BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to methods of electroplating metals onto a substrate and more particularly to electrodeposition of metals using pulsed reverse current for controlling evolution of hydrogen.
2. Brief Description of the Prior Art
Electrodeposition of metal coatings onto a substrate is a process that is widely used in modern industry. Such electrodeposited coatings are usually applied to metallic substrates and are generally intended to provide enhanced surface properties to the base metal. For example, metal coating layers are applied to a base metal to prevent corrosion, enhance surface hardness, provide a smooth surface having a relatively low coefficient of friction, and the like.
Metal coatings are ordinarily deposited by providing a plating bath which is an aqueous solution of metal-bearing ions, typically simple ions such as Cr3+, Zn2+, Au3+, Cd2+, which are generally present as aquo complexes, or complex ion containing the metal, such as Cr2O7 —. The metals are present in these ions in a positive oxidation state, e.g. Cr(III) or Cr(VI). The substrate to be plated is immersed in the plating bath and made the cathode of an electrolytic cell. The metal ions are reduced at the surface of the cathode and deposited thereon as a layer of metal.
The actual mechanism of the cathodic reduction and deposition can be complex, and is not well understood for many practical systems. Because the plating baths are aqueous solutions, electrolysis of water with evolution of hydrogen is usually a competing reaction at the cathode. The hydrogen formed may itself present problems, such as hydrogen embrittlement of the deposited metal coating or interference with the metal deposition caused by bubbles. The removal of hydrogen, with concomitant formation of OH− ions, also increases the pH of the plating solution adjacent to the surface of the cathode. A high pH in the plating layer may also produce problems such as formation of insoluble metal hydroxide layers on the cathode surface which also interfere with the transportation of the metal-bearing ions and the deposition of metal atoms on the surface.
In order to prevent the problems associated with hydrogen evolution at cathodic plating surfaces, the plating industry has adopted a number of expedients. The metal-bearing ions that are commonly used in industrial electroplating have been found to minimize the effects of hydrogen evolution. For example, chromium is conventionally plated from a chromate bath in which the metal is present in the hexavalent state (Cr(VI)), and gold is generally plated as the cyanide salt.
Moreover, certain metals present problems in depositing layers having satisfactory properties such as uniformity, luster and hardness, especially at useful plating rates. To overcome these problems the plating industry has developed plating baths that contain various additives that enhance the rate or ease of electrodeposition and the properties of the coatings.
Some of these expedients have resulted in the use of plating baths that are hazardous to use and difficult to dispose of by environmentally benign procedures. For example, it is has been found that the best results in gold electroplating are achieved using cyanide solutions which are evidently hazardous to use and difficult to remediate for disposal. Similarly, it is customary to plate chromium layers, particularly functional chromium layers, from solutions wherein the chromium is in the hexavalent state, typically as chromate or dichromate ions, a hazardous and carcinogenic form of the metal.
The workplace and environmental problems experienced with chromium plating are especially pressing because of the very extensive use of electroplated chromium coatings in industry.
Chromium coatings on a base metal are widely used in automotive, aerospace and other industries to provide a finished article with surface properties that are not inherent in the base metal itself or are attainable only by using expensive alloys. Such coatings are usually deposited on the base metal by electroplating. Two types of chromium coatings are used, conventionally identified as functional coatings and decorative coatings. Functional coatings consist of a relatively thick layer of chromium (typically 1.3 to 760 micrometers thick) to provide a surface with functional properties such as hardness, corrosion resistance, wear resistance, and low coefficient of friction. Such functional coatings are used on automotive strut and shock absorber rods, hydraulic cylinders, crankshafts, and industrial rolls. Carbon steel, cast iron, stainless steel, copper, aluminum, and zinc are substrates commonly used with functional chromium layers.
Decorative coatings consist of a thin layer of chromium (typically 0.003 to 2.5 micrometers, plated over a nickel layer) to provide a bright surface with wear and tarnish resistance. Such coatings are used on automobile bumpers and trim, bath fixtures, and small appliances.
Chromium is generally plated from an aqueous solution containing soluble chromium species wherein the chromium is in the hexavalent state (Cr(VI)). Such a hexavalent chromium plating bath is a chromic acid solution containing various chromate ions, dichromate ions, dichromate and trichromate. Sulfuric acid has been recognized as an essential ingredient of Cr(VI) plating baths.
Although plating from Cr(VI) baths has been the dominant commercial procedure for a long time, the process has certain disadvantages. A Cr(VI) plating bath is typically operated at a temperature significantly above room temperature and produces a mist of chromic acid. Consequently, measures to protect the workers from exposure to the toxic fumes are required by safety rules and by law. Exhaust/scrubber systems must be installed to keep the chromium concentration in the workplace atmosphere no greater than the prescribed limit of 0.01 mg/m3. The amount of chromium that can be emitted to the air and water of the environment is also strictly regulated by federal and local law. Some deposition of decorative chromium plating is done from trivalent chromium (Cr(III)) baths. Plating from Cr(III) baths instead of Cr(VI) baths has several environmental advantages.
1.) Cr(III) is non-toxic, non-hazardous and is not an oxidizing agent. Therefore, meeting air quality regulations is easier and working conditions are greatly improved. The exposure limit for Cr(III) is an order of magnitude higher than for Cr(VI).
2.) Waste disposal costs for Cr(III) plating are significantly less than for Cr(VI) plating. Hydroxide sludge generation is reduced ten to twenty times because a Cr(III) bath generally contains only about 4-20 g/liter of chromium, as opposed to 150-300 g/liter for a Cr(VI) bath.
3.) A Cr(III) bath may be used without additives, which permits the rinse water from the plating operation to be recycled readily.
Plating from a Cr(III) bath also has certain technical advantages.
1.) Current interruptions have little effect on the plating.
2.) A Cr(III) bath is not affected by drag-in of chloride and sulfate from any previous nickel plating operations. In contract, chloride and sulfate drag-in upset the catalyst balance in Cr(VI) baths.
3.) The throwing power of a Cr(III) bath, i.e., is ability to provide a uniform coating of chromium to recesses on the surface of the object to be plated, is greater than that of a Cr(VII) bath.
The effect of the plating bath chemistry, i.e., the composition of the solution, on the plating thickness, brightness, hardness, and corrosion resistance of chromium layers deposited from a Cr(III) bath have been studied by several authors. The effect of the waveform of the plating current on the structure of the chromium deposit, and its distribution, brightness and hardness have also been studied. Commercial Cr(III) baths are available that incorporate certain proprietary organic compounds as additives in order to provide baths for decorative chromium coating applications. However, the concentration of the additives is difficult to control because they are present in very small amounts. Furthermore, the additives react and break down with the passage of time to form contaminants. Consequently, the used Cr(III) bath and the rinse water from such plating operations cannot be replenished and/or recycled because the concentration of the contaminants would build up to unsatisfactory levels. Finally, decorative plating from a Cr(III) bath suffers from low current efficiency.
Currently, functional chromium coating from a Cr(III) bath is not commercially practical because it is difficult to plate thick chromium coatings with appropriate properties. Furthermore, the low current efficiency and low plating rate of Cr(III) baths lead to unfavorable economics.
Attempts to plate gold from non-cyanide solutions have also experienced difficulties. Gold plating baths that do not employ cyanide usually contain sulfite. Gold is deposited from the sulfite complex according to the equation
where M is an alkali metal or ammonium ion. The sulfite ion is itself in equilibrium with sulfur dioxide according to the equation
Because this reaction forms hydroxyl ions, the equilibrium is pH-dependent, and the sulfite ion is ordinarily stable only at alkaline pH. Because the plating reaction generates OH−, the pH near the plated surface (cathode) is usually very high. At alkaline pH, sulfite ions accumulate as gold is consumed and the specific gravity of the solution tends to increase continuously as the bath is operated. This is undesirable for high speed operation, or for applications requiring selectivity. It would clearly be desirable to operate a sulfite gold plating bath under conditions such that sulfur dioxide is volatilized at approximately the same rate at which gold is plated out. In such a case, the process tends to be self-regulating, and would operate in a fashion analogous to that of the cyanide gold plating solutions.
Another electroplating application wherein it is desirable to control hydrogen evolution and the local pH in the region of the cathode is the developing attempts to substitute zinc-nickel or zinc-tin plating for anti-corrosion coatings of cadmium in order to eliminate the use of that toxic metal. Zinc-based alloys, such as Zn—Ni and Zn—Sn are strong candidates to replace cadmium. However the current electroplating process for zinc alloy coatings suffers from two main difficulties:
1) It requires a hydrogen-relief bake post-treatment to eliminate hydrogen embrittlement.
2) It is difficult to control the composition of the alloy as deposited.
Zinc alloy plating suffers from what is known in the plating industry as anomalous deposition. The anomaly involved is the tendency in such systems for the less noble metal to be deposited preferentially. In the case of the zinc-based alloys the result is a coating that contains more zinc and less nickel or tin than desired. According to one of the leading proposed mechanisms the problem is caused by the formation of a zinc hydroxide film within the double layer adjacent to the cathode surface that inhibits the electrodeposition of the more noble metal. Attempts have been made to correct the anomalous deposition by adjusting the composition of the plating bath, but the results have not permitted zinc alloys to replace cadmium extensively.
Still another industrial use of hexavalent chromium compounds in coating applications is the formation of anti-corrosive chromate conversion coatings on aluminum. A recent process developed for replacing chromium in such coatings is the formation of a cerium-molybdenum alloy coating on aluminum. This “Ce+Mo” process involves a chemical treatment of an aluminum alloy surface with Ce(NO3)3 solution for several hours, followed by an electrochemical treatment (anodic polarization) in Na2MoO4 solution and finally a chemical treatment in CeCl3 solution. The treatment process requires about six hours to complete and it is difficult to control the chemical treatment step. In addition it is difficult to control the Ce—Mo composition and coating distribution due to the chemical treatment process. A cerium coating can be electrodeposited on aluminum. However the conventional electrolytic method, which uses direct current (DC), involves a large amount of hydrogen evolution due to the very negative reduction potential of Ce3+ (−2.335 V vs Standard Hydrogen Electrode (SHE)). The evolved hydrogen produces hydrogen embrittlement of the aluminum, stress corrosion cracking (SCC) and corrosion fatigue of aluminum alloys.
Accordingly, a need has continued to exist for a method of controlling the deleterious effects of hydrogen evolution in electroplating processes and, in particular, for plating functional chromium coatings from a Cr(III) plating bath that does not suffer from the disadvantages of current processes.
SUMMARY OF THE INVENTION
The problems of controlling evolution of hydrogen and its direct and indirect effects on the properties of the electroplated coatings and the adverse interaction of hydroxide ion with metal-bearing ions in the plating solution have now been alleviated by the process of the present invention wherein metal layers are deposited from a plating bath onto a cathode substrate using a pulsed reverse current (PRC). The process is especially applicable to electrodeposition of functional chromium coatings from a Cr(III) plating bath.
Accordingly, it is an object of the invention to provide a method of controlling the evolution of hydrogen in electrodeposition of metals at a cathode.
A further object is to control the pH in the vicinity of a cathode at which metals are being deposited electrolytically.
A further object is to provide a functional chromium layer on a substrate.
A further object is to provide a method for depositing a functional chromium layer on a substrate using a Cr(III) plating bath.
A further object is to provide a method of depositing a functional chromium layer using a pulsed reverse current waveform.
A further object is to provide a method for depositing gold on a substrate.
A further object is to provide a method for depositing gold on a substrate from a plating bath containing sulfite.
A further object is to provide a method for depositing Zn—Ni and Zn—Sn alloy layers on a substrate.
A further object is to provide a method of depositing a cerium-molybdenum anticorrosive layer on aluminum and aluminum alloys.
Further objects of the invention will become apparent from the description of the invention that follows.