EFFECTS OF OXYGEN AND WATER VAPOR PRESSURES ON OXIDATION OF IRON-CHROMIUM ALLOYS AT 573 K

Abstract

The oxidation of iron-chromium alloys (0,1 and 5 wt%Cr) was investigated by means of Rutherford backscattering spectroscopy at 573 K in the oxygen partial pressure range from 10 −1 to 10 5 Pa without and with water vapor at 10 4 Pa of water vapor pressure for 1 – 48 h. The oxidation of iron-chromium alloys (0 – 5 wt%Cr) obeyed the parabolic rate law after the rapid oxidation in the first stage. When iron-chromium alloys were oxidized under various oxygen partial pressures without water vapor, the parabolic rate constant decreased with increasing chromium concentrations at constant oxygen partial pressure probably due to the decrease of iron diffusion through chromium oxide, and the mechanism of oxidation was devided into three regions : Below 10 Pa the decrease of the oxidation rate with increasing oxygen partial pressure is due to the formation of dense hematite. Between 10 and 10 3 Pa the protective hematite coverage over magnetite kept the parabolic rate constants low regardless of oxygen partial pressure. At 10 5 Pa an increase of the parabolic rate constant was observed probably due to the simultaneous growth of hematite and magnetite. The remarkable increase of the parabolic rate constant for the oxidation of iron-chromium alloys by water vapor was observed at lower oxygen partial pressures. This fact may be explained by vapor-phase diffusion of valatile Fe(OH) 2 from the metal-oxide interface to the outer layer crystals. The corrosion behavior of iron-chromium alloys in gas phase containing water vapor may be closely related to that in high-temperature and high-pressure water around 573 K.