Effect of SO2 Addition on Air Oxidation Behavior of CM247 and CMSX-4 at 1050°C

In the present work, the oxidation behavior of two commercial Ni-base superalloys, CMSX-4 and CM247, in synthetic air with and without 2 vol.% SO2 at 1050°C has been studied. The corrosion reactions in the presence of SO2 could not be explained simply in terms of the contents of the main scale‐forming alloying elements, Cr and Al. The far better resistance of CMSX-4 is related to the formation of a rather pure and dense alumina scale after a very short period of transient oxidation. Rapid development of an alumina scale prevents access of molecular SO2 to the metal surface thus effectively suppressing internal sulfidation. In contrast, CM247 with a similar Al-content formed an Al-rich oxide scale with local intrusions and/or inhomogeneities caused by the underlying alloy microstructure, which deteriorated its resistance to internal sulfidation and resulted in rapid failure in synthetic air + 2% SO2

Phase Transformations in Co-Ni-Cr-W Alloys During High Temperature Exposure to Steam Environment

Three model alloys, Co-10Ni-20Cr-15W, Co-30Ni-20Cr-15W and Ni-20Cr-15W (all in wt.%) were investigated in Ar-50%H2O at 700 and 750 °C for up to 3000 h reaction. The results showed the formation of thin chromia scales on the sample surfaces in all cases. For the Co-base alloys, accompanied by the Cr2O3 formation, a chromium depletion zone was detected underneath the oxide scale along with Co3W with some amount of dissolved Cr and Ni. This kind of Co3W was enriched along the oxide/alloy interface. The formation of this intermetallic phase was considered to follow nucleation and subsequent growth based on morphology and composition analyses. Increasing nickel content reduced the amount of Co3W formation. For the nickel-base alloy Ni-20Cr-15W, no intermetallic phase was detected. The formation of the Co3W intermetallic was discussed based on phase transformation induced by chromium depletion. The effect of nickel on this phenomenon was also discussed according to thermodynamic phase equilibrium analysis

Simulating the effect of aluminizing on a CoNiCrAlY-coated Ni-base superalloy

MCrAlY (M = Ni, Co) coatings are commonly used on gas-turbine components as oxidation resistant overlay coatings and bondcoats for thermal barrier systems. The present work focuses on the effect of the aluminizing process on the CoNiCrAlY coating microstructure. In the as-received condition the outer part of the coating consisted mostly of β-(Ni,Co)Al with interspersed precipitates of Cr-rich carbide and Cr-rich boride precipitates. Formation of σ-CoCr was observed at the interface between the β-layer and the inner initial CoNiCrAlY microstructure. Scanning electron microscopy (SEM) combined with energy and wavelength-dispersive X-ray spectroscopy (EDX/WDX) was employed to characterize the aluminized CoNiCrAlY coating. Phases were then identified by electron backscatter diffraction (EBSD). Detailed microstructural studies of the coating were corroborated with the help of coupled thermodynamic-kinetic calculations to model the aluminizing process. The calculations were performed with the in-house developed code employing the commercially available thermodynamic and kinetic databases (ThermoCalc). The mechanisms of the observed microstructural changes were elucidated with the help of the modelling results

Predicting Oxidation-Limited Lifetime of Thin-Walled Components of NiCrW Alloy 230

Using alloy 230 as an example, a generalized oxidation lifetime model for chromia-forming Ni-base wrought alloys is proposed, which captures the most important damaging oxidation effects relevant for component design: wall thickness loss, scale spallation, and the occurrence of breakaway oxidation. For deriving input parameters and for verification of the model approach, alloy 230 specimens with different thicknesses were exposed for different times at temperatures in the range 950–1050 °C in static air. The studies focused on thin specimens (0.2–0.5 mm) to obtain data for critical subscale depletion processes resulting in breakaway oxidation within reasonably achievable test times up to 3000 h. The oxidation kinetics and oxidation-induced subscale microstructural changes were determined by combining gravimetric data with results from scanning electron microscopy with energy dispersive X-ray spectroscopy. The modeling of the scale spallation and re-formation was based on the NASA cyclic oxidation spallation program, while a new model was developed to describe accelerated oxidation occurring after longer exposure times in the thinnest specimens. The calculated oxidation data were combined with the reservoir model equation, by means of which the relation between the consumption and the remaining concentration of Cr in the alloy was established as a function of temperature and specimen thickness. Based on this approach, a generalized lifetime diagram is proposed, in which wall thickness loss is plotted as a function of time, initial specimen thickness, and temperature. The time to reach a critical Cr level at the scale/alloy interface of 10 wt% is also indicated in the diagrams

Effect of Specimen Thickness on Microstructural Changes During Oxidation of the NiCrW Alloy 230 at 950-1050°C

An accurate procedure for predicting oxidation-induced damage and lifetime limits is crucial for the reliable operation of high-temperature metallic components in practical applications. In order to develop a predictive oxidation lifetime model for Ni–Cr alloys, specimens of wrought NiCrW alloy 230 with different thicknesses were cyclically oxidized in air at 950–1050°C for up to 3000 h. After prolonged exposure, two types of carbides as well as a Cr-rich nitride (π-phase) precipitated in the γ-Ni matrix. The oxidation-induced loss of Cr from the alloy resulted in the formation of subscale zones, which were free of the Cr-rich carbide and nitride but also of the Ni-W rich M6C. The width of the M6C-free zone was smaller than that free of the Cr-rich precipitates. Thermodynamic and diffusion calculations of the observed time- and temperature-dependent Cr depletion processes identified that back diffusion of C occurred which resulted in an increased volume fraction of M23C6 in the specimen core. With increasing time and temperature, the amount of π-phase in the specimen core increased. The subscale depletion of the initially present Cr-nitrides and the formation of Cr-nitrides in the specimen center is believed to be related to a mechanism which is qualitatively similar to that described for the Cr carbide enrichment. However, with increasing time and decreasing specimen thickness, N uptake from the atmosphere becomes apparent. As a result, the precipitates present in the specimen center eventually consisted almost exclusively of nitride