Oxidation Of Ni-Cr Alloys Research Articles

On the role of surface deformation in the oxidation of NiCr alloys at 340–600 °C

The effect of surface deformation on selective oxidation is studied on a Ni-30Cr alloy prepared by grinding (SiC paper) vs. polishing (diamond suspensions), and oxidized in air at 340−600 °C. Imaging and subsurface composition profiling by transmission electron microscopy are combined to investigate the relationships between microstructure, diffusion and oxidation. The depth of deformed material is found to be critical to selective oxidation. Atypical Cr depletion profiles reflect an abrupt change in diffusion properties between the severely deformed subsurface and the bulk. The relative contributions of grain boundary, dislocation and bulk diffusion to the Cr flux are examined based on short-circuit diffusion theory.

Modelling of the effect of grain boundary diffusion on the oxidation of Ni-Cr alloys at high temperature

Grain boundaries in oxide scales have a strong effect on oxidation kinetics when they act as diffusion short circuits. This study proposes a quantitative evaluation of the phenomenon by modelling. Various cases of oxide microstructure evolution are treated using both analytical and numerical resolutions. Results showed that the effect of oxide grain growth on the oxidation kinetics can be analysed considering a transitory stage for which the oxidation kinetics is not purely parabolic. Some guidelines for choosing the appropriate post-treatment method for the analysis and extrapolation of experimental oxidation kinetics are given.

Oxidation of Ni-Cr alloy at intermediate oxygen pressures. II. Towards the lifetime prediction of alloys

The formation of protective Cr2O3 layer on Ni-Cr alloys depends on (i) the chromium concentration at the alloy/oxide interface and (ii) the chromium diffusion in the alloy from the core towards the surface. A sufficient interfacial chromium content and chromium gradient are necessary to sustain the selective external oxidation of chromium. Ni-30Cr samples have been oxidised at 1150°C in 10−13 up to 10−3atm O2. Analytical methods are used to model the diffusion in steady-state conditions and to predict the lifetime of the alloy. The interdiffusion coefficient is found equal to 1.2×10−10cm2s−1.

Oxidation of Ni-Cr alloy at intermediate oxygen pressures. I. Diffusion mechanisms through the oxide layer

The oxidation behaviour of Ni-30Cr alloy and the kinetics of the sole growth of Cr2O3 have been studied at 1150°C up to 289h at oxygen pressures comprised between 10−13 and 10−3atm, thanks to Rhines pack device. Oxide morphology, growth mechanisms and kinetics are discussed in terms of nature, concentration and diffusion of point defects. The criteria ruling the nature of the major diffusing point defects are considered through the influence of temperature and oxygen pressure. Diffusion coefficients are calculated from the oxidation kinetics data and compared to the literature.

Adsorption equilibrium at the metal-oxide film interface in the oxidation reactions of Ni, Cr, and their alloys

An adsorption thermodynamic model of the oxidation of Ni-Cr alloys is proposed. According to this model, the adsorption of the alloy component with a lower surface energy (Ni) at the alloy-oxide film interface shifts the equilibrium of the solid-phase reaction 3NiO + 2Cr = Cr2O3 + 3Ni (1) toward the enrichment of the oxide film in NiO. It was demonstrated that the total Gibbs energy change for reaction (1) can be presented as ΔGT, S = ΔGT + ΔGS, where ΔGT 0 is the contribution from the surface Gibbs energy of formation of the alloy associated with the replenishment of the surface layer of the alloy during its oxidation. Calculations of ΔGS are based on the published data on the surface energy of the pure metal ΔGSo and results of authors’ theoretical studies. It was found that the dependence of \({{a_{NiO}^3 } \mathord{\left/ {\vphantom {{a_{NiO}^3 } {a_{Cr_2 O_3 } }}} \right. \kern-\nulldelimiterspace} {a_{Cr_2 O_3 } }}\) on the content of Cr in the alloy determined from calculated equilibrium characteristic of reaction (1) at 1373 K proved to be in satisfactory agreement with the available experimental data on the composition of the oxide film on Ni-Cr alloys. In addition, the values of the potentials of metal-oxide Ni and Cr electrodes in an aqueous solution at 298 K are calculated, which nearly coincide with the published values of the Flade potential for the passivation of these metals.

Effect of NaCl vapor on the oxidation of Ni-Cr alloys

Ni-Cr alloys are known for their resistance to high temperature oxidation. The kinetics of scale formation and the nature of the scale in these alloys are affected by NaCl liquid or vapor. There have been a few investigations dealing with the influence of NaCl on long-time exposure. But the nature of reaction at short times can provide information on the initiation of such attack. In this investigation, Ni-Cr alloys with Cr varying from 0 to 25 wt% were exposed to NaCl vapor at 850°C for a few minutes. The surface chemistry of these alloys along with the unattached ones was analyzed by Auger electron spectroscopy. The nature of scale and the distribution of chlorine was found to vary with the Cr content in the alloys, which has a direct bearing on the rate of oxidation of these alloys in NaCl vapor.

Compositional changes in the underlying alloy produced by the oxidation of Ni-Cr alloys

Compositional changes in the alloy beneath scales have been examined for the oxidation of Ni-27.4%Cr and Ni-40.2% Cr in 1 atm oxygen in the temperature range 1073–1473°K. Calculations of the rate of approach of the interfacial alloy composition to a constant value are compared with experimental data. Theoretical chromium depletion profiles obtained using both a finite difference analysis and an analytical expression are shown to be essentially equivalent and in good agreement with experimental measurements. The consequences of alloy depletion for the scaling behavior, when the protective scale is ruptured, are discussed.

Oxidation of Ni-Cr alloys between 800° and 1200°C

Oxidation of Ni-Cr alloys between 800° and 1200°C