Kinetics and mechanism of the reaction of iron-chromium and iron-chromium-molybdenum alloys with chlorine gas

Abstract

This report deals with the kinetics and mechanism of the reaction of iron-chromium and iron-chromium-molybdenum alloys (containing 1.25 pct Cr-0.5 pct Mo, 5 pct Cr-0.5 pct Mo, and 12 pct Cr) with low partial pressure chlorine gas in the temperature range 270 to 550 °C. The rate of reaction generally decreases with exposure time and, in the case of alloys containing five pct Cr and higher chromium content, the kinetics follows a parabolic rate law. The reaction kinetics is influenced by surface films containing FeCl₂ and CrCl₃ which have very low vapor pressure in the temperature range studied. The effect of molybdenum on the reaction is negligible. In the case of alloys containing five pct Cr-0.5 pct Mo and 12 pct Cr, the reaction product film consists essentially of CrCl₃ (outer layer) and much smaller concentration of FeCl₂ (inner layer). For these alloys, the observed parabolic kinetics is attributed to kinetic control of the overall reaction by solid-state diffusion through the reaction product surface film. In the case of the 1.25 pct Cr-0.5 pct Mo alloy, the film consists essentially of a mixture of FeCl₂ and CrCl₃. In this case, an outer CrCl₃ film layer is not formed and the overall kinetics is influenced by the rate of formation of volatile FeCl₃ speciesvia the reaction 2 FeCl₂ + Cl₂ (FeCl₃)₂.