INFLUENCE OF BISMUTH ON THE AGE-HARDENING AND CORROSION BEHAVIOR OF LOW-ANTIMONY LEAD ALLOYS IN LEAD-ACID-BATTERY SYSTEMS

The effects of bismuth additions in the range 0.006-0.086 wt.% on the metallurgical and electrochemical properties of Pb-1.5 wt.% Sb alloy a re investigated. The self-discharge behaviour of batteries produced wi th grids of the doped alloys is also evaluated. Addition of bismuth is found to exert no significant effects on the age-hardening behaviour, general microstructure or grain size of the alloy. It does, however, influence the morphology of the eutectic in the inter-dendritic region s. The latter changes from a mainly lamellar to an irregular type with increasing bismuth content. The corrosion rate of the grid decreases with increase of the bismuth content. Attack occurs preferentially in the inter-dendritic regions where there is an enrichment of both antim ony and bismuth. Electron-probe microanalysis shows that the corrosion zone consists of a tri-layered structure, namely: a dense, continuous , inner layer (PbO1.1); a central layer (PbO1.8.PbSO4); a porous outer layer n(PbO1.8) PbSO4, with n = 2-8. In the latter, the value of n in creases in the direction of corrosive penetration into the grid. Data from atomic absorption spectrometric analysis reveal that bismuth, aft er oxidative leaching from the grid substrate, is retained mainly in t he corrosion layer. A key observation is that bismuth (i.e., up to sim ilar to 0.09 wt.%) does not affect the self-discharge behaviour of bat teries.