Microstructural Studies of Alloy Oxidation I. Selective Etching

Abstract

The oxide film formed at 600°c on iron-chromium alloys consists of a thin inner chromium-rich layer with a much thicker outer layer of Fe² 0³. The Fe²O³ can be removed from stripped oxide films by etching with solutions of hydrochloric acid in methanol; about 90–95% of the iron is removed, but no chromium. Etching avoids the difficulties in characterising the films introduced by the Fe²O³, which confuses both electron microscopy and X-ray analysis. This technique yields information on the oxidation process which cannot be obtained by other means, and it is particularly useful with oxide features too thick to be characterised directly. Consideration of the kinetics of the etching process suggests that the main factor affecting the rate is the stoichiometry of the Fe² 0³, so that etching also indicates changes in stoichiometry as oxidation proceeds. Initiation of etching shows some dependence on the chromium-rich oxide sheet, but the effects are short-lived. The mechanism is discussed with reference to published work on the pitting of stainless steels.