Alloys for VRLA batteries

Following the studies on pure lead [57,64], a great deal of work has been undertaken to determine the effects of tin on the corrosion layer of lead-calcium-tin alloys, which are the major alloys for VRLA batteries. One study [82] showed that the corrosion rate of lead-calcium alloys was significantly reduced by the addition of tin and the thickness of the a-PbO layer was substantially reduced. It was further found that tin enrichment at grain boundaries in cast alloys induced a high level of tin in the corrosion layer that was able to suppress passivation. Finally, it was suggested... 

For many years, alloys with V values above 9 1 were not utilized for VRLA batteries. Large battery grids and tubular grids required alloys which would harden rapidly. Alloys of 0.08-0.12wt.% Ca and 0.3. 6wt.% Sn were most commonly utilized for VRLA batteries for standby and telecommunications service. These alloys harden rapidly and the alloys can be handled and pasted after a short ageing time at room temperature compared with weeks for materials with high r values. 

The basis for the performance of the alloy in VRLA batteries is corrosion of the lead-cadmium-antimony alloy to produce antimony in the corrosion layer of the positive grid, which thus eliminates the antimony-free eifect of pure lead or lead-calcium alloys. During corrosion, small amounts of antimony and cadmium present in the lead matrix are introduced into the corrosion product and thereby dope it with antimony and cadmium oxides. The antimony and cadmium give excellent conductivity through the corrosion product. The major component of the alloy, the CdSb intermetallic alloy, is not significantly oxidized upon float service, but may become oxidized in cycling service. 

As the markets for VRLA batteries continue to expand, higher demands will be placed upon the quality of the raw materials used in the manufacture of such batteries. It has been well documented that raw materials of high quality, i.e., lead, alloys, acid, and separators, are essential in order to guarantee battery performance... 

To close the recycling loop, the majority of the production from secondary lead producers returns to the battery manufacturer as alloys and soft lead. At present, secondary producers experience no problems in achieving the purity requirements for most automotive battery alloys, particularly those that are antimony-based. The increased purity requirement in VRLA batteries will, however, make the refining stages more critical. 

The grid metallics should be stored and recovered separately from the battery paste so that metals from the alloys in the battery grids do not contaminate the relatively pure lead paste, which is ideal for producing soft lead. Alloys used to manufacture VRLA batteries do not contain either antimony or arsenic, and this means that the potential hazard of stibine (antimony hydride, SbHs) and arsine (arsenic hydride, AsHs) formation during the storage of the metallics is removed. Many automotive batteries with antimonial and arsenical alloys are still in use. 

At the end of the twentieth century, maintenance-free VRLA batteries were invented. The first VRLA batteries employed lead—calcium grids. The antimony-free effect exhibited fully, which forced metallurgists to switch back to Pb—Sb alloys for the positive grids, minimising... 

Lead—tin alloys with Sn content from 0.7 to 1.2 wt% Sn are used for casting grids for spirally wound and prismatic VRLA batteries for automotive, stationary and special purpose applications. Table 4.6 presents a summary of the mechanical properties of Pb—1.0 wt% Sn alloys with or without 0.06 wt% Ca [91]. 

Lead product development efforts have been devoted to the development of new lead alloys to reduce the rate of corrosion, improve the conductivity between the grids and active material, and constrain the active material to increase life. Lead product development efforts for batteries are also aimed at improved oxides for the active materials leading to improved material utilization. Process development to improve the purity of lead from recycled batteries is also a major factor in improving the life of VRLA batteries. 

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