Investigation On Oxidation Behaviour Of Ni-Cr-Al Based Superalloy Applied To Clean Coal Technology

Ni-Cr-Al based alloy is a kind of ideal material to be used as the high temperature gas filters in the Coal Clean Combustion Technology, since it possesses good strength, excellent welding ability, as well as outstanding resistance ability to corrosion under high temperature environment. This paper studied the oxidation kinetics of the Ni-Cr-Al based alloys with the powder state, the porous state and the bulk state, respectively, by static weight gain method. The compositions and microstructures of the oxide films formed on the surface of the alloys after oxidation were observed and analyzed by XRD, FE-SEM(EDS) and XPS methods, respectively. The oxidation and pre-oxidation mechanisms of Ni-Cr-Al based alloy at a high temperature were further discussed. The optimized compositions and pre-oxidation parameters of the Ni-Cr-Al based alloy were obtained. The following conclusions can be drawn:(1)The oxidation kinetics curves of the Ni-Cr-Al based alloys with the bulk and powder states at600℃all obeyed the parabolic law. Both the increase of Al content and the pre-oxidation process to the bulk and powder materials can increase its resistant ability to oxidation. However, the pre-oxidation process has been shown better improved resistant ability to oxidation than that of the increasing Al content.(2)When the content of Al increased from4.5wt.%to9wt.%, the compositions of the oxide layer formed on the surface of the Ni-Cr-Al based alloy powders changed from the mixture of Cr2O3and Al2O3to the single Al2O3. The single Al2O3oxide layer was more effective to improve the oxidation resistance ability of the alloy than that of the mixture oxide layer. After the pre-oxidation process, a continuous compact oxide layer was formed on the surface of the Ni-Cr-Al based powder, which effectively inhibited the further oxidation of the powders.(3)When the Ni-Cr-Al based alloy was oxidized at600℃, the calculated critical Al content that the formation of an integrated protective Al2O3layer needed was5.1at.%. While in the Ni-Cr and Ni-Al alloy, the critical values that the single integrated protective oxide layer (Cr2O3for Ni-Cr and Al2O3for Ni-Al) formed were9.26at.%and12.02at.%, respectively.(4)After being oxidized at the temperatures from850to1000℃for8hours in the air, the porous Ni-Cr-Al based material pre-oxidized at1000℃with a more flat kinetics curve showed the best effect. Al2O3, Cr2O3, NiCr2O4generated almost simultaneously during the initial state of the oxidation at the temperature of1000℃. As the corrosion kept on, the main oxide on the surface of the porous material with4.5wt.%Al changed to main Cr2O3, while the ratio of Al2O3became higher and higher on the surface of the porous material with9.0wt.%Al.(5)When the porous Ni-Cr-Al based samples were exposed at the temperatures from850to1000℃in the air, the open porosity, permeability, as well as the maximum porous diameter decreased with the oxidation time extended. What’s more, the higher the oxidation temperature increased, the more these parameters decreased. The samples with9.0wt.%Al decreased slower than that with less Al.