Rita G. Blatt The Pennsylvania State University The Capitol Campus Middletown, Pennsylvania 17057
Anodizing Aluminum An ektrolyti~oxidation experiment for general chemistry
In many introductory level college textbooks, electroplating is frequently dpscrihed in chapwrs dealing with oxidation rrduction reactions. corrosion. and electro~hemistrv.~.'This experiment deals with anodizing aluminum, a somewhat related. vet auite different. electrolvtic Anodizinn es- Drocess. . sentiaily inwlves an electrolytic process where the surface of the aluminum anode is converted to an oxide coating. The Anodizing Process In many respects, the general mechanisms of anodizing are the opposite of electroplating. Anodic coatings, which are porous in nature, start on the outside of the surface of the metal and progress inward, whereas in electroplating the coating begins a t the surface and continues to huild outward. Anodic coatings are oxides of the metal being treated, whereas electroplated coatings are metallic coatings of a totally different metal from the base material. In anodizing, the sample to he anodized is the anode of the electrolytic cell, whereas, in electroplating, the metal to he plated is the cathode of the
stores., etc.1. in a 100-mlbeaker and add 40 ml of water. Heat tn hoiline ~. ..~~~~~~
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and maintain the temperature of the dye bath near the boilingpoint. When the anodizing process is completed, remove the aluminum sample from the sulfuric acid solution exercising care so as not to touch the anodized surface. Immediately rinse the sample in water and place it in the hot dye bath. After 10 mi", remove the sample from the dye bath, rinse it well, and then dry it. The anodized areashould acquire the bright color of the dye while the unoxidized area should remain its original silverly, metallic color. Calculations The thickness of the film is a function of the amount of current employed. Knowing both time and current, and using Faraday's Law, one can calculate the theoretical weight of aluminum that is oxidized. From this value, one can calculate the weight of A1203 formed. In addition, using a density value for A1203, one can calculate a theoretical film thickness.
Discussion This experiment was originally designed to bring relevance to the chemistry laboratory experiences of students enrolled ^^I, GC,,. in the ~ n ~ i n e e r ~i enc~h n o i o ~ ~ l ' r o g r a m a t sthe Pennsylvania Oxide coatings of aluminum can be produced in an elecState University's Capitol Campus. Several hundred students trolyte, such as a sulfuric acid solution a t room temperature. have performed this experiment and it has provided them with The thickness of the commercial aluminum oxide (A12 03) films tends to range from 0.0001-0.001 in. (0.002%0.025 ~ n m ) . ~ the opportunity to deal with an electrochemical oxidation reduction system using materials which are familiar to Anodizing results in a thin porous film formed on the surface them. of a thin, non-porous, passive film which is referred to as the The students have responded favorably to the experiment. harrier layer.' Anodized surfaces are relatively maintenance The brightly colored samples clearly indicate that they have free with excellent resistance to weathering. In addition, ansuccessfully completed the experiment. The intense difference odic coatings provide a surface which can he colored with orbetween the bright color of the anodized area and the original ganic dyes and mineral pigments." Anodized aluminum is used color of the portion which is not oxidized provides a startling extensively in industry because of these desirable propercontrast which clearly indicates the changes which occur when tin. aluminum is anodized. This experiment provides a valuable, relevant laboratory learning experience for students enrolled Procedure in general chemistry courses. Approximately 250 ml of a 28% sulfuricacid solution is placed in a 400-ml beaker. Several layers of folded household aluminum foil are Acknowledgment placed in the solution so that the foil provides a somewhat loose lining Theauthor wishcs r o itcknstst:~ntPn>tPssor01' Chemistry st i h ~Ptmniylrania State 0.15 cin,which should first be buffed with steel wool and then brushed Ilniversity's Capitol Cimpr~s. with acetone in order to ensure a clean surface.The aluminum is then suspended in the solution in the center of the beaker. A dc power supply capable of supplying 10Vshauld be used to provide the elecBn~wn,T. L., and LeMay, Jr., H. E., "Chemistry, the Central trical enerev onemust Science," Prentice Hall, Inc., Englewood Cliffs, N. J., 1977, p. 561. .., to ooerate . the cell. In assembline..theaooaratus .. ~,PCWPIIII nut rn immprsetht lead aims in thesulturicacid solution. Masterton. W. L., and Slowinski, E. J., "Chemical Principles," Only the part d t h r s:,mplt which ir immarsrd in rhc~olutionwdl hr 4th Ed., W. B. Saunders Co., Philadelphia, 1977, p p 531-539. . ........- ... Uhlig, H. H., "Corrosion and Corrosion Control," John Wiley and As n rriult of trial and error, it has hren found rhnt a putential of Sons, Ine., New Yark, 1967, p. 210. 10 V ul'dccurrrnr vprritting ior 15-20mm pn,videsi,ptimumc
