Passivation of Lead by its Oxides

The production of tubular positive plates is in principle similar to that of pasted plates. A number of manufaeturers use the same gray oxide as the basic filling substance. Sometimes the share or red lead or minium (PbjO ) is increased above 25 or even to lOOwt.%. The latter is more economic when the manufacturer runs his own minium plant then the expense of the chemical oxidation of lead oxide (PbO) to minium (Pb304) may be compensated by reduced formation cost. Furthermore, curing is not required, because of the high oxidation state, and the battery starts with full capacity when formed. 

Different methods are in use for plate filling. The material can be filled as a powder with the aid of vibrators. Other techniques use a slurry of lead oxide or even a paste, as described above [27]. 

When dry material or a slurry has been filled, pickling is required, which means that the plate is stored in sulfuric acid for a short time. The material is soaked by the acid and transformed, at least partly, into lead sulfate (PbS04), as in the pastemixing process (Section 4.4.2.1). When minium is used, during the pickling process lead dioxide is also formed according to Eq. (4). 

The subsequent procedures, formation, washing, drying, and battery assembly are similar to those described above. 

Corrosion of the current-conducting elements in the positive electrode, as of the plate support (grid), bus bars, and termi- 

Corrosion of the current-conducting elements in the positive electrode, as of the plate support (grid), bus bars, and terminals, is a side-effect of the high cell voltage of this battery system, which implies a high potential of the positive electrode. Metals that are usually applied as current conductors, and even noble metals like gold, would be dissolved by oxidation when connected to the positive electrode of the lead-acid battery. 

Lead can be used, because the corrosion itself forms a rather dense passivating layer of lead dioxide that protects the underlying material against fast corrosion [27]. If foreign metals like copper are used they have to be covered thoroughly by a dense layer of lead. 

The Pb02 /PbOx border slowly penetrates into the metal, but only at a very slow rate as a solid-state reaction. Cracks are formed when the oxide layer exceeds a given thickness, on account of the growth in volume when lead becomes converted into lead dioxide (Table 6.7). Underneath the cracks the corrosion process starts again and again. As a whole, the corrosion proceeds at a fairly constant rate. It never comes to a standstill, and a continually flowing anodic current, the corrosion current, is required to re-establish the corrosion layer. 

When the grid material (Pb) is converted into lead dioxide (Pb02), the basic electrochemical reaction is 

Twice the amount of electricity is required compared with the discharge reaction at the negative electrode according to Equation 6.18, since corrosion involves four valences, which means 4F= 107.21 Ah per multiple of Equation 6.31. Consequently, for the corrosion reaction according to Equation 6.31 the equivalent values are  

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Lead s durability (its chemical inertness) and malleability make it useful in the construction industry. The inertness of lead under normal conditions can be traced to the passivation of its surface by oxides, chlorides, and sulfates. Passivated lead containers can be used for transporting hot concentrated sulfuric acid but not nitric acid, because lead nitrate is soluble. Another important property of lead is its high density, which makes it useful as a radiation shield because its numerous electrons absorb high-energy radiation. The main use of lead today is for the electrodes of rechargeable storage batteries (see Box 12.1). 

Therefore, passivation of the positive electrode by poorly conducting PbS04 can be reduced [348]. The porosity is important because it enables the expansion during the solid phase volume increase, which accompanies the transformation of Pb02 to PbS04. In the most popular construction, the electrode paste material (mixture of metallic lead with lead oxides) is held in a framework composed of lead alloys with additions of tin, antimony, selenium, and calcium [348]. Antimony improves the mechanical stability however, it increases the resistance and facilitates the selfdischarge of the battery. Better results are obtained for low antimony content and/or for lead-calcium alloys [203]. Methods of positive electrodes improvement, from the point of view of lead oxide technology have been discussed [350]. Influence of different factors on life cycle, nature, and composition of the positive active mass has been studied by Pavlov with coworkers [200, 351, 352]. 

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