Roles of Nb on the oxidation characteristics and oxide scale evolution of Fe‐25Cr‐35Ni‐2.5Al‐_XNb alloys at 1000°C and 1100°C

Abstract

The oxidation characteristics of Fe-25Cr-35Ni-2.5Al-X Nb (0, 0.6, and 1.2 wt%) alumina-forming austenitic alloys at 1000°C and 1100°C in air were investigated. Results show that Nb has an important effect on the hightemperature oxidation resistance. A bilayer oxide scale with a Cr 2 O 3 -rich outer layer and Al 2 O 3 -rich internal layer forms on the surface of the Nb-free alloy and exhibits a poor oxidation resistance at 1000°C and 1100°C. With Nb addition, both the 0.6Nb-addition and 1.2Nb-addition alloys exhibit better oxidation resistance at 1000°C. Because of the third element effect, Nb addition reduces the critical Al content and forms a single external protective Al 2 O 3 scale, which greatly improves the oxidation resistance. After oxidation at 1100°C, niobium oxides (mainly Nb 2 O 5 ) are formed on the surface of the 1.2Nb-addition alloy and destroy the integrity of the Al 2 O 3 scale, which causes the formation of Cr-rich oxide nodules and eventually develops to be a loose bilayer oxide scale with NiCr 2 O 4 , Cr 2 O 3 , and Fe 2 O 3 outer layers and Al 2 O 3 inner layer. K E Y W O R D S AFA alloys, Al 2 O 3 scale, high-temperature oxidation, Nb addition, Nb oxide | INTRODUCTIONIncoloy 800 is a typical Cr 2 O 3 -forming Ni-Fe-Cr-based alloy that has excellent oxidation resistance and creeprupture properties in the range of specified temperatures. [1][2][3] Incoloy 800 is widely used in furnace components and equipment, petrochemical furnace cracker tubes, and nuclear reactors. [2,4] However, when oxidizing at higher temperatures or water vapor conditions, the Cr 2 O 3 scale will evaporate and lose its protective effect on the matrix. [5][6][7][8] To solve this type of limitation of chromia-forming alloys, the Oak Ridge National Laboratory invented special alloys named alumina-forming austenitic (AFA) alloys. [9,10] AFA alloys can form a stable protective alumina (Al 2 O 3 ) scale, which offers excellent oxidation resistance at high temperatures. [5,11,12] Thus, AFA alloys have become potential alloys for elevated temperature applications. [13] Research to promote the formation of a protective alumina scale has been ongoing for several years. First of all, Al is recognized as the key point of the AFA alloys, so enough Al is needed to form a continuous and compact Al 2 O 3 scale. [14] Elements such as Nb, Y, V, Zr, Hf, Ce, Cu, Ti, and Ta have been added to investigate their effects on the oxidation behavior of AFA alloys, and many of these elements play an important role in the oxidation