Effect of Cobalt Content on the Oxidation and Corrosion Behavior of Ni-Fe-Based Superalloy for Ultra-Supercritical Boiler Applications

The oxidation and corrosion behavior of three model alloys with different cobalt contents (6-20 wt%) were investigated in static air and a simulated coal ash/gas environment at 750 A degrees C. The model alloys follow a parabolic law approximately during the oxidation in static air. High cobalt level improves the oxidation resistance, however, without noticeable improvement in coal ash/gas corrosion resistance. The sample with the highest cobalt content grows the thinnest oxide layer and the fewest internal oxidation products in the oxidation test. Cobalt in the model alloys promotes the establishment of a protective chromium oxide scale during the oxidation test, but did not show much difference in restraining the inward diffusion of sulfur by increasing its content. The oxidation and corrosion products formed on the sample surface consist mainly of a protective chromium oxide film. Internal aluminum oxide particles have been found especially along the grain boundaries of the base alloy.