Polyaniline Coating Promises Excellent Corrosion Protection
By Marie Pompili
Finding the right conductive coating for such applications as EMI shielding or protecting metal substrates from corrosion has been quite a challenge for surface finishers for some time. Finally, after more than a decade of experimentation, one solution may hold promise, particularly for the corrosion protection of aluminum.
In 1985, a scientist named David DeBarry studied the corrosion resistance of electrochemically deposited polyaniline in acid solution onto pre-passivated stainless steel. Realizing positive results, DeBarry also conducted tests on bare iron and copper. DeBarry also discovered that a reproducible shift in the corrosion potential of the metal by at least 100-200 mV was possible with a decrease in the specific corrosion current. For stainless steel, the corrosion potential shifts reached as high as 800 mV. According to DeBarry, because the corrosion potential shifts towards the noble region, the surface of the metal becomes many times more resistant to corrosive attack. Corrosion potential shifts were also seen on metal samples that were coated for proper reaction with the polyaniline and then had the coating removed. Additional studies were conducted using these coatings over the years and they became quite popular in Europe. Polymer Alloys LLC, Ardsley-on-Hudson, NY, recently obtained a U.S. patent containing claims covering the use of a particular form of polyaniline, the non-conductive or "emeraldine base form", on aluminum as a corrosion protection agent.1
According to DeBarry, polyaniline "is a conjugated polymer having a specific conductivity of 5 S/cm, making it an organic metal. It can also be blended to have specific conductivities ranging from 10 S/cm to 102 S/cm. The corrosion potential of polyaniline is close to silver, thus making it behave like a noble metal. Coating metals with polyaniline serves to ennoble the surface of the host metal." Polymer Alloys' technology contribution relies upon using the emeraldine base (EB) form of the material that can be easily formed by the oxidative polymerization of aniline in excess hydrochloric acid by ammonium persulfate followed by treatment with ammonium hydroxide. The material will have recurring backbone monomeric units that are substituted or unsubstituted phenyl rings and amine linkages, and substituted or unsubstituted quinoid rings and imine linkages".
Zipperling Kessler & Co. (now Ormecon Chemie), based in Arhrensburg, Germany, has been a pioneer in the use of polyaniline coatings. For example, when electrochemical deposition became difficult, the company discovered a way to manufacture the polyaniline coatings in a processable and dispersable form. Now, the material can be applied through simple coating methods avoiding the pre-passivation of the host metal with the acidic solution common with electrochemical deposition. The Ormecon polyaniline product is marketed under the trademark ORMECON. Their focus appears to be primarily directed to using the conductive form of polyaniline that contains acid dopant molecules. However, unlike the Polymer Alloys technology, it does not rely upon use of the emeraldine form of polyaniline, which is devoid of acid dopant additives that are believed by some investigators to possibly contribute to corrosion in the long term. Still, their tests have been invaluable to the industry.
Materials engineer, David Brumbaugh, reported in a recent issue of the AMPTIAC (Advanced Materials and Process Technology Information Analysis Center) Newsletter that the Zipperling/Ormecon tests where bare metal surfaces were coated with dispersed polyaniline exhibited the same shifts in corrosion potential as DeBarry's. "Furthermore, although no oxide layer was present on the metal before coating, sequential examination showed that a passive oxide layer had developed between the bare metal surface and the polyaniline, proving that pre-passivation of the metal surface was not necessary for the polyaniline coating to be effective." The studies also showed that the corrosion resistance increased by a factor of between 5 and 10, and data showed that the coating system continued to provide corrosion protection even after the surface was severely damaged.
Ormecon's CORREPAIR dispersed polyaniline containing primer system for the repair of rusted surfaces. CORRPASSIV is a dispersed polyaniline that contains primer for the ennoblement and passivation of metal surfaces where a sealing topcoat is applied. They are available in the United States via an agreement with DuPont, which also includes the emeraldine base form used by Polymer Alloys, which is available under the trademark ZYPAN EB.
Another company that manufacturers polyaniline, including the emeraldine base form that Polymer Alloys' technology requires, is Panipol Oy (www.panipol.com), of Parvoo, Finland.
Polymer Alloys' coating is applied by brush or roller; air or airless spray; electrostatic; hot or steam spraying; aerosol spray; dip, flow or electro deposition techniques, roller coating machines or by powder. In addition, heat treatment at temperature from about 120ºC to 300ºC can be used to enhance the adhesion of the coating to the metal as well as to enhance the corrosion protection. When the polyaniline is undoped, a chemical reaction may happen between the coating and the metal, changing the polyaniline to a reduced form. The company used a neat coating of neutral, undoped polyaniline in the emeraldine base form on the metal body to coat a metal body prone to corrosion to determine the effectiveness of the emeraldine polyaniline. These metals can include mold steel, compound metal (or alloy), copper, steel alloys, aluminum and its alloys (the preferred metal species), and other non-noble metals.
Polymer Alloys' polyaniline emeraldine base was dissolved in 9.5 gm of N-methylpyrrolidinone (NMP) by stirring. Any undissolved solid was removed via filtration. Approximately 10 drops of the resulting solution were placed along one end of an aluminum coupon (Aluminum 6061T6 measuring 4- x 4- x 0.32-in. cut and unpolished), provided by Advanced Coatings Technology, Hillside, MI. The polymer solution was spread over the coupon using a #24 wire-wound, 0.25-in. in diameter rod, provided by RDS of Webster, NY. The samples were then placed in a Blue M vented drying oven provided by General Signal, Blue Island, IL, at 150ºC for five to 10 minutes. They were then cooled by immersion in water at room temperature and then air-dried. The coatings were smooth and even, and did not flake when scratched. In addition, when rubbed with solvents, they did not easily come off. The same procedure was used for cold rolled steel with the same results.
Ten experiments were conducted using the polyaniline emeraldine base (PANI) dissolved in NMP and bar-coated on cold rolled steel and aluminum coupons. The samples were dried by various methods, including: "standing overnight at ambient temperature; oven-drying at 82ºC for 2 ½ hours, then cooled to room temperature; or dried for five minutes at 155ºC and then quenched by immersion in a room temperature water bath and then allowed to air dry."2
Adhesion was tested by the tape method, which involves the use of a single-edged razor blade to cut lines through the coatings into the steel substrate. Researchers cut two sets of parallel lines about 1 mm apart to create 1 cm2 boxes in the coating. SCOTCH brand Magic tape was applied with thumbnail pressure over the scribed area and then removed by slow peeling in a direction parallel to the plane of the coating. Interestingly, the coated aluminum samples all showed superior adhesion results (98%-99%) even for those samples subjected to ambient cure, whereas the steel samples that were subjected to ambient cure showed inferior adhesion performance (only 75%-88%).
Spin-coated steel samples, naked steel, and cold-rolled steel were tested further in a salt fog corrosion test and, with and without the polyaniline coating. All tests indicated that the polyaniline EB material is may be a viable coating for use on aluminum. Earlier work by IBM research personnel verified analogous good corrosion performance for the undoped or non-conductive form of polyaniline on copper and silver (Journal of the Electrochemical Society, Vol. 144, No.2, February 1997, pp. 436-442). Polymer Alloys is searching for partners to further develop, under license, their patented technology covering the corrosion protection of aluminum articles with the emeraldine base form of polyaniline.
Contact Information: Polymer Alloys LLC; Licensing and Business Development Office; E-mail: fenquarry@aol.com; Box 286, Shrub Oak, New York 10588; Phone: 914-962-5026; Fax: 914-962-1019.
1United States Patent number 5,928,795, July 27, 1999, Corrosion Resistant Aluminum Article Coated with Emeraldine Base Polyaniline. A full text version can be retrieved after doing a patent number search on the web site of the U.S. Patent and Trademark Office: www.uspto.gov/patft/index.html.
2 Ibid.