Thermal Oxidation Kinetics of Nickel and Dilute (Ni-Al) Alloys

Abstract

Oxidation of high purity nickel and of a dilute Ni-Al (0.11 wt%) polycrystalline alloy were performed in the temperature range 700 - 1200°C, under PO2=1 atm. At T<1100°C, Al leads to a reduction of the oxidation kinetic, while at higher temperatures its beneficial effect disappears. The results were analysed from a formal treatment, taking into account the thermodynamic and transport properties of undoped and Al-doped Ni1-xO single crystals. At temperatures below 1000°C, it was found that the results are in agreement with the thermal oxidation of undoped and doped Ni1-xO samples. They are due to the kinetic demixing of cations in the alloy layer growth, leading to both, a lower concentration of aluminium and a decrease of DNi in the outer oxidation scale. At higher temperatures, the Ni-Al alloy oxidises faster than Ni. This increase was explained by an inward oxygen gas transport within the layer through fissures, whose formation was attributed to compressive stresses due to the growth of nickel oxide units in grain boundaries. The mechanism whereby nickel oxide units form is explained.

Keywords

• Oxidation rate
• Kinetic demixing
• Thermodynamic
• Diffusion
• Point defects

References

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