The impedance of anodic pro-cesses on passive NiSi-electrode in sulfuric fluoride containing electrolyte

Abstract

The mechanism and kinetics of anodic oxidation of the Ni-Si electrode in solutions of 0.5 M H₂SO₄ + (0,005 – 0,05)M NaF in the passive state were investigated by methods of polarization and impedance measurements.Theimpedance spectra are interpreted on the assumption about the formation of the bilayer oxide film on the surface of the silicide nickel, the outer layer which has a porous structure.The growth of the porous layer with the increase of the electrode potential in the investigated solutions is linearly (constant anodizing is 2.2 nm/V).The increase in NaF concentration leads to a decrease in the thickness of the porous layer. The growth of the barrier layer of the oxide film was described in the framework of the model of point defects.The diffusion coefficient of oxygen vacancies inside the barrier layer of the film is 8.5∙10-16 cm²/s and varies weakly with the potential and the content of sodium fluoride in solution. Microscopic and profilometric studies show the development of the surface of the NiSi electrode during anodic etching.