Abstract
The electrochemical and corrosion behavior of Pb and Pb-In alloys in both phosphoric and sulfuric acid solutions containing various concentrations of phosphoric acid (0.05 to 0.20 M) at different temperatures was studied. Tafel plot and electrochemical impedance spectroscopy (EIS) techniques were used to obtain the experimental data, and the corrosion products formed on the surface were characterized by scanning electron microscopy (SEM). The results of both Tafel plot extrapolation and impedance measurements showed the same trend. Minor indium (0.5 %) alloying with lead significantly reduced corrosion rate in pure phosphoric acid solution. However, opposite behavior arises in the case of alloys containing indium more than 0.5 %, that is, the corrosion is higher than that of lead and alloy I. The corrosion current density decreases in both Pb and alloy I (0.5 %) in 4 M sulfuric acid with increasing the concentration of phosphoric acid up to 0.1 M, and then starts to increase relatively with increasing the additive concentration up to 0.2 M (but still lower than that in pure H2SO4). This exhibits that the higher concentration (0.2 M) of H3PO4 is less inhibitive of alloy I corrosion in H2SO4 solution. However, addition of phosphoric acid in various concentrations to sulfuric acid has little influence to inhibit the corrosion of alloys containing higher indium content (1 to 15 %). SEM photographs showed that the presence of indium as a minor alloying element retards the formation of PbSO4, but the formation of beta-PbO2 enhances.