Oxide film growth on manganese in neutral aqueous solutions

The open circuit potential of the Mn electrode is followed in aerated solutions of Cl(-), Br(-), I(-), SO(4)(-2), CrO(4)(-2), CO(3)(-2) and NO(2)(-) with different concentrations, till steady state values are attained. In all solutions studied, the steady state potentials are approached from negative values indicating oxide film growth. The rate of oxide film thickening is determined from the linear relationship between the open circuit potential and the logarithm of immersion time, t, as evident from the relation: E=a+b logt, where a and b are constants. Oxide film growth is assumed to occur by ion conduction under a high field. The rate of oxide film thickening depends on anion type and concentration. The concentration of the inhibitive anions, CrO(4)(2-), CO(3)(2-) and NO(2)(-) that can withstand a certain concentration of the aggressive ions, Cl(-), Br(-), and I(-), varies according to the relation: logC(inh.)=A+n logC(agg.), where A and n are constants.