Effect of Nb on Microstructure and Corrosion Resistance of Fe22Cr5Al3Mo Alloy

Fe-Cr-Al alloy is a promising candidate accident tolerant fuel (ATF) cladding material. It is helpful to developing Fe-Cr-Al alloy with excellent corrosion resistance by investigating the effect of chemical composition on its corrosion behavior under the normal condition of simulated nuclear reactor operation. In this work, the effect of Nb content (0.5%, 1.0%, 2.0%, mass fraction) on the corrosion resistance of Fe22Cr5Al3Mo alloys in 500 degrees C and 10.3 MPa superheated steam was investigated by static autoclave tests. The microstructures of the alloys and oxide films formed on the alloys after different exposure time were observed and analyzed by EBSD, TEM, EDS and SEM. The results showed that the second phase particles (SPPs) of Nb(C, N), Fe-2(Nb, Mo) and Cr-2(Nb, Mo) were precipitated and the grains were finer in the Nb-containing alloys. The corrosion resistance of the alloys was further improved with the increase of Nb content (in the case of Nb >= 1.0%). The oxides of Fe, Cr and Al were formed separately in the oxide layer from outside to inside. Compared with the Nb-free alloy, the interfaces of different oxides were clearer and had a more obvious stratification resulting from the oxides with different composition appeared in the oxide film on the alloy with 1.0%Nb. The thickness of oxide film on the alloy with 1.0%Nb was more uniform than that on the alloy without Nb. At the oxide film and metal matrix interface, the alumina layer on the Nb-free alloy was dispersive, and the alumina particles were detected in both the matrix and chromium oxide scale, which illustrated the internal oxidation of aluminum occurred. While the alumina layer on the alloy with 1.0%Nb was more uniform and continuous. This indicated that the addition of Nb inhibited the internal oxidation of aluminum, and promoted the formation of uniform and continuous alumina layer, therefore reduced the oxidation rate of the alloy.