Effect of phosphoric acid concentration on conductivity of anodic passive film formed on surface of lead–indium alloy

The addition of phosphoric acid into sulfuric acid solution is mentioned to be helpful in the reduction of sulfation after deep discharge of lead-acid battery. The anodic behavior of Pb and Pb–In alloys was studied in pure phosphoric acid and sulfuric acid containing various concentrations of phosphoric. The electrochemical measurements were performed using potentiodynamic, potentiostatic and cyclic voltammetric techniques. The composition and morphology of passive layer formed on the surfaces of Pb and Pb–In alloys were characterized by X-ray diffraction (XRD), energy dispersive X-ray spectroscopy analysis (EDX) and scanning electron microscopy (SEM). The potentiodynamic study shows that the passive current density increases with increasing the indium content in the alloy in the examined solutions. The addition of 0.1 mol/L H3PO4 into the electrolyte is more effective to decrease the thickness of passive film on the surface of alloys containing higher indium content (10% and 15%). The XRD, EDX and SEM data reveal that the formation of PbSO4 and PbO on the surface decreases with increasing the indium level in the alloy and is completely prevented at higher indium content (15%) in mixed acid.