Grid corrosion, lead oxides

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. 

These equations are based on the thermodynamically stable species. Further research is needed to clarify the actual intermediate formed during overcharge. In reality, the oxygen cycle can not be fully balanced because of other side reactions, that include grid corrosion, formation of residual lead oxides in the positive electrode, and oxidation of organic materials in the cell. As a result, some gases, primarily hydrogen and carbon dioxide (53), are vented. 

Based on the above-discussed experimental results, it can be concluded that alloying additives influence the rate of the reaction of lead oxidation at the interface grid metal I corrosion layer. Probably, they induce changes in the structure of the interface as well and thus create conditions for the reaction of lead oxidation to proceed faster or slower. Since, this is a solid-phase reaction, the changes in volume of the newly formed phase will also play their role. Considering the unit cell structures presented in Fig. 2.43 and the presumed incorporation of oxygen in the crystal lattice of lead and formation of tet-PbO, it can be expected that alloying additives wiU affect this elementary process of transformation... 

X 10 Q cm [4]. Oxidation of the CL proceeds further to the formation of Pb02 (10 to 1.2 X 10 Q cm [4]). Additives to the grid lead alloys affect the oxidation rate of the oxides in the CL. Hence, alloying additives should be selected so as to retard oxidation of the lead grid (i.e., the grid corrosion process) and accelerate (facilitate) the processes of oxidation of PbO to Pb02. 

It has been established that thermopassivation of the positive plates is a result of ehanges in composition of the lead oxide corrosion layer [1]. This is a very thin film formed during the formation process and is composed mostly of PbOi- When the plates are heated to above 80 °C, Pb of the grid reacts with Pb02 via a solid-state reaction yielding non-stoichiometric PbOn. When the stoichiometric coefficient decreases below a critical value (n < 1.5), the resistance of the corrosion layer increases significantly, which causes thermopassivation of the plates. 

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. 

In some cases, the electrode reaches some potential value at which an electrochemical dissolution of its components into the electrolyte occurs for example, the positive grid of lead-acid batteries contains antimony at the high potential of the grid, Sb can be oxidized to soluble SbO which can be reduced at the negative lead electrode. An antimony deposit then occurs leading to an acceleration of the self-discharge corrosion reaction. 

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. 

The grid lead and the free lead in the paste are oxidized. The lead in the grid alloy is oxidized, forming a corrosion layer on the grid rib surface, which is tightly bound to the paste skeleton. The free lead in the paste is also oxidized. 

The paste free lead is normally first oxidized by Oj, then the oxidation of the grid lead alloy follows. These two processes should be the same except they proceed at different rates because alloy additives affect the rate of grid corrosion. For these two oxidation reactions, moisture water serves as the catalyst, and the heat produced can help with water removal from the paste. 

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. 

The EV range is obviously affected by the level of selfdischarge. The latter is mainly due to the reduction of Pb02 by lead of the grid and to the formation of anodic oxidation products (e.g., of Sb) which diffuse to the negative plate. Their deposition decreases the hydrogen overpotential and results in corrosion of... 

At high pH values, lead is oxidized to Pb(OH)2 in a first step, and tin is oxidized, first to SnO, and subsequently to Sn02. The Pb(OH)2 may be reduced by SnO back to lead with consequent formation of Sn02, in a simple redox procedure [67]. In this reaction, the PbO layer becomes thinner and more enriched with Sn02. Significant conductivity of the grid active-material interface requires a concentration of tin of lOwt.% or higher in the corrosion product [65]. 

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. 

---