AN IN-SITU STUDY OF THE EFFECT OF TIN ON THE PASSIVATION OF LEAD-TIN ALLOYS

In lead/acid batteries, positive grids made of pure lead or low-antimony lead alloy have the disadvantage of developing a passive layer which impedes the electronic conduction through the electrode. The addition of tin to lead alloys has proved to overcome this passivation phenomenon and to increase the charge and discharge capacity of the lead batteries. In this work, lead-tin alloys were prepared with increasing tin levels up to 2.5 wt.%. These alloys were first passivated in a de-aerated solution of sodium tetraborate (pH-9.1). Electronic conduction through this passive layer was evaluated by measuring the polarization currents of the ferro/ferricyanide couple which was added to the solution (pH-9.1) in the passive potential range of the alloy electrode. The modification of the electronic conductivity of the passive layer by alloying with tin led to a change in the kinetics of the redox-couple reaction. From the kinetic parameters, it was found that no electron transfer was observed on passive lead alloyed with less than 0.8 wt.% tin. Electronic conductivity of the passive layer, evaluated by the exchange-current density of the redox system, increased sharply when the alloying tin level increased from 0.8 to 1.5 wt.%; it reached a plateau at higher tin levels. The instability of the passive layer, as evaluated by the passive current density, varied inversely with the electronic conductivity.