Chemisorption model of electrochemical passivity of metals and thermodynamic calculation of the flade potential of metals Ni and Cr taking into account their surface Gibbs energy

Abstract

A model of the chemisorption passivity of metals based on the consideration of equality of the electrochemical potentials of the metal ion at the interfaces between the bulk of a metal, the monoatomic surface layer (SL) of a metal, and the surface metal oxide has been proposed. Thereby, unlike the Vetter model, this approach takes into account the occurrence of the Galvani potential of the metal atoms (ions) in the SL in the form of the surface Gibbs energy ΔGS(hkl) of a particular crystal face. On this basis, a formula of the Flade potential EF(S) of a metal has been derived; this formula, along with the chemical Gibbs energy of formation of oxides, makes allowance for the ΔGS(hkl) value (of a low-index face of the metal). Normal potential EF(S)0(a pH of 0) has been calculated using the data of the first-principle calculation of the surface energy of metals. The calculated EF(S)0 values of 0.46 and−0.19 V for Ni and Cr, respectively, are in good agreement with the literature data. The calculated EF(S) values and the critical points in the anodic curves of passivation and activation of nickel in a 0.5 M H2SO4 solution and in the presence of KSCN additives have been compared.