Oxidation behaviour of thin Ni/Al2O3 nanocomposite coatings electrodeposited on steel substrate

Abstract

This paper presents investigations of the oxidation behaviour of nanocomposite Ni/Al2O3 coatings electrodeposited on steel substrates. The high-temperature processes of the coating oxidation were studied using the thermo-gravimetric analysis, scanning electron microscopy and XRD phase analysis. Simultaneous DSC-TGA oxidation experiments were conducted with continuous heating within the temperature range 200 °C–1200 °C and isothermally at 750 °C and 900 °C in oxygen flux. A proposed oxidation mechanism of Ni matrix nanocomposite coatings that includes diffusion of the substrate element was developed. The oxidation of the Ni/Al2O3 coating occurred via complex routes such as the outward diffusion of Ni and Fe ions and the inward diffusion of oxygen ions. The iron atoms diffused outward from the steel substrate through the Ni/Al2O3 coating to the surface and formed iron and nickel oxides layers. This thermally grown scale on the Ni/Al2O3 coating deposited on the steel substrate consisted of a Fe2O3 layer at the surface, a (Fe,Ni)3O4 spinel sublayer, and an inner NiO layer. Temperature-dependent isothermal kinetics at the steady-state stage of the oxidation process could be described by the linear growth law at 750 °C, while at 900 °C the quasi-linear growth rate is slightly decreased by the quadratic function of time.