Film Thickening on Nickel in Aqueous Solution in Relation to Anion Type and Concentration

Abstract

The variation of the open circutt potential of the Ni electrode in strongly aerated soluiion of different concentrations of Cl⁻, Br⁻, I⁻, NO⁻ ₃, SO²⁻ ₄ and CrO²⁻ ₄ is followdd till steady-stete values are established. In all solutions the steady potentials, E_sb, are approached from negative values, denoting film repair and thickening. Except in relatively concentrated Cl⁻ media, no Ni²⁺ is detected in solution. Plots of the Ni electrode potential, E, as a function of the logarithm of immersion time, t, are linear, satisfying the general relationship:

E = a_l + b₁log 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 promoee the diffusion of Ni²⁺ through the oxide. The rates of oxide thickening in the presenee of different concentrations of the various anions are computed and discussed. Expressions are deduced relating E_st and the thicknsss of the oxide to the various parameters of the partial anodic and cathodcc reaction.The steady-stete potentials vary with the logarithm of the molar concentration of the various anions according to:E_st = a₂ − b₂ log 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 electrod. Agreement between theory and experiment is established for Cl⁻, I⁻ and SO²⁻ ₄ soluiion. The adsorption of Br⁻ is assumdd to occur in the Br²⁻ ₂ form. The results also indicaee that the CrO²⁻ ₄ ion has a partial monovalent character. The adsorbability of the different anions tested decreases in the order: I⁻ (strong)> Cl⁻> Br⁻> SO²⁻ ₄> NO⁻ ₃ (weak)