Influence of alloying with bismuth on electrochemical behaviour of lead-calcium-tin grid alloys

The effect of bismuth, in the range 0.002 to 0.073 wt.%, on the electrochemical properties of lead-calcium-tin-aluminium alloy in sulfuric acid solutions at room temperature and 50 °C is investigated by potentiodynamic and a.c. impedance methods. Bismuth is added to a common grid alloy of lead-calcium-tin-aluminium, and the amount of bismuth in the alloys depends essentially on whether the alloying process is governed by equilibrium or kinetic processes. Potentiodynamic measurements provide a qualitative overview of the activating, passivating and transpassivating processes that correspond to Pb → PbSO4, PbSO4→ PbO2 reactions, respectively. Addition of bismuth increases the passivating current density, ipass, and modifies the conductivity of the passivating film on the electrode surface. An increased anodic attack is observed from cyclic voltammetric measurements and a more porous oxide layer is detected by scanning electron microscopy for various bismuth additions. The results show that the relationship between the extent of the anodic attack and bismuth content is not linear.