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

Abstract

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, Fe2O3, NiFe2O4, and MoO2, while NiCr2O4, MnCr2O4, and Cr2O3 are the main phases in the inner oxide layer. At temperatures above 700 °C, the formation of gaseous MoO3 produces cracks and spallation of the outer oxide layer. Moreover, Al2O3 and SiO2 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 Cr3+ and O2− in the bulk alloy. The formation mechanism of each oxide layer is discussed.