| The high-temperature corrosion of iron aluminide (Fe3Al) in environments containing chlorine and oxygen has been investigated using the thermogravimetric method. The corrosion experiments were performed to study three effects on the corrosion behavior: (1) the effects of chlorine in the gas mixture with a trace level of oxygen; (2) the effects of oxygen in the gas mixture with a certain amount of chlorine; and (3) the yttrium-doping effect.; Two distinct stages of weight change during the corrosion in the gas mixtures containing chlorine and a trace amount of oxygen in the temperature range of 650∼900°C were observed. The first is a slow linear weight loss stage and the subsequent second stage is a fast linear weight loss stage. The first stage is characterized by localized attack by chlorine and repassivation or defect-healing including alumina-forming active oxidation. The depletion of aluminum in the alloy near the metal/oxide interface and the subsequent evaporation of iron chlorides may result in the weight loss during the second stage in which a significant amount of porous nonprotective aluminum oxide is produced by active oxidation. This mechanism has been proved through mathematical interpretation and investigation of corrosion products.; The oxygen-effect experiments were performed in Ar-0.5%Cl2 mixtures containing various levels of oxygen from trace amounts to 20% at 700°C. In general, the corrosion rate decreased with increasing oxygen content in the gas mixture. Several kinetic modes of corrosion were observed depending on the oxygen content. To explain these kinetic modes, a modified quasi-stability diagram was developed for the Fe-Al-Cl2-O 2 system. In addition, the corrosion of Y-doped Fe3Al has been performed under the same environments to investigate the effects of reactive elements. It has been found that the Y-doped alloy has much longer duration time of the first stage, which shows yttrium improves corrosion resistance in the first stage of corrosion. However, after the corrosion mode is transferred to the second stage, severe intergranular corrosion is observed. Thus, Y-doping could be harmful to the application of the alloy to structural parts due to this localized attack along grain boundary. Despite this disadvantage, Y-doped alloys could be effective in the application to coating materials. |
