In situ and ex situ characterisation of the passive film on a ferritic stainless steel in molten sodium hydroxide

Voltammetric and electrochemical impedance spectroscopic measurements in combination with ex situ surface analysis are used to characterise the passive state of Fe-18% Cr in molten NaOH at 470 °C. The areas of corrosion, passivity and transpassivity are derived from experimental voltammetric data and thermodynamic considerations. Impedance measurements in a wide potential range are performed to study the conductivity mechanism of the anodic layers formed. The surface composition of the passive film is estimated by X-ray photoelectron spectroscopy and its in-depth composition by Auger electron spectroscopy. A transfer function based on a tentative kinetic model is proposed to describe the impedance spectra. The parameters of the transfer function are determined by best-fit calculation and some of the kinetic and transport parameters characterising the film growth process are estimated.