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REVIEW ARTICLE
CURRENT SCIENCE, VOL. 92, NO. 1, 10 JANUARY 2007 51
*For correspondence. (e-mail: sajivs@platinum.met.iisc.ernet.in)
Nanomaterials for corrosion control
V. S. Saji* and Joice Thomas
Corrosion Science and Engineering, Department of Metallurgy and Material Science, Indian Institute of Technology Bombay,
Mumbai 400 076, India
Nanomaterials are important due to their unique pro-
perties that may lead to new and exciting applications.
Current scenario of application of nanotechnology in
the field of corrosion prevention of metals is reviewed
here. Recent research and developments in this area
are discussed in designing efficient coating materials
and alloys, which provide superior resistance to cor-
rosion.
Keywords: Corrosion, coating, nanomaterials, R&D.
NANOSTRUCTURED materials (1–100 nm) are known for
their outstanding mechanical and physical properties due
to their extremely fine grain size and high grain boundary
volume fraction1. Significant progress has been made in
various aspects of synthesis of nano-scale materials. The
focus is now shifting from synthesis to manufacture of
useful structures and coatings having greater wear and
corrosion resistance. Existing PVD and CVD processes
for preparing microcrystalline coatings can be used to
produce nano-structured coatings by modifying the process
parameters or using feedstock powders having nano-grai-
ned structure. Deposition of coatings by thermally activa-
ted processes like high velocity oxy-fuel thermal spraying
(HVOF) has been successfully used for producing
nanocrystalline (nc) coatings2,3. Nanostructures promote
selective oxidation, forming a protective oxide scale with
superior adhesion to the substrate. A polymer nanocom-
posite coating can effectively combine the benefits of organic
polymers, such as elasticity and water resistance to that
of advanced inorganic materials, such as hardness and
permeability4. Improvements in environmental impact
can be achieved utilizing nanostructure particulates in
coating and eliminating the requirement of toxic solvents.
Nano-sized silica has proved to be an alternative to phos-
phate–chromate pretreatment that is hazardous due to
toxic hexavalent chromium5. Nano cobalt–phosphorus is
compatible with most existing electroplating equipment
and positioned as an effective replacement for the hexa-
valent chromium6. Medical benefits of nanostructured
diamond, hydroxyapatite and metalloceramics coatings are
well reviewed7. Nanocomposite coatings based on hydro-
xyapatite nanoparticles can provide better corrosion pro-
tection of titanium that can be utilized for fabrication of
advanced biomedical implants8. Development of paints
and finishing materials with self-cleaning properties, dis-
coloration resistance, graffiti protection and high scratch
and wear resistance caused a major revolution in the con-
crete world9. Work is in progress for developing nanopar-
ticle-based organic corrosion inhibitors, biocidal coatings
and non-skid coatings10. Wear-resistant protective coatings
based on nc-materials show considerable promise as a result
of the combination of strength and ductility that can be
achieved. However, the viability of protective nc-coatings
will ultimately depend upon the general corrosion resis-
tance exhibited by these materials over extended periods
of service.
Corrosion behaviour of nc-alloys
Electrodeposition is a versatile technique for producing
nc-materials. It is a technologically and economically viable
production route to metals, alloys and metal matrix com-
posites, both in bulk form and as coatings. Properties of
nano-structured electrodeposits such as hardness, wear
resistance and electrical resistivity are strongly grain size-
dependent.
Corrosion behaviour of nc-alloys has been assessed by
several techniques in various environments11–15. Thorpe
et al.11 reported an enhanced corrosion resistance of nc-
Fe32-Ni36-Cr14-P12-B6 than that of its amorphous counter-
part. The authors attributed this improved corrosion resis-
tance to the observed greater Cr-enrichment of the
electrochemical surface film via rapid interphase boundary
diffusion. Bragagnolo et al.12 reported improved corrosion
resistance with nc-Fe72Si10B15Cr3 metallic glass wires. In
their study, the beneficial effects of nc processing for cor-
rosion resistance were not evident with a non-passivating
alloy composition. Zeiger et al.13 reported an enhanced
corrosion resistance of nc-Fe-8 wt% Al in Na2SO4 solu-
tion. The study showed that defect density (density of
grain boundaries) promotes metal dissolution. As the dif-
fusion of aluminum is fast enough in the grain boundaries,
it is possible that the oxide film on nc-FeAl8 alloy pro-
vides better protection than on the same polycrystalline
(pc) alloy. The passive film formation of nc-FeA18 is dis-
tinctly eased in the pH range (weakly acidic to weakly
basic) where Al forms stable passive film. The study in-
dicates that nc-materials can be more easily passivated in
REVIEW ARTICLE
CURRENT SCIENCE, VOL. 92, NO. 1, 10 JANUARY 2007 52
weakly acidic medium compared with the corresponding
pc-material; whereas the situation is opposite in a strongly
acidic medium, where higher dissolution rates are meas-
ured. Barbucci et al.14 investigated corrosion behaviour of
Cu90Ni10 alloy in neutral media containing chlorides.
They reported a decrease in the protective properties of
the passive layer in the nanostructured alloy that was
found to depend on the oxygen concentration. The passive
oxides, which grow on the nanostructured metal surface,
are found not as compact as expected and detected in
coarse-grained alloys. According to the authors, the in-
creased amount of grain boundary in the nanostructured
alloy could justify the loss of oxide compactiveness as a
result of its irregular growth on the surface. Alves et al.15
found that nc-(Ni70Mo30)90B10 alloys are less sensitive to
corrosion in alkaline solutions than the coarse-grained
material.
Corrosion behaviour of nc-metals is investigated in dif-
ferent environments16–18. Rofagha et al.16 investigated
corrosion behaviour of nc-nickel (99.99%, 32 nm grain
size) in 2N H2SO4 in de-aerated media and found that the
corrosion potential of nc-nickel was shifted about
200 mV to the positive than that of pc-nickel. The study
also showed that nc-processing of nickel catalyses hydrogen
reduction processes, reduces kinetics of passivation, and
compromises passive film stability. According to the authors,
the excellent corrosion performance typically observed
with conventional pc-Ni and Ni-based alloys appears to
be retained in the nc-state, providing considerable promise
for the development of protective (wear, fracture and cor-
rosion resistant) nc-coatings. Youssef et al.17 compared
corrosion behaviour of nc-zinc produced by pulse current
electrodeposition with electro-galvanized steel in de-aerated
0.5 N NaOH electrolyte. The estimated corrosion rate of
nc-zinc (90 µA/cm2) was found to be about 60% lower
than that of electro-galvanized steel, and the passive film
formed on the nc-zinc surface seems to be the dominating
factor for this. The nc-structure enhances both kinetics of
passivation and stability of the passive film formed. Anodic
and cathodic Tafel slopes of nc-zinc (~40 and 107 mV/
decade) are found to be lower than that of electro-
galvanized steel (~59 and 128 mV/decade), indicating
higher activation energy for dissolution for nc-zinc. The
corroded surface of nc-zinc shows discrete etch pit mor-
phology, while uniform corrosion was observed on the
electro-galvanized steel surface. Mishra and Balasubra-
maniam18 compared corrosion behaviour of nc-nickel of
different grain sizes (8–28 nm) in 1 mol H2SO4 electrolyte
with that of bulk nickel. Zero current potential, passive
current density and breakdown potential were found to
increase with decrease in grain size. The increase in pas-
sive current density indicates defective nature of passive
film formed on nc-nickel. Tendency for localized corro-
sion was lower in the case of nc-nickel as indicated by an
increased breakdown potential. The corrosion rate of
freshly exposed nc-nickel was found to be lower com-
pared to bulk nickel, indicating a higher hindrance to anodic
dissolution from the nc-nickel surfaces. XRD analysis in-
dicated that the nc-nickel deposits were compressively
strained, with microstrain increasing with decreasing
grain size.
The reported studies on corrosion resistance of nc-alloys
showed mismatching results. It can be noticed that both
in weakly acidic and alkaline media the nc-material main-
tains a more effective passive layer, whereas in media
containing aggressive ions, the passive film stability is
decreased compared to the pc-material. As the defect
density is high, it can be assumed that the corrosion resis-
tance of the pc-material is better than that of the nc-
material. However, the diffusion rate of alloying atoms in
nc-alloys will be higher compared with pc-alloys. For nc-
alloys, where corrosion resistance originated from the
formation of passive films (oxide films), it can be expected
that their corrosion resistance is higher compared to con-
ventional pc-alloys. It should be noted that corrosion re-
sistance depends on the electrode, the aggressiveness of
the electrolyte and also how effective is the passive layer
formed. In conventional plating, unavoidable impurities
in metal alloys spread throughout a coating, migrating
naturally to grain boundaries and precipitating. In nano-
coatings, the size of the grains is much smaller and their
number is increased exponentially. As a result the impurities
are super-diffused; homogenization by segregation. Such
a coating is stronger and more resistant to stress and cor-
rosion cracking6.
Ceramic coatings
Ceramic coatings are attractive as they possess good
thermal and electrical properties, and are more resistant
to oxidation, corrosion, erosion and wear than metals in
high-temperature environments. Nanoparticles of diamond
as well as chemical compounds used for hard coatings
(SiC, ZrO2, and A12O3) are commercially available19, with
typical particle sizes in the range 4-300 nm. Within tribo-
logy, a new development has been to deposit nanocoatings
from colloids, e.g. of graphite. Nano-sized silica has
proved to be an alternative to toxic chromate conversion
coating5. Metal precoat based on the combination of a
nanostructured metallic oxide of ceramic-type, with metals
like Ti and Zr produces nanometre-range conversion coat-
ing, while the conventional phosphate layers are within
micron range20.
Incorporation of suitable nanoparticles in paints for
improved properties is well commercialized. During the
painting process, e.g. of automobiles, the ceramic nano-
particles float around freely in the liquid paint. When the
automobile body is baked at a higher temperature, the ceramic
nanoparticles crosslink into a dense network instead of
the long molecular chains found in conventional paint.
This allows the lacquer to provide a much more effective
REVIEW ARTICLE
CURRENT SCIENCE, VOL. 92, NO. 1, 10 JANUARY 2007 53
scratch protection against normal wear and tear and allows
the paint to retain its gloss.
Shen
et al.21 have re-emphasized the importance of
nano-TiO2 in the development of high corrosion resistance
and hydrophobic coatings. Hydrophobic coatings with
low wet ability are possible to effectively prevent the water
on the substrate surface, and exhibit excellent corrosion
resistance in wet environments. Hydrophobicity of the
porous coatings is attributed to air trapped in the nanopores
that limits water accessibility and concentration of corro-
sive species in the stainless steel holes, and hence causes
a retardation of the anodic dissolution process. The corro-
sion potential of the nano-TiO2 and fluoro alkyl silanes/
nano-TiO2-coated electrodes is found much nobler than
that of the 316L stainless steel substrate in Ringer’s solution.
Co-deposition of ceramic nano-scaled particles during
the electroplating process brings improvements in technical
properties at reasonable cost. However, the corrosion resis-
tance deteriorated when the particles were co-deposited.
Euler et al.22 produced a series of nickel nano-ceramic
composites, with co-deposition of particles of Al2O3 and
TiO2 as a single primary particle in the nanometre range
(10–30 nm) at one end of the scale and as agglomerates
up to a size of a micrometre at the other. Successful in-
corporation of particles up to 2-volume % has been estab-
lished despite the problem of possible agglomeration. The
decrease in corrosion resistance is explained by an accel-
erated diffusion of chloride ions along the interface bet-
ween nickel and the incorporated particles. The high
surface energy and agglomeration tendency of the nano-
particles in highly conductive metal electrolytes will tend
to impede uniform distribution of the particles.
Polymer coatings
Conducting polymers have evoked a great deal of interest
due to their electrochemical properties and their mixed
ionic/electronic conductivity properties23. They have been
used as host matrices in various composite films. Organic
or inorganic particles can be mixed with or incorporated
in the conducting polymers to modify their morphology,
conductivity and different physical properties depending
upon the application, such as corrosion protection. Poly-
crystalline nanocomposites that consist of conductive
polymers were found to display novel properties. Nano-
particulate dispersions of organic metal polyanilines in
various paints at low concentrations can cause tremendous
effects in corrosion protection24. Melt dispersion of poly-
aniline leads to fine particles, which self-organize into
complex ultra fine networks. Some specific nanoconducting
polymers which enhance corrosion resistance are polyani-
line, polythiophene and polypyrrole. To enhance the oxi-
dizing power of the polymers, incorporation of strong
oxidizing species in the polymer has been envisaged.
Polypyrrole nanocomposites with oxides, especially with
Fe3O4 have prospects for use in corrosion protection of
iron25. Polypyrrole nanocomposites with montmorillonite
clay showed better corrosion protection compared to un-
doped polypyrrole26.
Nanostructured materials engineering extends the pos-
sibility of engineering ‘smart’ coatings that can release
corrosion inhibitors on demand when the coating is
breached, stressed or an electrical or mechanical control
signal is applied to the coating27. Inherently conducting
polymer (ICP) films containing inhibiting anions as the
dopant anions can release them when the film is coupled
to a breach in the coating. Research has developed chro-
mate-free corrosion inhibiting additives in which organic
corrosion inhibitors are anchored to nanoparticles with
high surface areas that can be released on-demand28.
Self-assembled nanophase coating
In the traditional sol–gel method, hydrolysis–condensation
processes are followed by condensation polymerization
upon film application. However, the evaporation process
results in voids and channels throughout the solid gel and
cannot provide adequate corrosion protection due to the
high crack-forming potential. Sol–gel technology has an
important limitation related with the maximum coating
thickness attainable; typically lower than 2 mm. Studies
showed that incorporation of nanoparticles to the sol can
make it possible to increase the coating thickness, without
increasing the sintering temperature29. Electrophoretic
deposition of commercial SiO2 nanoparticles suspended
in an acid-catalysed SiO2 sol on AISI 304 stainless steel
substrates leads to coatings as thick as 5 mm with good
corrosion resistance29.
Incorporation of nanoparticles in the hybrid sol–gel
systems increases the corrosion protection properties due
to lower porosity and lower cracking potential30. Incorpo-
ration of inorganic nanoparticles can be a way to insert
corrosion inhibitors, preparing inhibitor nanoreservoirs
for self-repairing pre-treatments with controlled release
properties30,31. Studies showed that sol–gel films contain-
ing zirconia nanoparticles present improved barrier pro-
perties. Doping this hybrid nanostructured sol–gel coating
with cerium nitrate brings additional improvement to corro-
sion protection. Zirconia particles present in the sol–gel
matrix act as nanoreservoirs providing a prolonged re-
lease of the cerium ions32.
The recent discovery of a method of forming function-
alized silica nanoparticles in situ in an aqueous sol–gel
process, and then crosslinking the nanoparticles to form a
thin film, is an excellent example of a nanoscience approach
to coatings. This self-assembled nanophase particle
(SNAP) surface treatment based on hydrolysed silanes,
containing a crosslinking agent substantially free of organic
solvents and Cr-containing compounds promotes adhesion
of overcoat layers more effectively. Unlike chromate-
REVIEW ARTICLE
CURRENT SCIENCE, VOL. 92, NO. 1, 10 JANUARY 2007 54
based treatments, SNAP coatings provide barrier-type
corrosion resistance but do not have the ability to leach
corrosion inhibitors upon coating damage and minimize
corrosion of the unprotected area30. The SNAP surface
coating could replace the currently used chromate contain-
ing surface treatment and can provide the basis of long-
lived coating for aluminum alloys5,33. The ability to design
coating components from the molecular level upward offers
potential for creating multifunctional coatings. Molecular
simulation approaches have been used to enhance the un-
derstanding of complex chemical interactions in coatings-
related processes34.
Self-cleaning paints and biocidal coatings
There is a great interest in the design and development of
surfaces that not only provide biocidal activity but are
also easy to clean and even self-cleaning. Most of such
coatings acquire their biocidal/self-cleaning capacity by
incorporating specific nanoparticles: basically silver (Ag)
and titanium oxide (TiO2)35,36. Nano TiO2 is used for de-
veloping anti-UV, anti-bacterial and self-cleaning paints.
This possesses self-cleaning hydrophobic properties,
which causes water droplets to bead-off of a fully cured
surface picking up dirt and other surface contaminants
along the way. This self-cleaning action helps clean and
maintain important surfaces and to accelerate drying,
leaving the surface with minimal spotting. A recent study
by Cai et al.37 utilizes corona treatment technique, inert
sol–gel coating and anatase TiO2 layer. With the corona
treatment, an organic surface was activated to allow a
uniform TiO2 sol–gel coating. Nanoparticles of surface-
treated Al2O3 molecules help increase hydrophobicity and
increase scratch resistance.
Microbial evolution on a wide variety of surfaces can
cause corrosion, dirt, bad adour and even serious hygiene
and health problems. AMBIO (Advanced Nanostructured
Surfaces for the Control of Biofouling), a European Un-
ion research project38 is investigating how to prevent the
build-up of organisms on surfaces under marine conditions
to avoid biofouling. The project aims to use nanostructuring
to significantly reduce the adhesion of organisms to sur-
faces in aquatic environments, and thus control the foul-
ing process without the use of toxic biocides such as
copper and organotin compounds that prevent fouling by
killing organisms. Nanostructuring of the surface alters
the wetting properties and is intended to signal that the
site is not suitable for the organisms to settle. The project
aims to synthesize new nanostructured polymers that are
stable under marine conditions. Although no alternatives
to the use of biocides are available at present, creation of
nanostructured surfaces could offer an innovative and en-
vironment-friendly solution to the problem of biofoul-
ing38. Research has developed new biocidal coating
systems that prolong biocidal activity by immobilizing
such additives on nanoparticles; the embedded biocides
are designed to be released into the environment only when
needed, thus extending the lifetime of the biocidal acti-
vity10.
Nanostructured alloy and composite coatings for
high-temperature applications
Nanostructures form protective oxidation scales with supe-
rior adhesion to the substrate39. The high density of grain
boundaries provides fast diffusion paths, promoting selec-
tive oxidation of protective oxide scales. The fine-grained
coatings and/or the fine-grained oxide scales show a fast
creep rate at high temperatures, which can release the
stresses accumulated in the scales, therefore reducing the
scale spallation tendency. The oxides formed on nanocry-
stalline coatings are micro pegged onto the grain bounda-
ries to form a complex interface that results in better scale
adhesion to the metal substrate. Nanocrystalline alloy coat-
ings, oxide-dispersive alloy coatings and metal-oxide
composite coatings show superior high-temperature corro-
sion resistance.
Engineering alloys rely on the formation of protective
oxide films such as Al2O3 and Cr2O3 to resist high tem-
perature and corrosive environments. Unfortunately, rela-
tively large concentrations of Al or Cr are needed to form
a complete Al2O3 or Cr2O3 scale. In the Ni-20Cr-Al alloy
system, for instance, greater than 6 wt% Al is required to
form a complete Al2O3 scale. With nc-alloy coatings, the
Al content that is required to form a complete protective
oxide scale can be substantially reduced. Experimental
results indicate that when the grain size of Ni-20Cr-Al
coatings was ~60 nm, alloys containing ~2 wt% Al could
form a complete
α
-Al2O3 scale at 1000°C in air. This
concentration is only one-third of the required Al% for
the Ni-20Cr alloy with normal grain size39.
Ti alloys and Ti–Al intermetallics having advantages
of high strength, lightweight and high melting point have
lower oxidation resistance at elevated temperatures. They
have potential applications in the aerospace and automotive
industry due to their excellent mechanical properties at
high temperatures and corrosion resistance. Nano- or sub
micro-alloy coatings produced by electro-spark deposi-
tion provide a powerful tool for Ti–Al intermetallics to be
used as high temperature structural materials39.
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ACKNOWLEDGEMENT. We thank Prof. V. S. Raja, Corrosion Sci-
ence and Engineering, IIT Bombay for suggestions.
Received 29 January 2005; revised accepted 20 November 2006
