The Formation of Porous Oxides on Metals

Abstract

The porosity of the oxide scales formed on a number of metals during oxidation in dry oxygen has been investigated by measurements of specific surface and of density. The oxides formed during the linear oxidation of the eight metals: calcium, cerium, lead, magnesium, niobium, thorium, tungsten, and uranium are porous whereas those formed during the parabolic oxidation of copper and cobalt are impervious to oxygen gas.The oxidative behavior of a metal in oxygen depends to a marked extent on the physicochemical properties of the oxide layer formed on its surface. If the layer of reaction product remains continuous during oxidation, reaction can proceed only by the diffusion of either one or both of the reactants in the ionized form through the oxide lattice. In some cases, as with aluminum (1, 2) at 600~ the rate of oxidation falls off to a negligibly small value as time proceeds, while in others, as for example copper (3) at 1000~ the oxidation follows a parabolic rate law and the rate of attack is such that a sheet of material is oxidized completely in a few hours.On the other hand, some metals, usually above a certain characteristic temperature, oxidize at a constant rate which is maintained until the whole of the metal is consumed, a behavior exemplified by magnesium (4) at 500~This "linear oxidation" usually is interpreted in terms of the formation of an oxide scale which is porous in the sense that oxygen gas is able to pass freely through its thickness; the rate of oxidation is believed to be controlled by the rate of diffusion either of metal ions or oxygen anions through a thin barrier film of oxide adhering to the metal.The present study was undertaken in order to provide experimental evidence for the postulated porosity of the oxide layer formed during linear, or almost linear, oxidation and to show that the oxide layer formed during parabolic oxidation is, in fact, impervious to oxygen gas. The metals whose oxides were investigated were calcium, cerium, lead, magnesium, niobium, thorium, tungsten, and uranium, which oxidize at a linear rate, and cobalt and copper which oxidize at a parabolic rate. Two distinct experimental techniques were used to investigate each of the oxide products: the specific surface was measured by the method of krypton sorption (5) at --195~ and the density (6) by immersion in mercury and in carbon tetrachloride, respectively. ExperimentalEach oxide was prepared by oxidizing the metal in dry oxygen at atmospheric pressure in a glass or silica reaction chamber on a thermal balance. The experimental arrangement was the same as that previously described (4) except that the oxygen was further dried by passing it through a column packed with Linde molecular sieves (7).The percentage purity of the metals was: Ca, ~99.4; Ce, ~99.99; Co, 99.999; Cu, 99.999; Pb, 99.998; Mg, 99.95; Nb, ~99.8; Th, ~99.6; W, >99.95; and U, ~99.9. Except for calcium, cerium, and lead, the metals were in the form of sheet, 1/32 in. thick for niobium and tungsten and 1/16 in. thick for the others; a sample in ...

Oxidation kinetics of the quasicrystalline i-AlCuFe phase compared with that of crystalline ω-AlCuFe and pure aluminum

“…Thomas et al [34] contributed to the distinction of the phenomena: they measured a temperature of 400°C for the crystallization of oxide on the aluminum substrate and showed in parallel that the withdrawal of the substrate generated a delay for nucleation and a consecutive increase in this temperature of crystallization. Eventually, when the oxide film was formed on its substrate, all the authors agreed on a crystallization temperature of about 400-450°C [35][36][37]. This definitely demonstrates the influence of the substrate in the appearance of c-Al 2 O 3 crystallites.…”

The influence of oxide thickness on the early stages of the massive uranium–hydrogen reaction

“…Danon et al [14c] reported that uranium air-oxidised over several years produces an oxide ∼10-15 m thick with a surface area calculated to be 500-1000 times that expected, based upon the geometry of the massive uranium sample. Aylmore et al [33] reported that UO 2+x chipped from cooled, oxidised (240 • C, atmospheric pressure oxygen) samples of uranium exhibited a specific surface area of 6.5 ± 0.3 m 2 g −1 . This oxide when subjected to density measurements in Hg and CCl 4 was found to be significantly less dense (8.08 and 9.33 g cm −3 , respectively) than the theoretical density of UO 2 (10.96 g cm −3 ).…”

Oxidation of High-Purity Aluminum and 5052 Aluminum-Magnesium Alloy at Elevated Temperatures

“…Of the more comprehensive studies, Gulbransen and Wysong (1), Smeltzer (2), and Aylmore, Gregg, and Jepson (3) found that weight gains of metallographically polished high-purity aluminum in oxygen exceeded the 30 ~g/cm ~ level before the rate diminished sharply. Gulbransen and Wysong found in 2-hr tests that a parabolic law was obeyed from 350 ~ to 475~ and a linear law from 500 ~ to 550~ Smeltzer found that oxidation followed a two-step parabolic law during the early stages, then decreased to a lower rate after the weight gain had reached 30-40 ~g/cm ~.…”