OIL-IMPREGNATED NANOPOROUS OXIDE COATING FOR INHIBITING ALUMINUM CORROSION
A process includes means for depositing an anti-corrosion coating filled with liquid oil on an aluminum substrate. Aluminum is anodized and then treated with a thin hydrophobic sub-coating. The pores created through anodization are then impregnated with liquid oil. Oil penetration is maximized and residual air is minimized by first filling the pores with a filling solution, replacing the filling solution with an exchange fluid, and then replacing the exchange fluid with perfluorinated oil. The oil gives the surface coating anti-wetting properties and self-healing properties, thereby protecting the aluminum substrate underneath from corrosion.
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This application claims priority to U.S. Provisional Patent Application Ser. No. 62/627,042 filed Feb. 6, 2018, the entire disclosure of which is incorporated herein by reference.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCHThis invention was made with government support under Grant No. N00014-14-1-0502 awarded by the Office of Naval Research (ONR). The government has certain rights in the invention.
FIELD OF THE INVENTIONThis invention relates to a surface treatment coating for inhibiting corrosion of aluminum or other metal substrates and a method for preparing same.
BACKGROUND OF THE INVENTIONBecause corrosion is one of the most critical drawbacks of aluminum-based metallic material, various techniques have been applied to protect against corrosion. As a surface treatment method of aluminum, anodizing has been extensively employed in the manufacturing industry to improve surface properties and passivate the metallic surface. Anodizing processes form a thin coating of aluminum oxide that is composed of an inner thin compact layer and an outer thick layer with hexagonal columnar cells and cylindrical pores of nanoscale. Unfortunately, the porous layer may still be prone to corrosion, because corrosive media may be easily absorbed in the pores or adsorbed on the wall surface. Therefore, to seal the porous layer, various post-treatment methods have been applied and used.
Conventional sealing methods used in industrial fields include boiling water, steam, dichromate, nickel acetate and cold nickel fluoride sealing. Solid state oxide materials are formed in the pores by those sealing methods, improving corrosion resistance of the aluminum substrate. However, since the anodic aluminum oxide is naturally hydrophilic, corrosive media can still be absorbed in the pores. Recently, the entrapment of air in the pores via hydrophobic coatings on the anodic aluminum oxide surface was also reported to be effective for the prevention of corrosion. However, when entrapped air is exposed to water for a long time, it can be dissolved into the water. The hydrophobic nature of the surface is also vulnerable to physical damage.
SUMMARY OF THE INVENTIONThe invention relates to an oil-impregnated nanoporous aluminum oxide coating, which shows enhanced corrosion resistance and durability. In one embodiment, water-repellent and/or anti-corrosive liquid oil is filled partially or completely in nanopores of an anodic aluminum oxide of an aluminum substrate for inhibiting corrosion thereof. Since the pores are filled with water-immiscible oil within the high-aspect-ratio dead-end nanoscale pores, the oil is retained stably within the pores and passivates the pore walls from corrosion. Due to liquidity of the oil, the oil can effectively flow and fill damaged areas, thereby exhibiting a self-healing capability. In one embodiment, the nanopores are filled with oil completely with no air void within the pores. Due to the geometric effects of the pores (e.g., the high-aspect-ratio dead-end nanoscale pores) and the pressure of air initially occupying the pores, typical dip coating or spin coating may not be sufficient to completely fill the pores with oil. Therefore, a novel solvent exchange method is also provided for the complete filling of the pores with oil in accordance with one embodiment.
For a more complete understanding of the present invention, reference is made to the following detailed description of an embodiment considered in conjunction with the accompanying drawings, which are described briefly below. The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
The following disclosure is presented to provide an illustration of the general principles of the present invention and is not meant to limit, in any way, the inventive concepts contained herein. Moreover, the particular features described in this section can be used in combination with the other described features in each of the multitude of possible permutations and combinations contained herein.
All terms defined herein should be afforded their broadest possible interpretation, including any implied meanings as dictated by a reading of the specification as well as any words that a person having skill in the art and/or a dictionary, treatise, or similar authority would assign thereto.
Further, it should be noted that, as recited herein, the singular forms ‘a,’ “an,” and “the” include the plural referents unless otherwise stated. Additionally, the terms “comprises” and “comprising” when used herein specify that certain features are present in that embodiment, however, this phrase should not be interpreted to preclude the presence of additional steps, operations, features, components, and/or groups thereof.
To verify the imbibition of the oil in the nanopores of the anodic aluminum oxide (“AAO”) layer, a curable photoresist solution is mixed into the exchange fluid (e.g., the VERTREL® XF fluid) (solvent for oil) so that the solution can be solidified and thus be visualized with a scanning electron microscope (“SEM”).
In the case of the Teflon-coated AAO with oil impregnation achieved by simple dipping in oil for 24 hours (see
Ultrasonication for 20 minutes was also applied in the simple dipping process to compare with the solvent exchange method. The results illustrated in
In the case of the Teflon-coated AAO with oil impregnation by the solvent exchange method described above (see
The oil-impregnated AAO prepared without using the solvent exchange method but using a simple dipping for tens of minutes showed significant wetting of water droplets (pinning even at vertical (90°) inclination) in a couple of minutes by the shear flow of water (see
The anti-corrosion performance of the oil-impregnated Teflon-coated AAO (O-T-AAO) surface was evaluated using electrochemical impedance spectroscopy (EIS), compared with aluminum substrate (Al), AAO (inherently hydrophilic), Teflon-coated AAO (T-AAO), and oil-impregnated AAO (O-AAO) surfaces.
Anti-corrosion performance of the O-T-AAO surface was further evaluated using a potentiodynamic polarization method.
Surface damage of a coating layer is regarded as a potential issue for anti-corrosive surface treatments, including anodization, since the metal surfaces are likely to be exposed after being damaged. In one embodiment, the oil-impregnated surface of the present invention may autonomously recover damaged surface areas by allowing the impregnated liquid oil to immediately flow and cover the exposed areas upon damage. To evaluate the corrosion tolerance to such surface damage, cracks were deliberately created in the AAO layer by bending the AAO samples (T-AAO and O-T-AAO) against a cylindrical tube (diameter 2 cm), as shown in
To visually demonstrate the advantage of the self-healing property for anti-corrosion, a highly corrosive liquid (35 wt. % HCl+saturated CuSO4) was placed on the B-O-T-AAO and B-T-AAO surfaces, respectively, which contain many defects and cracks. Appearances of the corrosive liquid droplet on the surfaces over time are shown in
Microstructures of the surface area of each corrosion mark are shown for B-T-AAO and B-O-T-AAOs in
Supplemental details and further experimental verification are presented in the publication by Lee, J et al. entitled “Oil-Impregnated Nanoporous Oxide Layer for Corrosion Protection with Self-Healing,” Advanced Functional Materials, Vol 27, Apr. 18, 2017, Article No. 1606040 [online], <URL: https://onlinelibrary.wiley.com/doi/abs/10.1002/adfm.201606040><DOI:10.1002/adfm.201606040>, the entire contents of which publication are incorporated herein by reference.
It will be understood that the embodiments described herein are merely exemplary and that a person skilled in the art may make many variations and modifications without departing from the spirit and scope of the invention. All such variations and modifications are intended to be included within the scope of the invention.
Claims
1. A coating, comprising an inner layer containing aluminum oxide; an outer layer containing aluminum oxide; and a plurality of nanopores distributed throughout said outer layer, each nanopore of said plurality of nanopores having a respective lumen which contains a quantity of liquid oil.
2. The coating of claim 1, wherein said quantity of liquid oil is sufficient to fill each respective lumen, whereby each respective lumen is substantially free of air.
3. The coating of claim 1, wherein each of said plurality of nanopores has a sidewall which terminates at a dead-end.
4. The coating of claim 1, wherein said liquid oil is water-repellant.
5. The coating of claim 1, wherein said liquid oil is water immiscible.
6. The coating of claim 1, wherein said liquid oil is anti-corrosive.
7. The coating of claim 1, wherein said liquid oil is a perfluorinated oil.
8. The coating of claim 1, wherein said outer layer has a pair of surfaces, one surface being in contact with said inner layer and another surface being opposite said one surface.
9. The coating of claim 8, wherein said another surface of said outer layer has non-wetting properties.
10. The coating of claim 1, further comprising a hydrophobic coating layer provided on said sidewall of each nanopore of said plurality of nanopores.
11. The coating of claim 10, wherein said liquid oil is adapted to fill surface defects in said hydrophobic coating.
12. A process for applying a liquid oil to a coating layer having nanopores, said process comprising the steps of:
- providing said nanopores with a filling solution having low surface tension;
- replacing said filling solution with an exchange fluid that is miscible with both said exchange fluid and said liquid oil; and
- replacing said exchange fluid with said liquid oil, such that said nanopores contain only said liquid oil.
13. The process of claim 12, wherein said nanopores of said coating layer are completely filled with said liquid oil.
14. The process of claim 12, wherein said filling solution has a low Henry's constant.
15. The process of claim 12, wherein said process is conducted in a liquid environment.
16. The process of claim 12, wherein said liquid oil is a perfluorinated oil.
17. The process of claim 12, wherein said filling solution is ethanol.
18. A method for forming a coating on a surface of a metal substrate, said method comprising the steps of:
- anodizing said substrate in oxalic acid to form an oxide layer having a plurality of nanopores, each nanopore having a sidewall; and
- filling said plurality of nanopores with a liquid oil.
19. The method of claim 18, further comprising the step of coating the sidewall of each nanopore of said plurality of nanopores with a hydrophobic coating material.
20. The method of claim 18, wherein said metal substrate is aluminum.
21. The method of claim 18, wherein said liquid oil is a perfluorinated oil.
22. The method of claim 18, wherein filling said nanopores with said liquid oil further comprises the steps of:
- providing said nanopores with a filling solution having low surface tension;
- replacing said filling solution with an exchange fluid that is miscible with both said exchange fluid and said liquid oil; and
- replacing said exchange fluid with said liquid oil, such that said nanopores contain only said liquid oil.
23. The process of claim 22, wherein said nanopores are completely filled with said liquid oil.
24. The process of claim 22, wherein said filling solution has a low Henry's constant.
25. The process of claim 22, wherein said process is conducted in a liquid environment.
26. The process of claim 22, wherein said liquid oil is a perfluorinated oil.
27. The process of claim 22, wherein said filling solution is ethanol.
Type: Application
Filed: Feb 6, 2019
Publication Date: Aug 8, 2019
Applicant: THE TRUSTEES OF THE STEVENS INSTITUTE OF TECHNOLOGY (Hoboken, NJ)
Inventors: Chang-Hwan Choi (Tenafly, NJ), Junghoon Lee (Palisades Park, NJ)
Application Number: 16/269,348