A study of the oxidation mechanisms of some austenitic stainless steels in carbon dioxide at 1123k by means of charged-particle nuclear techniques. II

Abstract

The oxidation behaviour in carbon dioxide at 1123 K and a pressure of approximatey 0.1 MPa of a sample of 20% (by mass) Cr, 25% (by mass) Ni, Nb-stabilized steel (the standard steel) coated with ceria has been investigated by means of chargedparticle nuclear techniques and conventional methods. The nuclear techniques were used to study the growth mechanism, thickness and surface composition of the oxide. The scale consisted of an outer spinel layer and an inner layer of Cr 2 O 3 . It is not known if there was a silicon-rich layer at the oxide—metal interface because the oxide was adherent. The growth-mechanism of the oxide was studied with 18 O as a tracer and was found to grow primarily by oxygen diffusion, although there was indirect evidence for some cation diffusion. The growth-mechanism of the oxide is quite different from that on the standard steel without the ceria, and it is suggested that this is because ceria segregates to dislocations and grain-boundaries in the oxide. In the light of the results for the standard steel it is not clear if cerium reduces the cation diffusion and increases the oxygen diffusion, or merely reduces the cation diffusion.