The influence of aluminium on the oxidation of a Cr 2O 3-forming nickel-chromium alloy

Abstract

A study has been undertaken of the influence of 1% Al on the oxidation of Ni-28% Cr at 800 and 1000° C. Following the initial, transient oxidation period, a protective healing Cr 2O 3 layer is established on both alloys. The tertiary element facilitates this process, by acting as a secondary oxygen acceptor. Precipitates of its oxide may also act as preferential sites for nucleation of the protective oxide. Although the oxide scales thicken relatively slowly on the two alloys, considerable damage is sustained in the underlying alloy substrate. This involves formation of voids and internal oxide, particularly in the alloy grain boundaries, and results from depletions of chromium in the underlying alloy as the element is taken into the scale. Differential diffusion rates between chromium to the surface and nickel into the substrate lead to a Kirkendall effect and condensation of vacancies to precipitate voids in the chromium-depleted regions. The Cr 2O 3 scale is able to supply oxygen at a high enough activity, due to the chromium depletions in the adjacent substrate, to form new Cr 2O 3 internally in regions of higher chromium activity. The voids in the grain boundaries provide easy diffusion paths for oxygen, resulting in deep penetration of intergranular oxide. The presence of aluminium in the alloy increases the extent of such oxidation, due to the greater stability of Al 2O 3 compared with that of Cr 2O 3.