Morphology and Structure of Oxides Grown on Aluminum In Superheated Steam

Abstract

Abstract Four methods (light microscopy, electron microscopy, electron diffraction and X-ray diffraction) have been employed to identify and estimate the-products of reaction of pure aluminum with steam; in a few instances gravimetric measurements were also made. At temperatures between 400 and 450 C (752 and 842 F) and intermediate to high pressures aluminum disintegrated to a white powder product within 6 hours. This powder product was a mixture of α- and γ-alumina together with a small amount of aluminum. At higher temperatures and over the whole pressure range only γ-alumina was formed. The morphology of this material was threefold. A thin, highly resistant film rapidly grew to approximately 300 Å and drastically slowed down the reaction process. Outcropping from this film were oxide crystallites between 0.1 and 1.0 micron in size, as well as thin platelets which grew out from the surface as much as 10 microns. After reaction, specimen surfaces were found to contain numerous gas blisters as well as pits of various shapes and sizes. These pits ranged in size from less than 1 micron up to 300 microns and were located at grain boundaries as well as within metal grains.