Microstructures and cyclic hot corrosion behavior of laser deposited Inconel 718 alloy under different heat treatment conditions

Abstract

In this research, the hot corrosion behavior of laser deposited Inconel 718 alloy under different heat treatment conditions (solution plus aging, direct aging and as-deposited) is investigated in a salt mixture of Na2SO4 (87.5 wt.%) + NaCl (5 wt.%) + NaVO3 (7.5 wt.%) at 650 °C. The weight changes of the Inconel 718 specimens that have experienced different heat treatments are measured during the hot corrosion experiments and the data of the weight changes are used to determine the kinetics of the hot corrosion. Field emission scanning electron microscope (FE-SEM) with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) are employed to characterize the corrosion products of the specimens, thus the corrosion mechanisms are analysed. It is shown that the hot corrosion kinetic of the as-deposited Inconel 718 alloy follows a parabolic law. The element mapping results of the corroded surfaces reveal that the corrosion products of as-deposited Inconel 718 include two distinct layers, of which the outermost layer is mainly composed of Ni2CrO4, Cr2O3 and Fe3O4, while the inner layer contains Ni3S2. The laser deposited Inconel 718 alloys with different heat treatments behave differently in the hot corrosion. The hot corrosion performance of the solution treated specimens is better than that of the directly aged specimens, while the latter has better hot corrosion resistance than the as-deposited specimens. According to the hot corrosion test results and theoretical analyses, the hot corrosion behavior of as-deposited Inconel 718 alloy with different heat treatments in the salt mixture solution is closely related to basic fluxing mechanism, sulphidation and oxidation as well.