The Transport Properties and Defect Structure of the Oxide (Fe, Cr)₂O₃formed on FeCr alloys

Abstract

Abstract The protective properties of a scale forming on a metal are intimately relat‐ed to the way in which matter is transported through the oxide. This report re‐views the oxidation behaviour the bi‐nary iron‐chromium alloys which form the basis of many technologically important materials and attempts to relate it to the defect structure and transport properties of (Fe, Cr) 2 O 3 oxides. Existing information on this system is mainly restricted to the end members, Cr 2 O 3 and Fe 2 O 3 . The former is a cation deficient, p‐type oxide and cation transport dominates. Fe 2 O 3 is n‐type, probably containing iron interstitials and oxygen vacancies, with comparable transport rates for both species, but the defect structure and properties of both oxides have been only partly characterized. New measurements by thermogravimetric, electrical and high temperature deformation techniques of these properties are briefly described. Electrical measurements support the hypothesis that certain solid solutions of Cr 2 O 3 ‐Fe 2 O 3 have lower defect concentrations and ionic transport rates than the pure oxides. There is evidence that scales of about the required composition form on the binary alloy containing about 20% Cr and are responsible for its low oxidation rate. Possible extensions of the theory to the design of highly stoichiometric scales for the protection of tornary and more complex alloys are briefly mentioned.