Accelerated oxidation rate of chromium induced by sodium chloride

Abstract

The oxidation rate of NaCl-coated chromium was measured over the temperature range of 823–1043 K. Although the melting point of NaCl is 1074 K, accelerated oxidation was observed at every temperature. Oxidation mass-gain curves were divided into two types. Type I involved only accelerated oxidation during the initial stage followed by the formation of a thin protective Cr₂O₃ film. Type II pertained to an acceleration over a long time, forming a thick and nonprotective Cr₂O₃film containing Na₂CrO₄. The former type of oxidation occurred at lower temperatures or with a small amount of NaCl, whereas the latter occurred at higher temperatures and with large amounts of NaCl. A comparison of these oxidation processes with those by CaCl₂ or BaCl₂, revealed two problems: (1) Why was the oxidation rate of chromium so high in the presence of NaCl?, and (2) Why did the high oxidation rate continue for such a long time?