Development of grain structure in nickel oxide scale

Abstract

Outward diffusion of metal cations through the oxide scale controls the oxidation of a number of transition metals including Ni, Cr, and Fe. Grain boundary diffusion of cations is more predominant than that in the oxide lattice at temperatures in the range 500–1000°C. Thus, the grain structure of the scale is an important factor at these temperatures. The Ni/NiO system is focused on since much is known concerning this system and it can be used as a model for technologically important protective scales, such as Cr2O3. Grain growth in bulk ceramics is outlined and the possible differences from that in scales are highlighted. Factors controlling the initial grain size in oxide scales are discussed and observations of grain structure and grain growth are surveyed. Recent parallel and transverse TEM results on NiO scale and quantitative image analysis results for traced grain boundary networks are used to develop the discussion. Surface morphology observations are briefly reviewed and research on the microstructure of scales grown on transition metals other than Ni is summarised.MST/949