PROCESS FOR ACID COPPER PLATING OF ZINC
United States Patent 3664933
A process for the acid copper plating of zinc, particularly complex zinc parts having recessed areas, wherein a displacement copper deposit is formed on the zinc surface by treatment with an immersion copper plating bath and, thereafter, the thus-plated surface is electroplated with copper from an acid copper electroplating bath.
US Patent References:
Plated zinc base articles and method of making
Harrover - November 1966 - 3282659
Method of copper-plating metal surfaces
Strauss - September 1959 - 2903403
Anticorrosion process for zinc base castings
Ford - August 1938 - 2128550
Plated zinc base articles and method of making
Harrover - November 1966 - 3282659
Method of copper-plating metal surfaces
Strauss - September 1959 - 2903403
Anticorrosion process for zinc base castings
Ford - August 1938 - 2128550
Application Number:
04/834909
Publication Date:
05/23/1972
Filing Date:
05/19/1969
View Patent Images:
Export Citation:
Assignee:
Udylite Corporation (Warren, MI)
Primary Class:
Other Classes:
106/1.220, 106/1.260, 205/187, 205/191, 106/1.230, 106/1.270
International Classes:
C23C18/38; C25D5/34; C23C18/31; C23C3/00; C23F17/00
Field of Search:
117/13E 106/1 148/6.14 204/38R,38B,38S,40,6 29/199
Other References:
Metal Finishing Guidebook Directory, 1966, pages 422-425 .
Handbook of Chemistry & Physics, 32nd edition, 1950-1951, page 1521 .
Electroless Plating for Electronic Applications by Lordi, Plating, April 1967 pages 382-383.
Primary Examiner:
Mack, John H.
Assistant Examiner:
Andrews R. L.
Claims:
What is claimed is
1. A process for the copper electroplating of complex zinc die castings having recessed areas which comprises applying an electrolytic copper strike to the zinc surface to be treated, treating the thus-strike plated zinc surface with an immersion copper plating bath, forming a displacement copper deposit on the uncoated, recessed, zinc surface and, thereafter, electrodepositing copper on the thus-treated surfaces from an aqueous acidic copper electroplating bath.
2. The process as claimed in claim 1 wherein subsequent to the formation of the displacement copper deposit on the zinc surface and prior to the copper electrodeposit from the acid copper plating bath, the surface is treated with an immersion nickel plating bath to form a displacement nickel deposit on the surface.
3. The process as claimed in claim 2 wherein the immersion copper plating bath is an aqueous solution comprising from about 8 to 30 grams per liter of copper supplied by a water soluble copper salt, from about 30 to 350 grams per liter of a primary complexing agent for cupric ions, from about 5 to 80 grams per liter of an alkali metal hydroxide, from about 5 to 70 grams per liter of an accelerator selected from alkali metal chlorides and from about 0.2 to 5 grams per liter of a secondary complexing agent selected from alkali metal cyanides and ammonium hydroxide.
4. The process as claimed in claim 3 wherein the water soluble copper salt is copper sulfate, the primary complexing agent is sodium gluconate and the accelerator is sodium chloride and the secondary complexing agent is sodium cyanide.
5. A process for the copper electroplating of complex zinc die castings having recessed areas which comprises applying an electrolytic copper strike to the zinc surface to be treated, treating the thus strike plated zinc surface with an immersion copper plating bath which is an aqueous solution comprising from about 8 to 30 grams per liter of copper supplied by a water soluble copper salt, from about 30 to 350 grams per liter of a primary complexing agent for cupric ions, from about 5 to 80 grams per liter of an alkali metal hydroxide, from about 5 to 70 grams per liter of an accelerator selected from alkali metal chlorides and 0.2 to 5 grams per liter of a secondary complexing agent selected from alkali metal cyanides and ammonium hydroxide, forming a displacement copper deposit on the uncoated, recessed, zinc surface and, thereafter, electrodepositing copper on the thus treated surfaces from aqueous acidic copper electroplating bath.
6. The process as claimed in claim 5 wherein the water soluble copper salt is copper sulfate, the primary complexing agent for cupric ions is sodium gluconate and the accelerator is sodium chloride and the secondary complexing agent is sodium cyanide.
1. A process for the copper electroplating of complex zinc die castings having recessed areas which comprises applying an electrolytic copper strike to the zinc surface to be treated, treating the thus-strike plated zinc surface with an immersion copper plating bath, forming a displacement copper deposit on the uncoated, recessed, zinc surface and, thereafter, electrodepositing copper on the thus-treated surfaces from an aqueous acidic copper electroplating bath.
2. The process as claimed in claim 1 wherein subsequent to the formation of the displacement copper deposit on the zinc surface and prior to the copper electrodeposit from the acid copper plating bath, the surface is treated with an immersion nickel plating bath to form a displacement nickel deposit on the surface.
3. The process as claimed in claim 2 wherein the immersion copper plating bath is an aqueous solution comprising from about 8 to 30 grams per liter of copper supplied by a water soluble copper salt, from about 30 to 350 grams per liter of a primary complexing agent for cupric ions, from about 5 to 80 grams per liter of an alkali metal hydroxide, from about 5 to 70 grams per liter of an accelerator selected from alkali metal chlorides and from about 0.2 to 5 grams per liter of a secondary complexing agent selected from alkali metal cyanides and ammonium hydroxide.
4. The process as claimed in claim 3 wherein the water soluble copper salt is copper sulfate, the primary complexing agent is sodium gluconate and the accelerator is sodium chloride and the secondary complexing agent is sodium cyanide.
5. A process for the copper electroplating of complex zinc die castings having recessed areas which comprises applying an electrolytic copper strike to the zinc surface to be treated, treating the thus strike plated zinc surface with an immersion copper plating bath which is an aqueous solution comprising from about 8 to 30 grams per liter of copper supplied by a water soluble copper salt, from about 30 to 350 grams per liter of a primary complexing agent for cupric ions, from about 5 to 80 grams per liter of an alkali metal hydroxide, from about 5 to 70 grams per liter of an accelerator selected from alkali metal chlorides and 0.2 to 5 grams per liter of a secondary complexing agent selected from alkali metal cyanides and ammonium hydroxide, forming a displacement copper deposit on the uncoated, recessed, zinc surface and, thereafter, electrodepositing copper on the thus treated surfaces from aqueous acidic copper electroplating bath.
6. The process as claimed in claim 5 wherein the water soluble copper salt is copper sulfate, the primary complexing agent for cupric ions is sodium gluconate and the accelerator is sodium chloride and the secondary complexing agent is sodium cyanide.
Description:
This invention relates to a method for the copper plating of zinc surfaces and more particularly it relates to an improved process for the copper electroplating of complex zinc dicastings having recessed areas.
Acid copper electroplating baths have long been used in industry, particularly in the decorative field as an undercoat for nickel-chromium deposits. With the development in recent years of processes which will produce a fully bright, ductle, high leveling copper deposit, there has been an even wider use of acid copper plating solutions. With such solutions, it is now possible to process large zinc components from the leveling acid copper solutions directly to nickel and chromium plating baths, without the necessity for finishing the die casting to the degree formerly required. In addition to the savings resulting from the reductions in polishing or buffing cost, it has further been found that these copper deposits contribute to the durability of decorative coatings, thus making it possible to substitute copper for a portion of the nickel plating with micro-porous or micro-cracked chromium, without loss of durability.
Although these acid copper plating solutions offer many advantages, they are subject to one major difficulty which has limited their even greater use. This difficulty is their inherent tendency, because of the acidic nature of the solutions, to vigorously attack the zinc surfaces, with the resulting deposition of a loosely adherent displacement or immersion copper deposit on the zinc surfaces which are treated therewith. Although in some instances, this difficulty can be overcome by applying an electroplated strike of copper or nickel, where complex zinc parts are treated, particularly complex zinc die castings having blind holds and closed cavities, the strike deposit will not cover or will not be sufficiently thick to retard attack by the acid copper solution. There is, therefore, a deposition in such areas, of a non-adherent copper deposit which is easily dislodged when the part is subjected to some movement, as in rinse tanks or agitated nickel plating solutions. When this occurs, the effects in the nickel plating solution are roughness and pitting and undesirable solution contamination of the nickel bath. It is because of this difficulty that the use and application of this acid copper plating solution has been limited to the plating of relatively simple shapes which permit complete and adequate coverage by the electrodeposition of the copper strike, preceding the electrodeposition from the acid copper electroplating solution.
It is, therefore, an object of the present invention to provide an improved process for the electrodeposition of acid copper plating solutions which permit the use of such solutions on complex zinc parts.
Another object of the present invention is to provide an improved process for forming an adherent deposit from an acid copper plating solution, even in recessed areas of complex zinc parts.
A further object of the present invention is to provide an improved immersion plating composition for forming a displacement copper coating on zinc surfaces, prior to the electrodeposition of copper from an acid copper plating solution.
These and other objects will become apparent to those skilled in the art from the description of the invention which follows.
Pursuant to the above objects, the present invention includes applying an electrolytic copper strike to the zinc surface, treating the thus-strike plated zinc surface with an immersion copper plating bath, forming a displacement copper deposit on the zinc surface and, thereafter, electrodepositing copper on the thus-treated surface from an aqueous acidic copper electroplating bath. In this manner, an adherent copper plate is obtained from the acid copper plating bath over the entire zinc surface being treated, even in those areas, such as recesses and blind holes, which receive little or no electrical current.
More specifically, in the practice of the method of the present invention, the zinc surface is desirably cleaned, using various conventional cleaning techniques, prior to treatment with the plating solutions of the present invention. As are known to those in the art, such cleaning techniques may utilize alkaline, acidic, or organic solvent cleaning compositions, and may include spraying, scrubbing, vapor degreasing, ultrasonic cleaning, steam cleaning, and the like. Once the cleaning and or other surface preparation of the zinc surface has been completed, the electrolytic copper strike is applied to the surface.
Where a copper strike is applied, the various conventional copper strike plating baths, such as the copper cyanide baths, may be used. Such baths generally contain copper cyanide, an alkali metal cyanide, and an alkali metal hydroxide. Additionally, such baths may also contain Rochelle salts or other addition agents which may aid in the operation of the bath by modifying the structure of the deposits, retarding carbonate buildup in the bath, or assisting in anode corrosion. The operation of such copper strike baths is conventional and well known to those in the art. Typically, these baths are operated at temperatures of from room temperature up to about 70° C. for plating times of about 1/2 to 4 minutes at a tank voltage of from about 4 to 6 volts. It is to be appreciated, of course, that the copper strike plate may be either copper or a copper alloy, e.g., brass, or the like.
Following the application of the electrolytic copper strike plate, the zinc surfaces are treated with an immersion copper plating bath. Various immersion copper plating baths which will form a displacement copper deposit on the zinc surfaces may be used for this purpose, provided, however, that the plating solution does not unduly attack the zinc surface being treated, while still providing sufficient attack of the surface that the formation of the desired displacement copper deposit can be effected in a reasonable period of time. The immersion copper plating solutions used may be either acidic or alkaline solutions and may be operated over a wide range of pH values. In general, it has been found that better control of these plating solutions, and hence a more satisfactory displacement copper deposit is formed, where the solutions used are those containing a complex or chelate of copper, rather than merely simple copper salts. Such solutions may contain a water soluble copper salt, a primary complexing agent for cupric ions, an alkali metal hydroxide, and an accelerator and/or secondary complexing agent for cuprous ions.
Typical of the water soluble copper salts which may be used in such copper immersion plating solutions are copper sulfate, which is preferred, copper chloride, copper acetate, copper carbonate and the like. Desirably, the solutions will contain the copper salts in amounts giving a copper metal content within the range of about 8 to 30 grams per liter, with amounts within the range of about 12 to 24 grams per liter being preferred.
Various complexing agents, which will complex the cupric ions in the immersion plating solution, may be used. Typical of such complexing agents are the alkaline metal gluconates, such as sodium gluconate, citric acid, tartrates, such as Rochelle salts, ethylene diamine, diethylene triamine, diethanol glyoxime, ethylene diamine tetraacetic acid, lactonitrile and the like. Desirably, such complexing agents are included in the plating solution in amounts within the range of about 30 to 350 grams per liter, with amounts within the range of about 60 to 280 grams per liter being preferred.
Although the alkali metal hydroxides, such as sodium hydroxide, are preferred as components for the copper immersion plating baths, other alkaline materials, such as the alkali metal carbonates, may also be used. Such alkaline materials are desirably present in the plating bath in amounts within the range of about 5 to 80 grams per liter and preferably in amounts within the range of about 10 to 60 grams per liter.
In addition to the above components, the immersion copper plating baths for use in the method of the present invention also desirably contain a component which acts to accelerate the plating action of the bath and a component which acts to complex any cuprous ions formed during plating. The alkali metal chlorides, such as sodium chloride, or the alkaline earth metal chlorides, such as magnesium chloride, have been found to be particularly preferred accelerating materials. As complexing agents for cuprous ions, the alkali metal cyanides, such as sodium cyanide, have been found to give excellent results, although similar results are also obtained when using ammonium hydroxide.
The accelerator is desirably present in the bath in amounts within the range of about 5 to 70 grams/liter, with amounts within the range of about 10 to 50 grams/liter being preferred. Desirably, the cuprous ion complexing agent is present in the bath in amounts within the range of about 0.2 to 5 grams/liter, with amounts within the range of about 0.5 to 2.5 grams/liter being preferred.
Exemplary of particularly preferred immersion copper plating baths of the above type is a bath wich is an aqueous solution containing the following components in the amounts indicated:
Copper sulfate 40 to 90 grams per liter sodium gluconate 150 to 250 grams per liter sodium hydroxide 25 to 80 grams per liter sodium cyanide 0.5 to 2 grams per liter pH 10.5 to 12.5 grams per liter
As has been previously noted, these immersion copper plating baths may be operated over a relatively wide pH range, depending upon the quantity and type of the components which are used to formulate the bath. pH values for the plating bath within the range of about 4 to 13 are typical, although in many instances, pH values on the acid side of from about 5 to 6 or on the alkaline side or from about 10 to 13, are preferred.
The copper strike electroplated zinc surface is contacted, preferably by immersion with the immersion copper plating bath for a period sufficient to form the desired displacement copper deposit on the surface. Desirably, the immersion copper plating solution is at an elevated temperature, temperatures within the range of about 45° to 70° C. being typical, and the contact times used are desirably from about 2 to 5 minutes. It is to be appreciated, however, that in some instances, depending upon the specific operating conditions used, temperatures and times which are outside of these ranges may also be utilized to produce satisfactory results. During the formation of the displacement copper deposit, the immersion copper plating bath may be operated as a still bath or it may be agitated, using mechanical agitation or air agitation, whichever is desired.
Desirably, the immersion copper plating solution is maintained in contact with the zinc surfaces being treated for a period sufficient to produce a displacement copper coating on the surface having a thickness of from about five to thirty-five millionths of an inch. It is found that the displacement copper coatings produced are quite porous and this inherent porosity permits a controlled attack by the subsequently applied acid copper plating solution through the pores, to produce a second adherent copper layer. Although it is important that the displacement copper deposit formed from the immersion copper plating solution have some porosity, where the porosity is too great, the subsequently applied acid copper electroplating solution may corrode the zinc surface too rapidly through these pores at too many points, resulting in undercutting of the immersion copper displacement deposit with resulting poor adhesion of the total copper deposit. This has been found to occur where the parts treated are quite complex in shape, having numerous internal surfaces on which immediate direct coverage with the immersion copper plating solution cannot be obtained. In such instances, this difficulty can sometimes be overcome by utilizing longer immersion times in the copper immersion plating solution and/or by the use of agitation of the immersion copper plating solutions so as to build up a displacement copper plating having the necessary thickness to control the subsequent attack of the acid copper plating solution.
In some cases, depending upon the complexity of the shape of the parts being treated, the above techniques may not be sufficient to effect the desired reduction in the porosity in the immersion copper deposit. In such instances, prior to the treatment with the acid copper electroplating solution, it is desirable to treat the parts in an immersion nickel plating solution so as to form a displacement nickel coating on the previously formed displacement copper deposit, thereby sealing some of the porosity of the displacement copper coating. Although various immersion nickel plating baths may be used, such baths are desirably aqueous solutions having a pH of from about 3 to 5 and containing up to about 500 grams per liter of nickel chloride, up to about 300 grams per liter nickel sulfate, up to about 100 grams per liter sodium chloride and up to about 45 grams per liter boric acid. Preferably, the immersion nickel plating bath contains from about 60 to 200 grams per liter nickel chloride, from about 75 to 175 grams per liter nickel sulfate, from about 20 to 40 grams per liter boric acid and up to about 60 grams per liter of sodium chloride.
The zinc surfaces, which have been previously given a copper or nickel electro strike plate and the displacement copper coating, are contacted, preferably by immersion, with the immersion nickel plating solution. Desirably, this immersion nickel plating solution is at a temperature within the range of about 25° to 60° C., temperatures of from about 40° to 55° C. being typical, and the contact times are typically within the range of about 1 to 3 minutes. In some instances, it has been found that by the use of this treatment with the immersion nickel plating solution, in combination with the immersion copper plating solution, it is possible to obtain satisfactory results with shorter contact times in the immersion copper plating solution than are possible when using this solution alone. Thus, when using the combination of the immersion copper and immersion nickel plating steps, total contact times in the two immersion plating solutions of from about 2 to 4 minutes have been found to be typical.
Following the application of the displacement copper coating or the application of the displacement copper and the displacement nickel coating, the zinc surface is then electroplated, using an acid copper plating bath. Various acid copper electroplating baths, as are known to those in the art, may be used for the application of the copper electroplate in accordance with the method of the present invention.
Typically, such acid copper electroplating baths are aqueous acidic solutions of copper sulfate, copper fluoborate, copper nitrate, copper sulfamate, the copper alkyl sulfonates and disulfonates and the like. Generally, such acid copper plating baths will also contain one or more additives which are effective in improving the lustre, leveling, ductility, and the like of the copper electroplate obtained. Typical of such acid copper electroplating baths and the additives which they may contain are the plating baths described in U.S. Pat. Nos. 2,707,166; 3,267,010; and 3,288,690. It is to be appreciated, of course, that other acid copper electroplating baths, other than those specifically set forth, may also be used to effect the desired copper plating of the zinc surfaces.
The copper electroplate is applied to the zinc surfaces by the electrolysis of the acid copper electroplating baths in the conventional manner, as is known to those in the art. Generally, such baths will be operated so as to produce a copper plate having a thickness of from about 0.0002 to .0015 inches, typical operating conditions to produce such a plate including bath temperatures of from about 18° to 60° C., average current densities of from about 15 to 300 amps/square foot and plating times of from about 10 to 40 minutes. The thus-copper electroplating surfaces may then, if desired, be further electroplated with nickel and/or chromium to produce the final plated zinc parts.
The copper electroplate produced by the electrolysis of the acid copper plating bath, in accordance with the method of the present invention, is found to be bright, ductle, and adherent. Moreover, the adherency of this copper plate is found to be excellent, even in cavities and other recessed areas of the zinc surface where there is very low or even no current density during the electroplating process. Thus, by means of the method of the present invention, wherein a displacement copper or a displacement copper and a displacement nickel coating are applied to the zinc surface subsequent to the application of a copper or nickel electrostrike and prior to the application of the copper electroplate from an acid copper plating solution, the areas in which such bright acid copper electroplating solutions can be successfully used are greatly extended so that the advantages resulting from the use of such electroplating solutions can now be realized even in the plating of complicated and complex zinc surfaces.
In order that those skilled in the art may better understand the present invention and the manner in which it may be practiced, the following specific examples are given. In these examples, unless otherwise indicated, parts and percents are by weight and temperatures are in degrees centigrade. It is to be appreciated, however, that these examples are merely exemplary of the present invention and the manner in which it may be practiced and are not to be taken as a limitation thereof.
EXAMPLE 1
A hollow zinc die cast nozzle approximately 6 inches in length and open at both ends was plated for 2 minutes in a standard copper cyanide strike solution, followed by a 2-minute immersion copper deposit from a solution of the following composition:
CuSO 4 . 5H 2 O 77.8 g./l. NaC 6 O 7 H 11 290 g./l. NaOH 30.5 g./l. NaCl 23 g./l.
at a temperature of 130° F. and a pH of 12.5. The part was then plated in a bright acid copper solution for 15 minutes. The deposit on the interior of the nozzle was smooth and adherent.
EXAMPLE 2
Another hollow zinc die cast nozzle was processed exactly as above but the 2-minute immersion copper treatment was omitted. After plating in the acid copper solution, the deposit on the inside of the nozzle was rough and loosely adherent copper flaked off the zinc surface.
EXAMPLE 3
Zinc die cast mirror support arms having a deep cavity completely closed at one end even given a 2-minute copper strike deposit from an alkaline copper solution followed by a six minute immersion treatment in a solution of the following composition:
CuSO 4 . 5H 2 O 80.5 g./l. NaC 6 O 7 H 11 252 g./l. NaOH 11 g./l. NaCl 45 g./l. NaCu 1 g./l.
at a pH of 11.5 and a temperature of 150° F. The parts were then plated in a bright acid copper solution for 20 minutes. The deposits on the interior of the deep closed cavity were smooth and adherent.
EXAMPLE 4
Similar zinc die cast mirror support arms were treated as above with a 2-minute copper strike deposit and a 2-minute immersion copper deposit, from the same solution as described in Example 3, followed by a 2-minute nickel immersion deposit from a solution of the following composition:
190 g./l. NiSO 4 . 6H 2 O 190 g./l. NiCl 2 . 6H 2 O 30 g./l. H 3 BO 3
at a pH of 4.5 and a temperature of 110° F. Following plating in an acid copper solution for 15 minutes, a smooth, adherent deposit was present on the inside surfaces of the deep cavity.
EXAMPLE 5
Similar zinc die cast support arms were processed as in Example 3 but the copper immersion treatment was omitted. After acid copper plating for 15 minutes, the inside surfaces of the casting were covered with rough, loosely adherent deposits which readily flaked loose.
EXAMPLE 6
Small cup shaped zinc die cast buttons having an opening at one end were processed with a 2-minute alkaline copper strike and then immersed for 3 minutes in a solution of the following composition:
CuO 4 . 5 H 2 O 70 g./l. Na 3 C 6 H 5 O 7 248 g./l. Na 2 CO 3 80 g./l. NaCl 15 g./l.
at a pH of 10 and a temperature of 130° F. Following plating for 20 minutes in a bright acid copper solution, adherent deposits were found on the inside of the button.
EXAMPLE 7
Similar zinc die cast buttons processed in an identical manner but without the immersion copper deposit produced rough loosely adherent deposits on the inside surfaces of the die cast button.
While there have been described various embodiments of the present invention, it is to be appreciated that these specific embodiments are intended to be merely exemplary of the preferred manner in which the invention, as described by the following claims, may be practiced.
Acid copper electroplating baths have long been used in industry, particularly in the decorative field as an undercoat for nickel-chromium deposits. With the development in recent years of processes which will produce a fully bright, ductle, high leveling copper deposit, there has been an even wider use of acid copper plating solutions. With such solutions, it is now possible to process large zinc components from the leveling acid copper solutions directly to nickel and chromium plating baths, without the necessity for finishing the die casting to the degree formerly required. In addition to the savings resulting from the reductions in polishing or buffing cost, it has further been found that these copper deposits contribute to the durability of decorative coatings, thus making it possible to substitute copper for a portion of the nickel plating with micro-porous or micro-cracked chromium, without loss of durability.
Although these acid copper plating solutions offer many advantages, they are subject to one major difficulty which has limited their even greater use. This difficulty is their inherent tendency, because of the acidic nature of the solutions, to vigorously attack the zinc surfaces, with the resulting deposition of a loosely adherent displacement or immersion copper deposit on the zinc surfaces which are treated therewith. Although in some instances, this difficulty can be overcome by applying an electroplated strike of copper or nickel, where complex zinc parts are treated, particularly complex zinc die castings having blind holds and closed cavities, the strike deposit will not cover or will not be sufficiently thick to retard attack by the acid copper solution. There is, therefore, a deposition in such areas, of a non-adherent copper deposit which is easily dislodged when the part is subjected to some movement, as in rinse tanks or agitated nickel plating solutions. When this occurs, the effects in the nickel plating solution are roughness and pitting and undesirable solution contamination of the nickel bath. It is because of this difficulty that the use and application of this acid copper plating solution has been limited to the plating of relatively simple shapes which permit complete and adequate coverage by the electrodeposition of the copper strike, preceding the electrodeposition from the acid copper electroplating solution.
It is, therefore, an object of the present invention to provide an improved process for the electrodeposition of acid copper plating solutions which permit the use of such solutions on complex zinc parts.
Another object of the present invention is to provide an improved process for forming an adherent deposit from an acid copper plating solution, even in recessed areas of complex zinc parts.
A further object of the present invention is to provide an improved immersion plating composition for forming a displacement copper coating on zinc surfaces, prior to the electrodeposition of copper from an acid copper plating solution.
These and other objects will become apparent to those skilled in the art from the description of the invention which follows.
Pursuant to the above objects, the present invention includes applying an electrolytic copper strike to the zinc surface, treating the thus-strike plated zinc surface with an immersion copper plating bath, forming a displacement copper deposit on the zinc surface and, thereafter, electrodepositing copper on the thus-treated surface from an aqueous acidic copper electroplating bath. In this manner, an adherent copper plate is obtained from the acid copper plating bath over the entire zinc surface being treated, even in those areas, such as recesses and blind holes, which receive little or no electrical current.
More specifically, in the practice of the method of the present invention, the zinc surface is desirably cleaned, using various conventional cleaning techniques, prior to treatment with the plating solutions of the present invention. As are known to those in the art, such cleaning techniques may utilize alkaline, acidic, or organic solvent cleaning compositions, and may include spraying, scrubbing, vapor degreasing, ultrasonic cleaning, steam cleaning, and the like. Once the cleaning and or other surface preparation of the zinc surface has been completed, the electrolytic copper strike is applied to the surface.
Where a copper strike is applied, the various conventional copper strike plating baths, such as the copper cyanide baths, may be used. Such baths generally contain copper cyanide, an alkali metal cyanide, and an alkali metal hydroxide. Additionally, such baths may also contain Rochelle salts or other addition agents which may aid in the operation of the bath by modifying the structure of the deposits, retarding carbonate buildup in the bath, or assisting in anode corrosion. The operation of such copper strike baths is conventional and well known to those in the art. Typically, these baths are operated at temperatures of from room temperature up to about 70° C. for plating times of about 1/2 to 4 minutes at a tank voltage of from about 4 to 6 volts. It is to be appreciated, of course, that the copper strike plate may be either copper or a copper alloy, e.g., brass, or the like.
Following the application of the electrolytic copper strike plate, the zinc surfaces are treated with an immersion copper plating bath. Various immersion copper plating baths which will form a displacement copper deposit on the zinc surfaces may be used for this purpose, provided, however, that the plating solution does not unduly attack the zinc surface being treated, while still providing sufficient attack of the surface that the formation of the desired displacement copper deposit can be effected in a reasonable period of time. The immersion copper plating solutions used may be either acidic or alkaline solutions and may be operated over a wide range of pH values. In general, it has been found that better control of these plating solutions, and hence a more satisfactory displacement copper deposit is formed, where the solutions used are those containing a complex or chelate of copper, rather than merely simple copper salts. Such solutions may contain a water soluble copper salt, a primary complexing agent for cupric ions, an alkali metal hydroxide, and an accelerator and/or secondary complexing agent for cuprous ions.
Typical of the water soluble copper salts which may be used in such copper immersion plating solutions are copper sulfate, which is preferred, copper chloride, copper acetate, copper carbonate and the like. Desirably, the solutions will contain the copper salts in amounts giving a copper metal content within the range of about 8 to 30 grams per liter, with amounts within the range of about 12 to 24 grams per liter being preferred.
Various complexing agents, which will complex the cupric ions in the immersion plating solution, may be used. Typical of such complexing agents are the alkaline metal gluconates, such as sodium gluconate, citric acid, tartrates, such as Rochelle salts, ethylene diamine, diethylene triamine, diethanol glyoxime, ethylene diamine tetraacetic acid, lactonitrile and the like. Desirably, such complexing agents are included in the plating solution in amounts within the range of about 30 to 350 grams per liter, with amounts within the range of about 60 to 280 grams per liter being preferred.
Although the alkali metal hydroxides, such as sodium hydroxide, are preferred as components for the copper immersion plating baths, other alkaline materials, such as the alkali metal carbonates, may also be used. Such alkaline materials are desirably present in the plating bath in amounts within the range of about 5 to 80 grams per liter and preferably in amounts within the range of about 10 to 60 grams per liter.
In addition to the above components, the immersion copper plating baths for use in the method of the present invention also desirably contain a component which acts to accelerate the plating action of the bath and a component which acts to complex any cuprous ions formed during plating. The alkali metal chlorides, such as sodium chloride, or the alkaline earth metal chlorides, such as magnesium chloride, have been found to be particularly preferred accelerating materials. As complexing agents for cuprous ions, the alkali metal cyanides, such as sodium cyanide, have been found to give excellent results, although similar results are also obtained when using ammonium hydroxide.
The accelerator is desirably present in the bath in amounts within the range of about 5 to 70 grams/liter, with amounts within the range of about 10 to 50 grams/liter being preferred. Desirably, the cuprous ion complexing agent is present in the bath in amounts within the range of about 0.2 to 5 grams/liter, with amounts within the range of about 0.5 to 2.5 grams/liter being preferred.
Exemplary of particularly preferred immersion copper plating baths of the above type is a bath wich is an aqueous solution containing the following components in the amounts indicated:
Copper sulfate 40 to 90 grams per liter sodium gluconate 150 to 250 grams per liter sodium hydroxide 25 to 80 grams per liter sodium cyanide 0.5 to 2 grams per liter pH 10.5 to 12.5 grams per liter
As has been previously noted, these immersion copper plating baths may be operated over a relatively wide pH range, depending upon the quantity and type of the components which are used to formulate the bath. pH values for the plating bath within the range of about 4 to 13 are typical, although in many instances, pH values on the acid side of from about 5 to 6 or on the alkaline side or from about 10 to 13, are preferred.
The copper strike electroplated zinc surface is contacted, preferably by immersion with the immersion copper plating bath for a period sufficient to form the desired displacement copper deposit on the surface. Desirably, the immersion copper plating solution is at an elevated temperature, temperatures within the range of about 45° to 70° C. being typical, and the contact times used are desirably from about 2 to 5 minutes. It is to be appreciated, however, that in some instances, depending upon the specific operating conditions used, temperatures and times which are outside of these ranges may also be utilized to produce satisfactory results. During the formation of the displacement copper deposit, the immersion copper plating bath may be operated as a still bath or it may be agitated, using mechanical agitation or air agitation, whichever is desired.
Desirably, the immersion copper plating solution is maintained in contact with the zinc surfaces being treated for a period sufficient to produce a displacement copper coating on the surface having a thickness of from about five to thirty-five millionths of an inch. It is found that the displacement copper coatings produced are quite porous and this inherent porosity permits a controlled attack by the subsequently applied acid copper plating solution through the pores, to produce a second adherent copper layer. Although it is important that the displacement copper deposit formed from the immersion copper plating solution have some porosity, where the porosity is too great, the subsequently applied acid copper electroplating solution may corrode the zinc surface too rapidly through these pores at too many points, resulting in undercutting of the immersion copper displacement deposit with resulting poor adhesion of the total copper deposit. This has been found to occur where the parts treated are quite complex in shape, having numerous internal surfaces on which immediate direct coverage with the immersion copper plating solution cannot be obtained. In such instances, this difficulty can sometimes be overcome by utilizing longer immersion times in the copper immersion plating solution and/or by the use of agitation of the immersion copper plating solutions so as to build up a displacement copper plating having the necessary thickness to control the subsequent attack of the acid copper plating solution.
In some cases, depending upon the complexity of the shape of the parts being treated, the above techniques may not be sufficient to effect the desired reduction in the porosity in the immersion copper deposit. In such instances, prior to the treatment with the acid copper electroplating solution, it is desirable to treat the parts in an immersion nickel plating solution so as to form a displacement nickel coating on the previously formed displacement copper deposit, thereby sealing some of the porosity of the displacement copper coating. Although various immersion nickel plating baths may be used, such baths are desirably aqueous solutions having a pH of from about 3 to 5 and containing up to about 500 grams per liter of nickel chloride, up to about 300 grams per liter nickel sulfate, up to about 100 grams per liter sodium chloride and up to about 45 grams per liter boric acid. Preferably, the immersion nickel plating bath contains from about 60 to 200 grams per liter nickel chloride, from about 75 to 175 grams per liter nickel sulfate, from about 20 to 40 grams per liter boric acid and up to about 60 grams per liter of sodium chloride.
The zinc surfaces, which have been previously given a copper or nickel electro strike plate and the displacement copper coating, are contacted, preferably by immersion, with the immersion nickel plating solution. Desirably, this immersion nickel plating solution is at a temperature within the range of about 25° to 60° C., temperatures of from about 40° to 55° C. being typical, and the contact times are typically within the range of about 1 to 3 minutes. In some instances, it has been found that by the use of this treatment with the immersion nickel plating solution, in combination with the immersion copper plating solution, it is possible to obtain satisfactory results with shorter contact times in the immersion copper plating solution than are possible when using this solution alone. Thus, when using the combination of the immersion copper and immersion nickel plating steps, total contact times in the two immersion plating solutions of from about 2 to 4 minutes have been found to be typical.
Following the application of the displacement copper coating or the application of the displacement copper and the displacement nickel coating, the zinc surface is then electroplated, using an acid copper plating bath. Various acid copper electroplating baths, as are known to those in the art, may be used for the application of the copper electroplate in accordance with the method of the present invention.
Typically, such acid copper electroplating baths are aqueous acidic solutions of copper sulfate, copper fluoborate, copper nitrate, copper sulfamate, the copper alkyl sulfonates and disulfonates and the like. Generally, such acid copper plating baths will also contain one or more additives which are effective in improving the lustre, leveling, ductility, and the like of the copper electroplate obtained. Typical of such acid copper electroplating baths and the additives which they may contain are the plating baths described in U.S. Pat. Nos. 2,707,166; 3,267,010; and 3,288,690. It is to be appreciated, of course, that other acid copper electroplating baths, other than those specifically set forth, may also be used to effect the desired copper plating of the zinc surfaces.
The copper electroplate is applied to the zinc surfaces by the electrolysis of the acid copper electroplating baths in the conventional manner, as is known to those in the art. Generally, such baths will be operated so as to produce a copper plate having a thickness of from about 0.0002 to .0015 inches, typical operating conditions to produce such a plate including bath temperatures of from about 18° to 60° C., average current densities of from about 15 to 300 amps/square foot and plating times of from about 10 to 40 minutes. The thus-copper electroplating surfaces may then, if desired, be further electroplated with nickel and/or chromium to produce the final plated zinc parts.
The copper electroplate produced by the electrolysis of the acid copper plating bath, in accordance with the method of the present invention, is found to be bright, ductle, and adherent. Moreover, the adherency of this copper plate is found to be excellent, even in cavities and other recessed areas of the zinc surface where there is very low or even no current density during the electroplating process. Thus, by means of the method of the present invention, wherein a displacement copper or a displacement copper and a displacement nickel coating are applied to the zinc surface subsequent to the application of a copper or nickel electrostrike and prior to the application of the copper electroplate from an acid copper plating solution, the areas in which such bright acid copper electroplating solutions can be successfully used are greatly extended so that the advantages resulting from the use of such electroplating solutions can now be realized even in the plating of complicated and complex zinc surfaces.
In order that those skilled in the art may better understand the present invention and the manner in which it may be practiced, the following specific examples are given. In these examples, unless otherwise indicated, parts and percents are by weight and temperatures are in degrees centigrade. It is to be appreciated, however, that these examples are merely exemplary of the present invention and the manner in which it may be practiced and are not to be taken as a limitation thereof.
EXAMPLE 1
A hollow zinc die cast nozzle approximately 6 inches in length and open at both ends was plated for 2 minutes in a standard copper cyanide strike solution, followed by a 2-minute immersion copper deposit from a solution of the following composition:
CuSO 4 . 5H 2 O 77.8 g./l. NaC 6 O 7 H 11 290 g./l. NaOH 30.5 g./l. NaCl 23 g./l.
at a temperature of 130° F. and a pH of 12.5. The part was then plated in a bright acid copper solution for 15 minutes. The deposit on the interior of the nozzle was smooth and adherent.
EXAMPLE 2
Another hollow zinc die cast nozzle was processed exactly as above but the 2-minute immersion copper treatment was omitted. After plating in the acid copper solution, the deposit on the inside of the nozzle was rough and loosely adherent copper flaked off the zinc surface.
EXAMPLE 3
Zinc die cast mirror support arms having a deep cavity completely closed at one end even given a 2-minute copper strike deposit from an alkaline copper solution followed by a six minute immersion treatment in a solution of the following composition:
CuSO 4 . 5H 2 O 80.5 g./l. NaC 6 O 7 H 11 252 g./l. NaOH 11 g./l. NaCl 45 g./l. NaCu 1 g./l.
at a pH of 11.5 and a temperature of 150° F. The parts were then plated in a bright acid copper solution for 20 minutes. The deposits on the interior of the deep closed cavity were smooth and adherent.
EXAMPLE 4
Similar zinc die cast mirror support arms were treated as above with a 2-minute copper strike deposit and a 2-minute immersion copper deposit, from the same solution as described in Example 3, followed by a 2-minute nickel immersion deposit from a solution of the following composition:
190 g./l. NiSO 4 . 6H 2 O 190 g./l. NiCl 2 . 6H 2 O 30 g./l. H 3 BO 3
at a pH of 4.5 and a temperature of 110° F. Following plating in an acid copper solution for 15 minutes, a smooth, adherent deposit was present on the inside surfaces of the deep cavity.
EXAMPLE 5
Similar zinc die cast support arms were processed as in Example 3 but the copper immersion treatment was omitted. After acid copper plating for 15 minutes, the inside surfaces of the casting were covered with rough, loosely adherent deposits which readily flaked loose.
EXAMPLE 6
Small cup shaped zinc die cast buttons having an opening at one end were processed with a 2-minute alkaline copper strike and then immersed for 3 minutes in a solution of the following composition:
CuO 4 . 5 H 2 O 70 g./l. Na 3 C 6 H 5 O 7 248 g./l. Na 2 CO 3 80 g./l. NaCl 15 g./l.
at a pH of 10 and a temperature of 130° F. Following plating for 20 minutes in a bright acid copper solution, adherent deposits were found on the inside of the button.
EXAMPLE 7
Similar zinc die cast buttons processed in an identical manner but without the immersion copper deposit produced rough loosely adherent deposits on the inside surfaces of the die cast button.
While there have been described various embodiments of the present invention, it is to be appreciated that these specific embodiments are intended to be merely exemplary of the preferred manner in which the invention, as described by the following claims, may be practiced.