High‐Temperature Oxidation Behavior of Hastelloy N Alloy for Molten Salt Reactor at 650–980 °C

Isothermal oxidation of Hastelloy N, a nickel‐based superalloy with low Cr content, is conducted at 650–980 °C in air. The oxide film exhibits a complex structure with outer and inner oxide layers. The outer oxide layer consists of NiO, Fe 2 O 3 , NiFe 2 O 4 , and MoO 2 , while NiCr 2 O 4 , MnCr 2 O 4 , and Cr 2 O 3 are the main phases in the inner oxide layer. At temperatures above 700 °C, the formation of gaseous MoO 3 produces cracks and spallation of the outer oxide layer. Moreover, Al 2 O 3 and SiO 2 are identified along the grain boundary under the inner oxide layer owing to internal oxidation. The oxidation behavior of Hastelloy N alloy at 650–980 °C is jointly controlled by the chemical reactions at the oxide/alloy interface and the diffusion of Cr 3+ and O 2− in the bulk alloy. The formation mechanism of each oxide layer is discussed.