FILM THICKENING ON NICKEL IN AQUEOUS-SOLUTION IN RELATION TO ANION TYPE AND CONCENTRATION

The variation of the open circuit potential of the Ni electrode in strongly aerated solutions of different concentrations of clࢤ. Brࢤ, Iࢤ. NOࢤ3, SO2ࢤ4 and CrOr is followed till steady-state values are established. In all solutions the steady potentials, Est, are approached from negative values, denoting film repair and thickening. Except in relatively concentrated clࢤ media, no Ni2+ is detected in solution. Plots of the Ni electrode potential, E, as afunction of the logarithm of immersion time, t, are linear, satisfying the general relationship: E = a1 + b1 log t …… (1) where a1 and b1 are constants. Equation (1) is derived theoretically by taking into account the p-conducting properties of NiO, and assuming that the adsorption of the anions on the oxide film generates the electric field necessary to promote the diffusion of Ni2+ through the oxide. The rates of oxide thickening in the presence of different concentrations of the various anions are computed and discussed. Expressions are deduced relating Est and the thickness of the oxide to the various parameters of the partial anodic and cathodic reactions. The steady-state potentials vary with the logarithm of the molar concentration of the various anions according to: Est = a2 -b2log C …… (2)This relation is reached from a consideration of the structure of the double layer, and the effects produced thereon when the anions are specifically adsorbed on the surface of the electrode. Agreement between theory and experiment is establishedfor clࢤ, Iࢤ and SO2ࢤ4 solutions. The adsorption of Brࢤ is assumed to occur in the Br2ࢤ2 form. The results also indicate that the CrO2ࢤ4 ion has a partial monovalent character. The adsorbability of the different anions tested decreases in the order: Iࢤ (strong) > Clࢤ > Brࢤ > SO2ࢤ4> NOࢤ3 (weak). © 1979 Maney Publishing.