Electrolytic Preparation Of A Metal Substrate For Subsequent Electrodeposition
A method of plating a workpiece, the method includes electrochemically removing any oxide on the surface of the workpiece by applying a first waveform to the workpiece and a cathode both placed in a first electrolyte solution, and electroplating the workpiece surface by applying a second waveform to the workpiece and an anode both placed in a second electrolyte solution including a plating material.
This application claims benefit of and priority to U.S. Provisional Application Ser. No. 62/854,635 filed May 30, 2019, under 35 U.S.C. §§ 119, 120, 363, 365, and 37 C.F.R. § 1.55 and § 1.78, which is incorporated herein by this reference.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENTThis invention was made in part with U.S. Government support from U.S. Air Force SBIR Contract No. FA8501-16-P-0047 and U.S. Department of Energy Grant No. DE-SC0017751 and U.S. Department of Energy Grant No. DE-SC0015201. The U.S. Government may have certain rights in the invention.
FIELD OF THE INVENTIONThis invention relates to a new process to directly deposit a metallic coating onto a metal substrate while minimizing the number of surface pretreatment steps, and may in one embodiment particularly relate to a new process to directly deposit a metallic coating onto a passive oxide-forming metal substrate while minimizing the number of surface pretreatment steps.
BACKGROUND OF THE INVENTIONA need derives from the responsibility the US Air Force Air Logistics Complexes has for corrosion control and improved lifecycle sustainability of aircraft parts. A specific material that has been used for decades due to its excellent corrosion resistance, lubricity, electrical conductivity, and ability to be conversion coated and ability to withstands thermal and electrical shock is cadmium. Cadmium plating offers an effective barrier protection to the substrate, especially in the marine environment. Cadmium also offers sacrificial protection to the steel components under corroding conditions.
However, cadmium deposition from cyanide baths gives rise to unacceptably high hydrogen intake by plated components of high strength, leading to hydrogen embrittlement. Also cyanide waste treatment is very expensive and the cadmium is often conversion coated with hexavalent chromium—also highly toxic. Furthermore, cadmium and its salts are known carcinogens and were part of the Annex XVII REACH list as well as Executive Order 13431.
Therefore, the US Air Force and others desires replacement of cadmium plating with zinc-nickel alloy (Zn—Ni), specifically for steel and aluminum electrical connectors, back-shells, components on aircraft, and propeller system components. Corrosion resistant alloys such as Zn—Ni (10-15% Ni) offer low susceptibility to stress-corrosion cracking and good abrasion resistance, formability, weldability, paint adhesion, and corrosion resistance. The corrosion resistant Zn—Ni coating must adhere strongly to the aluminum substrate with the ability to resist delamination in order to maintain strong electrical connection during operation. The challenge in plating on aluminum is the tenacious oxide-forming passive film that readily forms on the surface. This is overcome in current practice through a large number of aggressive precleaning steps that include hydrofluoric acid or zincating.
Another interest is the direct deposition of nickel onto aluminum during manufacturing of neutrino focusing horns for the Department of Energy (DOE). The DOE requires novel coating technologies and processes for neutrino focusing horns that are cost effective, less complex and use a smaller volume of chemicals to replace the most successful functional coating thus far—an electroless nickel coating.
Another interest is electrodeposition of copper onto niobium for fabrication of copper-niobium superconducting radio frequency (SRF) cavities. The DOE seeks SRF cavity fabrication techniques that reduce use of expensive metals such as niobium while achieving equivalent performance as bulk niobium cavities. One approach is to electroform copper onto a thin niobium cavity shell. Electroforming is a well-established industrial processing technology, and is not only low cost, but is also flexible and adaptable to many cavity sizes and shapes. The challenge in plating copper on niobium is the tenacious oxide-forming passive film that readily forms on the niobium surface. This is overcome in current practice through a large number of aggressive precleaning steps.
Another interest is the plating of coatings such as nickel on oxide-forming passive substrates such as titanium. Another interest is the plating of coatings such as nickel on oxide-forming passive substrates such as silicon.
BRIEF SUMMARY OF THE INVENTIONFeatured, in one aspect, is an environmentally benign pretreatment and deposition process to clean and prepare the aluminum surface and subsequently directly electrodeposit nickel or a nickel alloy onto the aluminum surface. In another example, disclosed is an environmentally friendly pretreatment and deposition process to clean, prepare and de-passivate a titanium surface and subsequently directly electrodeposit nickel on the titanium surface. In still another example, disclosed is an environmentally friendly pretreatment and deposition process to clean, prepare and de-passivate a niobium surface and subsequently directly electrodeposit copper on the niobium surface. In still another example, disclosed is an environmentally friendly pretreatment and deposition process to clean, prepare and de-passivate a silicon surface and subsequently directly electrodeposit nickel on the silicon surface.
Finally, disclosed is an environmentally friendly pretreatment and deposition process to clean, prepare and de-passivate an oxide-forming passive surface and subsequently directly electrodepositing a metal or metal alloy on the surface.
Other objects, features and advantages will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which:
One challenge in plating a metal coating directly onto metal substrates including aluminum, titanium, niobium, silicon and the like is the tenacious passive oxide film that readily forms on the surface when it is exposed to air. If the oxide film is not removed prior to plating, the plated deposit generally demonstrates poor performance characteristics such as corrosion resistance, wear and adhesion. Consequently, plating lines generally include various pretreatment processes to prepare and remove oxide from the oxide-forming substrate prior to plating illustrated in the process flow in
As evident from the process flow of the instant invention in
An exemplary functional process flow is presented in
As noted in the discussion, electrolytic activation step 800 may include a pulse reverse waveform and electrolytic metal plating step 700 may use a direct current, pulse current, pulse reverse current. A depiction of a general pulse reverse current waveform of the instant invention is shown in
iavg=icλc−iaλa (1)
In pulse/pulse reverse process there are numerous combinations of peak current densities, duty cycles, and frequencies to obtain a given average current. These parameters provide the potential for much greater process control compared to direct current (DC).
The instant invention preferably comprises a pretreatment/plating process for an oxide-forming substrate. As illustrated, the pretreatment/plating process is simpler, contains fewer processing steps, and eliminates the need for hazardous and difficult to control chemicals including hydrofluoric acid, nitric acid, sulfuric acid and cyanide containing solutions.
In one embodiment, the process reduces and eliminates the need for aggressive chemical precleaning steps to prepare a passive metal substrate for deposition of another metallic coating. The process includes applying a pulse reverse waveform that removes the native oxide film prior to deposition of the desired metallic coating. See also U.S. Pat. Nos. 6,080,504 and 5,084,144 incorporated herein by this reference.
Example I: Nickel Coating on TitaniumTitanium is an oxide-forming substrate and usually requires extensive pretreatment similar to that depicted in
In the next experiment, a nickel coating was applied to a titanium substrate after the electrolytic pretreatment (800,
Aluminum is an oxide-forming passivating substrate and usually requires extensive pretreatment similar to that depicted in
The nickel (Ni) coating was directly deposited onto the electrolytically activated Al T6061 coupons from a Watts nickel plating bath. The Watts nickel bath contained nickel sulfate (NiSO4 300 g/L), nickel chloride (NaCl 60 g/L) and boric acid (H3BO3 45 g/L) in water. In this Ni deposition trial, a nickel anode was spaced 4″ from the electrolytically activated Al T6061 substrate. The plating bath temperature was maintained at 80° F. and the substrate was plated under direct current conditions for 35 minutes at a current density of 4 A/dm2.
The nickel-phosphorous (Ni—P) was directly deposited onto the electrolytically activated Al T6061 coupons from a Umicore NIPHOS 968 plating bath obtained from UYEMURA Corp. The anode was nickel sulfide pellets in a titanium basket and was spaced 4″ from the Al T6061 substrate. The plating bath temperature was maintained at 140° F. and the substrate was plated under direct current conditions for 35 minutes at a current density of 4 A/dm.
As a baseline for comparison, a high phosphorous (9 to 10%) electroless nickel coating was obtained from a commercial surface finishing facility, Techmetals Inc. Prior to applying the electroless nickel coating, the Al T6061 substrates were subjected to the standard pretreatment as generally depicted in
As measures and indicators of adhesion and therefore indicators of the effectiveness of the electrolytic activation pretreatment process, the samples and baseline were evaluated using 1) Tape Adhesion Test according to ASTM D3359 Tape Adhesion Test, 2) Bend Test around a 0.5″ radius, 3) Bend Test around a 0.25″ radius, and 4) Taber Wear Test according to ASTM D4060. The data are presented in TABLE I.
Both the Watts nickel and nickel-phosphorous coatings on the electrolytically activated Al T6061 substrates exhibited comparable adhesion to the baseline electroless nickel coating on the standard pretreated Al T 6061 substrate. While the nickel-phosphorous coating exhibited slight edge blistering in the 0.25″ bend test, this may be attributed to the lack of plating uniformity on the Al T6061 edge. The Taber Wear Index is a measure of the wear resistance of the coating and is indirectly related to coating adhesion to the substrate. Specifically, if the coating does not exhibit good adhesion to the substrate, then the coating will spall during the Taber Wear Test and the resulting Index will be extremely high. The Taber Wear Index of the nickel-phosphorous coating on the electrolytically activated Al T6061 is comparable to the baseline electroless nickel high phosphorous. Since the nickel coating is different than the nickel-phosphorous coating, the nickel sample was not tested for Taber Wear.
Example III: Zinc-Nickel Coating on AluminumAluminum is an oxide-forming substrate and usually requires extensive pretreatment similar to that depicted in
The zinc-nickel (Zn—Ni) coating was directly deposited onto the electrolytically activated Al T6061 coupons from a Dipsol IZ-C17+ plating bath obtained from DIPSOL OF AMERICA, Inc. The plating bath temperature was maintained at 80° F. and the substrate was plated under direct current conditions at a current density of 5.09 A/dm2. The plating times were 15, 25 and 40 minutes resulting in coating thicknesses of 10, 20 and 28 μm, respectively. All direct current plated coatings the electrolytically activated passed the ASTM D3359 Tape Adhesion Test.
Finally, zinc-nickel (Zn—Ni) coatings were directly deposited onto the electrolytically activated Al T6061 coupons from the Dipsol IZ-C17+ plating bath using cathodic pulse currents at frequencies of 1, 100 and 1000 Hz and duty cycles of 25, 50 and 75%. The average current density for all nine samples was 5.09 A/dm2. The plating duration was 25 minutes. All pulse current plated coatings on the electrolytically activated Al T6061 substrates passed the ASTM D3359 Tape Adhesion Test.
Example IV: Nickel Coating on SiliconSilicon is an oxide-forming substrate and usually requires extensive pretreatment similar to that depicted in
Aside from the preferred embodiment or embodiments disclosed below, this invention is capable of other embodiments and of being practiced or being carried out in various ways. Thus, it is to be understood that the invention is not limited in its application to the details of construction and the arrangements of components set forth in the following description or illustrated in the drawings. If only one embodiment is described herein, the claims hereof are not to be limited to that embodiment. Moreover, the claims hereof are not to be read restrictively unless there is clear and convincing evidence manifesting a certain exclusion, restriction, or disclaimer.
Although specific features of the invention are shown in some drawings and not in others, this is for convenience only as each feature may be combined with any or all of the other features in accordance with the invention. The words “including”, “comprising”, “having”, and “with” as used herein are to be interpreted broadly and comprehensively and are not limited to any physical interconnection. Moreover, any embodiments disclosed in the subject application are not to be taken as the only possible embodiments. Other embodiments will occur to those skilled in the art and are within the following claims.
In addition, any amendment presented during the prosecution of the patent application for this patent is not a disclaimer of any claim element presented in the application as filed: those skilled in the art cannot reasonably be expected to draft a claim that would literally encompass all possible equivalents, many equivalents will be unforeseeable at the time of the amendment and are beyond a fair interpretation of what is to be surrendered (if anything), the rationale underlying the amendment may bear no more than a tangential relation to many equivalents, and/or there are many other reasons the applicant cannot be expected to describe certain insubstantial substitutes for any claim element amended.
Claims
1. A method of plating a workpiece, the method comprising:
- electrochemically removing any oxide on the surface of the workpiece by applying a first waveform to the workpiece and a cathode both placed in a first electrolyte solution; and
- electroplating the workpiece surface by applying a second waveform to the workpiece and an anode both placed in a second electrolyte solution including a plating material.
2. The method of claim 1 further including cleaning the workpiece surface prior to electrochemically removing any oxide on the workpiece surface.
3. The method of claim 1 further including rinsing the workpiece between electrochemically removing any oxide on the workpiece surface and electroplating the workpiece surface.
4. The method of claim 1 further including rinsing the workpiece surface after electroplating.
5. The method of claim 1 in which the first waveform is a pulsed reverse waveform.
6. The method of claim 1 in which the workpiece is aluminum, titanium, silicon, or other oxide forming conductive materials/alloys.
7. The method claim 1 in which the plating material is a zinc-nickel alloy, nickel, nickel phosphorus, or other platable elements/alloys.
8. The method of claim 1 in which the first electrolyte solution includes sodium chloride, sodium bromide, sulfuric acid, sodium sulfate, potassium hydroxide, and/or other water-soluble salts.
9. The method of claim 1 in which the second electrolyte solution includes hydrochloric acid, water, and/or boric acid or other constituents required to electrodeposit a given material.
10. A system for plating a workpiece, the system comprising:
- an electrochemical tank including an electrolyte solution about the workpiece and a cathode;
- a first power supply for applying a first waveform to the workpiece and the cathode to electrochemically remove any oxide on the surface of the workpiece;
- an electrochemical plating tank including an electrolyte solution including a plating material about the workpiece and an anode; and
- a second power supply for applying a second waveform to the workpiece and the anode to electroplate the workpiece with the plating material.
11. The system of claim 10 further including a cleaning station for the workpiece for cleaning the workpiece surface prior to electrochemically removing any oxide on the workpiece surface.
12. The system of claim 10 further including a first rinsing station for rinsing the workpiece between electrochemically removing any oxide on the workpiece surface and electroplating the workpiece surface.
13. The system of claim 10 further including a second rinsing station for rinsing the workpiece surface after electroplating.
14. The system of claim 10 in which the first waveform is a pulsed reverse waveform.
15. The system of claim 10 in which the workpiece is aluminum, titanium, silicon, or other oxide forming conductive materials/alloys.
16. The system of claim 10 in which the plating material is a zinc-nickel alloy, nickel, nickel phosphorus, or other platable elements/alloys.
17. The system of claim 10 in which the first electrolyte solution includes sodium chloride, sodium bromide, sodium fluoride, sulfuric acid, sodium sulfate, and/or potassium hydroxide, and/or other water-soluble salts.
18. The system of claim 10 in which the second electrolyte solution includes hydrochloric acid, water, and/or boric acid or other constituents required to deposit a given material.
Type: Application
Filed: May 7, 2020
Publication Date: Dec 3, 2020
Inventors: Maria E. Inman (Yellow Springs, OH), Jing Xu (Englewood, OH), Timothy D. Hall (Englewood, OH), Earl Jennings Taylor (Troy, OH), Alan Bonifas (Vandalia, OH), Rajeswaran Radhakrishnan (Miamisburg, OH)
Application Number: 16/869,014