GB2058836A - Lead based alloys and battery grids made therefrom - Google Patents

Abstract

Lead based alloys suitable for lead-acid battery grids consist, by weight, of: 1.0 to 2.8% antimony, 0.1 to 0.4% tin, 0.005 to 0.03% selenium 0.004 to 0.012% silver and the balance lead and unavoidable impurities. The alloy may contain no more than impurity levels of arsenic or may contain 0.1 to 0.2% arsenic to facilitate parting of directly cast grid pairs or to minimise bowing during processing.

Description

SPECIFICATION Lead based alloys and battery grids made therefrom The present invention is concerned with certain lead based alloys and with grids for lead-acid batteries made therefrom.

For many years lead based alloys containing 4.5%-i 2% antimony (all percentages in this specification are by weight) have been used for the production of lead-acid battery grids. The principal function of the antimony constituent is to afford adequate grid strength to permit satisfactory casting and processing of the formed grids. Lithium and combination of lithium and tin have also been employed as is shown in U.S. Patent 3,647,545.

In recent years, considerable emphasis has been laid on the development of maintenance-free lead-acid batteries. These batteries require no servicing or water additions throughout the life of the battery and are typically provided in sealed or substantially sealed condition since there is no need to have access to the interior of the battery after assembly has been completed. To achieve this maintenance-free objective, it is necessary to reduce substantially water losses from the battery during use. This requires that the grids used in the battery have the effect of reducing the current draw at a fixed over-charge voltage so that only a minimum of gas is generated and the water loss that accompanies gassing is concomitantly minimized.With antimony-lead grids containing about 4.5% antimony, the current draw at the completion of charging is unacceptably high for a maintenance-free battery. In addition, it is known that self-discharge of a wet lead-acid battery having antimony-lead grids caused primarily by the dissolution of antimony from the grids and its subsequent deposition on the negative plates, where it causes electrochemical reactions that discharge the lead to form lead sulphate.

For these reasons, the development of suitable materials for grids in maintenance-free batteries has primarily been directed towards the use of lead based alloys containing no antimony or a reduced level of antimony.

However, when antimony is the only alloying constituent used in a lead based alloy, it is generally impractical to reduce the antimony level significantly below the 4.5% level as grids cast from lead based alloys having significantly lower antimony contents tend to crack. Such cracking has been avoided in low antimony alloys, however, by the use of lead based alloys containing other alloying constituents in addition to antimony.

British Specification 1,427,660 describes an alloy which has a reduced antimony content and which is suitable for forming the grids of maintenance-free batteries. The lead based alloys disclosed therein contain from 1.0 to 1.9% antimony and from 1.2 to 2.0% cadmium, the cadmium being present in an amount at least equal to the antimony present. The addition of cadmium avoid the cracking phenomenon and the resulting alloy provides superior grids for maintenance-free batteries. However, the toxicity of cadmium necessitates special handling precautions.

A number of patents suggest the use of lead based, low antimony alloys containing selenium for grain refinement as well as several other alloying ingredients. These patents include the following: British Specification 622,512 and U.S. Patents 3,801,310; 3,879,217; 3,912,537; 3,993,480 and 3,990,893. The selenium contents described in these patents vary significantly as do the levels of the other alloying ingredients proposed.

These low antimony, selenium lead-based alloys require an alloying ingredient to provide the requisite strength characteristics (including instantaneous handling strength) and minor amounts of arsenic are employed for this purpose in several of the cited patents. Unfortunately, achievement of the requisite strength characteristics in this fashion is obtained at the expense of the desired ductility. The use of arsenic in such alloys at the levels suggested thus results in grids which are too brittle to allow easy handling of the grids in further processing. Arsenic can also detract from the characteristics desired for maintenance-free applications.

Three of the cited patents further suggest, as an optional alloying ingredient, the inclusion of silver at a level of from 0.025 to 0.1%. Silver addition is said to stabilize the fine structure of such alloys and to improve corrosion resistance. Such addition is also desirable for batteries subject to high requirements with respect to mechanical strength, ductility and electrochemical behaviour of the grid alloys.

However, the inclusion of silver at such levels increases the alloy cost to what may be an unacceptable degree. Moreover, and importantly, such silver levels appear to cause brittleness as well as detracting from the characteristics required for maintenance-free applications.

In addition to providing a grid with an appropriately low full charge current draw, there are, of course, other characteristics which must be achieved to provide a truly commercially acceptable maintenance-free battery system. As regards the alloy itself, it should desirably be capable of being rapidly cast directly into weli-formed, thin grids (for example,12~18 castings/minute of 0.065 inch or less thick grids and particularly for negative grids less than about 0.055 inch) and should be resistant to excessive drossing.

Resistance to excessive drossing is particularly needed when grids are formed by direct casting in order to maintain the desired alloy composition and also to ensure good melt flow characteristics during casting. The cast grid must also possess sufficient instantaneous handling characteristics, such as stiffness, for removal from the mould and subsequent processing, such as trimming. In addition, the grid must be highly conductive and be corrosion resistant and morphologically structured so as not to influence adversely the capacity retension characteristics of the battery upon cycling.

We have now developed certain novel low antimony, #lead based alloys which have a combination rf properties which make them suitable for lead-acid storage battery grids and, in particular, for the grids of maintenance-free storage batteries.

According to the present invention, there is provided a lead based alloy consisting, by weight, of: 1.0 to 2.8% antimony, 0.1 to 0.4% tin, 0.005 to 0.03% selenium, 0.004 to 0.012% silver, and the balance lead and unavoidable impurities.

The present invention also comprises a battery grid for supporting an electrochemically active material in a lead-acid battery, the grid being formed of an alloy according to the invention.

Whilst the present invention will be particularly described in relation to grids for maintenance-free lead-acid batteries, it is not limited thereto and the alloys according to the invention may be used in any lead-acid battery. Further, while the alloys of the present invention are particularly useful for making directly cast battery grids, the alloys may be formed into grids by other techniques, such as by mechanical working.

The alloys according to the invention can be easily cast at commercially acceptable rates and at the same time provide grids with superior ductility characteristics. Moreover, and significantly, grids formed from such alloys exhibit reduced gassing and water loss characteristics in comparison with other alloys of this general type.

To obtain the advantages of the present invention, the antimony content of the alloy should be from 1.0 to 2.8%. Grids made using alloys containing more than 2.8% antimony give rise to greater gassing and water loss than is desirable for maintenance-free batteries. On the other hand, if the alloy contains less than 1% antimony, it is difficult, often impossible, to cast at commercially acceptable casting rates.

To improve the casting properties of the alloy by improving the flow properties of the molten alloy, tin is included in an amount of 0.1 to 0.4%. No additional benefit is obtained by using more than 0.4% of tin, while use of less than 0.1% can give rise to grid cracking.

Satisfactory grain structure in the grid is provided by including selenium in an amount of from 0.005 to 0.03%, preferably 0.01 to 0.03%. Use of more than 0.03% selenium does not provide additional improvements in grain refinement.

In accordance with the present invention, the amount of arsenic in the alloy is minimized, while silver is included in an amount of from 0.004 to 0.012%, preferably 0.005 to 0.01%. The silver present acts with selenium to provide the superior grain structure which is characteristic of grids formed from the alloys of the present invention. These alloying ingredients thus serve to provide the alloy with a uniform and consistent distribution of the antimony-rich second phase. Grain refinement through inclusion of silver is achieved in a different way from that resulting from use of selenium and, accordingly, the desired grain structure cannot be obtained by omitting silver and increasing the selenium level.

The inclusion of silver, in addition to obviating the brittleness characteristic of alloys containing excessive amounts of arsenic, imparts to the grids an optimum combination of strength and ductility.

The gassing characteristics are likewise improved in comparison with alloys of this type containing excessive amounts of arsenic. Use of silver above the 0.012% level should be avoided, as the gassing characteristics deteriorate somewhat with higher silver contents and the grids become undersirably brittle relatively soon after casting. On the other hand, the use of less than 0.004% silver leads to undesirable interdentritic voids. It should be appreciated that commercial grades of lead for battery manufacture, and perhaps other of the alloying ingredients used, may contain minor amounts of silver and this should be taken into account in determining the amount of silver to be added. Typically, the silver content, as an impurity, in lead is about 0.003% or less. This fortuitous circumstance serves to decrease the amount of silver that has to be added to lead to obtain an alloy according to the invention.

This is also true of the other alloying ingredients employed and impurity levels should accordingly be taken into account in determining the amount of alloying ingredients to be used to obtain a desired alloy composition.

The most preferred alloy compositions in accordance with the present invention for making maintenance-free battery grids are: antimony 1.4~1.6%, tin 0.2-0.3%, selenium 0.018-0.025%, silver 0.008-0.010%, lead balance With respect to the arsenic content, for most applications, no arsenic content is intentionally added and, indeed, it is preferred that no arsenic content be present whatsoever. However, as a practical matter, arsenic is present as an impurity in commerical grades of lead typically used in battery grid applications and total removal would increase the cost of the alloy. Accordingly, a minor amount of arsenic can be tolerated, but the amount should be kept at a level insufficient to cause any brittleness problems or undesirably increase the gassing characteristics.Arsenic also appears to interfere with grain refinement and its presence increases the tendecy for undesirable dendritic solidification; various toxicity problems can also increase with increasing arsenic content. It is accordingly preferred for most applications to maintain the arsenic level below 0.025 or 0.03%, based upon the total weight of the alloy.

However, in direct casting applications, it is conventional to cast grids in pairs joined by parting areas (e.g. two or more thin areas of alloying joining the grids together). After casting, trimming and the like, the grid pairs are pasted with the desired active material, parted and then stacked for curing. The flexibility and ductility of the essentially arsenic-free alloys of the present invention provide grids, whether as single grids or as grid pairs, which have a tendency to bow under the weight of the paste during movement through conventional pasting machines. While adjustments in the transporting system can minimize any processing difficulties due to the bowing, flexible grid pairs can be difficult to part, especially where the operation is manually carried out; this can give rise to a reject rate which is higher than desirable.

Where parting the grid pairs or bowing of grids is a problem, such problems can be obviated by increasing the arsenic content above the trace impurity level previously described. As may be appreciated from the prior discussion, such an increase in the arsenic content to obviate these problems brings with it certain concommitant disadvantages (as also previously described) so the level of arsenic used should be kept as low as possible. The objective can be achieved in most cases by increasing the arsenic content to 0.1 to 0.15%; sometimes it may be necessary to increase the arsenic content to 0.2% or even slightly higher. In such circumstances, it is necessary to increase the silver content to the higher portion of the range previously set forth, a content of 0.008 to 0.01% being preferred.It appears that this slight increase in the silver content compensates for the arsenic content present. Also, it may be desirable to further increase the silver content above 0.012%, up to perhaps 0.015%, to accommodate arsenic levels significantly above 0.2%. However, further increases in the silver and arsenic contents not only do not provide advantages but can be highly detrimental as they detract from the desired maintenance-free characteristics.

Alloys according to the invention can contain impurities in amounts typically present in commercially available battery grade lead. Additional impurities may also be present in the alloys as a result of impurities typically present in the antimony and other alloying constituents. Further, additional ingredients can be intentionally added to the alloys of the invention as long as they do not significantly adversely affect the desirable features attributable to the present invention.

For example, copper is typically present as an impurity in battery grade lead. Copper can be present in the alloy of the invention in an amount up to 0.06 or 0.08% without significant detrimental effects. Above the level of 0.08%, the copper precipitates at grain boundaries and can cause corrosion problems.

The alloys may be produced using conventional techniques by adding the antimony, tin, selenium, silver and, if needed, arsenic to molten lead and mixing until the mass is homogeneous. Production of grids using the alloy can be effected by commercially available, high speed grid manufacturing equipment. The alloy can be satisfactorily cast in this fashion using pot temperatures of 780 to 8500 F, ladle temperatures of 900 to 10000 F, and mould temperatures of 300 to 4000 F.

The grids formed from the alloys of the invention possess an optimum combination of strength of ductility. When the grids are formed by a direct casting technique, their instantaneous handling strength is sufficient to allow casting at commercially accepted speeds and to allow the necessary trimming and stacking of the soidified casting to be performed without adverse distortion of the grid configuration.

Adequate stiffness for conventional pasting operations is achieved within two days of casting. The ductility of the grids ensures that pasting and battery assembly procedures may be carried out without breakage. Indeed, when the arsenic content is at the trace impurity level, the grids are sufficiently ductile to allow bending into extremely tight rolls in a repeated fashion without cracking occurring at any of the wire intersections in a typical grid configuration. This ductility may be particularly useful when making grids by expanded metal techniques.

The structure of the battery need not be modified to accommodate grids formed from the alloys of the present invention. The grids may thus be simply substituted for the grids used in any of the many commercially available batteries.

In some applications, it may be desired to use both positive and negative grids formed from the alloys described herein. Other applications may make it suitable to form only some of the grids from such alloys. As an example, in particularly rigorous maintenance-free applications, it may be desirable to form only the positive grids from the alloys of this invention. The negative grids may be formed of any non-antimony alloy useful for maintenance-free applications, calcium-tin-lead alloys being preferred.

Such alloys may suitably contain 0.1 to 0.4% tin and 0.06 to 0.20% calcium, the balance lead.

Satisfactory alloys are disclosed in British Specification 1,439,888.

In order that the invention may be more fully understood, the following example is given by way of illustration. Of the alloys described in the example, Alloys 1 A and 2A are in accordance with the invention and Alloys 1 B and 2B are not in accordance with the invention and are given for the purpose of comparison.

EXAMPLE Alloys having the compositions indicated in Table I were prepared and cast into standard ASTM bars used for determining mechanical properties of alloys. Following standard ASTM test procedures, the ultimate tensile strength, yield strength, and percent elongation were determined for each composition for various period of aging at ambient temperature after casting. The results are given in Table II.

TABLE I

ALLOY 1A ALLOY ie ALLOY 2A ALLOY 2B Antimony 1.4 1.4 2.5 2.5 Tin 0.2 0.2 0.2 0.2 Selenium 0.018 0.018 Silver 0.01 0 0.01 0 Lead Balance Balance Balance Balance TABLE II

Ultimate Tensile Yield Days Strength Strength Elongation Aging Alloy (psi) (psi) (o/o) 1 1A 4600 2150 10 IB 4100 1600 15 2A 5400 3240 12 2B 5200 1650 21 2 1A 4600 2900 8 1 B 4100 2000 12 2A 5400 3300 12 2B 5100 1850 28 3 1 A 4700 2800 9 1 B 4000 2000 12 2A 7100 3300 10 2B 5000 2100 15 Alloys 1 B and 2B, which were tested for comparative purposes, differ from alloys 1 A and 2A in that the silver component was omitted. The test results given in Table II show that addition of silver in the small amount required by the present invention has a significant effect on the mechanical characteristics of the alloy.

Preliminary casting studies and battery performance data indicate the advantages of the alloys of the present invention. Thus, these alloys are extremely stable in the molten state and exhibit relatively low levels of drossing so that the nominal composition formulated for a particular alloy will be close to the actual composition after casting. Castability is excellent and useful casting temperatures appear to be relatively wide. Cast grids are extremely flexible and do not exhibit any sensitivity to grid cracking, even at the wire intersections or where relatively differently sized cross-section members are joined together. The microstructure is much more refined than other alloys of this type not including silver as an alloying ingredient and preliminary corrosion and battery performance characteristics indicate performance equal to or better than that of other commercially used low antimony, lead based alloys.

Batteries using grids formed from alloys of the present invention exhibit, in prelimary studies, about 20 to 30% reduced gassing and water loss characteristics in comparison with similar alloys having excessive amounts of arsenic.

Claims (16)

CLAIMS 1. A lead based alloy consisting, by weight, of:

1.0 to 2.8% antimony, 0.1 to 0.4% tin, 0.005 to 0.03% selenium, 0.004 to 0.12% silver, and the balance lead and unavoidable impurities.

2. An alloy according to claim 1, which contains from 1.4 to 2.6% of antimony.

3. An alloy according to claims 1 or 2, which contains from 1.4 to 1.6% of antimony.

4. An alloy according to any of claims 1 to 3, which contains from 0.2 to 0.3% of tin.

5. An alloy according to any of claims 1 to 4, which contains from 0.01 to 0.03% of selenium.

6. An alloy according to any of claims 1 to 5, which contains from 0.018 to 0.025% of selenium.

7. An alloy according to any of claims 1 to 6, which contains from 0.005 to 0.01% of silver.

8. An alloy according to any of claims 1 to 7, which contains from 0.008 to 0.01% of silver.

9. An alloy according to any of claims 1 to 6, which contains from 0.008 to 0.012% of silver and from 0.1 to 0.2% of arsenic.

10. An alloy according to any of claims 1 to 9, which additionally contains up to 0.08% of copper.

11. A lead based alloy substantially as herein described with reference to Alloy 1 A or 2A of the Example.

12. A battery grid for supporting an electrochemically active material in a lead-acid battery, the grid being formed of an alloy according to any of claims 1 to 11.

13. A battery grid according to claim 12 which has been directly cast from the molten alloy.

14. A pair of directly cast battery grids for supporting an electrochemically active material in a lead-acid battery system, the grids being joined together by parting areas and being formed of an alloy consisting, by weight, of:

1.0 to 2.8% antimony, 0.1 to 0.4% tin, 0.005 to 0.03% selenium, 0.008 to 0.012% silver, arsenic in an amount sufficient to allow the grid pair to bye readily separated at parting areas, and balance lead and unavoidable impurities.

15. A lead-acid battery containing positive and negative sets of grids, at least one of the set of grids being grids according to any of claims 12 to 14.

16. A lead-acid battery according to claim 1 5, in which the negative set of grids are formed of a calcium-tin-lead alloy.