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Ni-base superalloy IN 718 was cyclically oxidized in laboratory air at temperatures ranging from 750 to 950°C for up to 12 cycles (14 h/cycle). The kinetic behaviour as well as the surface morphology, and the oxide phases of the scales were characterized by means of weight gain measurements, cyclic oxidation kinetics, scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDS), and X-ray diffraction (XRD) analysis techniques. The results showed that as the oxidation temperature increased, the oxidation rate, the external scale thickness, and internal oxidation zone increased. It was suggested that the oxidation rate was controlled by the diffusion of substrate elements in the alloy and the inward diffusion of oxygen through the oxide scale. The oxidation kinetics followed a sub-parabolic rate law and, the activation energy of oxidation was 249 ± 20 kJ mol -1 . The scaling process was controlled mainly by the diffusion of chromium, titanium, manganese, and oxygen ions through the chromia scale. IN 718 showed low weight gain and very slow reaction rates of substrate elements at 750°C. At 850°C, a continuous and very thin oxide scale was formed. At 950°C, XRD and EDS-elemental mapping analysis revealed that a complex oxide scale had formed. It consisted of an outermost layer of TiO 2 -MnCr 2 O 4 spinels, inner layer of Cr 2 O 3 , and the inner most layer composed of Ni 3 Nb enriched with Nb, Ti and Al oxides underneath the chromia layer. The oxide scale at this temperature seemed to be thicker layer, significant spallation and volatilization had apparently occurred, and greater internal corrosion was identified. The doping effect of titanium was observed, where it was found to be diffused through the chromia scale to form TiO 2 at the oxide-gas interface as well as internally and at the oxide alloy interface.

The oxidation behavior of the IN600 Ni-Cr-Fe superalloy was investigated in air at temperatures ranging from 750 to 950°C, for up to 12 cycles. Oxidation kinetics and oxide scale morphologies were examined using weight gain measurements, SEM-EDS, and X-ray diffraction. The cyclic oxidation kinetic results suggested that the oxidation behavior of the IN600 alloy approximately followed a sub-parabolic rate and the scaling process was controlled by the formation of a chromia scale. At 850°C, SEM-EDS observations indicated that the formed oxide scale was primarily composed of Cr 2 O 3 , and the internal oxidation of Cr and Ti occurred. At 950°C, a fast initial stage with high weight gain was observed, followed by a steady-state stage with gradual weight gain. Additionally, a considerable change in the oxidation kinetic occurred. SEM-EDS observations and XRD results indicated that the external scale was relatively thick with a localized porous, preferential adherent, and a complex oxide scale was developed. This complex oxide scale consisted of an outermost thin layer composed of MnCr 2 O 4 -Cr 2 O 3 mixed together with a small amount of isolated TiO 2 , an intermediate relatively thick layer, composed of Cr 2 O 3 , and an innermost discrete layer formed at the scale/ alloy interface, which enriched by Ni/NiO mixed with Ti-, Al-, and Fe-oxides. Finally, only the Al alloying element was internally oxidized to form Al 2 O 3 fingers, which create a discrete and narrow internal oxidation zone. Al oxide was observed as a dark area and primarily grows along the alloy grain boundaries in the vicinity of the inward chromia pegs.

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