Battery grids lead alloy, oxidation

A simple, rapid and seleetive eleetroehemieal method is proposed as a novel and powerful analytieal teehnique for the solid phase determination of less than 4% antimony in lead-antimony alloys without any separation and ehemieal pretreatment. The proposed method is based on the surfaee antimony oxidation of Pb/Sb alloy to Sb(III) at the thin oxide layer of PbSOyPbO that is formed by oxidation of Pb and using linear sweep voltammetrie (LSV) teehnique. Determination was earried out in eoneentrate H SO solution. The influenee of reagent eoneentration and variable parameters was studied. The method has deteetion limit of 0.056% and maximum relative standard deviation of 4.26%. This method was applied for the determination of Sb in lead/aeid battery grids satisfaetory. 

Finally, the conversion of the primary metal into the product and the form which are actually utilized in the battery system should be considered. For example, the electrode materials in lead acid batteries are normally cast lead or lead-alloy grids. The materials utilized in NiCd batteries are cadmium oxide and high surface area nickel foams or meshes. Technically, all of these factors should be considered to produce a detailed life cycle analysis. However, again, these differences are generally quite small compared to the principal variables - composition, performance and spent battery disposal option. 

As a rule, national battery standards stipulate only Pb purity grade of 99.99% without specifying the type and amount of allowable impurities. The specific infiuence of additives to and impurities in lead alloys has been in the focus of attention of many researchers [6—12]. Table 4.3 summarises tbe maximum allowable impurity levels for both primary and secondary lead for battery use [10]. Secondary lead comes from recycling batteries after purification. Lead of the purity grade presented in Table 4.3 can be used for the manufacture of leady oxide and lead alloys for both positive and negative grids. 

A semiconductor mechanism was proposed for the electrocatalytic action of tin on the oxidation of PbO to PbO and Pb02. Tin has substituted antimony as an additive to the alloys for lead—acid battery grids. It is added in considerably lower concentrations than Sb and in combination with calcium which improves the mechanical properties of the alloys. Thus the interface problem of lead—calcium grids was resolved and the way was open for the manufacture of maintenance-free wet-charged batteries. 

Positive grid corrosion. The oxygen evolved on the positive plates penetrates through the interface grid/active material and oxidizes the lead alloy of the positive grid (see Chapter 2.11, p. 91). The rate of this process depends on grid alloy composition, cell temperature, positive plate potential and battery duty. 

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. 

However the battery structure has changed substantially from initial ones. In the early days of lead acid batteries, the corrosion layers formed on the surface of lead sheet were used as active materials. But at present, the pasted type electrodes, which are made from lead-oxide paste and lead-alloy grid, are used generally. Then such pasted type electrodes are charged in sulfuric acid to make positive and negative plates and have much larger effective surface area which leads to larger capacity compared to the batteries of the early days. 

Rea.ctivity ofLea.d—Ca.lcium Alloys. Precise control of the calcium content is required to control the grain stmcture, corrosion resistance, and mechanical properties of lead—calcium alloys. Calcium reacts readily with air and other elements such as antimony, arsenic, and sulfur to produce oxides or intermetaUic compounds (see Calciumand calciumalloys). In these reactions, calcium is lost and suspended soHds reduce fluidity and castibiUty. The very thin grids that are required for automotive batteries are difficult to cast from lead—calcium alloys. 

Since the early days of using PVC separators in stationary batteries, there has been a discussion about the generation of harmful substances caused by elevated temperatures or other catalytic influences, a release of chloride ions could occur which, oxidized to perchlorate ions, form soluble lead salts resulting in enhanced positive grid corrosion. Since this effect proceeds by self-acceleration, the surrounding conditions such as temperature and the proneness of alloys to corrosion as well as the quality of the PVC have to be taken carefully into account. 

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. 

When the lead—antimony alloy commonly used for the positive grids of lead—acid batteries was substituted for lead—calcium alloy, the cycle life of the batteries decreased dramatically. Obviously, antimony influenced the electrochemical behaviour of Pb02- One of the hypotheses was that it affected the hydration of Pb02 PbO(OH)2 particles. This hypothesis was verified through oxidation of Pb—Sb alloys with different content of antimony and determining the water content in the anodic Pb02 layer formed. The obtained results of these investigations are presented in Fig. 10.27 [34]. 

The first working lead cell manufactured in 1859 by the French scientist, Gaston Plante, consisted of two lead plates separated by a strip of cloth coiled and inserted into a jar with sulfuric acid. A surface layer of lead dioxide was produced by electrochemical reactions in the first charge cycle. Later developments led to electrodes made by pasting a mass of lead oxides and sulfuric acid into grids of lead-antimony alloy (for lead acid batteries the electrodes are often ealled plates). 

The battery components are fabricated and processed as shown in the flowsheets of Fig. 23.10. The major starting material is highly purified lead. The lead is used for the production of alloys (for subsequent conversion to grids) and for the production of lead oxides [for subsequent conversion first to paste and ultimately to the lead dioxide positive active material (Fig. 23.10a) and the sponge lead negative active material]. 

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