Composition and Properties of Oxide Films on a Ferriti Steel and a Nickel-Based Alloy in Molten Hydroxide - Carbonate Electrolytes

Abstract

The composition of the anodic films formed on Ni-10%Cr and Fe- 18%Cr in molten NaOH with and without the addition of carbonate have been characterised ex-situ by X-ray electron spectroscopy (XPS) and Auger electron spectroscopy (AES). Voltammetric and electrochemical impedance spectroscopic measurements were employed to characterise the conduction mechanism in the oxide layers formed.. The oxide formed on Ni-10%Cr and Fe-18%Cr in molten hydroxide is essentially a duplex layer, the inner part of which grows via solid-state diffusion-migration of point defects. This inner part behaves as a p-type semiconductor on Ni-10%Cr and a n-type semiconductor on Fe-18%Cr. The outer part of the layer grows via dissolution-precipitation mechanism to a certain extent analogous to that operative for films formed in high-temperature aqueous environments. The ability of the Mixed- Conduction Model to account for the electrochemical data points to a general similarity of the oxide film formation process in high-temperature electrolytic media.