Positive-grid corrosion

Silver reduces the oxygen evolution potential at the anode, which reduces the rate of corrosion and decreases lead contamination of the cathode. Lead—antimony—silver alloy anodes are used for the production of thin copper foil for use in electronics. Lead—silver (2 wt %), lead—silver (1 wt %)—tin (1 wt %), and lead—antimony (6 wt %)—silver (1—2 wt %) alloys ate used as anodes in cathodic protection of steel pipes and stmctures in fresh, brackish, or seawater. The lead dioxide layer is not only conductive, but also resists decomposition in chloride environments. Silver-free alloys rapidly become passivated and scale badly in seawater. Silver is also added to the positive grids of lead—acid batteries in small amounts (0.005—0.05 wt %) to reduce the rate of corrosion. 

The excellent corrosion-resistant lead dioxide, Pb02, film formed on anodes and lead—acid battery positive grids in sulfuric acid has enabled lead insoluble anodes and lead—acid batteries to maintain the dominant positions in their respective fields. 

Other alloying ingredients in lead, eg, arsenic (0.5—0.7%) and silver [7440-22-4] (0.1—0.15%), inhibit grid growth on overcharge and reduce positive grid corrosion. Tin added to a lead alloy produces well-defined castings that are readily adapted to mass production techniques (84). 

Corrosion of the positive grid [Eq. (28)1 occurs equivalent to about 1 mA/lOOAh at open-circuit voltage and intact passivation layer. It depends on electrode potential, and is at minimum about 40-80mV above the PbS04/Pb02 equilibrium potential. The corrosion rate depends furthermore to some extent on alloy composition and is increased with high anti-monial alloys,... 

The network of lead wires must provide optimum mechanical support to the pellets of active material that fill the void space. Sufficient conductivity has also to be provided by the grid. Grids for positive and negative electrodes are usually similar. In batteries designed for extended service life, the positive grid is made heavier to provide a corrosion reserve. For very thin electrodes, a lead foil is used as the substrate and current conductor. 

Figure 9 illustrates the consequences for battery practice. The above penetration rate would reduce the cross—section of a grid spine in a tubular electrode by about 50% within the usual service life of 15 years. This result is confirmed by field experience and shows that long-life batteries must have a corresponding "corrosion reserve" in their positive grids. 

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. 

Let us note finally, that tellurium has been considered as an appropriate component for the lead grid alloy in lead-acid batteries, as improving its durability, mechanical strength, and anti-corrosive ability. In investigating Pb-Te binary alloys with different contents of Te (0.01-1.0 wt%) in sulfuric acid solution it was shown recently [104] that the introduction of Te can inhibit the growth of Pb02 and increase corrosion resistance of the positive grid alloy of a lead-acid battery. By the... 

Corrosion of the positive grid can occur on charging and overcharging if the metal becomes exposed to the electrolyte. This leads to a progressive weakening of the plate structure and to an increase in the internal resistance of the cell. 

FM5. Corrosion. The positive grid is subject to corrosion. The rate of this debilitating process is influenced by grid composition and microstructure, plate potential, electrolyte composition, temperature. The corrosion product is generally more electrically resistive than the grid and thus diminishes the output of the battery. In extreme cases, corrosion results in disintegration of the grid and collapse of the plate. 

Even higher tin contents (up to 2wt.%) have been reported [89] to provide reduction in the rate of corrosion and growth of positive lead-calcium grids in VRLA batteries employed in standby service at elevated temperatures. The beneficial effects of high tin on positive-grid corrosion in VRLA batteries have recently been confirmed [90]. It is proposed that the improved corrosion resistance is due to the large number of fine precipitate particles and better accommodation of the stresses of corrosion by the high mechanical properties of the alloys. 

Silver additions to lead-calcium-tin alloys. Silver has been added to lead-calcium-tin alloys to increase the resistance to creep and corrosion, and to prevent growth of the positive grids at elevated temperatures. Valve-regulated lead-acid batteries often operate at elevated temperatures and/or produce high internal... 

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. 

Universite de Nancy, CEAC-France, Riole Nationale Superieure de Chimie de Toulouse, Metaleurop Recherche, Bright Euram Project BE-7297, Task 8 Effect of alloying elements on the mechanical properties, corrosion resistance arui passivation of the positive grid. Final Report, 1 January 1994 to 31 December 1997, Advanced Lead-Acid Battery Consortium, Research Triangle Park, NC, USA, 1998. 

To ensure reliability over long periods of service, good purity standards are essential. With proper control of temperature and float voltage, service-lives in excess of 10 years are normal and positive grid corrosion is the usual failure mode. At high... 

In general, VRLA batteries use lead alloys that are virtually free of antimony. This is because antimony is released from the positive grid by corrosion and migrates to the... 

As positive grid corrosion is an important influence on the expected lifetime of standby batteries, there have been many investigations of the parameters that influence the corrosion rate. It has been established that many parameters influence grid corrosion and growth. The most important are (i) alloy composition (ii) grid design (iii) casting conditions (iv) positive active material (v) impurities that accelerate corrosion (vi) battery temperature and (vii) potential of the positive plate. 

Flooded batteries operated in RAPS systems require water maintenance on a regular basis. Unfortunately, it is not uncommon for owner/operators to neglect this tedious task. This situation can affect the performance of the system and can ultimately result in premature failure of the batteries. Problems can also arise if the water used for topping up contains excessive amounts of impurities. For example, the presence of chloride ion leads to enhanced corrosion of the positive grids [11]. Also, some RAPS systems are located in areas where access is very difficult, e.g., microwave repeater stations on the tops of mountains. Hence, the cost of maintaining flooded batteries can be very high. 

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