Growth of nodular corrosion products on Fe-Al alloys in various high-temperature gaseous environments

The mechanisms for nodular corrosion-product development were investigated in various high-temperature gaseous environments. Fe-Al alloys, with 5-20 w t.%Al, were exposed in both oxidizing and sulfidizing [p(S-2) = 10(-4) atm, p(O-2) = 10-25 atm] atmospheres at 700 degrees C for times up to 100hr. Th e corrosion kinetics were monitored by the use of a thermogravimetric balan ce and the morphological development through light-optical and scanning-ele ctron microscopies, energy-dispersive spectroscopy, electron-probe microana lysis, and quantitative-image analysis. Under both conditions, the eliminat ion of nodule formation was observed by increasing the aluminum content of the alloy, above 5 and 7.5 wt.% Al for oxidizing and sulfidizing environmen ts, respectively, which promoted the growth and maintenance of a continuous surface scale of alumina. For those alloys that were observed to develop n odular corrosion products, their morphological appearance was similar in na ture regardless of the corroding species. The nodules typically consisted o f an outer iron-rich product, either sulfide or oxide, that was randomly di spersed across an alumina scale. Samples fr om the oxidizing atmosphere dis played a single growth-rate time constant from the kinetics data, suggestin g that the nodule growth mechanism was by the simultaneous or codevelopment of two different (Fe and Al) oxides from the onset of exposure. Measuremen t of nodule planar diameter and depth of penetration into the alloy indicat ed that growth occurred through diffusional processes. Kinetics data from t he development of sulfide nodules in the reducing environment revealed a di fferent type of mechanism. Multiple growth-rate time constants were found d ue to the localized mechanical failure of an initially formed surface scale . At early times in the sulfidizing atmosphere, a low corrosion rate was re corded as a continous-alumina scale afforded protection front excessive pro duct development. However, with the mechanical failure of the scale, sulfur was able to attack the underlying substrate through a short-circuit diffus ion mechanism that resulted in rapid weight gains from nonprotective, iron sulfide growth. The sulfide morphologies observed from very complex as cont inued growth of the nodule did not solely depend upon the diffusing species through the previously, formed corrosion products, but also, continued mec hanical failure of the oxide scale. It is suggested that the difference in development mechanisms between the two environments may lie in the relative growth rates of the nonprotective, Fe-base corrosion products formed.