IE49204B1 - Low concentration trivalent chromium electroplating solution and process - Google Patents

Abstract

The Cr concn. is 0.03M and is sufficiently weak to give a deposit whose colour is at least as bright as that of vapour deposited Cr. Bright Cr coatings can be formed (without the attendant problems of using Cr6+ baths) for decorative applications, and thick, coherent, smooth coatings can be formed for structural and mechanical applications. The ratio of the molar concns. of Cr/thiocyanate is pref. 1 : 2-4. The soln. pref. also contains an amino-acid (pref. aspartic acid) as buffer, in a molar concn. 1.25 times that of Cr. The bath temp. for a bright coating is pref. 40-60 degrees C, the brightest coloured coatings being formed using a current density >50 mA/cm2. The Cr concn. is pref. =0.02M. The pH of the bath is pref. 3.8 - 4.5.

Description

This invention relates to chromium electroplating solutions and processes in which the source of chromium comprises an aqueous solution of a chromium (III) thiocyanate complex.

Background Art Conventionally chromium has been plated from aqueous chromic acid baths prepared from chromic oxide (CrO^) and sulphuric acid. Such baths, in which the chromium is in hexavale.nt form are characterized by low current efficiency. The chromic acid fumes emitted as a result of hydrogen evolution als;> present a considerable health hazard. Furthermore the concentration of chromium in such baths is extremely r :gh leading to problems of waste or recovery because of ---called draqout of chromium compounds into the rinse tanks which follow the plating bath.

To overcome many of the disadvantages of hexavalent chromium plating, it has been proposed to plate chromium in trivalent form. One such process for plating chromium from an aqueous solution of a chromium (III) - thiocyanate complex is described in UK Patent 1 431 639. Another such process is described i.n our Irish Patent Specification No. 47088 which '.escribes a chromium plating solution and process in which an ., ,?'!□ solution of a chromium (III) - thiocyanate complex is again employed but in which a buffer material supplies one of the ligands to the chromium complex. The buffer material is selected from amino acids (eg glycine, aspartic acid), peptides, formates, acetates and hypophosphites.

These trivalent chromium plating processes do not give off chromic acid fumes. They are of high efficiency with a wide plating range and good covering power. A very much lower amount of chromium is needed in the bath than is the case with hexavalent processes thus reducing the problems associated with drag-out. Concentrations of chromium have ranged from 0.03 to 0.5 Molar.

Disclosure of the Invention Although the trivalent chromium plating processes of DK Patent 1 431 639 and UK Patent Application No 24734/77 overcome all the major disadvantages of hexavalent plating, the appearance of the deposited chromium is generally somewhat darker. While this colour is quite acceptable or even preferable for many applications, it would be advantageous in decorative applications to be able to plate lighter coloured chromium with a trivalent process.

Chromium plating, besides its decorative applications, is also used for engineering purposes where colour may be unimportant. Because of its hardness, low friction and corrosion resistance it is used to provide, for example, a wear resistant coating on the surface of a sliding machine part or to provide such a coating on screws or bolts. For such applications, it is generally necessary that the thickness of the plated chromium is very much greater than in decorative applications. Typically decorative chromium is less than one micron in thickness whereas engineering chromium needs to be of the order of tens of microns of thickness. Such thicknesses have hitherto been achievable only with hexavalent chromium plating. Attempts to plate thick chromium (above 5 microns) from trivalent baths such as those of UK Patent 1 431 639 and Irish Patent Specification No. 47088 have resulted in coarse, matt deposits with poor cohesion.

Thus, two problems exist with trivalent chromium from thiocyanate baths as described in the prior art, namely of colour for decorative applications and of thickness for engineering applications, The basis of the present invention is the unexpected discovery that chromium (III) - thiocyanate baths whose chromium concentration is far below the generally accepted level for efficiency and bath stability not only give a significantly lighter coloured deposit but also a deposit which enables the subsequent deposition of smooth coherent thick layers from a higher concentration bath.

According to one aspect, the present invention provides a chromium electroplating solution in which the source of chromium comprises an equilibrated aqueous solution of a chromium (III) - thiocyanate complex, the concentration of chromium being less than 0.03 Molar and sufficiently low to be capable of producing a deposit of a colour substantially as light or lighter than an evaporated chromium deposit.

According to another aspect, the present invention 2θ provides a chromium electroplating solution in which the source of chromium comprises an equilibrated aqueous solution of a chromium (III) - thiocyanate complex, the chromium concentration being less than or equal to 0.02 Molar.

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

Another preferred feature is that the solution includes an amino acid as a buffer material. The preferred amino acid is aspartic acid in molar concentration 1.25 times that of chromium. 48204 The invention also provides a process of plating chromium comprising the step of passing an electric plating current between an anode and a cathode in such a plating solution.

The preferred temperature range for achieving a light colour is 40 to 60°C. Again for the lightest colour it is preferred that the current density is greater than 50 mAcm 2.

According to another aspect, the present invention also provides a process of electroplating an article with a layer of chromium exceeding five microns in thickness comprising the step of electroplating the article with an initial relatively thin layer of chromium from an equilibrated aqueous solution of a chromium (III) - thiocyanate complex of relatively low chromium concentration, and plating the major proportion of the remaining thickness in one or more steps from an equilibrated aqueous solution of a chromium (III) thiocyanate complex of relatively higher concentration.

Preferably, the low concentration is less than 0.03 Molar and the higher concentration above 0.03 Molar. In the preferred process, only the initial thin layer is deposited from the lower concentration solution, the entire remaining thickness being plated from more concentrated solution.

Alternatively, alternate thick and thin layers may be plated from the higher and lower concentration baths respectively.

Detailed Description In studies which have been carried out, chromium has been plated, according to the invention, from solutions of chromium-thiocyanateamino acid complexes in which the 4 concentration of the complexes is very low. Aspartic acid and glycine are amino acids which have been employed.

Bright, white coherent deposits have been obtained from solutions of chromium concentration up to 0.02 M. These deposits are significantly lighter in colour than deposits from 0.1M solutions of the same complexes. The colour of the deposits to the eye is at least as light as that of an evaporated chromium deposit. This subjective impression is supported by reflectivity measurements which show that '0 deposits from baths having chromium concentrations up to 0.02 M were generally equally or more reflective than evaporated chromium though less reflective than electroplated hexavalent chromium.

The colour of the deposit has been found to be dependent to some extent on other factors besides chromium concentration. In particular the deposit is lighter the lower the ratio of thiocyanate to chromium. The deposit colour has also been found to lighten with increased solution temperature, 40° - 60°C giving the lightest deposits without causing other adverse effects. Increased current density has also been found to lighten the deposit.

Some experiments have shown deposits from a 0.03M bath to be significantly darker than evaporated chromium though still lighter than trivalent bath deposits from higher concentration baths. It is not possible to give a precise quantitative limit between 0.02 and 0.03M chromium concentration below which light deposits can be produced because, as discussed above, the colour depends to some extent upon the composition of the remainder of the solution and upon the process conditions. However, isolated experiments and purely visual observations indicate that by careful optimisation of variables, reflectivity or colour approximating to that of evaporated chromium could be obtained from trivalent solutions of chromium concentration approaching 0.03M.

Samples of brass, and evaporated copper on glass have been plated from the low chromium concentration solutions and the darker deposit obtained from a higher concentration trivalent chromium bath has also been overplated from the low concentration solutions. In the latter case, the primary bath was optimised for current efficiency and stability over a long period rather than for colour. A bath optimised for light colour would be somewhat inefficient and slow and would need frequent careful replenishment if used to plate thicknesses of chromium which are normally required commercially. Besides the lighter colour of the overplated coating, it has been found that the corrosion resistance of overplated samples is superior to samples which have not been overplated.

Process conditions have been varied widely and satisfactory plating still obtained. Baths have been operated at temperatures from 20 - 70°C and current densities in a Hull cell have ranged from 20 - 800 mAcm 2.

Studies of the parameters affecting the efficiency of the low chromium concentration bath indicate that both current density and solution pH have an effect. The optimum current density is 30 - 40 mAcm2 for efficiency although a current density above 50 mAcm”2 produces a still lighter colour. A pH range of 3.8 - 4.5 is generally the most efficient though any pH between 2 and 5 is acceptable and there is no marked effect on colour 9 2 0 4 Chromium has also been plated in thicknesses greater than 5 micron for engineering applications where colour is not as important as surface qualities such as smoothness, hardness and coherence. Such thick coatings, plated predominantly from a higher chromium concentration bath are found to have improved properties where an initial thin layer is deposited from a lower chromium concentration solution. Again, although a thick coating with good surface qualities could in theory be plated from a low concentration bath, the time involved would be very long and the efficiency very low.

Electron Spectroscopic Chemical Analysis measurements of the deposit from low chromium concentration solutions according to the invention indicate unexpectedly that the chromium is substantially not chemically bound with any other codeposited elements whereas '5 deposits from high chromium concentration solutions include a significant amount of chromium which is chemically bound with oxygen and sulphur. It is believed that, since the initial thin layer is very pure and uniform, it can act as a seeding layer for subsequent deposits from a higher concentration .10 solution and limits the granularity of the resultant hybrid deposit. The overall thick film is thus more cohesive and less friable than film of the same thickness deposited from the higher concentration bath alone. The light colour of the deposited chromium from low concentration solutions according /5 to the invention is also believed to be related to the presence of chemically unbound chromium.

The invention will now be described further with reference to the following examples and comparative examples: 49204 Comparative Example I This is an example of a trivalent chromium bath optimised for efficiency and lifetime rather than colour. It is not an example of the invention as such but may be used to carry out the first step of a process according to one aspect of the invention.

A chromium plating solution was prepared in the following manner:a) 50 grams of boric acid (Η^ΒΟ,ρ were added to 750 ml of deionised water which was then heated and stirred to dissolve the boric acid. b) 33.12 grams of chromium sulphate (Cr2(SO4)j.ISHjO) and 32.43 grams of sodium thiocyanate (NaNCS) were added to the solution which was then heated and stirred at approximately 70°C for about 30 minutes. c) 13.3 grams of DL aspartic acid (ΝΙ^Ο^Οί(COOH) / were added to the solution which was then heated and stirred at approximately 75°C for about 3 hours. During this time the pH was adjusted from pH 1.5 to pH 3.0 very slowly with 10% by weight sodium hydroxide solution.

Once the pH of 3.0 was achieved it was maintained at this value for the whole of the equilibration period. d) Sufficient sodium chloride was added to the solution to make it approximately IM concentration and 0.1 grams of FC 98 (a wetting agent produced by 3M Corporation) was also added. The solution was heated and stirred for a further 30 minutes. 9 2 0 4 e) The solution pH was again adjusted to pH 3.0 with sodium hydroxide solution. f) The solution was made up to 1 litre with deionised water which had been adjusted to pH 3.0 with 10% by volume of hydrochloric acid.

The final solution composition may be expressed as:0.1 M chromium sulphate - Cr2(SO^)15H2O 0.4 M sodium thiocyanate - NaNCS 0.125 M aspartic acid - ΝΗ2<3Η2<3Η (COOH) 2 60 g/1 boric acid - H^BO^ g/1 sodium chloride - NaCl 0.1 g/1 FC 98 - (wetting agent product of 3M Corp) As a result of the equilibration process, the bulk of the chromium in the final solution is believed to be in the form of chromium/thiocyanate/aspartic complexes.

An electroplating bath containing the above electroplating solution was operated at around pH 2.1 and 25°C to plate chromium onto a nickel plated brass plate connected as cathode in a Hull cell. The current density was 50 mA cm 2 and current was applied for 2 minutes. A relatively dark deposit of chromium approximately 0.35 microns thick was produced.

Example I This example is an example of an electroplating solution according to the invention which was made up as follows: 49204 A solution was prepared in exactly the same manner as comparative described in^Example I except that one half the quantity of sodium thiocyanate was used, resulting in a sodium thiocyanate concentration of 0.2M. 30 mis of this solution were made up to 1 litre with a solution containing 60 grams per litre of boric acid and 60 grams per litre of sodium chloride.

The final electroplating solution had essentially the following composition:0.003 M chrome sulphate 0.006 M sodium thiocyanate 0.00375 M aspartic acid 60 g/1 boric acid 60 g/1 sodium chloride A plate which had been plated with chromium as described comparative in^Example I was transferred without rinsing to a second Hull cell which contained the electroplating solution of the present example. The increase in concentration of chromium due to drag-out from the first solution was not precisely determined but is estimated not to have increased the concentration by more than 0.001 M. Plating current was passed through the cell for 2 minutes. Because of the arrangement of the plate in the cell, the current densities across the plate ranged from 20 to approximately 150 mA cm2. The temperature of the bath was 25°C. A bright white coherent deposit was formed which obscured the initial deposit obtained from the bath of Example X. The thickness of the overplated deposit was estimated to be a few hundred angstoms.

Example ΓΓ A sample plate was plated in the manner described in Comparative Example I. The plate was transferred without rinsing to a second solution as described in Example I and parti.ally ima.rseJ therein. A thin layer of chromium was plated on the immersed portion of the plate in the manner described in Example I. The overplated layer obscured the criginally plated layer and was significantly lighter in colour than, the portion of the originally plated layer which was not over-plated.

Measurements were made with a spot meter of ambient reflected light intensity from the surface of the overplated (light) area and the singly plated (dark) area of the plate. Similar measurements were also made on light reflected from a specular evaporated chromium reflector and also from a white diffuse reflector. These were used as standards. By comparing the measured light intensity from the reflectors and from the light and dark areas of the sample plate, it was found that the reflectance ratio of light to dark areas of the sample plate was 2.26 to 1.

Example III A number of chromium plating solutions were made up as described in Example I, except that each solution had a different chromium concentration. In each case the molar ratio of chromium/thiocyanate/ aspartic acid was 1/4/1.25.

The chromium was plated onto a substrate consisting of an evaporated copper layer on glass at a current density of 50 mA cm2. The temperature of the solution during plating lay in the range 40±5°C. Measurements of the percentage reflectivity of the plated samples at various wavelengths were made using a Beckman Spectrophotometer Acta MVI with 198900 double-beam variable angle specular reflectance accessory. The standard used was a magnesium fluoride overcoated aluminised glass mirror. The results are given in the following table of percentage reflectivity:- Cr Concentration 550nm 800nm 350nm 725nm 10 .001M 62.2 77.7 62.2 71.1 .003M 66.2 77.7 65 70.8 .005M 64 75.7 61.8 68.3 .010M 62.1 73.7 58.8 66.7 • 015M 60 71.6 56.6 64.8 15 .020M 56.6 68.5 51.2 61.9 By way of comparison another table gives identically obtained percentage reflectivity figures for higher chromium concentration trivalent plated samples, for a hexavalent chromium plated sample and for an evaporated chromium sample:- Sample 550nm 800nm 350nm 725nm Trivalent (.03M) 35.7 44.3 30.1 39.9 Trivalent (.04M, 23.1 32.2 16.3 28.2 Hexavalent 73.7 80.9 82.5 Evaporated 57.7 63.3 61.1 The hexavalent samples were commercially obtained and were on different substrates which may have affected the reflectivity measurements. A relatively stronger short wavelength (blue) component was noted. The evaporated samples were produced by evaporation onto copper/glass substrates identical to those used for plating. r It can be seen that the reflectivity of the trivalent chromium is roughly as good or better than that of evaporated chromium up to a concentration of 0.02M. At 0.03M and above, the reflectivity of the plated samples is significantly worse than that of evaporated chromium under the plating conditions of this example. From other isolated experiments and purely visual observations of colour, it seems probable that by careful optimisation of other solution components, such as thiocyanate, and of process conditions such as temperature and current density, a reflectivity approximately to that of evaporated chromium could be obtained from trivalent solutions of chromium concentration approaching 0.03M. However, no precise limit can be given.

Example IV In one further set of experiments a number of chromium plating solutions according to the invention were made up in the manner of Example I with a chromium concentration of 0.003 M and with thiocyanate concentrations ranging between 0.020 and 0.120 M. In all cases the aspartic acid concentration was 0.00375M. Deposits of chromium were plated from each of these solutions under the same conditions as for Example III. Percentage reflectivity measurements were made on each plated sample and the results were as follows;- 49204 NCS Concentration 550nm 800nm 350nm 725nm .020 62.2 74.3 57 67.3 .040 56.3 69.8 46.4 62.8 .080 53.1 64.9 48.1 58.3 .100 52.8 64.4 48.5 57.8 .120 46.3 56.9 42.6 50.9 It can be seen that excess thiocyanate reduces the percentage reflectivity but that the effect is gradual. Even when the thiocyanate molar concentration is fifty times that of the chromium molar concentration the percentage reflectivity is still better than from the 0.03M solution of Example III, Example V Xn a further set of experiments a number of chromium plating solutions of different concentrations were made up in the manner of Example I. The molar ratio of chromium to thiocyanate to aspartic acid was 1:4:1.25. Each solution was pH adjusted to pH 3.0 and a number of samples were plated from each solution at different current densities. In all cases the bath temperature was 45eC. The results were as follows:Current Cr Concentration Density mAcm~2 % Efficiency 1 25 30 3 3.5 1.5 003 M 4!)4ul 120 1.5 180 1 .007 Μ 40 120 180 2.3 .022 Μ 120 18 0 11.6 .6 ' 5 .0 30 Μ 120 180 .6 12 .7 6.6 These results show that the optimum current density for plating efficiency is in the range 30-40 mA cm 2. However visual observation indicates that current densities above 50 mAcrn”2 produced the lightest colours.

Examde VI In a further set of experiments, two chromium plating solutions of 0.003 M and 0.012 M were made up in the manner of Example X. The molar ratio of chromium to thiocyanate to aspartic acid was 1:4:1.25.

Samples of each solution were adjusted to different pH's by addition of acids or bases and the effect of pH variation studied by plating deposits of chromium. In each case the temperature was maintained at 45°C and the plating current density was 40 mA cm”2. The results were as follows:10 Cr Concentration pH % Efficiency 2.0 3.0 3mM 3.0 2.5 3.8 3.6 4.5 3.0 2.0 5.2 12mM 3.0 5.9 3.8 6.4 4.5 7.6 The results were not completely consistent but generally indicate that a pH in the range 3.8 - 4.5 is the most efficient. There was no marked effect on colour.

Comparative Example II A solution prepared as in Comparative Example I (ie with 0.1 M chromium concentration) was introduced into a plating cell. A platinised titanium anode and a steel sample panel as cathode were immersed in the cell. The steel panel had an overcoating of 10-12 microns of bright nickel. A plating current of 75 mA cm 2 was passed between the electrodes for 4-9 2 0 4 'in minutes. Λ layer of chromium of 20.9 microns thickness «:s iepositei. l« depus.t was lull and matt in appearance and proved yxuremc-l/ friable. Profile measurements of the surface (CLA) .’autre line average^measurements in the range 62-75 ---,-: (..5 - 1.9 microns).

Example 711 A second lower concentration chromium (0.003 M) plating solution, according to the invention, was made up as iescnbed in Example I.

The lower concentration electroplating solution was . r ·_ ’ .need into a plating cell having a platinised titanium ar.cd,- -.....: .. -:-:01 sample panel as cathode. In a process according to the ii.v-.’ition, a plating current of 40 mA cm 2 was passed through tlie cell for 240 seconds to deposit an initial layer of chromium estimated to be not more than 1000 angstroms in thickness.

The panel was then transferred without rinsing to a seconi plating cell containing a higher concentration 2C chromium electroplating solution of the same composition as that of Comparative Examples I and II. A plating current of 75 mA cm 2 was passed through the cell for 180 minutes to deposit a much thicker layer of chromium on top of the initial thin layer. The final thickness of the chromium layer was 21.6 microns.

This thick layer appeared smooth and reflective to the eye. The CLA of the surface was 7 microinches (0.178 microns). The deposit was less friable and more cohesive than that of Comparative Example II.

Example VIII The two step plating described in Example VII was 5 repeated in a series of experiments using the same two plating solutions, although in some cases the wetting agent was omitted. This appeared to improve the characteristics of the deposit even further by reducing granularity. Films ranging from 10 to 75 microns thickness were plated. Current densities for plating from the low concentration bath were in the range 40-50 mA cm2. Current densities for plating from the high concentration bath were in the range 50-120 mA cm2.

CLA measurements on some of these samples lay in the range 7-11.2 microinches.

Example IX Using the same solutions as for Example VII, and starting with the lower concentration solution alternate layers of chromium were deposited on a steel sample panel from the two solutions.

The steel panel was first connected as cathode in the low concentration bath and a current of density 40 mA cm 2 was passed for 240 seconds to produce a thin initial layer of chromium of no more than 1000 angstroms thickness. The panel was transferred, without rinsing, to the high concentration bath and plated at a current density of 50 mA cm 2 for 30 minutes to produce a thicker layer of chromium. The panel ί 9 2 Ο 4 was then transferred back to the low concentration bath and plated for 2 minutes at 40 mA cm 2. The alternate plating for 30 minutes in the high concentration bath and 2 minutes in the low concentration bath was continued for a total time of 215 minutes.

In all a thickness of 16.9 microns of chromium was deposited. The final deposit was cohesive, smooth and non friable and had a CLA of 8 microinches (0.2 microns).

Claims (5)

1. J. Λ chromium electroplating solution in which the source of chromium comprises an aqueous solution oi' a chromium (III) - thiocyanate complex, the concentration of chromium being less than 0.03 M and sufficiently low to give a perceptibly lighter coloured deposit than deposits from such solutions of concentration above 0.03 M, the solution further including a pH buffer material.
2. 2. An electroplating solution us claimed in claim J in which the ratio uf the molar concentrations of chromium to Lhiocyuiia' o is between 1:2 and 1:4. J. An electroplaiing solution as claimed in any preceding claim in which the buffer material includes an amino acid providing at least one of the ligands for the complex.
3. 3. 4. An electroplating solution as claimed in claim 3 in which the amino acid is aspartic acid in a molar concentration 1.25 times the concentration of chromium.
4. 4. 5. A chromium electroplating solution substantially as hereinbefore described with reference to Examples I to IV. G. A process of plating chromium comprising the step of passing an electric plating current between an anode and a cathode in a solution as claimed in any one of claims 1 to 5.
5. 5. 7. A process as claimed in claim 6 in which the temperature ia in the range 40°C to 60°C. H. A process as claimed in claim 6 or claim 7 in which the current, density is greater than 50 mA cm 4 9 2 0 4 pi·-.- .-1 , ι j..: 'Mi'inium .1:· vlairaud xn I'ljus ι, und nuhnLanl i.iHy ι·.; .. ? j·.., m ι : wj'Ij reference Lo Examples III to VI. Λι. article plated by a process as claimed in any one of claims 6 to