Reversible catastrophic oxidation of a 38Fe–34Ni–25Cr alloy induced by sodium sulphate at low oxygen potential atmospheres

Abstract

The chromia-forming nickel-based alloy Haynes® HR-120 was oxidised with and without Na2SO4 deposit in a CO/H2/CO2 (45/45/10%vol.) simulated process atmosphere at 900°C for 96h. During the first hours of oxidation, samples covered by sodium sulphate exhibit higher oxidation rate than non-covered ones. However, after 24h both sulphate-covered and uncovered specimens follow the same linear kinetics. In this very low oxygen partial pressure environment (10−18atm), the presence of Na2SO4 promotes the growth of localized iron-rich oxide nodules leading to the observed accelerated oxidation. The development of these nodules is discussed to be the result of the chromia dissolution induced by a basic fluxing mechanism. As soon as the salt is evaporated, slower kinetics are observed and the nodules disappear. In these specific conditions, the oxidation could be considered as a self-healing process.