Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Forming nonmetal coating
Reexamination Certificate
1998-01-05
2001-12-11
Gorgos, Kathryn (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Forming nonmetal coating
C205S332000
Reexamination Certificate
active
06328874
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to coatings for oxidizable metals, and more particularly to corrosion-resistant coatings containing intrinsically conductive polymers; to methods for using such coatings to protect oxidizable metals from corrosion; to oxidizable metal surfaces protected from corrosion by such coatings; and to compositions that may be used to form such coatings.
2. Description of Related Art
Coatings for metals are important in a number of applications, one of which is the protection of the metal from corrosion. Corrosion resistant coatings for metals are designed: (1) to form a barrier between the metal and the environment; (2) to provide cathodic protection of the metal, as with a zinc-rich primer; and/or (3) to passivate the metal, as with a heavy metal oxide of, for example, chromium or molybdate. Problems in providing barrier coatings free of pin-holes and environmental concerns about heavy metals have resulted in increased interest in protective coatings that include intrinsically conductive polymers.
Intrinsically conducting polymers (ICP's), are polymers such as polyaniline, polypyrrole and polythiophene that have poly-conjugated &pgr;-electron systems and that conduct electricity in at least one valence state. It is believed that coatings for metals that contain ICP's have conductivity properties that passivate and protect the metal from corrosion; even where the coating is penetrated by pin-holes or scratches. However, limitations in the processability of ICP's along with the brittleness and lack of strength and adherence of films composed only of ICP's has limited their commercial application. See, for example, Deng, et al.,
J. Electrochem. Soc
., 136:2152-2157, 1989; Ren and Barkey,
J. Electrochem. Soc
., 139:1021-1026, 1992; Wessling, Adv. Mater., 6:226-228, 1994; and Lu, et al.,
Synth. Metals
, 71:2163-2166, 1995.
ICP's can be incorporated into corrosion resistant coatings by applying a film that includes a dispersion or solution of the polymer, or by polymerization of an ICP monomer into an ICP in situ onto the surface to be protected, by either chemical or electrochemical means.
A film containing an ICP in polymer form can be applied to a surface in the form of a paint or liquid coating formulation. The carrier solvents can then be evaporated leaving the ICP in the form of a film on the surface to be protected. Use of an ICP in polymer form has the advantage of permitting the production of the ICP under conditions that are optimum for the desired molecular weight and conductivity properties, but such method also requires the use of a solvent and the evaporation of the solvent can cause air pollution problems.
Alternative methods of application of ICP's in polymer form can be electrodeposition (See, e.g., U.S. Pat. Nos. 5,543,084 and 5,556,518), or incorporation of ICP's into formulated coatings, (See, e.g., U.S. Pat. Nos. 5,494,609, 5,290,483, 5,006,278, 5,532,025, JP 5003138 A, JP 6045196 A and JP 6045195 A), or paints, (See, e.g., U.S. Pat. No. 5,441,772 and PCT Publ. No. WO93/14166).
It has been reported that anti-corrosion films containing polyaniline having improved thermal and pH stability against de-doping can be produced by the use of so-called, “double-strand” polyaniline. (See, e.g., Sun, et al.,
Mat. Res. Soc. Symp. Proc
., 328, 1993; Racicot, et al.,
Synth. Metals
, 85:1263-1264, 1997; U.S. Pat. No. 5,489,400 to Liu et al.; Racicot et al., SPIE Reprint:
Optical and photonic applications of electroactive and conducting polymers
, 2528:251-258, 1995; Racicot, et al.,
Mat. Res. Soc. Symp. Proc
., 413:529, 1996, and Racicot et al.,
Corrosion protection comparison of a chromate conversion coating to a novel conductive polymer coating on aluminum alloys
, Paper No. 531, Corrosion 97, NACE International, 1997). Such polyanilines are produced by chemical polymerization of aniline in the presence of a multifunctional polymeric acid (“polyacid”) such as polystyrenesulfonic acid or poly(methylacrylate-co-acrylic acid) to form a complex of the polyaniline and the polymeric acid. It is believed that the tight binding of the large multi-anionic polymeric acid with the amine groups of the polyaniline enhanced the stability of the doped form of the polyaniline to de-doping by high temperatures and basic pH values. The group reported promising use of a polyaniline/poly(methylacrylate-co-acrylic acid) complex as an anti-corrosion coating for aluminum when the complex was dissolved in ethyl acetate and applied to cleaned and polished aluminum alloys AA7075 and AA2024 as a liquid. Upon evaporation of the solvent, the complex formed a film that was reported to have a corrosion current density that was 2 to 3 orders of magnitude lower than a conventional anodized aluminum oxide film. None of these references, however, reported film formation by in situ electrochemical polymerization. In fact, U.S. Pat. No. 5,489,400 contrasted the process of electrochemical polymerization as being quite different from those of the “template guided” chemical polymerization preferred in that invention. It was also stated that the molecular complexes made by chemical polymerization are also quite different from those made by electrochemical polymerization.
Formation of polyaniline by electropolymerization from aqueous solutions that also contained a polyacid has been reported by Hyodo et al., in
Electrochemical Acta
, 36:87-91, 1991, and by Hwang and Yang, in
Synthetic Metals
, 29:E271-E276, 1989. Neither of these references deposited ICP films on aluminum, however, and all electrooxidation was carried out at electrical potentials under 0.75 volts. In fact, Hyodo et al. stated that polyaniline degradation was suppressed by limiting cell potential to 0.75 volts.
In situ chemical polymerization of ICP's into films on metal surfaces has been reported for the construction of electrical batteries and capacitors. Although the art of producing batteries and capacitors is not closely related to the formation of corrosion-resistant coatings, it has been found that such solid state devices can be formed from a layer of ICP's deposited on a valve action metal, such as aluminum or tantalum, with an interlayer of a dielectric, such as the metal oxide. These techniques first require the formation of a metal oxide layer on the surface of the metal and then coat the oxide with the ICP. The ICP can be applied in polymer form as previously described, or it may be polymerized in situ (i.e., polymerized on the surface of the oxide to form an ICP film). In situ polymerization can be carried out by applying to the metal oxide either a monomer polymerizable into an ICP (ICP monomer) followed by contact with a separate solution of a chemical oxidant, (See, e.g., U.S. Pat. No. 5,567,209), or first applying a chemical oxidant followed by a solution containing an ICP monomer, (U.S. Pat. No. 4,780,796). Other methods form a first ICP layer by chemical polymerization and then apply an additional layer of a second ICP by electrochemical polymerization. See, e.g., EP 591035, U.S. Pat. No. 4,780,796, JP 6045200 A and JP 6045199 A. The advantage cited for such two-stage application of the ICP layer is that the insulating properties of the metal oxide layer make it impossible, or, at least, difficult, to deposit the ICP electrochemically, (See, e.g., U.S. Pat. No. 4,780,796).
Ohsawa et al. (U.S. Pat. No. 4,948,685 and U.S. Pat. No. 4,999,263), however, used electrochemical polymerization of polyaniline to form sheet-shaped electrodes and demonstrated the electrochemical polymerization of polyaniline onto nickel, stainless steel and substantially pure, surface-roughened aluminum at electrode potentials of up to 1.2 volts versus a saturated calomel electrode (V/SCE) in aqueous solutions of acids having pK
a
values between −2.5 and +2.5. Sulfuric and p-toluenesulfonic acids were successfully used for the electropolymerization, but no polyaniline film was produced on aluminum when acids having pK
Kinlen Patrick J.
Lawless Lawrence M.
Menon Vinod P.
Alston & Bird LLP
Gorgos Kathryn
McDonnell Douglas Corporation
Nicolas Wesley A.
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