CORROSION PROTECTION OF ALUMINUM AND ALUMINUM ALLOYS USING EMERALDINE BASE POLYANILINE

Abstract: The present invention includes a coating composition for protecting aluminium and alloys thereof from corrosion. The composition comprises at least one substance selected from the group consisting of emeraldine base polyaniline and oligomers thereof, or sulfonated polyanilines and salts and oligomers thereof. The invention also includes an aluminum metal or aluminum alloy piece bearing such a coating.

Description: CORROSION PROTECTION OF ALUMINUM AND ALUMINUM ALLOYS USING EMERALDINE BASE POLYANILINE Related Application Data This application claims the benefit of U.S. Provisional Application Serial No. 60/036,685 filed on March 11, 1997, hereby incorporated herein by reference. Technical Field of the Invention The present invention relates to the protection of aluminum and its alloys from corrosion. Background of the Invention Aluminum and its alloys are widely used in a variety of applications, including aerospace and automotive construction, building construction, and public utilities. Many of these applications involve the use of aluminum and its alloys in exposure to the environment or otherwise in corrosive atmospheres. Aluminum typically undergoes relatively rapid oxidation which, over time, can detract from its functional and/or structural viability, and its appearance. Accordingly, to preserve the functional and/or structural viability, as well as the appearance in some cases, of aluminum and its alloys, it is desirable to reduce or eliminate the corrosion of aluminum Although the advantages and goals of the present invention are described with reference to aluminum and its alloys, the present invention is not limited to either general or specific uses. Indeed, the potential uses of the present invention are numerous as may become apparent to one of ordinary skill in the fields of endeavor to which the present invention might be applied. Accordingly, additional advantages or the solution to other problems may become apparent to one of ordinary skill in these arts from the present disclosure or through practice of the present invention. Summary of the Invention Aluminum and aluminum alloys normally prone to corrosion when subjected to potentially corrosive conditions (acid, alkaline or neutral) can be protected by coats of emeraldine base polyaniline (EB) (see Figure 1), and derivatives thereof. The EB containing film protects the aluminum surface it is coated on, but may also protect the uncoated surface areas of the metal object, including the opposite side of the coated surface, the sides and edges. In broadest terms, the present invention includes a coating composition for protecting aluminum and alloys thereof from corrosion. The composition comprises at least one substance selected from the group consisting of emeraldine base polyanilines and oligomers thereof. Such oligomers may be trimers, tetramers, octamers, hexadecamers, and/or mixtures thereof. The fraction of imine units in the emeraldine base may range from about .3 to about .75 units. The ring structures of emeraldine base or oligomers thereof may also be provided with additional ring substituents for purposes such as providing polymer cross linking, improved or in some cases even reduced solubility and/or better bonding with the carrier resin. Such substituents may be, for example, carbonate groups, alkyl groups, epoxy groups, and combinations thereof. Nominally, it will be preferred that the coating composition also include a resin which is used as a carrier for the active ingredient. Examples of such resin include acrylic and epoxy resins. Resins and blends containing one or more of the polymers/oligomers mentioned herein may also offer corrosion protection of aluminum and aluminum alloys. In most applications, a top coat sealing the corrosion protecting polymers/oligomers from the environment is likely to be used, enhancing the durability of the corrosion protecting coat. Adhesion-improving surface pretreatment prior to the coating with the corrosion protecting polymer/oligomer also is likely to be utilized in order to improve the effectiveness of the protective coat. The present invention also includes a coated metal piece which may be of aluminum or any alloy thereof which has at least one surface adapted to receive a coating. The coated metal piece also has on at least one such surface a coating comprising at least one substance selected from the group consisting of emeraldine base polyaniline and oligomers thereof. The coating may also be provided with a coating sealant such as a lacquer or epoxy sealant. The present invention also includes, in broadest terms, a coating composition for protecting aluminum and alloys thereof from corrosion, the composition comprising at least one substance selected from the group consisting of sulfonated polyaniline and salts and oligomers thereof. Such oligomers may include trimers, tetramers, octamers, hexadecamers, and/or mixtures thereof. The sulfonated polyaniline coatings may also be born by resin such as acrylic and epoxy resins. Typically the sulfonated polyaniline coatings will have a degree of sulfonation in the range of from about 50% to about 100% (i.e. expressed in terms of the percentage of ring structures bearing a sulfonate group). The ring structures of the sulfonated polyaniline may also be provided with additional ring substituents for purposes such as providing polymer cross linking and/or better sulfonation or bonding with the carrier resin. Such substituents may be, for example, carbonate groups, alkyl groups, epoxy groups, and combinations thereof. The present invention also includes a coated metal piece comprising a metal selected from the group consisting of aluminum and alloys thereof, said metal piece having at least one surface adapted to receive a coating; and a coating on said surface which comprises at least one substance selected from the group consisting of sulfonated polyaniline and oligomers thereof. Such oligomers may be trimers, tetramers, octamers, hexadecamers, and/or mixtures thereof. The coated metal piece may also have a coating sealant atop the coating, and such sealants may be, for instance, lacquers and epoxy sealants. Brief Description of the Drawings Figure 1 shows the schematic chemical structure of emeraldine base polyaniline (EB). Figure 2 shows the schematic chemical structure of sulfonated (100%) polyaniline (SPAN) in the emeraldine base form. Figure 3 shows the XPS Al 2p core level depth profile of a polished A13003 sample that has not been exposed to a corrosive environment. Figure 4 shows the XPS Al 2p core level spectra of (a) an A13O03 sample immersed in the 0.1 M HCl acid bath for 2 hours, (b) an EB/A13003 sample immersed in the 0.1 M HCl acid bath for 2 hours (the metal side being depth profiled), and (c) an A13003 control sample not exposed to a corrosive environment. Data were acquired after argon ion sputtering in ultra-high vacuum to a depth of about 15 nm. Figure 5 shows the XPS Al 2p core level spectra of (a) A13003 exposed to HCl and (b) the uncoated metal side of A13003/EB exposed to HCl where the EB film failed during the acid exposure. Data were acquired after argon ion sputtering in ultra-high vacuum to a depth of about 28 nm. Figure 6 shows a graph of a potentiodynamic scan showing the results of a potentiodynamic study on an uncoated aluminum 2024-T3 sample and an emeraldine base-coated aluminum 2024-T3 sample, in accordance with one embodiment of the present invention. The surface was polished with 600 grit emery paper and degreased with ethanol before application of any coating. Reproducibility is evidenced by scans of multiple samples. Figure 7 shows a graph of a potentiodynamic scan showing the results of a potentiodynamic study on an uncoated aluminum 2024-T3 sample and an emeraldine base trimer-coated aluminum 2024-T3 sample, in accordance with one embodiment of the present invention. Reproducibility is evidenced by scans of multiple samples. Figure 8 shows a graph of a potentiodynamic scan showing the results of a potentiodynamic study on a scribed emeraldine base-coated aluminum 2024-T3 sample in accordance with one embodiment of the present invention, as compared to an epoxy-coated aluminum 2024-T3 sample. The surface was polished with 600 grit emery paper and degreased with ethanol before application of any coating. Reproducibility is evidenced by scans of multiple samples. Figure 9 shows a graph of a potentiodynamic scan showing the results of a potentiodynamic study on a scribed emeraldine base trimer-coated aluminum 2024-T3 sample in accordance with one embodiment of the present invention, as compared to an epoxy-coated aluminum 2024-T3 sample. The surface was polished with 600 grit emery paper and degreased with ethanol before application of any coating. Reproducibility is evidenced by scans of multiple samples. Detailed Description of the Preferred Embodiments In accordance with the foregoing summary of the invention, the following presents a detailed description of the one embodiment of the invention which is also presently considered to be the best mode of the invention. Example 1 EB films were coated onto A13003 alloys and were found to reduce the corrosion damage on the uncoated backside as well as the coated side of the Al samples when immersed in an 0.1 M HCl bath at 80 C for 2 hours, as compared to corresponding uncoated A13003 control samples. Likewise, samples of pure aluminum vapor deposited onto EB coated glass slides were also found to be more resistive to corrosion when exposed to hot acid vapor (0.1 M HCl) than the control samples consisting of Al vapor deposited onto glass. Sample Preparation Samples consisting of A13003 (alloy) coupons were polished on both sides using emery paper and subsequently ultrasound washed in acetone and propanol. Some of the coupons were then coated by emeraldine base polyaniline (EB) in an N- methylpyrrolidinone solution through drop-coating and dried over night under a hood. EB films also were drop-coated from N-methylpyrrolidinone solution onto glass slides and dried over night under a hood. Thin (-1000 A) Al films were then vapor deposited onto the EB coated glass slides and non-coated control glass slides. The samples to be corroded where immersed in an acid bath kept at 80° C for two hours. The acid used was 0.1 M HCl. After the acid exposure, the samples were quickly blow dried with N 2 gas before being inserted into the UHV chamber. Experimental Procedure X-ray photoelectron spectroscopy (XPS) was carried using a VG Scientific ESCALAB Mkll system and Mg K α X-rays (1253.6 eV). Depth profiling was carried out using an argon ion sputtering gun. The spectroscopy was carried out at a background pressure of 10 ' mbar. XPS depth profiling experiments were carried out on the metal side for EB/A13003 and Al/EB/glass samples as well as for A13003 and Al/glass control samples exposed to the corrosive environments mentioned in the sample preparation section. Depth profiling was carried out for polished Al and A13003 samples that had not been exposed to acid. In the case of the EB/A13003 samples, depth profiling was carried out both the A13003 side and the EB side. Results In Figure 3 is depicted the Al 2p core level depth profile of a polished A13003 sample that has not been exposed to a corrosive environment. The peak situated at 72.9 eV represents metallic Al and the peak at 76 eV results from Al 2 O 3 . The relative amount of metallic Al increases as the surface Al 2 O 3 is sputtered away, with the metallic Al being the dominating feature at depths of -10 nm giving an estimate of the oxide layer's thickness. When aluminum and aluminum alloys are exposed to corrosive environments, the protective Al 2 O 3 oxide layer is damaged and Al + ions from the underlying metal bulk are dissolved into the solution. Various new chemical species are formed at the surface layers, including chlorine containing aluminum hydroxides and aluminum oxides, with binding energies ranging from 76-79 eV. This new feature in the Al 2p core level spectrum will thus be a signature of corrosion, and the depth into the sample with which it extends will give an estimate of the severity of the corrosion damage. In Figure 4 is shown the Al 2p core level spectra of (a) an A13003 sample immersed in the 0.1 M HCl acid bath for 2 hours, (b) an EB/A13003 sample immersed in the 0.1 M HCl acid bath for 2 hours (the metal side being depth profiled), and (c) an A13003 control sample not exposed to a corrosive environment. All spectra were taken at a depth of -15 nm. Whereas the intensities of the features at 75 eV and higher binding energies are practically the same for the control sample (c) and the EB protected one (b) with only a slight tail extending from 77 eV and out signifying the possible existence of corrosion products, the unprotected A13003 sample (a) has a much more intense peak in the non-metallic energy range giving evidence to a corrosion attack. Qualitatively, the same results were obtained for the pure Al samples, where the features in the Al 2p spectrum signifying corrosion damage extended much further down into the Al films for the unprotected samples than for the EB protected ones. A series of immersion tests were made and it was noted that the adhesion between the polymer and oligomer films, EB included, and the aluminum alloy substrates was quite poor leading to variations in the effectiveness of corrosion protection depending on the rate of film failure. In a practical application, this problem can be circumvented by crosslinking the polymers/oligomers, applying them in blends, pretreating the aluminum alloy surface to increase the adhesion or applying top coats typically used in the paint industry to protect the primer layer. An example of such a case where parts of the EB film failed during the acid exposure is shown in Figure 5. Here Al 2p spectra of (a) unprotected A13003 and (b) A13003/EB samples were taken at a depth of ~28 nm. Unlike the case depicted in Figure 4, the peak signifying corrosion damage (76-78 eV) is of higher intensity than the metallic peak (-73 eV), indicating that the corrosion protection was weaker in this case. The EB protected sample, however, still shows less corrosion damage as is evident from the larger relative metallic/corrosion damage peak intensity as compared to the unprotected sample. The unprotected sample has a distinct shoulder at -78 eV, significantly broadening the non-metallic peak giving further proof of corrosion damage since the number of different Al containing oxides and hydroxides has increased compared to the A13003/EB sample. It should be mentioned that in all cases studied, there was always some corrosion damage visible. This is not surprising, since, although not limited by theory, it is believed that the corrosion protection is of anodic origin (although based on our EB/steel studies we expect that there are other factors as well that contribute to the corrosion protection when using this class of polymers/oligomers) and anodic protection only reduces the corrosion current, generally not bringing it to zero. Potentiodynamic Studies For the electrochemical experiments, EB and trimer films were drop cast from N-methylpyrrolidinone solutions on to 1.5 x 1.5 x 0.19 cm 3 A12024-T3 coupons that had been polished with 600 grit emery paper and degreased with ethanol. The thickness of the EB films were -20 micrometers, and -40 micrometers for the trimer films. The samples were stored for up to 24 hours in air prior to the electrochemical experiments. The EB-coated, trimer-coated and non-coated A12024-T3 samples were placed in a holder at the bottom of a vessel with the coated side exposed to a deaerated (argon) 0.1 M NaCl solution. The (coated) A12024-T3 coupons functioned as the working electrode in a three electrode set up. A platinum foil was used as the counter electrode and a saturated calomel electrode (SCE) was used as the reference electrode. The potentiostat used for the potentiodynamic experiments was a Gamry Instruments model PC3 potentiostat galvanostat equipped with the CMS 100 corrosion system software. The scanning speed for the experiments was 5 mV/s. The potentiodynamic experiments carried out in a deaerated 0.1M NaCl solution gave open circuit potentials of - -0.9 V vs SCE for the non-coated A12024- T3 coupons and - -0.5 V vs SCE for the EB-top-coated A12024-T3 samples, Fig. 6. Trimer top coated A12024-T3 coupons had open circuit potentials of - -0.75 V vs SCE, see Fig. 7. The aluminum alloy is ennobled in the presence of an EB or a trimer top coat, i.e., some form of anodic protection is taking place. The pitting potential was —0.6 V for the uncoated A12024-T3 coupons. In contrast, no obvious pitting was found to occur for the EB-coated or trimer coated samples, even at potentials as high as 0.2 V. Furthermore, the corrosion current was more than an order of magnitude lower for the EB-coated coupons than the non-coated ones, clearly demonstrating the corrosion protecting capability of EB coats in salt environments. The corrosion current was roughly five times less for the trimer coated coupons compared to the non-coated ones, also demonstrating an anti-corrosion effect. Identical potentiodynamic studies were carried out for A12024-T3 samples coated with EB, trimer and epoxy (Buehler Limited, epoxide resin #20-8130-032 and epoxide hardener #20-8132-008) where a 0.2 cm scratch had been scribed into the films exposing the bare A12024-T3 metal to the electrolyte. Potentiodynamic experiments carried out in a deaerated 0.1M NaCl solution gave evidence that the EB and trimer films have throwing power, i.e., the ability to protect a metal against corrosion even for areas where the metal is exposed (cracks in the coating, etc). In Fig. 8 are shown the potentiodynamic scans for scribed EB and epoxy, and the corrosion current was reduced by more than a factor of ten for the EB coated samples as compared to the epoxy coated coupons. The same anti-corrosion effect was found for the scribed trimer coatings, as depicted in Fig. 9, where the corrosion current for the trimer samples was reduced by an order of magnitude compared to the epoxy- coated coupons. Summary There is evidence that EB and derivatives thereof spun/cast as films/coats reduce the corrosion damage of aluminum and aluminum alloys when exposed to, or resident in, corrosive environments. Sulfonated (50% - 100%) polyanilines (SPAN), depicted in Figure 2, oligomers of EB and SPAN (trimers, tetramers, octamers, hexdecamers, etc.), as well as ring-modified derivatives of EB, SPAN and their oligomers are expected to offer similar corrosion protection given their electronic and chemical similarities to EB. The potentiodynamic studies also indicated that the coatings in accordance with the present invention are capable of achieving throwing power levels such that the coatings are capable of providing an anti-corrosive effect even when there are exposed gaps in the coating. In view of the foregoing disclosure, it will be within the ability of one skilled in the art to make alterations and variations to the present invention, such as through the substitution of equivalent materials and processing steps, without departing from the spirit of the invention as reflected in the following claims, the substance of which is included herein.

Claims: What is claimed is: 1. A coating composition for protecting aluminum and alloys thereof from corrosion, said composition comprising at least one substance selected from the group consisting of emeraldine base polyaniline and oligomers thereof. 2. A coating composition according to claim 1 wherein said at least one substance is an oligomer of emeraldine base polyaniline selected from the group consisting of trimers, tetramers, octamers, hexadecamers, and mixtures thereof. 3. A coating composition according to claim 1 additionally comprising a resin. 4. A coating composition according to claim 3 wherein said resin is selected from the group consisting of acrylic and epoxy resins. 5. A coating composition according to claim 1 wherein said at least one substance is provided with additional ring substituents. 6. A coating composition according to claim 5 wherein said ring substituents are selected from the group consisting of carbonate groups, alkyl groups, alkoxy groups, and combinations thereof. 7. A coated metal piece, said coated metal piece comprising: (a) a metal piece comprising a metal selected from the group consisting of aluminum and alloys thereof, said metal having piece having at least one surface adapted to receive a coating; and (b) a coating on said at least one surface, said coating comprising at least one substance selected from the group consisting of emeraldine base polyaniline and oligomers thereof. 8. A coated metal piece according to claim 7 wherein said at least one substance is an oligomer of emeraldine base polyaniline selected from the group consisting of trimers. tetramers. octamers, hexadecamers, and mixtures thereof. 9. A coated metal piece according to claim 7 additionally comprising a coating sealant atop said coating. 10. A coated metal piece according to claim 9 wherein said sealant is selected from the group consisting of lacquers and epoxy sealants. 11. A coated metal piece according to claim 7 wherein said at least one substance is provided with additional ring substituents. 12. A coated metal piece according to claim 11 wherein said ring substituents are selected from the group consisting of carbonate groups, alkyl groups, alkoxy groups, and combinations thereof. 13. A coating composition for protecting aluminum and alloys thereof from corrosion, said composition comprising at least one substance selected from the group consisting of sulfonated polyaniline and salts and oligomers thereof. 14. A coating composition according to claim 13 wherein said at least one substance is an oligomer of sulfonated polyaniline selected from the group consisting of trimers, tetramers, octamers, hexadecamers, and mixtures thereof. 15. A coating composition according to claim 13 additionally comprising a resin. 16. A coating composition according to claim 15 wherein said resin is selected from the group consisting of acrylic and epoxy resins. 17. A coating composition according to claim 1 wherein the degree of sulfonation of said at least one substance is in the range of from about 50% to about 100%. 18. A coating composition according to claim 13 wherein said at least one substance is provided with additional ring substituents. 19. A coating composition according to claim 18 wherein said ring substituents are selected from the group consisting of carbonate groups, alkyl groups, alkoxy groups, and combinations thereof. 20. A coated metal piece, said coated metal piece comprising: (a) a metal piece comprising a metal selected from the group consisting of aluminum and alloys thereof, said metal having piece having at least one surface adapted to receive a coating; and (b) a coating on said at least one surface, said coating comprising at least one substance selected from the group consisting of sulfonated polyaniline and salts and oligomers thereof. 21. A coated metal piece according to claim 20 wherein said at least one substance is an oligomer of sulfonated polyaniline selected from the group consisting of trimers, tetramers, octamers, hexadecamers, and mixtures thereof. 22. A coated metal piece according to claim 20 additionally comprising a coating sealant atop said coating. 23. A coated metal piece according to claim 22 wherein said sealant is selected from the group consisting of lacquers and epoxy sealants. 24. A coated metal piece according to claim 20 wherein the degree of sulfonation of said at least one substance is in the range of from about 50% to about 100%. 25. A coated metal piece according to claim 20 wherein said at least one substance is provided with additional ring substituents. 26. A coated metal piece according to claim 25 wherein said ring substituents are selected from the group consisting of carbonate groups, alkyl groups, alkoxy groups, and combinations thereof. 27. A coated metal piece according to claim 20 wherein said metal is selected from the group consisting of A13003 and A12024-T3. 28. A process of providing a coating on a metal piece comprising aluminum and alloys thereof, said process comprising the steps: (a) obtaining a metal piece comprising a metal selected from the group consisting of aluminum and alloys thereof, said metal having piece having at least one surface adapted to receive a coating; (b) polishing said at least one surface so as to remove any oxidation formed thereupon; and (c) placing a coating on said at least one surface, said coating comprising at least one substance selected from the group consisting of emeraldine base polyaniline and oligomers thereof. 29. A process according to claim 28 wherein said at least one substance is an oligomer of emeraldine base polyaniline selected from the group consisting of trimers, tetramers, octamers, hexadecamers, and mixtures thereof. 30. A process according to claim 28 additionally comprising placing a coating sealant atop said coating. 31. A process according to claim 30 wherein said sealant is selected from the group consisting of lacquers and epoxy sealants. 32. A process according to claim 28 wherein said at least one substance is provided with additional ring substituents. 33. A process according to claim 28 wherein said ring substituents are selected from the group consisting of carbonate groups, alkyl groups, alkoxy groups, and combinations thereof. 34. A process of providing a coating on a metal piece comprising aluminum and alloys thereof, said process comprising the steps: (a) obtaining a metal piece comprising a metal selected from the group consisting of aluminum and alloys thereof, said metal having piece having at least one surface adapted to receive a coating; (b) polishing said at least one surface so as to remove any oxidation formed thereupon; and (c) placing a coating on said at least one surface, said coating comprising at least one substance selected from the group consisting of sulfonated polyaniline and oligomers thereof. 35. A process according to claim 34 wherein said at least one substance is an oligomer of sulfonated polyaniline selected from the group consisting of trimers, tetramers, octamers, hexadecamers, and mixtures thereof. 36. A process according to claim 34 wherein said at least one substance additionally comprises a resin. 37. A process according to claim 36 wherein said resin is selected from the group consisting of acrylic and epoxy resins. 38. A process according to claim 34 wherein the degree of sulfonation of said at least one substance is in the range of from about 50% to about 100%. 39. A process according to claim 34 wherein said at least one substance is provided with additional ring substituents. 40. A process according to claim 39 wherein said ring substituents are selected from the group consisting of carbonate groups, alkyl groups, alkoxy groups, and combinations thereof.

Inventors: EPSTEIN ARTHUR J, FAHLMAN MATS A P, EPSTEIN, ARTHUR, J., FAHLMAN, MATS, A., P.
Applicants: UNIV OHIO STATE RES FOUND, THE OHIO STATE UNIVERSITY RESEARCH FOUNDATION
Application year: 1998
Main classification: G11B5
IPC codes: B05D7, C09D5, C09D179, C23C22, C23F11, G11B5