JOURNAL OF CHEMICAL EDUCATION
FORMATION OF ANODIC FILMS ON ALUMINUMA LECTURE DEMONSTRATION R. C. SPOONER and H. P. GODARD Aluminium Laboratories Ltd., Kingston, Ontario, Canada
IT
IS well known that the good corrosion resistance of aluminum is due t o the presence of a natural film of oxide which begins t o form as soon as the metal is exposed to air. This 6lm is about one hundred thousandth of a millimeter thick (0.01 microns).
INDUSTRIAL PROCESSES
Through electrochemical action this oxide layer may he made much thicker, as much as a thousand times or more, to produce films from 0.005 to 0.025 mm. thick. This is usually done by making aluminum the anode and an inert metal the cathode in a solution of sulfuric or chromic acid under suitable experimental conditions, a process termed "anodizing"; and the resultant film is described as the "anodic" film. Anodic films on aluminum impart a number of advantages to the metal due to the harrier which the thickened oxide layer interposes between the metal and its environment. The film possesses marked ability to resist the action of weather, abrasion, and many chemicals and is applied commercially to such objects as refrigerator ice cube trays, washing machine tubs, building spandrels, and knitting needles. The continued growth of the oxide film depends upon the porosity of the film formed so that the high electrical resistance of the aluminum oxide does not seriously decrease the current flo~v. The dyeing of the freshly formed
anodic film also depends upon the adsorptive and porous properties of the oxide layer. The anodic oxidation of aluminum comprises the following processes. cleaning, anodizing, dyeing, and sealing, with a thorough rinse in water hetween each process. The cleaning removes all grease and dirt so yielding a clean surface which is essential for uniform anodic oxidation. While dirert current is customarily used in this process, alternating current may he employed also, in which case the two electrodes may both be of aluminum. Under the latter conditions each electrode becomes alternately anode and cathode and so the formation of the oxide layer proceeds more slowly. On anodizing with the acids mentioned above there is little, if any, "valve" or rectifying action such as aluminum exhibits when immersed in solutions of weak electrolytes such as potassium alum. By varying the electrolyte, its concentration, the temperature, current density and duration of electrolysis, oxide films with a wide range of physical properties can be produced. The film after oxidation is quite porous and if immersed in certain acid dyes, becomes strongly colored. By immersion in boiling water, or "sealing" as it is called, the aluminum oxide loses this adsorptive property due t o the closing of the pores by the formation of the aluminum oxide monohydrate. In this way the dye is sealed into the pores. This chemically inert oxide layer with its in-
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creased electrical, corrosion, and abrasion resistance is a very valuable and attractive finish vhich has found an increasing' number and variety of commercial applicat,ions. LECTURE DEMONSTRATIONS
The formation of this'anodic film follolved by tpsts of its porosity and electrical resistance may easily be demonstrated within a lecture period and shorild effectively arouse stmudentinterest in these unique anodic oxide coatines. Apparatus required: variac transformer; a x . ammeter (5 amperes); Pyrex beakers, 600 ml.; electrode clips with side hooks for hanging on wall of the beaker; glass spiral water cooling coil. Chemicals required: NaOA solution, 50 g./l, 65%; HzSOa solution, 150 ml. conc. acid/l., 25°C.; acid dye solution,* 3 g./l., 65%; 2 pieces of commercial alnminum sheet,'3 in. X 1.5 in. The electrode clips are attached to -the aluminum sheet and 350 ml. of the solutions required are poured into three beaken and heated to thd recommended temperature. Three other beakers are required, filled with cold water for rinsing purposes. The clips should not dip into the solution but be just above the surface, so ensuring that a small section of the metal is not anodized. The aluminum is immersed in the sodium hydroxide solution for 30 seconds. If the metal is quite dirty it may he cleaned with carbon tetrachloride prior to this treatment. After rinsing, the metal is hung from the opposite sides of the beaker containing sulfuric acid solution and is connected by clips and wires from the ammeter to the output of the variac which has been set at approximately 15 volts. g-he variac is then connected to 110-volts a. c.; by rhanging the voltage the current is maintained a t 1.5 amperes for the duration of the electrolysis. Due to the heating effect of the current it is, necessary to cool the acid by the insertion of an internal glass ~mter-coolingcoil so as to maintain a temperature of 2s0 for the duration of the electrolysis. Aft& electrolysis for 20 minutes one piece of alumi-
num is rinsed and immersed in the dye solution for a 15minute period followed by a rinse and sealing in boiling water for 10 minutes. The second piece is sealed first for 10 minutes, rinsed and then dyed for 10 minutes. The difference in color of the two pieces illustrates the decrease in porosity caused by the spaling process.
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VARIAC
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Xany acid dyes are recommended for anodic dyeing-as Anodal Light Red No. 2 or Aluminum Blue A (both Sandoa Co. dyes): ~ ~sapphire i 5. E. ~(xittionrtl ~ ~ " i l i ,~, ~ chemical i Corp.).
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Anodic (hidation of ~
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An interesting variation is to use a quick drying lacquer *nd paint a design on the dry anodized but unsealed metal. After the lacquer has dried, the metal is dyed abd sealed in the customary manner. The design is readily visible against the colored background, The increased electrical resistance of the oxide fXm as compared with the metal can be shown by connecting a 1.5-volt bulb to one terminal of a 1.5-volt.dry cell. Prongs are attached to the other terminal and the unconnected side of the bulb. One prong is clipped to the hare metal surface and the other allowed to touch the unoxidized metal. If the latter prong is moved to the oxide roating the light goes out because of electrical re~ ~ sistance of the oxide film.
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