Isothermal oxidation kinetics of nanocrystalline and coarse grained nickel: Experimental results and theoretical approach

In this research, isothermal oxidation kinetics of nanocrystalline (NC) nickel prepared by electroplating as well as coarse grained (CG) nickel was investigated. Oxidation tests were carried out in static air for various durations at 700 degrees C, 800 degrees C and 900 degrees C. The structure of oxidized specimens was characterized by means of SEM and XRD. Moreover, a theoretical model based on Atkinson approach as well as Maxwell Equation was proposed. The derived model was checked by fitting the theoretical curves to the experimental data using Matlab software. Good agreement between theoretical model and obtained experimental results as well as reported data was observed. The results indicated that the weight gain per unit area (w = Delta m/A) of NC samples was considerably higher than the CG nickel. Moreover, the decrease in the parabolic growth rate constant (k(p)) of NC nickel by increasing the oxidation time was sharper than the microcrystalline samples. In spite of the overall data of log (k(p)), the logarithms of k(p) values corresponding to a given w was a linear function of the reciprocal temperature resulted an activation energy of similar to 108 kJ/mol for oxide layer growth on NC nickel as well as CG samples. In addition, the values for the activation energy for grain boundary diffusion of Ni in NiO layer which are determined from the fitting parameters for NC and CG nickel, both were close to that for the growth of NiO layer. (C) 2011 Elsevier B.V. All rights reserved.