The Use of Additional Redox Couples in the Investigation of the Electronic Conductivity of Passive Layers

Abstract

Passive layers developed at the alloy surface may be undesirable in some industrial applications. In lead-acid batteries, the passivation of lead alloy electrodes has been a problem to resolve. The addition of tin to lead alloys has overcome the passivation phenomenon. To follow in situ the electrical properties of the passive layer, the ferro-ferricyanide couple was added to the electrolyte (tetraborate solution, pH 9.1) and polarization currents were measured in the passive potential range. The modification of the electronic conductivity of the passive layer by alloying lead with tin altered the exchange current density of the redox reaction. No electron transfer was observed on passive lead alloyed with less than 0.8wt% tin. The electronic conductivity of the passive layer increased sharply when the alloying tin level increased from 0.8 to 1.5 wt% and reached a plateau at higher tin levels. Ferrous-ferric and cerous-ceric couples were used in sulfuric solutions, to follow the conductivity of various passive layers formed at various potentials for various times. Potentiodynamic studies showed that, for lead alloys containing more than 1wt% tin, the passive layer formed by polarization from -400 to 1000mV/Hg-Hg 2 SO 4 was non-conducting. But, after an incursion into high potential, i.e 1550 mV, the passive layer became electron conducting, even at low potentials, in the range 1000 to -400 mV. The change with time of the conductivity of the passive layer was followed by the oxidation rate of ferrous ions dissolved in sulfuric solutions. It was shown that the level of the conductivity was closely related to the proportion of tin in the lead-tin alloys.