Oxidation behavior of several chromia-forming commercial nickel-base superalloys

Several commercially available Ni-base superalloys were exposed isothermally in air at temperatures between 750 degrees and 1000 degrees C and also under cyclic conditions at 1000 degrees C. The kinetics of oxidation were determined and the scales were analyzed by electron microscopy and X-ray diffraction. Thin adherent chromia-rich scales for med on the alloys at 750 degrees C after 1000 hr. Although Waspaloy showed rite lowest weight gain in this test, it also showed the deepest internal corrosion due to oxidation of the grain-boundary car bides. Ar temperatures up to 1000 degrees C the external scales were also chromia-rich but there was greater internal corrosion. Titanium in the alloys oxidized, diffusing through the chromia scale to form faceted rutile (TiO2) grains at the surface as well as forming TiO2 and TiN internally. The amount of rutile at the oxide surface increased with temperature and alloy Ti concentration. Alumina formed as discrete internal oxides below the chromia scale, although Astroloy when oxidized isothermally at 1000 degrees C developed a semicontinuous internal layer of alumina due to its higher Al content. Under cyclic conditions Astroloy formed a thicker, less-protective scale of transition oxides probably due to its lower Cr content.