DE1034445B - Baths and processes for the electrolytic production of chrome coatings on metal objects, as well as salt mixtures for the production of the baths - Google Patents

Description

The invention relates to baths for the production of chrome coatings on metal objects as well as salt mixtures of such baths. The baths contain chromium trioxide and also strontium sulfate and Alkali silicofluoride in amounts exceeding their solubility.

Such baths for the chrome plating of metals are known per se. The excess should be in these baths of strontium sulfate and alkali silicofluoride ensure that the dissolved portion of these during the electrolysis Salts and thus the concentration of acid ions remains constant. You can only have one in these baths maintain a certain saturation concentration, but cannot do this itself according to the respective requirements vary in chrome plating. But this is extremely desirable.

It is also known to add strontium chromate to saturation in chromium baths which contain strontium. In this way, however, no control of the concentration of sulfate ions is achieved. A good even and crack-free chromating requires control of the concentration of sulfate ions. It was So far not even known how large the concentration of sulfate ions is at all to achieve optimal Results should be.

The present invention now provides a new composition of such baths for chrome plating, in which the saturation limit of the two salts mentioned is reduced to a considerably lower value, but still to a finite value, by means of additives. In this way, depending on the amount of additives added, the saturation concentration of strontium sulfate or alkali silicofluoride can be set to a desired level below the saturation concentration without these additives. The added salts must either be strontium and / or alkali compounds, but have a different acid residue than is present in the strontium sulfate or alkali silicofluoride, so that the solubility product of these two salts is shifted to a lower value and thus the concentration of the catalytically active acid ions SO ₄ "and SiF ₆ " is lowered. By means of the baths according to the present invention, a much more precise setting of the constant maintenance of the composition of the chromium plating baths can be achieved. The chrome plating itself is then carried out under the usual conditions. The temperatures used are not critical, so that it is also possible to work at room temperature or at higher temperatures of 60 ° C. or higher.

The additives are said to be the saturation concentration of strontium sulfate and sodium or potassium silicofluoride strong, but reduce it to a still finite value. The saturation concentrate baths are preferred and methods for the electrolytic production of chromium coatings

Metal objects, as well as salt mixtures

for making the baths

Applicant:

Metal & Thermit Corporation, New York, N.Y. (V. St. A.)

Representative: Dipl.-Ing. W. Meissner, Berlin-Grunewald, and Dipl.-Ing. H. Tischer, Munich 2, Tal 71,

Patent attorneys

Jesse E. Stareck, Royal Oak, Mich. (V. St. A.), has been named as the inventor

Tration of strontium sulfate to one tenth of its original value, d. H. the saturation concentration in the bath the same composition without the addition of the additives mentioned, and those of sodium or Potassium silicofluoride reduced to a quarter of its original value. For example, is used for Reduction of the saturation concentration of the strontium sulfate strontium chromate, while that of the alkali silicofluoride by that of the corresponding alkali chromates or bichromates.

The following advantages, among others, are achieved by the additives according to the invention:

1. Self-regulation of the content of the catalytically active acid ions to or near the optimum Concentration in baths with any chromic acid content;

2. Little or no analytical control necessary;

3. the ability to produce significant amounts of catalytically active acid ions that act as impurities or Can be added to compensate without exceeding the working limits of the bath;

4. superior hiding power of chromium deposits;

5. high efficiency, low power consumption, few rejects and low operating costs;

6. wide bright chrome plating area;

7. high chrome plating speed;

8. good cathode activation;

9. direct bright chrome plating on stainless steel;

10. Avoiding an iridescent film on objects that are only partially chrome-plated.

809 577/350

Example VI

Chromium trioxide 62.5 to 87.5

Potassium dichromate 5.7 to 17.7

Strontium chromate 4.3 to 12.3

Potassium Silicon Fluoride - 2.0 to 6.0

Strontium sulfate 0.5 to 1.5

Example VII

Chromium trioxide 85 to 95

Potassium dichromate 2.4 to 6.6

Strontium chromate 1.5 to 4.5

Potassium silicofluoride 0.9 to 2.5

Strontium sulfate 0.4 to 1.2

Example VIII

Chromium trioxide 85 to 95

Sodium dichromate 2.3 to 6.7

Strontium chromate 0.6 to 1.8

Potassium silicofluoride 1.5 to 4.5

Strontium sulfate 0.6 to 2.0

Example IX

Chromium trioxide 79 to 95

Sodium dichromate 2.3 to 11.7

Sodium silicofluoride 1.5 to 5.1

Strontium sulfate 1.2 to 4.2

The total amount of the catalytically active acid ions in the baths is according to the invention of several substances, each of which contributes a part of this total amount. The amount of Acid ion is determined by the solubility of the salt in question.

The solubility of the strontium sulfate can be determined in a known manner by other strontium compounds such as SrCO ₃ , Sr (OH) ₂ , SrCrO ₄ , and that of potassium silicofluoride by adding potassium compounds such as KOH, xo K ₂ CO ₃ , K ₂ CrO ₄ , K ₂ Cr ₂ O ₇ , can be decreased.

It is desirable to regulate the solubility of the catalytically active salts in the bath, so that self-regulation is achieved of the catalytically active acid ions at or near the optimal concentration at any chromic acid concentration with the particular advantages of these various concentrations is achieved. The baths advantageously contain 100 to 500 g of chromium trioxide per liter, with some of the chromium trioxide also can be replaced by alkali chromate or bichromate.

Mixtures of salts which result in such baths when dissolved in water contain 72 to 97 percent by weight Chromium trioxide as well as strontium sulfate and alkali silicofluoride in such amounts that when the salt mixtures dissolve some of these salts remain undissolved in the prescribed amount of liquid. Also included according to the invention, these salt mixtures are not catalytic

acting strontium and / or alkali compounds, so The mixtures given in Example I are used for

that the saturation concentrations of the catalytically active solutions with a chromium trioxide concentration of seed salts corresponding to the above-mentioned conveyance 30 about 150 to about 400 g / l, the optimum depending on ranks of the present invention are reduced. the base metal to be chrome plated and accordingly the type of chrome plating is slightly different. For example, if stainless steel is chrome-plated, then usually a chromium trioxide concentration of about

According to the invention, (in% of the total mixture): 35 ISO to about 200 g / l are preferred. Will the chrome plating

made on an intermediate nickel layer, a chromium trioxide concentration of about 200 to about 250 g / l preferred. For the generation of a chromium deposit with maximum compensatory Effect, a chromium trioxide concentration of about 250 to about 350 g / l is usually preferred.

The mixture according to Example II gives satisfactory solutions with a chromium trioxide concentration of about 250 to about 500 g / l. The main application of the mixture according to Example II is to use it as a top-up mixture to be used for baths according to Example I, although they can of course also be used for the production of chrome-plating baths can serve. Used for a chrome plating bath when chrome plating over a nickel interlayer is used, the optimum chromium trioxide range is between about 275 and about 350 g / l. If it is used for the production of heavy chromium deposits with a maximum balancing effect, so a concentration of from about 350 to about 500 g / l is usually preferred. The higher chromium trioxide concentrations, made possible by Example II

Here, too, some of the chromium trioxide can be replaced by alkali chromate or bichromate.

Examples of salt mixtures corresponding to the present invention

Example I.

Chromium trioxide 70 to 90

Potassium dichromate 5.5 to 15.5

Strontium chromate 0.6 to 3.6

Potassium silicofluoride 2.5 to 7.5

Strontium sulfate 1.4 to 3.4

Example II

Chromium trioxide 85 to 95

Potassium dichromate 2.5 to 7.5

Strontium chromate 0.5 to 1.5

Potassium silicofluoride 1.3 to 4.1

Strontium sulfate 0.7 to 1.9

Example III

Chromium trioxide 18 to 72.8

Sodium dichromate 23.4 to 70

Strontium chromate 0.8 to 3

Sodium silicofluoride 1.8 to 6

Strontium sulfate 1.2 to 3

Example IV

Chromium trioxide 59 to 86.4

Sodium dichromate 11.6 to 35

Strontium chromate 0.5 to 1.5

Sodium silicofluoride 1.0 to 3.0

Strontium sulfate 0.5 to 1.5

Example V

Chromium trioxide 85 to 95

Sodium dichromate 1.6 to 5

Strontium chromate 1.1 to 3.3

Sodium silicofluoride 1.9 to 5.5

Strontium sulfate 0.4 to 1.2

result in a bath with high conductivity and minimal tendency for the precipitates to flake off with heavy chrome plating.

Example III represents a mixture according to Example I with the difference that sodium salts are used Place of potassium salts are used. Everything that has been said in relation to Example I also applies Example III too.

In the same way, the mixture of Example IV corresponds to that of Example II with the difference that the sodium salts are used in place of the potassium salts of Example II. Everything related to Example II and the baths made thereafter also apply to Example IV.

5 6

The mixture of example V is particularly suitable for Trom curves A and AI as well as B and BI result in similar

melchroming usable. You can borrow within a ratio.

Chromium trioxide concentration of about 300 to 500 g / l. In FIG. 2, curve B shows the regulating effect used, with the optimum at about 400 g / l being the one that lowers the saturation concentration. These baths can also be used for the usual 5 substitutes in baths with 200 and 250 g of chrome trioxide decorative chrome plating and for heavy chrome plating per liter according to Example III, depending on the temperature. changes in nature. Curve A represents a bath which is produced with a mixture according to Example III, but solutions for chromium trioxide concentrations of approximately without the reducing additives. From curve B 100 to about 250 g / l, the optimum at about 10 in FIG. 2 showing that at 250 g of chromium trioxide 150 g / l is. This bath operates at low chromium of SiF _e "content of the solution at a satisfactory Temperaturtrioxydkonzentrationen and is particularly advantageous in small change from 32.2 to 54.4 ° C only about 1.4 to cost and high efficiency. 2 , 7 g / l changes, while curve A shows that when the mixture according to Example VII changes in temperature, the SiF ₆ "content is mainly used to refill baths according to Example VI. They 15 changes from 9.8 to 13.9 g / l. It can also be seen from curve B for chromium plating baths with one of FIG. 2 that at 200 g of chromium trioxide per liter the chromium trioxide concentration rises from about 200 to about SiF ₆ "per liter only from 1.7 to 3.4 g 400 g / l are used. In all baths according to the invention, Example VIII changes a mixture for chromium-SiF ₆ "content only slightly with temperature changes, baths with chromium trioxide concentrations of about 250 20 The SO _{4 also changes in the same way} "Content of up to about 500 g / l, with the optimum at around 400 g / l temperature changes, the size of the change being slight. In this bath, only the saturation concentration is even lower than with SiF ₆ " of strontium sulfate , through the strontium content.

Fig. 3 illustrates the effect of the saturation concentration of the silicofluoride. In this example, the concentration-reducing agent on the content of only the strontium chromate is a reducing agent, acid ions in the solution. Curve A shows the effect that has a depressant effect on sulfate; that of the sodium content of the reducing agent on the sodium dichromate, that part of the chromium trioxide silicofluoride content. Curve B shows the effect of the replaced, does not act as a depressing agent because the potassium of the depressant agent is present on the silicofluoride-silicofluoride as the potassium salt and not as the sodium salt, and curve C shows the effect of the strontium and the sodium from the Sodium the reducing agent on the sulphate content. The bichromate is present in insufficient quantity, according to the curves of FIG. 3, the effects described represent a reducing effect on the silicofluoride in a bath that contains 250 g of chromium trioxide per liter. This mixture can be used for jewelry chrome- 43.3 ° C. So the bath, which contains 8 g / l barrels, is used for barrel chrome plating and for strong 35 shots of strontium (this is the strontium of the chromium plating agent used) and, as shown by curve C, example IX is the same as example VIII with the Deviating from this, 0.5 g of sulphate per liter, an amount such that only the saturation concentration of the silico-sulphate (for example from sulfuric acid, a common fluoride is adjusted by sodium dichromate. This substitute) is added, the 4 g / l strontium The equivalent is the mixture can be used for jewelry and barrel chrome plating (ie 4.4 g / l sulfate) in the range of 8 to 4 g / l gene as well as for heavy chrome plating. Strontium at curve C, the amount of sulfate that goes down into solution, is equal to the difference between the sulfate-setting agent for the silicofluoride is not only 8 g / l strontium and 4 g / l strontium, in that its concentration in the solutions to the ie 0.9 g / l minus 0.5 g / l; therefore only 0.4 g / l sulphate. The desired value is set, but also in it, 45 so only 0.4 g / l sulphate goes with a total addition of that the content of Silicofhiorionen less of temperature 4.4 g / l sulphate in solution, the rest is as strontium temperature changes and from changes in the chromium trioxide sulfate precipitated. It can therefore be seen that the concentration of the bath is affected. Saturation levels are kept to a minimum Some of the beneficial properties of baths are. In the same way it is clear from the curves A and B, according to the invention, it can be seen on the diagrams 50 that an identical relationship is established between the. added amount of SiF ₀ "and that going into solution

In Fig. 1, curves A, B and C represent the amount there is.

Change in the sulfate ion concentration of baths Quite generally, it can be seen from Fig. 3 that the

according to Example I with the mixture 200, 250 and concentration of sulfate ions through a feed

300 g of CrO ₃ per liter, if you have more sulfate ions in 55 of strontium ions up to a concentration below

added in an amount of up to a little over 3 g / l. That of the non-reduced saturation concentration, however

Curves AI, BI and CI represent the effect of the addition to be reduced with a finite value

there would be of sulfate ions on the sulfate concentration can. The non-reduced saturation value is on

ordinary solutions, which are given only from chromium trioxide of the curve C at point zero, namely represents

and sulphate are produced, with a concentration of 60 this point the sulphate content of the strontium

ions similar to those of the baths with curves A, B and C sulfate in the absence of other strontium or

correspond. With solutions represented by the curves AI, sulphate compounds represent saturated solution. With others

5-1 and C-I are shown, the sulfate ions increases - words can up to the concentration of sulfate ions

concentration proportional to the addition of sulfate ions. a value in the range from 2.1 to 0.2 g / l is reduced

Looking at curve C it can be seen that at 65, the latter value being at the right end

an addition of 1 g of SO ₄ "per liter, the SO ₄ " -concentration curve and about a tenth of the

tration here only from about 0.83 to about 1.0, i.e. by not reducing the saturation concentration.

0.17 g / L increases; the increase is only about. Of course, this value can be a little lower,

20% of the SO / 'concentration, while in bath C-I it is when the curve is lengthened to the right. In the same

120%. When comparing between the 70 ways, the concentration of silicofluoride ions

(cf. curve B) can be reduced by adding potassium ions to a lower, but above zero concentration, the concentration extending down to about a quarter or even less of the non-reduced saturation concentration. The concentration of silicofluoride ions can also be reduced by sodium ions (cf. curve A) to about a quarter or a little lower than the non-reduced saturation concentration. These remarks apply very generally to the invention.

The complete mixtures of Examples I through IX are preferred in manufacture and use. However, the advantages of the invention can also be achieved if, for example, the mixture of Example I with the omission of chromium trioxide, then the other components of this example be added to a separately prepared chromium trioxide solution. In the same way, an or several components can be added to the bath independently of the others. It can also, as already emphasized, degrading agents other than those specifically mentioned in the examples may be used.

Furthermore, with regard to the examples in which a TeE of the total CrO ₃ content is added by CrO ₃ as such and a part is provided by the non-catalytic salts such as sodium dichromate or strontium chromate, it should be noted that the information relating to CrO ₃ in the claims, unless expressly stated otherwise, both as such to the mixture or the bath added CrO ₃ and the added as part of the non-catalytic compound CrO are intended to include _{the third} The CrO ₃ content, therefore can range from a minimum value of about _72/0 in Example III to a highest value of about 97% in Example VII rich °.

Claims (6)

Patent claims: exceed their solubility, characterized in that they also do not act catalytically Contain strontium and / or sodium or potassium compounds in such amounts that the Saturation concentrations of strontium sulfate and / or alkali silicofluoride are still finite Are reduced in value. 2. Baths according to claim 1, characterized in that the amounts of non-catalytically active Strontium and / or alkali compounds are dimensioned so that the saturation concentrations of strontium sulfate up to a tenth and / or alkali silicofluoride up to a quarter of the saturation concentration of these salts are reduced without these additional components. 3. Baths according to claim 1, characterized in that part of the chromium trioxide is replaced by sodium or Potassium chromate or bichromate is replaced. 4. Baths according to claim 1 to 3, characterized in that they contain 100 to 500 g / l of chromium trioxide. 5. Salt mixtures for preparing or supplementing aqueous chromium baths according to claim 1 and 2, characterized in that they contain 72 to 97 percent by weight of chromium trioxide, which can be partially replaced by alkali chromate or bichromate, so much strontium sulfate and alkali metal silicate that the produced therefrom Chromium baths are saturated with both salt groups and contain further amounts of both salt groups undissolved and the salt mixtures also contain non-catalytically active strontium and / or alkali compounds in amounts corresponding to the requirements of claims 1 and 2. 6. The method using baths according to claim 1 to 4, characterized in that at Temperatures up to about 55 ° C are used. 1. Baths for the electrolytic production of publications under consideration: Chromium coatings on metal objects, including 40 United States Patent No. 2,042,611; Chromium trioxide and next to it strontium sulfate and Bilfinger, manual of the electrolytic chromium Sodium or potassium silicofluoride in quantities, the mung, 1952, p. 41 ff. 1 sheet of drawings © 809 577/350 7.58 iiiiil