THE ROLE OF REACTIVE ELEMENTS ON SCALE ADHERENCE IN HIGH-TEMPERATURE OXIDATION BEHAVIOUR OF METALS AND ALLOYS

摘要

A coating of oxide particles of certain reactive elements (e.g. Hf, Yt, Ce, La, etc.) applied to metals and alloys may improve the growth and spallation of its oxide scale. Oxide particle size and spacing, and the concentration of reactive elements (RE) segregation are, among the others, the controlling parameters in the reactive elements effect (REE) on grain boundary diffusion. The physico-chemical properties of the reactive elements (crystallographic structure, free energy of formation of compounds, solubility of RE in the metallic substrate, etc.) can explain their beneficial effect. In this paper the introduction of Cr, Al and Hf into the surface of the RENE 80 and 2.25Cr-Mo steel by pack cementation in a single step process has been described. The coated alloys were subject to cyclic oxidation test. The coated specimens were characterised by optical microscope, SEM and XRD analysis. Cyclic oxidation tests showed that the presence of Hf on the surface of the alloys enhanced their oxidation resistance.