ELECTRIC PROPERTIES OF a-Fe_2O_3, Cr_2O_3 AND α-(Cr,Fe)_2O_3 AND THEIR RELEVANCE TO CORROSION

摘要

Electrochemical Impedance Spectroscopy (EIS) was used to analyze the semi-conducting properties of oxide-layers formed at 600℃ on Cr-containing iron based steels (X20 and 310). One (310) forming chromium rich oxides while the other (X20) tend to form oxides richer in iron. In order to explain the obtained results concerning these steels, synthetic oxide-layers on Au substrate were made, containing mixtures of Fe_2O_3/Cr_2O_3 ranging from pure Fe_2O_3 to pure Cr_2O_3 and analogous EIS measurements were performed. Three different electrical behaviors were observed. Firstly, oxides containing 100% to 75% chromium and exhibiting only p-type character, secondly, pure iron oxide with only n-type properties and thirdly, single phase solid solutions of α-(Cr,Fe)_2O_3, which exhibits both p- and n-type conduction. The oxides formed on X20 and 310 have similar semi-conducting properties as the synthetic oxides that contained a solid solution of Fe_2O_3 and Cr_2O_3. These oxides in turn show very complex behavior in terms of degree of doping and flat-band potential.