INDUSTRIAL ALVDENGINEERISG CHEMISTRY
December. 1930
1321
Deposition of Black Copper Oxide on Brass’ Henry L. Kellner and Harry A. Curtis DEPARTMENT OF CHEMICAL ENGINEERISG, YALEUNIVERSITY, KEW HAVEN,COXN.
The “black-bronzing” of brass is accomplished coman apparatus was built to dePROCESS in common mercially by dipping the cleaned brass in a bath comtermine the composition of use for the deposition posed of basic copper carbonate and aqua ammonia. these complexes by the wellof a black, adhering A study of the bath reveals that the basic copper cark n o TP n v a p o r -pres s u r e coating on brass consists in bonate does not dissolve as such, but is decomposed, method. It was found that, dipping the cleaned brass into the carbonate and some of the hydroxide going into when ammonia gas at a fracan ammoniacal solution of solution, leaving a residue of partially dehydrated tion of an atmosphere presbasic copper carbonate. A hydroxide. The bath corrodes the surface of the bath, sure was brought into contact typical bath for this process, and the black deposit, which consists of partially hywith basic copper carbonate, known c o m m e r c i a l l y as drated cupric oxide, is laid down over the roughened there was indeed a slow de“black bronzing,” is made up surface. The seemingly black and continuous deposit crease in the vapor pressure of by mixing 40 gallons of 26” is curiously translucent when viewed under the microammonia. When the ammoBB. ammonia with 100 gallons scope. nia in the system is less than of water and adding thereto enough to convert all the basic 170 pounds of commercial basic copper carbonate. The basic copper carhonate does not carbonate present to a complex having at least 1 mol of ammoentirely dissolve in the ammoniacal solution. In fact, the suc- nia per gram atom of copper, the vapor pressure registered a t cessful operation of the bath is said to depend on there being an equilibrium should be the vapor pressure of the complex having undissolved residue. This residue is a t first light green, the smallest mol ratio of ammonia to copper. Actually it the same color as the basic carbonate, but presently begins was found that under these conditions the ammonia gas to darken and eventually becomes a black mud. I n practical pressure did sink to a constant value. There is considerable operation of the black-bronzing process there seems to be doubt, however, as to the significance of this minimum no chemical control of the bath. Copper carbonate or pressure, for it mas found when using high initial amounts of ammonia is added as the operator may think necessary, and ammonia that the basic copper carbonate slowly blackened, occasionally the whole bath is thrown away when it fails to due presumably to decomposition. This behavior is in line with the decomposition which function properly. The present investigation was undertaken t o learn more occurs in the black-bronzing solution mentioned above. of the chemical reactions occurring in the bath, and of the It was also observed that basic copper carbonate immediately blackens when brought into contact with liquid nature of the black coating deposited on the brass. anhydrous ammonia.
A
Basic Copper Carbonate
The basic copper carbonate or synthetic malachite of commerce is made by interaction of sodium carbonate and copper sulfate in solution. It usually carries a little sodium sulfate and often a trace of iron as impurities. The formula but the commercial product is theoretically CuC03.CU(OH)~, is said to vary somewhat in composition. An analysis was made of a sample of commercial basic copper carbonate obtained from an industrial plant using the black-bronzing process and of a sample of c. P. grade basic copper carbonate, with results given in Table I. Table I-Analyses of Basic Copper Carbonate CONSTITU- THEORETICAL COMMERCIAL COMMERCIAL PRODUCTC P. EST CuCO3 Cu(OH)z PRODUCT WASHED AND DRIED GRADE Per cent Per cent Per cent Per cent 69.5 70.3 CUO 71.94 69.0 19.2 19.5 COa 19.95 17.1 9.7 9.9 H10 ~ - . 8.11 12.2 ~
___
100.00
--
__
__
98.3
98.4
99.7
It will be noted that the composition of the washed and dried sample of the commercial product corresponds to 1CuC03:1.0017C11(OH)~,with 1.3 per cent of uncombined water and 1.6 per cent of impurities, while in the c. P. grade the ratio of CuC08 to C U ( O K )is ~ 1:0.9937. It is apparent that in both samples this ratio is very close to unity. Action of Dry Ammonia Gas on Dry Basic Copper Carbonate
On the assumption that ammonia would combine with dry basic copper carbonate to form molecular complexes, 1 Received August 21, 1930. M o s t of the experimental data used in this paper are taken from a dissertation submitted to the Graduate School of Yale University by Henry L. Kellner in candidacy for the Ph D . degree.
How Basic Copper Carbonate Dissolves in Aqua Ammonia
Basic copper carbonate may be dissolved completely in aqua ammonia if a sufficiently large volume of liquid is used. In the black-bronzing bath, however, more of the basic carbonate is used than will dissolve. It is therefore important to know whether basic copper carbonate dissolves as a whole in aqua ammonia or splits up so as to give a copper-carbon dioxide ratio in solution different from that existing in basic copper carbonate. To throw light on this question two series of mixtures were made up. I n one various known weights of basic copper carbonate were used, each with 100 cc. of aqua ammonia of fixed concentration; in the other series various known weights of basic copper carbonate were also used, each with 200 cc. of aqua ammonia of the same concentration as used in the first series. The aqua ammonia used was in all cases 0.366 normal. The mol ratio of ammonia to basic copper carbonate in the several systems varied from 29.16 to 6.10. These mixtures were sealed in glass-stoppered bottles and tumbled for various lengths of time-from 20 to 80 hours-in a thermostat at 25” C. I n every case it soon became evident that the undissolved residue was changing in composition, the color passing from the original light green of basic copper carbonate t o a brownish black. I n general the change of color was the more rapid the less the mol ratio of ammonia t o basic copper carbonate in the system. Of all the systems only the one having the highest ammonia-to-copper mol ratio retained finally any of the original green color in the undissolved residue. At the end of the tumbling period the bottles were allowed t o stand
INDUSTRIAL A N D ENGINEEMNG CHEMISTRY
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If there are selected from the several systems in both series those wherein equilibrium was most nearly approached, as judged by the per cent of the total carbon dioxide in ' solution, the data of Table I1 are obtained. As an explanation of the fact that there was more copper than corresponded to the carbonate in solution, i t may be postulated that the free ammonia in solution exerted a solvent action on the copper hydroxide. S o t all of the ammonia in the system may be considered as free, Itowever, for it is well known that ammonia-copper complexes exist in solution, and the solutions now under discussion all had the deep blue color characteristic of copper-ammonia complexes. If i t is assumed that when copper carbonate dissolves in aqua ammonia each copper atom links with 4 ammonia molecules to form a complex, then the amount of ammonia remaining free in each of the systems may be calculated, and the amount of copper hydroxide dissolved front the residue by this free ammonia may likewise bo calculated from the known solubility of copper hydroxide in aqua ammonia. Of course, the equilibrium must actually be more complex than indicated by the assumption now made, but i t is of interest to note how nearly the explanation made above fits the facts regarding the copper.
in the thermostat for a t least 3 days, after which a sample of each of the clear supernatant liquids was withdrawn and analyzed for copper, ammonia, and carbon dioxide. I n the first series, including four systems, the mol ratios of ammonia to basic copper carbonate usere, respectively, 29.16, 17.31, 9.62, and 7.77. In these four, the carbon dioxide in solution was found to be, respectively, 81.7, 89.1, 91.1, and 98.1 per ccnt of the total carbon dioxide in the system, the lowest percentage being in the solution in contact with the greenish residue. I n the second series, where a longer time was allowed for the systems to reach equilibrium, the carbon dioxide in solution was in every case more than 90 per cent of the total carbon dioxide in the system. Table 11-Solufion
of Basic C o m e r Carbonate i n Aaua Ammonia
Aqua ammonia. CC. CuCGi. Cu(Ofi).. grams
ii 'being cokbined with the undissolved residue. The ammonia actually found by analysis varied from 94 to 99 per cent of the total, small losses being difficult to avoid. A special apparatus was constructed whereby ammonia loss could be completely avoided, and in this apparatus it was shown conclusively that all the ammonia used in making up the system is subsequently found in the solution and none in the undissolved residue. It appears to be fairly definitely established by the above data that, on bringing basic copper carbonate into contact
Vol. 22, No. 12
(1) M O i
Calculated
.
Found
0 0515
0.0420
(2) Mol
0.0691 0.0019
(3)
(4)
Mol 0.0538
Md
0.0623
0.1311 0.1312
Using Bonsdorff's data (1) on the solubility of cupric hydroxide in aqua ammonia, and assuming that every mol of carbon dioxide in solution corresponded to 1 gram-atom of copper and 4 mols of ammonia, the mols of copper which should be in solution may be calculated and these values compared with thoseactually obtained by analysis (Table 111). The assumption has been made that the ammonia-copper carbonate complex in solution is one in which 4 mols of ammonia are associated with 1 mol of copper carbonate. An investigation of this point by the chloroform-distribution method of determining free and combined ammonia in solution indicated that the number of mols of combined ammonia pcr gram-atom of copper in solution was greater than 3. Carbon Dioxide-Copper Mol Ratio in Black Bronzing Bath
If basic copper carbonate dissolved as a whole in aqua ammonia, the mol ratio of carbon dioxide to copper would be 0.5. On the other hand, if only the copper carbonate dissolved, leaving the copper hydroxide as a residue, this ratio would be unity. Actually the ratio was found to he an intermediate one, the aqua ammonia taking all the copper carbonate into solution and a part of the copper hydroxide. By passing carbon dioxide into the solution, more Flgures I and 2-Cross Sections of Film between Brass and Lead. 1000 x of the hydrated copper oxide was carried with aaua ammonia. the basic comaound is slowlv de- into solution. and as long as the carbon dioxide-copper mol
December, 1930
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figures 3, 4, and 5-Crosa
Section of Nickel Plate between Brass and Sealing War. 1000 x FI*ure 6-Croas
Sections of Film between Brass and Solder.
Figure 7 -Surface of Brass Covered with Black Film. 670 x
of basic copper carbonate, practically all of the copper going into solution. A satisfactory bath was made up by this procedure, and i t appears that commercial concerns using the black-bronzing process may save a part of the cost of the nrocess in this way. Chemical Nature of Black Deposit
A small sample of the black deposit was secured by carefully bending and scraping heavily coated brass strips. The serapings were examined under a low-power microscope and the few particles of brass present removed. The sample was then divided into two parts. On one part the total water was'determined, 7.4 per cent being found. The other part was then weighed and dried for an hour a t 105" C., the loss in weight being 1.4 per cent. The dried sample was then analyzed for copper, 76.5 per cent being found. The samples were so small that the analytical data are not highly accurate. It is apparent, however, that the copper
1323
I000 X
Fi)?ure8-Surface of Double Crystai of Brass Covered with Black Film. 75 x
oxide is only partially hydrated, for even the monohydrate CuO.H,O would yield 18.4 per cent mater instead of the 7.4 per cent actually found. If the original black deposit were of the composition CuO 92.6, and H,O 7.4 per cent, then the samole dried at 105" C. should analvse CuO 93.9 and H,O 6.1 ber cent. As noted above, the l o p p a on the dried sample was 76.5 per cent, which would correspond to 95.8 per cent CuO on the dried sample as against a t h e e retical93.9 per cent. On the other hand, if the dried sample were Cu10 93.9 and H206.1 per cent, the per cent of copper would be 83.4 copper as against 76.5 found. The evidence is, then, that the black deposit is a partially hydrated cupric oxide Physical Nature of Black Deposit
Strips of brass carrying the black deposit were mounted in solder or sealing wax, cut so &8 to expose a cross section of the brass and black deposit, and then ground, polished,
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etched, and photographed under high magnification by the methods commonly used in metallographic studies. The results are given in Figures 1to 5, inclusive. These photomicrographs bring out a number of interesting facts. It will be noted that the black-bronzing solution has etched the brass deeply, leaving crystals of brass sticking up above the surface. Over the peaks and valleys of this roughened surface the black deposit lies in an irregular blanket. I n some instances, as in Figures 4 and 5 , crystals of brass have been loosened from the surface and lie buried in the black deposit. The thickness of the black deposit is of the order of 0.0002 inch (0.0050 mm.). For comparison, a photomicrograph of nickel-plated brass was made by the same procedure used in making Figures 1 to 5 . This is shown in Figure 6 and is in sharpcontrast with the black deposits shown in the other figures. I n observing a black-bronzed surface of brass under the microscope, it was noted that the outlines of brass crystals and scratches on the brass could apparently be seen through the black deposit. Figure 7 is a photomicrograph of such a surface. I n order to show this curious fact the more clearly, a piece of brass containing but two crystals was polished, etched, and a deposit of black bronze placed on it. The surface was then photographed a t 75 diameters with the result shown in Figure 8. As a matter of fact, the boundary line between the two crystals can be discerned with the
Vol. 22,
KO.
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unaided eye, although on an ordinary brass surface the black deposit appears to be continuous and opaque. The black deposit itself shows no crystal forms a t magnifications up to 1000. The black deposit was also examined for crystal structure by the x-ray method, using both molybdenum and copper targets. KOevidence of crystal structure other than that of brass was observed. Etching of Brass Surface during Deposition of Black Bronze
These photomicrographs clearly show that the surface of the brass is deeply etched during the deposition of the blackbronze coating. As a check on this conclusion a blackbronzing solution initially free from zinc was analyzed and zinc was found to be present. As to whether a black-bronzing solution gains or loses copper during use would presumably depend on time of dip, concentration of ammonia, and other factors. I n an actual test case it was found that a strip of brass lost weight on being coated with the black bronze and the solution actually gained in copper content. Literature Cited ( 1 ) Bonsdorff, Z. anoyg. Chem., 41, 132 (1904).
Effect of Various Metallic Sulfates upon the Throwing Power of a Chromium Plating Bath' Lawrence E. Stout and Jonas Carol WASHINGTON UNIVERSITY, ST.LOUIS,Mo.
ARLY workers in chromium plating were faced with the fact that the throwing power of the chromic acid bath was quite poor. Empirical ratios for the various constituents soon were worked out so that the process could be put on a production basis. Various researches have been carried out and published to indicate the best conditions for plating this metal as uniformly as possible over an irregular surface. Farber and Blum ( 1 ) have made a very valuable investigation on the effect of a number of addition agents upon the throwing power of a chromium plating bath. They determined the optimum sulfate content, and also reported upon the effect upon the throwing power of several added materials other than sulfuric acid. I n some plants it has been the practice for the past year to produce the desired sulfate content in the plating bath by the addition of zinc sulfate rather than sulfuric acid. Extravagant claims have been made by the operators for such procedure. Since the literature contains no specific information on this point, the purpose of this investigation was to determine the effect of various metallic sulfates upon the throwing power of a chromium plating bath.
E
Apparatus
The throwing power of the baths was measured by a glass throwing-power box such as devised by Farber and Blum. The glass container was 20 em. long, 5 em. wide, and 10 em. ~
Received August 9, 1930. Presented before the Division of Industrial and Engineering Chemistry at the 80th Meeting of the American Chemical Society, Cincinnati, Ohio, September S to 12, 1930. 1
deep, and was fitted with an anode of perforated platinum foil (5 em. square). Polished nickel cathodes were used; they were 5 em. wide and were placed on both sides of the anode, one a distance of 5 em. and the other 10 em. This gave a primary current ratio of 2. The whole apparatus was placed in a thermostat. Early attempts to use a 6 per cent antimonial lead anode failed to give results that could be checked. It was concluded that a sufficiently varying non-conducting film at such anode made check results improbable. Materials Used
Chromic acid, CrOs, contained 0.03 per cent 804. Zinc sulfate, ZnSO4.7Hz0, analyzed 32.3 per cent Sod. Cadmium sulfate, 3CdSOa.8Hz0, tested 37.3 per cent SO,. Cobalt sulfate, CoS04.7H20, and nickel sulfate, NiS04.7H20, analyzed 34.2 and 34.8 per cent Sod, respectively. All chemicals were of the highest purity obtainable, having negligible traces of any other heavy metal. Experimental Procedure
Throwing powers were determined from chromic acid baths using Cr03-S04ratios of 15:1, 50:1, 100:1, 200:1, and 300:l. I n each bath the sulfate was added as metallic sulfate instead of as sulfuric acid. All runs were made a t 55" C. and a t a current density of 35 amperes per square decimeter using 250 grams CrOs per liter (33 ounces per gallon)' Farber and B1um found these conditions to give the maximum throwing power.
