Abstract:
An alloy for casting terminals for lead-acid batteries which has good corrosion resistance, produces less dross during manufacturing, has resistance to polypropylene degradation, as well as other improved features. The alloy is composed of antimony present in the range of about 2.5-4.75%; arsenic present in the range of about 0.15-0.35%; tin present in the range of about 0.23-0.6%; copper present in the range of about 0-200 ppm; sulfur present in the range of about 0-40 ppm and the balance lead. In an alternative embodiment, copper and sulfur are replaced with selenium in the range of about 40-90 ppm.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The field of invention is lead-acid storage batteries. More particularly, it relates to an improved alloy for producing side terminals for these batteries. 
     2. Background Art 
     A lead alloy commonly used to make terminals for lead-acid storage batteries is composed of 2.75-3.25% antimony, 0.05-0.20% arsenic, 0.15-0.40% tin, 400-600 ppm copper, less than 30 ppm sulfur and less than 20 ppm selenium. Actual composition specifications for the referenced alloy are those placed into the caster, rather than the finished bushing composition. When parts were produced by gravity casting at temperatures typically above 800° F., this necessitated the use of grain refiners (Cu, S, and/or Se) for suitable metallurgical structure. When parts are produced via a die casting process typically at temperatures below 800° F., such grain refiners (Cu, S, and/or Se) form dross and are reduced or completely lost from the finished part. Unwanted reductions in other alloying elements can occur as dross forms. Drossing losses are an expense that is figured into the bushing cost. 
     Side terminal bushing properties that are important to safety and functional performance are nut rotation, weldability, weld torque, age hardening, and corrosion resistance. Particularly in hot climate areas which accelerate corrosion processes, improved corrosion resistance is desirable for extended product life and performance. A hypothesis of corrosion sensitivity was developed, which involves the composition of the bushing alloy, welding temperatures (particularly at the lead-to-poly interface), and catalytic or protective interaction of alloy components in the degradation of polypropylene container, and associated lead alloy corrosion due to the interaction of polypropylene decomposition products and lead alloys. 
     Thus, there is a need for an improved alloy for manufacturing terminals in lead-acid storage batteries. 
     The objects of the invention therefore include: 
     a. providing an alloy for manufacturing terminals for lead-acid storage batteries which have reduced drossing during die casting operations; 
     b. providing an alloy of the foregoing type which reduces material losses and cost of recycling; 
     c. providing an alloy of the foregoing type which has improved composition stability due to reduced pull-out of alloy components; 
     d. providing an alloy of the foregoing type which has improved compatibility with polypropylene at elevated temperatures by reducing copper, which catalytically oxidizes polypropylene and increasing tin, which provides anti-oxidant protection to polypropylene; 
     e. providing an alloy of the foregoing type which has improved grain refinement for the terminal weld joint; 
     f. providing an alloy of the foregoing type which has improved intrinsic corrosion resistance; 
     g. providing an alloy of the foregoing type which has improved corrosion resistance when assembled into a finished battery. 
     h. providing an alloy of the foregoing type which has improved tolerance to high temperature welding conditions; and 
     i. providing an alloy of the foregoing type which retains minimum void volume, acceptable nut rotation torque, weld torque, and weld current range. 
     BRIEF SUMMARY OF THE INVENTION 
     The shortcomings of the prior art and the objects of the invention are accomplished in one aspect by an alloy for use in casting a lead-acid battery terminal which is composed of antimony which is present in the range of about 2.5-4.75%; arsenic present in the range of about 0.15-0.35%; and tin present in the range of about 0.2-0.6%, the percentages being based upon the total weight of the lead-based alloy. Copper is present in the range of about 0-200 ppm, and sulfur is present in the range of about 0-40 ppm. The balance of the alloy is essentially lead. 
     In a preferred embodiment, antimony is present in the alloy in the range of about 2.75-3.25%; arsenic is present in the range of 0.18-0.28%; and tin is present in the range of about 0.20-0.35%, the percentages being based upon the total weight of the lead-based alloy. Copper is present in the range of about 0-100 ppm, and sulfur is present in the range of about 0-40 ppm. The balance of the alloy is essentially lead. 
     In a more preferred embodiment, antimony is present in the alloy in an amount of about 3.0%; arsenic is present in an amount of about 0.2%; and tin is present in an amount of about 0.3%. Copper is present in an amount of less than about 50 ppm and sulfur is present in an amount of less than about 15 ppm. Such copper and sulfur ranges are background concentrations which are typically found in smelted lead. 
     In an alternative embodiment, copper and sulfur are not present in the alloy above such typical background concentrations and the alloy further includes selenium in a range of about 40-90 ppm. 
     In another aspect, there is a side terminal for a lead acid storage battery comprising a fastening member and a bushing surrounding the nut member. The bushing is molded into a side wall of the battery and is composed of an alloy comprising about 2.5-4.75% antimony; about 0.15-0.35% arsenic; and about 0.2-0.6% tin. Copper is present in the range of about 0-200 ppm, and sulfur is present in the range of about 0-40 ppm. The balance is essentially lead. 
     In another preferred embodiment, the fastening member of the side terminal is a nut and the side terminal is connected to a wall of a lead-acid storage battery which has a container composed of polypropylene. 
     In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown by way of illustration preferred embodiments of the invention. Such embodiments do not necessarily represent the full scope of the invention. Therefore, reference is made to the claims herein for interpreting the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a lead-acid battery container with side terminals composed of the alloy of this invention. 
     FIG. 2 is a view in partial cross-section of one of the side terminals shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     The corrosion failures of a side terminal joint are expected to drop as much as 90% through the use of this alloy. The success of this alloy is attributed to reduction of copper (catalyst for poly decomposition the products of which increase lead alloy corrosion), increased tin (poly anti-oxidant protection), increased arsenic (hardness recovery at increased tin content for resistance to corrosion, induced stresses and retains other mechanical requirements of the terminal), and reduction of sulfur (improved composition stability during casting). As arsenic is increased from its minimum of 0.15%, for every 0.01% increase in arsenic, tin is increased by 0.01% until a maximum of 0.35% is reached at which point tin can increase up to 0.6% with no further increase in arsenic preferred. The improved alloy of the invention has been optimized to preserve nut rotation and weld torque performance within currently acceptable ranges. Weldability and age hardening have not been adversely affected. 
     The alloys of the present invention may be produced in the conventional manner by adding the constituents to molten lead in the prescribed amounts and mixing to obtain a homogenous melt. In a preferred manner, 3.0% antimony, 0.3% tin, 0.2% arsenic, 0.005% copper, and 0.0015% sulfur are in the molten lead. Note that copper and sulfur are normally present in secondary lead at no more than these concentrations. Therefore, they are either added to or not refined from the molten lead to attain such concentrations. The percentages are based upon the total weight of the lead alloy. A preferred method for casting the alloy of this invention is die casting. 
     Bench marking analyses of several side terminal alloys, regardless of the tombstone-terminal design, were conducted for elemental composition, polypropylene interaction, and limited tank corrosion resistance. The results are summarized in attached Table 1. 
     
       
         
               
             
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Competitive Side Terminal Alloy Comparison 
               
             
          
           
               
                   
                 Anti- 
                   
                   
                   
                   
                   
                   
               
               
                   
                 mony 
                 Arsenic 
                 Tin 
                 Copper 
                 Sulfur 
                 Selenium 
                 Poly 
               
               
                 Alloy 
                 % 
                 % 
                 % 
                 PPM 
                 PPM 
                 PPM 
                 Degrad 
               
               
                   
               
             
          
           
               
                 A 
                 3.05 
                 0.167 
                 0.125 
                 67 
                 &lt;1 
                 132 
                 Moderate 
               
               
                 B 
                 2.8 
                 0.12 
                 0.13 
                 300 
                 &lt;5 
                 &lt;20 
                 Severe 
               
               
                 C 
                 3.1 
                 0.195 
                 0.43 
                 207 
                 80 
                 &lt;20 
                 Severe 
               
               
                 D 
                 4.6 
                 0.125 
                 0.24 
                 621 
                 22 
                 &lt;20 
                 Moderate 
               
               
                 E 
                 3.2 
                 0.206 
                 0.21 
                 576 
                 68 
                 &lt;20 
                 Moderate 
               
               
                 F 
                 4.5 
                 0.112 
                 0.23 
                 117 
                 86 
                 &lt;20 
                 Slight 
               
               
                 G 
                 3.1 
                 0.036 
                 0.43 
                 24 
                 19 
                 &lt;20 
                 Minimal 
               
               
                   
                   
                   
                   
                   
                   
                   
                 (best) 
               
               
                 H 
                 3.0 
                 0.200 
                 0.30 
                 &lt;100 
                 &lt;40 
                 &lt;20 
                 Slight 
               
               
                   
               
             
          
         
       
     
     Most notable of the group was the low incidence of corrosion related failure in the G alloy compared to the D alloy, which were both of similar construction. The superior performance of the G alloy was attributed to the high tin and very low copper concentrations observed. 
     A range of alloys were prepared in standard side terminal bushings (FIG. 2) to evaluate overall performance from molding through welding and corrosion testing. During this process, a series of parameters were observed to assess the suitability of varied alloys through processing. The parameters and results are summarized on the following Tables 2-4. Tables 2 and 3 set forth the composition of eight alloys numbered 1-8. Alloys  1  and  5  represent benchmark or comparative alloys. Alloy  2  is the alloy of this invention. Table 4 sets forth the tests conducted and observations made. 
     Alloy  1  is an alloy of the prior art. Alloy  2  is an exemplary alloy of the present invention. Alloy  3  is a competitive alloy. Alloy  4  is a high arsenic version of alloy  3 . Alloy  5  is an experimental high tin alloy. Alloy  6  is an experimental high antimony alloy. Alloys  7  and  8  are also experimental alloys. 
     In addition to the observations and comments made in Table 4, the following observations are made with respect to the indicated testing: 
     Void Volume: All within specification of &lt;1%. 
     Nut Rotation: The numbered designations show torque values and are in terms of inch-pounds. All are within existing specification, except for alloy  5 . 
     Weld Torque: The Current Range is in terms of amps. Torque values are expressed as in-lb. All are within existing specification for immediately after welding. 
     Metallography: Slightly coarser grain structure with improved alloy  2  when compared to benchmark alloy  1 . 
     Poly Oxidation: Polypropylene in contact with 2% tin alloy demonstrated no visible discoloration/oxidation over a two week period at 150° C. and is believed to impart protective qualities to polypropylene at elevated temperatures. The 2% tin alloy  5  is not suitable for side terminal use due to its inherent softness, but was chosen for test based on perceived superior corrosion resistance. The improved alloy  2  ranked third of the viable candidate alloys in this test based on time to discoloration and extent of discoloration at fixed intervals. 
     Coupon Corrosion: Of the four alloys tested, the intrinsic corrosion of the improved alloy  2  was significantly less than the benchmark alloy  1  or alloy  3 . The improved alloy  2  was comparable to alloy  4 . Intrinsic corrosion assesses the corrosion rate of the as cast alloy. 
     Tank Corrosion: Improved alloy  2  demonstrated a single failure (6%) in a relatively high temperature/high amperage weld. Its composition is believed to provide improved tolerance to higher temperature welding extremes then the other alloys examined. Failure rates between 53% and 26% for standard alloy  1  composition and 47% for alloy composition  7  were found. Results for alloys  3  and  4  exhibited failure rates of 25-26% with sensitivity to high welding temperatures and/or scattered results. Tank corrosion assesses the corrosion induced failures of the fully processed terminal, including poly molding, brushing, and welding. 
     Conversion Factors: For purposes of converting percentages to ppm or ppm to percentages, the following equations can be employed: 
     
       
         %×10,000=ppm 
       
     
     
       
         ppm/10000=%  
       
     
     
       
         
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Alloy Test Plan 
               
             
          
           
               
                   
                 1 
                   
                 1a 
                 1b 
                 1c 
                 2 
                   
                   
                 3 
                   
                   
                 4 
                   
                   
               
               
                   
                 Nom 
                 +/− 
                 Meas. 
                 Meas. 
                 Meas. 
                 Nom 
                 +/− 
                 Meas. 
                 Nom 
                 +/− 
                 Meas. 
                 Nom 
                 +/− 
                 Meas 
               
               
                   
                   
               
             
          
           
               
                 Sb 
                 3.00  
                 0.25  
                 2.76  
                 3.05  
                 3.08  
                 3.00  
                 0.25  
                 2.96  
                 3.25 
                 0.25 
                 3.05  
                 3.25  
                 0.25 
                 2.95 
               
               
                 Sn 
                 0.275 
                 0.125 
                 0.277 
                 0.251 
                 0.236 
                 0.300 
                 0.100 
                 0.273  
                 0.45 
                 0.05 
                 0.401  
                 0.45  
                 0.05 
                 0.399 
               
               
                 As 
                 0.125 
                 0.075 
                 0.133 
                 0.135 
                 0.137 
                 0.200 
                 0.05  
                 0.194  
                 0.05 
                 max 
                 0.029  
                 0.290 
                 0.05 
                 0.211 
               
               
                 Cu 
                 0.050 
                 0.010 
                 0.048 
                 0.043 
                 0.047 
                 0.005 
                 max 
                 0.0022 
                  0.005 
                 max 
                 0.0019 
                 0.005 
                 max 
                 0.0018 
               
               
                 S 
                  0.0055 
                 0.003 
                  0.0028 
                  0.0016 
                  0.0019 
                 0.002 
                 max 
                 0.0022 
                  0.002 
                 max 
                 0.0016 
                 0.002 
                 max 
                 &lt;.0005 
               
               
                 Se 
               
               
                 Ag 
                 0.01  
                 max 
                 0.002 
                 0.003 
                 0.003 
                 0.01  
                 max 
                 0.0009 
                   
                   
                 0.0013 
                   
                   
                 0.0013 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 Alloy Test Plan (Continued) 
               
             
          
           
               
                   
                 5 
                   
                   
                 6 
                   
                   
                 7 
                   
                   
                 8 
                   
                   
               
               
                   
                 Nom 
                 +/− 
                 Meas. 
                 Nom 
                 +/− 
                 Meas. 
                 Nom 
                 +/− 
                 Meas. 
                 Nom 
                 +/− 
                 Meas. 
               
               
                   
                   
               
             
          
           
               
                 Sb 
                 &lt;.0010 
                 0 
                 0.0025 
                 4.25  
                 0.25  
                 4.23 
                 3.00  
                 0.25  
                 3.01 
                 3.100  
                 0.150  
                 2.92 
               
               
                 Sn 
                 2.00 
                 0.05 
                 2.109 
                 0.300 
                 0.100 
                 0.283 
                 0.300 
                 0.100 
                 0.309 
                 0.1250 
                 0.0250 
                 0.148 
               
               
                 As 
                 &lt;.0005 
                 0 
                 &lt;.0005 
                 0.200 
                 0.05  
                 0.206 
                 0.125 
                 0.075 
                 0.121 
                 0.1500 
                 0.0500 
                 0.155 
               
               
                 Cu 
                 &lt;.0010 
                 0 
                 0.00054 
                 0.005 
                 max 
                 0.003 
                  0.0175 
                  0.0075 
                 0.017 
                 0.0200 
                 max 
                 0.0059 
               
               
                 S 
                 &lt;.005 
                 0 
                 &lt;.0005 
                 0.002 
                 max 
                 &lt;.002 
                 0.004 
                 max 
                 &lt;.0005 
                 0.0015 
                 max 
                 &lt;.005 
               
               
                 Se 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                 0.0100 
                 0.003 
                 0.0064 
               
               
                 Ag 
                 &lt;.0050 
                   
                 0.0034 
                   
                   
                 &lt;.0024 
                   
                   
                 &lt;.0024 
                   
                   
                 0.0020 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 Summary of Experimental Results 
               
             
          
           
               
                   
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
               
               
                   
                   
               
             
          
           
               
                 Void 
                 0.64 
                 0.52 
                 0.86 
                 0.82 
                 0.38 
                 0.94 
                 0.43 
                 0.52 
               
               
                 Volume 
               
               
                 Nut 
               
               
                 Rotation 
               
               
                 1 hr after 
                 191 
                 192 
                 165 
                 196 
                 100 
                 No data 
                 175 
                 173 
               
               
                 casting (in-lb 
               
               
                 before nut 
               
               
                 rotation) 
               
               
                 Weld 
               
               
                 Current Range 
                 800 
                 500 
                 800 
                 500 
                 no data 
                 no data 
                 500 
                 no data 
               
               
                 (amps) 
               
               
                 Torque-fresh 
                 90 
                 81 
                 85 
                 88 
                 86.5 
                 93.2 
                 no data 
                 88.7 
               
               
                 (in-lb to break 
               
               
                 assembly) 
               
               
                 Metallography 
                 Benchmark 
                 coarser 
                 similar 
                 coarser 
                 similar/ 
                 similar 
                 coarser 
                 finest 
               
               
                   
                   
                   
                   
                   
                 larger 
               
               
                   
                   
                   
                   
                   
                 grain 
               
               
                   
                   
                   
                   
                   
                 structure 
               
               
                 Corrosion 
                 Benchmark 
                 best 
                 worse 
                 similar 
                 no data 
                 no data 
                 similar 
                 no data 
               
               
                 Alloy 
               
               
                 Coupon 
                 Benchmark 
                 Better 
                 Similar 
                 Better 
                 no data 
                 no data 
                 no data 
                 no data 
               
               
                 corrosion 
               
               
                 Poly Oxidation 
                 6th 
                 3rd 
                 2nd 
                 2nd 
                 1st 
                 4th 
                 5th 
                 4th 
               
               
                 Rank (2 weeks) 
               
               
                 1 = best, 6 = worst 
               
               
                   
               
             
          
         
       
     
     While the alloy of this invention has been described for use with a side terminal battery, it can be employed with all lead-acid battery terminals including top posts, marine, and flag types. The improved alloy described is suitable for any die cast termination, as well as gravity casting. This broad application is important, as many bushing/terminal sources use common lead pots to feed various molding dies. With multiple alloys, potential for increased part costs exists if casting pot alloy must be changed frequently to manufacture different parts. 
     Although the incorporation of grain refiners, such as copper and sulfur, are not specifically needed for bushing quality and are generally undesirable in the die cast process, evidence indicates that the inclusion of selenium will assist in producing a desirable fine grained weld without the detrimental effects of copper and sulfur at levels required for grain refinement as described in the prior art. A practical working concentration of 50-70 ppm Se in die cast processes is feasible and produces desired results. The lower level is set well above background to provide grain refinement. The upper limit is set at a level below that which causes poly degradation and associated corrosion and by excessive drossing in the die cast process. Accordingly, a range of 40-90 ppm selenium can be employed for die cast terminals. 
     Referring to FIG. 1 of the drawing, there is shown a lead-acid battery  10  which includes a polypropylene container  11  with side terminals  12 . One of the side terminals is illustrated in FIG.  2  and has a bushing  14  into which is secured a stainless steel nut  16 . A weld button  18  is provided at the back for attachment to the usual tombstone joint. The bushing is composed of the alloy of this invention. 
     INDUSTRIAL APPLICABILITY 
     An alloy composition is provided for lead-acid battery terminals which alloy composition affords a reduction of dross in manufacturing; improved corrosion resistance as well as resistance to polypropylene degradation and other improved features.