In Situ Study of Salt Film Stability in Simulated Pits of Nickel by Raman and Electrochemical Impedance Spectroscopies

Abstract

The changes in solution composition and the stability of a salt film in a 10 mm deep, simulated pit electrode made of nickel in a 0.5 M HCl solution were monitored as a function of applied potential using a 532 nm fiber‐optic Raman system. Raman spectroscopy was able to clearly distinguish dissolved Ni(II)Cl species from the salt film of . Furthermore, the gradient of Ni(II)Cl in the pit was measured as a function of potential. At an applied potential of , a salt film was observed at the bottom of the pit after about 5200 s, corresponding to the first big drop in current density. The salt film was not observed below an applied potential of . However, repassivation did not occur above a potential of about . The concentration gradient inside the pit decreased dramatically at and the chloride concentration at the bottom of the pit at repassivation was found to be about 17% of saturation value with respect to . Electrochemical impedance spectroscopy of the simulated pit electrode in the same solution at various applied potentials was used to determine the electronic properties of the salt film and the thickness of the film. EIS suggested that the salt film had a dual structure, with a compact film underneath a thicker porous layer.