Page 222 - Handbook of Battery Materials

6.5 Passivation of Lead by Its Oxides 191

Figure 6.9 Conversion of grid material into lead
New spine dioxide (PbO 2 ) by corrosion: spine of a positive
of a tubular grid tubular plate. New plate: 3 mm diameter means
2
2
7.1 mm cross-section (πr with r = 1.5mm).
Sectional area: Aged plate: reduction of r by 0.03 × 15 = 0.45 mm
7,1 mm 2 2 2
means πr = 3.5mm .

After 15 years
of service life
Sectional area:
3,5 mm 2

2,1 mm
3 mm PbO 2

Figure 6.9 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 grid material is converted into lead dioxide, a slight increase in the actual
capacity is often observed with lead–acid batteries. The reduced cross-section in
Figure 6.9 does not affect the performance of batteries that are used for discharge
durations in the order of 1 h or more. Attention must, however, be paid to batteries
that are loaded with high currents, because the conductivity of the grid gains
importance with increased current flow.
The ‘corrosion capacity of the positive grid’ can be estimated from the above
figures. The positive grid in lead–acid batteries for stationary and traction appli-
cations contains about 10 g of lead/Ah −1 (usually slightly more). This means a
positive-grid weight of about 1 kg/100 Ah. With the values of Equation 6.32, the
‘corrosion capacity’ is 500/100 Ah of battery capacity. The corrosion rate of 1 or
2 mA/100 Ah means 8.76 and 17.52/100 Ah per year respectively. Related to the 500
Ah of the total ‘corrosion capacity,’ 2–4% of the grid material would be converted
into lead dioxide per year under these assumptions.
6.5.1
Disintegration of the Oxide Layer at Open-Circuit Voltage

In Section 6.3.3 it has been shown that corrosion is one of the reactions that cause
self-discharge of the positive electrode. In connection with Figure 6.8 it has been
mentioned that an anodic current, the corrosion current, must flow continuously