Electrochemical and surface analytical study of the anodic oxidation of Fe-18% Cr steel in molten NaOH-Li2CO3 mixtures

The anodic oxidation behaviour of a Fe-18%Cr steel in molten NaOH-Li2CO3 (50:50 mol%) at 470 °C was characterised by in-situ and ex-situ methods. In-situ electrochemical impedance spectroscopic (EIS) measurements showed that the steady state ionic conductivity of the oxide decreased with potential in the passive range. This indicates an increasing layer thickness or a decrease in the amount of mobile current carriers in the oxide with increasing potential. The two high-frequency time constants in the impedance spectra are most probably related to the semiconductor properties of the corrosion film and interfacial charge transfer, whereas the low-frequency time constant corresponds to the ionic defect transport through the oxide. The surface composition of the corrosion layers was estimated by X-ray Photoelectron Spectroscopy (XPS). The results show that the films can be regarded as mixed iron oxides containing a certain amount of chromium, sodium and lithium.