium (both types), polycrystalline zinc with 0.1% indium plated on the zinc particles, polycrystalline zinc with 0.02% indium added to the mercury, and a control of polycrystalline zinc with no indium and 7% mercury (a typical alkaline cell). Each of the cells of AA standard size contains a 2.7 gram anode (1.75% starch graft copolymer gelling agent), 2.6 grams of a 37% KOH electrolyte and a manganese dioxide cathode with the cell being anode limited. Five cells are tested for gassing for varying periods at 71° C. without discharge and five are similarly tested but after partial discharge at 3.9 ohm for one hour. Total gas volume and gassing rates are set forth in Table 3 below. 
     
                                           TABLE 3                                 
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(GASSING)                                                                 
                                                  POLYCRYSTALLINE         
                                                  ZINC (With Pb)          
          SINGLE CRYSTAL ZINC POLYCRYSTALLINE ZINC (No Pb)                
                                                         No Indium        
          No Indium                                                       
                0.1% Indium                                               
                       0.1% Indium                                        
                              No Indium                                   
                                    0.1% Indium                           
                                           0.1% Indium                    
                                                  0.02%                   
                                                         7% Hgm           
          1.5% Hg                                                         
                in 1.5% Hg                                                
                       +1.5% Hg                                           
                              1.5% Hg                                     
                                    in 1.5% Hg                            
                                           +1.5% Hg                       
                                                  1.5% Hg                 
                                                         Control          
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GAS VOLUME mL/2 g AMALGAM (OUT-OF-CELL)                                   
DAYS (71° C.)                                                      
 7        1.55  0.44   0.41   4.10  1.50   1.45   1.21   0.49             
14        3.52  0.95   0.98   7.73  2.52   2.68   2.49   0.92             
21        5.88  1.57   1.70   --    3.60   3.96   3.91   1.42             
28        7.75  2.25   2.36   --    4.64   5.11   5.29   1.96             
IN-CELL (UNDISCHARGED)                                                    
 0        --    0.17   0.19   --    --     --     0.29   0.28             
 7        --    0.48   0.37   --    --     0.40   0.78   0.37             
14        --    0.56   0.58   --    --     0.76   0.94   0.47             
28        --    0.92   0.94   --    --     1.00   1.28   0.71             
IN-CELL (DISCHARGED AT 3.9 OHM FOR ONE HOUR)                              
 0        --    0.21   0.18   --    --     0.28   0.23   0.18             
 7        --    0.31   0.30   --    --     0.60   0.91   0.39             
14        --    0.43   0.42   --    --     1.02   1.25   0.49             
28        --    1.01   0.98   --    --     2.53   4.37   1.00             
HOURS AT 90° C.                                                    
(OUT-OF-CELL)                                                             
24        0.53  0.20   0.20   1.89  0.47   0.60   0.50   0.17             
48        1.88  0.68   0.69   4.73  1.21   1.49   1.20   0.56             
GASSING RATE uL/g-day (OUT-OF-CELL)                                       
DAYS (71° C.)                                                      
0-7       111   31     29     293   107    104    86     35               
7-14      141   36     41     259   73     88     91     31               
14-21     169   44     51     --    77     91     101    36               
21-28     133   49     47     --    74     82     99     39               
0-14      126   34     35     276   90     96     89     33               
14-28     151   46     49     --    76     87     100    37               
0-28      138   40     42     --    83     91     94     35               
IN-CELL (UNDISCHARGED)                                                    
0-7       --    16     10     --    --      7     26      5               
7-14      --     4     11     --    --     19      8      5               
0-14      --    10     10     --    --     13     17      5               
14-28     --    10     10     --    --      6      9      6               
0-28      --    10     10     --    --     10     13      6               
IN-CELL (DISCHARGED AT 3.9 OHM FOR ONE HOUR)                              
0-7       --     5      6     --    --     17     36     11               
7-14      --     6      6     --    --     22     18      5               
0-14      --     6      6     --    --     20     27      8               
14-28     --    15     15     --    --     40     83     13               
0-28      --    11     11     --    --     30     55     11               
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     EXAMPLE 5 
     Cells are made as in Example 4 and are each discharged to various cutoff-voltages with a 3.9 ohm load with the capacities in service hours set forth in Table 4 below. 
     
                                           TABLE 4                                 
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(Discharge Characteristics)                                               
(Service Hours at 3.9 ohms)                                               
       SINGLE CRYSTAL ZINC                                                
                     POLYCRYSTALLINE ZINC                                 
CUT-OFF                                                                   
       0.1% In +                                                          
              0.1% In                                                     
                     0.02% In                                             
                          0.1% In +                                       
                                No In                                     
VOLTAGES                                                                  
       1.5% Hg                                                            
              in 1.5% Hg                                                  
                     1.5% Hg                                              
                          1.5% Hg                                         
                                7% Hg Ctrl                                
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1.2    0.660  0.668  0.533                                                
                          0.688 0.623                                     
1.1    1.616  1.638  1.457                                                
                          1.712 1.553                                     
1.0    2.826  2.859  2.480                                                
                          2.926 2.598                                     
0.9    3.535  3.652  3.235                                                
                          3.619 3.295                                     
0.8    3.800  3.967  3.498                                                
                          3.883 3.599                                     
 0.65  3.986  4.179  3.594                                                
                          4.020 3.684                                     
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     EXAMPLE 6 
     Polycrystalline zinc is prealloyed with 550 ppm of gallium and 100 ppm of indium. A first sample thereof is then amalgamated with 1.5% mercury. A second sample is made into individual single crystal alloy particles, as described above, prior to the amalgamation with mercury. Two grams of each of the samples are placed in a 37% KOH electrolyte solution with gassing at the end of 24 and 48 hours being measured at 90° C. as being representative of corrosion. As control an amalgam is made with polycrystalline zinc with 7% mercury similar to that commonly used in alkaline type cells. Results of such tests are given in Table 5. 
     
                       TABLE 5                                                     
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             VOLUME OF GAS (mL), 90&#39;C.                                    
SAMPLE         24 Hours    48 Hours                                       
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polycrystalline alloy                                                     
               0.7         1.9                                            
single crystal alloy                                                      
               0.3         1.0                                            
control (7% Hg)                                                           
               0.2         0.5                                            
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     EXAMPLE 7 
     A first portion of polycrystalline zinc powder containing 0.04% lead is amalgmated with 2% Hg and a second portion is converted to individual single crystal particles prior to the amalgamation. The amalgams are then tested for corrosion rate in 10M KOH containing 2% ZnO. The gassing rates at 71° C. are 225 uL/gm per day and 80 mL/gm per day respectively. 
     It is evident that the corrosion reduction of anode metals such as zinc by the prealloying with corrosion reducing additive materials is greatly enhanced by the formation of single crystals from the anode metal-additive alloy. 
     It is further evident from the above examples and tables that the single crystal zinc with one or more additives of the present invention, whether as part of the single crystal structure or as an additive thereafter, is markedly effective in permitting large mercury reductions without significant increase in cell gassing while at the same time enhancing cell discharge characteristics when such cells are compared to the current commercial alkaline cells having high mercury amalgam content. 
     It is understood that the above examples are for illustrative purposes only and details contained therein are not to be construed as limitations on the present invention. Changes in cell construction, materials, ratios and the like may in fact be made without departing from the scope of the present invention as defined in the following claims.