The influence of calcium, tin and grid thickness on corrosion-induced grid growth

Antimony-free lead alloys are the choice material for the positive and negative grids of valve-regulated, stationary, lead/acid (VRLA) batteries. A lead alloy suitable for this task is a ternary lead-calcium-tin alloy with additions of aluminium. The influence of two levels of calcium and tin, as well as the grid thickness, on the corrosion rate (i.e., growth) is investigated through tests on bare and pasted grids. The results show that a combination of a low calcium (≈0.07 wt.%) and a high tin (≈0.7 wt.%) content in the alloy, together with thick grids (≈4 mm), yields low growth-rates that are adequate for long-life VRLA batteries (3–6% growth in 54 weeks at +60 °C). The microstructure of the alloy, i.e., the number of grain boundaries per unit volume determines the magnitude of the resulting corrosion attack. Lead-calcium-tin-aluminium alloys with an approximate composition of 0.07 wt.% Ca and 0.7 wt.% Sn have a lower number of grain boundaries when compared with alloys of the same system with 0.09 wt.% Ca and 0.3 wt.% Sn. Although a penetrating grain-boundary attack is observed with the Pb-0.07wt.%Ca-0.7wt.%Sn alloy, its overall performance, in terms of resulting VRLA battery life, is far superior to that of the Pb-0.09wt.%Ca-0.3wt.%Sn alloy.