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

Abstract

Oxidation of chemically polished high-purity aluminum in dry oxygen, water vapor, and moist air at temperatures from 450 ~ to 640C ~ is characterized by a near-linear reaction rate to a weight gain of 3 ~g/cm 2, followed by a rate that decreases rapidly with further weight gain. Oxidation beyond the 5-7 ~g/cm ~ weight gain range is very slow. Oxidation is slightly faster in the moist atmospheres than in dry oxygen at temperatures above 550~ Oxidation of a commercial aluminum-magnesium alloy (5052) in dry oxygen and in moist air is much faster than for high-purity aluminum and proceeds to much higher weight gains. The rate does not conform to any recognized oxidation law, but it is more nearly parabolic than linear. The much higher weight gains obtained by other workers who used mechanically polished samples of both high-purity aluminum and aluminum-magnesium alloy are attributed to surface roughness. Electron diffraction examination of oxidized specimens show only eta alumina on the high-purity metal and magnesium oxide on the aluminum-magnesium alloy.Previous studies of the oxidation of solid aluminum at elevated temperatures showed differences in the amount of oxide formed and in the oxidation law followed. 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 ~. The time required for completion of the initial rapid rate decreased from 20 hr at 450~ to 1 hr at 600~Aylmore, Gregg, and Jepson found that oxidation curves were parabolic after the first 2 hr at 400~ but had three distinct branches at higher temperatures, each with a succeedingly lower rate. They explained their results in terms of an amorphous oxide which forms initially and crystallizes to eta alumina.Using chemically polished samples of high-purity aluminum, Hunter and Fowle (4) learned that, upon oxidation, eta alumina formed to a thickness of 160-210A (2.7-3.4 ~g/cm -~ for an oxide density of 3.4 g/cm ~) at temperatures above 475~ Below 450~ the oxide was amorphous and did not grow beyond 50A. They noticed no difference between oxidation in air and in oxygen, but oxidation in moist air from 125 ~ to 275~ was slower than in dry air, although the same ultimate thickness was reached. Their oxide thickness determination was to measure the voltage required to cause normal current leakage through oxide films in a nondissolving electrolyte, and to convert this to oxide thickness by the relationship that 1 v = 14A for barrier oxides on aluminum.A possible explanation for the differences in the references just cited was contained in the work of Lewis and Plumb...

Oxidation of aluminum foil under simulated high temperature annealing using thermogravimetry

“…However, it should be emphasized that with the exception of the work by Cochran and Sleppy [5] the bulk of the literature data are concerned with pure aluminum of known roughness. No data on surface roughness was available in this work.…”

Effect of simulated combustion atmospheres on oxidation and microstructure evolution of aluminum alloy 5052

“…Finally, the microstructrures of aluminum alloy after high temperature exposure are illustrated in Figure . As can be seen, a fine and elongated precipitates are distributed in the matrix along rolling direction in the as‐received alloy . On one hand, the rolling texture was retained as the temperature was maintained as low as 300°C.…”

A phenomenological description of the rate of the aluminum/oxygen reaction in the reaction-bonding of alumina

“…5,[11][12][13]22 As discussed above this corresponds to the growth of an amorphous oxide layer which is controlled by aluminum ion diffusion through the layer. 14,22 This is also valid for the initial weight gain region above 450 • C, but as will be discussed below the fit does not extend to higher conversions.…”

“…[11][12][13][14][15][16][17][18][19][20][21] Most of them have used thin, flat samples of aluminum, but some work has also been reported in powder samples. The rate of oxidation of aluminum has been observed to be extremely temperature sensitive, and various kinetic models with different mechanisms have been used to describe the oxidation at various temperatures.…”

“…This is a purely experimental observation which has been clearly seen when taking most weight gain data in the literature. [11][12][13][14][15]17,18 and replotting them in terms of (kt = −ln(1 − α)) Note that in Fig. 4a and b, the small variations in the degree of conversion at zero time arise due to corresponding differences in heat-up period prior to reaching the isothermal test temperature.…”