Title:
ELECTRODEPOSITION OF METALS
United States Patent 3772167
Abstract:
The specification discloses an improved method and an improved electrolyte solution for providing a metal coating on a metallic substrate. The metal coatings are provided by electrodeposition from solutions comprising water, a dipolar organic solvent, metal ions, halide ions, and ammonium ions. Specific dipolar organic solvents disclosed include dimethylsulphoxide, dimethylacetamide, tetrahydrothiophen dioxide, dimethylformamide, propylene carbonate, tetramethylurea and hexamethyl phosphormamide.
US Patent References:
Superconductive articles
Husni - August 1967 - 3336658
Electrodeposition of chromium and alloys thereof
Snavely et al. - November 1954 - 2693444
Electroplating from an organic electrolytic solution
Micillo - April 1964 - 3131134
Superconductive articles
Husni - August 1967 - 3336658
Electrodeposition of chromium and alloys thereof
Snavely et al. - November 1954 - 2693444
Electroplating from an organic electrolytic solution
Micillo - April 1964 - 3131134
Inventors:
Bharucha, Nanabhai Rustomji (Montreal, Quebec, CA)
Ward, John Joseph Bernard (Kent, EN)
Ward, John Joseph Bernard (Kent, EN)
Application Number:
05/175771
Publication Date:
11/13/1973
Filing Date:
08/27/1971
View Patent Images:
Export Citation:
Primary Class:
Other Classes:
205/284, 205/241, 205/253, 205/311, 205/244, 205/272, 205/255, 205/240, 205/260, 205/274, 205/259, 205/270, 205/302, 205/238, 205/245, 205/313, 205/281, 205/239, 205/246, 205/287, 205/296, 205/269, 205/271, 205/262
International Classes:
A41C3/00; C25D3/02; C25D3/06; C25D3/10; C25D3/56; C23B5/32
Field of Search:
204/14N,45R,45A,43,51
Other References:
R D. Blue et al., Trans. Electrochemical Soc., Vol. 43, pp. 231-238, (1933)..
Primary Examiner:
Kaplan G. L.
Parent Case Data:
This is a continuation-in-part of our co-pending applications Ser. Nos. 840,823 filed July 10, 1968 and 718,234 filed Apr. 2, 1968, both now abandoned. This invention is directed to the electrodeposition of metal coatings and to electrolyte solutions suitable for the depositions of metallic coatings.
Claims:
We claim
1. An electrolyte solution for electrodeposition of a chromium-nickel alloy on a substrate, said solution comprising at least about 40 percent of an organic dipolar aprotic solvent selected from the group consisting of dimethylformamide, dimethylsulphoxide, dimethylacetamide, tetrahydrothiophen dioxide, propylene carbonate, tetramethyl urea, and hexamethyl phosphoramide, trivalent chromium ions in a concentration of at least about 0.8M, nickel ions in a concentration of at least about 0.05M, and at least about 20 percent water.
2. An electrolyte solution according to claim 1 containing at least 0.2M ammonium ions, and a complexing agent which consists essentially of said organic dipolar aprotic solvent.
3. An electrolyte solution according to claim 2 wherein the concentration of chromium ions is from about 0.8 M to about 1.3 M.
4. An electrolyte solution according to claim 2 wherein the concentration of nickel ions is from about 0.8 M to 1.2 M.
5. An electrolyte solution according to claim 2 wherein the concentration of ammonium ions is at least 0.2 M and the concentration of water is from about 20 percent to about 60 percent.
6. An electrolyte solution according to claim 2 containing boric acid in a concentration of at least about 0.1 M.
7. An electrolyte solution according to claim 1 wherein said dipolar aprotic organic solvent is dimethylformamide.
8. An electrolyte solution according to claim 7 containing at least 0.8 M sodium chloride.
9. A method for electrodepositing a chromium-nickel alloy on a substrate which comprises immersing said substrate in an electrolyte solution comprising at least about 40 percent of a dipolar aprotic organic solvent selected from the group consisting of dimethylformamide, dimethylsulphoxide, dimethylacetamide, tetrahydrothiophen dioxide, propylene carbonate, tetramethyl urea, and hexamethyl phosphoramide, trivalent chromium ions in a concentration of at least about 0.8M, nickel ions in a concentration of at least about 0.05M, ammonium ions in a concentration of at least 0.2M, from about 20 percent to about 60 percent water, and a complexing agent which consists essentially of said dipolar aprotic solvent, the pH of the solution being from about 1 to about 3.5, said solution being free of other complexing agents, and passing an electric current through said solution thereby to deposit said metal ions on a substrate in the form of a metallic coating.
1. An electrolyte solution for electrodeposition of a chromium-nickel alloy on a substrate, said solution comprising at least about 40 percent of an organic dipolar aprotic solvent selected from the group consisting of dimethylformamide, dimethylsulphoxide, dimethylacetamide, tetrahydrothiophen dioxide, propylene carbonate, tetramethyl urea, and hexamethyl phosphoramide, trivalent chromium ions in a concentration of at least about 0.8M, nickel ions in a concentration of at least about 0.05M, and at least about 20 percent water.
2. An electrolyte solution according to claim 1 containing at least 0.2M ammonium ions, and a complexing agent which consists essentially of said organic dipolar aprotic solvent.
3. An electrolyte solution according to claim 2 wherein the concentration of chromium ions is from about 0.8 M to about 1.3 M.
4. An electrolyte solution according to claim 2 wherein the concentration of nickel ions is from about 0.8 M to 1.2 M.
5. An electrolyte solution according to claim 2 wherein the concentration of ammonium ions is at least 0.2 M and the concentration of water is from about 20 percent to about 60 percent.
6. An electrolyte solution according to claim 2 containing boric acid in a concentration of at least about 0.1 M.
7. An electrolyte solution according to claim 1 wherein said dipolar aprotic organic solvent is dimethylformamide.
8. An electrolyte solution according to claim 7 containing at least 0.8 M sodium chloride.
9. A method for electrodepositing a chromium-nickel alloy on a substrate which comprises immersing said substrate in an electrolyte solution comprising at least about 40 percent of a dipolar aprotic organic solvent selected from the group consisting of dimethylformamide, dimethylsulphoxide, dimethylacetamide, tetrahydrothiophen dioxide, propylene carbonate, tetramethyl urea, and hexamethyl phosphoramide, trivalent chromium ions in a concentration of at least about 0.8M, nickel ions in a concentration of at least about 0.05M, ammonium ions in a concentration of at least 0.2M, from about 20 percent to about 60 percent water, and a complexing agent which consists essentially of said dipolar aprotic solvent, the pH of the solution being from about 1 to about 3.5, said solution being free of other complexing agents, and passing an electric current through said solution thereby to deposit said metal ions on a substrate in the form of a metallic coating.
Description:
In recent times substantial improvements have been made in the appearance and the performance of electrodeposited metal coatings. Particularly significant improvements have been achieved in connection with the deposition of chromium. However, substantial problems and disadvantages still exist in the presently available processes for the preparation of metallic coatings. For example, conventional chromic acid baths deposit the metal at low cathode efficiency and provide coatings having poor covering. Moreover, conventional baths often have poor throwing power. Chromium deposits from conventional baths have been found to be in a highly stressed condition so that they are subject to crack development when the thickness reaches a few hundredths of a milliinch. Higher current efficiencies, which may approach 40 percent, can be obtained with aqueous solutions of trivalent compounds but the deposits from such solutions are satisfactory in appearance only when deposited over a very narrow range of pH values. The large amount of hydrogen gas evolved during deposition tends to cause substantial changes in the pH of the solution in the vicinity of the cathode, thus making pH control difficult and restricting the permissible current density range which can be used.
It is recognized by those in the art that considerable economic advantage could be achieved if the current efficiency could be increased. This advantage is particularly attractive with respect to chromium plating baths. It is also recognized that the evolution of hydrogen from the cathode during deposition of the metal is a serious problem and that coatings of improved corrosion resistance and appearance can be provided if the evolution of hydrogen can be controlled. In the past, experimentation with the use of modified chromic acid and trivalent chromium baths has shown little prospect of improvement in the quality of the deposit without a concomitant reduction in current efficiency.
Chrome nickel alloy coatings have excellent resistance to oxidation and corrosion. Prior methods of applying such coatings have involved electrophoresis or cladding of a layer of the alloy to the metal object to be coated. Methods for applying such coatings by electrodeposition have been put forward, but have not been satisfactory, due, as is the case with chromium plating, to the fact that, at the pH range at which nickel is readily deposited, chromic acid solutions do not readily deposit coherent chromium coatings and solutions of trivalent chromium evolve excessive amounts of hydrogen. There is therefore a need for a bath from which coherent chromium nickel alloy coatings can be electrodeposited over a wide range of current densities at adequate current efficiencies.
The general object of this invention is to provide an improved process for the electrodeposition of metallic coatings and to provide improved plating baths for use in electroplating processes.
A particular object of the invention is to provide improved electrolyte solutions for the deposition of metallic coatings by electrolytic techniques.
A further object of the invention is to provide metallic coatings which are characterized by improved corrosion resistance.
Another object of the invention is to provide metallic coatings by a method characterized by improved current efficiency and throwing power.
A further object of the invention is to provide a method for the deposition of metallic coatings by an electrolytic process characterized by reduced hydrogen evolution at the cathode during deposition.
A still further object of the invention is to satisfy the need for chromium plating solutions capable of providing crack free deposits over a wide range of current densities and to provide chromium plating baths capable of depositing a suitable chromium deposit under conditions of adequate current efficiency, improved covering characteristics, and improved throwing power.
A still further object of the invention is to provide an electrolyte solution for the plating of chromium nickel alloy and nickel metallic coatings.
A still further object of the invention is to provide a method by which significantly improved coatings can be deposited upon articles of complex shape.
Broadly, the instant invention is directed to the plating of any metal which can be electroplated or deposited from solution by passing an electric current through a solution containing ions of the metal to be deposited. Such metals include chromium, manganese, iron, zinc, copper, lead, nickel, cobalt, tin, and cadmium. Chromium is of particular interest in view of its desirable appearance and high resistance to tarnish and corrosion under varied indoor and outdoor environmental conditions. It has been discovered that metallic deposits, particularly chromium deposits, having greatly improved corrosion resistance and exhibiting a very good appearance can be provided by electrolytic techniques through the use of a plating bath containing chromium ions, particularly trivalent chromium ions, ammonium ions, halide ions and a homogeneous mixture of water and a dipolar organic solvent, the molecules of which contain a highly electronegative oxygen atom as hereinafter defined.
Although the invention is particularly described herein with respect to the deposition of chromium, chromium nickel, and nickel coatings, it will be appreciated that in view of the broad scope of the problems of hydrogen evolution at the cathode, plating efficiency, throwing power, and covering power, this development is of general application in and to the electroplating of metals and is of particular advantage in connection with the plating of metals which are difficult to plate due to their position in the standard electromotive series.
The plating baths of this invention comprise a homogeneous mixture of water and a dipolar organic solvent, preferably a dipolar aprotic organic solvent, ammonium ions, and halide ions in addition to the ions of the metal to be deposited. The proportion of water in the electrolyte solvent can range from about 20 to about 60 percent by volume and the amount of organic solvent can range from about 40 to about 80 percent by volume. The preferred ratio of water to dipolar organic solvent is from 45 : 55 to about 55 : 45 by volume.
The term "dipolar organic solvent," as used herein, refers to an organic liquid which is capable of dissolving a substantial amount of a salt of the metal to be deposited and which does not donate a substantial amount or quantity of hydrogen ions. It is to be understood that the solvent must permit ionization of the dissolved metal salts in the bath. The dipolar organic molecule must contain at least one highly electronegative oxygen atom. Illustrative examples of suitable dipolar organic solvents include dimethylsulphoxide and tetrahydrothiophen dioxide. The oxygen atoms of these materials are activated by the electron donating properties of the sulphur atoms. A ketonic oxygen atom is not generally sufficiently electronegative to constitute a suitable dipolar organic solvent unless there is present an activating group adjacent to the carbon atom, for example, an amino group. Accordingly, compounds such as tetramethylurea, tertiary butyl formamide, and compounds of the formula R 1 R 2 N.OCR 3 , wherein R 1 , R 2 and R 3 may be the same or different members selected from the group consisting of hydrogen atoms, aryl groups, e.g., phenyl, or alkyl groups, preferably lower alkyl, are suitable. Such solvents include dimethylformamide (DMF) and dimethylacetamide (DMAC). It will be appreciated that mixtures of several of the above-mentioned organic solvents may be employed. A particularly preferred group of dipolar organic solvents are the dipolar aprotic organic solvents. Such aprotic solvents include dimethylsulphoxide, tetrahydrothiophen dioxide, tetramethylurea, dimethylacetamide and dimethylformamide. Of these particular solvents, dimethylformamide is especially convenient. Dimethylformamide, in addition to possessing an excellent balance of physical, chemical, toxicological, and economic properties, has shown excellent electroplating properties. Dimethylformamide is characterized by a relatively wide liquid range, a boiling point of 150° C., a high dielectric constant of about 37.5, a low vapor pressure of about 3.6 millimeters of mercury at 25° C. Dimethylformamide is also completely miscible with water in all proportions and has the ability to dissolve substantial amounts of metal salts, e.g., about 450 grams per liter of hexahydrated chromium trichloride. Dimethylformamide also interacts with such salts to form stable complexes of the type CrCl 3 .6DMF and CrCl 3 .4DMF. It is important to note that because of the ability of the DMF to function as a complexing agent, no other complexing agent need be utilized.
The metal to be deposited can be conveniently incorporated in the plating bath in a form of a metal salt which is soluble in the water-organic solvent mixture. Halide salts such as bromides, chlorides, or iodides, nitrates, acetates, formates, oxalates, and sulfates are suitable. The bromides, chlorides, and iodides with their characteristic water of hydration are preferable due to the rather limited solubility of the acetates, formates, oxalates, sulfates, and chrome alums in the dipolar solvents. In cases where the metal salt has limited solubility due to the presence of the organic liquid, solubility can be increased by the use of water concentrations near the upper end of the ranges above mentioned.
Solutions containing trivalent metal salts, such as chromic chloride, in combination with organic dipolar solvents and water, may be unstable with respect to composition and plating performance. The physical characteristics of the solution, e.g., color, pH, and viscosity, have been found to change during storage and the color and adhesion of successive metal deposits from the same bath may deteriorate. Moreover, plating baths containing low concentrations of water have a relatively low conductivity, while solutions with higher concentration of water tend to evolve excessive amounts of hydrogen at the cathode during plating. It has been found that the presence of an ammonium salt improves the stability of the solution and reduces the tendency of the bath to evolve hydrogen at high water content. Moreover, the presence of ammonium salts has been found to significantly reduce the effect of changes in pH on the lower limiting plating current density. The ammonium ion should be present at a concentration of at least about 0.2 molar and preferably from about 0.6 to about 1 molar. Solutions containing over 80 percent by volume of dimethylformamide provide limited solubility for ammonium salts. It is therefore necessary to provide sufficient water to dissolve the required amount of ammonium salt. The use of about 20 percent by volume of added water and about 80 percent by volume of dimethylformamide allows solutions containing about 1 molar concentrations of ammonium ions to be prepared at 55° C. The preferred proportion by volume of added water to dipolar organic compound is from 10 : 90 to 50 : 50.
A prerequisite of successful chromium, chromium nickel, and nickel plating from a dipolar organic solvent is that the trivalent chromium and nickel ions shall form small moderately stable complexes with the solvent molecule. If there is no complex formation, then the chromic and nickel salts are unlikely to be sufficiently soluble. If the complexes formed are excessively stable, then electrodeposition may be difficult. It is believed that the highly electronegative oxygen atoms which characterize the dipolar organic compounds of this invention may act as covalent links in the formation of complexes between the chromic ions and the organic molecules. Such solutions by themselves do not, however, give smooth coherent metallic deposits. The addition of water probably generates the polynuclear olated and oxalated chromic species usually found in solutions of trivalent chromium compounds, which again do not readily give good chromium deposits. The effect of the ammonium ion may be attributed to its structure-disordering properties simplifying the nature of the trivalent chromium and nickel species in solution, possibly with the formation of mononuclear Cr 3 +--DMF--H 2 O complexes. The water may be prevented from showing its full protic tendencies by reason of the formation of complexes with the dipolar organic molecules. The halogen ions may be partly solvated, although the ready evolution of chlorine at the anode indicates solvation is not complete.
The pH of the solution should be from 1 to 3.5 and preferably about 2. If the pH is too low, hydrogen tends to be evolved at the cathode in preference to chromium. If the pH is too high, basic chromium compounds are liable to precipitate out. The pH can be adjusted by the use of hydrochloric acid or sodium hydroxide as required.
The current efficiency of the solution may be improved by the addition of boric acid, preferably to a concentration of at least O.I.M. Boric acid is not normally soluble to the extent of more than about 0.2M.
In addition to the ammonium ions mentioned above, electroplating baths of the present invention contain a sodium halide which has been found to increase the plating range and current efficiency. Sodium halides are also beneficial in that they enhance the covering power of the bath. It is preferred that the sodium halide be present in a concentration of at least about 0.8 molar.
When applying a chromium nickel coating, the concentration of chromium in the solution is not critical, and is preferably about from 0.8M to 1.3M, particularly about 1M. Nickel ions should be present in the solution at a concentration of at least 0.05M, and a preferred concentration range is about 0.8M to 1.2M.
The invention also provides a method of electrodepositing a chromium nickel alloy, which method comprises providing a cathode in the solution described above, and an anode, and passing an electric current through the solution so as to deposit a chromium nickel alloy on the cathode.
Electroplating baths as described herein have useful current density ranges of from 0.5 to about 20 amperes per square decimeter, depending on the composition and temperature of the particular solution and on the desired composition of the deposited metal. Optimum current densities are generally within the range from about 6 to about 10 amperes per square decimeter. The useful plating current range for a particular plating solution varies with temperature, being larger at lower temperatures. Above about 30° C. narrower plating ranges are encountered. The amount of organic solvent in the bath influences the plating range and the temperature at which optimum plating occurs. For example, a solution containing a high ratio of dimethylformamide to water provides suitable plating at temperatures of about 50° C. Preferred plating temperatures are in the range of 40° to 80° C.
When depositing a chromium nickel alloy the composition of the alloy deposited depends on the composition of the plating solution, on the temperature and on the plating current density. In general, the higher the current density, the greater is the percentage of chromium in the alloy deposited, and the lower the current density, the greater is the percentage of nickel in the alloy deposited. This may be due to the fact that the threshold current density for plating with the two separate metals differs. It may be desirable to replenish the plating solution as the metals are removed from its, and this replenishment will normally be designed to maintain the relative concentrations of nickel and chromium in the solution, so as to keep constant the composition of the alloy being deposited.
When the anode of the electroplating apparatus is to be immersed in a chromium or chromium nickel solution, it is preferred that the anode be of graphite or titanium or of some other similar inert material. Unfortunately, however, the use of graphite anodes is disadvantageous because of the evolution of chlorine gas at the anode. Since chlorine is very soluble in some of the organic solvents, particularly dimethylformamide, large concentrations of chlorine can build up in the solution. The use of a nickel chromium anode prevents chlorine formation, but the chromium may dissolve off as chromic acid which is liable to oxidize the DMF. Also, if nickel or chromium anodes are used, some indefinite and largely uncontrollable contribution will be made to the concentration of these elements in the electrolyte. Passivated lead anodes have been examined, but the chloride ion tends to break the passive layer, exposing fresh metal to the solution which dissolves it intensively. Accordingly, it is preferred to immerse the anode in an anolyte which is separated from the chromium solution by means of a porous diaphragm within the cell. A suitable aqueous anolyte comprises a molar solution of ammonium or sodium acetate separated from the plating solution by a porous ceramic diaphragm. The use of such a diaphragm in the plating apparatus has provided a successful means of preventing chlorine evolution. The presence of a more highly conducting aqueous portion, i.e., the anolyte, compensates for the increased resistivity of the apparatus due to the presence of the diaphragm.
The invention described above is further illustrated by the following examples.
EXAMPLE 1
Bright nickel plated copper cathodes were plated with chromium in a bath comprising 240 grams per liter of chromic chloride hexahydrate, 58 grams per liter of sodium chloride, 50 grams per liter ammonium chloride, and 8 grams per liter of boric acid, the solvent being a 50 percent by volume mixture of water and dimethylformamide (DMF). The bath was operated at temperatures from 15° to 30° C., a current density of from 1 to 20 amperes per square decimeter, and a pH from 1 to 3. Current efficiencies of up to 50 percent were achieved.
Examples 2 - 28, in Table I below, illustrate bath compositions and plating conditions found to provide a good smooth adherent deposit of chromium. The deposit in examples 2 - 5 was initially covered by a green slime which was easily removed by dipping in a 50 percent solution of hydrochloric acid without impairing the appearance of the coating. The plating range ratio was calculated from the upper and lower current densities at which a bright plate was obtained. ##SPC1##
EXAMPLE 29
In this example, the plating performance of a trivalent organic solution according to the invention is compared with a conventional hexavalent aqueous bath. ##SPC2##
It is apparent that the organic trivalent solutions of this invention showed a wider plating range ratio, a higher current efficiency, and better covering power than the conventional hexavalent aqueous bath.
EXAMPLE 30
DMSO (Dimethylsulphoxide) Solution
Bath Composition
1 M CrCl 3 .6H 2 O
1 m naCl
0.6 M NH 4 Cl
50 percent v/v DMSO
50 percent v/v H 2 O
Conditions of Plating
Temp. : 25°C.
pH : 2.4
Current : 2 A
Voltage : 20 V
Results
C.D. (A/dm 2 ) 9.9 8.1 3.8 2.6 1.3 Efficiency % 13 9 10.1 7 Small Plating Range : 1.3-11.7 A/dm 2 Plating Range Ratio : 9
EXAMPLE 31
60 percent H 2 O, 40 percent DMF Solution
Bath Composition
1 M CrCl 3 .6H 2 O
0.5 m nh 4 cl
1 M NaCl
40 percent v/v DMF
60 percent v/v H 2 O
Conditions of Plating
Temp. : 30°C.
pH : 1.6
Current : 4 A
Voltage : 25 V ##SPC3##
EXAMPLE 32
DMAC (Dimethylacetamide) Solution
Bath Composition
1 M CrCl 3 .6H 2 O
0.8 m naCl
0.2 M NH 4 Cl
70 percent v/v DMAC
30 percent v/v H 2 O
Conditions of Plating
Temp. : 35°C.
pH : 2.3
Current : 3 A
Voltage : 30 V
Results
C.D. (A/dm 2 ) 10.6 7.25 3.98 1.66 Efficiency % 28.4 22.4 7.7 Small Plating Range : 1.66-14.6 A/dm 2 Plating Range Ratio : 9.1
EXAMPLE 33
Formamide Solution
Bath Composition
1 M CrCl 3 .6H 2 O
1 m naCl
0.5 NH 4 Cl
50 percent v/v Formamide
50 percent v/v H 2 O
Conditions of Plating
Temp. : 25°C.
pH : 2.1
Current : 4 A
Voltage : 30 V ##SPC4##
EXAMPLE 34
In an example of a plating bath for the electrodeposition of a chromium nickel alloy, the solution comprised 60 percent of dimethylformamide by volume and 40 percent of water, and contained the following salts in solution:
1 M CrCl 3 .6H 2 O
0.4 m niCl 2 .6H 2 O
0.5 m nh 4 cl
A bright adherent deposit containing 70 percent of chromium and 30 percent of nickel and having high resistance to corrosive attack by cold 50/50 v/v hydrochloric acid was obtained by electrodeposition from this solution under the following operating conditions:
pH 2.0 Temperature 20°C. Plating current density 2 A/dm 2
EXAMPLE 35
In another example of a bath according to this invention, the solution comprised 50 percent DMF by volume and 50 percent water and contained the following salts:
1 M CrCl 3 .6H 2 O
1 m niCl 2 .6H 2 O
0.5 m nh 4 cl
A bright adherent deposit containing 80 percent nickel and 20 percent chromium and having a high resistance to corrosive attack by cold 50 percent v/v HCl was obtained by electrodeposition from this solution, under the following operating conditions:
pH 2.0 Temperature 50°C. Plating current density 6.8 A/dm 2
EXAMPLE 36
Composition Operating Conditions 1 M CrCl 3 .6H 2 O pH -- 1.1 0.5 M NiCl 2 .6 H 2 O Temperature -- 25°C. 25 g/l NH 4 Cl Current Density -- 100 A/Sq.ft.
This solution yields a dark deposit containing, approximately 20 percent chromium, 80 percent nickel.
EXAMPLE 37
Composition Operating Conditions 1 M CrCl 3 .6 H 2 O pH -- 1.0 1 M NiCl 2 .6 H 2 O Temperature -- 25°C. 25 g/l NH 4 Cl Current Density -- 100 A/Sq.ft. 50 percent DMF v /v
This solution yields deposits containing, approximately 10 percent chromium, 90 percent nickel.
EXAMPLE 38
Composition Operating Conditions 1 M CrCl 3 .6 H 2 O pH -- 1.0 1 M NiCl 2 .6 H 2 O Temperature -- 25°C. 25 g/l NH 4 Cl Current Density -- 100 A/Sq.ft. 5 g/l H 3 BO 3 50 percent DMF v /v
This solution yields deposits containing, approximately 15 percent chromium, 85 percent nickel.
EXAMPLE 39
The invention also encompasses electrolyte solutions for nickel plating. For electroforming applications, e.g., the lining of dies, etc., the following proved suitable:
Composition Operating Conditions 1 M NiCl 2 .6 H 2 O Operating temperature -- 40°C. 50 g/l NH 4 Cl pH 1.9 Water 50 percent v /v Plating range 0.1 -- 400 A/sq.ft. DMF 50 percent v /v
EXAMPLE 40
Composition Operating Conditions 1 M NiCl 2 .6 H 2 O Operating temperature -- 40°C. 50 g/l NH 4 Cl pH 1.9 - 4.5 10 g/l H 3 BO 3 Plating range 0.1 - 400 A/sq.ft. Water 50 percent v /v DMF
the hardness of the deposit depends on pH, bath, two giving 570 VPN at pH 1.9 and 616 at pH 4.5, both values being recorded at a current density of 200 A/sq.ft.
It is recognized by those in the art that considerable economic advantage could be achieved if the current efficiency could be increased. This advantage is particularly attractive with respect to chromium plating baths. It is also recognized that the evolution of hydrogen from the cathode during deposition of the metal is a serious problem and that coatings of improved corrosion resistance and appearance can be provided if the evolution of hydrogen can be controlled. In the past, experimentation with the use of modified chromic acid and trivalent chromium baths has shown little prospect of improvement in the quality of the deposit without a concomitant reduction in current efficiency.
Chrome nickel alloy coatings have excellent resistance to oxidation and corrosion. Prior methods of applying such coatings have involved electrophoresis or cladding of a layer of the alloy to the metal object to be coated. Methods for applying such coatings by electrodeposition have been put forward, but have not been satisfactory, due, as is the case with chromium plating, to the fact that, at the pH range at which nickel is readily deposited, chromic acid solutions do not readily deposit coherent chromium coatings and solutions of trivalent chromium evolve excessive amounts of hydrogen. There is therefore a need for a bath from which coherent chromium nickel alloy coatings can be electrodeposited over a wide range of current densities at adequate current efficiencies.
The general object of this invention is to provide an improved process for the electrodeposition of metallic coatings and to provide improved plating baths for use in electroplating processes.
A particular object of the invention is to provide improved electrolyte solutions for the deposition of metallic coatings by electrolytic techniques.
A further object of the invention is to provide metallic coatings which are characterized by improved corrosion resistance.
Another object of the invention is to provide metallic coatings by a method characterized by improved current efficiency and throwing power.
A further object of the invention is to provide a method for the deposition of metallic coatings by an electrolytic process characterized by reduced hydrogen evolution at the cathode during deposition.
A still further object of the invention is to satisfy the need for chromium plating solutions capable of providing crack free deposits over a wide range of current densities and to provide chromium plating baths capable of depositing a suitable chromium deposit under conditions of adequate current efficiency, improved covering characteristics, and improved throwing power.
A still further object of the invention is to provide an electrolyte solution for the plating of chromium nickel alloy and nickel metallic coatings.
A still further object of the invention is to provide a method by which significantly improved coatings can be deposited upon articles of complex shape.
Broadly, the instant invention is directed to the plating of any metal which can be electroplated or deposited from solution by passing an electric current through a solution containing ions of the metal to be deposited. Such metals include chromium, manganese, iron, zinc, copper, lead, nickel, cobalt, tin, and cadmium. Chromium is of particular interest in view of its desirable appearance and high resistance to tarnish and corrosion under varied indoor and outdoor environmental conditions. It has been discovered that metallic deposits, particularly chromium deposits, having greatly improved corrosion resistance and exhibiting a very good appearance can be provided by electrolytic techniques through the use of a plating bath containing chromium ions, particularly trivalent chromium ions, ammonium ions, halide ions and a homogeneous mixture of water and a dipolar organic solvent, the molecules of which contain a highly electronegative oxygen atom as hereinafter defined.
Although the invention is particularly described herein with respect to the deposition of chromium, chromium nickel, and nickel coatings, it will be appreciated that in view of the broad scope of the problems of hydrogen evolution at the cathode, plating efficiency, throwing power, and covering power, this development is of general application in and to the electroplating of metals and is of particular advantage in connection with the plating of metals which are difficult to plate due to their position in the standard electromotive series.
The plating baths of this invention comprise a homogeneous mixture of water and a dipolar organic solvent, preferably a dipolar aprotic organic solvent, ammonium ions, and halide ions in addition to the ions of the metal to be deposited. The proportion of water in the electrolyte solvent can range from about 20 to about 60 percent by volume and the amount of organic solvent can range from about 40 to about 80 percent by volume. The preferred ratio of water to dipolar organic solvent is from 45 : 55 to about 55 : 45 by volume.
The term "dipolar organic solvent," as used herein, refers to an organic liquid which is capable of dissolving a substantial amount of a salt of the metal to be deposited and which does not donate a substantial amount or quantity of hydrogen ions. It is to be understood that the solvent must permit ionization of the dissolved metal salts in the bath. The dipolar organic molecule must contain at least one highly electronegative oxygen atom. Illustrative examples of suitable dipolar organic solvents include dimethylsulphoxide and tetrahydrothiophen dioxide. The oxygen atoms of these materials are activated by the electron donating properties of the sulphur atoms. A ketonic oxygen atom is not generally sufficiently electronegative to constitute a suitable dipolar organic solvent unless there is present an activating group adjacent to the carbon atom, for example, an amino group. Accordingly, compounds such as tetramethylurea, tertiary butyl formamide, and compounds of the formula R 1 R 2 N.OCR 3 , wherein R 1 , R 2 and R 3 may be the same or different members selected from the group consisting of hydrogen atoms, aryl groups, e.g., phenyl, or alkyl groups, preferably lower alkyl, are suitable. Such solvents include dimethylformamide (DMF) and dimethylacetamide (DMAC). It will be appreciated that mixtures of several of the above-mentioned organic solvents may be employed. A particularly preferred group of dipolar organic solvents are the dipolar aprotic organic solvents. Such aprotic solvents include dimethylsulphoxide, tetrahydrothiophen dioxide, tetramethylurea, dimethylacetamide and dimethylformamide. Of these particular solvents, dimethylformamide is especially convenient. Dimethylformamide, in addition to possessing an excellent balance of physical, chemical, toxicological, and economic properties, has shown excellent electroplating properties. Dimethylformamide is characterized by a relatively wide liquid range, a boiling point of 150° C., a high dielectric constant of about 37.5, a low vapor pressure of about 3.6 millimeters of mercury at 25° C. Dimethylformamide is also completely miscible with water in all proportions and has the ability to dissolve substantial amounts of metal salts, e.g., about 450 grams per liter of hexahydrated chromium trichloride. Dimethylformamide also interacts with such salts to form stable complexes of the type CrCl 3 .6DMF and CrCl 3 .4DMF. It is important to note that because of the ability of the DMF to function as a complexing agent, no other complexing agent need be utilized.
The metal to be deposited can be conveniently incorporated in the plating bath in a form of a metal salt which is soluble in the water-organic solvent mixture. Halide salts such as bromides, chlorides, or iodides, nitrates, acetates, formates, oxalates, and sulfates are suitable. The bromides, chlorides, and iodides with their characteristic water of hydration are preferable due to the rather limited solubility of the acetates, formates, oxalates, sulfates, and chrome alums in the dipolar solvents. In cases where the metal salt has limited solubility due to the presence of the organic liquid, solubility can be increased by the use of water concentrations near the upper end of the ranges above mentioned.
Solutions containing trivalent metal salts, such as chromic chloride, in combination with organic dipolar solvents and water, may be unstable with respect to composition and plating performance. The physical characteristics of the solution, e.g., color, pH, and viscosity, have been found to change during storage and the color and adhesion of successive metal deposits from the same bath may deteriorate. Moreover, plating baths containing low concentrations of water have a relatively low conductivity, while solutions with higher concentration of water tend to evolve excessive amounts of hydrogen at the cathode during plating. It has been found that the presence of an ammonium salt improves the stability of the solution and reduces the tendency of the bath to evolve hydrogen at high water content. Moreover, the presence of ammonium salts has been found to significantly reduce the effect of changes in pH on the lower limiting plating current density. The ammonium ion should be present at a concentration of at least about 0.2 molar and preferably from about 0.6 to about 1 molar. Solutions containing over 80 percent by volume of dimethylformamide provide limited solubility for ammonium salts. It is therefore necessary to provide sufficient water to dissolve the required amount of ammonium salt. The use of about 20 percent by volume of added water and about 80 percent by volume of dimethylformamide allows solutions containing about 1 molar concentrations of ammonium ions to be prepared at 55° C. The preferred proportion by volume of added water to dipolar organic compound is from 10 : 90 to 50 : 50.
A prerequisite of successful chromium, chromium nickel, and nickel plating from a dipolar organic solvent is that the trivalent chromium and nickel ions shall form small moderately stable complexes with the solvent molecule. If there is no complex formation, then the chromic and nickel salts are unlikely to be sufficiently soluble. If the complexes formed are excessively stable, then electrodeposition may be difficult. It is believed that the highly electronegative oxygen atoms which characterize the dipolar organic compounds of this invention may act as covalent links in the formation of complexes between the chromic ions and the organic molecules. Such solutions by themselves do not, however, give smooth coherent metallic deposits. The addition of water probably generates the polynuclear olated and oxalated chromic species usually found in solutions of trivalent chromium compounds, which again do not readily give good chromium deposits. The effect of the ammonium ion may be attributed to its structure-disordering properties simplifying the nature of the trivalent chromium and nickel species in solution, possibly with the formation of mononuclear Cr 3 +--DMF--H 2 O complexes. The water may be prevented from showing its full protic tendencies by reason of the formation of complexes with the dipolar organic molecules. The halogen ions may be partly solvated, although the ready evolution of chlorine at the anode indicates solvation is not complete.
The pH of the solution should be from 1 to 3.5 and preferably about 2. If the pH is too low, hydrogen tends to be evolved at the cathode in preference to chromium. If the pH is too high, basic chromium compounds are liable to precipitate out. The pH can be adjusted by the use of hydrochloric acid or sodium hydroxide as required.
The current efficiency of the solution may be improved by the addition of boric acid, preferably to a concentration of at least O.I.M. Boric acid is not normally soluble to the extent of more than about 0.2M.
In addition to the ammonium ions mentioned above, electroplating baths of the present invention contain a sodium halide which has been found to increase the plating range and current efficiency. Sodium halides are also beneficial in that they enhance the covering power of the bath. It is preferred that the sodium halide be present in a concentration of at least about 0.8 molar.
When applying a chromium nickel coating, the concentration of chromium in the solution is not critical, and is preferably about from 0.8M to 1.3M, particularly about 1M. Nickel ions should be present in the solution at a concentration of at least 0.05M, and a preferred concentration range is about 0.8M to 1.2M.
The invention also provides a method of electrodepositing a chromium nickel alloy, which method comprises providing a cathode in the solution described above, and an anode, and passing an electric current through the solution so as to deposit a chromium nickel alloy on the cathode.
Electroplating baths as described herein have useful current density ranges of from 0.5 to about 20 amperes per square decimeter, depending on the composition and temperature of the particular solution and on the desired composition of the deposited metal. Optimum current densities are generally within the range from about 6 to about 10 amperes per square decimeter. The useful plating current range for a particular plating solution varies with temperature, being larger at lower temperatures. Above about 30° C. narrower plating ranges are encountered. The amount of organic solvent in the bath influences the plating range and the temperature at which optimum plating occurs. For example, a solution containing a high ratio of dimethylformamide to water provides suitable plating at temperatures of about 50° C. Preferred plating temperatures are in the range of 40° to 80° C.
When depositing a chromium nickel alloy the composition of the alloy deposited depends on the composition of the plating solution, on the temperature and on the plating current density. In general, the higher the current density, the greater is the percentage of chromium in the alloy deposited, and the lower the current density, the greater is the percentage of nickel in the alloy deposited. This may be due to the fact that the threshold current density for plating with the two separate metals differs. It may be desirable to replenish the plating solution as the metals are removed from its, and this replenishment will normally be designed to maintain the relative concentrations of nickel and chromium in the solution, so as to keep constant the composition of the alloy being deposited.
When the anode of the electroplating apparatus is to be immersed in a chromium or chromium nickel solution, it is preferred that the anode be of graphite or titanium or of some other similar inert material. Unfortunately, however, the use of graphite anodes is disadvantageous because of the evolution of chlorine gas at the anode. Since chlorine is very soluble in some of the organic solvents, particularly dimethylformamide, large concentrations of chlorine can build up in the solution. The use of a nickel chromium anode prevents chlorine formation, but the chromium may dissolve off as chromic acid which is liable to oxidize the DMF. Also, if nickel or chromium anodes are used, some indefinite and largely uncontrollable contribution will be made to the concentration of these elements in the electrolyte. Passivated lead anodes have been examined, but the chloride ion tends to break the passive layer, exposing fresh metal to the solution which dissolves it intensively. Accordingly, it is preferred to immerse the anode in an anolyte which is separated from the chromium solution by means of a porous diaphragm within the cell. A suitable aqueous anolyte comprises a molar solution of ammonium or sodium acetate separated from the plating solution by a porous ceramic diaphragm. The use of such a diaphragm in the plating apparatus has provided a successful means of preventing chlorine evolution. The presence of a more highly conducting aqueous portion, i.e., the anolyte, compensates for the increased resistivity of the apparatus due to the presence of the diaphragm.
The invention described above is further illustrated by the following examples.
EXAMPLE 1
Bright nickel plated copper cathodes were plated with chromium in a bath comprising 240 grams per liter of chromic chloride hexahydrate, 58 grams per liter of sodium chloride, 50 grams per liter ammonium chloride, and 8 grams per liter of boric acid, the solvent being a 50 percent by volume mixture of water and dimethylformamide (DMF). The bath was operated at temperatures from 15° to 30° C., a current density of from 1 to 20 amperes per square decimeter, and a pH from 1 to 3. Current efficiencies of up to 50 percent were achieved.
Examples 2 - 28, in Table I below, illustrate bath compositions and plating conditions found to provide a good smooth adherent deposit of chromium. The deposit in examples 2 - 5 was initially covered by a green slime which was easily removed by dipping in a 50 percent solution of hydrochloric acid without impairing the appearance of the coating. The plating range ratio was calculated from the upper and lower current densities at which a bright plate was obtained. ##SPC1##
EXAMPLE 29
In this example, the plating performance of a trivalent organic solution according to the invention is compared with a conventional hexavalent aqueous bath. ##SPC2##
It is apparent that the organic trivalent solutions of this invention showed a wider plating range ratio, a higher current efficiency, and better covering power than the conventional hexavalent aqueous bath.
EXAMPLE 30
DMSO (Dimethylsulphoxide) Solution
Bath Composition
1 M CrCl 3 .6H 2 O
1 m naCl
0.6 M NH 4 Cl
50 percent v/v DMSO
50 percent v/v H 2 O
Conditions of Plating
Temp. : 25°C.
pH : 2.4
Current : 2 A
Voltage : 20 V
Results
C.D. (A/dm 2 ) 9.9 8.1 3.8 2.6 1.3 Efficiency % 13 9 10.1 7 Small Plating Range : 1.3-11.7 A/dm 2 Plating Range Ratio : 9
EXAMPLE 31
60 percent H 2 O, 40 percent DMF Solution
Bath Composition
1 M CrCl 3 .6H 2 O
0.5 m nh 4 cl
1 M NaCl
40 percent v/v DMF
60 percent v/v H 2 O
Conditions of Plating
Temp. : 30°C.
pH : 1.6
Current : 4 A
Voltage : 25 V ##SPC3##
EXAMPLE 32
DMAC (Dimethylacetamide) Solution
Bath Composition
1 M CrCl 3 .6H 2 O
0.8 m naCl
0.2 M NH 4 Cl
70 percent v/v DMAC
30 percent v/v H 2 O
Conditions of Plating
Temp. : 35°C.
pH : 2.3
Current : 3 A
Voltage : 30 V
Results
C.D. (A/dm 2 ) 10.6 7.25 3.98 1.66 Efficiency % 28.4 22.4 7.7 Small Plating Range : 1.66-14.6 A/dm 2 Plating Range Ratio : 9.1
EXAMPLE 33
Formamide Solution
Bath Composition
1 M CrCl 3 .6H 2 O
1 m naCl
0.5 NH 4 Cl
50 percent v/v Formamide
50 percent v/v H 2 O
Conditions of Plating
Temp. : 25°C.
pH : 2.1
Current : 4 A
Voltage : 30 V ##SPC4##
EXAMPLE 34
In an example of a plating bath for the electrodeposition of a chromium nickel alloy, the solution comprised 60 percent of dimethylformamide by volume and 40 percent of water, and contained the following salts in solution:
1 M CrCl 3 .6H 2 O
0.4 m niCl 2 .6H 2 O
0.5 m nh 4 cl
A bright adherent deposit containing 70 percent of chromium and 30 percent of nickel and having high resistance to corrosive attack by cold 50/50 v/v hydrochloric acid was obtained by electrodeposition from this solution under the following operating conditions:
pH 2.0 Temperature 20°C. Plating current density 2 A/dm 2
EXAMPLE 35
In another example of a bath according to this invention, the solution comprised 50 percent DMF by volume and 50 percent water and contained the following salts:
1 M CrCl 3 .6H 2 O
1 m niCl 2 .6H 2 O
0.5 m nh 4 cl
A bright adherent deposit containing 80 percent nickel and 20 percent chromium and having a high resistance to corrosive attack by cold 50 percent v/v HCl was obtained by electrodeposition from this solution, under the following operating conditions:
pH 2.0 Temperature 50°C. Plating current density 6.8 A/dm 2
EXAMPLE 36
Composition Operating Conditions 1 M CrCl 3 .6H 2 O pH -- 1.1 0.5 M NiCl 2 .6 H 2 O Temperature -- 25°C. 25 g/l NH 4 Cl Current Density -- 100 A/Sq.ft.
This solution yields a dark deposit containing, approximately 20 percent chromium, 80 percent nickel.
EXAMPLE 37
Composition Operating Conditions 1 M CrCl 3 .6 H 2 O pH -- 1.0 1 M NiCl 2 .6 H 2 O Temperature -- 25°C. 25 g/l NH 4 Cl Current Density -- 100 A/Sq.ft. 50 percent DMF v /v
This solution yields deposits containing, approximately 10 percent chromium, 90 percent nickel.
EXAMPLE 38
Composition Operating Conditions 1 M CrCl 3 .6 H 2 O pH -- 1.0 1 M NiCl 2 .6 H 2 O Temperature -- 25°C. 25 g/l NH 4 Cl Current Density -- 100 A/Sq.ft. 5 g/l H 3 BO 3 50 percent DMF v /v
This solution yields deposits containing, approximately 15 percent chromium, 85 percent nickel.
EXAMPLE 39
The invention also encompasses electrolyte solutions for nickel plating. For electroforming applications, e.g., the lining of dies, etc., the following proved suitable:
Composition Operating Conditions 1 M NiCl 2 .6 H 2 O Operating temperature -- 40°C. 50 g/l NH 4 Cl pH 1.9 Water 50 percent v /v Plating range 0.1 -- 400 A/sq.ft. DMF 50 percent v /v
EXAMPLE 40
Composition Operating Conditions 1 M NiCl 2 .6 H 2 O Operating temperature -- 40°C. 50 g/l NH 4 Cl pH 1.9 - 4.5 10 g/l H 3 BO 3 Plating range 0.1 - 400 A/sq.ft. Water 50 percent v /v DMF
the hardness of the deposit depends on pH, bath, two giving 570 VPN at pH 1.9 and 616 at pH 4.5, both values being recorded at a current density of 200 A/sq.ft.