Chloridation and oxidation of iron, chromium, nickel and their alloys in chloridizing and oxidizing atmospheres at 400–700°C

Abstract The corrosion behavior of Fe, Cr, Ni, the ferritic alloys Fe–15Cr and Fe–35Cr and the austenitic alloys Alloy 800, Alloy 825 and Alloy 600 was investigated in N 2 –5 vol% O 2 and He–5 vol% O 2 with additions of 500–1500 vppm HCl at temperatures between 400 and 700°C using discontinuous exposures and thermogravimetric experiments. The corrosion products were analyzed by scanning electron microscopy with EDX-analysis, X-ray analysis and Auger electron spectroscopy. The main corrosion process, especially at temperatures above 500°C, is ‘active oxidation’, i.e. the formation of volatile metal chlorides at the metal/oxide interface, which evaporate, diffuse outwards and are converted into oxides in regions of higher oxygen partial pressure. The characteristic differences concerning the corrosion behavior of the different metals and alloys at different temperatures are largely determined by thermodynamics, that means the free energy of chloride formation, the vapor pressure of the chlorides and the oxygen pressure necessary for conversion of the evaporated chloride to the oxide.

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