Due to the massive economic impact of corrosion degradation on metallic structures, the area of active protective coatings has been developed very fast in the past few years. The goal is to significantly reduce the maintenance costs in many industrial applications by applying active sensing coatings. The indication of corrosion activity by these coatings will allow optimization of the maintenance operations avoiding excessive unnecessary preventive operations in the cases when coating is still able to protect the metallic structures.
Today we look at a novel sensing
active coating on the basis of nanocapsules containing pH-indicating agent.
The main idea of this work by a research team from the University of Aveiro
in Portugal, is to create a novel active protective coating which is able to
indicate when corrosion processes start under the coatings or in different
defects.
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The nanoreactors being introduced
in the coating change their color in the zones where corrosion processes
start. An important feature of these nanoreactors is that the indicating
molecules are not released from the mesoporous nanocarriers, thereby
preventing spontaneous leaching and ensuring long service time.
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Scheme
of pH sensing response from coating. (© IOP Publishing)
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The team, led by Mikhail Zheludkevich, a senior researcher at the university's Department of
Materials and Ceramics Engineering, CICECO, published their findings in the
September 17, 2013 online edition of Nanotechnology ("Nanocontainer-based corrosion sensing
coating").
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"The corrosion sensing can be
considered as independent functionality of a protective coating or can also
be considered as an important additional functionality which can complement
the self-healing coatings," Zheludkevich tells Nanowerk. "For
example, the new coating developed by our group in collaboration with several
European partners from academic and industrial sectors is counting on the
multi-level self-healing effect based on 'smart' nanocontainers. The main
idea is that several mechanisms of self-healing can be integrated in the same
coating providing effective active protection which is proportional to the
external impacts such as corrosive attack and mechanical impacts.
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In such a coating, the active
healing agents responsible for different mechanisms are encapsulated in
micro- or nano-containers and then integrated into the polymer coating.
Different nanocontainers can introduced to different layers of the coating or
can be used in a single layer system.
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Pictures
of coated substrates doped with Si NC–PhPh and tested during 3 days in 0.5 M
NaCl: (a) and (b) coated aluminium alloy with artificial scribe, (c) and (d)
coated magnesium alloy without scribe. (© IOP Publishing)
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These coatings were developed in
the frame of large-scale European project MUST ('Multi-Level Protective Materials for
Vehicles by 'Smart' Nanocontainers').
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In their work, the team
encapsulated phenolphthalein – a colorless crystalline solid often used as an
acid-base indicator – in mesoporous silica nanocontainers.
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"With the onset of corrosion,
pH increases in the local cathodic areas due to the formation of hydroxide
ions whilst acidification often occurs at anodic sites as a result of
hydrolysis reactions," explains Zheludkevich. "The respective pH
variations can be used to detect and locate the active corrosion spots in
confined defects of the coatings or under the coatings if pH indicators are
incorporated in a polymer protective layer."
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The main idea of the team was to
provide a color change signal as a result of pH change in the vicinity of a
nanocontainer with consecutive diffusion of hydroxide ions into the mesopores
reacting with pH indicator within the container.
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As Zheludkevich points out, the
introduction of corrosion sensing functionality to the self-healing coating
is an important step which allows detecting the moment when the coating is
not able to heal the defects anymore and an external intervention is needed
to avoid an extensive corrosive damage.
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One challenging future direction
for this research area is the integration of several functionalities into the
protective coatings. The objective would be to develop coatings which not
only integrate several self-healing mechanisms at the same time but also
provide additional self-monitoring tasks with optional antibacterial or
antifouling functionalities.
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