R e la tiv e Energy of th e F la sh (4000-3000 A.) as a F unction of Timej F la sh Energy, 4800 J o u le s. .. . 50 16. A b so rp tio n S p e c tra of Azomethane Before and A fte r th e F l a s h .

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...

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