ABSTRACT

As mentioned earlier. Sb migration from Pb-Sb-based positive grid alloys to negative electrode results in the reduction of hydrogen overvoltage and consequent decrease in cell voltage. This led to increased degassing and water loss. The move to purer systems with no poisoning of negative plates led to the use of Pb-Ca alloys. The cast and wrought Pb-Ca binary alloys are significantly inferior to cast Pb-Sb alloys in hardness. creep resistance. and corrosion resistance. The cycle life of lead-acid batteries is mainly limited by the performance of the positive plate. the capacity of which decreases on cycling. especially under deep discharge. The common degradation mechanisms [357] include (1) loss of interparticle contact. (2) shedding of active material due to morphological changes and grid corrosion. (3) grid deterioration and growth. and (4) irreversible plate sulfation due to acid stratification effects. The growth of positive plates due to corrosion in service reduced the cycle life in batteries with Ca grids and low-Sb alloy grids. This is attributed to (1) the formation of high-resistance (X-PbD on the grid. (2) the increased tendency for cracking and delamination of corrosion layers. and (3) the structural changes in active material that is aggravated by the absence of Sb in the grid alloy. This effect together with other plate mechanisms that reduce battery life to fall well short of design life is known as the antimony-free effect or premature capacity loss (PCL). The antimonyfree effect underlined an urgent need for additions to Pb-Ca. The mechanical properties in Pb-Ca binary alloys peak at 0.07% Ca. Above 0.06% Ca. cellular precipitation of Pb3Ca leads to the fine-grain size. In VRLA batteries. using thin grids of high Ca have been used to aid processing (faster aging rate). but they produce fine grains. Increasing Ca contents above the 0.07% level accelerates corrosion and this is believed to be due to fine grains and primary Pb3Ca. Sn additives increase mechanical properties by changing the mode of precipitation as the precipitate phase is changed from Pb3Ca to more stable Pb(Sn.Ca)3' Sn addition aids electrochemical properties by preventing passivation of the grid and permitting recharge of batteries from the deeply discharged condition. Pb-Ca-Sn alloys have become established in traditional automotive batteries and in VRLA batteries. Pb-Ca-Sn alloys are inferior to Pb-Sb-As alloys in terms of mechanical properties. but the properties are adequate. Additions of Ag to Pb-Ca and Pb-Ca-Sn alloys increases creep and corrosion resistance and the durability of batteries. Pb-(0.025-0.06)Ca-(0.3-0.7)Sn-(0.015-0.045)Ag positive grid alloys show improved creep and corrosion resistance. Increased Ag additions are being considered for severe deep cycling conditions. The addition of Al tends to stabilize drossing loss of Ca. Pb-Sr alloys are better than corresponding

Table 1 Typical Composition Range of Battery-Grid Alloys

Alloy Composition

Conventional high-Sb alloy-cast Low Sb alloys-cast Cast/wrought Pb-Ca-Sn

Pb-(9-12)Sb-(0.35-0.5)As Pb-2.5Sb with other minor additions Pb-(0.025-0.06)Ca-(0.3-0.7)Sn-(0.015-

0.045)Ag with minor Al addition

Pb-Ca alloys, but the high cost of Sr is an inhibiting factor as long as PbCa-based alloys are behaving adequately.