Process for the electrodeposition of chrome plate using fluorine-containing wetting agents

In the electrodeposition of a chrome layer wherein the chrome is deposited from an electrolyte solution containing a hexavalent chromium compound in the presence of a fluorine-based surfactant, the improvement which comprises employing as the fluorine-based surfactant a quaternary ammonium perfluoralkane sulfonate of the formula EQU [R.sub.F SO.sub.3 ].sup.- [NR.sup.1 R.sup.2 R.sup.3 R.sup.4 ].sup.+ in which R.sup.1,R.sup.2, R.sup.3 and R.sup.4 each individually is alkyl, alkenyl, cycloalkyl or aralkyl of up to 18 carbon atoms, or two or three of R.sup.1, R.sup.2, R.sup.3 and R.sup.4 together with the nitrogen atom to which they are attached form a heterocyclic ring, and R.sub.F is a perfluorinated alkyl radical with 6 to 12 carbon atoms. Additives such as Na.sub.2 CO.sub.3, NaHCO.sub.3, Na.sub.2 CrO.sub.4, K.sub.2 CrO.sub.4, Na.sub.2 Cr.sub.2 O.sub.7 and K.sub.2 Cr.sub.2 O.sub.7 or water may also be admixed with the quaternary ammonium perfluoralkane sulfonate.

In the electrodeposition of chrome plate from hexavalent Cr-electrolytes, 
the vigorous evolution of hydrogen at the cathode and, to a lesser extent, 
the evolution of oxygen at the anode results in the formation of spray 
mists containing chromic acid which have a highly corrosive effect and can 
contaminate other surface treatment baths in the vicinity of a 
chrome-plating installation. Furthermore, on account of the toxic 
properties of hexavalent chromium these spray mists are physiologically 
unacceptable. It is particularly the last of these disadvantages which, in 
the course of efforts to protect the environment, has prompted numerous 
investigations into ways and means of suppressing the formation of spray 
mists during the electrodeposition of chrome plate or of effectively 
eliminating the mists formed. In the latter case, elaborate extraction 
systems are required for recovering the spray losses which amount to as 
much as 5 to 10 % of the CrO.sub.3 -consumption (cf. R. Weiner, Die 
galvanische Verchromung, Leuz-Verlag Saulgau/Wurtt, 1974, page 13, 192). 
The formation of spray mists may be reduced for example by introducing 
small hollow plastic beads on to the surface of the bath. 
Another method adopted in practice for alleviating the difficulties 
referred to above is to use wetting agents which are resistant both to 
chromic acid and to anodic oxidation and which, on the one hand, reduce 
the surface tension of the highly viscous electrolyte solution, thereby 
greatly reducing entrainment losses, especially in bright chrome plating 
with its short exposure times, and on the other hand form a dense covering 
of foam which effectively prevents the droplets of chromic acid entrained 
by the hydrogen from escaping. By virtue of their high chemical and 
thermal stability, fluorine surfactants have proved to be particularly 
suitable for this purpose, for example in accordance with German Patent 
Specifications Nos. 937,210 and 939,611, cf. also C. G. Klaus, Chem. Age 
77 (1957), 883; Belgian Pat. No. 814,801. Salts of perfluoralkane sulfonic 
acids, more especially the potassium salts of perfluoroctane sulfonic 
acid, are primarily used in practice. Unfortunately, the potassium salt of 
perfluoroctane sulfonic acid is substantially insoluble in water 
(approximately 2 g/l). In concentrated acids, such as aqueous chromic 
acid, its solubility is even lower. However, our own investigations have 
shown that the maximum reduction in surface tension amounts to 
approximately 23 dyn/cm (in water at 25.degree. C) for a concentration of 
4 g/l (with sediment). The reason why it is only supersaturated solutions 
of C.sub.8 F.sub.17 SO.sub.3 K which reduce surface tension to the maximum 
extent is that the manufacturing method of electrochemical fluorination 
results in the formation of isomer mixtures of straight-chain and branched 
perfluoralkane sulfonyl compounds (cf. for example German 
Offenlegungsschrift No. 2,238,152), the branched isomers being more 
soluble in water. 
Unfortunately, conventional wetting agents also give rise to disadvantages 
when used in baths for the electro-deposition of chrome plate. Thus, their 
use is not recommended for the deposition of relatively thick, 
commercial-grade chrome layers with thicknesses of more than 15 microns, 
for example in hard chrome plating, because pores can be formed (resulting 
in inadequate protection against corrosion), cf. H. Dettner, 
Galvanotechnik 63 (1972), page 188, and Handbuch der Galvanotechnik, Vol. 
II, (1966), pages 225, 241, C. Hanser Verlag, Munich. On account of their 
inadequate solubility in electrolytes, there are also limits to the manner 
in which these agents are added for the suppression of chromic acid mists. 
Undissolved particles can be incorporated in the Cr-deposit where they 
give rise to roughness. Finally, it is not always possible to suppress the 
formation of chromic acid mists in hard chrome plating on account of the 
high current densities involved. 
Accordingly, the present invention provides a process for the production of 
electrodeposited metal layers, more especially hard and bright chrome 
layers, from electrolyte solutions containing hexavalent chromium 
compounds using fluorine-based surfactants, which is characterized by the 
fact that quaternized ammonium perfluoralkane sulfonates are added as the 
fluorine-based surfactant. 
It has surprisingly been found that the disadvantages of conventional 
products can be obviated by using fluorine-based surfactants corresponding 
to the general formula 
EQU [R.sup.1 R.sup.2 R.sup.3 R.sup.4 N] .sup.+ R.sub.F SO.sub.3 .sup.-, 
in which R.sup.1, R.sup.2, R.sup.3 and R.sup.4 each individually is alkyl, 
alkenyl, cycloalkyl or aralkyl of up to 18 carbon atoms, or two or three 
of R.sup.1, R.sup.2, R.sup.3 and R.sup.4 together with the nitrogen atom 
to which they are attached form a heterocyclic ring, and R.sub.F is a 
perfluorinated alkyl radical with 6 to 12 carbon atoms, in the 
electrodeposition of chrome plate. 
These quaternary ammonium perfluoralkane sulfonates are obtained by 
reacting perfluoralkane sulfonyl fluorides with tertiary amines and with 
silicic acid esters, cf. German Offenlegungsschrift No. 1,929,655 and Ann. 
731, 58 - 66 (1970). By virtue of this method of production, the products 
according to the invention only contain extremely small quantities of 
fluoride ions (for example less than 0.005 % of F) and other impurities, 
because the inorganic fluoride liberated during the reaction escapes in 
the form of readily volatile silicon-fluorine compounds. Apart from the 
fact that the purity of surfactants is of considerable significance in the 
production of optimum surface-tension effects (cf. K. J. Mysels and A. T. 
Florence, Journal of Colloid and Interface Science, Vol. 43, No. 3 (1973), 
page 577, and I. Kloubek and A. W. Neumann, Tenside 6 (1969), 1, 4/10), 
fluoride ions play an important part as catalysts in various types of 
baths in the electrodeposition of chrome plate, cf. R. Weiner, loc. cit., 
page 46. In cases where the fluorine-based surfactants according to the 
invention are used, their high purity avoids any uncontrolled input of 
fluoride ions. In contrast, other commercially available fluorine-based 
wetting agents have a considerably greater fluoride ion content, for 
example 0.15 % of F, because, in the hydrolysis of the perfluoralkane 
sulfonyl fluorides with potassium hydroxide for example, it is not 
possible completely to separate the perfluoralkane sulfonates from the 
potassium fluoride which is also formed. 
The following are examples of compounds suitable for use in accordance with 
the invention: tetramethyl ammonium perfluoroctane sulfonate, tetraethyl 
ammonium perfluorheptane sulfonate, N-methyl pyridinium perfluordecane 
sulfonate, N-dimethyl morpholinium perfluoroctane sulfonate and trimethyl 
octyl ammonium perfluorhexane sulfonate. 
It is preferred to use the derivatives with short C-chains in the cation, 
for example (C.sub.2 H.sub.5).sub.4 N .sup.+ R.sub.F SO.sub.3 .sup.-, and 
with 8 carbon atoms in the perfluorinated carbon radical. 
One typical method of preparing tetraethyl ammonium perfluoroctane 
sulfonate for example is described in the following: 
750 ml of chlorobenzene, 0.5 mole of perfluoroctane sulfonyl fluoride, 0.53 
mole of triethyl amine and 0.175 mole of triethoxy methyl silane are mixed 
and the resulting mixture is heated with stirring for 1 hour at 
100.degree. C, gaseous methyl trifluorosilane being evolved. After the 
evolution of gas has ceased, the reaction mixture is cooled with stirring 
to room temperature, as a result of which the compound (C.sub.2 
H.sub.5).sub.4 N .sup.+ C.sub.8 F.sub.17 SO.sub.3 .sup.- crystallizes out. 
The solubility of the product in water amounts to more than 100 g per 100 g 
of H.sub.2 O, in other words is higher by powers of 10 than the solubility 
of conventionally used fluorine-based surfactants, for example C.sub.8 
F.sub.17 SO.sub.3 K. The maximum surface-tension reductions amount to 22.6 
dyn/cm (in water at 25.degree. C) for a very small input of only 0.72 g/l. 
The maximum reduction in the surface tension of a commercial-grade chromic 
acid solution (250 g/l of CRO.sub.3, 1 % of H.sub.2 SO.sub.4, 50.degree. 
C) to approximately 20 dyn/cm is obtained with quantities of only 0.1 to 
0.15 g/l. 
In addition to their high solubility, their extreme purity and the minimum 
input required to obtain a reduction in surface tension to almost 20 
dyn/cm and to form a dense layer of foam in commercial-grade chromic acid 
electrolytes, the products suitable for use in accordance with the 
invention are surprisingly characterized by the fact that even relatively 
thick layers of chrome can be deposited without any danger of pore 
formation. Since hard chrome plating is playing an increasingly more 
significant role in electroplating, the process according to the invention 
affords considerable advantages. 
The concentration of the tetraalkyl ammonium perfluoralkane sulfonates used 
for the electrodeposition of chrome plate generally amounts to about 10 to 
300 mg/l and preferably to about 50 to 150 mg/l. With high current 
densities and in stirred baths, the concentration required to obtain a 
dense layer of foam effectively preventing chromic acid mists from 
escaping is at the upper limit of the specified range. 
Finally, it is also surprising that the products used in accordance with 
the invention, in the form of organic nitrogen compounds, are resistant 
both to chromic acid and to anodic oxidation. Any replenishment of the 
additive which is necessary is essentially a function of the entrainment 
losses. By virtue of the considerable effectiveness of these 
fluorine-based surfactants, it is advisable, in order to improve their 
dosability, to use the products in admixture with inert substances which 
do not interfere with the electro-deposition process, for example soda, 
sodium hydrogen carbonate, sodium chromate, sodium dichromate, potassium 
chromate or potassium dichromate, or to use an aqueous solution. 
The baths used in the process according to the invention are the bright and 
hard chrome baths commonly used for the electrodeposition of chrome plate 
(cf. for example B. R. Weiner, Die galvanische Verchromung, Leuze Verlag 
Saulgau/Wurtt, 1974, pages 15, 16). 
In general, baths of this kind contain: 
1. (Chromium(VI)oxide = CrO.sub.3 the concentration of which may vary 
within wide limits. CrO.sub.3 -concentrations of about 200 g/l to 400 g/l 
are normally preferred. 
2. Catalysts (foreign ions) such as sulfate, fluoride, silicofluoride and 
mixtures thereof which are responsible for the actual deposition of 
chrome. 
The most frequently used catalyst is sulfate which is generally added in 
such quantities that the weight ratio of CrO.sub.3 to SO.sub.4 amounts to 
between about 120:1 and 80:1. Instead of or in addition to sulfate, chrome 
baths also contain additions of fluoride or silicofluoride ions. The most 
favorable concentration of fluoride ions amounts to between about 1.5 and 
2.5 %, while the most favorable concentration of silico-fluoride ions 
amounts to between about 1.2 and 4 %, based on the CrO.sub.3 -content. The 
foreign ions may also be present in chrome baths in the form of 
substantially insoluble salts, for example SrSO.sub.4, K.sub.2 SiF.sub.6 
(SRHS-baths -- Self-Regulating High-Speed).

The process according to the invention is illustrated by the following 
Examples. 
EXAMPLE 1 
95 kg of sodium hydrogen carbonate and 5 kg of tetraethyl ammonium 
perfluoroctane sulfonate were pre-mixed in a drum and subsequently ground 
in a disc-attrition mill. 
3.2 kg (1 g/l) of the fluorine surfactant mixture were added to a sulfuric 
acid hard chrome-plating bath (250 g/l of CrO.sub.3 and 1 % of H.sub.2 
SO.sub.4, based on CrO.sub.3) with an electrolyte volume of 3200 l which 
was operated with agitation of the electrolyte at 45.degree. to 55.degree. 
C and at a current load of an average 12,000 Ah per day. An impervious 
cover of foam was formed on the surface of the bath in a more or less 
considerable thickness, depending upon the quantity of current used. The 
effectiveness of the fluorine-based wetting agent was judged on the basis 
of the dryness of the anode ends situated above the bath level and on the 
basis of spray-mist formation tested with filter paper 5 cm above the bath 
level on the cathodes. 9 kg of the 5 % fluorine-based surfactant mixture, 
including the starting quantity (3.2 kg) were used over a period of 95 
working days for maintaining an impervious foam cover over the surface of 
the hard chrome electrolyte. This corresponds to an average consumption of 
80 g of mixture = 4 g of tetraethyl ammonium perfluoroctane sulfonate for 
10,000 Ah or approximately 2.5 g of mixture = 0.125 g of fluorine-based 
wetting agent per 100 liters of electrolyte per day. 
The parts hard-chrome-plated in this bath did not show any reduction in the 
hardness of the chrome layer. There was no evidence of any roughness, 
inclusions or pore formation of the kind observed where other wetting 
agents are used for hard chrome plating. 
EXAMPLE 2 
100 mg/l of tetraethyl ammonium perfluoroctane sulfonate were dissolved in 
4 liters of chrome electrolyte containing 250 g/l of CrO.sub.3, 5g/l of 
SrSO.sub.4 and 14 g/l of K.sub.2 SiF.sub.6. The surface tension amounted 
to 24 dyn.cm.sup.-.sup.1 at 50.degree. C. Electrolysis was carried out 
with agitation of the bath at 50.degree. to 55.degree. C with a current 
density of 50 A/dm.sup.2. The current load amounted to 400 Ah/day. After a 
load of 1000 Ah, it was not possible to measure any increase in surface 
tension. 
EXAMPLE 3 
50 mg/l of tetramethyl ammonium perfluoroctane sulfonate were dissolved in 
4 liters of chrome electrolyte containing 400 g/l of CrO.sub.3 and 4 g/l 
of H.sub.2 SO.sub.4. Chrome plating was carried out with agitation of the 
bath at a temperature of 40.degree. C and at a current density of 15 
A/dm.sup.2, a 2.5 cm thick, impervious layer of foam being formed. After a 
load of 1000 Ah, it was still possible to measure an impervious layer of 
foam 1.5 cm thick. 
It will be appreciated that the instant specification and examples are set 
forth by way of illustration and not limitation, and that various 
modifications and changes may be made without departing from the spirit 
and scope of the present invention.