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).
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