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-antimo ny lead alloy have the disadvantage of developing a passive layer whic h impedes the electronic conduction through the electrode. The additio n of tin to lead alloys has proved to overcome this passivation phenom enon and to increase the charge and discharge capacity of the lead bat teries. In this work, lead-tin alloys were prepared with increasing ti n levels up to 2.5 wt.%. These alloys were first passivated in a de-ae rated solution of sodium tetraborate (pH-9.1). Electronic conduction t hrough this passive layer was evaluated by measuring the polarization currents of the ferro/ferricyanide couple which was added to the solut ion (pH-9.1) in the passive potential range of the alloy electrode. Th e 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 t ransfer was observed on passive lead alloyed with less than 0.8 wt.% t in. Electronic conductivity of the passive layer, evaluated by the exc hange-current density of the redox system, increased sharply when the alloying tin level increased from 0.8 to 1.5 wt.%; it reached a platea u at higher tin levels. The instability of the passive layer, as evalu ated by the passive current density, varied inversely with the electro nic conductivity.