CN1784512A - Electrodeposition of aluminum and refractory metals from non-aromatic organic solvents - Google Patents

Abstract

An electroplating solution includes a non-aqueous non-aromatic organic solvent and a mixture including soluble metallic salts and organic additives dissolved in the non-aqueous or non-aromatic organic solvent. Electrodeposition of a metal from the electroplating solution includes preparing an electroplating solution and electrodepositing the metal from the electroplating solution onto a conductive substrate under a cathodic current. An electroplating system has a plating chamber containing an electroplating solution, an entry point to the electroplating system, and a transporting system to convey a part to be electroplated from the entry point to the plating chamber. The electroplating solution includes a non-aqueous non-aromatic organic solvent and a mixture including soluble metallic salts and organic additives, the mixture dissolved in the non-aqueous non-aromatic organic solvent.

Description

Electrodeposition of Aluminum and Refractory Metals from Nonaromatic Organic Solvents

Cross References to Related Applications

[0001] This application claims the benefit of U.S. Provisional Patent Application No. 60/451,631 filed in the U.S. on March 5, 2003, which is hereby incorporated by reference in its entirety.

Background technique

Field of invention:

[0002] The method generally involves electrodeposition of coatings. More specifically, the present method relates to metallic coatings of aluminum, magnesium and refractory metals such as titanium, tantalum, zirconium and their alloys, obtained by electrodeposition from non-aromatic organic solvents.

Background of the invention:

[0003] In the following description of the background of the invention, reference is made to certain apparatus and methods, however, such reference is not necessarily to be construed as an admission that such apparatus and method properties are prior art. Applicants reserve the right to demonstrate that any subject matter referred to does not constitute prior art to the present invention.

[0004] Surface protection against corrosion and wear can be applied to ferrous and non-ferrous metal parts. One suitable method of obtaining this protection is coating. For example, in the aerospace industry, cadmium coatings are used to protect components such as landing gear or fasteners from corrosion. This cadmium coating acts as a sacrificial coating that provides protection even when scratched. However, cadmium is a toxic metal electrodeposited from cyanide-based baths, which affects handling and handling safeguards. Suitable metal coatings for cadmium are required.

[0005] One aspect of the protection afforded by such coatings is the result that the brine electrochemical potential of the cadmium coating is negative relative to the underlying material. Accordingly, suitable coating materials for replacing cadmium include those that have a similar, eg negative, brine electrochemical potential relative to the underlying material. Examples of candidate materials include zinc, manganese, beryllium and magnesium to replace cadmium in coatings. However, these candidate materials have drawbacks. Zinc, while being easy to plate, suffers from environmental embrittlement and is therefore unsuitable for use in high-strength steels. Manganese can be electroplated by aqueous methods, but exposure can cause lung and neurological effects. Similarly, beryllium also has environmental drawbacks, and magnesium is highly reactive in this application when not incorporated in the coating.

[0006] Aluminum and refractory metals such as titanium are alternative candidates for protective coatings and for replacing cadmium coatings. Aluminum coatings provide a sacrificial protective barrier against corrosion of ferrous metal parts. Refractory metal coatings can provide a protective barrier that prevents damage to underlying metals, such as ferrous and non-ferrous metal parts, through mechanisms such as corrosion, erosion, wear, abrasion, and embrittlement.

[0007] Aluminum and refractory metals such as titanium are typically obtained electrochemically from molten salt bath chemistries such as those based on lithium chloride, or from toxic organic solvents such as those based on benzene and toluene, or by electrophoresis. These known methods have negative drawbacks in terms of coating quality and cost. For example, molten salt bath methods can avoid embrittlement during electroplating, but trapped salts can be a source of subsequent corrosion and embrittlement.

[0008] Metallic coatings of aluminum and refractory metals can also be obtained by other methods. For example, physical techniques such as arc spraying, physical vapor deposition techniques such as ion vacuum deposition of aluminum (otherwise known as ivadizing) and chemical techniques such as sol-gel can be used, either from toxic organic solvents or through molten salts. Electrodeposition to form coatings of aluminum and refractory metals.

[0009] US Patent No. 4,145,261 discloses a plating solution that is a mixture of toluene, organics including benzene, and aluminum halides. US Patent No. 4,759,831 discloses a load-lock isolated electrolyte chamber for electrodeposition of aluminum. Properties of the bath such as environmental concerns and susceptibility to contamination when exposed to air make the disclosed solutions and processes expensive, inefficient and potentially hazardous to the environment and health.

[0010] Japanese Patent No. 55-158289 discloses various low molecular weight organic solvents in which synthetic aluminum salts are dissolved. However, specific coating properties are not provided.

[0011] Thus, there remains a need for coating technologies that can replace current cadmium layers, especially in high strength steel and aerospace applications. In addition, there is a need for an aluminum electrodeposition technique that is environmentally safe and clean, while also being cost and time efficient.

Summary of Invention

[0012] A representative method of metal electrodeposition includes preparing an electroplating bath and electrodepositing metal from the electroplating bath on a conductive substrate under cathodic current. The plating bath includes a mixture of soluble metal salts and organic additives dissolved in an anhydrous, non-aromatic organic solvent.

[0013] A representative electroplating bath includes an anhydrous, non-aromatic organic solvent and a mixture comprising a soluble metal salt and an organic additive dissolved in the anhydrous, non-aromatic organic solvent.

[0014] A representative embodiment of an electroplating system includes a plating chamber containing a plating solution, an inlet into the electroplating system, and a delivery system for transporting parts to be plated from the inlet to the plating chamber. The electroplating solution includes an anhydrous, non-aromatic organic solvent and a mixture including soluble metal salts and organic additives, the mixture being dissolved in the anhydrous, non-aromatic organic solvent.

Description of drawings

Can understand the preferred embodiment described in detail below in conjunction with accompanying drawing, wherein identical numeral represents identical part, wherein:

[0016] Figure 1 is a schematic diagram of the electroplating system.

Detailed Description of the Preferred Embodiment

[0017] Electrodeposition from non-aromatic organic solvents (NAOS) can deposit metals and alloys not obtainable by conventional (aqueous) electroplating. Since little or no water is present in solution (eg, in water in an aqueous solution), hydrogen embrittlement of high tensile strength materials can be reduced or minimized. In addition, the NAOS process has similar or identical characteristics to conventional electroplating in terms of part handling, certain equipment and methods, thereby allowing an effective replacement of NAOS in conventional electrodeposition equipment.

[0018] A representative method of metal electrodeposition involves preparing an electroplating bath and electrodepositing metal from the electroplating bath on a conductive substrate under cathodic current. The electroplating bath comprises a mixture of soluble metal salts and organic additives dissolved in an anhydrous, non-aromatic organic solvent.

[0019] The anhydrous, non-aromatic organic solvent can be any suitable solvent. For example, one suitable solvent includes low molecular weight non-aromatic organic solvents. Herein, low molecular weight means 200 g/mole or less. Such solvents include alcohols such as ethanol, propanol, isopropanol, butanol, 2-butanol and their corresponding alcohols having more than one OH functionality, ethanolamine and amines. Carboxylic acids such as oxalic acid, citric acid and ammonium citrate can be used as support electrolytes or also as solvents. In addition, the plating electrolyte may have a combination of two or more of these suitable solvents. Furthermore, although other organic solvents such as ketones, aldehydes, alkenes, alkynes, ethers, amides, and toxic aromatic compounds can also be used, the toxicity of these solvents makes them undesirable.

[0020] Soluble metal salts are dissolved in these solvents. These metal salts include metal alkoxides such as ethoxides, propoxides, isopropoxides, butoxides and their corresponding halides, phosphates, carbonates, and other Ionic inorganic and organic compounds. The metal salt may be present alone, or other salts and mixtures thereof may be used.

[0021] In representative methods, the target metals for electrodeposition are those metals that are not easily electroplated from aqueous solutions. Examples of target metals include aluminum, titanium, tantalum, zirconium, molybdenum, tungsten, niobium, osmium, hafnium, magnesium, alloys of any combination of these metals, or other metals from salts dissolved in the above solvents.

[0022] In the electroplating solution, the concentration of the metal salt can be 5%-100% of the saturation concentration of each metal salt in the solvent at the operating temperature of electrodeposition. Conductive additives such as oxalic acid, phosphoric acid, and other low molecular weight organic acids can also be added to this plating electrolyte. Herein, low molecular weight means 200 g/mol or lower. Any organic or inorganic compound that increases the conductivity of the solvent and is soluble in such a solvent may be added alone or in combination. For example, in ethanol the following by-products are soluble: sodium hydroxide, boric acid, ammonium chloride, calcium carbonate, sodium iodide, ammonium fluoride, aluminum nitrate, stearate or hydrochloride, chloride aluminum oxide, aluminum phosphate, and potassium aluminum phosphate.

[0023] Minimizing electrolyte contamination improves adhesion and coating performance. Contamination of the plating electrolyte can lead to uneven, porous, galling, pitting, foaming and even no deposition at all in the deposit. There are various ways to suppress or minimize electrolyte contamination by air, water or solution or by-products of electrodeposition. Electrolyte filtration and controlled atmospheres (eg, at potting baths) can be utilized to assist in maintaining proper operation of the electrodeposition bath. For example, plating electrolytes can be continuously filtered over molecular sieves to minimize water contamination. A suitable molecular sieve for this application is a 3 Angstrom molecular sieve. In another embodiment, an inert atmosphere is maintained over the plating bath at least during the electrodeposition of the metal on the substrate. The inert atmosphere can minimize contamination by oxygen and water, for example, by keeping the atmosphere in contact with the plating bath substantially free of oxygen and water. For example, the oxygen content may be 1-10 ppm, preferably less than 5 ppm, and the water content may be 1-10 ppm, preferably less than 5 ppm, although actual conditions may vary depending on the metal being electrodeposited. Preferably, when an inert atmosphere is used, the inert atmosphere is continuously maintained over the plating bath. An example of an inert atmosphere includes bubbling nitrogen gas through the electrolyte and/or gas-conditioning the nitrogen atmosphere on the electroplating bath.

[0024] In another embodiment, a positive pressure may be used within the plating chamber to reduce contamination. Preferably, the positive pressure is maintained at 1 atmosphere, but depending on the sensitivity of the process and the design of the plating chamber, the atmosphere can be lower or higher than 1 atmosphere.

[0025] In another embodiment, a glove box-type or closed double chamber chamber filled with an inert gas such as nitrogen and filtered out of water and oxygen with a suitable catalyst can be used for NAOS. The plating bath in this glove box-type or closed dual chamber chamber can be arranged as a single electrolyte bath, or it can be divided into separate catholyte and anolyte chambers. The separation into catholyte and anolyte compartments is optional, where decomposition of the solution due to electrolysis may occur. When separating the catholyte and anolyte compartments, possible dissociation can be obtained, for example, by a suitable ion-permeable membrane between the compartments. For plating chemistries that require separation of the electrolyte, suitable ion-permeable membranes are polymeric membranes, although a wide variety of commercially permeable or semi-permeable membranes are available. Membrane separation can also be used between the electrodes to use different anode and catholyte solutions.

[0026] In representative electrodeposition methods, suitable anode materials include metals that are electrochemically equivalent or similar to the metal being plated, as long as they do not form an insulating barrier on the surface, or, whether soluble or insoluble, are compatible with the electrolyte compatible metals. To avoid contamination of the electrolyte, suitable insoluble or soluble anodes formed from the material containing the metal to be plated may be used. Alternatively, other anode materials such as platinized titanium, DSA type anodes or other insoluble but conductive materials may be used.

[0027] Depending on the electrolyte composition and the metal to be plated, suitable cathodic current densities on the conductive substrate can vary between 0.05 and 1000 amps/ ^dm2 . In each NAOS process, the electrode surface ratio is determined so that the amount of metal dissolved from the anode and the metal to be plated is in balance, thereby maintaining the chemical balance of the electrolyte.

[0028] The conductive substrate optionally has a surface prepared prior to electrodeposition. For example, the surface can be grit blasted, masked, and then cleaned with alkali or acid prior to plating. After cleaning with alkali and acid, the conductive substrate is dipped or sprayed with alcohol to remove any aqueous cleaners. In some cases, a reverse etch can be used to improve adhesion.

[0029] Any type of agitation of the plating solution or part of the solution may be incorporated into the process. For example, moving or vibrating bus-bars, nitrogen or other inert gas sparging, or ultrasonic equipment may be used.

[0030] The bath temperature for each NAOS process can be adjusted so that the solvent surface tension does not exceed about 50% (± 10%) of its vapor pressure, or so that the bath is cooled to improve plating conditions and efficiency. For example, the plating bath can be conditioned by using a heat exchanger or cooler.

[0031] Evaporation of the electrolyte is reduced by adding flotation devices such as polyspheres or compounds on top of the liquid surface that prevent evaporation but allow the part to enter the bath.

[0032] Figure 1 is a schematic diagram of an electroplating system. A representative embodiment of an electroplating system 100 includes a plating chamber 102 containing an electroplating solution, an inlet 104 of the electroplating system 100 , and a delivery system 106 for transferring a part 108 to be electroplated from the inlet 104 into the plating chamber 102 .

[0033] The plating chamber 102 includes a single plating chamber, or, as discussed herein, separate chambers for the anolyte and catholyte. As shown in FIG. 1 , the chamber 102 comprises a separate chamber having two anodes 110, 112 disposed substantially opposite each other with the catholyte in between. Each anode 110,112 is separated from the catholyte by a membrane 114,116.

[0034] The plating solution is continuously agitated by circulation through an outer chamber 118 containing a molecular sieve, which scavenges water molecules. A cooperating pump (not shown) helps circulate and recirculate the electrolyte between the plating chamber 102 and the outer chamber 118, thereby assisting in controlling the purity and composition of the electrolyte.

[0035] Furthermore, an inert atmosphere is maintained at least within the plating chamber 102, preferably in the electroplating system 100, by gas conditioning. In FIG. 1 , the gas regulation is shown as a regulated nitrogen tank 120 . However, any suitable inert atmosphere and any suitable conditioning system may be used. A positive pressure, for example about 1 atm, is generally maintained in the plating chamber 102 .

[0036] The inlet 104 of the electroplating system 100 may be any suitable inlet that accommodates the member 108 and interfaces with the plating chamber 102 while maintaining proper contamination and bath vapor control. For example, FIG. 1 shows the entrance 104 as an airlock type transfer point having a first outer door 122 and a second inner door 124 . Generally, the operation of the outer door 120 and inner door 122 is coordinated with the vacuum pump and inert gas backfill valve to minimize the introduction of air into the plating chamber 102 from outside the electroplating system 100 .

[0037] Conveyor system 106 places part 108 into position within plating chamber 102 for electrodeposition. For example, as shown in FIG. 1 , the delivery system 106 places the component 108 into the catholyte and is electrically connected to a power source 126 . Depending on the electrochemical conditions, the power source 126 may be a gravimetric or potentiostatic power source. A power source 126 is also electrically connected to each anode 110,112.

[0038] Delivery system 106 may comprise any suitable delivery system for component 108. Examples of suitable conveyor systems include hydraulic lifts, chain elevators, conveyor belts, elevator systems, rack-up systems, rotary systems, and the like. For example, large parts can be pushed up and plated from the pushed position, and small parts can be plated by barrel plating. In barrel plating, the part is placed in a drum that is rotated in an electroplating bath so that the part is fully immersed in the bath. In barrel plating, the remaining unplated spots from the push step are minimized to plate the entire part.

[0039] The following examples (Example 1-Example 3) illustrate the possibility of electroplating aluminum with NAOS-based solutions:

Example 1: Element Chemical Name Concentration Remark Solvent ethanol - Aluminum source #1 Aluminum isopropoxide 0-5g/L Conductive Additive #1 Oxalic acid 0-100g/L Aluminum source #2 aluminum chloride 0-900g/L Conductive Additive #2 boric acid 0-50g/L temperature room temperature Anode material aluminum

Example 2: Element Chemical Name Concentration Remark Solvent ethanol-isopropanol 0-50V/V Aluminum source #1 Aluminum isopropoxide 0-5g/L Conductive Additive #1 Oxalic acid 0-100g/L Aluminum source #2 aluminum chloride 0-900g/L temperature room temperature Anode material aluminum

Example 3: Element Chemical Name Concentration Remark Solvent Butanol - Aluminum source #1 Aluminum butoxide 0-10g/L Conductive Additive #1 Oxalic acid 0-100g/L Aluminum source #2 aluminum chloride 0-500g/L temperature room temperature Anode material aluminum

[0040] The following example (Example 4) illustrates the possibility of electroplating titanium with a NAOS-based solution:

Example 4: Element Chemical Name Concentration Remark Solvent ethanol - Titanium source #1 Titanium Ethoxide 0-10g/L Conductive Additive #1 boric acid 0-50g/L Titanium salt #2 Titanium chloride 0-50g/L temperature room temperature Anode material Titanium

[0041] The following example (Example 5) illustrates the possibility of electroplating tantalum with NAOS-based solutions:

Example 5: Element Chemical Name Concentration Remark Solvent ethanol - Tantalum Source #1 Tantalum isopropoxide 0-100g/L Conductive Additive #1 Oxalic acid 90±15g/L Tantalum source #2 Tantalum pentachloride 0-5000g/L temperature room temperature Anode material Tantalum

[0042] The following example (Example 6) demonstrates that zirconium can be electroplated with a NAOS-based solution:

Embodiment 6: Element Chemical Name Concentration Remark Solvent ethanol - Zirconium source #1 Zirconium ethoxide 0-50g/L Conductive Additive #1 Oxalic acid 0-100g/L Zirconium source #2 Zirconium tetrachloride 0-500g/L temperature room temperature Anode material Zirconium

[0043] Although in metal deposition processes, a variety of anode, cathode and solution chemistries can be used, the preferred electrodeposition process for aluminum uses aluminum anodes, mild, organic and inorganic aluminum salts, and other conductivity enhancing chemistries . However, other metals and refractory metals can be obtained from organic solvents of the same functional groups by electrodeposition.

[0044] The characteristics of the electrodeposition method and the performance of the electroplated aluminum from this process were analyzed. The throwing power was found to be approximately equal to that of cadmium deposition, which is important for uniform coating on complex structures and inside diameters. Since the aluminum plating process is anhydrous, the process does not release free hydrogen at the cathode which can cause hydrogen embrittlement in high strength steel. In addition, unlike traditional cadmium, plated aluminum can be anodized, thereby increasing the hardness from a typical Vickers hardness (Hv) of 10-25 to about 500 Vickers hardness for such plated coatings (in this hardness range inside, using the ASTM standard test procedure for Vickers hardness), thus helping to reduce scratches and damage during manufacture and use.

[0045] One of the applications of the present invention is the electrodeposition of aluminum on high-strength steel parts used in aviation and aerospace. The NAOS process can be applied to any conductive substrate as long as the substrate is not susceptible to chemical attack by the chemistry of the plating solution.

[0046] Although the invention has been disclosed in connection with representative embodiments thereof, it should be appreciated that, without departing from the spirit and scope of the invention as defined in the appended claims, those skilled in the art may make unspecified Additions, deletions, modifications and substitutions are disclosed.

Claims (34)

1. The electrodeposition method of metal, this method comprises: preparing a plating solution comprising a mixture of soluble metal salts and organic additives dissolved in an anhydrous, non-aromatic organic solvent; and The metal is electrodeposited from the plating solution on a conductive substrate under cathodic current. 2. The method of claim 1, wherein the soluble metal salt is a metal alkoxide. 3. The method of claim 2, wherein the metal alkoxide is selected from the group consisting of metal ethoxides, propoxides, isopropoxides, butoxides, the corresponding halides, phosphates and carbonates, and mixtures thereof. 4. The method of claim 1, wherein the metal is selected from the group consisting of aluminum, titanium, tantalum, zirconium, molybdenum, tungsten, niobium, osmium, hafnium, magnesium, and alloys and combinations thereof. 5. The method of claim 1, wherein the anhydrous, non-aromatic organic solvent is a low molecular weight non-aromatic solvent. 6. The method of claim 1, wherein the anhydrous, non-aromatic organic solvent is an alcohol or an amine. 7. The method of claim 6, wherein the alcohol has more than one OH functional group. 8. The method of claim 1, wherein the anhydrous, non-aromatic organic solvent is selected from the group consisting of ethanol, propanol, isopropanol, butanol, 2-butanol, and ethanolamine. 9. The method of claim 1, wherein the concentration of the soluble metal salt in the electroplating bath is 5% to 100% of the saturation concentration of the metal salt in an anhydrous, non-aromatic organic solvent at the operating temperature of electrodeposition. 10. The method of claim 1, comprising adding a conductive additive to the plating solution to increase the conductivity of the solvent. 11. The method of claim 10, wherein the conductive additive is a low molecular weight organic solid. 12. The method of claim 10, wherein the conductive additive is oxalic acid, citric acid, ammonium citrate, or metal chlorides, pentachlorides, tetrachlorides, or organic or inorganic compounds soluble in the electroplating bath. 13. The method of claim 1, comprising continuously filtering the plating solution over molecular sieves and maintaining an inert atmosphere over the plating solution at least during electrodeposition. 14. The method of claim 13, wherein the inert atmosphere maintains the atmosphere in contact with the plating bath substantially free of oxygen and water. 15. The method of claim 1, comprising dividing the electroplating bath into separate catholyte and anolyte compartments, the catholyte and anolyte compartments being separated by a membrane. 16. The method of claim 1, comprising agitating the electroplating bath or agitating the part being electroplated. 17. The method of claim 1, comprising preparing the conductive substrate surface for electrodeposition by sandblasting the surface, masking the surface, cleaning with an alkaline or acidic cleaning solution, and removing the cleaning solution by alcohol immersion or spraying. 18. An electroplating solution comprising: Anhydrous, non-aromatic organic solvents; and Consists of a mixture of soluble metal salts and organic additives dissolved in an anhydrous, non-aromatic organic solvent. 19. The electroplating bath of claim 18, wherein the soluble metal salt comprises an aluminum salt capable of electrodepositing aluminum. 20. The electroplating bath of claim 18, wherein the soluble metal salt comprises a titanium salt capable of electrodepositing titanium. 21. The electroplating bath of claim 18, wherein the soluble metal salt comprises a tantalum salt capable of electrodepositing tantalum. 22. The electroplating bath of claim 18, wherein the soluble metal salt comprises a zirconium salt capable of electrodepositing zirconium. 23. The electroplating bath of claim 18, wherein the soluble metal salt comprises at least one refractory metal salt capable of electrodepositing a refractory metal. 24. The electroplating bath of claim 23, wherein the refractory metal is molybdenum, tungsten, niobium, osmium, hafnium, alloys thereof, or combinations thereof. 25. The electroplating bath of claim 18, wherein the soluble metal salt is a metal alkoxide. 26. The electroplating solution of claim 18, wherein the anhydrous, non-aromatic organic solvent is a low molecular weight non-aromatic solvent. 27. An electroplating system comprising: a plating chamber containing an electroplating solution; Inlets to plating systems; and a conveying system for conveying the parts to be plated from this inlet to the plating chamber, Wherein the electroplating solution includes anhydrous, non-aromatic organic solvent and a mixture including soluble metal salt and organic solvent, and the mixture is dissolved in the anhydrous, non-aromatic organic solvent. 28. The electroplating system of claim 27, wherein the plating chamber comprises a single plating chamber, or separate plating chambers for the anolyte and catholyte. 29. The electroplating system of claim 27, wherein the electroplating chamber comprises a separate electroplating chamber having two anodes disposed substantially opposite each other with a catholyte therebetween, each anode being separated from the catholyte by a membrane . 30. The electroplating system of claim 27, comprising an outer chamber containing the molecular sieve, wherein the electroplating solution is continuously agitated by circulation through the outer chamber. 31. The electroplating system of claim 27, comprising an inert gas source, and maintaining an inert atmosphere within at least the plating chamber by gas conditioning of the inert gas source. 32. The electroplating system of claim 31, wherein the inert atmosphere is maintained at a positive pressure. 33. The electroplating bath of claim 27, wherein the soluble metal salt is a metal alkoxide. 34. The electroplating bath of claim 27, wherein the anhydrous, non-aromatic organic solvent is a low molecular weight non-aromatic solvent.

Images