IAVDUSTRIAL AND ENGINEERING CHEMISTRY
1324
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.
12
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 t o 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 a t 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.
I S D U S T R I A L S S D ENGINEERIAVG CHEJilISTRY
December, 1930
The apparatus was standardized, using a Cr03-S04 ratio of 200:l. A throwing power of -24 per cent was obtained against -13 per cent as obtained by Farber and Blum. Such calibration must be made for each individual box so as to give results which may be interpreted in terms of the other work reported.
deposits, zinc sulfate gave good deposits with good throwing powers, and cadmium sulfate gave the best throwing powers obtained. It was hoped that the sulfates of mercury would give even better results, but they precipitated from the bath.
-
Data a n d Results
Data and results obtained from the use of nickel, cobalt, cadmium, and zinc sulfates are listed in the accompanying table. SULFATE ADDED
ZnSG..7HzO
Cr03-SOd RATIO
300 : 1 200:l
15O:l
1OO:l 7531 15 1 300:l 200:l 135:l
1OO:l 7531
l5:l CoSOd.7HzO
NiSOd. 7Hz0
300:l 2OO:l 15O:l 1OO:l 7531 l5:l 300 :1 2OO:l 15031
1OO:l 7 5 :1
VOLTAGE T’olts 6.5 8.0 9.0 10 10 13.5
7 5
7.5 8 5 9.0 9.5 13.0
8.0 9.0 9.0 10.0
11 14 8.0 8.5 9.0 10.0 11.0 13.0
THROWING POWER
Per cent - 50
- 10 --612
- 25 - 150 - 40 -4.5
0
- 48 - 82 - 200 - 53 - 30 -28 - 50 - 60 - 90
-55
- 35 - 30 - 5‘4 - 70 - 125
NATURE OF
DEPOSIT Very poor Shiny Shjny Shiny Shiny Milky Shiny Shiny Shiny Milky Milky Milky
1325
’
O
a
r
-150 i
F
b
$
i-/oo t -90
6?-80
sk -60 b-70
P -50 E -40 -30 -20
Milky Shiny Shiny Shiny Shiny Milky AIilky Shiny Shiny Shiny Shiny Milky
-10 0
f’’
4 /5l 75f / 2001 3-00/ io01
I50
Ratio Cr@:SO,
The use of mercuric and mercurous sulfates as addition agents was found to be impracticable owing to the formation of insoluble basic sulfates. Only two runs were made with magnesium sulfate as an addition agent because of the very poor deposits obtained.
Farber and Blum pointed out that the presence of trivalent chromium and iron increased the resistance of the Lath. A similar situation exists in the results herein reported. Increasing metallic contents of the bath (other than chromium as CrOa) decreases the conductivity of such a bath. It is evident, therefore, that such addition agents should not be used by the operator unless he has available a generator giving a potential of 10 to 15 volts.
Discussion of Results
Conclusions
l5:l
The observations made in this work present valuable information from two angles. I n the first place, deposits obtained from the use of nickel or cobalt sulfates were very resistant to the action of hydrochloric acid. Ordinary deposits obtained from chromic-sulfuric acid baths stripped almost instantly in hydrochloric acid, but deposits obtained from the baths to which cobalt or nickel sulfate had been added in a ratio of Cr03-S04 of 200:l or 15031 required, in some cases, as many as 12 hours to strip. Deposits obtained from baths containing zinc or cadmium sulfate were slightly more resistant to the action of hydrochloric acid, but did not approach those first mentioned. It seems unlikely that this decreasing solubility in hydrochloric acid is due to the co-deposition of the added metal with the chromium. I n no case did the deposit give a chemical test for the added metal when adherent deposits were obtained. However, in all four cases a t the 15:l Cr03-S04 ratio the deposits peeled and did give chemical tests for the other metals. I n the second place these data indicate that higher throwing powers are obtainable from chromic acid baths whose Cr03-S04 ratio is 200:l or 150:l than from the ordinary Cr03-H2S04 bath. Cadmium sulfate consistently gave the highest throwing powers and a t the same time gave beautifully adherent deposits that needed no buffing or coloring to develop the desired brilliance. It is interesting to note that the members of the magnesium family in the periodic table increase in throwing power in descending order. Magnesium sulfate gave unsatisfactory
1-The addition of nickel, cobalt, zinc, or cadmium sulfates to a chromic acid plating bath in the ratios of OsSO4 of 200:l or 150:l increases the throwing power of the bath. 2-Cadmium sulfate, used in the proportions mentioned above, gives the highest throwing power. 3-Nickel and cobalt sulfates, when used in the proportions mentioned above, give a deposit which is much more resistant to the solvent action of hydrochloric acid. 4--Metals, as sulfates, used in the proportions mentioned above, do not deposit a t the cathode with the chromium. L i t e r a t u r e Cited (1) Farber and Blum, Bur. Standards I Research, 4, 27 (1930).
Department of Commerce Cooperates with Drug Industry All of the research facilities of the Bureau of Foreign and Domestic Commerce which may be needed have been extended to the National Drug Store Survey Committee, representing thirty trade associations in the drug and allied fields. The Department of Commerce has agreed to undertake, beginning January 1 in St. Louis, Mo., a comprehensive survey of drugstore merchandising operations and problems. A complete analysis of drug-store merchandising operations will be made; the druggist’s relation to his sources of supply and market will be ascertained and many special problems incident to drug-store merchandising will be analyzed. The industry will furnish $50,000 and cooperate in an advisory capacity.
