US3772170A

US3772170A - Electrodeposition of chromium

Info

Publication number: US3772170A
Application number: US00226859A
Authority: US
Inventors: N Bharucha
Original assignee: Individual
Current assignee: Individual
Priority date: 1966-10-31
Filing date: 1972-02-16
Publication date: 1973-11-13
Anticipated expiration: 1990-11-13

Abstract

The specification discloses an improved method and electrolyte solution for providing metal coatings on metals. These metals are electrolytically deposited from solutions comprising aprotic dipolar organic solvents which may contain water. The specific dipolar aprotic organic solvents disclosed are dimethylsulphoxide, dimethylacetamide, tetrahydrothiophen dioxide, dimethylformamide, propylene carbonate, tetramethyl urea and hexamethyl phosphoramide.

Description

United States Patent 1 [111 3,772,170

harucha 1 Nov. 13, 1973 ELECTRODEPOSITION F CHROMIUM 902,755 11/190 Meyer 204/14 N 6] n entor: Nanabhai Rusmmji Bharucha 4530 3,336,658 8/1967 Husm 204/23 X Cote des Neige, Apt. 105, Montreal, OTHER CATIONS Q Canada R. D. Blue et al., Trans. Electrochemical Soc., Vol. 22 Filed: Feb. 16, 1972 231-238 (1933)- Donald B. Keyes et al., Studies in the Electrodepo- PP 226,859 sition of Metals, U. of Ill. Bulletin, No. 206, p. 14,

Related US. Application Data (1930)- [63] Continuation of Ser. No. 32,402, April 27, 1970, and

a continuation-in-part of Ser. No. 679,250, Oct. 30, Pnmary ExammerG' Kaplan 1967, abandoned Att0meyBrumbaugh, Graves, Donohue and Raymond Foreign Application Priority Data Oct. 31, 1966 Great Britain 48,761/66 [57] ABSTRACT The specification discloses an improved method and [52] US. Cl 204/51, 204/14 N, 204/45 R, electrolyte solution for providing metal coatings on 204/48, 204/49, 204/ R, 204/52 R, metals. These metals are electrolytically deposited 204/53, 204/54 R, 204/55 R from solutions comprising aprotic dipolar organic sol- [51] Int. Cl C23b 5/06 vents which may contain water. The specific dipolar [58] Field of Search 204/14 N, 51 aprotic organic solvents disclosed are dimethylsulphoxide, dimethylacetamide, tetrahydrothiophen diox- [56] References Cited ide, dimethylformamide, propylene carbonate, tetra- UNITED STATES PATENTS methyl urea and hexamethyl phosphoramide.

3,131,134 4/1964 Micillo 204/14 N 5 Claims, No Drawings ELECTRODEPOSITION OF CHROMIUM This is a continuation of application Ser. No. 32,402 filed Apr. 27, 1970 which is turn was a continuation-inpart of application Ser. No. 679,250 filed Oct. 30, 1967, both applications now abandoned.

The instant invention is directed to electroplating, i.e., the electrodeposition of metal coatings and to electrolyte solutions for providing coatings of such metals.

in recent times substantial improvements have been made in the appearance and performance of electrolytically deposited metal coatings. Particularly significant improvements have been achieved in the electroplating industry. However, substantial problems and disadvantages exist in the presently available processes for the preparation of metallic coatings, such as electrolytically deposited chromium coatings.

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 of chromium and other metals which are characterized by improved corrosion resistance.

Another object of the invention is to provide a method for the deposition of metallic coatings which is characterized by improved current efficiency and throwing power.

Another object of the invention is to provide a method for the deposition of metallic coatings by electrolytic techniques characterized by a reduction in the amount of hydrogen evolved at the cathode.

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.

It is recognized that considerable economic advantage would be achieved if the current efficiency of metal plating techniques could be increased. This is particularly so with respect to baths for theplating of bright chromium. it is also recognized that the evolution of hydrogen from the cathode during an electrolytic deposition process is a serious problem and that coatings of improved corrosion resistance and appearance could be provided if the evolution of hydrogen could be reduced. In the past, experimentation with the use of modified chromic acid electrolyte solutions has shown little prospect of improvement in the quality of the deposit without a concomitant reduction in current efficiency.

in the past, basic studies in typical reactions, particularly in connection with solutions containing trivalent chromium ions, have suggested several possible methods for increasing current density, extending the plating current density range, and improving the general appearance of the metallic coating. One such method, the addition of buffers to the plating bath to prevent or diminish a change in pH resulting from hydrogen evolution, produced only moderate improvements in plating performance. Another attempt at improvement involved the addition of organic complexing agents to the plating bath. Such additions were invariably associated with a substantial reduction in current efficiency.

Broadly, the instant invention is directed to the plating of metals which can be electroplated or deposited from solution such as chromium, iron, tin, manganese,

zinc, copper, lead, nickel, cobalt, and cadmium. Chromium is of particular interest in view of its desirable appearance and high intrinsic resistance to tarnish and corrosion in various indoor and outdoor environments. it has been discovered that metallic deposits, particularly chromium deposits having greatly improved corrosion resistance properties and exhibiting very good appearance, may be produced by electrolytic techniques from a plating bath containing chromium ions, preferably trivalent chromium ions, in which the electrolyte comprises a dipolar aprotic organic solvent. in view of the broad scope of the problems of hydrogen evolution at the cathode during electroplating processes, it is apparent that this development is of general application in and to the electroplating industry and is of particular advantage in connection with the plating of metals which are difficult to plate because of their position in the standard electromotive series.

The term dipolar aprotic organic solvent as used herein, including the appended claims, refers to an organic solvent which is capable of dissolving a substantial amountof 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. Preferred dipolar aprotic organic solvents are those having high dielectric constants, e.g., greater then about 15, possess one or more electron donating groups, and are capable of solvating cations in the bath. Although these solvents may contain hydrogen atoms they should not donate substantial quantities of labile hydrogen atoms to form strong hydrogen bonds or to be discharged at the cathode during electrolysis.

As examples of common dipolar aprotic solvents there may be mentioned dimethylformamide, dimethylacetamide, dimethylsulphoxide, tetrahydrothiophen dioxide, propylene carbonate, tetramethyl urea, hexamethylphosphoramide, and the like.

Preferred dipolar aprotic solvents are dimethylformamide and dimethylsulphoxide. Dimethylformamide has a relatively wide liquid range having a boiling point at C., a high dielectric constant of about 37.5 and a low vapor pressure at ordinary temperature. Dimethylformamide is also miscible with water in all proportions and has a substantial ability to dissolve chromic salts.

The dipolar aprotic organic solvents disclosed herein can be employed as the sole solvent in the plating bath. Alternatively, the plating bath may be formulated to contain water as a co-solvent. Generally, it is preferred that the amount of water be limited to amounts of up to 40 percent by weight, and it is particularly preferred to limit the amount of water present in the solution to a maximum of about 15 percent by weight.

It has been discovered that, in addition to functioning as a solvent, the dipolar aprotic solvent serves as a complexing agent. This unique quality makes the addition of any other complexing agent to the plating bath totally unnecessary and, in fact, undesirable.

The conductivity of the dipolar aprotic organic solvent can be conveniently raised to any desired degree in order to permit the passage of suitable plating currents by the addition of water alone or containing inorganic ions other than those of the metal to be deposited. Sodium chloride is one such additive which will have this effect.

The metal to be deposited can be conveniently incorporated in the plating bath in the form of a metal salt which is soluble in the dipolar aprotic solvent, e.g., a halide, nitrate, acetate, formate, oxalate, sulfate alums, and the like. Chloride salts such as hexahydrated chromic chloride are preferred. In general, the solubility of the metal salt should be sufficient to provide a metal ion concentration in the bath of at least about 0.5 molar. Satisfactory operation of plating baths, as herein disclosed, has been obtained with a concentration of about I molar with respect to chromic ions. In the case of partially aqueous solutions, the pH of the aqueous portion of the solution can be adjusted before addition of the non-aqueous solvent.

The vessel for the plating bath can be of any suitable design and may preferably be constructed of glass, ceramic, chemical stoneware, or any other suitable nonconducting material.

The plating baths of this invention are superior to conventional chromium plating baths in that they are characterized by enhanced current efficiency and plat ing rate. They have been found to function over a wider plating range, have greater covering power and improved throwing power. The chromium deposit obtained by the use of such baths has been found to be substantially crack-free.

Table 1, below, shows plating conditions and results for a series of tests carried out in a Hull type cell without a porous diaphragm and using an inert graphite anode. The cathode was nickel plated copper. The upper and lower limits of current density over which a bright plate was obtained were recorded and their ratio was recorded as the plating range ratio. Dimethylformamide (DMF) was employed as the dipolar aprotic solvent, and the solution in each case was 1 molar CrCI TABLE I Temper- Total Plating Solution ature Cell Cell Range DMF pH "C Current \roltage Ratio 10 2.2 40 3.0 40 7.2 30 2 l 42 3.0 40 7.0 50 2.0 45 3.5 42 8.0 70 2.3 55 4.0 38 6.0 90 2.3 56 4.5 70 7.0 95 L6 57 2.0 70 8.0 90 1.0 55 2.5 60 2.1 90 1.48 56 2.5 60 2.6 90 L4 55 2.5 6l 3.9 90 1.78 55 2.5 60 7.0 90 2.l 54 2.5 60 6.9 90 2.4 55 2.5 60 5.1

The invention also includes a method for the electrodeposition of a metal which comprises immersing an article which is to be cathodically plated in a plating bath containing ions of the metal to be deposited and an organic dipolar aprotic solvent, and electroplating the article by passing an electric current through the plating bath to effect electrodeposition of metal upon said cathode.

The operating conditions under which the plating of a metal upon a cathode is achieved are, at least in part,

determined by the type of equipment employed, the nature of the metallic ions to be deposited on the cathode, and the shape and composition of the cathode itself. Bath temperatures below the boiling point of the dipolar aprotic solvents are generally employed. Preferably such temperatures range from about F. to about 140 F. Plating is generally accomplished at a current density of from about I to about 20 and preferably 1 to about 15 amperes per square decimeter. The pH of the solution is generally maintained in the range from about 1 to about 2.5 and preferably about 2.

An illustrative bath may comprise dimethylformamide and water in a volume ratio of about 4:1 containing about 1 mole of a metal salt per liter of solvent.

I claim:

I. A process for depositing a chromium coating on an article which comprises the steps of immersing said article as the cathode of an electroplating apparatus into a plating solution comprising a dipolar aprotic organic solvent selected from the group consisting of dimethylformamide, dimethylsulphoxide, dimethylacetamide, and tetrahydrothiophen dioxide, trivalent chromium ions, and a complexing agent which consists essentially of said dipolar aprotic organic solvent, and passing an electric current through said plating solution thereby to effect electrodeposition of the chromium onto said article.

2. The process of claim 1 wherein chromium metal is electrodeposited from a solution comprising about one mole of chromic chloride hexahydrate per liter of a solution comprising dimethylformamide and water in a volume ratio of about 4:1.

3. An electrolyte solution for the electrodeposition of chromium comprising trivalent chromium ions, a dipolar aprotic organic solvent and a complexing agent which consists essentially of said dipolar aprotic organic solvent, said solution consisting essentially of about I mole of chromic hexahydrate per liter of solvent and said solent comprising dimethylformamide as complexing agent in water in a volume ratio of about 4:1.

4. A process for depositing a chromium coating on an article which comprises the steps of immersing said article as the cathode of an electroplating apparatus into a plating solution comprising a dipolar aprotic organic solvent, trivalent chromium ions, a complexing agent which consists essentially of said dipolar aprotic organic solvent and from about It) percent to percent by weight water, and passing an electric current through said plating solution thereby to effect electrodeposition of the chromium onto said article.

5. An electrolyte solution for the electrodeposition of chromium comprising trivalent chromium ions, a dipolar aprotic organic solvent and a complexing agent which consists essentially of said dipolar aprotic organic solvent and from about 10 percent to 90 percent by weight water.

izgigio j NI ED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,772,170 Dated November 13, 1973 Inventor(s) Nanabhai Rustomji Bharucha It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 4, line 40, solen t" should be -solvent--.

Signed and sealed this 9th day of April 197A.

(SEAL) Attest:

EDWARD I'-i'.FLETCHER,JR. C. MARSHALL DAMN Attesting Officer Commissioner of Patents

Claims

2. The process of claim 1 wherein chromium metal is electrodeposited from a solution comprising about one mole of chromic chloride hexahydrate per liter of a solution comprising dimethylformamide and water in a volume ratio of about 4:1.
3. An electrolyte solution for the electrodeposition of chromium comprising trivalent chromium ions, a dipolar aprotic organic solvent and a complexing agent which consists essentially of said dipolar aprotic organic solvent, said solution consisting essentially of about 1 mole of chromic hexahydrate per liter of solvent and said solent comprising dimethylformamide as complexing agent in water in a volume ratio of about 4:1.
4. A process for depositing a chromium coating on an article which comprises the steps of immersing said article as the cathode of an electroplating apparatus into a plating solution comprising a dipolar aprotic organic solvent, trivalent chromium ions, a complexing agent which consists essentially of said dipolar aprotic organic solvent and from about 10 percent to 90 percent by weight water, and passing an electric current through said plating solution thereby to effect electrodeposition of the chromium onto said article.
5. An electrolyte solution for the electrodeposition of chromium comprising trivalent chromium ions, a dipolar aprotic organic solvent and a complexing agent which consists essentially of said dipolar aprotic organic solvent and from about 10 percent to 90 percent by weight water.