Oxidation Behavior and Mechanism of Inconel 740H Alloy for Advanced Ultra-supercritical Power Plants Between 1050 and 1170 °C

Abstract

The oxidation behavior and mechanism of as-cast Inconel 740H superalloy, newly developed for advanced ultra-supercritical power plants superheater and reheater pipes, were investigated in the temperature range of 1050-1170°C in air. Isothermal oxidation testing, X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy were used in this study. The results showed that the oxidation of Inconel 740H was a diffusion-controlled process and controlled by the outward diffusion of metallic elements and inward penetration of oxygen. Owing to the complicated chemical composition of the alloy, different oxides tended to form in the oxidation process. Cross-sectional characterization investigation indicated that the oxide-scale layer consisted of three parts when the temperature was high or the oxidation time was long: an internal oxidation zone composed of Al 2 O 3 and TiO 2 , a continuous and dense middle layer of Cr 2 O 3 containing precipitates of TiO 2 and an outmost layer of NiCr 2 O 4 spinel. In the formation process of NiCr 2 O 4 , NiO was found in the early transient stage. With further oxidation, TiO 2 finally could not be detected in the internal oxidation zone as a result of outwards diffusion of Ti and its limited concentration in the matrix.

Oxidation Behavior of Inconel 740H Nickel Superalloy in Steam Atmosphere at 750 °C

“…Literature data [ 22 , 23 ] suggest that TiO 2 oxides form at the beginning of oxidation (before Cr 2 O 3 oxide). However, the higher Cr content in the chemical composition of the superalloy caused significant growth and dominance of Cr 2 O 3 oxide on the superalloy surface.…”

“…The investigations of the superalloy surface after 1000 h showed that the outer part of the external oxide layer was TiO 2 oxide crystallites, which grew with time ( Figure 5 d,e). The formation of TiO 2 at the steam-oxide layer border resulted from the easy diffusion of Ti ions through the Cr 2 O 3 layer [ 22 , 23 ]. Moreover, evaluation of the chemical analysis of the external oxide layer also revealed diffusion of Mn, whose beginning started earlier, i.e., after 1000 h of oxidation ( Figure 5 ).…”

“…Increased concentration of Mn was observed in the areas where TiO 2 was present. The authors of the papers [ 22 , 23 , 24 , 25 ] about the evaluation of the oxidation resistance of Inconel 740 and 740H superalloys described the presence of Mn in the chemical composition of the formed oxide layers. It was noted that the depletion of the surface layer of the alloy in Ti and Cr may cause diffusion along the layer of other components, e.g., Mn [ 14 , 16 ].…”

Effect of plasma nitriding process on the fatigue and high temperature corrosion resistance of Inconel 740H nickel alloy

“…It should be mentioned, that materials dedicated to the superheater/reheater should have an outstanding oxidation resistance. According to the literature, 60% of boiler failures occurred due to the steam oxidation and coal-ash corrosion [5]. Among the main reasons of the nickel-based alloys failure due to a high temperature corrosion, one can indicate a loss of the surface strength, crack initiation, dealloying and component failure [6].…”

“…Among the main reasons of the nickel-based alloys failure due to a high temperature corrosion, one can indicate a loss of the surface strength, crack initiation, dealloying and component failure [6]. Such mechanisms are associated with the occurrence of specific oxides: Cr 2 O 3 , NiCr 2 O 4 , and NiFe 2 O 4 (Inconel 718) [7], Cr 2 O 3 and NiCr 2 O 4 (Inconel 625) [8], NiCr 2 O 4 , Cr 2 O 3 and TiO 2 (Inconel 625 [9] and Inconel 740H [10]). However, the oxidation behaviour and its mechanism for the nitrided Inconel 740H nickel alloy at 1000 °C were not investigated as yet.…”