US3240688A

US3240688A - Aluminum alloy electrode

Info

Publication number: US3240688A
Application number: US361581A
Authority: US
Inventors: Michael J Pryor; Douglas S Keir; Philip R Sperry
Original assignee: Olin Corp
Current assignee: Olin Corp
Priority date: 1964-04-21
Filing date: 1964-04-21
Publication date: 1966-03-15
Anticipated expiration: 1983-03-15
Also published as: NO115857B; GB1099071A; DE1483273C3; JPS5112441B1; DE1483273B2; DE1483273A1

Description

United States Patent 3,240,688 ALUMINUM ALLOY ELECTRODE Michael J. Pryor and Douglas S. Keir, Hamden, and Philip R. Sperry, North Haven, Conn., assignors to Olin Mathieson Chemical Corporation No Drawing. Filed Apr. 21, 1964, Ser. No. 361,531 20 Claims. (Cl. 204-148) This application is a continuation-in-part of US. patent application Serial No. 251,024, filed January 14, 1963, now US. Patent 3,189,486, Serial No. 60,166, filed October 3, 1960, now US. Patent 3,180,728, Serial No. 171,114, filed February 5, 1962, now US. Patent 3,186,- 836, and Serial No. 304,923, filed October 27, 1963.

The present invention relates to an improved aluminum alloy which may be advantageously utilized in a number of applications. For example, the improved alloy of the present invention may be advantageously utilized in (l) a primary electric battery suitable for use with liquid electrolytes, such as aqueous electrolytes and especially sea Water, and (2) as sacrificial aluminum anodes in conjunction with a metallic cathode which thereby receives substantial protection against corrosion.

Magnesium and magnesium alloys in the form of sheet are generally used as the anodes of electric cells or batteries adapted to utilize sea Water or similar aqueous electrolytes. The cost of conventional sea Water batteries utilizing magnesium and magnesium alloys has been found to be prohibitively high except for military applications This prohibitively high cost is due in part to the high price of magnesium and also due to the difficulty in rolling the hexagonal metal down to light gage sheet of less than 0.020 inch thickness.

Further disadvantages of magnesium and magnesium alloys for this application include the fact that they generally corrode readily in saline mediums even When uncoupled. In addition, relatively low power efliciency on the order of about 60 percent is obtained. Further, they are accompanied by a marked hydrogen-evolution problem and are characterized by a power output which falls with time and for which special design allowances must be made.

Zinc is disadvantageous, inter alia, as it provides insufficient power output to be a useful anode material in this type of power cell.

The more widely used sacrificial anodes for protection of ferrous structures against corrosion are the zinc and magnesium anodes. Aluminum alloys have not been as Widely adopted for this purpose as the zinc and magnesium anodes because they have previously produced only low protective currents equivalent to those generated by zinc anodes but at a much higher unit cost. Furthermore, aluminum alloys have frequently shown the characteristic of becoming highly polarized due to the accumulation of insoluble corrosion products so that ultimately little useful protective current is delivered. However, the magnesium depresses the potential of steel in sea water into the hydrogen evolution range and stripping of protective coatings from the steel can result, for example, paint coatings. Furthermore, magnesium itself produces copious quantities of hydrogen when it serves an as anode in sea water. This is of particular significance in connection with protection of sea water ballast tanks in ships for which purpose magnesium anodes have been found to be hazardous. Zinc is undesirable due to the low galvanic ice currents delivered which necessitates the use of a plurality of anodes in order to provide acceptable current levels.

It is therefore an object of the present invention to provide an improved aluminum alloy having a wide variety of uses, for example, which is capable of being utilized as a sacrificial anode and in an improved electric cell or battery adapted to utilize sea water or other electrolytes.

It is a further object of the present invention to provide an improved battery as aforesaid which attains high average current density, high power output and high cur rent potential.

It is a still further object of the present invention to provide an improved battery as aforesaid which is inexpensive and economical while still attaining excellent results.

It is a still further object of the present invention to provide an improved cathodic protection system and an improved method of cathodically protecting a ferrous metal structure in contact with a medium corrosive thereto.

Further objects and advantages of the present invention will appear hereinafter.

In accordance with the present invention it has now been found that the foregoing objects and advantages may be readily accomplished by providing an aluminum base alloy containing at least percent aluminum, between 0.04 and 0.5 percent tin and from 0.005 to 1.0 percent gallium, with the tin being retained in solid solution to the maximum degree.

The alloy of the present invention is particularly useful for fabrication into a metal anode. The anode may be utilized in a primary electric cell containing, in addition to the anode, a consumable unpolarized cathode and a liquid-electrolyte. Still further, the anode may be utilized in a cathodic protection system comprising a cathodic metal structure and the aluminous sacrificial anode electrically connected thereto, with the metal structure and the anode being in contact with a medium corrosive to said metal structure.

The above identified co-pending application Serial No. 251,024 teaches that a metal anode comprising an aluminum base alloy containing at least 90 percent aluminum and between 0.04 and 0.5 percent tin, with the tin retained in solid solution to the maximum degree, is highly useful and advantageous and in fact attains surprising advantages over previously known systems. It has now been found that in accordance with the present invention still further improvements may be obtained by providing in addition to the aluminum and tin alloy a specified quantity of gallium. The gallium provides greatly improved current density characteristics over the alloy without the gallium present while retaining the highly desirable and advantageous and in fact surprising characteristics of the aluminum-tin alloy of co-pending application Serial No. 251,024.

In addition, the aluminum-tin-gallium alloy of the present invention provides: (1) a maintenance of the current plateau at a nearly consistent level during most of the useful life of the cell rather than the continual decline of current experienced with the magnesium alloys; (2) markedly reduced hydrogen-evolution and less temperature rise. These characteristics greatly simplify battery design and operation.

An additional advantage of the alloys of the present invention is that they can be readily fabricated by casting by either hot or cold rolling, and can be readily rolled to small gages desirable for power cell anodes in distinction to magnesium where its hexagonal lattice severely restricts its fabricating.

The improved aluminum alloy of the present invention contains tin in an amount from 0.04 to 0.5 percent, at least 90 percent aluminum and from 0.005 to 1.0 percent gallium, with the tin being retained in solid solution to the maximum degree, i.e., about 0.1 percent with the excess tin or a suitable third ingredient being provided as taught in co-pending application, Serial No. 60,166, to improve uniformity of corrosion and to improve anodic efiiciencies.

The preferred manner of preparing this alloy is to heat the aluminum tin sample at elevated temperatures, e.g., around 550 to 630 C. with 620 C. being preferred, for a sufficient period of time to dissolve the maximum amount of tin and to redistribute excess tin or other alloying additions in a coarse, particulate form which produces maximum uniformity of attack and power efficiency. Generally, the heating period within the preferred temperature range may vary between 15 minutes and 24 hours. After the heating period, the sample is cooled rapidly, for example, by immersion in a large volume of water at ambient temperatures or in the case of thin sheet, by cooling in air. For simplicity, this treatment may be termed homogenization treatment. Homogenization within the above temperature yields maximum tin in solid solid solution. Outside of this range the amount of tin in solid solution falls off markedly, thus yielding poorer electrochemical characteristics.

It has been found, as discussed in detail in the above co-pending applications, that the alloys of the present invention develop surface layers on oxidation of any portion thereof, which surface layers have an excess of n-type defects in a concentration effective to substantially increase the conductivity thereof.

The preferred amounts of tin are from 0.08 to 0.35 percent. In some instances high purity aluminum may be preferred, for example, in the primary cells; however, the present invention is not limited to the use of a high purity aluminum and may be prepared from lower purity aluminum containing up to 0.10 percent silicon and up to 0.1 percent iron. It should be naturally understood that the alloy of the present invention may contain in addition to the aluminum, tin, and gallium and incidental impurities, other metallic components which may be added to achieve particularly desirable results.

Generally, insoluble elements may be added to the alloy, i.e., elements which have less than 0.03 percent maximum solid solubility in aluminum. The total amount of these insoluble elements should be no greater than 0.5 percent. These insoluble elements have no significant effect on current output as they do not reduce the solid solubility of tin in aluminum, but they act as second phase particulate cathodes and large amounts ultimately reduce anodic efliciency by promoting local corrosion of the anode.

Soluble elements may also be added to the alloy. The soluble elements may be considered either lattice expanders or lattice contractors, i.e., ternary addition elements which either expand or contract the aluminum lattice. Generally lattice expanders stabilize tin in retained solid solution and permit high galvanic currents to be drawn from the alloy. Lattice expanders may be used in an amount from about 0.001 to 8 percent, with typical lattice expanders and amounts thereof which may be used including: magnesium from about 0.001 to 7.0 percent which is particularly preferred; zirconium from about 0.001 to 0.3 percent; bismuth from about 0.001 to 0.5 percent; indium from about 0.001 to 0.5 percent; and mixtures thereof. It should be noted that gallium should be considered as a lattice expander since it stabilizes tin in retained solid solution and permits high galvanic currents to be drawn from the alloy.

Lattice contractors generally reject tin from solid solution, but small amounts may be tolerated, for example, zinc up to 0.01 percent; copper up to 0.002 percent; silicon up to 0.10 percent; and manganese up to 0.05 percent.

The primary cell of the present invention employs a consumable unpolarized cathode, a liquid electrolyte and the improved metal anode of the present invention. As a cathode material any consumable and unpolarized cathode may be conveniently employed, and preferably a readily reducible and insoluble metal salt or oxide, for example, a silver salt or oxide or a copper salt or oxide, a catalyzed porous electrode, such as porous metal or carbon wherein oxygen from without is continually consumed.

In the primary cell of the present invention it is preferred to utilize solid, fused silver chloride as a cathode. Alternatively, any silver salt may be utilized as the cathode material, provided the salt is at least as soluble as silver chloride, but sufficiently insoluble to avoid disintegration of the cathode during operation of the cell. Among such other cathodic materials which may be employed are silver oxide, silver chromate, silver sulfate, silver phosphate, silver acetate and silver carbamate. Cells may be formed with cathodes of silver salts more insoluble than silver chloride such as silver bromide and silver iodide, but the voltage is considerably lower since the cathodic material is much more insoluble than the silver chloride. Exemplificative copper compounds include preferably copper oxides.

The electrolytes which may be employed are broadly any liquid electrolyte and preferably the liquid-aqueous type electrolytes. The electrolyte which should be employed should, in addition to being liquid at operating temperatures, be one which does not polarize the anode or the cathode and one free from inhibitive action on the anode.

The primary cell of the present invention is especially adapted to utilizing sea water as the electrolyte; however, it is apparent that the cells and batteries of the present invention will operate advantageously in electrolytes other than sea water, for example, any aqueous solution of sodium chloride may be conveniently employed, such as a 3.5 percent aqueous solution of sodium chloride. Similarly, other alkali metal chlorides or alkaline earth metal chloride will be satisfactory. Other suitable electrolytes, weak or strong, dilute or concentrated may be conveniently employed. Water also yields an operative cell although a considerable time may be required before the cell reaches its full capacity. Exemplificative of the nonaqueous type electrolytes include fused sodium chloride or potassium chloride, including low melting alkali halide eutectics.

Naturally, the primary cell of the present invention may be prepared by any of the conventional means well known in the art. In the preparation of the primary cell of the present invention, for example, the anode and cathode material may be separated or spaced apart by any conventional means, for example, thin films of a chemically stable material such as nylon may be adherred to the anode material. If the particular cell or battery under consideration is intended to operate at a high current density, the electrodes should be more closely spaced. In a cell or battery not intended to operate at high current densities, close spacing is not required. In the low current density batteries, rubber strips or tabs at the edges of the electrode sheets may be employed.

The cathode material may be prepared by any of the conventional means, for example, cast sheets of substantial thickness may be employed or rolled silver chloride may be produced by suspending a body of silver, such as silver screen, in a dilute chloride solution for a time sufficient to form a silver chloride coating of the desired thickness. Other means for preparing the cathodic material are well known in the art. It is preferred to set up a plurality of the primary cells spaced from one another so that individ ual cells are established between the plates of succeeding electrodes when immersed in an electrolyte.

' below.

the sheet.

, comprising a cathodic metal structure and at least one aluminous sacrificial anode electrically connected thereto, both the metal structure and the anode being in contact with a medium corrosive to said metal structure, said anode comprising the above aluminum alloy of the present invention.

The anodes of the present invention can be used in cathodic protection systems for underground structures, such as pipe lines, foundations, and the like. They may be used in fresh water or in saline aqueous media. They are particularly well suited for use in sea water, and provide for the first time, cathodic protection systems for protection of iron, such as ships hulls, ballast tanks, and commercial fishing devices, such as lobster pots, which are free from the shortcomings of previously used systems.

In carrying out the present invention, the sacrificial anode of the type previously described, is attached to a metal structure to be protected, such as, for example, a ferrous metal structure, by means of a suitable electrical conductor, and then immersed or imbedded in the surrounding corrosive medium, in accordance with the customary practice. The alloy anode may be of any desired shape or size, such as, for example, a cylindrical piece, or a trapezoidal shaped member.

The present invention and improvements resulting therefrom will be more readily apparent from a consideration of the following illustrative examples.

EXAMPLE I This example describes the preparation of the alloy of the present invention. The aluminum used was super purity aluminum containing 0.001 to 0.002 percent silicon and 0.002 and 0.003 percent iron. In all cases the alloying addition was pure tin and pure gallium where gallium was used.

The ingots were cast and after casting the ingots were rolled to sheet and were given a homogenization heat treatment of 1145 F. for one hour followed by rapidly air cooling in order to retain the tin in solid solution to the maximum degree. All samples are degreased before testing.

' EXAMPLE II lnthe following example primary cells were prepared in identical fashion except that the anodic material in each primary cell differed in composition as indicated In all cases the anode material was prepared as in Example I.

The test primary cells were constructed in the conventional manner. The anodes were shaped from sheet material generally about 0.012 inch thick. The cathodes were shaped from silver chloride sheet about 0.015 inch thick. The cathodes were modified to incorporate insulating spacers which would allow them to be placed in close proximity to the anodes without electrical short circuiting. This was done, in one of the conventional art manners, by using glass beads fastened to one side of A multiplicity of holes was drilled in the cathode to increase the reactive surface of the silver chloride. In addition, the silver chloride was partially reduced to silver in a conventional manner by immersion in a photographic film developer.

The electrical contact with the silver chloride cathode was furnished by a 0.001 inch thick 99.9 percent pure silver foil held in pressure contact in the conventional fashion. External electrical connections were made to the anode and cathode, suitably insulated from each other and from the electrolyte. The anode, cathode and spacers were then taped together to complete the cell structure.

The cell was activated by immersing it in a solution of 3.5 weight percent sodium chloride in distilled water. The circuit was completed through a recording ammeter 6 which provided a 1 ohm external load resistance for the cell.

The results are shown in the following table.

The foregoing experiments demonstrate that small amounts of gallium, for example, as low as 0.01 percent, gives a significant and impressive increase in current density and that a peak in the increase is obtained at about 0.05 percent gallium. The alloys with gallium continued to display the other favorable characteristics associated with aluminum-tin alloys as shown in the above co-pending applications, such as low gas evolution and evenly maintained level of current output over the useful period of operation of the cell. In addition, the alloys with gallium exhibited good current rise characteristics.

EXAMPLE III In the following example the tests of Example II were repeated with additional anode materials except that the time in seconds to reach peak current was also measured. In all cases the anode material was prepared as in Ex ample I.

The results are shown in the following table.

This example in addition to showing the same effect as Example II shows that the lattice expander, magnesium, increases the maximum current over the anode without magnesium and simultaneously shows a marked improvement in the current rise characteristics. The presence of the magnesium does not detract from the effect of the gallium with respect to current density and improves the current rise characteristics.

EXAMPLE IV The following example illustrates the effect of providing the tin component in solid solution to the maximum degree in the alloy of the present invention.

All samples were prepared in a manner after Example I except that the homogenization temperature varied as shown in the table below. In every case the alloy had the following composition: gallium, 0.05 percent; tin, 0.20 percent; magnesium, 0.5 percent, and the balance essentially aluminum. The current density characteristics were measured.

The results are shown in the table below.

Table III Homogenization Peak Current Temperature Density (amp/sq. in.)

1. 37 620 C 1. 40 1. 41 1. 40 565 C 1. 40 l' a 540 C 28 This example demonstrates the criticality of providing tin in solid solution to the maximum degree. With the sample homogenized at 540 C. the tin was not provided in solid solution to the maximum degree and much poorer characteristics were shown. Further, a sample without any homogenization at all shows still poorer characteristics.

EXAMPLE V Table IV Composition (percent) Coulombs Flow- Alloy ing in 48 hours Sn Fe 87 7 1,374,1,400,1,554 Nil This invention may be embodied in other forms or carried out in other ways without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered as in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency are intended to be embraced therein.

What is claimed is:

1. An aluminum base alloy consisting essentially of at least 90 percent aluminum, between 0.04 and 0.5 percent tin and between 0.005 and 1.0 percent gallium, with the tin being retained in solid solution to the maximum degree, said maximum degree being 0.1 percent.

2. An alloy according to claim 1 wherein the tin is present in an amount from 0.08 to 0.35 percent.

3. An alloy according to claim 1 containing silicon in an amount up to 0.10 percent and iron in an amount up to 0.10 percent.

4. An alloy according to claim 1 containing from 0.001 to 8.0 percent of a lattice expander which has greater than 0.03 percent maximum solid solubility in aluminum.

5. An alloy according to claim 4 wherein said lattice expander is selected from the group consisting of magnesium in an amount from 0.001 to 7.0 percent, zirconium in an amount from 0.001 to 0.3 percent, bismuth in an amount from 0.001 to 0.5 percent, indium in an '8 amount from 0.001 to 0.5 percent and mixtures thereof.

6. A metal anode comprising an aluminum base alloy consisting essentially of at least percent aluminum, between 0.04 and 0.5 percent tin and between 0.005 and 1.0 percent gallium, with the tin being retained in solid solution at a maximum degree, said maximum degree being 0.1 percent.

7. An anode according to claim 6 wherein said alloy contains tin in an amount from 0.08 to 0.35 percent.

8. An anode according to claim 6 wherein said alloy contains silicon in an amount up to 0.10 percent and iron in an amount up to 0.10 percent.

9. An anode according to claim 6 wherein said alloy contains from 0.001 to 8.0 percent of a lattice expander which has greater than 0.03 percent maximum solid solubility in aluminum.

10. An anode according to claim 9 wherein said lattice expander is selected from the group consisting of magnesium in an amount from 0.001 to 7.0 percent, zirconium in an amount from 0.001 to 0.3 percent, bismuth in an amount from 0.001 to 0.5 percent, indium in an amount from 0.001 to 0.5 percent and mixtures thereof.

11. In a primary electric cell containing a metal anode, a consumable unpolarized cathode and a liquid electrolyte, the improvement of a metal anode comprising an aluminum base alloy consisting essentially of at least 90 percent aluminum, between 0.04 and 0.5 percent tin and between 0.005 and 0.1 percent gallium, with the tin being retained in solid solution to the maximum degree, said maximum degree being 0.1 percent.

12. A primary electric cell comprising a consumable, unpolarized cathode selected from the group consisting of a silver salt, a silver oxide, a copper salt and a copper oxide, an aqueous electrolyte and a metal anode comprising an aluminum base alloy consisting essentially of at least 90 percent aluminum, between 0.04 and 0.5 percent tin and between 0.005 and 1.0 percent gallium, with the tin being retained in solid solution to the maximum degree, said maximum degree being 0.1 percent.

13. A primary electric cell according to claim 11 wherein the electrolyte is sea water.

14. A primary electric cell according to claim 11 wherein said aluminum base alloy contains from 0.001 to 8.0 percent of a lattice expander which has greater than 0.03 percent maximum solid solubility in aluminum.

15. A primary electric cell according to claim 14 wherein said lattice expander is selected from the group consisting of magnesium in an amount from 0.001 to 7.0 percent, zirconium in an amount from 0.001 to 0.3 percent, bismuth in an amount from 0.001 to 0.5 percent, indium in an amount from 0.001 to 0.5 percent and mixtures thereof.

16. A cathodic protection system comprising a cathodic metal structure and at least one aluminous sacrificial anode electrically connected thereto, both the metal structure and the anode being in contact with a metal corrosive to said metal structure, said anode comprising an aluminum base alloy consisting essentially of at least 90 percent aluminum, between 0.04 to 0.5 percent tin and between 0.005 and 1.0 percent gallium with the tin being retained in solid solution to the maximum degree, said maximum degree being 0.1 percent.

17. A cathodic protection system according to claim 16 wherein said anode contains tin in an amount from 0.08 to 0.35 percent.

18. A cathodic protection system according to claim 16 wherein said anode contains from 0.001 to 8.0 percent of a lattice expander which has greater than 0.03 percent maximum solid solubility in aluminum.

19. A cathodic protection system according to claim 18 wherein said lattice expander is selected from the group consisting of magnesium in an amount from 0.001 to 7.0 percent, zirconium in an amount from 0.001 to 0.3 percent,'bismuth in an amount from 0,001 to 0.5

percent, indium in an amount from 0.001 to 0.5 percent and mixtures thereof.

20. The method of cathodically protecting a ferrous metal structure in contact with a medium corrosive thereto which comprises: connecting to said metal structure an aluminous sacrificial anode and immersing said anode in said corrosive medium, said anode comprising an aluminum base alloy consisting essentially of at least 90 percent aluminum, between 0.04 to 0.5 percent tin and between 0.005 and 0.1 percent gallium, with the tin 10 References Cited by the Examiner UNITED STATES PATENTS 2,565,544 8/1951 Brown 204-148 2,886,432 5/1959 Schmitt et a1 75138 3,063,832 11/1962 Snyder 75138 3,172,760 3/1965 Sakano et al. 75-138 JOHN H. MACK, Primary Examiner.

T. TUNG, Assistant Examiner.