CA1214426A - Trivalent chromium electroplating solution and bath - Google Patents
Trivalent chromium electroplating solution and bathInfo
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- CA1214426A CA1214426A CA000415906A CA415906A CA1214426A CA 1214426 A CA1214426 A CA 1214426A CA 000415906 A CA000415906 A CA 000415906A CA 415906 A CA415906 A CA 415906A CA 1214426 A CA1214426 A CA 1214426A
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- chromium
- sulphate
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Abstract
ABSTRACT
TRIVALENT CHROMIUM ELECTROPLATING SOLUTION AND BATH
A chromium electroplating solution in which the source of chromium is an aqueous solution of chromium (III) complexes.
The complexes are selected from a solution of chromium (III) and at least one of aspartic acid or an organic compound having a -C=S group or a -C-S- group. A supporting electrolyte is chloride free and comprises a mixture of sodium and potassium sulphates in a concentration sufficient to provide electrical conductivity for the plating process. The concentration of sodium sulphate is in the range of about 0.1 to 1 Molar and the concentration of potassium sulphate is about 1 Molar.
TRIVALENT CHROMIUM ELECTROPLATING SOLUTION AND BATH
A chromium electroplating solution in which the source of chromium is an aqueous solution of chromium (III) complexes.
The complexes are selected from a solution of chromium (III) and at least one of aspartic acid or an organic compound having a -C=S group or a -C-S- group. A supporting electrolyte is chloride free and comprises a mixture of sodium and potassium sulphates in a concentration sufficient to provide electrical conductivity for the plating process. The concentration of sodium sulphate is in the range of about 0.1 to 1 Molar and the concentration of potassium sulphate is about 1 Molar.
Description
TRIVALENT CHROMIUM ELECTROPLATING SOLUTIOM AND BATH
The invention relates to chromium electroplating solutions and baths in which the source of chromium comprises an equilibrated aqueous solution of chromium ~III) complexes.
Background Art The advantages of plating chromium from an equilibrated aqueous solution of chromium ~III) - thiocyanate complexes over conventional chromic acid plating are elaborated in our U.K. Patent 1,431,639. Refinements and modifications of this basic process have been described in later patents among which are U.S. Patent 4,141,803 and 4,161,432. The benefits to the trivalent chromium process of an anolyte and catholyte separated by a cation exchange membrane are described in our Canadian Patent 1,120,427, issued March ~3, 1982 to D.J. Barclay et al. Finally our U.K. Patent
The invention relates to chromium electroplating solutions and baths in which the source of chromium comprises an equilibrated aqueous solution of chromium ~III) complexes.
Background Art The advantages of plating chromium from an equilibrated aqueous solution of chromium ~III) - thiocyanate complexes over conventional chromic acid plating are elaborated in our U.K. Patent 1,431,639. Refinements and modifications of this basic process have been described in later patents among which are U.S. Patent 4,141,803 and 4,161,432. The benefits to the trivalent chromium process of an anolyte and catholyte separated by a cation exchange membrane are described in our Canadian Patent 1,120,427, issued March ~3, 1982 to D.J. Barclay et al. Finally our U.K. Patent
2,033,427, issued May 6, 1982 to D .JO Barclay et al and Canadian Patent 1,150,185, issued July 9, 1983 to D.J.
Barclay et al describe a related solution and process in which beneficial effects are obtained from a reduction in the level of chromium and thiocyanate concentration to levels well below those originally contemplated.
The equilibrated chromium (III) - thiocyanate complexes from which plating takes place have been prepared from a variety of starting materials. The originally preferred starting salts of U.K. Patent 1,431,639 were chromium perchlorate and sodium thiocyanate. In order to make the solution sufficiently ~lectrically conductive additional sodium perchlorate was added as a supporting electrolyte.
U.S. Patent 4,141,803 proposed hexathiocyanatochromium salts of potassium or sodium (K3Cr~NCS)6 or Na3Cr(NCS)6) to UK9-80-00~X -1-which sodium perchlorate or sodium sulphate was a~ded as a conductivity salt. Po~assium sulphate was also mentioned as a possible conductivity salt but no example was yiven. In U.S. Patent 4,161,432 one preferred solution was prepared from chromium chloride (CrC13) and sodium thiocyanate.
Potassium chloride was added for conductivity. A second preferred solution was prepared from chromium sulphate ~Cr2(SO4)3) and sodium thiocyanate. In this case sodium sulphate was added for conductivity.
In Canadian Patent 1,120,427, in which a catholyte and anolyte are separated by a membrane, the catholyte was prepared from chromium sulphate ICr2(SO4)3) and sodium thiocyanate, and sodium chloride was added for conductivity.
The anolyte consisted of an aqueous solution of a depolarising agent to which sodium sulphate (Na2SO4) was added for conductivity. The advantage of having sodium sulphate in the anolyte rather than sodium chloride is that chlorine evolution from the anode is very much reduced. The electrolyte employed in Canadian Patent 1,150,185 has essentially similar constituents to that of Canadian Patent 1,120,427 except that the concentration of chromium is below 0.03 molar and the concentration of thiocyanate is also proportionally reduced.
It is found that in plating chromium from electrolytes as described in Canadian Patents 1,120,427 and 1,150,185, with catholyte and anolyte separated by a cation exchange membrane, chloride ions from the catholyte are, in practice, able to penetrate the membrane in sufficient numbers to give significant chlorine evolution at the anode~ This is not only environmentally undesirable but prevents the use of cheap lead anodes because of formation of lead chloride UK9-80~004X -2-UK9-8o-oo4E X
l thereon. Ins tead, platinized titanium anodes have had to be used. A ~urther problem with baths h3ving chloride anions in the catholyte is that pH stability is poor and needs frequent adjustment.
Disclosure of the Invention .
The above stated disadvantages of a chloride supporting electrolyte point to the use of a sulphate. Several examples of the use of sodium sulphate as a conductivity sal t for a supporting electrolyte are given in the above listed prior art~ This salt is cheap and readily soluble. No noxious anode gases are liberated and the pH stability of the bath is improved. However, the efficiency and plating current density ran~e,of trivalent chromium/thiocyanate plating baths employing sodium sulphate rather than the chloride are found to be materially reduced~ It is hypothesized that the reason for this deterioration in perfonmance may be complexing between the sulphate ions and the chromium-thiocyanate cGmplexes which tends to hinder mobility and electrochemical activity of the complexes in solution.
The present invention stems from the discovery that potassium sulphate as a conductivity salt for a supporting electrolyte does not cause such a deterioration in perfor-mance of the trivalent chromium plating process. Potassium sulphate had been suggested as a possible conductivity salt in US patent 4141803 but no examples of its use or suggestions of this advantage were given. Using potassium sulphate the efficiency of the ~ath was found to improYe.
, However it was also observed that, although plating was possible at much higher current densities than with the sodium sulphate bath, it was not possible at such low current densities as with the sodium sulphate bathO
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Barclay et al describe a related solution and process in which beneficial effects are obtained from a reduction in the level of chromium and thiocyanate concentration to levels well below those originally contemplated.
The equilibrated chromium (III) - thiocyanate complexes from which plating takes place have been prepared from a variety of starting materials. The originally preferred starting salts of U.K. Patent 1,431,639 were chromium perchlorate and sodium thiocyanate. In order to make the solution sufficiently ~lectrically conductive additional sodium perchlorate was added as a supporting electrolyte.
U.S. Patent 4,141,803 proposed hexathiocyanatochromium salts of potassium or sodium (K3Cr~NCS)6 or Na3Cr(NCS)6) to UK9-80-00~X -1-which sodium perchlorate or sodium sulphate was a~ded as a conductivity salt. Po~assium sulphate was also mentioned as a possible conductivity salt but no example was yiven. In U.S. Patent 4,161,432 one preferred solution was prepared from chromium chloride (CrC13) and sodium thiocyanate.
Potassium chloride was added for conductivity. A second preferred solution was prepared from chromium sulphate ~Cr2(SO4)3) and sodium thiocyanate. In this case sodium sulphate was added for conductivity.
In Canadian Patent 1,120,427, in which a catholyte and anolyte are separated by a membrane, the catholyte was prepared from chromium sulphate ICr2(SO4)3) and sodium thiocyanate, and sodium chloride was added for conductivity.
The anolyte consisted of an aqueous solution of a depolarising agent to which sodium sulphate (Na2SO4) was added for conductivity. The advantage of having sodium sulphate in the anolyte rather than sodium chloride is that chlorine evolution from the anode is very much reduced. The electrolyte employed in Canadian Patent 1,150,185 has essentially similar constituents to that of Canadian Patent 1,120,427 except that the concentration of chromium is below 0.03 molar and the concentration of thiocyanate is also proportionally reduced.
It is found that in plating chromium from electrolytes as described in Canadian Patents 1,120,427 and 1,150,185, with catholyte and anolyte separated by a cation exchange membrane, chloride ions from the catholyte are, in practice, able to penetrate the membrane in sufficient numbers to give significant chlorine evolution at the anode~ This is not only environmentally undesirable but prevents the use of cheap lead anodes because of formation of lead chloride UK9-80~004X -2-UK9-8o-oo4E X
l thereon. Ins tead, platinized titanium anodes have had to be used. A ~urther problem with baths h3ving chloride anions in the catholyte is that pH stability is poor and needs frequent adjustment.
Disclosure of the Invention .
The above stated disadvantages of a chloride supporting electrolyte point to the use of a sulphate. Several examples of the use of sodium sulphate as a conductivity sal t for a supporting electrolyte are given in the above listed prior art~ This salt is cheap and readily soluble. No noxious anode gases are liberated and the pH stability of the bath is improved. However, the efficiency and plating current density ran~e,of trivalent chromium/thiocyanate plating baths employing sodium sulphate rather than the chloride are found to be materially reduced~ It is hypothesized that the reason for this deterioration in perfonmance may be complexing between the sulphate ions and the chromium-thiocyanate cGmplexes which tends to hinder mobility and electrochemical activity of the complexes in solution.
The present invention stems from the discovery that potassium sulphate as a conductivity salt for a supporting electrolyte does not cause such a deterioration in perfor-mance of the trivalent chromium plating process. Potassium sulphate had been suggested as a possible conductivity salt in US patent 4141803 but no examples of its use or suggestions of this advantage were given. Using potassium sulphate the efficiency of the ~ath was found to improYe.
, However it was also observed that, although plating was possible at much higher current densities than with the sodium sulphate bath, it was not possible at such low current densities as with the sodium sulphate bathO
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1 Since there is a direct relationship bet~een current density and plating voltage for a given electrolyte, this higher minimum current density requirement dictates a higher minimum plating voltage.
-Accordingly, the present invention provides a chromiumelectroplating solution comprising an aqueous solution of chromium (III) - complexes as the source of chromium and a supporting electrolyte consisting essentially of a mixture of sodium and potassium sulphates in a concentration sufficient to provide electrical conductivity for the plating process.
By using a mixture of both these salts as the supporting electrolyte,~;~oth high efficiency and a wide plating range can be achieved without the need for high plating voltages.
In preEerred examples, efficiencies of up to 9.5~ (at 60 mAcm 2, 60 centigrade and pH 3.51 and a plating range of 10 - 1000 mAcm 2 have been achieved.
One reason for the beneficial effect of the potassium sulphate on efficiency and plating range is believed ~o be that the potassium preferentially ion-pairs with the sulphate in solution thus leaving the mobility of the chromium (III) complexes largely ~naffec-ted. To maximize the benefit, it is preferred that ~he potassium sulphate should be present in saturation concentration.
It is also preferred that the concentration of sodium sulphate is less than or equal to 1 Molar. Otherwise, with a i , greater proportion of sodium sulphate than this, efficiency hegins to fall off again. The optimum concentration of sodium sulphate appears to be around 0~5 Molar.
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oj 1 It has been found that a mixed sulphate electrolyte will be beneficial in any chromium (III) plating solution including, but not limited to, those containing thiocyanate.
In its broadest aspects, this invention comprises a chromium plating solution comprising an aqueous solution of chromium (III~ complexes as the source of chromium and a supporting electrolyte consisting essentially of a mixture of sodium and potassium sulphates in a concentration sufficient to provide electrical conductivity for the plating process.
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Considering now, in particular, a trivalent chromium/thiocyanate bath having anolyte and catholyte separa~ed by a cation exchange membrane, the basic reason for the use of such a membrane is to prevent anodic oxidation of bath constituents at the anode. As a result of the blocking of thiocyanate anions by the membrane, water, instead, is oxidised at the anode resulting in a steady input of hydrogen ions to the anolyte. The flux of these hydrogen ions through ,n the membrane into the catholyte is important in that it maintains the acidity of the catholyte which would otherwise decrease because of the steady evolution of hydrogen at the cathode. Thus the membrane acts to stabilize pH.
The presence of chloride ions in the catholyte but not the anolyte ~s believed to reduce this pH stabilizing effect on the catholyte somewhat. The reason for this is not entirely clear but could be connected with the concentration differential o chloride across the memhrane. AS noted above this leads to an inward flux of chloride ions to the anolyte. It is possible that the flux of chloride ions acts to reduce the outward flux of hydrogen ions from anolyte to catholyte. Also the rate of production of hydrogen ions in the anolyte by electrolysis of water will be reduced because of the preferential oxidation of the chloride ions.
This additional problem is solved according to another aspect of the present invention, without greatly affecting the bath efficiency, by providing a chromium electroplating bath comprising an anolyte and a catholyt~ separated by a cation exchange membrane, ~he c~tholyte being chloride free , and comprising an equilibrated aqueous solution of ~!
chromium (III) - thiocyanate complexes and a supporting elec- ~' trolyte comprising at least potassium sulphate in a concen .
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1 tration sufficient to provide electrical conductivity for the plating process, and the anolyte also being chloride free and comprisin~ sulphate ions in aqueous solution.
The plating range of an all potassium sulphate catholyte may be found inadequate in which case sodium sulphate is added in an amount sufficient to increase the range without reducing efficiency to an unacceptable degree.
Sulphate ions in the anolyte are preferably provided as an aqueous solution of sulphuric acidu One further important consequence of the chloride free bath i5 that its anode may be of lead rather than platinized titanium.
~uantitative results have been obtained from plating experiments performed in a Hull cell. The electrolyte employed was one of 0.012M chromium concentration including, thiocyanate and aspartic acid as complexants, the conductivity salts, and boric acid as a pH buffer.
In addition to Hull cell experiments, larger baths have been operated for periods of up to several months. In these baths both potassium sulphate alone and also a mixture of potassium and sodium sulphates have been used as conductivity salts. The larger baths have an anolyte and catholyte separated by a cation exchange membrane. Topping up of these baths with "chrometan" (hydrated chromium sulphate3 and thiocyanate anions replaces depleted chromium without altering the essential composition of the bath~ Adjustment of pH, when necessary, can be effected with a mixture of potassium and sodium hydroxides in the same proportion as the 29 conductivity salt mixtureO
l Detailed Description The invention will now be described further with reference to the following comparative examples and examples.
Comparative Example I
A concentrated chromium plating solution was first prepared in the following manner:-a) 60 grams of boric acid (H3BO3) were added to 750 mlof deionised water which was then heated and stirred to dissolve the boric acid.
b) 33.12 gràms of chromium sulphate (Cr2(SO4)3.15H2O) and 16.21 grams of sodium thio-cyanate (NaNCS) were added to the solution which was then heated and stirred at approximately 70C for about 30 minutes.
c) 16.625 grams of DL aspartic acid (NH2CH2CH(COOH)2) ~ere added to the solution which was then heated and stirred at approximately 75C for abut 3 hours. During this time the pH was adjusted from pH 1.5 to pH 3.0 very slowly with a 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 ~as added to the solution to make it approximately lM concentra~ion and 0.1 grams of FC 98*ta wetting agen~ produced by 3M Corporation) was also added. The solution was heated and stirred for a further 30 minutes.
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1 e) The solution pH was again adjusted to pH 3.0 with sodium hydroxide s~lution. -'-f) The solution was made up to 1 litre with deionised water which had been adjusted to pH 3.0 with a 10% by volume solution of hydrochloric acid.
The concentrated solution composition may be expressed as:-0.1 M chromium sulphate Cr2(SO4)3.15H70 0. 2 M sodium thiocyanate - NaNCS
0~125 M aspartic acid - NH2CH2CH(COOH)2 60 g/l boric acid - H3BO3 60 g/l so~`~ium chloride - NaCl ~;
0.1 g/l FC 98 - Iwetting agent product of 3M Corp) L
As a result of the equilibration process, the bul~ of the chromium in the final solution is believed to be in the form of chromium/thiocyanate/aspartic complexes.
' 120 mls of this solution were made up to 1 litre with a solution containing 60 grams per litre of boric acid and 60 E~
grams per litre of sodium chloride.
, The final solution composition (omitting the wetting agent~ was:-0.012 M chromium sulphate O ~ û24 M sodium thiocyanate 0.0'S M aspartic acid 60 g/l boric acid 60 g/l sodium chloride . ~ , ~4~
l This solution was introduced into a Hull cell having a standard brass Hull eell panel connected as a cathode and a platinized titanium anode. At a temperature of 60C and a solution pH adjusted to 3.5, a total current of 10 amps was passed through the Hull cell to produce a bright deposit of chromium on the test plate. To sustain the plating current required a voltage of 10.6 volts applied to the cell.
Examination of the Hull cell test panel indicated acceptably bright plating within a current density range of 10-700 mAcm 2. Efficiency measurements were made in a separate cell, employing an anode bag, and filled with a plating solution of the above eomposition as catholyte. The anode bag was a perfluorinated eation exehange membrane separating the catholyte from a separate anolyte eomprising an aqueous solution of ~ulphurie acid in 2~ by volume eoncentration. ~, The plating efficiency of this solution was calculated from the results of these separate experiments to be 8~ falling to 6% after plating for 4 Ampere hours per litre. The effieiency was measured at a eurrent density of 75 mAcm 2, a temperature of 60C and a pH of 3.5. Despite the memhrane ehloride ions were deteeted in the anolyte in eoneentrations up to approximately 0.5M, resulting in the evolution of ehlorine at the anode, furthermore the pH of the bath began to rise quickly and had to be adjusted frequently.
Comparative Example II
Two plating solutions were made up exactly as for Com-parative Example I exeept that sodium sulphate ~Na2SO4~ L
replaeed sodium ehloride as the ~onductivity salt2 One solution had a 1 molar eoneentration of sodium sulphate and . 30 the other had a 2 molar eoneentration. @
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UK9-80-004E 1~4~
1 The solutions were introduced as electrolytes into a Hull cell with the same anode as for Comparative Example I. Test panels were plated at 10 amps total current to produce bright chromium deposits. In all experiments, the temperature was 60C and the solution pH was adjusted to 3 5 i-For the lM sodium sulphate electrolyte, 15.2 volts were needed across the cell to sustain the current. The current density plating range in the Hull cell was 20-600 mAcm 2, For the 2M sodium sulphate electrolyte, 13.2 volts were needed to sustain the current of 10 amps. The plating range was reduced as compared with the chloride conductivity salt to 10-500 mAcm '.
In furthe~ experiments, efficiencies were measured in a separate cell ~aving an anode membrane and anolyte as or ~-Comparative Example 1 and employing the lM and 2M sodium sulphate plating solutions as catholytes. For the lM sodium sulphate catholyte, the initial efficiency of the solution, as measured at a current density o 50-55 mAcm 2, a temperature of 60C and a pH of 3.5 was 7.0%. For the 2M
sodium sulphate catholyte9 the initial efficiency measured separately under the same conditions as above was 7.5% but fell rapidly to a sustained efficiency of 4.53.
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Since no chloride was employed no chlorine could be evolved at the anode. However, the sustained efficiency and plating range of the sodium sulphate bath were reduced as compared with chloride bath. L
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l Exam~le I
A plating solution was made up in the manner of Com-parativ~ Example I except that potassium sulphate (K2504) replaced sodium chloride as the conductivity salt, potassium hydroxide was used instead of sodium hydroxide and potassium thiocyanate replaced sodium thiocyanate. The potassium sùlphate was present in saturation concentration and was prepared from potassium hydrogen sulphate.
This plating solution was introduced, as the catholyte, into a cell having the same anode, anolyte and membrane arrangement as for the Comparative Examples.
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Efficien~ measurements were made at a current density of 50-55 mAcm a, a temperature of 60C and an adjusted pH of 3.5. The initial efficiency of the solution was measured to be 9~ and fell only to 8.5~ over a long period of time.
Thus, a bath employing potassium sulphate for conductivity has significantly better current efficiency than one --employing sodium sulphate (c.f. ComparatiYe Example II~. ~
The pH stabili~y of this bath is also better than the ~i bath of Comparative Example I. The solution pH only rose from 3.5 to 4.0 a~ter 40 ampere hours per litre of charge had passed. It was then adjusted back to 3.5 using sulphuric acid. It will be recalled that the membrane acts to stabilize pH by allowing electrolysis of water at the anode instead of other reactions which would occur preferentially with catholyte components~ The hydrolysis produces hydrogen ions which can pass through the membrane to replace those lost by hydrogen evolution at the cathode. It is believed 29 that since sulphate will not pass through the membrane, the ? ~_ _ 1 flux of hydrogen ions is greater than it would be with chloride in the catholyte. Also sulphate, unlike chloride does not preferentially oxidise at the anode thereby allowing the maximum number of hydrogen ions to be generated.
In order to determine platïng range and minimum plating voltage, the plating solution of this example was introduced as the electrolyte into a Hull cell. Test panels were plated at a total current of 10 amps to produce bright chromium deposits. The solution temperature was 60C and its pH was adjusted to 3.5. A voltage of 11.9 volts was needed to sustain this plating current. The plating range in th~ Hull cell was from 25 to approximately 1000 m~cm ~ The upper limit could not be precisely determined because the test plate was plated right~to the top edge. As compared with a bath employing sodium sulpha-te for conductivity, a bath employing potassium sulphate has an extended upper limit of plating current density but the lower threshold for plating was raised.
Thus potassium sulphate has advantages as a conductivity salt particularly in a bath with a membrane. It does however have the disadvantage that the lower end of the plating range is rather high at 25 mAcm ~. As explained earlier this higher minimum current density requirement implies a high~r minimum plating voltage than would otherwise be required.
This may be a disadvantage in a working environmen~ where there is only a limited supply voltage available.
Example II
A plating solution was made up in the manner of Example I
29 but, in addifion to the potassium sulphate in 1 Molar ~:. ~J
l concentration, sodium sulphate was also added in 0.5 Molar concentration.
The mixed conductivity salt plating solution was intro-duced into an electroplating cell as the catholyte with the same anode, anolyte and membrane arrangement as for the previous examples. The initial efficiency of plating was measured, under the same conditions as for Example I, to be 8~.
In separate experiments, the same plating solution was introduced as the electrolyte into a Hull cell under the same conditions as for Example Io Test panels were plated at a total cell current of 10 amps to produce bright chromium deposits. A~oltage of 11.2 volts was needed to sustain this current. The plating range in the Hull cell was from 10 to approximately 100-0 mAcm a. ~his is wider than for Example L
I or Comparative Examples I and II. This implies a significantly lower minimum voltage for satisfactory plating in a working bath than would be needed for an all potassium bath. Thus, a bath employing a mixture of sodium and potassium sulphate as conductivity salts has both high efficiency and good plating range while overcoming the deficiencies of chloride conductivity salts.
Example III
Several plating solutions were made up in the manner of Example II but having different concentrations of sodium sulphate. -le Plating experiments were conducted in the manner of 28 Example II. In each case, he voltage needed to sustain a 1~ L
current of 10 amps and the current density plating range were determined in a Hull cell. The initial plating efficiencies were determined under the same conditions as for Example I, in a separate cell employing an anode membrane. Sustained efficiencies were not measured.
The following results were obtained:-Sodium Hull cell Plating Initial sulphate voltage Range Efficiency concentration mAcm 2 %
0.1 M 11.6 20-1000 7-8 0.3 M 11.3 10-1000 7-8 1.0 M 11~2 10-700 6 Example IV
A plating solution was made up in the manner of Example II but with the differenee that sodium thiocyanate, rather than potassium thioeyanate was employed. Another difference was that the concentration of boric acid was increased from 60 to 75 g/l.
Expressed in terms of its initial eonstituents the composition of the solution was:-0.012 M ehromium sulphate 0.012 M sodium thioeyanate 0.015 M aspartic acid 75 g/l boric aeid 0.5 M sodium sulphate 1.0 M potassium sulphate ..j~..
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1 ~ull cell experiments were conducted at a temperatUFe of 60C and a solution pH adjusted to 3.5. The plating range was 10 to approximately 1000 mAcm ~. Since the supporting electrolyte is the same as for Example II, this implies that a similar plating voltage as for Example II would be necessary to sustain an overali current of 10 amps, though this voltage was not, in fact, measured.
However, the initial efficiency measured separately in the manner of Example II, improved to 9.5%. The solution temperature was again 60C and the solution pH was 3.5 but the current density was 60 mAcm lr It was also observed that the bright chromium deposits produced in ~ese experiments were ligh~er in colour than tho~e produced in Example II.
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l The following comparative e~ample illllstra'_es t.-le beneficial effects of the mixed sulphate electrolyte ir. -particular embodiment.
CO~IPARATIVE EX~IPLE III
Comparative examples were carried out on a chromi~m (III)-thiourea bath employing a sodium sulphate electrol~te and on the same bath employing mixed sodium and potassium electrolytes. The benefit of mixed electrolytes was demonstrated in that the hull cell plating voltage was reduced from 17 volts to 13.5 volts. Two sulphate baths were made up, the first of composition 14 gm/L malic acid, 75 gm/L boric acid, 33 gm/L Cr2(SO4)3.15H2O, 50 mg/L
thiourea and 1.5M Na2SO4. The other of the same composition except that the molarity of the Na2SO~ was reduced to 0.5M, and the bath was made lM in K2SO4. Both baths had tergitol*
08 as we~ing agent and both were of pH 3.85.
The first bath (just Na2SO4) was warmed to 54C, and a hull cell was filled with it. lA was passed for two minutes with a voltage of 4.5Vo With another brass plate, with the same solution, a lOA test plate was run for two minutes at 50C, this time with a voltage of 17V.
The second plating bath was warmed to 52C, and this was added to the cleaned hull cell. A lA test plate was run for 2 minutes 20 seconds with this solution at a voltage of 4V. With the same solution and another brass plate, a lOA
test plate was run at 50C for 2 minutes, with a voltage of 13.5V.
Both solutions plated over a comparable range with the same quality deposit. Eloweverl the voltage of the mixed sulphate bath was significantly lower than that of the bath containing only Na2SO~. Plating current densities ranged - from 20-800 mA/cm2.
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1 Additional examples of other particular embodiments of the invention follow:
In following three groups of examples, a bath consisting of anolyte separated from a catholyte by a Nafion cation exchange membrane is used. The anolyte comprises an aqueous solution of sulphuric acid in 2% by volume concentration (pH 1.6). The anode is a flat bar of a lead alloy of the type conventionally used in hexavalent chromium plating processes.
The catholyte for each Example was prepared by making up a base electrolyte and adding appropriate amounts of chromium (III), complexant and the organic compound.
The base electrolyte for each example consisted of the following constituents dissolved in 1 litre of water:
Potassium sulphate lrl Sodium sulphate 0.05M
Boric Acid lM
Wetting Agent FC98 0.1 gram Example 1 The following constituents were dissolved in the base electrolyte:
Chromium ¦III) lOmM (from chromium) DL aspartic acid 10~1 Thiourea lmM
at pH 3.5 Although equilibration will occur quickly in normal use, initially the electrolyte is preferably equilibrated until there are no spectroscopic changes which can be detected. The bath was found to operate over a temperature range of 25 to 60C. Good bright deposits of chromium were obtained over a current density range of 5 to 800 mA/cm2.
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1 EY~ample 2 The following constituents were dissolved in the base electrolyte:
Chromium (III) lOmM (from chrometan) Iminodiacetic acid lOmM
Thiourea lm~1 at pH 3.5 The electrolyte is preferably equilibrated until there are no spectroscopic changes. The bath was found to operate over a temperature range of 25 to 60C. Good bright deposits of chromium were obtained.
Example 3 I
The following constituents were dissolved in the base 15 electrolyte:
Chromium ~III3 lOOmM (from chrometan3 DL Aspartic acid lOOmM
Mercaptoacetic acid lmM
at pH 3.5 The electrolyte is preferably equilibrated until there are no spectroscopic changes. The bath was found to operate over a temperature range of 25 to 60C. Good bright deposits were obtained.
:, Example 4 The following constituents were dissolved in the base electrolyte:
Chromium (III) lOOmM (from chrometan) DL Aspartic acid lOOmM
Thiourea lm~l at p~l 3.5 The electrolyte is preferably equilibrated until there are no spectroscopic changes. The bath was found to operate over a temperature range of 25 to 60C. Good bright deposits were obtained over a current density range of 10 to 800 mA/cm2.
The above examples illustrate a chromium electroplating electrolyte containing a source of trivalent chromium ions, a complexant, a buffer agent and organic compound having a -C=S
group or a -C-S yroup within the molecule for promoting chromium deposition, the complexant being selected so that the stability constant Kl of the chromium complex as defined herein ls in the range 10 ~ K1 ~ 10 M
By way of example, complexant ligands having K1 values within the range 108~ K1 < 1012 M 1 include aspartic acid, iminodiacetic acid, nitrilotriacetic acid and 5-sulphosalicylic acid.
The organic compound haviny -C-S group can be selected from thiourea, N-monoallyl thiourea, N-mono-p-tolyl thiourea, thioacetamide, tetramethyl thiuram monosulphide, tetraethyl thiuram disulphide and diethyldithiocarbonate. The organic compound having a -C-S~ group can be selected from mercaptoacetic acid and mercaptopropionic acid.
Since the plating efficiency of the electrolyte is relatively high, a commercial trivalent chromium electrolyte ean have as low as 5mM chromium. This removes the need for expensive rinse water treatment sinee the chromium content of the 'drag-out' from the plating electrolyte is extremely low.
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1 In general, the concentration of 'che constituents in the electrolyte are as follows:
Chromium (III) ions 10 to 1l`i Organic compound 10 to 10 M
A practical chromium/complexant ligand ratio is approximately 1:1.
Above a minimum concentration necessary for acceptable plating rates, it is unnecessary to increase the amount of the organic compound in proportion to the concentration of chromium in the electrolyte. Excess of the organic compound may not be harmful to the plating process but can result in an increased amount of sulphur being co-deposited with the chromium metal. This has two effects, firstly to produce a progressively darker deposit and, secondly, to produce a more ductile deposit.
GROUP II
~
The following constituents were dissolved in the base electrolyte:
Chromium (III) lOm~l (from chrometan) DL aspartic acid 1OmM
Sodium thiosulphate lmM
at pH 3.5 Although equilibration will occur quickly in normal use, initially the electrolyte is preferably equilibrated until there are no spectroscopic changes which can be detected. The bath was found to operate over a temperature range of 25 to 60C. Good bright deposits of chromium were obtained over a current density range of 10 to 800 mA/cm2.
1 Example 2 The following constituents were dissolved in the base electrolyte:
Chromium ~III) lOmM ~from chrometan) Iminodiacetic acid lOmM
Sodium thionate lmM
at pH 3.5 The electrolyte is preferrably equilibrated until there are no spectroscopic changes. The bath was found to operate over a temperature range of 25 to 60C. Good bright deposits of chromium were obtained.
Example 3 The following constituents were dissolved in the base electrolyte:
Chromium (III) lOOmM (from chrometan) DL aspartic acid lOOmM
Sodium thiosulphate ln~l at pH 3.5 The electrolyte is preferably equilibrated until there are no spectroscopic changes. The bath was found to operate over a temperature range of 25 to 60C. Good bright deposits were obtained.
1 Example 4 The following constituents were dissolved in the base electrolyte:
Chromium (III) lOOmM (from chrometan) DL aspartic acid lOOmM
Sodium thionate lm~l at pH 3.5 The electrolyte is preferably equilibrated until there are no spec-troscopic changes. The bath was found to operate over a temperature range of 25 to 60C. Good bright deposits were obtained over a current density range of 10 to 800 mA/cm2.
The above examples illustrate a chromium electroplating electrolyte containing a source of trivalent chromium ions, a complexant, a buffer agent and a sulphur species having S-O or S-S bonds for promoting chromium deposition, -the complexant being selected so that the stability constant K
of the chromium complex as defined herein is in the range 106 C Kl ~ 1012 M 1 and the sulphur species being selected from thiosulphates, thionates, polythionates and sulfoxylates.
By way of example complexant ligands having Kl values within the range 106 ~ Kl ~ 1012 M 1 include aspartic acid, iminodiacetic acid, nitrilotr~acetic acid, 5-sulphosalicylic acid and citric acid.
The sulphur species are provided by dissolving one or more of the following in the electrolyte: sodium thiosulphate, potassium thio sulphate, barium thiosulphate, ammonium thiosulphate, calcium thiosulphate, potassium polythionate, sodium polythionate, and sodium sulfoxylate.
1 Very low concentrations of the sulphur species are needed to promote reduction of the trivalent chromium ions.
Also since the plating efficiency of the electrolyte is rela-tively high a commercial trivalent chromium electrolyte can have as low as 5m~ chromium. This removes the need for expensive rinse water treatment since the chromium content o~ the 'drag~out' from the plating electrolyte is extremely low.
In general, the concentration of constituents in the electrolyte are as follows.
Chromium (III) ions 10 to 1~1 Sulphur species 10 to 10 M
A practical chromium/complexant ligand ratio is approximately 1:1.
Above a minimum concen-tration necessary for acceptable plating ranges, it is unnecessary to increase the amount of the sulphur species in proportion to the concentration of chromium in the electrolyte. Excess of the sulphur species may not be harmful to the plating process but can result in an increased amount of sulphur being co-deposited with the chromium metal. This has two effects, firstly to produce a progressively darker deposit and, secondly, to produce a more ductile deposit.
GROUP III
,, _ The following constituents were dissolved in the base electrolyte:
Chromium (III) 5mM (from chrometan~
DL aspartic acid SmM
Sodium sulphite 5mM
at pH 3.5 Although equilibration wil:L occur quickly in normal use, initially the electrolyte is preferably equilibrated until there are no spectroscopic changes which can be detected. The bath was found to operate over a temperature range of 25 to 60C. Good bright deposits of chromium were obtained over a current density range o~ 10 to ~00 n~/cm2.
1 Example 2 The following constituents were dissolved in the base electrolyte:
Chromium tIII~ 5mM (from chrometan) Iminodiacetic acid 5mM
Sodium dithionite 2mM
at pH 3.5 The electrolyte is preferrably equilibrated until there are no spectroscopic changes. The bath was found to operate over a temperature range of 25 to 60C. Good bright deposits of chromium were obtained.
Exam~le 3 The following constituents were dissolved in the base electrolyte:
Chromium (III~ 50mM (from chrometanJ
DL Aspartic acid 50mL~
Sodium sulphite lOmM
at pH 3.5 The electrolyte is preferably equilibrated until there are no spectroscopic changes. The bath was found to operate over a temperature range of 25 to 60C. Good bright deposits were obtained.
Example ~
The following constituents were dissolved in the base electrolyte:
Chromium (IIX) 50mM (from chrometan) 5-sulphosalicylic acid 50mM
Sodium sulphite lmM
at pH 3,5 The electrolyte is preferrably equilibrated until there are no spectroscopic changes. The bath was found to operate over a temperature range of 25 to 60C. Good bright deposits were obtained.
1 The abov~ examples illustrate a chromium electroplating electrolyte containing a source of trivalent chromium ions, a complexant, a buffer agent and a sulphur species haviny selected frorn sulphites and dithionites for promoting chromium deposition, the complexan-t being selec-ted so that the stability cons-tan-t K1 of the chromium complex as defined herein is in the range 106 ~ K1 ~ 1012 M 1 and the chromium ions having a molar concentration lower than O.OlM.
By way of example, complexant ligands having Kl values within the range 106 ~ K1 ~ 1012 M 1 include aspar-tic acid, iminodiacetic acid, nitrilotriacetic acid, 5~sulphosalicylic acid and citric acid.
The above examples also illustrate a chromium electrolyte containing trivalent chromium ions, a complexantt a buffer agent and a sulphur species selected from sulphites and dithionites, the complexant being selected from aspartic acid, 5-sulphosalicylic acid and citric acid.
The above examples further illustrate a chromium electroplating bath comprising an anolyte separated from a catholyte by a perfluorinated cation exchange membrane, the anolyte comprising sulpha~e ions and the catholyte comprising a source of trivalent chromium ions, a complexant, a buffer agent and a sulphur species selected from sulphites and dithionites, and in which the source of sulphate ions is chromium sulphate. Suitable complexant ligands are aspartic acid, iminodiacetic acid, nitrilotriacetic acid, and 5-sulphosalicylic acid and citric acid.
Sulphites can include blsulphites and metabisulphites.
l Low concentrations of sulphite or dithionite are needed to promote reduction of the trivalent chromium ions. Also since the plating efficiency of the electrolyte is relatively high a commercial trivalent chromium electrolyte can have less than 10mM chromium. This removes the need for expensive rinse water treatment since the chromium content of the 'drag-out' from the plating electroly-te is extremely low.
In general, the concentration of the cons-tituents in the electroly-te are as follows:
Chromium (III) ions 10 to lM
Sulphur species 10 to 10 M
A practical chromium/complexant ligand ratio is approximately 1:1.
Above a minimum concentration necessary for acceptable plating rates, it is unnecessary to increase the amount of -the sulphur species ln proportion to the concentration of chromium in the electrolyte. Excess of sulphite or dithionite may not be harmful to the plating process but can result in an increased amount of sulphur being co-deposited with the chromium metal. This has two effects, firstly to produce a progressively darker deposit and, secondly, to produce a more ductile deposit.
The preferred source of trivalent chromium is chromium sulphate which can be in the form of a commercially available mixture of chromium and sodium sulphates known as -tanning liquor or chrometan. Other trivalent chromium sal-ts, which are more expensive than the sulphate~ can be used, and include chromium chloride, carbonate and perchlorate.
l The preferred buffer agent used to maincain the pH of the bulk electrolyte comprises boric acid in high concentrations, iOe., near saturation. Typical pH range for the electroly-te is in the range 2.5 to 4.5.
The conductivity of the electrolyte should be as high as possible to minimize both voltage and power consumption.
Voltage is often critical in practical plating environments since rectifiers are often limited to a low voltage, e.g., 8 volts. In an electrolyte in which chromium sulphate is the source of the trivalent chromium ions a mixture of sodium and potassium sulphate is the optimum. Such a mixture is described in United Kingdom Patent 2,071,151.
A wetting agent is desirable and a suitable wetting agent is FC98, a product of the 3M Corporation. However, other wettinc3 agents such as sulphosuccina-tes or alcohol sulphates may be used.
It is preferred to use a perfluorinated cation exchange membrane to separate the anode from the plating electrolyte as described ln United Kingdom Patent 1,602,404. A suitable perfluorinated cation exchange membrane is Naf iOII tTrade Mark), a product of the Du Pont Corporation. It is particularly advantageous to employ an anolyte which has sulphate ions when the catholyte uses chromium sulphate as the source of chromium since inexpensive lead or lead alloy anodes can be used. In a sulphate anolyte, a thin conducting layer of lead oxide is formed on the anode.
Chloride salts in the catholyte should be avoided since the chloride anions are small enough to pass through the membrane ln sufficient amount to cause both the evolution of chlorine at the anode and the formation of a highly resistive film of lead chloride on lead or lead alloy anodes. Cation exchange membranes have the additional advantage in sulphate electrolytes that the pH of the catholyte can be stabilized by adjusting the pH of the anolyte to a:Llow hydrogen ion transport through the membrane to comp~nsate for the increase in pH of the catholyte by hydrogen evolution at the cathode. Using the combination of a membrane, and sulphate based anolyte and ca-tholyte a plating bath has been operated for over 40 Amphours/litre without pH adjustment.
!
1 Since there is a direct relationship bet~een current density and plating voltage for a given electrolyte, this higher minimum current density requirement dictates a higher minimum plating voltage.
-Accordingly, the present invention provides a chromiumelectroplating solution comprising an aqueous solution of chromium (III) - complexes as the source of chromium and a supporting electrolyte consisting essentially of a mixture of sodium and potassium sulphates in a concentration sufficient to provide electrical conductivity for the plating process.
By using a mixture of both these salts as the supporting electrolyte,~;~oth high efficiency and a wide plating range can be achieved without the need for high plating voltages.
In preEerred examples, efficiencies of up to 9.5~ (at 60 mAcm 2, 60 centigrade and pH 3.51 and a plating range of 10 - 1000 mAcm 2 have been achieved.
One reason for the beneficial effect of the potassium sulphate on efficiency and plating range is believed ~o be that the potassium preferentially ion-pairs with the sulphate in solution thus leaving the mobility of the chromium (III) complexes largely ~naffec-ted. To maximize the benefit, it is preferred that ~he potassium sulphate should be present in saturation concentration.
It is also preferred that the concentration of sodium sulphate is less than or equal to 1 Molar. Otherwise, with a i , greater proportion of sodium sulphate than this, efficiency hegins to fall off again. The optimum concentration of sodium sulphate appears to be around 0~5 Molar.
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oj 1 It has been found that a mixed sulphate electrolyte will be beneficial in any chromium (III) plating solution including, but not limited to, those containing thiocyanate.
In its broadest aspects, this invention comprises a chromium plating solution comprising an aqueous solution of chromium (III~ complexes as the source of chromium and a supporting electrolyte consisting essentially of a mixture of sodium and potassium sulphates in a concentration sufficient to provide electrical conductivity for the plating process.
L/~ ~ ~
Considering now, in particular, a trivalent chromium/thiocyanate bath having anolyte and catholyte separa~ed by a cation exchange membrane, the basic reason for the use of such a membrane is to prevent anodic oxidation of bath constituents at the anode. As a result of the blocking of thiocyanate anions by the membrane, water, instead, is oxidised at the anode resulting in a steady input of hydrogen ions to the anolyte. The flux of these hydrogen ions through ,n the membrane into the catholyte is important in that it maintains the acidity of the catholyte which would otherwise decrease because of the steady evolution of hydrogen at the cathode. Thus the membrane acts to stabilize pH.
The presence of chloride ions in the catholyte but not the anolyte ~s believed to reduce this pH stabilizing effect on the catholyte somewhat. The reason for this is not entirely clear but could be connected with the concentration differential o chloride across the memhrane. AS noted above this leads to an inward flux of chloride ions to the anolyte. It is possible that the flux of chloride ions acts to reduce the outward flux of hydrogen ions from anolyte to catholyte. Also the rate of production of hydrogen ions in the anolyte by electrolysis of water will be reduced because of the preferential oxidation of the chloride ions.
This additional problem is solved according to another aspect of the present invention, without greatly affecting the bath efficiency, by providing a chromium electroplating bath comprising an anolyte and a catholyt~ separated by a cation exchange membrane, ~he c~tholyte being chloride free , and comprising an equilibrated aqueous solution of ~!
chromium (III) - thiocyanate complexes and a supporting elec- ~' trolyte comprising at least potassium sulphate in a concen .
.~
1 tration sufficient to provide electrical conductivity for the plating process, and the anolyte also being chloride free and comprisin~ sulphate ions in aqueous solution.
The plating range of an all potassium sulphate catholyte may be found inadequate in which case sodium sulphate is added in an amount sufficient to increase the range without reducing efficiency to an unacceptable degree.
Sulphate ions in the anolyte are preferably provided as an aqueous solution of sulphuric acidu One further important consequence of the chloride free bath i5 that its anode may be of lead rather than platinized titanium.
~uantitative results have been obtained from plating experiments performed in a Hull cell. The electrolyte employed was one of 0.012M chromium concentration including, thiocyanate and aspartic acid as complexants, the conductivity salts, and boric acid as a pH buffer.
In addition to Hull cell experiments, larger baths have been operated for periods of up to several months. In these baths both potassium sulphate alone and also a mixture of potassium and sodium sulphates have been used as conductivity salts. The larger baths have an anolyte and catholyte separated by a cation exchange membrane. Topping up of these baths with "chrometan" (hydrated chromium sulphate3 and thiocyanate anions replaces depleted chromium without altering the essential composition of the bath~ Adjustment of pH, when necessary, can be effected with a mixture of potassium and sodium hydroxides in the same proportion as the 29 conductivity salt mixtureO
l Detailed Description The invention will now be described further with reference to the following comparative examples and examples.
Comparative Example I
A concentrated chromium plating solution was first prepared in the following manner:-a) 60 grams of boric acid (H3BO3) were added to 750 mlof deionised water which was then heated and stirred to dissolve the boric acid.
b) 33.12 gràms of chromium sulphate (Cr2(SO4)3.15H2O) and 16.21 grams of sodium thio-cyanate (NaNCS) were added to the solution which was then heated and stirred at approximately 70C for about 30 minutes.
c) 16.625 grams of DL aspartic acid (NH2CH2CH(COOH)2) ~ere added to the solution which was then heated and stirred at approximately 75C for abut 3 hours. During this time the pH was adjusted from pH 1.5 to pH 3.0 very slowly with a 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 ~as added to the solution to make it approximately lM concentra~ion and 0.1 grams of FC 98*ta wetting agen~ produced by 3M Corporation) was also added. The solution was heated and stirred for a further 30 minutes.
* Trade ~ark .,. ,~
_ _ .
1 e) The solution pH was again adjusted to pH 3.0 with sodium hydroxide s~lution. -'-f) The solution was made up to 1 litre with deionised water which had been adjusted to pH 3.0 with a 10% by volume solution of hydrochloric acid.
The concentrated solution composition may be expressed as:-0.1 M chromium sulphate Cr2(SO4)3.15H70 0. 2 M sodium thiocyanate - NaNCS
0~125 M aspartic acid - NH2CH2CH(COOH)2 60 g/l boric acid - H3BO3 60 g/l so~`~ium chloride - NaCl ~;
0.1 g/l FC 98 - Iwetting agent product of 3M Corp) L
As a result of the equilibration process, the bul~ of the chromium in the final solution is believed to be in the form of chromium/thiocyanate/aspartic complexes.
' 120 mls of this solution were made up to 1 litre with a solution containing 60 grams per litre of boric acid and 60 E~
grams per litre of sodium chloride.
, The final solution composition (omitting the wetting agent~ was:-0.012 M chromium sulphate O ~ û24 M sodium thiocyanate 0.0'S M aspartic acid 60 g/l boric acid 60 g/l sodium chloride . ~ , ~4~
l This solution was introduced into a Hull cell having a standard brass Hull eell panel connected as a cathode and a platinized titanium anode. At a temperature of 60C and a solution pH adjusted to 3.5, a total current of 10 amps was passed through the Hull cell to produce a bright deposit of chromium on the test plate. To sustain the plating current required a voltage of 10.6 volts applied to the cell.
Examination of the Hull cell test panel indicated acceptably bright plating within a current density range of 10-700 mAcm 2. Efficiency measurements were made in a separate cell, employing an anode bag, and filled with a plating solution of the above eomposition as catholyte. The anode bag was a perfluorinated eation exehange membrane separating the catholyte from a separate anolyte eomprising an aqueous solution of ~ulphurie acid in 2~ by volume eoncentration. ~, The plating efficiency of this solution was calculated from the results of these separate experiments to be 8~ falling to 6% after plating for 4 Ampere hours per litre. The effieiency was measured at a eurrent density of 75 mAcm 2, a temperature of 60C and a pH of 3.5. Despite the memhrane ehloride ions were deteeted in the anolyte in eoneentrations up to approximately 0.5M, resulting in the evolution of ehlorine at the anode, furthermore the pH of the bath began to rise quickly and had to be adjusted frequently.
Comparative Example II
Two plating solutions were made up exactly as for Com-parative Example I exeept that sodium sulphate ~Na2SO4~ L
replaeed sodium ehloride as the ~onductivity salt2 One solution had a 1 molar eoneentration of sodium sulphate and . 30 the other had a 2 molar eoneentration. @
.
UK9-80-004E 1~4~
1 The solutions were introduced as electrolytes into a Hull cell with the same anode as for Comparative Example I. Test panels were plated at 10 amps total current to produce bright chromium deposits. In all experiments, the temperature was 60C and the solution pH was adjusted to 3 5 i-For the lM sodium sulphate electrolyte, 15.2 volts were needed across the cell to sustain the current. The current density plating range in the Hull cell was 20-600 mAcm 2, For the 2M sodium sulphate electrolyte, 13.2 volts were needed to sustain the current of 10 amps. The plating range was reduced as compared with the chloride conductivity salt to 10-500 mAcm '.
In furthe~ experiments, efficiencies were measured in a separate cell ~aving an anode membrane and anolyte as or ~-Comparative Example 1 and employing the lM and 2M sodium sulphate plating solutions as catholytes. For the lM sodium sulphate catholyte, the initial efficiency of the solution, as measured at a current density o 50-55 mAcm 2, a temperature of 60C and a pH of 3.5 was 7.0%. For the 2M
sodium sulphate catholyte9 the initial efficiency measured separately under the same conditions as above was 7.5% but fell rapidly to a sustained efficiency of 4.53.
.
Since no chloride was employed no chlorine could be evolved at the anode. However, the sustained efficiency and plating range of the sodium sulphate bath were reduced as compared with chloride bath. L
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l Exam~le I
A plating solution was made up in the manner of Com-parativ~ Example I except that potassium sulphate (K2504) replaced sodium chloride as the conductivity salt, potassium hydroxide was used instead of sodium hydroxide and potassium thiocyanate replaced sodium thiocyanate. The potassium sùlphate was present in saturation concentration and was prepared from potassium hydrogen sulphate.
This plating solution was introduced, as the catholyte, into a cell having the same anode, anolyte and membrane arrangement as for the Comparative Examples.
~.
Efficien~ measurements were made at a current density of 50-55 mAcm a, a temperature of 60C and an adjusted pH of 3.5. The initial efficiency of the solution was measured to be 9~ and fell only to 8.5~ over a long period of time.
Thus, a bath employing potassium sulphate for conductivity has significantly better current efficiency than one --employing sodium sulphate (c.f. ComparatiYe Example II~. ~
The pH stabili~y of this bath is also better than the ~i bath of Comparative Example I. The solution pH only rose from 3.5 to 4.0 a~ter 40 ampere hours per litre of charge had passed. It was then adjusted back to 3.5 using sulphuric acid. It will be recalled that the membrane acts to stabilize pH by allowing electrolysis of water at the anode instead of other reactions which would occur preferentially with catholyte components~ The hydrolysis produces hydrogen ions which can pass through the membrane to replace those lost by hydrogen evolution at the cathode. It is believed 29 that since sulphate will not pass through the membrane, the ? ~_ _ 1 flux of hydrogen ions is greater than it would be with chloride in the catholyte. Also sulphate, unlike chloride does not preferentially oxidise at the anode thereby allowing the maximum number of hydrogen ions to be generated.
In order to determine platïng range and minimum plating voltage, the plating solution of this example was introduced as the electrolyte into a Hull cell. Test panels were plated at a total current of 10 amps to produce bright chromium deposits. The solution temperature was 60C and its pH was adjusted to 3.5. A voltage of 11.9 volts was needed to sustain this plating current. The plating range in th~ Hull cell was from 25 to approximately 1000 m~cm ~ The upper limit could not be precisely determined because the test plate was plated right~to the top edge. As compared with a bath employing sodium sulpha-te for conductivity, a bath employing potassium sulphate has an extended upper limit of plating current density but the lower threshold for plating was raised.
Thus potassium sulphate has advantages as a conductivity salt particularly in a bath with a membrane. It does however have the disadvantage that the lower end of the plating range is rather high at 25 mAcm ~. As explained earlier this higher minimum current density requirement implies a high~r minimum plating voltage than would otherwise be required.
This may be a disadvantage in a working environmen~ where there is only a limited supply voltage available.
Example II
A plating solution was made up in the manner of Example I
29 but, in addifion to the potassium sulphate in 1 Molar ~:. ~J
l concentration, sodium sulphate was also added in 0.5 Molar concentration.
The mixed conductivity salt plating solution was intro-duced into an electroplating cell as the catholyte with the same anode, anolyte and membrane arrangement as for the previous examples. The initial efficiency of plating was measured, under the same conditions as for Example I, to be 8~.
In separate experiments, the same plating solution was introduced as the electrolyte into a Hull cell under the same conditions as for Example Io Test panels were plated at a total cell current of 10 amps to produce bright chromium deposits. A~oltage of 11.2 volts was needed to sustain this current. The plating range in the Hull cell was from 10 to approximately 100-0 mAcm a. ~his is wider than for Example L
I or Comparative Examples I and II. This implies a significantly lower minimum voltage for satisfactory plating in a working bath than would be needed for an all potassium bath. Thus, a bath employing a mixture of sodium and potassium sulphate as conductivity salts has both high efficiency and good plating range while overcoming the deficiencies of chloride conductivity salts.
Example III
Several plating solutions were made up in the manner of Example II but having different concentrations of sodium sulphate. -le Plating experiments were conducted in the manner of 28 Example II. In each case, he voltage needed to sustain a 1~ L
current of 10 amps and the current density plating range were determined in a Hull cell. The initial plating efficiencies were determined under the same conditions as for Example I, in a separate cell employing an anode membrane. Sustained efficiencies were not measured.
The following results were obtained:-Sodium Hull cell Plating Initial sulphate voltage Range Efficiency concentration mAcm 2 %
0.1 M 11.6 20-1000 7-8 0.3 M 11.3 10-1000 7-8 1.0 M 11~2 10-700 6 Example IV
A plating solution was made up in the manner of Example II but with the differenee that sodium thiocyanate, rather than potassium thioeyanate was employed. Another difference was that the concentration of boric acid was increased from 60 to 75 g/l.
Expressed in terms of its initial eonstituents the composition of the solution was:-0.012 M ehromium sulphate 0.012 M sodium thioeyanate 0.015 M aspartic acid 75 g/l boric aeid 0.5 M sodium sulphate 1.0 M potassium sulphate ..j~..
~K9~80-004EX
1 ~ull cell experiments were conducted at a temperatUFe of 60C and a solution pH adjusted to 3.5. The plating range was 10 to approximately 1000 mAcm ~. Since the supporting electrolyte is the same as for Example II, this implies that a similar plating voltage as for Example II would be necessary to sustain an overali current of 10 amps, though this voltage was not, in fact, measured.
However, the initial efficiency measured separately in the manner of Example II, improved to 9.5%. The solution temperature was again 60C and the solution pH was 3.5 but the current density was 60 mAcm lr It was also observed that the bright chromium deposits produced in ~ese experiments were ligh~er in colour than tho~e produced in Example II.
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l The following comparative e~ample illllstra'_es t.-le beneficial effects of the mixed sulphate electrolyte ir. -particular embodiment.
CO~IPARATIVE EX~IPLE III
Comparative examples were carried out on a chromi~m (III)-thiourea bath employing a sodium sulphate electrol~te and on the same bath employing mixed sodium and potassium electrolytes. The benefit of mixed electrolytes was demonstrated in that the hull cell plating voltage was reduced from 17 volts to 13.5 volts. Two sulphate baths were made up, the first of composition 14 gm/L malic acid, 75 gm/L boric acid, 33 gm/L Cr2(SO4)3.15H2O, 50 mg/L
thiourea and 1.5M Na2SO4. The other of the same composition except that the molarity of the Na2SO~ was reduced to 0.5M, and the bath was made lM in K2SO4. Both baths had tergitol*
08 as we~ing agent and both were of pH 3.85.
The first bath (just Na2SO4) was warmed to 54C, and a hull cell was filled with it. lA was passed for two minutes with a voltage of 4.5Vo With another brass plate, with the same solution, a lOA test plate was run for two minutes at 50C, this time with a voltage of 17V.
The second plating bath was warmed to 52C, and this was added to the cleaned hull cell. A lA test plate was run for 2 minutes 20 seconds with this solution at a voltage of 4V. With the same solution and another brass plate, a lOA
test plate was run at 50C for 2 minutes, with a voltage of 13.5V.
Both solutions plated over a comparable range with the same quality deposit. Eloweverl the voltage of the mixed sulphate bath was significantly lower than that of the bath containing only Na2SO~. Plating current densities ranged - from 20-800 mA/cm2.
* Trade Mark .~
1 Additional examples of other particular embodiments of the invention follow:
In following three groups of examples, a bath consisting of anolyte separated from a catholyte by a Nafion cation exchange membrane is used. The anolyte comprises an aqueous solution of sulphuric acid in 2% by volume concentration (pH 1.6). The anode is a flat bar of a lead alloy of the type conventionally used in hexavalent chromium plating processes.
The catholyte for each Example was prepared by making up a base electrolyte and adding appropriate amounts of chromium (III), complexant and the organic compound.
The base electrolyte for each example consisted of the following constituents dissolved in 1 litre of water:
Potassium sulphate lrl Sodium sulphate 0.05M
Boric Acid lM
Wetting Agent FC98 0.1 gram Example 1 The following constituents were dissolved in the base electrolyte:
Chromium ¦III) lOmM (from chromium) DL aspartic acid 10~1 Thiourea lmM
at pH 3.5 Although equilibration will occur quickly in normal use, initially the electrolyte is preferably equilibrated until there are no spectroscopic changes which can be detected. The bath was found to operate over a temperature range of 25 to 60C. Good bright deposits of chromium were obtained over a current density range of 5 to 800 mA/cm2.
* Trade Mark .,~....
1 EY~ample 2 The following constituents were dissolved in the base electrolyte:
Chromium (III) lOmM (from chrometan) Iminodiacetic acid lOmM
Thiourea lm~1 at pH 3.5 The electrolyte is preferably equilibrated until there are no spectroscopic changes. The bath was found to operate over a temperature range of 25 to 60C. Good bright deposits of chromium were obtained.
Example 3 I
The following constituents were dissolved in the base 15 electrolyte:
Chromium ~III3 lOOmM (from chrometan3 DL Aspartic acid lOOmM
Mercaptoacetic acid lmM
at pH 3.5 The electrolyte is preferably equilibrated until there are no spectroscopic changes. The bath was found to operate over a temperature range of 25 to 60C. Good bright deposits were obtained.
:, Example 4 The following constituents were dissolved in the base electrolyte:
Chromium (III) lOOmM (from chrometan) DL Aspartic acid lOOmM
Thiourea lm~l at p~l 3.5 The electrolyte is preferably equilibrated until there are no spectroscopic changes. The bath was found to operate over a temperature range of 25 to 60C. Good bright deposits were obtained over a current density range of 10 to 800 mA/cm2.
The above examples illustrate a chromium electroplating electrolyte containing a source of trivalent chromium ions, a complexant, a buffer agent and organic compound having a -C=S
group or a -C-S yroup within the molecule for promoting chromium deposition, the complexant being selected so that the stability constant Kl of the chromium complex as defined herein ls in the range 10 ~ K1 ~ 10 M
By way of example, complexant ligands having K1 values within the range 108~ K1 < 1012 M 1 include aspartic acid, iminodiacetic acid, nitrilotriacetic acid and 5-sulphosalicylic acid.
The organic compound haviny -C-S group can be selected from thiourea, N-monoallyl thiourea, N-mono-p-tolyl thiourea, thioacetamide, tetramethyl thiuram monosulphide, tetraethyl thiuram disulphide and diethyldithiocarbonate. The organic compound having a -C-S~ group can be selected from mercaptoacetic acid and mercaptopropionic acid.
Since the plating efficiency of the electrolyte is relatively high, a commercial trivalent chromium electrolyte ean have as low as 5mM chromium. This removes the need for expensive rinse water treatment sinee the chromium content of the 'drag-out' from the plating electrolyte is extremely low.
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1 In general, the concentration of 'che constituents in the electrolyte are as follows:
Chromium (III) ions 10 to 1l`i Organic compound 10 to 10 M
A practical chromium/complexant ligand ratio is approximately 1:1.
Above a minimum concentration necessary for acceptable plating rates, it is unnecessary to increase the amount of the organic compound in proportion to the concentration of chromium in the electrolyte. Excess of the organic compound may not be harmful to the plating process but can result in an increased amount of sulphur being co-deposited with the chromium metal. This has two effects, firstly to produce a progressively darker deposit and, secondly, to produce a more ductile deposit.
GROUP II
~
The following constituents were dissolved in the base electrolyte:
Chromium (III) lOm~l (from chrometan) DL aspartic acid 1OmM
Sodium thiosulphate lmM
at pH 3.5 Although equilibration will occur quickly in normal use, initially the electrolyte is preferably equilibrated until there are no spectroscopic changes which can be detected. The bath was found to operate over a temperature range of 25 to 60C. Good bright deposits of chromium were obtained over a current density range of 10 to 800 mA/cm2.
1 Example 2 The following constituents were dissolved in the base electrolyte:
Chromium ~III) lOmM ~from chrometan) Iminodiacetic acid lOmM
Sodium thionate lmM
at pH 3.5 The electrolyte is preferrably equilibrated until there are no spectroscopic changes. The bath was found to operate over a temperature range of 25 to 60C. Good bright deposits of chromium were obtained.
Example 3 The following constituents were dissolved in the base electrolyte:
Chromium (III) lOOmM (from chrometan) DL aspartic acid lOOmM
Sodium thiosulphate ln~l at pH 3.5 The electrolyte is preferably equilibrated until there are no spectroscopic changes. The bath was found to operate over a temperature range of 25 to 60C. Good bright deposits were obtained.
1 Example 4 The following constituents were dissolved in the base electrolyte:
Chromium (III) lOOmM (from chrometan) DL aspartic acid lOOmM
Sodium thionate lm~l at pH 3.5 The electrolyte is preferably equilibrated until there are no spec-troscopic changes. The bath was found to operate over a temperature range of 25 to 60C. Good bright deposits were obtained over a current density range of 10 to 800 mA/cm2.
The above examples illustrate a chromium electroplating electrolyte containing a source of trivalent chromium ions, a complexant, a buffer agent and a sulphur species having S-O or S-S bonds for promoting chromium deposition, -the complexant being selected so that the stability constant K
of the chromium complex as defined herein is in the range 106 C Kl ~ 1012 M 1 and the sulphur species being selected from thiosulphates, thionates, polythionates and sulfoxylates.
By way of example complexant ligands having Kl values within the range 106 ~ Kl ~ 1012 M 1 include aspartic acid, iminodiacetic acid, nitrilotr~acetic acid, 5-sulphosalicylic acid and citric acid.
The sulphur species are provided by dissolving one or more of the following in the electrolyte: sodium thiosulphate, potassium thio sulphate, barium thiosulphate, ammonium thiosulphate, calcium thiosulphate, potassium polythionate, sodium polythionate, and sodium sulfoxylate.
1 Very low concentrations of the sulphur species are needed to promote reduction of the trivalent chromium ions.
Also since the plating efficiency of the electrolyte is rela-tively high a commercial trivalent chromium electrolyte can have as low as 5m~ chromium. This removes the need for expensive rinse water treatment since the chromium content o~ the 'drag~out' from the plating electrolyte is extremely low.
In general, the concentration of constituents in the electrolyte are as follows.
Chromium (III) ions 10 to 1~1 Sulphur species 10 to 10 M
A practical chromium/complexant ligand ratio is approximately 1:1.
Above a minimum concen-tration necessary for acceptable plating ranges, it is unnecessary to increase the amount of the sulphur species in proportion to the concentration of chromium in the electrolyte. Excess of the sulphur species may not be harmful to the plating process but can result in an increased amount of sulphur being co-deposited with the chromium metal. This has two effects, firstly to produce a progressively darker deposit and, secondly, to produce a more ductile deposit.
GROUP III
,, _ The following constituents were dissolved in the base electrolyte:
Chromium (III) 5mM (from chrometan~
DL aspartic acid SmM
Sodium sulphite 5mM
at pH 3.5 Although equilibration wil:L occur quickly in normal use, initially the electrolyte is preferably equilibrated until there are no spectroscopic changes which can be detected. The bath was found to operate over a temperature range of 25 to 60C. Good bright deposits of chromium were obtained over a current density range o~ 10 to ~00 n~/cm2.
1 Example 2 The following constituents were dissolved in the base electrolyte:
Chromium tIII~ 5mM (from chrometan) Iminodiacetic acid 5mM
Sodium dithionite 2mM
at pH 3.5 The electrolyte is preferrably equilibrated until there are no spectroscopic changes. The bath was found to operate over a temperature range of 25 to 60C. Good bright deposits of chromium were obtained.
Exam~le 3 The following constituents were dissolved in the base electrolyte:
Chromium (III~ 50mM (from chrometanJ
DL Aspartic acid 50mL~
Sodium sulphite lOmM
at pH 3.5 The electrolyte is preferably equilibrated until there are no spectroscopic changes. The bath was found to operate over a temperature range of 25 to 60C. Good bright deposits were obtained.
Example ~
The following constituents were dissolved in the base electrolyte:
Chromium (IIX) 50mM (from chrometan) 5-sulphosalicylic acid 50mM
Sodium sulphite lmM
at pH 3,5 The electrolyte is preferrably equilibrated until there are no spectroscopic changes. The bath was found to operate over a temperature range of 25 to 60C. Good bright deposits were obtained.
1 The abov~ examples illustrate a chromium electroplating electrolyte containing a source of trivalent chromium ions, a complexant, a buffer agent and a sulphur species haviny selected frorn sulphites and dithionites for promoting chromium deposition, the complexan-t being selec-ted so that the stability cons-tan-t K1 of the chromium complex as defined herein is in the range 106 ~ K1 ~ 1012 M 1 and the chromium ions having a molar concentration lower than O.OlM.
By way of example, complexant ligands having Kl values within the range 106 ~ K1 ~ 1012 M 1 include aspar-tic acid, iminodiacetic acid, nitrilotriacetic acid, 5~sulphosalicylic acid and citric acid.
The above examples also illustrate a chromium electrolyte containing trivalent chromium ions, a complexantt a buffer agent and a sulphur species selected from sulphites and dithionites, the complexant being selected from aspartic acid, 5-sulphosalicylic acid and citric acid.
The above examples further illustrate a chromium electroplating bath comprising an anolyte separated from a catholyte by a perfluorinated cation exchange membrane, the anolyte comprising sulpha~e ions and the catholyte comprising a source of trivalent chromium ions, a complexant, a buffer agent and a sulphur species selected from sulphites and dithionites, and in which the source of sulphate ions is chromium sulphate. Suitable complexant ligands are aspartic acid, iminodiacetic acid, nitrilotriacetic acid, and 5-sulphosalicylic acid and citric acid.
Sulphites can include blsulphites and metabisulphites.
l Low concentrations of sulphite or dithionite are needed to promote reduction of the trivalent chromium ions. Also since the plating efficiency of the electrolyte is relatively high a commercial trivalent chromium electrolyte can have less than 10mM chromium. This removes the need for expensive rinse water treatment since the chromium content of the 'drag-out' from the plating electroly-te is extremely low.
In general, the concentration of the cons-tituents in the electroly-te are as follows:
Chromium (III) ions 10 to lM
Sulphur species 10 to 10 M
A practical chromium/complexant ligand ratio is approximately 1:1.
Above a minimum concentration necessary for acceptable plating rates, it is unnecessary to increase the amount of -the sulphur species ln proportion to the concentration of chromium in the electrolyte. Excess of sulphite or dithionite may not be harmful to the plating process but can result in an increased amount of sulphur being co-deposited with the chromium metal. This has two effects, firstly to produce a progressively darker deposit and, secondly, to produce a more ductile deposit.
The preferred source of trivalent chromium is chromium sulphate which can be in the form of a commercially available mixture of chromium and sodium sulphates known as -tanning liquor or chrometan. Other trivalent chromium sal-ts, which are more expensive than the sulphate~ can be used, and include chromium chloride, carbonate and perchlorate.
l The preferred buffer agent used to maincain the pH of the bulk electrolyte comprises boric acid in high concentrations, iOe., near saturation. Typical pH range for the electroly-te is in the range 2.5 to 4.5.
The conductivity of the electrolyte should be as high as possible to minimize both voltage and power consumption.
Voltage is often critical in practical plating environments since rectifiers are often limited to a low voltage, e.g., 8 volts. In an electrolyte in which chromium sulphate is the source of the trivalent chromium ions a mixture of sodium and potassium sulphate is the optimum. Such a mixture is described in United Kingdom Patent 2,071,151.
A wetting agent is desirable and a suitable wetting agent is FC98, a product of the 3M Corporation. However, other wettinc3 agents such as sulphosuccina-tes or alcohol sulphates may be used.
It is preferred to use a perfluorinated cation exchange membrane to separate the anode from the plating electrolyte as described ln United Kingdom Patent 1,602,404. A suitable perfluorinated cation exchange membrane is Naf iOII tTrade Mark), a product of the Du Pont Corporation. It is particularly advantageous to employ an anolyte which has sulphate ions when the catholyte uses chromium sulphate as the source of chromium since inexpensive lead or lead alloy anodes can be used. In a sulphate anolyte, a thin conducting layer of lead oxide is formed on the anode.
Chloride salts in the catholyte should be avoided since the chloride anions are small enough to pass through the membrane ln sufficient amount to cause both the evolution of chlorine at the anode and the formation of a highly resistive film of lead chloride on lead or lead alloy anodes. Cation exchange membranes have the additional advantage in sulphate electrolytes that the pH of the catholyte can be stabilized by adjusting the pH of the anolyte to a:Llow hydrogen ion transport through the membrane to comp~nsate for the increase in pH of the catholyte by hydrogen evolution at the cathode. Using the combination of a membrane, and sulphate based anolyte and ca-tholyte a plating bath has been operated for over 40 Amphours/litre without pH adjustment.
Claims (6)
1. A chromium electroplating solution comprising an aqueous solution of chromium (III) complexes as the source of chromium, said complexes being formed from chromium (III) and at least one of aspartic acid and an organic compound having a -C=S group or a -C-S- group, and a supporting electrolyte which is chloride free and comprises a mixture of sodium and potassium sulphates in a concentration sufficient to provide electrical conductivity for the plating process, the concentration of sodium sulphate being in the range of about 0.1 to 1 Molar and the concentration of potassium sulphate being about 1 Molar.
2. A solution as claimed in claim 1 wherein the potassium sulphate is present in saturation concentration.
3. A solution as claimed in claim 1 wherein the sodium sulphate concentration is 0.5 Molar.
4. A solution as claimed in claim 2 wherein the sodium sulphate concentration is 0.5 Molar.
5. A solution as claimed in claims 1, 2 or 3 in which the source of chromium from which the complex is prepared is chromium sulphate.
6. A solution as claimed in claim 4 in which the source of chromium from which the complex is prepared is chromium sulphate.
Applications Claiming Priority (8)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB8134776 | 1981-11-18 | ||
| GB8134778 | 1981-11-18 | ||
| GB8134779 | 1981-11-18 | ||
| GB8134777 | 1981-11-18 | ||
| GB08134777A GB2110242B (en) | 1981-11-18 | 1981-11-18 | Electroplating chromium |
| GB08134778A GB2109816B (en) | 1981-11-18 | 1981-11-18 | Electrodeposition of chromium |
| GB08134776A GB2109815B (en) | 1981-11-18 | 1981-11-18 | Electrodepositing chromium |
| GB08134779A GB2109817B (en) | 1981-11-18 | 1981-11-18 | Electrodeposition of chromium |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1214426A true CA1214426A (en) | 1986-11-25 |
Family
ID=27449293
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000415906A Expired CA1214426A (en) | 1981-11-18 | 1982-11-18 | Trivalent chromium electroplating solution and bath |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1214426A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2016075287A1 (en) * | 2014-11-14 | 2016-05-19 | Maschinenfabrik Kaspar Walter Gmbh & Co. Kg | Production of chromium layers on intaglio printing cylinders |
-
1982
- 1982-11-18 CA CA000415906A patent/CA1214426A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2016075287A1 (en) * | 2014-11-14 | 2016-05-19 | Maschinenfabrik Kaspar Walter Gmbh & Co. Kg | Production of chromium layers on intaglio printing cylinders |
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