A Survey on the Oxidation Behavior of a Nickel-Based Alloy Used in Natural Gas Engine Exhaust Valve Seats

Abstract

This study reports the oxidation behavior of a Ni-based alloy used in the manufacture of valve seats for automotive engine exhaust systems. Isothermal thermogravimetric analyses were carried out at temperatures of 660, 740, 860, and 900 °C under an oxygen atmosphere for up to 1 h. At 660 and 740 °C, only one stage was observed during the whole time studied. At this stage, the oxide layer was formed mainly by NiO + Cr2O3, following a linear oxidation law with a rate constant (Kl) on the order of magnitude of 10−6 kg/m2s and an apparent activation energy (Ea) of ~47 kJ/mol. At 860 and 900 °C, an identical first stage was observed with a transition to a different stage. In the second stage, the oxidation layer was composed of Cr2O3, and a parabolic oxidation law was followed with a rate constant (Kp) on the order of 10−8 kg2/m4s and Ea of ~128 kJ/mol. Moreover, the Ni-based alloy formed a dense and compact oxide layer after oxidation, with no apparent cavities, pores, or microcracks. Characterization techniques such as Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDS), Fourier Transform Infrared Spectroscopy (FTIR), and Raman Spectroscopy were carried out to characterize the formed oxide layer.