Potential-dependent properties of the passivating film on nickel in sulphuric acid solution from multiple-angle-of-incidence reflectivity measurements

Abstract

Relative reflectivity changes due to the formation and removal of the passivating film on nickel in 1 M sulphuric acid were measured at various angles of incidence. Conditions were established under which errors due to roughening by corrosion become negligible. The optical constants and the thickness of the film depend on the electrode potential. Close to the passivation potential at U HSS=0.55 V vs. the hydrogen electrode in the same solution, the refractive index is n 2=1.9, the extinction index k 2, and the thickness d=3.15 nm. As the potential becomes more positive, the refractive index rises to a maximum n 2=2.3 at U HSS=1.05 V and the thickness decreases to a minimum d=1.7 nm while the extinction index stays constant. At more positive potentials the refractive index increases and both the extinction index and the thickness grow larger. The results are explained by gradual hydration of nickel(II) oxide at relatively negative potentials and gradual oxidation to NiOOH at positive potentials. During activation at negative potentials two processes of film transformation and of subsequent currentless film dissolution are observed. During transformation, the refractive index decreases to n 2=1.6. The film becomes almost transparent and its thickness may almost double. It is concluded that transformation yields nickel(II) hydroxide. The rate of film dissolution is constant with time and of the order 0.1 nm s −1 to 1 nm s −1. It depends on the potential of film formation.