Radiotracer studies of carbon permeation through oxide scales on commercial high temperature alloys and model alloys

Chromia- and alumina-forming commercial high temperature alloys and model alloys were preoxidized at 900 or 10000C in H2-H2O at a low oxygen potential. The oxide layers were characterized by different methods. The carbon permeation through the oxide layers was studied by exposing the preoxidized specimens to an atmosphere CO-CO2-H2-H2O, tagged with radiocarbon, for long time. The carbon was detected by stepwise polishing and measuring the radioactivity. A slow carbon ingress occurs through chromial layers, differences in the potential by the oxide scale could be tested by the radiotracer method for the different alloys. The alumina layer on Fe-6Al is not protective, but no carbon ingress could be detected for an alloy Fe-6Al-0.5Ti. Autoradiography, AES and X-ray structure analysis showed the presence of Ti(O,C) beneath the outer Al2O3-layer. The oxicarbide improves the nucleation and adherence of the Al2O3 and prohibits the carbon penetration. The results were confirmed by gravimetric experiments, after preoxidation samples were exposed to CO-CO2-H2-H2O at high carbon activity (asub(c) = 1.02), carburization and graphite deposition were retarded or prohibited by dense and well adherent oxide layers. (orig.)