INDUSTRIAL AND ENGINEERING CHEMISTRY
1602
under similar conditions from the concentrated distillery residues. Some fermentation tests were also set up wit'h unhydrolyzed and hydrolyzed blackstrap, but because of the small increases in the invert sugar upon hydrolysis the additional alcohol yields were erratic. Experiments of this nature would have to be made on a much larger scale t o furnish reliable results.
Vol. 39, No. 12
5 . Khen blackstrap. molasses is treated with acid under the same conditions as distillery residue, the reducing power also increases to a maximum and then decreases again, but the increases are small because t,he acid exerts a destructive effect on the sugars in the molasses. For this reason a marked increase in alcohol yield upon hydroll-sis cannot be expected. Whether alcohol can be produced economically from hydrolyzed distillery residues would have to be decided by large scale experiments.
CONCLUSIONS
1. When the reducing substances in the unfermented residue from blackstrap molassm are heated xyith acid, the reducing power first increases t o a maximum because of tmheformation of fermentable sugars by the hydrolysis of the sugar anhydrides and sugar-amino acid condensation products, and then decreases again t'hrough the destruction of the fermentable sugars. 2. Within the experimental limits of pH 0.2 to 2 and temperatures hctrveen 40" and 100" C. the maximum reducing power increases with the acidity a t constant t,emperature, and with the temperature a t constant acidity. The rate of destruction follows the same rule. 3. Increase in the concentration decreases the rate of hycirolysis and increases the rate of destruction. 4. Upon fermentation with yeast the sugars formed by hydrolysis yield alcohol, but the alcohol recovery is much smaller than the theoretical because fermentation inhibitors are formed from the sugars by theeffect of acid at high temperature.
LITERATURE CITED
(1) D a v i s , Intern. SugarJ., 40, 233 (1938). (2) J a c k s o n , Silsbee, and Proffitt, Bur. Standards Sei. Paper S o . 519 (1926). (3) Kerr, "Chemistry and Industry of Starch," pp. 264-88, 1944. (4) Mathews and Jackson, Bur. Standards J . Research, 11, 619 ( 1 9 3 3 ) ; R. P. 611. (5) Reindel and Frey, Z . Spiritusind., 57, 237 (1934). (6) S a d o v y i , T r u d y Voronezh Khim.-Tekhnoi. I n s t . , 3-4, S9 (19301. (7) Saeman. IND. ESG. CHEM.,37, 43 (1943). (8) Sattler and Zerban, I t i d . , 37, 1133 (1945). (9) Wohl, Ber., 23, 2054 (1890). (10) Zerban, J . Assoc. O f i c i a l A g r . Chem., 24, 656 (1941). i l l ) Z e r b a n , Sugar Research Foundation (X. Y.1, Technol. Xept Ser., 2 (1947). RBCEITED February 11, 1947. Report of work carried o u t under grant from the Sugar Research Foundation, Inc.. S e w Tork, S . 1 ' .
Anodic Reactions of Aluminum and Its Alloys in Sulfuric and Oxalic Akd Electrolytes '
RALPH B. RMSON AND CHARLES J. SLUNDERl .Jluminum Research Laboratories, S e w Kensington, Pu. T h i s article describes factors, such as temperature, concentration and agitation of electrolj te, time of treatment, current density, and type of alloy treated, which affect the formation of aluminum oxide produced by the anodic treatment of aluminum in sulfuric and oxalic acid electrol) tes. Electrolq tes used at elelated temperatures or in a more concentrated form tend to produce oxide coatings of lighter weight. The electrollte must be sufficiently agitated to remoie the heat deieloped a t the aluminum anode; otherwise, $ h e oxide coating is dissolied. Long time treatment in the electrolJte brings about a decrease in the efficienq of oxide coating formation. Lilicitise, a decrease in the current demit) results in the formation of a lighter weight oxide coating. The purit? of the aluminum anode has a marked effect upon the efficiency of oxide coating formation, the highest efficiency beiug obtained itith pure aluminum.
T
HE anodic oxidation of aluminum is now a well established coat'ing process, and has found extensive use for the decora-
tion and protection of aluminum surfaces ( 4 ) . The value of anodic coatings for specific applications is usually determined by measurement of one or more of the specific properties required. Properties such as hardness, abrasion resistance, thickness, adsorptivc capacity, or resistance to corrosion may tie varied widely by changing the coating conditions. The development of suitable t e i t niethods and determinntion of the effect of the operating variablcs on the properties of the coatings have been the subject of considerable research. Some 1
Present address, Battelle Memorial Institute, Columbus, Ohio.
of the reports (2, 3,4,7 ) on this work have indicated the accuracy of these test methods arid their field of application. iilthougli these methods have been found satisfactory, the information they supply on the mechanism of coating formation and the variables involved in the anodic oxidation procedure is limited. I n connection rr-ith the development of procedures for measuring coating thickness by stripping methods, R. B.Mason (.i) discovered that a solution of phosphoric and chromic acids quickly dissolves the oxide coaring and has practically no effect 011 tl!ci aluminum. The w i g h t of the coating can thus be readily alid accurately obtained. The value of such a method to detcmiiiic the efficiency oi anodic oxidation and to show the effect of citli~jr minor or major changes in the coating procedure \vas inimediately apparent. Edn-ards ant1 Iceller (6) u.icd the method t o detcrminc the apparent current efficiency of the anodic p r o r e ~ s : they showed that, while the aluminum was oxidized m i l tiissolved a t approximately 1007, cfficiency, only about 70'7' oi the aluminum lost \vas accounted for by the weight of the coating, assuming that it consisted entirely of aluminum oside. The authors pointed out, how r, that this assumption is only :In approsirnation because the coating, when it is formed in suli'uric acid, is known to contain some sulfate and water. GENERAL PROCEDURE
Preliminary experiments indicated that even very i~iiall changes in the operating conditions wcre readily dctectcd b v changes in the weight of coating or in the weight of m e t d removed. A convenient unit for expressing the results w:is obtained by dividing the weight of coating formed undcr tlii, couditions of test by the weight of mctal removed during ihc for-
INDUSTRIAL AND ENGINEERING CHEMISTRY
December 1947
niation of the coating. This “coating ratioJJzis a measure of the over-all anode efficiency with respect to coating formation. No assumptions regarding the possible composition of the coating are required in the calculation of this “coating ratio.” If, with pure aluminum, all the metal which reacts electrochemically were converted to alumina in the coating, the coating ratio would be 1.889. A coating ratio lower than this theoretical figure indicates that alumina has been dissolved bv the electrolyte either chemically or electrochemically. The general procedure followed in these experiments was to weigh the ingredients of the electrolyte into the glass cell M-hich Kas either cooled or heated, as required, by running water through a lead coil. The coil also served as the cathode. The acid content of the concentrated sulfuric acid which was used to make up the electrolyte was determined by chemical analysis. The sheet specimens to be anodized were 2 X 3 inches (5.1 X 7.6 cm.), and, in most cases, were thin gage so that the surface area of the edges was neglected in calculating the current density. The temperature of the electrolyte was maintained as close to the desired figure as possible and in no case varied more than =t1 F. (0.5’ C.). Stirring is important in order to remove heat and maintain a uniform temperature. Adequate stirring was supplied by a mechanical agitator. The time of anodic oxidation was measured by a stop clock, and the current was obtained by a potentiometer arrangement from a direct current line (motor generator). The recorded quantity of electricity passed through the cell was therefore subject only to errors in the ammeter readings and to the fluctuations in the line current. I n carrying out an experiment, the specimen was cleaned by an etching treatment or anodically coated and stripped in the pliosphoric-chromic acid mixture, weighed accurately on an analytical balance, anodically treated, rinsed dried, and reweighed. It was then placed in the stripping sohiion which contained 35 ml. of 85% phosphoric acid (H,PO,) and 20 grams of chromic acid ( G O l ) per liter a t a temperature of about 200” F, (93.3” C.) for 5 minutes or until the oxide was all dissolved. The specimen was then rinsed, dried, and weighed again. By subtraction the veight of coating and weight of metal lost were obtained and the “coating ratio” determined as mentioned above. A number of different variables were studied by this procedure. Surprisingly consistent results were obtained when all the conditions were carefully maintained. Indeed, the weight of metal dissolved from 99.9% aluminum sheet employed as anode The practical film efficiency of T a r r , Darrin, and Tubba ( 8 ) , multiplied by 1.899, equals the “coating ratio.” f
1.6 I
I
1
I
I
I
1
I
1.4
1.0
0.8
1.6
I
I
I
I
I
I
410
810
IJO
160
2b0
0.8
s 0.6
0.6
0.4
0.4
0.2 25%
60
The composition of the metal n a s expected to have an important effect on the coating ratio. .A few experiments were made for comparative purposes, but this point is not considered in detail here. I n these experiments the aluminum anodes \\ere suspended in the sulfuric acid by means of tantalum clips which show no leakage loss in the sulfuric acid electrolyte a t the voltages employed. For the nieasurement of current, a copper coulometer was bonnected in series with the aluminum-lead cell. Table I shows the variation in coating ratio caused by alloy composition and indicates the importance of this variable. Thc specinwns were oxidized in 157, sulfuric acid a t 12 amperes per squaic foot (1.3 amperes per sq. dm.) for 30 minutes a t 70” F. (21.1” C.). The metal, which approaches 100yo purity, is consumed anodically a t close to 1007, efficiency. Aluminum of commercial purity (2s)has been checked a number of times and found to dissolve a t an anode efficiency of about 97.5%. However, the effect of metal composition is brought out strikingly by the 17s-T and 24s-T specimens. I t was known previously that coatings on alloys of this type were not so thick or abrasion resistant as those on pure metal. These figures show that the coating ratio is markedly decreased and also that only about 800; of the current is effective in reacting with the aluminum. The pretreatment of the surface F a s also cansidered as a possible source of variation in the coating weight. Specimens of 2s-H sheet were cleaned by various treatments and oxidized in 15% sulfuric acid for 30 minutes a t 70 O F . and 12 amperes per square foot. The different surface cleaning treatments did not affect the efficiency of the anodic treatment. The small differ-
I.o
+ a
0
ALLOY COMPOSITION AND SURFACE PRErREATMENT
1.2
E
g-
in sulfuric acid was so Consistent that it appeared to be a more accurate measure of the quantity of electricity than was obtained with the ammeter and stop clock. Since a high purity aluminum anode acts as its own coulometer in the sulfuric acid electrolyte, it is desirable to use the weight of aluminum dissolved as a measure of the current actually employed in the electrochemical reaction. This method eliminates errors from leakage losses in the coated aluminum clips used for suspending the sample in the electrolyte as well as errors because of initial current surges.
I .4
I .2
2
1603
70
80
90
100
I 110
15”A
7%
0.2
H2504
120
130
TEMPERATURE OF ELECTROLYTE -OF.
Figure 1. Variation in Coating Ratio with Temperature of Electrolyte for 99.9% Aluminum Anodirally Treated for 30 Alinutes a t 12 Amperes per Square Foot
u h,§Q
- GRAMS
240
280
320
PER LITER
Figure 2. Variation i n Coating Ratio with Concentration of Electrolyte for 99.9% Aluminum Anodically Treated for 30 Rlinutes a t 12 Amperes per Square Foot
INDUSTRIAL AND ENGINEERING CHEMISTRY
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TABLL. I. CBFECTOF C o \ r r ~ o s i ~ ~OFo sh 1 ~ i . 1o~x .ISODC EFFICICXCY AND COATI>
