Growth and structure of nickel oxide scale formed on polycrystalline nickel

Abstract

<p>In this thesis the growth and structure of nickel oxide scales formed on polycrystalline nickel during continuous oxidation and oxidation interrupted by vacuum anneals at 800° and PO₂ = 400 Torr were studied. The morphological development of nickel oxide formed commencing with early stages of oxidation to scale growth at very long times was studied by thermogravimetry, scanning electron microscopy and X-ray diffraction measurements.</p> <p>The results demonstrate the influence of the metal structure on the anisotropy of oxidation, the influence of the defect structure of the oxide set up by the growth process itself on the transport mechanism of the reactants, and the preferential growth of oxide grains.</p> <p>The nickel oxide scales exhibited initially a bilayer structure or a simple equiaxed structure; these structures could be transformed into a monolayer structure of columnar oxide grains by different methods. The early stage cellular structure of nickel oxide developed into inner equiaxed and outer columnar oxide layers by a combined transport of nickel, through the oxide lattice and through boundaries of the oxide grains. The structural development of the nickel oxide scale was correlated with the nonparabolic oxidation behaviour of nickel.</p>