The oxidation of Co-20% Cr base alloys containing Nb or Ta

Abstract

Alloys based on Co-20% Cr containing approximately 4, 7 or 10wt% Nb or Ta were oxidized in oxygen and air at 900, 1000 and 100°C for times up to 350 h. In general, the addition of Nb accelerated the oxidation rate, although this effect was small at the lowest temperature. Futhermore, little evidence could be found for the development of a protective Cr 2O 2 layer. In contrast, the addition of Ta proved beneficial at all temperatures, promoting the development of protective oxide scales. A critical difference appeared to be the ability of the Ta-containing alloys to form a compound oxide, CrTaO 4, whereas no similar phase could be detected in the scales on the Nb-containing alloys. The Ta-rich oxides formed a layer adjacent to the metal, while a Cr-rich layer was formed outside it. It is possible that the Ta reduced the oxygen activity at the surface of the alloy, preventing the formation of cobalt-containing oxides which might otherwise disrupt the protective scale. Both elements have restricted solubility in Co-20% Cr, forming intermetallic compounds which oxidize internally. In the case of the Nb-containing alloys, a process occurs, during oxidation, which produces a change in the intermetallic deep in to the alloy, though there is no similar change in the Ta-containing alloys. This process has not yet been defined.