CA1162424A - Low antimony lead-based alloys and use thereof - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A lead base alloy containing 1.0%-2.8% antimony, 0.1%-0.4% tin, .005%-.03% selenium, and .004%-.012% silver is disclosed for lead-acid battery grids, the grids in turn being particularly useful for preparation of maintenance-free batteries having superior characteristics.
The alloy preferably contains no arsenic other than trace impurity levels; however, in some applications, the presence of minor amounts of arsenic may desirably be included to enhance parting of directly cast grid pairs or to minimize bowing during processing.

Description

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~1--BACKGROUND OF THE INVENTION
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The present invention relates to a novel alloy, and, more particularly, to an alloy for use in the manufacture of grids for lead-acid batteries. The invention is~particularly useful for making grids for S maintenance-free batteries.
~ For many years lead based alloys containing about 4.5%-12% antimony have been used for the prepar-ation of lead-acid battery grids. The principal func-tion of the antimony constituent is to afford adequate grid stren~th to permit satisfactory casting and pro-cessing of the fo~ned grids. Lithium and combinationsof lithium and tin have likewise been employed as is shown in U. S. Patent No. 3,647,545.
In recent years, considerable emphasis has centered on the preparation o~ maintenance-free lead-acid batteries. These batteries require no servicingor water additions throughout the li~e o~ 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 ~fter a~sembly has been ~I fi;~

completed. To achieve this maintenance-free objective, substantial reduction of watex losses must be achie~ed.
This requires that the grids employed in the maintenance-free battery have the effect of reducing the current draw at a fixed over-chaxge ~oltage so that only a minimum of gas is generated and thé water loss that accompanies gassing is concomitantly minimized. With antimony-lead grids containing about 4.5% by weight antimony, the current draw at the completion of charging is unaccept-ably high for maintenance-free battery applications.
In addition, it is known that self-discharge of a wet lead-acid battery employing an an~imony alloy is caused primarily by the dissolution of antimony from the grids and its subsequent deposition on the ne~ative plates, where it causes electrochemical reactions that discharge the lead to lead sulfate. For these reasons, the development of suitable materials for grids in mainte-nance-free batteries has primarily emphasized 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 generally becomes impractical to reduce the antimony level signi-ficantly below the 4.5% level, due to a tendency of grids cast from such low antimony lead based alloys to crack. The cracking phenomenon has been avoided in low antimony alloys, however, by use of lead based alloys containing other alloying constituents in addition to antimony.
Mao and Lannoye, C~ian Patent No. 1,028,531 issued on March 28, 1978, for:
"Cadmium-Antimony-Lead-Alloy for Maintenance Free Lead-Acid Battery" describes an alloy that has a reduced ! antimony content and is suitable for forming the grids B f maintenance-free batterles. The lead based alloys ~, 242~

disclosed therein contain from about 1.0 to about 1.9 wt.% antimony and fr~m a~out ~.~ to about-2.0 Wt.%
cadmium, the cadmium belng present in an amount at least equal to the antimony present. The addition of cadmium avoids the cracking phenomenon, and the resulting alloy provides superior grids for maintenance-free batteries. However, the toxicity of cadmium necessitates special handling precautions.
Still further, a number of patents suggest the use of lead based, low antimony alloys containing selenium for grain refinement as weIl as several other alloying ingredients. These patents include the follow-ing: GreAt Britain 622,512: U. S. 3,801,310, U. S.
3,879,217; U. S. 3,912,537; U. S. 3,993,48Q and U. S.
3,990,893. The amounts of selenium set forth vary sig-nificantly as do the levels of the other alloying in-gredients proposed.
These low antimony, selenium lead-based al-loys require an alloying ingredient to provide the req-uisite strength characteristics (including instantaneoushandling strength); and minor amounts of arsenic are em-ployed for this purpose in several of the cited patents.
Unfortunately, achievement of the re~uisite strength characteristics in this fashion is obtained at the expense of the desired ductility. The use of arsenic in such alloys in the levels suggested thus results in grids which are too brittle to allow easy handling of the grids in further processing. This is particularly significant in direct casting applications when either relatively thicker grids are needed or where the grid configuration involves the intersection o~ bars or wires of differing cross-sectio~s. At such inter~ections, the suscep~ibility to breaks due to excessive brittleness , is increased. Likewise, relatively brittle alloys are undesirable when forming grids using expanded metal and

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metal working techniques. Also, the presence of arsenic in significant amounts can detract from the characteri-stics des'ired for maintenance-free applications.
Three of the' cited patents further suggest, as an optional alloying in~redient, the inclusion of silver in a level of from 0.025 to 0.1 weight percent.
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 behavior of the grid alloys.
However, the' inclusion of silver in such levels increases the alloy cost which may be significant. More-over, and importantly, such sil~er levels appear to detract from the characteristics required for maintenance-free applications.
Aside from providing a grid with an appropriate-ly low ~ull charge current draw, there are, of course, ~ other characteristics which must be achieved to provide a truly commercially acceptable maintenance-free battery system. Insofar as the alloy itself, when direct casting techniques are used, it should desirably be capable of being rapidly cast directly into well-~ormed, thin grids (e.g., 12 18 castings/minute of about 0.065 inch thick grids or less and particularly for negative grids less than about 0.055 inch) and be resistant to excessive drossing.
Resistance to excessive drossing is particular-ly needed in direct cast applications in order to main-tain the desired alloy content and also to assure goodmelt flow characteristics during castiny. The cast grid must also possess sufficient instantaneous handling characteristics such as stiffness for removal ~rom the mold and subsequent processing such as trimming. In addition, the grid must be highly conductive and be corrosion resistant and morphblogically struc~ured so as not to adversel~ influence the capacity retention characteristics of the battery upon cycling.
It is accordingly an object of the present S invention to provide a low antimony, lead based alloy that can be used to provide lead-acid storage battery grids having improved handling characteristics and good mechanical properties.
A further object of the inven~ion provides an alloy suitable for making grids for maintenance-free battery applications. A more specific and related object is to provide battery grids that do not cause excessive gassing or water loss when used in maintenance-free, lead-acid storage batteries.
Another object lies in the provision of an alloy that is extremely stable in the molten state and exhibits little drossing.
Yet another object of this invention is to provide an alloy having improved corrosion resistance characteristics.
Other ohjects and advantages will become apparent from the following description of the invention and from the claims.
While the invention will be described in con-nection with certain preferred embodiments, it will beunderstood that it is not intended to limit the inven-tion to these embodiments. On the contrary, it is intended to CQVer all embodiments, modifications and equiualents as can be included within the spirit and scope of the invention as defined in the appended claims. Thus, as an example, while the present in-vention will be primarily descxibed in conjunction with maintenance~free applications, the invention is ! not so limited. The alloys disclosed herein may thus be employed for any leaa-acid battery application.

Further, while the pxesent invention will be described primarily in conjunction with forming grids b~ direct casting techniques, the allo~s may be formed into grids by other techniques, such as by mechanical working or the like. ~11 parts and percentages referred to herein are by weight unless otherwise specified.
The present invention, is, in general, pred-icated on the discovery that low antimony, selenium lead based alloys can be formed into battery grids h~-~ng exceptional properties by minimizing the arsenic cortent of such alloys and adding a relatively low level of silver. Such alloys can be easily cast at commer-cially acceptable rates, yet provide grids with superior ~ ductility characteristics. Moreover, and significantly, grids formed from such alloys exhibit reduced gassing ~ and water loss characteristics in comparison to other ; alloys of this general type.
To provide all of the advantages of the pre-sent invention, the antimony content of the alloy should be in the range from about 1.0 to about 2.8 wt.
~. Grids made using alloys containing more than about 2.8 wt.% antimony result in greater gassing and water loss than is desirable for maintenance-free batteries.
On the other hand, if the alloy contains less than about 1 wt~% 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 about 0.1 to 0.4~, based upon the weight of the alloy. No additional benefit is obtained by usi.ng greater than about 0.4 wt.
% of tin while use of less than about 0.1 wt.% can cause grid cracking.
Satisfactory grain structure in the grid is provided by includin~ in the alloy selenium in an ~J~:42~ZL

amount of from about 0.~5 to 0.03 wt.%, preferably O.01 to, 0.03 wt.%. Use of more than about 0.03 wt.%
selenium does not pxovide additional impro~ements in grain refinement.
In accordance with',the'present invention, silver is included in an amount of from about 0.004 to 0.012 wt.%, preferably 0.005 to 0.01, while the amount of arsenic is minimized. The'level of silver employed acts with the' selenium to provide the superior grain structure characteri~ed by 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 fashion different from that resulting from use of selenium; and, accordingly, the desired grain structuxe cannot be achieved by omitting silver and increasing the selenium level.
Moreover, 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 com-parison to alloys of this type containing excessive amounts of arsenic. Use of silver significantly above the 0.012 wt.~ level should be avoided, as the gassing characteristics deteriorate somewhat with higher levels.
On the other hand, use of silver in an amount less than 0.004 wt.% leads to undesirable interdentritic voids.
It should be appreciated that commercial grades of lead for battery manu~ac~uring, and perhap~ other of the alloying ingredients used, may contain minor amounts of silver; and this should be taken into account in ! determining the level of silver desired. Typically, the silver level as an impurity in lead will be about 0.003~ or less, based upon the weight of the resulting alloy. Of course, this ~ortuitous circumstance'serves to decrease the cost premium nece~'sary due to the'inclusion of silver. This is also true of the alloying ingredients employed, and impurity level's should accordingly be taken into account in determinin~ the composition of the alloy.
The most preferred composition for an alloy 10 in accordance with the'present invention for a mainte-nance-free application is:
Antimony 1.4-1.6 wt.%
Tin 0.2-0.3 wt.%
Selenium .018-.025 wt.%
Silver .008-.010 wt.%
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 commercial grades of lead typically used in battery grid applications, and total removal increases the costs involved. Accordingly, a minor amount of arsenic can be tolerated; but the amount should be kept at a level insufficient to cause any brittleness probIems or undesirably increase the gassing characteristics. Likewise, arsenic appears to interfere with grain refinement; and its presence in-creases the tendency for undesirable dendxitic solidiEi-cation. Still further, various toxicity problems can increase with incxeasing arsenic content. It is accordingly preferred for most applications to maintain the arsenic level be'low about 0 025 ox 0.03 wt.%, based , upon the total weight of the alloy.
However, in direct casting applications, it is L6~Z~

conventional to cast grids in pairs joined by parting areas (e.gO-two or more thin area~ of alloy ~oining the grids together). After casting, tximming ~nd 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't~pically used pasting machines. While adjustments in the trans-porting 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. Higher than desirable scrap levels can accordingly result.
In accordance with one aspect of the present invention, where parting the grid palrs or bowing of grids is a problem, such problems can be obviated hy increasing the arsenic content to above the trace impurity level previously described herein. As may be appreciated from the prior discussion, the increase in arsenic content to obviate problems such as discussed herein represents a trade-off in properties so the level used should be kept as low as possible. The ohjective can be achieved by increasing the arsenic content to ahout 0.1 to 0.15 wt.~. The content may perhaps be increased in some applications to about 0.2 wt.% or slightly higher. In such circumstances, it is necessary to increase the silver content to the higher portion of the range previously set forth, a level of about 0.00~ to 0.01 being preferred. It appears that the subtle'increase in the silver content compensates for the arser.ic content present. Also, it may be deslr-, able to further incre~se the silver content above about 0.012 wt.%l up to perhaps about 0.015 wt.%, to accommodate arsenic levels significantly above 0o2 wt.
%. However, further increases not only do ~ot proYide advantages but can be hi'ghl'y detrimenkal. Such'increased levels of silver and arsenic detract from the'desired maintenance'free characterist'ics.
Alloys prepared in accordance with the present invention can contain impurities in amounts typically present in commercially available battery grade'lead. Additional impurities may also be pxesent in the alloy as a result of impurities typically present in the antimony and other alloying constituents. Fur-ther, additional ingredients can be intentionally added ; to the alloy of the invention, so 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 pre-sent in the alloy of the invention in an amount up to about .06 or .08 wt.~ without significant detrimental effects. Above the level of .06 to .08 wt.~, the copper precipitates at grain boundaries and can cause corrosion problems.
The alloy described herein may be produced using conventional techniques by adding the antimony, tin, selenium, silver and, if needed, arsenic, to the molten lead and mixing until the mass is homogeneous.
Production of grids using the alloy can then be accom-plished by commercially available, high speed grid manufacturing equipment. The alloy can be satisfactor-ily cast in this fashion using pot temperatures in the range'of about 780 to 850F, ladle temperatures in therange of about 900 to 1000F, and mold temperatures of about 300 to 400F.
! The grids formed ~rom the alloys of the ~2~

present invention possess an optimum combination of strength and ductility. Whe~ the' grids are formed ~y direct oasting techniques,,the'ir instantaneous handling strength is sufficient to allow casting at commercially accepted speeds as well a~ to allow the necessary trimming and stacking oper~tions to be performed without adverse distortion of ~he grid configuration resulting.
Ade~uate stiffness for conventional pasting operations is achieved within tw~ days after casting. The ductility of the grids insures that pasting and battery assembly procedures may be carried out without breakage. Indeed, when the arsenic conten-~ is at the trace impuri~y level, the grids are sufficiently ductile to allow bending into extremely tight rolls in a repeated ~ashion without cracking occurring at any of the wire intersections in typical grid configurations. This ductility may particularly be useful in making grids by expanded metal techniques.
The structure of the battery need not be modi-fied to accommodate grids formed from the alloys of the present invention. The ~grids may thus be simply sub-stituted for the grids used in any of the many commer-cially 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 posi-tive grids from the alloys of this inventlon. The nega-~ive grids may employ 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 wt.% tin with from about 0.06 up to about 0.15 or 0.20 wt.% calcium. Sati~faotory alloys axe disclosed in c~ian Patent No. 1,028jS32 i5sued on March 28, 1978, or:
"Lead Base Calcium-Tin Alloy and Use Thereof."
The following Example is illustrative, and not in limitation, of the present invention. All percentages are based upon the weight of the alloy.

Alloys having the composltions 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 ~arious periods of aging at ambient temperature after casting. The re-sults are given in Table II.

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TABLE I

ALLOY lA ALLOY lB ALLOY 2A ALLOY 2B

Antimony 1.4 1.4 2.5 2.5 Tin .2 .2 .2 .2 Selenium .018 .018 .018 .018 Silver .01 0 .01 0 Lead Balance Balance Balance Balance TABLE II

ULTIMATE
TENSILEYIELD
DAYS STRENGTHSTRENGTHELONGATION
AGING ALLOY (psi)(psi) (~) 1 lA 4600 2150 10 lB 4100 1600 15 2 lA 4600 2900 8 lB 4100 2000 12

3 lA 4700 2800 9 lB 4000 2000 12 !

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Alloys lA and 2A are in accordance with the present in-vention. Alloys lB and 2B, which were tested ~or com-parative purposes, differ from alloys lA and 2A ln that the silver component wa5 omitted. The test results shown in Table II demonstr~te that addition of silver in the small amount required by the present invention has a sig-nificant effect on t~e mechanical characteristics of the allcy.
Preliminary casting studies and battery per-formance data support the uniqueness of the alloys of the present invention. Thus, such alloys are extremely stable in the molten state and exhibit relatively low levels o~ drossing so that the nominal composition for-mulated 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 differing siæed cross-section members are joined together. The microstructure is much more refined than other alloys of this type not including silver as an alloying ingre-dient, and preliminary corrosion and battery performance characteristics indicate performance equal or even superior to other commercially used low antimony, lead based alloys. Batteries using grids formed from alloys of the present invention in preliminary studies exhibit about 20 to 30% reduced gassing and water loss charac-teristics in comparison to similar alloys having excessive amounts of arsenic.

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Thus, as may be seen, the present invention provides alloys Which satisfy the many and dive~se characteristics necessary for lead-aci~ battery grids.
Such grids are particularly useful in maintenance-free battery applications.

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Claims (17)

1. The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
   
   l. A lead based alloy consisting essentially of:
   from about 1.0 to about 2.8 wt.% antimony;
   from about 0.1 to about 0.4 wt.% tin;
   from about .005 to about .03 wt.% selenium;
   from about .004 to about .012 wt.% silver;
   and the balance lead.
2. 2. The alloy of claim 1 wherein the selenium is present in an amount between about 0.01 wt.% and .03 wt.%.
3. 3. The alloy of claim 1 wherein the silver is present in an amount of from about .005 wt.% to about .01 wt.%.
4. 4. The alloy of claim 1 wherein the silver is present in an amount of from about .006 wt.% to about .008 wt.%.
5. 5. The alloy of claim 1 wherein the antimony is present in an amount between about 1.4 and 2.6 wt.%.
6. 6. The alloy of claim l wherein the antimony is present in an amount between about 1.4 and about 1.6 wt.%.
7. 7. A battery grid useful for supporting an electrochemically active material in a lead-acid battery system, said grid being comprised of an alloy that consists essentially of:
   from about 1.0 to about 2.8 wt.% antimony;
   from about 0.1 to about 0.4 wt.% tin;
   from about .005 to about .03 wt.% selenium;
   from about .004 to about .012 wt.% silver;
   and the balance lead.
8. 8. The battery grid of claim 7 wherein the grid is directly cast from said alloy.
9. 9. The battery grid of claim 7 wherein the silver is present in said alloy in an amount between about .006 and about .008 wt.%.
10. 10. A pair of directly cast battery grids useful for supporting an electrochemically active material in a lead-acid battery system, the grids being joined together by parting areas, said grids being comprised of an alloy that consists essentially of:
    from about 1.0 to about 2.8 wt.% antimony;
    from about 0.1 to about 0.4 wt.% tin;
    from about .005 to about .03 wt.% selenium;
    from about .008 to about .012 wt.% silver;
    arsenic in an amount up to about 0.2 wt.%;
    and the balance lead.
11. 11. In a lead-acid battery system containing positive and negative sets of grids the improvement wherein at least one of said sets of grids is comprised of grids as defined in claim 7.
12. 12. In a maintenance-free, lead-acid battery containing positive and negative sets of grids, the improvement wherein at least one of the sets of grids is comprised of grids as defined in claim 7.
13. 13. The maintenance-free, lead-acid battery of claim 12 wherein the negative sets of grids are formed from a calcium-tin-lead alloy.
14. 14. A battery grid useful for supporting an electro-chemically active material in a lead-acid battery system, said grid being comprised of any alloy that consists essentially of:
    from about 1.0 to about 2.8 wt.% antimony;
    from about 0.1 to about 0.4 wt.% tin;
    from about .005 to about .03 wt.% selenium;
    from about .005 to about .012 wt.% silver;
    and the balance lead.
15. 15. A 1 ad based alloy consisting essentially of:
    from about 1,0 to about 2.8 wt.% antimony;
    from about 0.1 to about 0.4 wt.% tin;
    from about .005 to about .03 wt.% selenium;
    from about .004 to about .012 wt.% silver;
    up to about 0.2 wt.% arsenic;
    and the balance lead.
16. 16. A battery grid comprised of the alloy of Claim 15.
17. 17. The battery grid of Claim 16, wherein said arsenic in said alloy is present in an amount sufficient to stiffen said alloy to prevent excessive bowing of said grid during the processing thereof.