The reactive element effect: ionic processes of grain- boundary segregation and diffusion in chromium oxide scales

The transport processes in chromium oxide reviewed here, are related to the basic ionic processes in oxides. Solid state science has been effective in describing the complicated diffusion-controlled oxide growth of chromium and chromia-forming alloys. Additions of reactive elements to chromia-forming alloys have a remarkably beneficial effect and this effect is also related to ionic transport processes in crystal lattices. Chromia-forming alloys are widely used in coal handling and conversion systems due to their good high-temperature corrosion properties; for industrial applications the requirement is that a good adherent protective oxide with a low diffusion of defects is formed. Recently, improved knowledge has been gained of the identity of the moving species and the values of the corresponding diffusion coefficients in chromia scales. STEM measurements give experimental support for the theory of grain-boundary segregation and blocking of grain-boundary diffusion by reactive elements. In this paper the current ideas on the transport processes in chromia and the role of rare earth additions on the corrosion behaviour are reviewed. In addition calculations on crystal lattices and grain boundaries are discussed. The calculations on chromia have been beneficially influenced by earlier theoretical considerations on nickel oxide. Consequently, results on (doped) nickel oxide as well as chromium oxide are discussed.