U.S. patent application number 11/027185 was filed with the patent office on 2005-09-15 for composition for the controlled release of inhibitors for corrosion, biofouling, and scaling.
This patent application is currently assigned to THE BOEING COMPANY. Invention is credited to Hon, Melitta M., Kendig, Martin W., Warren, Leslie F. JR..
Application Number | 20050201890 11/027185 |
Document ID | / |
Family ID | 29780139 |
Filed Date | 2005-09-15 |
United States Patent
Application |
20050201890 |
Kind Code |
A1 |
Kendig, Martin W. ; et
al. |
September 15, 2005 |
Composition for the controlled release of inhibitors for corrosion,
biofouling, and scaling
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.
Inventors: |
Kendig, Martin W.; (Thousand
Oaks, CA) ; Hon, Melitta M.; (Daly City, CA) ;
Warren, Leslie F. JR.; (Camarillo, CA) |
Correspondence
Address: |
ALSTON & BIRD LLP
BANK OF AMERICA PLAZA
101 SOUTH TRYON STREET, SUITE 4000
CHARLOTTE
NC
28280-4000
US
|
Assignee: |
THE BOEING COMPANY
|
Family ID: |
29780139 |
Appl. No.: |
11/027185 |
Filed: |
December 30, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11027185 |
Dec 30, 2004 |
|
|
|
10190932 |
Jul 8, 2002 |
|
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Current U.S.
Class: |
422/8 |
Current CPC
Class: |
C23F 11/165 20130101;
Y10T 428/31504 20150401; C23F 11/161 20130101; C09D 5/08
20130101 |
Class at
Publication: |
422/008 |
International
Class: |
C23F 011/00 |
Claims
What is claimed is:
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 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.
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.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] 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
[0002] 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
[0003] 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.
[0004] 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.
[0005] 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 Cr.sub.2O.sub.3. 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 Cr.sub.2O.sub.3). Preferably, the coating retains a
portion of the chromate that can be released to protect the metal
at imperfections in the film.
[0006] 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.
[0007] 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
[0008] 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.
[0009] 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.
[0010] 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.
[0011] 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.
[0012] 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.
[0013] 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.
[0014] 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.
[0015] A third embodiment of the invention includes a method of
coating the corrosion-inhibiting film upon a metallic
substrate.
[0016] 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.
[0017] 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.
[0018] 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.
[0019] 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.
[0020] 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
[0021] 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:
[0022] FIG. 1 is a cross-sectional view of a substrate including a
defect and a film according to an embodiment of the invention;
[0023] FIG. 2 is a molecular formulae for two anions according to
embodiments of the invention;
[0024] FIG. 3 is a molecular formula of a conductive polymer
according to an embodiment of the invention; and
[0025] FIG. 4 is a graph of the current densities of a solution
including anions according to various embodiments of the
invention.
[0026] FIG. 5 shows the structure of NH.sub.4 PYRR in accordance
with an embodiment of the invention;
[0027] FIG. 6 shows the structure of 2,5 dimercapto-1,3,4
thiadiazole dipotassium salt (DMTD) in accordance with an
embodiment of the invention;
[0028] FIG. 7 illustrates the experimental apparatus used in
Example 10;
[0029] FIG. 8 illustrates a decrease in the cathodic current
density for short diffusion lengths (high values of .delta..sup.-1)
in 5% NaCl solution containing 10 mM of NH.sub.4 PYRR according to
an embodiment of the invention;
[0030] 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;
[0031] FIG. 10 shows UV-visible spectra for various concentrations
of NH.sub.4 PYRR in water;
[0032] FIG. 11 shows UV-visible spectra of 0.5 M NaCl containing
different concentrations of the DMTD;
[0033] FIG. 12 shows UV-visible spectra of 0.5 M hydrazine
containing different concentrations of the DMTD;
[0034] FIG. 13 shows UV-visible spectra of supernatant solutions
after long term equilibration with the doped Ligno-PANI;
[0035] FIG. 14 shows residual DMDT release from PANI doped with
DMDT;
[0036] 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;
[0037] 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;
[0038] FIG. 17 shows the release of an ORR inhibitor from NH.sub.4
PYRR doped Ligno-PANI in a NaCl solution;
[0039] FIG. 18 shows potentiostatic currents for samples of Al
2024-T3 coated with PANDA doped with NH.sub.4 PYRR as evaluated
with the experimental setup of FIG. 7;
[0040] FIG. 19 shows a sample of Al 2024-T3 coated with PANDA and
doped locally for 24 h with 0.02 M NH.sub.4 PYRR was scribed and
exposed to a 48 h salt fog environment;
[0041] FIG. 20 presents a viable model for `smart`
corrosion-inhibiting coatings based on conducting polyaniline films
in accordance with the invention;
[0042] 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,
[0043] 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
[0044] 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.
[0045] 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.
[0046] 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 11a impregnated fibers or
fabric 11b 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 11a, whether used alone or as the resin matrix
of a composite, and a corresponding fiber or fabric layer is
generally referred to as item 11b if the inhibiting composition is
a composite.
[0047] 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 1/2 O.sub.2+H.sub.2O+2e.sup.-.fwdarw.2OH.sup.-.
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 1/2 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.
[0048] 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.
[0049] In addition, the conducting polymer 11a 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 11a allows the release of BA 26 which
forms a released active inhibitor or anion (RA) 28, from the BA 26.
Moreover, as the polymer 11a 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 11a, 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 11a 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.
[0050] A further exemplary mechanism for the release of the BA 26
may include deprotonation or deacidifying of the polymer 11a. Not
only does the production of the non-conducting product 24 release
the BA 26, but the hydroxyl group OH.sup.-, from the ORR occurring
on or in the coating, may help make basic the polymer 11a which
further helps release the BA 26. Generally, the hydroxyl group
OH.sup.- from the ORR attracts a proton H.sup.+ from the polymer
11a such that the BA 26 is released from the polymer 11a. In this
way, the BA 26 may also be released through the second mechanism to
help inhibit the corrosion of the metallic substrate 12.
[0051] 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.
[0052] 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 11a, thus releasing its anion which
becomes dispersed and intermingled or reacted yet fixed in the
polymer 11a as a "dopant" anion or the BA 26 when the polymer 11a
exists in its oxidized and conducting form. Moreover, the BA 26
must be able to migrate through the polymer 11a 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.
[0053] 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.
[0054] 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.
[0055] 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 11a. 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.sup.-H.sup.+, where RS.sup.- is the BA 26. One example of an
appropriate monothiol includes 2-mercaptothiazoline, which produces
the BA 26a, illustrated in FIG. 3. Dithiols have the general
formulation HSRSH, where R is any organic radical. Dithiols may
disassociate into HSRS.sup.-H.sup.+, where HSRS.sup.- is the BA 26.
An example of a dithiol includes 2,5-dimercapto-1,3,4-thiadiazole
which produces the BA 26b. 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-nitrobenzimid- azole, benzothiazole,
2-mercaptobenzoazole, and 2-mercapto-5-methylbenzimi- dazole. In
each of these examples the acid is deprotonated to produce a proton
and an anion, where the anion is the BA 26.
[0056] Composition 11 is preferably made of a conducting polymer
11a 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.
[0057] The BA 26 may be introduced into polymer 11a 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 11a that
is now protonated. This allows for an association of the BA 26 at
specific locations along the polymer chain of polymer 11a, but also
allows the BA 26 to be released when the composition 11 is treated
with base or is reduced electrochemically.
[0058] Appropriate methods for doping the polymer 11a include
synthesizing the coating to include the BA 26. The oxidized
protonated form of the polymer 11a has the BA 26 disbursed
throughout the polymer 11a 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 11a with the HA such that
polymer 11a becomes doped. This process occurs after the
composition 11 has placed in contact with the metallic substrate
12.
[0059] Another alternative for doping polymer 11a or otherwise
fixing a releasable inhibitor in or on the coating includes the
following: An oxidized polymer 11a 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 11a, 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.
[0060] 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.
[0061] 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.
[0062] 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.RTM., 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.
[0063] 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
[0064] 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.dbd.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
[0065] A three inch by three inch (76.2 mm.times.76.2 mm) test
panel of aluminum alloy Al2024-T3 was first degreased with acetone
and deoxidized in Sanchem 1000.RTM., available from Sanchem, Inc.,
Chicago, Ill., at about 37.degree. 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
[0066] A three inch by three inch (3".times.3") (76.2
mm..times.76.2 mm) test panel of aluminum alloy Al2024-T3.sup.- is
first degreased with acetone and deoxidized in Sanchem 1000.RTM. at
about 37.degree. 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).
[0067] 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 cm.sup.2 of 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
[0068] 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.
[0069] 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
[0070] A test panel of aluminum alloy Al2024-T3 of approximately
3".times.3" is substantially prepared as described in Example 2.
After the test panel is prepared properly, a coating of CorrPassiv
900226119.RTM. 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 cm.sup.2 is 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
[0071] 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
[0072] 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,
.delta..sup.-1. .delta.=1.75 .omega..sup.-1/2 v.sup.1/6 D.sup.1/2,
where .omega. is the rotation rates of the Cu rotating disk
electrode (RDE), D=2.times.10.sup.-5 cm.sup.2/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.
[0073] 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.
[0074] 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.
[0075] 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
[0076] A commercially available Ligno-PANI.TM. (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
(NH.sub.4 PYRR) and allowing reaction to take place for several
hours. The solid is removed from the filtrate and rinsed repeatedly
with deionized water. The NH.sub.4 PYRR whose structure appears in
FIG. 5 is used as-received from Aldrich without further
purification.
Example 9
Another Alternative Doping of PANI
[0077] 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
[0078] The UV-cured PANI or PANDA.TM. (14) provided by Crosslink
Polymer Research (Fenton, Mo.) is also doped by allowing the
coating to soak overnight in 0.5 M NH.sub.4 PYRR. 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
[0079] A 5% NaCl solution containing 10 mM of NH.sub.4 PYRR
exhibits inhibition of the ORR as shown by the decrease in the
cathodic current density for short diffusion lengths (high values
of .delta.-1) as shown in FIG. 9. Whereas the blank 5% NaCl gives a
current density of over 650 .mu.A/cm.sup.2 at .delta..sup.-1=2000
cm.sup.-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 .mu.A/cm.sup.2. This demonstrates the effectiveness of the
NH.sub.4 PYRR 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 NH.sub.4 PYRR and the DMTD, exhibit
excellent inhibition of the ORR in NaCl environments.
[0080] 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.
[0081] 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 NH.sub.4 PYRR in water appear in FIG. 10.
FIG. 10 shows a decrease in % transmittance in the UV region as the
concentration of the NH.sub.4 PYRR 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).
[0082] Spectra obtained from supernatant solutions after long term
equilibration (overnight) with the doped Ligno-PANI.TM. (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.
[0083] 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.TM. material.
[0084] 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.
[0085] Electrochemical data show that in the case of the NH.sub.4
PYRR doped Ligno-PANI.TM., 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.TM. 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.TM.. 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.TM.. The doped
Ligno-PANI.TM., 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).
[0086] Samples of coatings on Al 2024-T3 of PANDA.TM. doped with
NH.sub.4 PYRR 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 NH.sub.4 PYRR
treatment give relatively high currents at levels typical of the
blank 5% NaCl (.about.650 mA/cm.sup.2). However, when the coatings
have been treated with the NH.sub.4 PYRR, 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).
[0087] Finally, a sample of Al 2024-T3 coated with PANDA.TM., 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 NH.sub.4 PYRR 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.
[0088] These results show that the reaction of NH.sub.4 PYRR 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 NH.sub.4 PYRR 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.
[0089] 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.
[0090] 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.
* * * * *