Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Wenyan Li et al.
MICRO-ENCAPSULATION FOR CORROSION
DETECTION AND CONTROL
Wenyan Li and Luz M. Calle
Corrosion Technology Laboratory, NASA Kennedy Space Center, FL 32899, USA
e-mail: Luz.M.Calle@nasa.gov
Pitting can be one of the most dangerous forms of corrosion because it is difficult to anticipate and prevent,
relatively difficult to detect, occurs very rapidly, and can penetrate a metal without a significant amount of
weight loss. Failures of metals by pitting can occur very suddenly and can be catastrophic. One way of
preventing these failures is with a coating that can detect and heal localized corrosion. This work describes the
development of a smart coating that uses micro-encapsulation technology to detect and heal pitting corrosion in
the early stages. The dual function of the microcapsules is triggered by the pH changes that occur when pits
begin to form on the surface of a metal. The microcapsules are designed to respond to the pH changes by
breaking and releasing their contents. The contents can be a localized corrosion healing agent to terminate the
corrosion process or an indicator that signals its initiation. Corrosion indicator and corrosion inhibitor containing
microcapsules were formed and incorporated into representative paint systems. Test panels of selected steels and
aluminum alloys were painted using these paints. Testing of compatibility between the microcapsules and the
different paint systems are in progress. Initial results with the microcapsule containing paints show the presence
of visible color changes at induced corrosion sites.
1
Introduction
Corrosion is a serious and often overlooked problem that has enormous cost and safety
implications. The cost of corrosion damage includes manpower, materials used to repair it,
equipment downtime, and reduced productivity.
An extensive study by the U.S. Federal Highway Administration (FHWA) showed that the
total annual estimated direct cost of corrosion in the U.S. is a staggering $276 billion,
approximately 3.1% of the nation’s Gross Domestic Product (GDP).1 About one-third of
corrosion failures are due to localized corrosion such as pitting corrosion and crevice
corrosion caused by pitting.2 Numerous studies have been carried out to minimize losses and
failures due to corrosion and significant progress has been made in prolonging the service life
of materials. When localized corrosion takes place without being detected, the result can be a
catastrophic failure.
NASA uses different types of coatings to protect flight hardware, launch pad structures, and
ground support equipment. Barrier coatings, such as epoxies and urethanes, are used to isolate
the surface of a metal structure from the corrosive environment. Chromate conversion
coatings, such as the primer used for corrosion protection of areas throughout the Orbiter,
convert the surface into a hard, durable corrosion resistant layer. Sacrificial coatings, such as
the zinc-rich primers used at the launch pads, offer corrosion protection by corroding in
preference to the carbon steel.
Smart coatings represent the state-of-the-art of coating technology. These coatings sense the
environment and provide an appropriate response.
1
© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Wenyan Li et al.
This research is aimed at developing “smart coatings” for corrosion detection and control at
an early stage to prevent further corrosion.
2
Smart coatings for corrosion applications
The intelligence of the so-called “smart coatings” relies on their ability to respond to physical,
chemical or mechanical stimuli by developing readable signals which may often exert, in
addition to simple sensing, corrective action such as self-mending or healing.3
2.1
Smart coatings for corrosion sensing
Smart coatings for corrosion sensing purposes rely on a material undergoing a transformation
through its interaction with the corrosive environment. Such transformations can potentially
be used for indicating and detecting corrosion damage. Ideally, the sensing function could be
integrated with additional actuation and control functions, which are designed to control
corrosion damage.4 Since corrosion of metals is an electrochemical process, most of the
corrosion sensors that are relevant, include the following:
ƒ
ƒ
ƒ
2.2
Paint systems with color-changing compounds that respond to the pH changes that
result from corrosion processes.5
Changes of coating compounds from non-fluorescent to fluorescent states, upon
oxidation or complexation with metal cations. 6-9
Release of color dyes on coating damage from incorporated dye-filled microcapsules.
Smart coatings for corrosion protection
The best coatings for corrosion protection provide not only barriers to the environment, but
also a controlled release of a corrosion inhibitor, as demanded by coating damage and the
presence of a corrosive environment. Examples include chromate10 and metallic zinc
containing coatings, such as the zinc-rich paint systems used at the KSC launch pad
structures. When exposed to a corrosive environment, Chromate Conversion Coatings (CCCs)
release the inhibiting hexavalent chromium (Cr(VI))11-14 which passivates the metal exposed
at defects in the coating. The overwhelming success of CCCs can be attributed to their
performance as damage-responsive materials. CCCs release, Cr(VI), not simply by massaction dissolution from the coating, but as a result of electrochemical corrosion reactions that
concentrate alkali at cathodic sites, thereby stimulating its release.15 Unfortunately, Cr(VI)
has limited use for corrosion protection due to its toxic and carcinogenic properties.
Another damage-responsive coating technology involves the use of metallic zinc in coatings.
Metallic zinc, not only acts as a sacrificial material to electrochemically protect the substrate,
but its corrosion product is also inhibiting. The cost and weight of these coatings and their
general ineffectiveness for the lighter alloys, along with some concern for their environmental
impact, make them less than ideal for many aerospace applications.
Different approaches have been used to develop a damage-responsive protective coating, such
as semi-conducting coatings that would provide an electronic barrier at the metal coating
interface,16,17 and sol-gel coatings that include corrosion inhibitors in their controllable microor nano-structure.18
2
© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Wenyan Li et al.
Yang and van Ooij19 have encapsulated soluble corrosion inhibitors using plasma
polymerization. Such inhibitors can be incorporated in paints similar to the way in which
conventional solid inhibitors are used. The inhibitor is slowly released as it diffuses through
the thin polymer film. Some coatings may inhibit corrosion by stimulating the formation of
protective biofilms.20 Ion exchange corrosion inhibiting pigments have been considered for a
number of years. Recently Williams and McMurray demonstrated that hydrotalcite,
rehydrated in the presence of inhibitor anions such as phosphate and chromate, provides
excellent inhibition for filiform corrosion.21 The paints that include these pigments work to
limit filiform corrosion in at least two ways: by lowering the chloride activity through ion
exchange and by buffering the anodic head of the filiform. The most studied materials for this
application are probably conducting polymers and different inhibition mechanisms are being
considered.22-25 DeBerry et al. demonstrated that, in the conducting, oxidized form, such
materials could anodically protect stainless steel in sulfuric acid by maintaining its potential
in the passive region.26
Scott White at the University of Illinois, leads a team working on self-healing polymers. The
group has developed a microcapsule with healing agents that can be embedded in a polymer.27
Based on a similar principle, two smart system prototypes with “self-healing” properties have
been reported by Kumar and Stephenson.28 Both prototype coatings contain self-healing
microcapsules whose core constituents can be released when they are ruptured. The
microcapsules in the first prototype contain film forming compounds (healants) and corrosion
inhibitors, while those in the second prototype contain calcium hydroxide, (Ca(OH)2), which
can react with CO2 in the air and form a thin film of calcium carbonate (CaCO3). This film
acts as a healant to restore the coating integrity.
The smart coating being developed in this project will combine the functions of corrosion
sensing and corrosion protection by using pH-triggered release microcapsules29 for early
detection of corrosion and for corrosion mitigation.
3
Technical approach
3.1
Localized corrosion and local environment pH
Corrosion is largely an electrochemical process because, in most cases, it involves the transfer
of electrons between a metal surface and an aqueous electrolyte solution. For instance, when
iron corrodes in near neutral environments, the typical electrochemical reactions are:
Cathodic reaction:
O2 + 2 H 2 O + 4e − → 4OH −
(1)
Anodic reaction:
Fe → Fe 2+ + 2e −
(2)
In cases of localized corrosion, such as pitting corrosion, the anodic reaction happens in a
small confined area, the metal ions produced are precipitated as solid corrosion products, such
as Fe(OH)2 (often further oxidized to Fe(OH)3), which covers the mouth of the pit:
3
© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Wenyan Li et al.
2 Fe 2+ + 2 H 2 O + O2 + 4e − → 2 Fe(OH ) 2
(3)
4 Fe(OH ) 2 + 2 H 2 O + O2 → 3Fe(OH ) 3
(4)
This covering traps the solution in the pit and allows the buildup of H+, through a hydrolysis
reaction:
Fe 2+ + 2 H 2 O → 2 Fe(OH ) 2 + 2 H +
(5)
Then, chloride or other damaging negative ions diffuse into the pit to maintain charge
neutrality. Consequently, the solution in the pit becomes highly acidic, lowering the pH. The
overall effect is that, while localized corrosion occurs, the anode area often has an acidic pH
and the cathode area has an alkaline pH.30
3.2
pH-Triggered release microcapsules
The critical component of the new smart coating system presented on this paper is the pHtriggered release microcapsules. These microcapsules can be formed with a size of 1 micron
or larger, and their content can be completely released in a relatively short amount of time,
such as four hours, when the environmental pH is between 8-10, or 1-4. These pH ranges
match those present in localized corrosion sites in near neutral environments.
pH-triggered release microcapsules can be used to deliver healing agents to localized
corrosion sites to terminate the process at its early stage. They can also be used as corrosion
indicators themselves by releasing dyes at the localized corrosion sites. The dyes can be color
or fluorescent dyes, with or without pH sensitivity. A current study on corrosion sensing is
based on incorporating pH sensitive fluorescent dyes directly into paints. This approach has
encountered many challenges such as the low solubility of the dyes, the low pH sensitivity of
the dyes, and the loss of fluorescence in the cured coatings.8 These problems can be avoided
by incorporating the dyes into pH-sensitive microcapsules.
The type of smart coating being developed in this project uses pH-triggered release
microcapsules. Several functions can be added into the microcapsules through core contents:
film-forming compounds, corrosion-inhibiting compounds, as well as a regular pH indicator.
For indication applications, the microcapsules can be mixed with clear or light color paint so
that the color changes can be easily observed.
4
Experimental methods and results
Three steps are involved in the formation of the smart coating being developed by the authors.
The first step is the formation of the micro-emulsion.
4
© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Wenyan Li et al.
Normally, surfactants are needed to reduce the interfacial energy to enable the formation of
the micro-emulsion. There are various surfactant mechanisms that can be used for this
propose. Some examples are surfactant stabilizers, emulsifiers, polymeric stabilizers, and
thickening agents. By adjusting the amount of various surfactants and the stirring speed, it is
possible to form stable micro-emulsion with a desired size distribution. The second step is the
formation of the solid polymer capsule shell through interfacial polymerization.31,32 In our
process, the prepolymer and cross-linker are both dissolved in the oil phase, and the catalyst
of the polymerization reaction, an inorganic acid, is added to the water phase. Upon heating,
the polymerization reaction between the prepolymer and cross-linker takes place at the
interface between the oil and the water phase, where the catalyst is available. The reaction
stops when the polymer shell grows to a certain thickness and density. The chemical reaction
is presented in Figure 1. The pH sensitivity of the resulting structure lies in the ester or
thioester groups from the crosslinking agent. They can be cleaved through the nonreversible
hydrolysis reaction under basic pH conditions. The last step is to incorporate these
microcapsules into a coating system. After the microcapsules were prepared, their pH and
corrosion sensitivity was studied for their controlled release application; as well as their
compatibility with various paint system.
O
BuOH2C
N
H
O
N
O
N
HOH2C
N CH2OBu
O
N
H
CH2OBu
BuOH2C
SH
N
H
H
O
O
+
O
O
CH2OBu
O
O
N
H
O
O
O
O
HS
O
O
S
N
O
CH2OBu
O
N
CH2OBu
N
CH2OBu
N
CH2OBu
N
CH2OBu
BuOH2C
O
BuOH2C
N
N
H
O
O
H
H
N
HOH2C
O
N
+
BuOH
O
O
BuOH2C
H
N
N
O
O
S
SH
O
S
O
O
CH2OBu
S
O
N
O
O
O
N
O
N
HOH2C
N CH2OBu
O
H
O
N
O
N
SH
O
N
N
H
BuOH2C
N
N
H
O
N
CH2OBu
N
O
Figure 1: Possible reaction mechanism for the microcapsule wall formation
4.1
Preparation of microcapsules
After the initial experiments, different formulations and conditions have been varied to
achieve the desired microcapsule size and size distribution. While a homogenous size
distribution is normally preferred, different sizes might be required for different functions of
the microcapsule system. For example, for corrosion indication, a size of about 20 to 40
micron is suitable, and for inhibitor releasing, a smaller size might be better.
There are many factors that affect the stability and size of a micro-emulsion, such as oil-towater ratio, different surfactants, stirring speed, and stirring time. However, the extent to
which these factors influence the micro-encapsulation process varies. In our system, the oilto-water ratio is important for the size distribution. There is an upper limit of oil-to-water
ratio to get a stable and homogenous oil-in-water emulsion. Surfactants are critical for
emulsification.
5
© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Wenyan Li et al.
Their amount in the system affects the size, size distribution, and stability of the emulsion
system. It is possible to control the emulsion size by adjusting the amount of specific
surfactant at a given stirring speed. Stirring is important to initiate the emulsification process
and bring it to a stable stage in the presence of the surfactant. Normally, a higher stirring
speed results in a smaller size colloid. A certain time is needed for the emulsion to reach its
stable stage and optimum size distribution.
Based on the understanding of the micro-emulsion process, different formulations can be
chosen for different purposes. Figure 2 shows microcapsules, with various sizes ranging from
2 to 100 microns, made by the authors in their laboratory.
Figure 2: Microcapsules with various sizes
4.2
pH and corrosion sensitivity studies
To increase the pH sensitivity of the microcapsules, that is, to make microcapsules that break
down faster under basic pH conditions, two approaches are applied: increasing the
crosslinking agent content and decreasing the thickness of the microcapsule wall. Increasing
the amount of crosslinking agent will yield more ester groups in the wall structure. Decreasing
the reaction time will result in a thinner microcapsule shell.
6
© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Wenyan Li et al.
These two factors will result in higher pH sensitivity. It should be noted that a thinner
microcapsule shell also means low mechanical strength and this imposes a limitation that has
to be taken into account depending on the application. Figure 3 shows the color change
observed when the microcapsules are exposed to a solution that has a basic pH. The
microcapsules in Figure 3a had a thicker wall and fewer (about 25%) released the indicator
(as indicated by the intense red color), while those in Figure 3b had a thinner wall and more
(about 80%) showed color change.
(a)
(b)
(b)
Figure 3: (a) Microcapsule with thicker wall, color change under basic condition, about and (b) microcapsule
with thinner wall, stronger color change under the same basic condition after 5 minutes of exposure
After testing the pH sensitivity of the microcapsules, a simple test was performed on their
corrosion indication ability. A drop of pH-indicator containing microcapsules in water was
placed on a carbon steel panel. Based on prior experience, a rust spot would form under the
water drop. This is a simple way to observe corrosion on a metal surface. As seen in Figure 4,
the microcapsules indicated the presence of localized corrosion by changing color.
(a)
(b)
Figure 4: (a) A drop of microcapsule-containing solution indicating corrosion through color change, and (b)
corroded area after removal of the microcapsule-containing solution
7
© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
4.3
Wenyan Li et al.
Compatibility with Paint Systems
There are a few common concerns regarding the incorporation of microcapsules into paint
systems. One concern is that their interaction with paint constituents could lower the adhesive
and protective properties of the paint. Carbon steel test panels (4×6 inch2, sandblasted) were
prepared using four representative paint systems: Acrylic, Epoxy, Polyurethane, and Siloxane.
These panels were tested for adhesion. A PATTI (Pneumatic Adhesion Tensile Testing
Instrument) tester was used for pull-off strength measurements according to ASTM standard
D4541-85(89). The test results showed that, in most cases, the incorporation of these
microcapsules into the representative paint systems has no significant (more than 15%) effect
on the paint adhesion properties.
These results are shown in Figure 5.
Figure 5: PATTI Adhesion Test Results
The second concern is that the microcapsules might not survive the high shear mixing
process, that they will get broken in the process, and that they might clog the spray gun, and
not be sprayed properly. The third concern is that, the microcapsules might not keep the same
functionality in dried paint that they exhibited in the colloid system or dried powder form.
The first two concerns are related to the size and mechanical strength of the microcapsules.
When the size of the microcapsules is comparable to that of other solid particles in the paint
system, and the capsule shell is thick enough, the first two concerns do not pose a problem.
Microcapsules with an average size of 20 microns or smaller, can easily be added to
commercial paint products by mixing them with the paint using conventional high shear
stirring, and then painting metal panels using a spray gun.
The pH sensitivity of the microcapsules in dry paint was also tested. Figure 6 shows the vivid
color changes observed when the microcapsules in the dry paint were exposed to basic pH
conditions.
8
© Springer 2007
Proceedings of the First International Conference on Self Healing Materials
18-20 April 2007, Noordwijk aan Zee, The Netherlands
Wenyan Li et al.
Figure 6: Color change observed when microcapsules in dry paint were exposed to basic pH conditions
5
Summary
Microcapsules that respond by delivering an indicator that changes color at the basic pH
conditions present when localized corrosion occurs have been developed. Preliminary results
indicate that the incorporation of these microcapsules into representative paint system has no
adverse effects on the adhesive properties of the paints.
ACKNOWLEDGEMENTS
Funding for this project was provided by the NASA Kennedy Space Center Director’s Discretionary Fund
(CDDF) and NASA’s Space Operations Mission Directorate (SOMD).
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18-20 April 2007, Noordwijk aan Zee, The Netherlands
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