The use of ternary phase diagrams in the study of high temperature corrosion products formed on Fe-5WT% Al alloys in reducing and oxidizing environments

Ternary phase diagrams, in conjunction with microscopy techniques and react ion product chemistries, were used to describe the possible "diffusion path s" and resulting morphologies that may occur during formation of corrosion scales from high temperature gaseous exposure. Iron with 5 wt% of aluminum was exposed to reducing and oxidizing environments at 700 degrees C. Charac terization of the surface reaction products was conducted using microscopy techniques with energy dispersive spectroscopy and electron probe microanal ysis. Under both conditions, iron diffused outward to form surface reaction products, either iron sulfide or iron oxide. The ingress of sulfur or oxyg en through the previously formed reaction products was found to produce int ernal corrosion phases within the alloy. By plotting chemical information a cquired from the corrosion scales on ternary phase diagrams, development of the phase layer sequence and morphologies of the multiphase corrosion scal es was schematically explained. This analysis was conducted using conventio ns summarized by Clarke (Trans. Am. Inst. Min. Engrs, 1963, 227, 1250) for plotting diffusion paths in multiphase ternary diffusion studies from solid -stale reactions. By presenting the experimental data in this manner, the m orphological development of the scales was related to the composition path on the ternary phase diagrams. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.