| Summary: | 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 + Cr<sub>2</sub>O<sub>3</sub>, following a linear oxidation law with a rate constant (<i>K</i><sub>l</sub>) on the order of magnitude of 10<sup>−6</sup> kg/m<sup>2</sup>s and an apparent activation energy (<i>E<sub>a</sub></i>) 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 Cr<sub>2</sub>O<sub>3</sub>, and a parabolic oxidation law was followed with a rate constant (<i>K<sub>p</sub></i>) on the order of 10<sup>−8</sup> kg<sup>2</sup>/m<sup>4</sup>s and <i>E<sub>a</sub></i> 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.
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