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

Ternary phase diagrams, in conjunction with microscopy techniques and reaction product chemistries, were used to describe the possible “diffusion paths” 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°C. Characterization of the surface reaction products was conducted using microscopy techniques with energy dispersive spectroscopy and electron probe microanalysis. Under both conditions, iron diffused outward to form surface reaction products, either iron sulfide or iron oxide. The ingress of sulfur or oxygen through the previously formed reaction products was found to produce internal corrosion phases within the alloy. By plotting chemical information acquired from the corrosion scales on ternary phase diagrams, development of the phase layer sequence and morphologies of the multiphase corrosion scales was schematically explained. This analysis was conducted using conventions summarized by Clarke ( Trans. Am. Inst. Min. Engrs, 1963, 227, 1250) for plotting diffusion paths in multiphase ternary diffusion studies from solid-state reactions. By presenting the experimental data in this manner, the morphological development of the scales was related to the composition path on the ternary phase diagrams.