Effect of Ce addition on hot corrosion behaviours of a cast γ′-strengthened Co–Al–W–Mo–Ta–B alloy at 800 ​°C

Abstract

Hot corrosion behaviours of a novel Co–9Al-4.5W-4.5Mo–2Ta-0.02B alloy doped with 0.01, 0.05, 0.1 and 0.2 ​at% Ce exposed at 800 ​°C in a solution of 75%Na2SO4/25%NaCl were investigated. The alloys comprised a coherent γ-CoSS/γ′-Co3(Al, W) microstructure (0.01Ce and 0.05Ce alloys) and κ-Co3(W, Mo) precipitates (0.1Ce and 0.2Ce alloys) at grain boundaries. Hot corrosion kinetics curves demonstrated the parabolic time dependency profile with two stages: the first parabolic stage is within the beginning ∼50 ​h corrosion and follows by the second parabolic stage. With an increasing nominal Ce content the weight gain of the alloy significantly decreased from approximately 70.1 ​mg ​cm−2 (0.01Ce) to 40.8 ​mg ​cm−2 (0.2Ce) when exposed for 100 ​h. A two-layer corrosion scale formed, and the scale was composed of an outer layer of Co3O4 oxide with spinel compounds of CoAl2O4, CoWO4and CoSO4, and an inner γ/needle-like Co3W/sulphide layer adhered to the substrate. Heavy spallation of the corrosion scale occurred in the 0.01Ce∼0.1Ce alloys, however, spallation was slight in the 0.2Ce alloy. The excellent corrosion resistance of the 0.2Ce alloy could be attributed mainly to the formation of continuous Al2O3 lines in the corrosion scale, as well as the prolongation of the incubation period of the corrosion product spallation.