Estudo mecanístico de reações de estado sólido: o caso do sistema Pb/PbSO4/Na2SO4
Freitas, Heverson Renan de
In this work, the growth and reduction of PbSO4 in Na2SO4 solution under transient conditions was characterized. A novel methodology for the working-electrode surface preparation before stabilization enhanced the voltametric measurements reproducibility. It was demonstrated a nucleation process during the growth first steps. It is suggested a new mechanism for the film growth by injection of non recombining point defects. On the other hand, the reduction occurs by injection of recombining point defects. It was demonstrated the validity of the Tafel equation for characterization of the metal/film interface and the ohmic model for the film. It was determined the Flade potential for the system Pb/PbSO4/Na2SO4. From the linear region on the Tafel plot, it was possible to determine the charge transfer coefficient and the exchange current density of the reaction Pb2+(metal)/Pb2+(film) at the Pb/PbSO4 interface. The ionic resistivity of the film was calculated versus the film thickness and the sweep rate during the transient. The disruption, which is the partial rupture of the passivation film into nanometric particles, was also analyzed. The anodic and cathodic charges determined here showed that the disruption does not occur when there is no potentiostatic aging of the film. Variation of the solution internal potential also does not affect the passivation film stability. However, when the film is subjected to a potentiostatic aging step after the voltammetric growth, if the cathodic sweep rate is bigger than the anodic one, the disruption can reach 10% of the total anodic charge, depending on reduction rate. Those aspects are different from what happens in the Pb/H2SO4 system, for which the disruption process can reach 60% of the film.