Pitting Corrosion Currents of Tin in Relation to the Concentration of the Inhibitive and Corrosive Anions under Natural Corrosion Conditions

The changes in the pitting corrosion current density with time for tin electrode concerning the concentrations of both the passiviting oxalate and the aggressive chloride anions were followed using a simple electrolytic cell. The experimental data show that, at the moment of addition of the aggressive Cl- anions to the passivating oxalate anions solution, instantaneous high currents are observed. After that, the values of these currents were decreased to lower values until steady state was attained. These currents were dependent on the concentration of both the oxalate and Cl- anions. At a constant concentration of oxalate anions, the pitting corrosion current density varied with the concentration of Cl- anions according to the relation: log I-pit = a(1) + b(1) log C-Cl-. It also varies with the oxalate anions concentration at a constant Cl- ions concentration according to the relation: log I-pit = a(2)-b(2) log Coxalate. The constants a(1), a(2), b(1) and b(2) were experimentally determined for all systems studied.