Penetration rates, grid corrosion

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. 

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. 

Figure 8. Structure of the corrosion layer at the grid surface. Penetration and corrosion rates are approximated for room temperature and normal float voltage (2.23-2.25 V/cell) (see text).

Typical pore size distributions result in mean pore diameters of around 15 //m. Even long and intensive efforts did not succeed in decreasing this value decisively in order to enable production of micropo-rous pocketing material resistant to penetration [65, 66], In practice PVC separators prove themselves in starter batteries in climatically warmer areas, where the battery life is however noticeably reduced because of increased corrosion rates at elevated temperature and vibration due to the road condition. The failure modes are similar for all leaf separator versions shedding of positive active mass fills the mud room at the bottom of the container and leads to bottom shorts there, unless — which is the normal case — the grids of the positive electrodes are totally corroded beforehand. 

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