An aging factor in lead acid batteries is the corrosion of the current
collector grid.

The choice alloys for VRLA battery grids are lead-calcium-tin and lead-tin alloys with the calcium level ranging from 0.03 to 0.09 % and the tin level from 0.3 to 2.0 %.

More exotic alloys containing lead-antimony-cadmium and lead-strontium-tin are also in use or have been proposed.

The grids of the positive plate of a lead acid battery are subject to a corrosion process in which lead (Pb) is transformed into lead dioxide (PbO2).
The corrosion rate increases with temperature (doubling approximately every 10 °C), float voltage and acid concentration. The corrosion attack may be an area type attack or a grain boundary attack or a combination of both.
The corrosion product, lead dioxide, requires a 37 % higher volume then
the lead source material.

This volume expansion induces mechanical forces in the grid which will become deformed and stretched. This deformation is called growth and reduces the contact of the current collector (grid) with the active material.
A capacity decay occurs. The growth is not linear in time but accelerates
rapidly toward the end of the design life.

At a growth level of 4-7 %, grid fractures and capacity losses of the battery occur. In extreme case internal shorting and cell case rupture may ensue.

The transformation of lead into lead dioxide consumes also the water of
the electrolyte (1g Pb = 0.17g water) which further impinges on the perfor-
mance of the battery.

In VRLA cells and monoblocs a chemically aggressive condition may occur on the surface of the negative plate lugs and straps. This condition is triggered by the oxygen recombination reaction and the presence of inorganic sulfate salts in the electrolyte.

This aggressive condition causes the so called sulfate rot i.e. a catastrophic corrosion of the negative plate strap. An appropriate choice of high purity alloys (absence of antimony, arsenic) and tin levels at 1-3 % together with a grain size modifier can prevent this corrosion to occur.