Anodizing Aluminum with Frills Anne E. Doeltz, Stanley Tharaud, and William F. S h e e h a n University of Santa Clara, Santa Clara, CA 95053 "Anodizing Aluminum" ( I ) describes a vivid and relevant laboratory experience for students of general chemistry. It explains anodizing A1 in sulfuric acid and contrasts i t to electrowlatine. This anodizine ~rocedureis the kev feature of
alized crystal struct;re calculations involving close-packing. Besides finding interrelations among several such subjects, a student can also recognize how oxidation and reduction can he caused and controlled a t electrodes and thus used as a method of synthesis. Additions to Procedure The procedure of reference I is slightly modified by having three similar strips of Al as one anode. These are held as a unit immerskd part of each, the three pieces are fan;ed out so that the parts in the sulfuric acid do not overlap. At the end of electrolysis, these three pieces are rinsed with water and their total anodized areas are measured to allow eventual estimation of the average thickness of the A1203 coating. One of these is dyed as in the standard procedure ( I ) .The other two go to test tubes containing 1F NaOH and 3 F HC1 to compare relative chemical reactivities of almost clean and anodized Al. Oxide Coating Even so-called clean Al has a stable layer of oxide ahout 20 A thick after only a few minutes a t room temperature ( 3 ) . Oxidation a t 350-450°C in air yields surface layers of AlzOs about 400 A thick (4). Anodizing, as in this experiment, pro(11, the outer part of duces much thicker coatings (--lo") which typically is porous if produced a t highEMF ( 5 )or pH less than 7 ( 6 )as in this procedure. The amount of industrial work on anodizing Al is so great that the most thorough general reference is the General Subject Index of Chemical Abstracts under the heading: Anodization, of aluminum. Even reading the index entries is a mini-education. Anodizing is a complex process used widely in industry but only partly understood in its detail. Various chemicals in the
156
Journal of Chemical Education
electrolyte may he used to dye the coating as it is formed. There is ample evidence also of the existence of AIt as an evanescent and highly reactive species in the electrolyte during anodization. Indeed, in the extreme (71, two Al+ may be generated for every AIA+,but much of this Al+ can return to the anode to he oxidized there to A13+. The Alt liberates Hz in the solution near the anode (8) and may he the cause of the pink coloration generally observed in this procedure. In general, the loss in anode mass is seldom less than 1.2 times what is expected of Faraday's law (8) because of Al+ and hecause of direct loss of matter from the anode through solution. In other words, neither weighing the anode nor measuring the charge can give a true measure of A120s formed on the anode. Perhaps this limitation is compensated by offering the students a chance to see a pink coloration that may he a complex form of Alf in the electrolyte. This procedure in 3 F H2S04 generates a goodly amount of aluminum oxide that adheres tightly to the underlying Al. The innermost laver of A1701 is dense and without cracks. Bv a~mtr:1>1, ruht.3 Iiydnttd g.aidt i t t i r , m . c ~ m a i s rI~;tp3rnted ~ rl\.~nrirm irtjn ,,uid~~ r v - r d -nhith rtr, I V L ~I I I I 111 the 11nc1~ surface and db not cover and protect it from further attack (9). Some metals which do not form a continuous protective oxide coating are Fe, Na, Mg, and Ca. Some metals whose oxide coatings are typically continuous and adherent are Al, Be, Ti, V, Cr, Ni, and Ta. Such a paint-like oxide coating protects the underlying metal from corrosion and weathering. Increasing the thickness of such oxide layers hy gaseous oxidation or by anodization requires that ions and electrons (or their vacancies) move through the oxide coating because of intense electric fields developed across that oxide layer as ions are generated a t the surface ( 3 ) .These ideas should be mentioned to students who use this multifaceted experiment. Idealized Electrochemistry For the sake of student understanding and straightforward calculations, the half-reactions in this experiment are assumed to he simply these:
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At the cathode. 6 e t 6H+ 3H2 A t the anode: 2A1+ 3HzO * AlzO.,+ 6Htt 6e
Since the layer-to-layer distance is 2.21 A, this suggests about 6.1 X lo4 layers of oxide ions in the AlzOz. At the start, in 20 A on "clean" Al, there were about 9 layers.
Electrical Circuit of One Cell
T h e A1203 is assumed to stick to the Al'without loss as a nonporous, even layer of &03.The mass of AlzOs is estimated from the numher of coulombs of charge and the idealized anode half-reaction. To emphasize the idea of equivalents and equivalent weight, the report sheet has an entry for the numher of equivalents. If the layer produced by anodizing is assumed to he pure dry AlzOs, then its density is 4.00 g ~ m - ~ . From this and the mass of A1203 comes the volume of A1203. This volume, with the measured area anodized, then yields the average thickness of the oxide layers on the three pieces of Al. Structural Interpretation The real structure of AllO1 (assumed here to be corundum) is rhombohedral, wherein the oxide ions are almost hexagonallv closelv Merelv as a calculational device. cubic -nacked. . close packing of oxide ions-is used to determine thelayerto-laver distance of close-oacked oxide ions in A1703. Since the same distance between laye's and ultimately t h e same numher of such layers in the calculated average thickness of the anodized coating on the Al. A face-centered cuhic structure (like NaCl and CaO) has four close-packed anions ( C l or 02-) per unit cell. Such a unit cell has 413 as many oxide ions as the formulaAlzOs; hence, each such "unit cell" of A1205contains 2 X (413) A1"ions (on the average). That is, in this arbitrarily adopted "unit cell" there would be effectivelv (413) A1101. or four oxide ions and (813) AI~' io& From this assumption and the density of A1203 (4.00 g cm-9, it is straightforward to find the volume of the "unit cell" and then its edge length a. The familiar calculation with this NaC1- or CaO-tvue cuhic "unit cell" vields a 1 4 2 = 2.71 A as the distance between centers of close-packed oxide ions and a/& = 2.21 A as the distance hetween close-packed layers of oxide ions (in Alz03).The observed average thickness of the anodized region is then compared to alJ3 as if the anodized region has such layers parallel to the metallic surface. Typical results are 0.145 amp for 1312 sec to give an estimated 0.0335 g A1203 with avolume of 0.00838 cm3 and (if the anode area is 6.26 em2) an average thickness of about 1.34 X lo5 A. ~
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Current Control T h e current is held almost constant for each student's ue. riod of electrolysis (despite changes of load on the one common storage battery as other students begin and end their electrolyies a t various times) by the circuit of the figure. Six such cells are run in parallel from one 12-V automobile storage battery S. The transistor (NPN) is a 100-W audio hi-fi type with emitter E, collector C connected to aluminum foil acting as cathode in the electrolytic cel1,and base B connected to variable resistance (500Q) R2. Resistance R,, fixed a t 2200Q, limits the maximum current through B. Ammeter A reads up to 500 mamp. Between about 6 V and 10 V, the electrolysis current varies only slightly, namely from about 0.22 amp to 0.23 amp. The maximum variation in the battery voltage is of the order of 0.5 V; this causes variations of ahout 0.001 ampere in cell current for those students still on the line. This variation is less than the error in reading the meters. Chemical Results
gins a t the unanodized end where tiny bubbles of Hz form. Within a few minutes, bubbling is more rapid and the anodized area also begins to bubble. Much later, the whole strip is generating HZa t a goodly pace. The reaction in 1F NaOH is much faster. Slight bubbling of H z begins almost a t once all along the A1 strip; within a few minutes, bubbles form vigorously over the entire strip. Still later, the action may be so vigorous that the 1 F NaOH must he diluted. Students are asked to write equations that show: (1) evolution of HZby A1 in acidic and basic solution and (2) the amphoteric behavior of '41203. More The more capable students should he invited to read (7) about various oxidizing agents that can he added to modify the pink color that is probably associated with Al+. It might he possihle to show that the change in mass of the anode (as Alz03 is formed, as A1203 dissolves, and as Al+ forms with A13+) does not match the exact predictions of Faraday's laws. In these regards, the procedure is open ended. It is recommended that the anodizing he done in a hood and that only oxidizing agents already used (7) be used again here after due and mature consideration by the instructor of how they may react. Literature Cited (11 B1aLL.R. 6..d. CHEM. EDUC..56,268 (1979). 12) Dodtr, A. E.,and Sheehan. W. P.,"Experrment A. AnodizingAlurninum.(( A. E. DoelU and W. F.Shcohan. @ 1980Allri~ht9reserved. 13) Grimley.T.R.,"0xidationoiMetsls.((m "ChemiLiyolLheSolidState,i. W E Garner lgdiior). London. Butterworths Scientfic I'ublicatiuns. 1955. D. Sfil. 14) "Chemirtry of the Snhd Sfate:p. 360. ( 5 ) Tumashov. N. D.. and Chrrnous. G.P.. "Passivity and Protection oiMelalr Against Co~rosinn,"New Ynrk.Plenum Press, 1967. p. 43. 16) ~ ~ ~ ~ ~ ~ kR i ~ . U~ C~. . I. .,. S~U ~,T~, ~d ~ , 1980, , I . In (i1,11-24 [chfm. ~ b ~ t i . 9 2 ,
Volume 60
Number 2
February 1983
157
