Thermodynamics of chemisorption of sulfur from thiocyanate on nickel during electrochemical passivation

The effect of potassium thiocyanate content in an aqueous H2SO4 solution on the activation potential of a passive film formed on the nickel surface is studied. Based on the results, a thermodynamic model of chemisorption in electrochemical passivation of nickel in an aqueous H2SO4 solution containing surface-active KSCN thiocyanate additive is proposed. In the model, the formation of a NiO-NiS film is considered as a result of the equilibrium processes at the interfaces of the Ni|NiO-NiS|H2O, SCN- system. The total Gibbs energy of the chemisorptive phase formation of NiO-NiS solid solution is determined by partial chemisorptive formation energies of NiO and NiS. In turn, partial formation energy of either component is estimated as a sum of the Gibbs energy of the chemical formation of the component and the surface Gibbs energy of nickel, which depends on the KSCN concentration in the solution in the case of NiS formation. The system can be controlled due to the relation between the Gibbs energy and Flade electrode potential. As was experimentally found, the latter value determines the quantitative NiO: NiS ratio in the passive film.