US20050201890A1

US20050201890A1 - Composition for the controlled release of inhibitors for corrosion, biofouling, and scaling

Info

Publication number: US20050201890A1
Application number: US11/027,185
Authority: US
Inventors: Martin Kendig; Melitta Hon; Leslie Warren
Original assignee: Boeing Co
Current assignee: Boeing Co
Priority date: 2002-07-08
Filing date: 2004-12-30
Publication date: 2005-09-15
Also published as: US20040005478A1; EP1382721B1; EP1382721A1; US6942899B2

Abstract

A polymer composite structure, wherein the composition releases an anionic dopant upon application of an electrochemical potential, such as when in contact with a metallic substrate in a corrosive environment. The composite actively inhibits corrosion at the point of contact of the composite with a metal substrate by release of “active” or “smart” corrosion inhibitors which migrate to the corrosion area. Composites having anionic dopants having biocidal or scale-inhibiting properties may be used to inhibit biofouling and scaling wherein the dopants are released upon application of a electrochemical potential.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part application and claims the benefit of U.S. patent application Ser. No. 10/190,932, filed Jul. 8, 2002, which is hereby incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to coatings and composites that inhibit biofouling, scaling, and corrosion, and more particularly relates to coatings and composites that inhibit biofouling, scaling and corrosion when in galvanic contact therewith as well as when coated thereon.

BACKGROUND OF THE INVENTION

In many applications, a metal structure or substrate may become corroded or oxidized when exposed to a particular atmosphere. Precautions can be taken to prevent such corrosion, but generally it occurs if a particular metal is exposed to a particular environment for an extended period of time. Many applications exist, such as automotive bodies and frames and aerospace application, where metal structures of vehicles are constantly exposed to extremely corrosive atmospheric conditions.
One mechanism of metallic corrosion is a galvanic reaction between the metal and the environment surrounding the metal. For example, oxygen in the atmosphere oxidizes the metal through a transfer of electrons from the metal to the oxygen at electrocatalytic sites on the metal surface and subsequent combination of the resulting metal cation with the oxygen anion to form a non-structural metal oxide corrosion product. In particular, water vapor acts as the electrolyte allowing oxygen to react with the metal. It has been proposed that conducting polymers can provide a pacification of this reaction by creating an anodic pacification. A coating is placed over the metallic substrate such that no reaction may occur unless there is an imperfection in the coating. When an imperfection occurs and the electrode reaction begins the conducting polymer acts as a cathode so as to supply a low but sufficient current to form a protective metal oxide film on the surface of the metal within the exposed defect.
Other films or coatings have been used which include hexavalent chromium, also referred to as chromate. In these coatings, the chromate acts as an inhibitor since it is water-soluble and reacts with the metal to form a barrier layer composed of the metal oxide and Cr₂O₃. In this reaction the metal is oxidized (looses electrons) and the chromate is reduced (gains electrons to go from the six valent chromate to the three valent Cr₂O₃). Preferably, the coating retains a portion of the chromate that can be released to protect the metal at imperfections in the film.
In this way, when an imperfection occurs and the electrode reaction is initiated, the chromate remaining in the film may migrate through the film and block the corrosion reaction with the atmosphere. In particular, the chromate moves into the imperfection of the coating to react with the metallic substrate thereby forming a protective layer. Nevertheless, a slow release of the chromate over time can reduce its availability to be released at the appropriate time. Although chromate is useful in this application, chromate may be toxic if ingested in sufficient amounts in a living organism. Therefore, strict and costly standards must be adhered to when using and disposing of the materials coated with the chromate.
Therefore, it is desirable to provide a coating which provides the blocking effects of chromate, but which is not substantially harmful to living organisms. In addition, it is desirable to provide a composition which produces an active or “smart” inhibition of the galvanic reaction created between a metallic substrate and the atmosphere even when that composition is not coated upon the particular section of metal undergoing the galvanic reaction. It is desired to produce a composition which provides substantial inhibition to corrosion of a metallic substrate by the release of a blocking or inhibiting constituent into the defect to stop the corrosion of the metallic substrate.

SUMMARY OF THE INVENTION

The present invention is a corrosion inhibiting composition comprising an electrically conductive polymer doped with an anionic dopant that may be placed on or in contact with a metallic substrate to inhibit or help prevent corrosion of the substrate when in a corrosive environment or prevent scaling and biofouling upon and after application of an electrochemical potential.
The inhibiting composition may be provided as a coating of the doped polymer or as a composite material that uses the doped polymer as a resin matrix. The inhibiting composition may also advantageously inhibit or help prevent biofouling or scaling of a metallic substrate when in contact with that substrate. The invention also encompasses metallic substrates coated with or in galvanic contact with the corrosion-inhibiting composition, the method of coating metallic substrates with the composition, and the method of forming the corrosion inhibiting composite structures. When applied as a coating, the coating not only passively inhibits corrosion where the coating is undamaged, but also actively inhibits corrosion at the site of a defect in the coating.
When applied as a coating, the coatings of the present invention include dopants which act as “active” or “smart” corrosion inhibitors which may migrate to the defect to prevent corrosion in that area. The dopants are referred to as “active” or “smart” because the migration occurs after the defect has occurred in the coating and an oxidation/reduction reaction has begun. The reaction generally creates a galvanic reaction of the metallic substrate. In particular, the metal substrate becomes oxidized due to the oxidizing atmosphere. Once this reaction has begun, the active inhibitors, generally anions of various formulas, for example 2,5 dimercapto-1,3,4 thiadiazole anion, 1-pyrrolidine carbodithioic acid anion, and dialkyl dithiocarbamate, migrate to the reaction site and become participants in the reaction.
The invented composition may be present in the form of a composite material in galvanic connection with a metal substrate. Upon oxidation of the metal substrate, the resulting galvanic action on the composite serves to release the active inhibitors of the composition to migrate to the surface of the metal substrate undergoing oxidation. In the case of oxidation, inhibiting ions block the oxygen reduction upon the surface of the metal substrate so as to slow the corrosion reaction. The anions of the coating are released when the coating is galvanically coupled to the metal defect. Subsequently the anions block the oxygen reduction in the defect so as to slow the corrosion reaction.
In addition to corrosion inhibition, the composition provides a marker or indication that such corrosion has begun to occur. Generally, the color of the electrochemically reduced composition adjacent to the defect is different than the surrounding composition. This distinguishes the corrosion or defect area from the other areas of the composition. Therefore, the owner of the object is made aware that corrosion has occurred and can be provided with an opportunity to address the situation so that further damage to the metallic substrate can be avoided.
A first embodiment of the invention is a coated metal substrate that includes a corrosion inhibition film for a metallic substrate that inhibits oxidation on the substrate when a portion of the metallic substrate is exposed through the film. The film comprises a conducting polymer and a dopant. The dopant includes an anion that is a basic anion of an organic or inorganic acid that may associate with the polymer. Furthermore, the dopant disassociates from the polymer when the metallic substrate begins to oxidize.
A second embodiment of the invention includes a film to inhibit an oxidation of a metallic substrate at a region of the metallic substrate not substantially covered by the film. The film substantially coats the metallic substrate. The film includes a conductive polymer and an anionic dopant. The anionic dopant may associate with the conducting polymer. The anionic dopant is also releasable from the film when the film becomes reduced as a result of being galvanically coupled in the presence of a corrosive electrolyte to the metal existing at the base of a defect in the coating.
A third embodiment of the invention includes a method of coating the corrosion-inhibiting film upon a metallic substrate.
A fourth embodiment of the invention includes a system and method for inhibiting corrosion of a metallic substrate with a coating when an imperfection occurs in the coating. First, a metallic substrate is coated with a film that includes a releasable inhibiting anion. The inhibiting anion includes a basic anion of an organic or inorganic acid. Next, the film is reduced via electron uptake from a galvanic reaction between the metallic substrate and the coating acting as an oxidizer surrounding the metallic substrate where a portion of the metallic substrate is not coated. Then, inhibiting anions are released from the film when the film is reduced because it is galvanically connected to the metal through a corrosive electrolytic environment.
A fifth embodiment of the invention includes composite structures and a method of forming composite structures that inhibit corrosion of metallic surfaces or components when placed in contact with the metallic components. The resin matrix of the composite structure includes a conductive polymer and an anionic dopant. The anionic dopant may associate with the conducting polymer. The anionic dopant is also releasable from the polymer matrix portion of the composite when the polymer becomes reduced as a result of being galvanically coupled to metal in the presence of a corrosive electrolyte.
A sixth embodiment of the invention includes composite structures and a method of forming composite structures that inhibit biofouling or scaling of surfaces or components. The resin matrix of the composite structure includes a conductive polymer and an anionic dopant having biocidal or anti-scaling properties. The anionic dopant may associate with the conducting polymer. The anionic dopant is also releasable from the polymer matrix portion of the composite when the polymer becomes reduced as a result of an electrochemical potential being applied to the polymer. Thus, biocides or anti-scaling dopants may be controllably released from the composite by selective application of the electrochemical potential.
A seventh embodiment of the invention includes a corrosion resistant metal article comprising a metallic substrate in galvanic contact with the inhibiting composite structure for inhibiting corrosion of the metallic substrate under corrosive conditions. The metallic substrate may comprise practically any metal article in a corrosive environment, and is advantageously selected from a metallic component of a cooling tower, radiator cap, aircraft, watercraft, or pipeline.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:
FIG. 1 is a cross-sectional view of a substrate including a defect and a film according to an embodiment of the invention;
FIG. 2 is a molecular formulae for two anions according to embodiments of the invention;
FIG. 3 is a molecular formula of a conductive polymer according to an embodiment of the invention; and
FIG. 4 is a graph of the current densities of a solution including anions according to various embodiments of the invention.
FIG. 5 shows the structure of NH₄PYRR in accordance with an embodiment of the invention;
FIG. 6 shows the structure of 2,5 dimercapto-1,3,4 thiadiazole dipotassium salt (DMTD) in accordance with an embodiment of the invention;
FIG. 7 illustrates the experimental apparatus used in Example 10;
FIG. 8 illustrates a decrease in the cathodic current density for short diffusion lengths (high values of δ⁻¹) in 5% NaCl solution containing 10 mM of NH₄PYRR according to an embodiment of the invention;
FIG. 9 illustrates a decrease in the cathodic current density for short diffusion lengths in 5% NaCl solution containing 2,5 dimercapto-1,3,4 thiadiazole dipotassium salt (DMTD) according to an embodiment of the invention;
FIG. 10 shows UV-visible spectra for various concentrations of NH₄PYRR in water;
FIG. 11 shows UV-visible spectra of 0.5 M NaCl containing different concentrations of the DMTD;
FIG. 12 shows UV-visible spectra of 0.5 M hydrazine containing different concentrations of the DMTD;
FIG. 13 shows UV-visible spectra of supernatant solutions after long term equilibration with the doped Ligno-PANI;
FIG. 14 shows residual DMDT release from PANI doped with DMDT;
FIG. 15 shows a UV-visible spectra analysis of solid DMDT-doped and multiply rinsed PANI equilibrated with deionized water, 0.5 M NaCl, and 0.5 M NaCl+0.5 M hydrazine, respectively;
FIG. 16 shows a UV-visible spectra analysis of PANI doped in 1M sulfuric acid, then 0.5M DMDT, equilibrated with 0.0001 M NaCl, 0.001 M NaCl, 0.01 M NaCl, 0.1 M NaCl, and 0.5 hydrazine, respectively;
FIG. 17 shows the release of an ORR inhibitor from NH₄PYRR doped Ligno-PANI in a NaCl solution;
FIG. 18 shows potentiostatic currents for samples of Al 2024-T3 coated with PANDA doped with NH₄PYRR as evaluated with the experimental setup of FIG. 7;
FIG. 19 shows a sample of Al 2024-T3 coated with PANDA and doped locally for 24 h with 0.02 M NH₄PYRR was scribed and exposed to a 48 h salt fog environment;
FIG. 20 presents a viable model for ‘smart’ corrosion-inhibiting coatings based on conducting polyaniline films in accordance with the invention;
FIG. 21 shows a sample of Al 2024-T3 coated with oxidized PANI (emeraldine base) with the dark area indicating oxidation of the metal; and,
FIG. 22 shows a sample of Al 2024-T3 coated with oxidized PANI (emeraldine base), subsequently treated with an aqueous solution of ammonium 1-pyrrolidine dithiocarbamate, and subjected to a 168 hour salt fog test.

DETAILED DESCRIPTION OF THE INVENTION

The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout.
A corrosion inhibition system 10 is illustrated in FIG. 1. The corrosion inhibiting system 10 generally includes a composition 11 placed on a substrate 12. The substrate 12 is generally a metallic substrate formed of a unitary piece of metal or several portions affixed together. Also, the particular alloy is not of particular relevance, except that the alloy is generally one that may be oxidized. Moreover, the metallic substrate may also include a pure metal. An exemplary metallic substrate 12 may be an aluminum alloy including copper.
The composition 11 is embodied in either a polymer film applied as a coating upon the surface of the metal substrate or a composite material comprised of polymer 11 a impregnated fibers or fabric 11 b in galvanic contact with the metal substrate as shown in FIG. 1. For convenience, the inhibiting composition is generally referred to as item 11, the conducting polymer is generally referred to as item 11 a, whether used alone or as the resin matrix of a composite, and a corresponding fiber or fabric layer is generally referred to as item 11 b if the inhibiting composition is a composite.
The coating or composite material in contact with the metal includes a conducting polymer such that a current or electron transfer may occur through the coating or composite. One mechanism for oxidation of the metallic substrate is the Oxygen Reduction Reaction (ORR) which is the reduction of oxygen from the atmosphere to produce negative hydroxyl ions. ORR is illustrated in the chemical equation ½ O₂+H₂O+2e⁻→2OH⁻. When ORR occurs, electrons transfer from a metal atom of the metallic substrate 12 that is oxidized to produce a metal ion 22. The ORR can occur both on the exposed surface of the metal 20, for example at surfaces of microscopic secondary phases in the alloy, or on the surface of the conducting polymer 16. The two electrons required by the reaction of ½ of one molecule of oxygen by the ORR come from oxidation of metal to a metal cation. The oxidized metal ion 22 is generally not strongly adhered to the metallic substrate 12 and easily comes away from the metallic substrate 12. When this occurs, corrosion continues in the metallic substrate 12. This reaction induces the metallic substrate 12 into an anodic activity.
The inhibiting composition 11 is preferably conductive so that some of the electrons being driven away from the metallic substrate 12 transfer into the molecular structure of the composition 11 producing a non-conductive product 24 near the substrate oxidation site 20. This increases the electronic resistance between the metallic substrate 12, which is acting as the anode, and the oxygen from the atmosphere at the polymer in or on the composition 11, which is acting as the cathode.
In addition, the conducting polymer 11 a of the composition may be doped or synthesized to include a bound active inhibitor or inhibiting anion (BA) 26 in a form that is first bound in the polymer. Although only two BAs 26 are illustrated, it will be understood that a plurality may be dispersed throughout the composition 11. When the non-conducting product 24 is produced, the polymer in contact with the metallic site undergoing oxidation 18 becomes more basic. Without being bound by the theory, the reduction of the polymer 11 a allows the release of BA 26 which forms a released active inhibitor or anion (RA) 28, from the BA 26. Moreover, as the polymer 11 a is reduced the faster or greater the amount of the RA 28 is produced upon the release of the BA 26. In particular, the BA 26 may be released from the polymer 11 a, thereby forming the RA 28, such that the RA 28 enters the oxidation site 20 to further inhibit the corrosion or oxidation of the metallic substrate 12. In this way, the RA 28, which was doped into the polymer 11 a as BA 26, helps slow the oxidation or corrosion of the metallic substrate 12 when the metallic substrate 12 is exposed to a corrosive environment.
A further exemplary mechanism for the release of the BA 26 may include deprotonation or deacidifying of the polymer 11 a. Not only does the production of the non-conducting product 24 release the BA 26, but the hydroxyl group OH⁻, from the ORR occurring on or in the coating, may help make basic the polymer 11 a which further helps release the BA 26. Generally, the hydroxyl group OH⁻ from the ORR attracts a proton H⁺ from the polymer 11 a such that the BA 26 is released from the polymer 11 a. In this way, the BA 26 may also be released through the second mechanism to help inhibit the corrosion of the metallic substrate 12.
An electrically conductive polymer is desired to produce the composition 11 to help release the BA 26 under the appropriate conditions. Essentially, the conductive polymer allows the composition 11 to react to a transfer of electrons in a unique way. A non-conductive coating may not allow for such an easy transfer of the BA 26 to the metallic defect 20 to inhibit the corrosion thereof.
Any number of inorganic or organic acids (HA) may be used as a reactant to produce the BA 26. Provided that the HA is strong enough to protonate the polymer 11 a, thus releasing its anion which becomes dispersed and intermingled or reacted yet fixed in the polymer 11 a as a “dopant” anion or the BA 26 when the polymer 11 a exists in its oxidized and conducting form. Moreover, the BA 26 must be able to migrate through the polymer 11 a so that it migrates to the area of the metal substrate being oxidized to inhibit further corrosion. This type of interaction is found with a conducting polyaniline system.
Although the inhibiting compositions are generally described herein in the context of corrosion inhibition, it has been found that many of the particular dopants described herein also effectively inhibit biofouling and scaling of metal components upon exposure to environments otherwise conducive to biofouling or scaling. The invention encompasses methods of inhibiting biofouling and scaling by use of the inhibiting composition described herein.
The act of incorporating inhibiting anions (BAs 26) into a polymer is referred to herein as “doping” and is assumed to be part of an ion exchange process. There may be other reactions that serve to fix the dopant in the polymer so that it is released upon the reduction of the polymer at a later time. Regardless of the actual mechanism, the methods of formulating the inhibiting composition herein are valid.
Although many appropriate BAs 26 may be used, exemplary BAs 26 include mono and dithiols which are derived from mono and dithiol organic acids (HA). The BA 26 is the anion of the HA which is the dopant in the polymer 11 a. Thiols have been found to be effective when aluminum alloys are being protected. This is especially the case when the aluminum alloys include Cu-rich secondary phases. Monothiols have the general formulation RSH, where R is an organic radical and may disassociate into RS⁻H⁺, where RS⁻ is the BA 26. One example of an appropriate monothiol includes 2-mercaptothiazoline, which produces the BA 26 a, illustrated in FIG. 3. Dithiols have the general formulation HSRSH, where R is any organic radical. Dithiols may disassociate into HSRS⁻H⁺, where HSRS⁻ is the BA 26. An example of a dithiol includes 2,5-dimercapto-1,3,4-thiadiazole which produces the BA 26 b. Other examples of acids which produce appropriate BAs 26 include: 6-ethoxy-2-mercaptobenzothiazole, 1,3,4 thiadiazole, 6-ethoxy-2-mercaptobenzothiazole, dimethyldithiocarbamic acid, o-ethylxanthicacid, 2-mercaptobenzothiazole, 2-mercaptoethanesulfonic acid, diethyldithiocarbamic acid, 5-amino-1,3,4,-thiadiazole-2-thiol, 2-mercaptobenzoxazole, 2,1,3-benzothiazole, 1-pyrrolidinecarbodithioic acid, 1-(4-hydroxyphenyl)-1H-tetrazol-5-thiol, 2 mercapto-5-nitrobenzimidazole, benzothiazole, 2-mercaptobenzoazole, and 2-mercapto-5-methylbenzimidazole. In each of these examples the acid is deprotonated to produce a proton and an anion, where the anion is the BA 26.
Composition 11 is preferably made of a conducting polymer 11 a or a polymer resin/conducting polymer blend that is inherently conductive. Although many appropriate polymers or polymer blends may be used to form the composition 11, one example is a polyaniline having a formula illustrated at 30 in FIG. 3. This polyaniline may either be synthesized from aniline monomers or may be purchased from a supplier such as Aldrich. In any case, the polyaniline, depending on its oxidation state or degree of protonation can then be reacted with the appropriate HA or BA 26 base, as described further herein.
The BA 26 may be introduced into polymer 11 a using a means suitable to provide the interaction of the HA with the conducting polymer of the composition 11. Generally, the conducting polymer in the oxidized and unprotonated form may accept a proton from the HA, such that the polymer becomes positively charged and the BA 26, derived from the HA, becomes associated with the polymer 11 a that is now protonated. This allows for an association of the BA 26 at specific locations along the polymer chain of polymer 11 a, but also allows the BA 26 to be released when the composition 11 is treated with base or is reduced electrochemically.
Appropriate methods for doping the polymer 11 a include synthesizing the coating to include the BA 26. The oxidized protonated form of the polymer 11 a has the BA 26 disbursed throughout the polymer 11 a before the composition 11 is placed in contact with the metallic substrate 12. This allows for an even and substantially thorough distribution of the BA 26 in the composition 11. Another exemplary method includes saturating an area of the oxidized unprotonated form of polymer 11 a with the HA such that polymer 11 a becomes doped. This process occurs after the composition 11 has placed in contact with the metallic substrate 12.
Another alternative for doping polymer 11 a or otherwise fixing a releasable inhibitor in or on the coating includes the following: An oxidized polymer 11 a may be contacted with the metallic substrate 12 including an original or coating anion. A salt of the BA 26 may be placed in contact with the polymer 11 a, as described herein, or using other appropriate methods. The original anion is then exchanged with the BA 26 from the salt solution. More particularly the composition 11, in this instance, is a cationic film that includes the original anion as a counter anion. The BA 26 then replaces the original anion to become the counter anion in the film 11.
The inhibiting composition is advantageously provided in the form of a composite material. The term “composite material,” generally refers to a fiber-reinforced material disposed in, or impregnated with, a resin matrix material. In the context if the invention, the resin matrix material can be any of a number of electrically conductive thermoplastic or thermoset polymeric resins, and is preferably a polyaniline-based polymer as described herein. Composite materials are often used as structural materials in the aerospace industry and are often used in physical contact with metallic components or substrates. Thus, the inhibiting composition, though incorporated within a composite material, provides corrosion inhibition to metallic materials in galvanic contact with the composite.
The electrically conductive polymer may be mixed with compatible polymers during production of the composite. The polymer may also be mixed with fillers and other additives through known methods, such as via melt compounding. Fillers and other additives may be incorporated to increase the strength, stiffness, UV-resistance, or other physical properties of the composite material.
The reinforcement material of the composite, which can be provided as fibrous pieces or strands, tows, woven or nonwoven mats, and the like, can be any of a variety of fibrous materials such as glass, metal, minerals, conductive or nonconductive graphite or carbon, nylon, aramids such as Kevlar®, a registered trademark of E. I. du Pont de Nemours and Company, and the like. The resin matrix and reinforcement material can be selected according to the desired mechanical, physical, chemical, thermal, and electrical requirements of any particular application. For example, some fiber additives provide additional strength, while others provide enhanced electromagnetic and radio frequency shielding.
Regardless of the method used to apply the BA 26 to the composition 11 once the composition 11 has been doped with the BA 26, it can be used to inhibit corrosion on the metallic substrate 12. The general method of the inhibition has been described above. The following examples provide specific examples of particular BA 26 dopants which are appropriate with a particular conductive coating placed on an aluminum alloy substrate. It will be understood the other substrates may be placed in contact with coatings or composite materials including the inhibitors described herein to carry out the present invention.

EXAMPLES

Example 1

Application of PANI Polymer

10 to 20 gm of polyaniline emeraldine base, the oxidized unprotonated form of polyaniline (PANI), which may be obtained from Aldrich, is dissolved in 100 ml of N-methylpyrrolidone so as to make a thick paint-like suspension. A No. 13 Meyer bar is used to draw a portion of the solution into a film covering a surface of an Al 2024-T3 aluminum alloy test panel, which may be prepared as described herein, and allowed to dry and cure to form the solid film of PANI. The PANI film includes a polymeric structure that allows for the transfer of electrons through the film once the film is protonated. With reference to FIG. 3, the PANI film may have a general structure that is reduced and unprotonated (that is also non-conducting) represented by 30. The reduced and unprotonated PANI film 30 also has, and is generally received in, an oxidized and unprotonated or basic form (emeraldine base, which is non-conducting) PANI 32, wherein at least one of the nitrogens of the reduced PANI 30 has lost a previously bound proton and has been oxidized to form the N═C bond. This forms a site where a proton and the BA 26 may associate with the nitrogen, through reaction 34, to produce the oxidized and protonated PANI 36 which has been doped with the BA 26. This reaction may proceed by the reaction of HA with the emeraldine base oxidized and deprotonated PANI 32 or by first protonation of the emeraldine base oxidized and deprotonated PANI 32 with a strong acid (including an acid ion) followed by ion exchange of the acid ion with the BA 26. For the oxidized PANI structure 32 a number of the monomer units of the polymer chain can accept one molecule of the singly-charged BA 26. Therefore, the density of the BA 26 can be fairly high in the composition 11. The reaction 34 shows how the appropriate BA 26 interacts with the oxidized PANI 32 to form the doped PANI 36.

Example 2

Application of PANI Polymer from Solution

A three inch by three inch (76.2 mm×76.2 mm) test panel of aluminum alloy Al2024-T3 was first degreased with acetone and deoxidized in Sanchem 1000®, available from Sanchem, Inc., Chicago, Ill., at about 37° C. for about 15 minutes folloed by a de-ionized water rinse. The panel was then dried and coated (FIG. 21) with oxidized PANI (emeraldine base) dissolved in a 80:20 formic acid:dichloroacetic acid solution that is spray coated. The acidic solution chemically anodizes the alloy as evidenced by its transient color change during drying. The excess acids volatilize. The color returns to the dark oxidized form as a result of air reoxidation. Hence, the resulting film remains reactive. Subsequent treatment of the resulting surface with an aqueous solution of a dithiocarbamate, here ammonium 1-pyrrolidine dithiocarbamate fixes, by oxidative polymerization, an insoluble disulfide linked polymer of the dithiocarbamate on the surface and within the film. In a corrosive environment the disulfide polymer depolymerizes to give the dithiocarbamate oxygen reduction reaction (ORR) inhibitor that renders the intermetallic phases on the metal surface inactive for the oxygen reduction half of the corrosion reaction. Conversion coatings formed by this process on Al 2024-T3 give an apparent salt fog life (FIG. 22) of about 168 hours.

Example 3

Doping of the PANI Polymer

A three inch by three inch (3″×3″) (76.2 mm.×76.2 mm) test panel of aluminum alloy Al2024-T3⁻ is first degreased with acetone and deoxidized in Sanchem 1000® at about 37° C. for about 15 minutes followed by a de-ionized water rinse. The panel is then dried and coated with oxidized PANI 32 as prepared and described above. The coating of the oxidized PANI 32 is allowed to air dry and cure at room temperature. The PANI coating is then doped with 2,5 dimercapto-1,3,4,-thiadiazole (2,5 dopant).
The 2,5 dopant is provided at a concentration of about 0.02M. The 2,5 dopant is placed in a sealed and gasketed cell such that approximately 8 cm²of the test panel is exposed to the 2,5 dopant. The 2,5 dopant is expected to reduce the oxidized PANI 32 and dope the coating via reaction 34. The cell is affixed to the test panel for approximately 24 hours. After the 24 hour exposure period, the cell is removed and the panel rinsed and dried. After the doping procedure, a change in hue of the area doped is visible.

Example 4

Corrosion Inhibition Test of Coated Substrate

The test panel is then scribed, such that a mark is made in the coating which passes through the coating and creates a defect in the aluminum substrate, such that bare metal is exposed through the coating. After scribing the test panel such that the scribe intersects both the doped and undoped area, the test panel was exposed to a salt fog. The salt fog met the standards of ASTM B117 for testing.
The test panel is exposed to the salt fog for at least 130 hours. At various times throughout the testing phase the test panel is observed for corrosion. At no time during the test phase, nor after the test phase, is a large quantity of corrosion product noted in the doped area. While outside of the doped area, extensive corrosion product is found in the scribe. Therefore, the doped region of the coating substantially decreases or inhibits any harmful corrosion of the metallic substrate.

Example 5

Doping of the PANI Polymer

A test panel of aluminum alloy Al2024-T3 of approximately 3″×3″ is substantially prepared as described in Example 2. After the test panel is prepared properly, a coating of CorrPassiv 900226119® by Zipperling is applied to the test panel and allowed to cure at room temperature. After the coating cures at room temperature for about 24 hours, a cell having an open surface area of approximately 8 cm²is affixed to a portion of the test panel. The cell includes about 0.02M concentration of a 2-mercaptothiazoline dopant. The dopant is exposed to the test panel for approximately 24 hours. After the dopant is applied, the doped area includes a visible color change from the undoped area.

Example 6

Corrosion Inhibition of Coated Substrate

After the test panel is doped, a scribe is placed upon the test panel that intersects both the doped and undoped areas. After the test panel is scribed it is placed in the salt fog as described above. The test panel is again exposed to the salt fog for at least about 130 hours. During the test process, the test panel is observed at several time intervals. Substantially no corrosive product is found in the scribe in the doped area either during or at the end of the test process. Nevertheless, found in the scribe in the undoped area is a significant amount of corrosive product. Therefore, the area of doped coating is a significant inhibitor to corrosive activity in the salt fog.

Example 7

Cathodic Current in the Presence of Anion Dopant

Cathodic currents of the ORR, flowing to a rotating copper electrode polarized to −0.7 V vs a saturated calomel electrode (SCE) were determined as a function of the rotation rate in 5% sodium chloride in the presence of both the anions from Examples 3 and 5 at 0.01 M concentration and in their absence. The following results are exemplary of the dopants described in Examples 3 and 5 above, they are not meant to limit the scope of the present invention in any way. These cathodic currents are proportional to the rate of the ORR. The results of these galvanic measurements are shown graphically in FIG. 4. FIG. 4 illustrates the current densities plotted as a function of the inverse diffusion length, δ⁻¹. δ=1.75 ω^−1/2v^1/6D^1/2, where ω is the rotation rates of the Cu rotating disk electrode (RDE), D=2×10⁻⁵cm²/s, and v=1 cP. The results of both are compared to the results in the presence of chromate, which is generally known. The current density dramatically increases over the range of inverse diffusion lengths observed of the rotating copper electrode if there is no inhibitor. The anionic dopants, exemplary of the current invention, significantly decrease or almost eliminate current density over the observed range of inverse diffusion lengths. Moreover, the inhibition due to the RA 28 is substantially similar or better than that of the currently known chromate inhibitor. Therefore, the use of the RA 28 substantially reduces the current density for the ORR that drives the oxidation of a metal or alloy, when exposed to a corrosive environment.
Without being bound by the theory as described above, it is believed that the composition 11 does not anodically passivate the metallic substrate 12, but rather blocks the oxygen reduction half of the corrosion reaction responsible for transforming the metallic substrate 12 into some form of the oxidized ion 22, be it an oxide, hydroxide or aquated metal ion complex. As the galvanic reaction occurs, the composition 11 is reduced to produce the non-conductive product 24, which occurs substantially near the substrate defect 20 in the metallic substrate 12. This increases the electronic resistance between the site of metal ion formation and ORR, thereby decreasing the corrosive reaction rate of the metallic substrate 12.
In addition, when the composition 11 is reduced and becomes non-conductive, it releases the dopant BA 26 to form RA 28 which moves into the defect where it inhibits the ORR occurring at cathodic sites which exist within the metallic surface of the defect 20. Furthermore, the ORR generates a basic byproduct, which can further deacidify the composition 11. This in turn creates an additional release of the RA 28 near the substrate defect 20. Both of these actions cause the release of the BA 26 near the coating defect 18. As the RA 28 enters the coating defect 18, it further slows the reduction of oxygen that drives the anodic dissolution at 20.
Thus, the present invention allows for an “intelligent” release of the BA 26, which is the corrosion inhibitor, into a substrate defect 20 only after the substrate defect 20 occurs and a corrosive environment is present. Rather than having a time release or steady release of the inhibiting product, the BA 26 of the present invention, when placed in the composition 11, is released only when a galvanic reaction occurs near the coating defect 18. Substantially only when the coating defect 18 occurs and a corrosive environment is present is there a substantial possibility of the metallic substrate 12 becoming corroded. Thus, the composition 11 of the present invention does not lose its corrosion inhibiting properties over time, but rather retains its inhibiting properties until they are needed. The BA 26 is generally needed or released when the coating defect 18 and the substrate defect 20 are produced.

Example 8

Alternative Doping of PANI

A commercially available Ligno-PANI™ (Lignin sulfonate-doped polyaniline) (13) obtained from GeoTech Chemical Company, LLC (Tallmadge, Ohio) is doped by adding the insoluble solid to a 0.5 M solution of ammonium pyrrolidinedithiocarbamate (NH₄PYRR) and allowing reaction to take place for several hours. The solid is removed from the filtrate and rinsed repeatedly with deionized water. The NH₄PYRR whose structure appears in FIG. 5 is used as-received from Aldrich without further purification.

Example 9

Another Alternative Doping of PANI

An oxidized, protonated PANI, doped with proprietary organic sulfonate is obtained from Aldrich and used without further purification. A 100 gm portion of the solid is equilibrated with 1 M sulfuric acid. The resulting solid was separated by filtration. The residue is rinsed several times with deionized water. Then 2.5 g of the resulting PANI sulfate is equilibrated with 0.5 M 2,5 dimercapto 1,3,4 thiadiazole dipotassium salt (DMTD) (the structure which appears in FIG. 6). The resulting solid is separated via filtration and then washed 7 times with several 250 mL portions of deionized water. Each portion of the wash is saved for subsequent UV-visible spectroscopic analysis using a 1 cm quartz cell and a Cary 5 spectrophotometer. A 0.3 g portion of the resulting DMTD-doped PANI is equilibrated overnight with 6 mL each of deionized water, deionized water containing various concentrations of NaCl, deionized water containing NaCl plus 0.5 M hydrazine, and a 0.5 M solution of hydrazine containing no NaCl. A sample of the DMTD-doped PANI equilibrated with the hydrazine-containing solutions produces bubbles and the solid PANI turns green. In all cases the solid is separated from the supernatant and the supernatant is subject to spectroscopic analysis to determine release of the dopant.

Example 10

Cathodic Current in the Presence of Anion Dopant

The UV-cured PANI or PANDA™ (14) provided by Crosslink Polymer Research (Fenton, Mo.) is also doped by allowing the coating to soak overnight in 0.5 M NH₄PYRR. The subsequent release of an ORR inhibitor by the doped PANDA is determined using a Cu RDE cathode (rotated at 2000 rpm and biased at −0.7 V vs SCE) placed within a calibrated distance from the coated surface in aerated 5% NaCl. The decrease in the cathodic ORR current at the rotating cathode indicates release of an inhibiting species. A schematic of the experimental apparatus appears in FIG. 7.

Example 11

Demonstrated Anticorrosive Effect of Inhibitor Anions

A 5% NaCl solution containing 10 mM of NH₄PYRR exhibits inhibition of the ORR as shown by the decrease in the cathodic current density for short diffusion lengths (high values of δ−1) as shown in FIG. 9. Whereas the blank 5% NaCl gives a current density of over 650 μA/cm²at δ⁻¹=2000 cm⁻¹, the corresponding value of the current density for the ORR in the presence of 10 mM of the inhibitor is more than an order of magnitude less, giving a corresponding current density of about 20 μA/cm². This demonstrates the effectiveness of the NH₄PYRR to inhibit the diffusion limited ORR. Similar results are obtained when the 5% NaCl contained a portion of the DMTD as shown in FIG. 9. The DMTD decreases the current density of the oxygen reduction reaction by a slightly greater extent. Clearly both of these inhibitors, the NH₄PYRR and the DMTD, exhibit excellent inhibition of the ORR in NaCl environments.
Were these materials to be held by a coating that would release them in the presence of corrosive conditions, the coating would be considered a ‘smart’ active corrosion inhibitor. Corrosive conditions as seen by a coating would include: the presence of water, chloride, and a reducing potential due to galvanic coupling of the coating to the aluminum alloy substrate at a defect.
Increases in absorbance (decrease in % transmittance) in the 230 to 450 nm region of the UV-visible spectra result when water or NaCl solutions take up concentrations of these PYRR and DMTD inhibiting anions. For example, UV-visible spectra for various concentrations of the NH₄PYRR in water appear in FIG. 10. FIG. 10 shows a decrease in % transmittance in the UV region as the concentration of the NH₄PYRR increases. The compound has a very strong absorbance at 280 nm with a weaker band at 340 nm (FIG. 10). UV-visible spectra of 0.5 M NaCl in deionized water containing different concentrations of the DMTD appear in FIG. 12. Similarly, significant absorbance (decrease in the % transmittance) occurs in the 260 to 400 nm region (max. absorbance around 317 nm) of the spectrum due to the presence of DMTD. The spectra appear to be similar regardless of whether they are taken in the presence of oxygenated deionized water or 0.5 M hydrazine in deoxygenated water (see FIG. 11 and FIG. 12).
Spectra obtained from supernatant solutions after long term equilibration (overnight) with the doped Ligno-PANI™ (Lignin sulfonate-doped polyaniline) appear in FIG. 13 along with a 0.5 M NaCl blank. The NaCl blank shows that, like deionized water, NaCl does not absorb in this spectral region (giving a 100% transmittance down to and below 250 nm). The decrease in the transmittance between 270 and 400 nm occurs in the following for water, NaCl and NaCl+hydrazine. The decrease with NaCl is greater than that for water and the decrease is greatest in the presence of the hydrazine reductant and NaCl. This result shows that the degree of release of the inhibitor from the doped Ligno-PANI occurs in the following order: hydrazine+NaCl>NaCl>water.
Emeraldine salt which had first been converted to the sulfate PANI by treatment with 1 M sulfuric acid followed by a treatment in 0.5 M DMDT so as to convert the sulfate to the DMDT analog through ion exchange was rinsed several times to assure the removal of residual adsorbed DMDT. After about 6 or 7 rinses, only a relatively low level of the inhibitor appeared to be released with the water rinse as evidenced by a residual spectrum with a minimum in the transmittance at 320 nm (FIG. 14). Portions of the solid DMDT-doped and multiply rinsed PANI were then equilibrated overnight with the following: deionized water, 0.5 M NaCl, and 0.5 M NaCl+0.5 M hydrazine. The supernatants from these equilibrations gave the spectra shown in FIG. 15. The water rinse shows the presence of an absorbing compound, though possibly not the DMDT since the minimum transmittance appears below 300 nm. On the other hand, equilibration of the doped PANI with the 0.5 M NaCl results in a solution deeply UV absorbing in the 300-400 nm region. The apparent absorption in the region becomes even greater if the DMDT-doped PANI is equilibrated with the NaCl in the presence of the hydrazine reductant. Furthermore it was observed that the hydrazine reduced the PANI since solid material went from a dark blue to a green color. It is most important to point out that the presence of both chloride alone and a chloride and a reductant cause the release of the inhibitor. The combined presence of the chloride and the reductant appears to be much more effective as noted also for the Ligno-PANI™ material.
FIG. 16 provides additional evidence that both the ion exchange mechanism alone, and the reduction or deprotonation mechanism appear to release the inhibitor. Increasing the chloride concentration appears to bring out more of the UV absorbing material as shown in FIG. 16. The presence of 0.5 M hydrazine alone in the absence of chloride appears to be most effective in extracting the inhibitor. Again this is seen as accompanying the reduction of the doped PANI since the doped PANI turned green with the treatment with the hydrazine.
Electrochemical data show that in the case of the NH₄PYRR doped Ligno-PANI™, an ORR inhibitor is indeed released to a NaCl solution. Potentiostatic current densities for the ORR occurring at a Cu RDE (2000 rpm) cathode in 5% NaCl slurries of the doped and un-doped Ligno-PANI™ material as well as in the 5% NaCl blank appear in FIG. 17. There appears to be an initial anomalously large current density for the as-received Ligno-PANI™. It is unlikely that this is due to the ORR, but more likely due to the electrochemical reaction of reducible compounds released by the un-doped Ligno-PANI™. The doped Ligno-PANI™, however, releases an inhibitor for the ORR since the cathodic current at the Cu RDE falls significantly below the baseline when the solid is in equilibrium as a slurry with the 5% NaCl. Furthermore, when the solid is removed it leaves a residual inhibitor as evidenced by a current density lower than the baseline (FIG. 17).
Samples of coatings on Al 2024-T3 of PANDA™ doped with NH₄PYRR were evaluated for ORR release with a Cu RDE cathode placed within a well-defined distance from the coating surface as shown schematically in FIG. 7. The resulting potentiostatic currents for the doped and undoped materials appear in FIG. 18. As can be seen the currents for the coating without the NH₄PYRR treatment give relatively high currents at levels typical of the blank 5% NaCl (˜650 mA/cm²). However, when the coatings have been treated with the NH₄PYRR, they release a compound that inhibits the ORR as evidenced by a dramatic decrease in the ORR currents at the RDE held at a calibrated distance from the coating surface in the 5% NaCl (FIG. 18).
Finally, a sample of Al 2024-T3 coated with PANDA™, a commercial polyaniline containing a proprietary anion dopant provided by Crosslink Polymer Research (Fenton, Mo.), and doped locally for 24 h with 0.02 M NH₄PYRR was scribed and exposed to a 48 h salt fog environment. The resulting sample showed a voluminous white corrosion product in the scribe placed through the un-doped region but only a slight brown staining in the scribed region as shown in FIG. 19.
These results show that the reaction of NH₄PYRR with PANI, both as a coating and the solid powder of Ligno-PANI, produces a material that releases an ORR inhibitor, presumably the anionic form of NH₄PYRR with exposure to NaCl. This point has been demonstrated both electrochemically and spectroscopically for the Ligno-PANI. Furthermore the release, in the case of the Ligno-PANI appears to be greater in the presence of a reductant, in this case in the presence of hydrazine. Salt fog exposure of the PANI coating clearly shows enhanced corrosion inhibition in the doped region of a specimen.
Similar results are obtained for a DMDT doped PANI, which was thoroughly washed and subsequently treated with solutions of NaCl and hydrazine. Both the chloride ion exchange and the hydrazine reduction of the doped PANI result in the release of the inhibitor. The reductively induced release appears to be more effective, but both reduction and ion exchange appear to release the UV absorbing dopant, also shown to be an effective ORR inhibitor. These results show that the schematic of FIG. 20 presents a viable model for ‘smart’ corrosion-inhibiting coatings based on conducting polyaniline films. Such films can be tailored to release on demand an appropriate corrosion inhibitor. The model proposes that both anion exchange with chloride and hydroxide, and galvanic reduction can release an inhibitor for the oxygen reduction reaction (ORR) if the ICP is appropriately doped.
Many modifications and other embodiments of the invention will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims

1. A composite structure that releases an anionic dopant upon application of an electrochemical potential, said composite comprising:

a fibrous material impregnated with a resin matrix, wherein the resin matrix comprises a conducting polymer and an anionic dopant associated with said polymer, wherein said dopant is dissociable from said polymer upon contact with a metal substrate under oxidating conditions.

2. The composite structure of claim 1, wherein said dopant includes a compound selected from the group consisting of monothiols and dithiols.

3. The composite structure of claim 1, wherein said dopant includes the anion of an organic acid.

4. The composite structure of claim 1, wherein said dopant selectively disassociates from said conducting polymer when said conducting polymer becomes more basic than when protonated by said dopant acid or is reduced.

5. The composite structure of claim 1, wherein said dopant becomes associated with said conducting polymer when said dopant acid protonates said conducting polymer.

6. The composite structure of claim 1, wherein the conducting polymer includes polyaniline.

7. The composite structure of claim 1, wherein the fibrous material is selected from the group consisting of glass, metal, minerals, conductive or nonconductive graphite or carbon, nylon, polyaramids.

8. The composite structure of claim 1, wherein the dopant has biocidal properties or is an effective scaling inhibitor.

9. A corrosion resistant metal article, comprising:

a metal substrate galvanically connected to a composite structure, wherein the composite structure comprises

a fibrous material impregnated with a resin matrix, wherein the resin matrix comprises a conducting polymer and an anionic dopant associated with said polymer, wherein said dopant is dissociable from said polymer upon oxidation of the metallic substrate.

10. The metal article of claim 9, wherein said dopant includes a compound selected from the group consisting of monothiols and dithiols.

11. The metal article of claim 9, wherein said dopant includes the anion of an organic acid.

12. The metal article of claim 9, wherein said dopant selectively disassociates from said conducting polymer when said conducting polymer becomes more basic than when acidified by said dopant or is reduced.

13. The metal article of claim 9, wherein said dopant becomes associated with said conducting polymer when said dopant protonates said conducting polymer.

14. The metal article of claim 9, wherein the conducting polymer includes polyaniline.

15. The metal article of claim 9, wherein the fibrous material is selected from the group consisting of glass, metal, minerals, conductive or nonconductive graphite or carbon, nylon, polyaramids.

16. The metal article of claim 9, wherein the metallic substrate is a component of a cooling tower.

17. The metal article of claim 9, wherein the metallic substrate is a component of a radiator cap.

18. The metal article of claim 9, wherein the metallic substrate is a component of an aircraft.

19. The metal article of claim 9, wherein the metallic substrate is a component of a watercraft.

20. The metal article of claim 9, wherein the metallic substrate is a component of a pipeline.

21. A method for inhibiting corrosion of a metallic substrate, comprising:

galvanically contacting the metallic substrate with a composite structure comprising

22. The method of claim 21, wherein said inhibiting anion is formed from an acid that is able to become associated with said polymer when said acid protonates said polymer.

23. The method of claim 22, wherein said inhibiting anion is disassociable from said polymer when said polymer is made more basic than when it is protonated.

24. The method of claim 22, wherein said inhibiting anion is disassociable from said polymer when said polymer is reduced.

25. The method of claim 21, wherein said inhibiting anion is derived from a molecule selected from the group consisting of acidic thiols and non-acidic thiols.

26. The method of claim 21, wherein said inhibiting anion is formed from dissociation of an organic acid.

27. The method of claim 21, wherein the conducting polymer is associated with the dopant by:

doping a cationic polymer with a first anion;

converting the cationic film into an oxidized form; and,

exchanging the first anion with the inhibiting anion from a solution of a salt of the inhibiting anion.

28. The method of claim 20, wherein the dopant has biocidal properties or is an effective scaling inhibitor.

29. The method of claim 21, wherein the step of doping the cationic material comprises:

protonating the film with an acid including an acid anion; and,

exchanging the acid anion with the inhibiting anion.