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

The corrosion behavior of Fe, Cr, Ni, the ferritic alloys Fe-15Cr and Fe-35 Cr and the austenitic alloys Alloy 800, Alloy 825 and Alloy 600 was investi gated in N-2-5 vol% O-2 and He-5 vol% O-2 With additions of 500-1500 vppm H Cl at temperatures between 400 and 700 degrees C using discontinuous exposu res and thermogravimetric experiments. The corrosion products were analyzed by scanning electron microscopy with EDX-analysis, X-ray analysis and Auge r electron spectroscopy. The main corrosion process, especially at temperat ures above 500 degrees C, is 'active oxidation', i.e. the formation of vola tile 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 d etermined by thermodynamics, that means the free energy of chloride formati on, the vapor pressure of the chlorides and the oxygen pressure necessary f or conversion of the evaporated chloride to the oxide. (C) 2000 Elsevier Sc ience Ltd. All rights reserved.