FI65286B - ELEKTROPLAETERINGSFOERFARANDE VARI ANVAENDS TRIVALENT KROM I LAOG KONCENTRATION - Google Patents

Description

6 5 2 8 6 Electroplating method using trivalent chromium at low concentrations

The invention relates to a chromium electroplating process in which the chromium source is an aqueous solution of a chromium (li) thiocyanate complex.

It has been common to perform chromium plating starting from aqueous chromic acid baths made of chromium oxide (CrO 2) and sulfuric acid. Such baths, in which the chromium is in the hexavalent form, are characterized by a low current output. In addition, the chromium content in such baths is extremely high, which causes waste or recovery problems in the so-called chromium compounds. due to "withdrawal" in rinsing tanks used after a plating bath.

In order to overcome several disadvantages associated with plating with hexavalent chromium, plating with chromium in trivalent form has been proposed.

One such method of chromium plating starting from an aqueous solution of a chromium (III) -thiocyanate complex is described in GB Patent 1,431,639. The buffering agent comprises an amino acid, (e.g., glycine, aspartic acid), peptides, formates, acetates, and hypophosphites. These plating methods using trivalent chromium do not remove chromic acid fumes. They have high efficiency, wide plating range and good coverage.

A much lower amount of chromium is required for the bath than in methods using hexavalent chromium, which reduces recovery problems. The chromium concentrations are 0.03-0.5 mol / l.

Although the plating methods using trivalent chromium according to the aforementioned GB patent overcome all the major disadvantages associated with plating with hexavalent chromium, the alloyed 65286 chromium generally has a somewhat darker appearance. Although this color is perfectly acceptable or may even be preferred for many uses, it may be advantageous in spasmodic contexts to be able to plate lighter-colored chromium by a trivalent process.

In addition to decorative purposes, chrome plating is also used for practical purposes where color is irrelevant. Due to its hardness, low friction and corrosion resistance, it is used, e.g., to provide a wear-resistant coating on the surface of a sliding machine part or to provide such a coating on screws or nuts. In such applications, it is generally necessary that the thickness of the plated chromium is clearly much greater than in the case of decoration 15. Normally, the thickness of the decorative chromium is less than 1 μm, while the thickness of the "practical" chromium must be in the order of tens of micrometers. Such thicknesses have hitherto been achieved by plating with hexavalent chromium alone. , for example according to GB patent 1,431,639, has resulted in coarse, opaque deposits with poor adhesion.

Thus, trivalent chromium previously obtained from prior art thiocyanate baths is associated with two problems, namely color in decorative applications and thickness in practical applications.

The present invention is based on the unexpected finding that chromium (III) thiocyanate baths with a chromium-30 content well below the generally accepted level for efficiency and bath stability not only give a much lighter color precipitate, but also a precipitate that allows post-exposure. purchasing smooth cohesive thick layers sa-35 from a bath with a higher concentration.

3 65286

The process according to the invention uses a chromium electroplating solution in which the chromium source is a balanced aqueous solution of a chromium (III) -thiocyanate complex with a chromium content of less than 0.03 mol / l and low enough to give a precipitate whose color is essentially as light .

According to the invention, a chromium-electroplating solution is also used, in which the chromium source is a balanced aqueous solution of a chromium (III) -10 thiocyanate complex having a chromium content of less than or equal to 0.02 mol / l

The preferred ratio of chromium to thiocyanate molar concentrations is 1: 2 to 1: 4.

another good feature is that the solution contains 15 amino acids as a buffering agent. The preferred amino acid is sparagaric acid at a molar content of 1.25 x the molar content of chromium.

According to the invention, there is provided a chromium plating method comprising the step of passing an electrical plating current between an anode and a cathode in the plating solution described above. The preferred temperature range for obtaining a light color is 40-60 ° C. For the lightest color, it is preferred that the current density be greater than 2 mA / cm.

In practice, chromium has been plated according to the invention since solutions of chromium-thiocyanate-amino acid complexes in which the content of the complexes is very low.

In this case, aspartic acid and glycine have been used as amino acids. Glossy, white, permanent precipitates have been obtained from solutions with a chromium content of not more than 0.02 mol / l. These precipitates are considerably lighter in color than those from 0.1 molar solutions of the same complex. For the eye, the color of the precipitates is at least as pale as the color of the vaporized chromium precipitate. This subjective impression is confirmed by reflection measurements, which show that the deposits from baths with chromium concentrations up to 0.02 mol / l were generally as reflective or reflective as heat / knurled chromium, but less reflective than electroplated hexavalent chromium.

The color of the precipitate has been found to depend to some extent on factors other than the chromium content.

More specifically, the lower the thiocyanate / chromium ratio, the lighter the precipitate becomes. In addition, it has been found that the color of the precipitate becomes lighter as the solution temperature increases, with 40-60 ° C giving the lightest precipitates without any other adverse effects. An increase in current density has also been shown to whiten precipitation.

Certain experiments have shown that the precipitates from a molar 0.03 bath become substantially darker than the ground chromium but still lighter than the trivalent precipitates from baths with a higher concentration. It is not possible to give an exact quantitative limit between the chromium content of 0.02 and 0.03 mol / l at which light precipitates can be obtained, which, as indicated above, is due to a certain effect on the color by the composition of the rest of the solution and the process conditions. However, individual experiments and purely visual observations show that by accurately optimizing the variable, solutions of trivalent chromium in the vicinity of 0.03 mol / l can give a reflectivity or color that roughly corresponds to the corresponding properties of evaporated chromium.

Brass samples and vaporized copper samples on glass have been plated with solutions with a low chromium content, and the darker precipitate obtained from a bath with a higher trivalent chromium content has also been plated with low-content solutions. In the latter case, the primary bath was optimized for current performance and stability over a longer period of time instead of optimizing for color. A light-optimized bath would be somewhat inefficient and slow and would require 35 dense precision fillings if used to plate the chromium thicknesses normally required in the 65286 market. In addition to the lighter color of the plated coating, it has been found that plated samples have greater resistance to corrosion than non-plated samples.

5 The process conditions have varied greatly and a good plating result has been achieved continuously. In this case, baths in the temperature range of 20-70 ° C and currents of 20-800 mA / cm in the Hull cell have been used.

Examining the parameters that affect the efficiency of 10 in a bath with a low chromium content, it turned out that both the current density and the pH of the solution are important. The optimal current density is 2 30-40 mA / cm in terms of efficiency, although a current density of more than 2 50 mA / cm gives an even lighter color.

The pH range of 3.8-4.5 normally gives the highest efficiency, although each pH value in the range of 2-5 is acceptable and does not appreciably affect the color.

Chromium has also been plated according to the invention as a first step in a process for plating thick (more than 5 μm) coatings for practical applications where color is not as important as surface properties, e.g. smoothness, hardness and cohesiveness. Such thick coatings, which in many cases are plated from a bath with a higher chromium content, have been shown to have improved properties when a thin starting layer is alloyed with the solution of the present invention and the process thereof. Although - as mentioned above - in theory, a thick coating with good surface properties could be plated from a bath with a low concentration, the real time would become very long and the efficiency very low.

When measuring the precipitate obtained from solutions with a low chromium content according to the invention, it was surprisingly shown that chromium is not mainly chemically bound to some of the other elements precipitated together with it, while precipitates obtained from solutions with a high chromium content contain a substantial amount of chromium that is chemically bound to oxygen and sulfur. It is assumed that since the thin starting layer is very clean and uniform, it can act as a seed layer for subsequent layers from a solution with a higher concentration and limit the granularity of the resulting hybrid layer. The whole thick film is thus more cohesive and less brittle than a film of the same thickness. , precipitated solely from a bath of higher concentration. The light color of the precipitated chromium from the solutions with the low concentration used according to the invention is also assumed to be associated with the presence of chemically unbound chromium.

The above and other objects, features and advantages of the invention will become apparent from the following examples and comparative examples.

Comparative Example 1 20 This is an example of a trivalent chromium bath that is optimized for efficiency and longevity rather than color. This is not an example of the invention as such, but may be used to perform the first step in a process according to an embodiment of the invention.

The chromium spreading solution was prepared as follows: a) 60 g of boric acid (H-jBO 2) were added to 750 ml of deionized water, which was then heated and stirred to dissolve the boronic acid.

b) 33.12 g of chromium sulphate (Cr2 (SO4) 3 ♦ 32.43 g of sodium thiocyanate (NaNCS) was added to the solution, which was then heated and stirred at about 70 ° C for about 30 minutes.

c) 16.625 g of DL-aspartic acid (NH 2 CH 2 CH (COOH) 2) was added to the solution and it was then heated and stirred at about 75 ° C for about 30 hours, during which time the pH was adjusted to 1, 5 to 3, Ora very slowly 5 with 10% w / w sodium hydroxide solution As soon as pH 3.0 was reached, this value was maintained throughout the smoothing period.

d) Sufficient sodium chloride was added to the solution to a concentration of about 1 mol / l * and 0.1 g of FC98 wetting agent was also adjusted. The solution was heated and stirred for a further 30 minutes.

e) The pH of the solution was again adjusted to 3.0 with sodium hydroxide solution.

f) The volume of the solution was made up to 1 liter with deionized water adjusted to pH 3.0 with 10% v / v hydrochloric acid.

The final composition of the solution was: 0.1 mol / l chromium sulphate - Cr2 (SO2) ^ · 15H2 ° 0.4 mol / l sodium thiocyanate - NaNSC20 0.125 mol / l aspartic acid - NH2CH2CH (C00H) 2 60 g / l boric acid - H BO ^ 60 g / l sodium chloride - NaCl 0.1 g / l wetting agent.

The result of the smoothing process is assumed to be in the form of a chromium / thiocyanate / asparagine complex in the final solution of the chromium-25 mass.

An electroplating bath containing the above-mentioned electroplating solution having a pH of about 2.1 and a temperature of 25 ° C was used to plate chromium on a nickel-30 plated brass plate fitted as a cathode to a Hull cell. The current density was 50 mA / cm and the current was switched on for 2 minutes. A relatively dark precipitate about 0.35 microns thick formed.

Example I

This example illustrates an electroplating solution used in the method of the invention, which is prepared as follows;

A solution was prepared in exactly the same manner as described in Comparative Example I, with the exception that only half of the amount of sodium thiosvanate 5 was used, giving a sodium cyanate content of 0.2 mol / l. This solution was made up to 1 l with a solution containing 60 g of boric acid and 60 g of sodium chloride / l.

The final composition of the electroplating solution 10 was essentially as follows: 0.003 mol / l chromium sulphate 0.006 mol / l sodium thiocyanate 0.00375 mol / l aspartic acid 60 g / l boric acid 60 g / l sodium chloride.

The plate plated with chromium according to Example I was transferred without rinsing to another Hull cell containing a plating solution according to this example. The increase in chromium content due to chromium removed from the first solution was not accurately determined but was not expected to increase concentration greater than 0f001 mol / l · The plating current was allowed to pass through the cell for 2 minutes. Due to the placement of the plate in the cell, the current density was in the range of 20 to about 150 25 mA / cm 2, the bath temperature was 25 ° C. A glossy, white, co-precipitate formed which covered the precipitate obtained from the bath of Example I. The thickness of the plated precipitate was estimated to be a few hundred Angstroms.

Example II

The test plate was plated as shown in Comparative Example I. The plate was transferred without rinsing to another solution of Example I and partially immersed therein. A thin layer of chromium was plated on the embedded part of the plate as described in Example I. The top-35 plated layer covered the originally plated layer and had a clearly lighter color than the portion of the originally plated layer that was not top-plated.

65286 The spot meter was used to take measurements of the intensity of the reflected ambient light from the area of the plated (light) surface and from the simply plated (dark) area of the plate. Similarly, light reflected from a partially reflective vaporized chromium reflector, a partially white, indirect reflector, was also measured. These measurements were used as standards. A comparison of the measured light intensity with the reflectors and the light and dark areas of the test plate showed that the reflection coefficient ratio between the light and dark areas on the test plate was 2.26: 1.

Example III

A number of chromium plating solutions were prepared according to Example I, with the exception that each solution had a different chromium content. In all individual cases, the chromium / thiocyanate / aspartic acid molar ratio was 1/4 / 1.25.

Chromium was plated on a substrate formed of a layer of copper vaporized on glass with a current density of 2 20 50 mA / cm. The temperature of the solution during plating was 40 + 5 ° C. Measurements of the percentage reflectance of the plated samples were performed at different wavelengths. Magnesium fluoride polished aluminized glass mirror was used as standard. The following table 25 shows the results of the percentage reflectance.

Cr Concentration 550nm 800nm 350nm 725nm 0.001 mol / 1 62.2 77.7 62.2 71.1 0.003 "66.2 77.7 65 70.8 0.005" 64 75.7 61.8 68.3 30 0.010 "62 , 1 73.7 58.8 66.7 0.015 "60 71.6 56.6 64.8 0.020" 56.6 68.5 51.2 61.9

For comparison, yet another table is shown with similarly obtained percent volatility figures for samples plated with 10 65286 higher trivalent chromium, a sample plated with hexavalent chromium, and a sample of vaporized chromium.

5 Experiment 5St) 0nm 8Q0nm 3S0nm 725nm trivalent (0.03 mol / l) 35.7 44.3 30.1 39.9 trivalent (0.04 mol / l) 23.1 32.2 16.3 28.2 hexavalent 73.7 80.9 82.5 10 vaporized 57.7 63.3 61.1

Samples of hexavalent chromium were commercially available and were on different substrates, which may have affected the reflectance measurements. A relatively stronger shortwavelength component (blue) could be detected. Evaporated samples were prepared by steaming on copper / glass substrates similar to those used for plating.

It is clear from this that the hi-20 solubility of trivalent chromium is in fact as good as or better than that of evaporated chromium up to a concentration of about 0.02 mol / l. At a concentration of 0.03 mol / l and more, the reflectivity of the plated samples is clearly worse than that of the vaporized chromium under the plating conditions of this example. , by accurately optimizing, it would probably be possible to obtain a reflectivity corresponding to that of vapor-30 chromium from solutions of trivalent chromium at a concentration close to 0.03 mol / l. however, no exact limit can be given.

Example IV

In another series of experiments, a series of 35 chromium ether solutions for use in accordance with the invention were prepared as described in Example I, having a chromium content of about 0.003 mol / L and a thiocyanate content of 0.020-0.160 mol / L.

In all cases, the concentration of aspartic acid was 0.00375 mol / l ·

Chromium precipitates were plated from each of these solutions under the same conditions as in Example III.

5 Percent reflectance measurements were performed on each plated sample and the results were as follows: NCS ^ concentration 10 0.020 62.2 74.3 57 67.3 0.040 56.3 69.8 46.4 62.8 0.080 53.1 64.9 48, 1 58.3 0.100 52.8 64.4 48.5 57.8 0.120 46.3 56.9 42.6 50.9 15 It follows from the above that the excess thiocyanate reduces the percentage reflectance, but that this is gradual. Even when the molarity of the thiocyanate is 50 times the molarity of chromium, the percentage reflectance is consistently better than that of the 20 comparative 0.03 molar solutions in Example III.

Example V

In another series of experiments, a series of chromium-plating solutions of different concentrations were prepared as a solution as described in Example I. The chromium / thiocyanate / aspartic acid ratio was 1: 4: 1.25. The pH of each solution was adjusted to 3.0 and a number of samples were plated in each solution at different current densities. The temperature in the sheath was 45 ° C in all cases. The results were as follows: 2 30 Cr content Current density mA / cm Efficiency% 20 1 30 3 0.003 mol / 1 40 3.5 80 1.5 120 1.5 180 1 12 65286 2Q 7 30 7 0.007 mol / 1 40 9 80 4 120 3 180 2,3 5 20 15 30 22 0,030 mol / 1 40 23 80 12 120 10,7 180 6,6 10 These results show that the optimal current density for the plating result is between 30 and 40 mA / cm.

2 However, visual observations show that wave densities above 50 mA / cm gave the brightest colors.

Example VI

In another series of experiments, two chromium spreading solutions of 0.003 and 0.012 mol / l were prepared as described in Example I.

The molar ratio of choroyl / thiocyanate / aspartic acid was 1: 41.25.

Samples of each solution were adjusted to different pH values by the addition of acids or bases, and the effect of the pH change was studied by plating a chromium precipitation team.

The temperature was maintained in all cases at 45 ° C and the 2 plating current density was 40 mA / cm.

The results were as follows: 25 Cr content pH Efficiency% 2.0 3.0 3 mmol / l 3.0 2.5 3.8 3.6 4.5 3.0 30 2.0 5.2 12 mmol / l 3 .0 5.9 3.8 6.4 4.5 7.6

The results were not entirely consistent, but li 13 65286 generally show that a pH of 3.8-4.5 gives the highest yield. No clear effect on color could be observed.

Comparative Example II

A solution was prepared according to Comparative Example I (i.e., with a chromium content of 0.1 mol / L) and placed in a plating cell. A platinum-plated titanium anode and a steel test plate as a cathode were immersed in the cell. The steel plate had 2 10 -12 μm polished nickel coatings. A plating current of 75 mA / cm 10 was allowed to pass between the electrodes for 90 minutes. A 20.9 [mu] m thick chloro layer then precipitated.

The appearance of this precipitate was unpolished and opaque and proved to be very brittle. The surface profile measurements gave a value of 1.5-1.9 μm as the center line.

Example VII

A second chromium plating solution for use according to the invention was prepared with a lower concentration (0.003 mol / 11 according to Example I).

20 A lower concentration electroplating solution was placed in a plating cell having a platinum-plated titanium anode and a steel test plate as a cathode. During the process of the invention, a plating current of 40 mA / cm was allowed to pass through the cell for 240 seconds to precipitate a chromium initiation layer having an arbored thickness of not more than 1000 Angstroms.

A plate plated by the method of the invention from the solution of the invention was then transferred without rinsing to another plating cell containing an electroplating solution having a higher chromium content but the same composition as in Comparative Examples I and II. A plating current of 75 mA / cm was allowed to pass through the cell for 180 minutes to deposit a much thicker chromium layer on top of a thin starting layer.

The final thickness of the chromium layer was 21.6.

14 65286 This thick layer looked visually smooth and reflective. The mean surface area was 0.178 μm. The precipitate was less brittle and better cohesive than the precipitate in Comparative Example II.

5 Example Fruit

The two-stage plating described in Example VIII was repeated in a series of experiments using the same two plating solutions, although the wetting agent was omitted in a particular case. This appeared to further improve the precipitation properties by reducing the granularity. Films with thicknesses of 10-75 μm were plated.

The current densities in plating from the low concentration 2 bath were 40-50 mA / cm. Current densities of plating yarticle with high concentration, 2 15 oliyat 40–120 mA / cm. Mean averages for some of these samples range from 0.178 to 0.273,

Example IX

Using the same solutions as in Example VII and starting from a solution having a lower concentration according to the invention, varying chromium layers of each solution were deposited on a steel test plate.

The steel plate was first arranged as a cathode in a low concentration bath, and a current with a density of 2 40 mA / cm was allowed to flow for 240 seconds to develop a thin layer of starting 25 chromium with a thickness not exceeding 1000 Angstroms. The plate was transferred without shaking to a high concentration bath and plated at a current density of about 50 mA / cm for 30 minutes to obtain a thicker chromium layer. The plate was returned to a low concentration bath and 2 was plated for 2 minutes at 40 mA / cm. Alternating plating for 30 minutes in a high concentration bath and 2 minutes in a low concentration bath was continued for a total of 215 minutes. 35 Total chromium was deposited to a thickness of 16.9 microns, The final deposit was cohesive, smooth, and not brittle, and had a mean centerline of 0.2 microns.

i

Claims (5)

652 8 6 A method for chromium electroplating / causing an electric current to flow between an anode and a cathode 5 in a solution, characterized in that an electric current having a flow density of 30-120 mA / cm is made between the anode and the cathode in an aqueous solution of chromium (III) thiocyanate complexes, the chromium content is less than 0.03 mol / l and the temperature is in the range of 40-60 ° C. Process according to Claim 1, characterized in that the chromium content is less than or equal to 0.02 mol / l. Process according to Claim 1 or 2, characterized in that the molar content ratio of chromium to thiocyanate is between 1: 2 and 1: 4. Process according to one of Claims 1 to 3, characterized in that the amino acid is present as a buffering agent and forms at least one of the complex ligands. Process according to Claim 4, characterized in that the amino acid is aspartic acid having a molarity of 1.25 times the chromium content.