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REFERENCES (1) BANCROFT: Science 98, 98 (1943). (2) BONNIER: Bull. BOC. botan. France 27, 103 (1880). (3) COCKERELL: Kature 43, 207 (1881). (4) COMBES:Compt. rend. 167, 1002 (1914); 168, 272 (1914). (5) COSTERCS:Nature 26, 482 (1882). (6) DCCHARTRC: Bull. soc. botan. Paris 7, 152 (1860). (7) EVERST: Proc. Roy. Soc. (London) 90B, 251 (1918). ( 6 ) FISCHER: Flora 98,380 (1908). (9) LAURENT: Compt. rend. SOC. roy. botan. Belg. 29, 11, 71 (1890). (10) MAYER AND COOK:The Chemistry of Natural Coloring Matters, p. 20. Reinhold Publishing Corporation, New York (1943). (11) M ~ B I U SBiol. : Zentr. 1892, 48. AND STREETER: J. Biol. Chem. 92, 713 (1931). (12) PEARCE (MRs.) : The Anthocyanin Pigments o j Plants, 2nd edition, p . 121. University (13) ONSLOW Press, Cambridge (1925!, (14) ROSESHEIM:Biochem. J. 14, 73 (1920). (15) SHIBATA: Botan. Mag. Tokyo 29, 118 (1915). ASD KISHIDA: Botan. Mag. Tokyo 29, 301 (1915). (16) SHIBATA AXD SAGAI:Botan. &lag. Tokyo 30, 149 (1916). (17) SHIBATA (18) SHIBhT.4, S A G A I , A S D I ~ I S H I D J. A :Biol. Chem. 28, 93 (1916). (19) WHELDALE: J. Genetics 1, 43 (1911). The Old Dirt Dobber’s Garden Book, p. 226. I?. M. McBride and Company, (20) WILLIAMS: S e w York (1947).
T H I S OXIDE FILMS ON ALUMINUM EBRL A GULBRBNSEN
AND
W. S. WYSONG
Westinghouse Research Laboratories, East Pittsburgh, Pennsylvania Received April 11, 1947
illthough aluminum is one of the more reactive metals, the reaction with oxygen a t room temperature and even a t temperatures up to 600°C. is extremely slow (15). This has led to the view that the oxidation reaction of aluminum is different from that of other metals (1, 15). Since aluminum is found in some heater and other high-temperature alloys, it is of interest to study the reaction kinetics. This communication \vi11 present the results of a vacuum microbalance ( 5 , i ) study of the osidation behavior of aluminum in the temperature range 200550°C. These results mill be correlated with observations on the physical and chemical structure of the oxide film. If the oxide film theory is accepted as a basis of discussion, there are many favorable factors for the view that the aluminum oxide film is protective. They are the following: ( 1 ) the ratio of oxide volume to metal volume is greater than 1; (2) the oxide is very stable thermodynamically in regard to decomposition, reduction, and solid-phase reactions with other metals; ( 3 ) the relatively high
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melting point and boiling point of the oxide; (4) the fact that alumina is a reduction semiconductor; and ( 5 ) the relatively few oxide structures which may form. The unfavorable factors for the oxide on aluminum are the low melting point and the relatively high vapor pressure of the metal. LITERATURE SURVEY
A number of papers during the last twenty-five years have dealt with the oxidation of aluminum and the structure of the protective film. KO critical study has concerned itself with the reaction kinetics. Pilling and Bedworth (15) studied the oxidation a t 600°C. by a weight-gain method. One series of measurements showed that the oxidation could be broken down into two distinct stages. The first is the formative stage, during which a thin layer of oxide is formed slowly on the surface, the action lasting 60-80 hr. The second is the protective stage, during which the oxidation ceases. The parabolic rate law is found to hold during the first stage. A value of the rate constant of 0.30 X (g./cm.')* per hour is given. The stable oxide film is 2000 A. ih thickness. Krylova (11) has studied this reaction by means of the polarization method. Above 400°C. the oxide film on aluminum increased continuously. The details of this work are not available to us a t this date. Numerous workers have studied the thickness of the equilibrium room-temperature film on various types of aluminum specimens, using a variety of techniques. Vernon (21) studied the reaction of aluminum in industrial atmospheres at ambient temperatures, using a weight-gain method. The film reached a thickness of about 100 A . after 10-14 days, This thickness mas found to be independent of the purity of the sample and the nature of the atmosphere. Treadwell and Obrist (20) determined the thickness of the oxide film on aluminum by removing the metal with anhydrous ethereal hydrogen chloride. An estimate of 10-100 A. is given for the thickness of the air-formed film. Steinheil (IS) found somewhat thicker films on specimens of aluminum foil heated gently in a Bunsen flame. Podgurski (16) has recently measured the air-formed film on a freshly evaporated aluminum film by the use of a vacuum microbalance technique. Assuming a surface roughness ratio of 1, a thickness of 32 A. after 24 hr. of reaction is calculated. The structure of the oxide film on aluminum has been studied in some detail by Steinheil (18), Yamaguti (24), Preston and Bircumshaw (17), Darbyshire and Cooper (2), Hass (lo), and more recently by de Brouckkre (l),using the electron-diffraction technique. The following results are found: The film found on molten aluminum is usually crystalline and corresponds to y-Ald&. Rapid heating to 700°C. for short times gives a mixture of amorphous AL03 and y.41203. The a-A12O3 is found on heating the oxide to 14OO0C. The roomtemperature .films are amorphous. APPAFLAl'US
The microbalance with its auxiliary apparatus is essentially, the same as previously described (5, 7 ) . The method is to suspend a, 10- or 12-mil sheet of
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aluminum from the beam of a sensitive quartz microbalance operating in an allglass vacuum system. The weight change of the specimen is followed continuously as the several operations are performed upon it. These operations include evacuation to pressurcs of mm. of mercury or better, degassing, and oxidation of the metal specimen. Each of these operations can be carried out on aluminum over a temperature range of 254360°C. (m.p.) and over a pressure range of mm. to 0.1 atm. The balance has a sensitivity of 0.8B division (1 division equals 0.001 cm.) per microgram for a sample weight of 0.3659 g. The period of the balance is 8 sec. The balance is checked periodically for a stablc zero point and negligible temperature and pressure corfficients. A scale micrometer microscope is used to observr the beam and to record the deflection relativc to a fixed point on a quartz supporting frame. The readings are reproducible to 1/4 of a division or approximately 0.3 X loe6 g. METHOD
The weighed aluminum sample is placed on the balance beam and the balance checked for alignment. l h e specimen glass tube is sealed off'. The apparatus is evacuated to mm. of mercury, liquid air being placed in the traps during the evacuation proccss. All samples are given a preliminary degassing treatment by heating the quartz specimen tube to 575°C. over a period of 30 min. n-ith an auxiliary furnace. The regular furnace, controlled at the desired temperature, is raised around the specimen tube. After thermal equilibrium is attained, a preliminary set of readings of the balance beam and furnace temperatures is taken. Oxygen is now admitted from the dosing system to the desired pressure. Readings of the balance beam are taken a t fixed intervals during the oxidation. At the end of the oxidation the system is evacuated and a final set of readings is taken. SAMPLES
Four samples of aluminum' were used in the experiments. Samples Xo. 1 and 2 are stated to have an analysis of 99.985 per cent aluminum, as determined on the ingot before rolling. The t n o samples are taken from different sections of the same sheet roll. Samples S o . 3 and So. 4 are specially prepared. Elaborate precautions are taken to prevent the inclusion of oxide during the rolling processes. Both samples are prepared from blocks 2 in. x 2 in. x 1 in., machined from a highpurity aluminum ingot having the following analysis: silicon, 0.002 per cent; iron, 0.001 per cent; copper, 0.005 per cent; barium, 0.004 per cent; magnesium, 0.008 per cent; sodium, 0.000 per cent; carbon, 0.001 per cent; phosphorus,
