Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/56—Electroplating: Baths therefor from solutions of alloys
  • C25D3/58—Electroplating: Baths therefor from solutions of alloys containing more than 50% by weight of copper
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/56—Electroplating: Baths therefor from solutions of alloys
  • C25D3/565—Electroplating: Baths therefor from solutions of alloys containing more than 50% by weight of zinc

Definitions

• the present invention relates generally to the field of electroplating and more particularly to electroplating surfaces with copper and zinc alloys.
• Electroplating is a process for putting a metallic plate or coating on a conducting surface by using an electric current.
• the surface to be plated is connected to the negative end of a source of electricity and it is placed in a plating solution containing ions of the metal with which it is to be plated.
• the conducting surface to be plated when thus connected is referred to as the "cathode".
• the positive end of the electrical source is connected to another conductor which is of the same composition as the metal plate desired. It is commonly referred to as the "anode” and it is also placed in the plating bath.
• a direct electrical current is passed through the bath to separate metal ions from the plating bath and cause the metal to be deposited on the cathode.
• Alkaline plating baths containing cyanide are employed for the commercial electrodeposition of copper and zinc alloys, such as, brass which contains approximately 70% copper and 30% zinc, white brass which contains 50% zinc and about 50% copper and high copper alloys which contain about 90% copper and 10% zinc.
• the primary objects of the present invention are to disclose novel cyanide-free alkaline plating baths and a process for electrodepositing copper and zinc alloys from said baths.
• the novel plating baths of the present invention are aqueous mixtures containing a suitable source of the metal ions such as a mixture of copper and zinc salts, a chelating agent selected from glucoheptonic acid and its salts, which chelating agent is present in an effective amount which is in excess of the stoichiometric equivalent of at least one of the metal ions and sufficient base to render the mixture alkaline.
• the plating bath contains in each liter about 3.75 to about 15.0 grams of copper; about 1.25 to about 6.0 grams of zinc; about 10 grams to about 75 grams of the chelating agent and about 20 to about 37.5 grams of caustic and has a pH of about 10 to about 13.5.
• the mixture is preferably supplied as a bath concentrate which is diluted with 4 parts of water to yield the plating bath.
• the process of the present invention for electrodepositing copper and zinc alloys basically comprises preparing a plating bath of the desired composition, connecting the conductive surface to be plated to the negative end of a source of electricity to form a cathode, placing the cathode in the plating bath, inserting into the bath an anode, which is of the same metal mixture as that which is to be deposited, and which is connected to the positive end of a source of electricity.
• a direct current is passed through the bath to electrically deposit the metal alloy upon the cathode.
• the excess metal ions are removed from the spent plating baths by introducing carbonate ions into the spent bath and reducing the pH of the bath to precipitate the metal and carbonate ions as a readily removable sludge.
• the plating solution after dilution has a pH of about 10 to 13.5 and may have the following composition:
• the plating solution is made up as follows:
• the preferred bath described above requires either 70/30 bar anodes or titanium baskets with 70/30 ball anodes. Steel should not be allowed to come in contact with the plating solution as it will dissolve slowly under reverse current conditions.
• Plating with the bath of the present invention requires a clean lined plating tank and related equipment. Rubber, Koroseal or other plastic tanks are suitable. When the bath is used in tanks that have previously been used for cyanide plating the tank, anodes, anode bars must be free of cyanide. After the removal of the old cyanide bath the equipment should be washed with hypochlorite solution and the tank soaked for 24 hours in 2% sodium hypochlorite to destroy all cyanide. Hoods, barrels, and filtering equipment must also be free of cyanide.
• a dilute 1% to 2% sulphuric acid rinse should follow and after rinsing with water a 5% caustic rinse should be used to eliminate the acid.
• the caustic rinse should be removed and the concentrated solution added and diluted for use.
• Control of the caustic content of the bath is the most frequent adjustment required.
• the caustic content should be checked daily and corrected (this takes place of the cyanide check in usual brass solutions). The analysis only takes minutes and should not be ignored. Control of the caustic is very important for anode corrosion. Liquid KOH is the preferred caustic because of ease of addition.
• the metal content of the bath is replenished by adding the bath concentrate whenever the copper metal content decreases. Adding additional bath concentrate also adds caustic. Caustic adjustments should be made, if required, after a metal addition if analysis shows the caustic to be below the desired level. Adding concentrate automatically adjusts the other chemical components.
• the amount of brightener agent to be added is determined by the ampere hours plated. One gallon of brightener solution is added for every 12-15,000 A/H. The additions should be made every 4 hours or by automatic feed for best and most economical results. A dark smutty deposit indicates a high brightener content (assuming bath composition is satisfactory) and a low concentration results in loss of proper yellow green brass color.
• the copper and zinc ions in the bath may be supplied by using water soluble salts such as the sulfate, chloride, fluoroborate, fluorosilicate and fluoride.
• water soluble salts such as the sulfate, chloride, fluoroborate, fluorosilicate and fluoride.
• Other copper and zinc compounds which can be used include carbonates, phosphates, pyrophosphates, hydroxides and zinc oxide.
• Especially preferred for use are the mono or polycarboxylic aliphatic acid salts or mono or polyhydroxy aliphatic acid salts of the metals. Such salts are not generally available in large enough quantities to allow their commercial use, however they can be formed in situ. It is well known that as long as the metal ions are present, i.e.
• any salt can be used to supply the metal ions which does not contain other ions which interfere with the electroplating process.
• copper and zinc chromates or cyanides would not be suitable as sources of the metal ions, as the chromate would inhibit the metal deposition and the cyanides would introduce cyanide ions into the plating bath and defeat an important purpose of the invention.
• the mixture of copper and zinc salts employed may have copper to zinc ion ratios ranging from 99 to 1 to 1 j to 99.
• the ratio to be used will depend upon the nature and properties of the alloy desired to be deposited. There seems to be no limitation on the ratios of the metals in the mixtures that can be deposited using the baths of the present invention.
• the chelating agents which can be used in the practice of the present invention are glucoheptonic acid and its alkali metal salts. Although the sequestering or chelating abilities of the glucoheptonic acid and its salts has been known for some time, such compounds have only been used in the past in plating baths in relatively small amounts to chelate undesirable metal ions introduced by impurities or as a means of chelating small amounts of desirable metals used as grain refiners and brighteners. In the baths of the present invention the chelating agents are used in much higher quantities which maintain in the bath a reservoir of metal ions of sufficient concentration to permit the electrodeposition of the particular metal over a wide range of operating conditions.
• the plating bath contain an amount of chelating agent which is in excess of a stoichiometric equivalent of at least one of the metals and preferably the copper.
• the preferred chelating agent is the sodium salt of glucoheptonic acid (sodium 1,2,3,4,5,6, hexahydroxy hexane-1-carboxylate). If desired, the bath may contain additional chelating agents.
• Representative plating bath formulations of the present invention are the following:
• the electrodeposition of the copper and zinc alloys over a wide range of temperatures using the above formulations was evaluated. As a result it was found that the metals could be deposited at temperatures ranging from approximately 20°C to 65°C. The high temperatures were found to be more suitable for high copper alloys and the lower temperatures more suitable for the higher zinc alloys. However, for most purposes a temperature range of 30°C to 50°C provides a range in which all alloys may be codeposited depending upon bath composition and other operating conditions.
• the bases that may be used to adjust the pH of the plating baths are preferably the alkaline metal hydroxides of sodium, potassium and lithium.
• the use of ammonium hydroxide is not recommended because it does not increase the pH to the level required for successful operation of the inventive process.
• ammonium hydroxide is difficult to remove as the copper complex from the effluent in the pollution treatment process.
• amines also may be used but they also can cause difficulties in the subsequent pollution treatment process.
• a convenient process for removing metal ions from a spent bath of the present invention involves introducing carbonate ions into the spent bath.
• carbonate ions When carbonate ions are added to the spent bath and the pH of the bath is reduced to about 8.5, a reduction in chelating power causes the remaining metal ions to precipitate as carbonates allowing for their easy removal as sludge. This facilitates upgrading the effluent without requiring the usual costly cyanide destruction process which is required with alkaline cyanide baths.
• the organic ions which remain are carboxylic aliphatic acid salts which are relatively nontoxic. Many of them are contained in large quantities in natural foods or are added to prepared foods to provide desired characteristics or maintain quality and freshness.
• the object to be plated is connected to the negative end of a source of electricity as the cathode and an anode of the desired metal or metal alloy is connected to the positive end of the electrical source.
• a direct current is then passed through the solution resulting in an electrodeposit of a uniform plate or layer of metal upon the cathode.
• the tank voltages and amperages used in the process are similar to those used on cyanide processes; 2-6 volts on tank operations and 9-12 volts on barrel operations.
• the plating is preferably conducted at 100°F, with either mild air or cathode agitation in the tank or barrel agitation at 2-6 RPMs.
• filtering must be continuous for smooth plating and the filter should be lined or of non-metallic construction. Ventilation is required because of the caustic nature of the solution.
• the plate of alloy obtained through practice of the invention is uniform and acceptable for many purposes, however, it may be duller in appearance than desired. If so, a bright, smooth uniform deposit which may be more commercially desirable can be obtained by adding conventional brightening agents to the plating bath.
• aldehydes such as anisaldehyde, benzaldehyde, crotonaldehyde, veratraldehyde etc.
• any aldehyde or compound containing a carbonyl group which is capable of producing a smoothening and brightening effect in a commercial zinc plating process, appears to be capable of promoting brightness in cooperation with the metal ions mentioned in the preceding paragraphs.
• the quantity of compound required varies from .01 g/L to about .25 g/L and may vary with the compound selected.
• the aldehyde used may be added as the bisulfite adjunct or added dissolved in a suitable organic solvent, such as alcohol, glycol or a commercial solvent such as methoxy-ethanol, ethoxy-ethanol, etc.
• the above described carbonyl group containing compounds may be used along with small amounts of sulphur containing compounds to further enhance brightness.
• Examples of compounds having mercaptan groups include dl homocystine (2-amino, 4-mercaptobutyric acid), 2-mercaptoethanol, and 2-mercaptobenzimidazole.
• sulphur bearing compounds which showed brightening effects with less effectiveness were thiobenzanilide, 5-sulfosalicylic acid and sulfamide. These compounds are preferably used in amounts of .1 to 5ppm as auxiliary brighteners.
• the copper and zinc alloy plating baths can include any or all of the addition agents mentioned and they may be added as water soluble compounds.
• a typical brightener system for a 70:30 copper/zinc deposit would be:
• a typical brightener system for a 50:50 copper/zinc deposit would be:
• a typical brightener system for a 10:90 copper/zinc deposit would be:
• lead and tellurium or antimony and tellurium must be used in the brightener for grain refinement and smoothening effects.
• auxiliary sulphur compounds to give additional brightness is accomplished by adding them to the plating bath in quantities of .1 to 2 ppm.
• the plating baths of the present invention is a significant advance in the field of electroplating because the baths make possible the economical commercial plating of copper and zinc alloys of commercially acceptable quality without the use of baths containing cyanide ions or special equipment.
• the plating baths may be used for barrel plating, or with air or cathode rod agitation for rack plating heavy brass. Ball burnishing of the plate is required for a bright, barrel finish or, it may be oxidized to obtain brushed finishes.
• my plating baths provide all the advantages previously obtained only with cyanide baths without the pollution control problems associated with the use of such cyanide baths. Furthermore, my baths make it possible to deposit copper and zinc alloys directly on the copper, iron and zinc alloys customarily used in industry without the need for a preliminary copper cyanide strike to improve adhesion. Still further, my plating baths when used in the place of cyanide baths avoid the problem of cyanide spotting of porous areas.
• my plating baths also provides significant energy savings over the use of alkaline cyanide baths because there is no need when using my baths to destroy the cyanide ions.
• the spent baths of the present invention are simply adjusted to a pH of 8.5, the solution is treated with slaked lime, and sodium polysulfide is added to precipitate the remaining metals. The resulting effluent can be disposed of conventionally.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Electroplating And Plating Baths Therefor (AREA)

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Citations (3)

• 1982
  • 1982-08-31 EP EP82304561A patent/EP0101769A1/de not_active Withdrawn

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