Corrosion inhibitor system for alkaline aluminum cells
United States Patent Kordesch et al.  
 CORROSION INHIBITOR SYSTEM FOR ALKALINE ALUMINUM CELLS Inventors: Karl V. Kordesch, Lakewood; Akiya Kozawa, Middleburg Heights, both of Ohio Union Carbide Corporation, New York, NY.  
 Filed: Oct. 26, I972 App]. No.: 301,110  
 Assignee:  
 US. Cl. 136/100 R; 136/154 Int. Cl. .Q HOlm 11/00 Field of Search 136/86 A, 20, 83 R, 154,  
 136/100 R; 252/396, 389 R; 204/45 R, 58  
 &#39; References Cited UNITED STATES PATENTS 7/1928 Sicgmund 136/154 X Apr. 29, 1975 2.018.563 10/1935 Martus 136/154 x 3,043.898 7/1963 Miller 3,563,803 2/1971 Katoh 136/86 A OTHER PUBLICATIONS Horiguchi, K. et al., Denki Kagaku, 34(11), 911-913, (1966), (Japan).  
 Primary Examiner-Winston A. Douglas Assistan! Examiner,lohn F. Niebling Attorney, Agent, or Firm lohn R. Doherty 15 Claims, 6 Drawing Figures Scale for reading the [electrolyte level Thermometer Aluminum sheet anode gectrolyfe level 1111l1111l11 11l CORROSION INHIBITOR SYSTEM FOR ALKALINE ALUMINUM CELLS The invention relates to a corrosion inhibitor system for an alkaline aluminum galvanic cell.  
  The use of aluminum electrodes in cells employing alkaline electrolytes under limited conditions has been reported. For example, see Zaromb, U.S. Pat. No. 3,554,810, J. Electrochem. Soc. 109, No. 12, 1125 (1962), and ibid 109, No. 12,1191 (1962);Zaromb et al., ibid 110, No. 4, 267 (1963); and Katoh, U.S. Pat. No. 3,563,803. One of the problems encountered when aluminum electrodes are employed in alkaline cells is corrosion of the aluminum, especially when the cell is on open circuit or when it is discharging at a low rate such that the current density on the aluminum electrode is below about 20 milliamperes per square centimeter. This corrosion problem has prevented the wide commercial use of alkaline aluminum cells.  
  It has been proposed to employ certain inhibitors in the alkaline electrolyte as a means for controlling the above-discussed corrosion problem. For instance, in U.S. Pat. No. 3,563,803, Katoh has proposed the use of plumbites, plumbates, or stannates in the electrolyte. The present invention is based on the discovery that an inhibitor system including an alkali metal citrate plus a lead compound or a tin compound or both a lead compound and a tin compound provides more effective and practical corrosion inhibition for alkaline aluminum galvanic cells when the cells are on open circuit and when they are discharging (at both low and high rates of discharge), than the corrosion inhibitor systems heretofore proposed. Accordingly, it is an object of this invention to provide a corrosion inhibitor system for use in alkaline aluminum galvanic cells to inhibit wasteful corrosion of the aluminum electrode, both when the cell is on open circuit and when it is discharging.  
  This and other objects of the invention are accomplished by the provision of a corrosion inhibitor system for alkaline aluminum galvanic cells, which system comprises an alkali metal citrate plus either a lead compound, a, tin compound, or both a lead and a tin compound.  
  The principles of the invention will be described in detail hereinafter with reference to the accompanying drawings, wherein:  
  FIG. 1 is a cut-away front elevations view of one type of cell which embodies the principles of the invention;  
  FIG. 2 is a partially schematic sectional side view of the cell of FIG. 1; and  
  FIGS. 3-6 are graphs on which electrolyte levels are plotted versus current for the cell of FIGS. 1 and 2, with four different electrolytes.  
  In FIGS. 1 and 2, an aluminum-air cell is shown. The cell includes an aluminum sheet as the anode, two air electrodes 12 and 14, and polypropylene grid spacers 16 and 18. The cell also includes aqueous alkali metal hydroxide electrolyte 20, which contains the corrosion inhibitor system of the invention. The cell container 22 is made of a meterial that is inert to the electrolyte, such as polymethyl methacrylate plastic.  
  The corrosion inhibitor system of the invention is applicable to known types of alkaline aluminum cells. In such cells, the anode is preferably made of aluminum having a purity of at least 99.99 per cent. The electrolyte is aqueous alkali, e.g., sodium hydroxide or potassium hydroxide, preferably potassium hydroxide, in concentrations of from about 4N to about 9N.  
  The preferred cathode is an oxygen-depolarized electrode, often referred to as an air electrode. The air electrode can be any of the types that are known in the art, for instance, the air electrode can be a porous acti vated carbon plate, a phenolic resin-bonded carbon plate, or a thin, flat, plastic-bonded carbon plate of the fixed zone type as disclosed by Darland et al. in U.S. Pat. No. 3,423,247. The air electrode can also be constructed of a sintered metal such as sintered nickel or silver. Customary air electrode catalysts can be used, such as A1 0 CoO spinel, silver, noble metals, and ferric phthalocyanine. The preferred air electrode is a thin, flat, plastic-bonded carbon electrode of the fixed zone type employing ferric phthalocyanine catalyst. Other cathodes, such as manganese dioxide cathodes, can also be used in the cell.  
  The corrosion inhibitor system of the invention includes an alkali metal citrate such as sodium citrate or preferably, potassium citrate. The system also includes either a lead compound, a tin compound, or both. Illustrative lead compounds which can be employed include alkali metal (e.g., sodium or potassium) plumbites and plumbates, lead oxide, and lead acetate. Illustrative tin compounds include alkali metal (e.g., sodium or potassium) stannates and tin oxide. The preferred compounds for use in the corrosion inhibitor system of the invention are potassium citrate, lead acetate, and potassium stannate.  
  The corrosion inhibitor compounds are employed in amounts effective to inhibit corrosion of the aluminum electrode in alkaline aluminum galvanic cells when the cell is on open circuit and when the cell is discharging at a rate such that the current density on the aluminum electrode is below about 20 milliamperes per square centimeter. (Corrosion inhibition at higher discharge rates is also obtained, although corrosion is less of a problem at higher discharge rates.) As an illustration of the effective amounts of the corrosion inhibitors, the alkali metal citrate can be employed in the electrolyte in an amount within the range of from about 0.5 per cent to about 10 per cent, and preferably from about 0.5 per cent to about 2 per cent. The tin compound can be employed in the electrolyte in amounts of from about 0.5 per cent to about 5 per cent, and preferably from about 0.5 per cent to about 2 per cent. The lead compound can be employed in amounts of from about 0.1 per cent to about 2 per cent, and preferably from about 0.5 per cent to about 2 per cent, as lead. All percentages are based on weight of electrolyte.  
  Alkaline aluminum cells inhibited against corrosion in accordance with the invention are particularly useful as reserve cells of high ampere-hour capacity.  
 The Examples set forth below illustrate the invention.  
 EXAMPLE 1 Utilization Test Using a pure aluminum (99.99%) sheet (5 cm X 15 cm X 2.0 mm) as the anode, and two 7 cm X 6.5 cm X 1 mm fixed zone, plastic-bonded carbon air cathodes, various aqueous electrolytes (shown below in Table I) were tested in a box type aluminum-air cell (shown in FIGS. 1 and 2). The total area of the aluminum anode that was in contact with the electrolyte was square centimeters. The cell was operated at a constant current (4.0 amperes) for 2 hours and the weight decrease of the aluminum anode was measured. The utilization shown in Table l was calculated from the weight loss based on the three-electron process (Al Al 3e). It should be noted that in pure 6 M KOH (no ad- Electrolyte: 50 cc of6 M aqueous KOH with or without additives.  
  Procedure: At 24C (room temperature), the cell was&#39;filled with electrolyte. A constant current of 4.0  
 ditives), the cell temperature rose to 45C. within 20 amps was passed for 10 to 12 minutes until the cell minutes. Once the cell temperature exceeded 45C., electrolyte temperature reached 40C. or a stable lower the anode corrosion became uncontrollably heavy and temperature level. Then the circuit was open and the hydrogen evolution was so violent that it was impossielectrolyte level was read (recorded as current (0) in ble to continue cell operation. When the electrolyte FIGS. 3-6). Another reading was taken -60 seconds contained additives which were effective in reducing l0 later (recorded as current 0 in FIGS. 3-6). The current the anode corrosion, the cell temperature stayed at was then increased stepwise to 2, 4, 6 and 8 amperes, about 40C. at 4 amperes (cathode: about 50 and the electrolyte level at each current was measured. mA/cm anode: about 50 mA/cm during the 2-hour In order to obtain a series of consistent measurements operation. at ilC., the level reading was always made 30-40 Five grams of additive is equivalent to 1.6 weight per 15 seconds after adjusting the current. cent, based on weight of electrolyte, when added to The results are shown in FIGS. 3-6 for the following 250 milliliters of 6 M aqueous KOH. four solutions, wherein each inhibitor additive was used TABLE I Base Solution *Temp. Utilization Electrolyte 6 M KOH Na stannate K stannate K citrate Pb acetate (&#34;C (7:)  
 No. l 250ml 46 2 250 ml 5 g 37.5 87.5 35 90.2 3 250 ml 5 g 37 93 4 250 ml 5 g 39 74 5 250 ml 5 g 46 6 250 ml 5 g 5 g 35 103 7 250 ml 5 g 0.5 g 36 90 Sodium stannate Na SnO;,.3H O Potassium stannate K- SnO;,.3H O Potassium citrate K;,C,;H -,O,.H2O Lead acetate Pb(C. ,H O- .3H O  
  all the end of 2 hours operation. after 2 hours continuous operation at about 50 mA/cm anode current density.  
  lt is evident from the utilization values shown in 35 in p p ti of ght p cent, based on weight Table I that a combination of potassium citrate and p0- of electrolyte: tassium stannate is significantly better than each com- 6 M KOH ponent alone (compare electrolytes No. 3, 5 and 6). A 6 M K 0-075 M sodlul&#39;fl stannate combination of lead acetate and potassium citrate 40 6 M l&#39; 0:066 M Potassium stannate 0.062 (electrolyte No. 7 in Table I) also produced utilization M Potasslum Citrate better than either component alone (electrolytes 4 and 6 M KOH 0-062 M Potasslum curate 0-053 M 5 lead acetate These tests demonstrate the effectiveness of the in- It 15 evldem that y above. SolutlOnS. the hibitor system of this invention is significantly reducing 45 electrolyte level decreases wlth&#39;mcl&#39;easmg current In wasteful aluminum corrosion during cell operation, and FIGS- ht represents the dlfference between F in substantially increasing the efficiency of utilization electrolyte levels at 0 and 8 amperes P Cell, and IS a of the aluminum anode. rough measure of the hydrogen gas evolution due to aluminum anode corrosion on open circuit, since at 8 5O amperes very little gas was evolved except in solution EXAMPLE 2 (a). In solutions (c) and (d), racticall no h dro en P Y Y 8 gas was evolved at 6 amperes or greater. ln solution (a), even at 8 amperes, the amount of gas evolved was Anode Corrosion at Various Current Densities considerable. The difference in the electrolyte levels at It iS known that, at the aluminum anode, ectro- 2 and 8 amperes is represented by h It should be noted chemical utilization becomes higher at higher current that the I1 values of solutions (c) and (d) are smaller densities; that is, wasteful corrosion is reduced at than those for solutions (a) and (b). This indicates that higher current densities. addition to the electrolyte of potassium citrate plus ei- The electrolyte level in the Cel deSCflbed above In ther potassium stannate or lead acetate considerably Example 1 was used to estlmate the degree of anode reduces the aluminum corrosion during cell operation, corrosion at various current densities. The electrolyte wh compared i h h ddi i f potassium t level becomes higher as the degree of wasteful corronate l sion becomes higher, since vigorous hydrogen gas evowh i l i d i lution raises the electrolyte level. Therefore, a change 1, A l i ll comprising an l i anode, a in the electrolyte level can be p y as an pp cathode, and an electrolyte in contact with said anode mate measure of the degree of wasteful corrosion of the aluminum anode.  
 The experimental procedure was:  
 and said cathode, said electrolyte comprising an aqueous solution of an alkali metal hydroxide and a corrosion inhibitor system in an amount effective to inhibit corrosion of said aluminum anode when said cell is on open circuit and when said cell is discharging, said corrosion inhibitor system comprising (a) an alkali metal citrate, and (b) at least one compound selected from the group consisting of lead compounds and tin compounds.  
  2. The galvanic cell of claim 1 wherein said alkali metal hydroxide is potassium hydroxide.  
  3. The galvanic cell of claim 1 wherein the lead compounds are selected from the group consisting of alkali metal plumbites, alkali metal plumbates, lead oxide, and lead acetate, and wherein the tin compounds are selected from the group consisting of alkali metal stannates and tin oxide.  
  4. The galvanic cell of claim 2 wherein the lead compounds are selected from the group consisting of alkali metal plumbites, alkali metal plumbates, lead oxide, and lead acetate, and wherein the tin compounds are selected from the group consisting of alkali metal stannates and tin oxide.  
  5. The galvanic cell of claim 1 wherein the corrosion inhibitor system is potassium citrate plus at least one number selected from the group consisting of lead acetate and potassium stannate.  
  6. The galvanic cell of claim 2 wherein the corrosion inhibitor system is potassium citrate plus at least one number selected from the group consisting of lead acetate and potassium stannate.  
  7. The galvanic cell of claim 1 wherein the alkali metal citrate is employed in the electrolyte in an amount within the range of from about 0.5 to about weight per cent, wherein the tin compound is employed in the electrolyte in an amount within the range of from about 0.5 to about 5 weight per cent, and wherein the lead compound is employed in the electrolyte in an amount within the range of from about 0.1 to about 2 weight per cent, all percentages being based upon weight of electrolyte.  
  8. The galvanic cell of claim 2 wherein the alkali metal citrate is employed in the electrolyte in an amount within the range of from about 0.5 to about 10 weight per cent, wherein the tin compound is employed in the electrolyte in an amount within the range of from about 0.5 to about 5 weight per cent, and wherein the lead compound is employed in the electrolyte in an amount within the range of from about 0.1 to about 2 weight per cent, all percentages being based upon weight of electrolyte.  
  9. The galvanic cell of claim 3 wherein the alkali metal citrate is employed in the electrolyte in an amount within the range of from about 0.5 to about 10 weight per cent, wherein the tin compound is employed in the electrolyte in an amount within the range of from about 0.5 to about 5 weight per cent, and wherein the lead compound is employed in the electrolyte in an amount within the range of from about 0.1 to about 2 weight per cent, all percentages being based upon weight of electrolyte.  
  10. The galvanic cell of claim 4 wherein the alkali metal citrate is employed in the electrolyte in an amount within the range of from about 0.5 to about 10 weight per cent, wherein the tin compound is employed in the electrolyte in an amount within the range of from about 0.5 to about 5 weight per cent, and wherein the lead compound is employed in the electrolyte in an amount within the range of from about 0.1 to about 2 weight per cent, all percentages being based upon weight of electrolyte.  
  11. The galvanic cell of claim 5 wherein said potassium citrate is employed in said electrolyte in an amount within the range of from about 0.5 to about 2 weight per cent, wherein said potassium stannate is employed in said electrolyte in an amount within the range of from about 0.5 to about 2 weight per cent, and wherein said lead acetate is employed in said electrolyte in an amount within the range of from about 0.5 to about 2 weight per cent, all percentages being based on said electrolyte.  
  12. The galvanic cell of claim 11 wherein said alkali metal hydroxide is potassium hydroxide.  
  13. The galvaniccell of claim 1 wherein the cathode is an oxygen-depolarized cathode.  
  14. The galvanic cell of claim 2 wherein the cathode is an oxygen-depolarized cathode.  
  15. A galvanic cell comprising an aluminum anode, a cathode, and an electrolyte in contact with said anode and said cathode, said electrolyte comprising an aqueous solution of an alkali metal hydroxide and a corrosion inhibitor system comprising (a) an alkali metal citrate selected from the group consisting of sodium citrate and potassium citrate, in an amount within the range of from about 0.5 to about 10 weight per cent, and (b) at least one compound selected from the group consisting of lead compounds in an amount within the range of from about 0.1 to about 2 weight per cent, and tin compounds in an amount within the range of from about 0.5 to about 5 weight per cent, all percentages being based upon weight of electrolyte.