A microstructural study of the oxidation of Fe-Ni-Cr alloys I. protective oxide growth

Abstract
The oxidation behaviour of a range of Fe—Ni~Cr alloys has been studied by correlating ‘edge-on’ tem observations of the retained metal—oxide interface with data obtained by conventional low resolution techniques for studying oxidation. Results are presented for ‘protective’ scale formation on three Fe-20Cr—Ni alloys as oxidized at 600 °C in air and in 1 % CO—CO2. The temporal development of thin layers of chromium-rich sesquioxides in air was followed and it was shown that growth occurs by a process essentially dominated by the outward diffusion of chromium. Results obtained by ‘edge-on’ tem demonstrated that decreased chromium gradients in the alloy and higher chromium concentrations at the metal—oxide interface are associated with some inward development of the scale. These minor microstructural changes are paralleled by increases in the rate constants with a tendency towards cube law kinetics. A reverse trend in the mass gain data, as a function of the nickel content of the alloy was characterized for oxidation in 1 % CO-CO2. Despite this, chemically similar oxides are seen to form although carbides were observed beneath the scales on the higher nickel content alloys. Significantly the trend with increasing nickel content was found to be paralleled by an increasing tendency for inward oxide growth. The iron activity in the alloy of lowest nickel content is sufficiently high for chemically unstable behaviour and the formation of duplex oxide nodules. Mechanisms by which simultaneous inward oxidation and carburization can take place during the formation of both chromium-rich sesquioxides and duplex spinel scales are discussed.