Research Article • DOI: 10.2478/shm-2013-0001 • SHM • 2013 • 1-18

Self-Healing MaterialS

1

* E-mail: m.abdolahzadeh@tudelft.nl

1. Introduction

Corrosion is one of the main processes leading to material

destruction and economic losses, the latter being estimated

between 2 to 5% of the world’s GNP. Although corrosion

is inevitable, the costs can be significantly reduced using

appropriate protection methods. For years passive and/or active

protective coatings have been used as an inexpensive and

efficient way to prevent corrosion of materials. Coatings protect

substrates from corrosion according to one or several of the

following passive and active mechanisms [1-5]:

1. providing strong adhesion to avoid delamination

2. providing barrier to minimize mass-transport of corrosive

species (e.g. H2O)

3. carrying sacrificial pigments (i.e. imparting anodic

protection)

4. carrying inhibitive species (i.e. anodic and cathodic

corrosion inhibitors capable of reacting with the underlying

metallic surface).

Several coating technologies are available; solvent and

waterborne liquid paints, cataphoretic paints and powder

coatings being the most extended ones. Apart from these

well-established technologies, sol-gel coatings have lately

attracted considerable interest due to their advantageous

features such as low processing temperature, high chemical

versatility, easiness of application and possibility of coating

complex shapes, strong interaction of coating with metallic

substrates and the environmental friendliness nature of the

process.

The sol-gel process was initially established for production

of ceramic and glass-like structures and can be traced back

to 1842. Generally, the sol-gel process can be described as

the evolution of an oxide network by continuous condensation

reactions of molecular precursors in a liquid medium, via

hydrolytic or non-hydrolytic procedures [6,7]. The non-hydrolytic

method is based on the formation of a colloidal suspension,

followed by gelation through condensation of precursor

molecules via alkyl halide elimination, ether elimination, ester

elimination, C-C bond formation between benzylic alcohols and

alkoxides and aldol condensation reactions, depending on the

precursor and solvent molecules [8-10]. Hydrolytic approaches,

which are more common in comparison to non-hydrolytic ones,

are mainly based on hydrolysis and condensation reactions of

metal or metalloid alkoxides (M(OR)n) (Figure 1). In this kind of

precursor molecules, M and R represent a network-forming

element and an alkyl/allyl group, respectively. In spite of the

versatility of precursor molecules, silicone alkoxides are the

most studied ones due to their stability and gentle reactions,

which facilitate the control of hydrolysis and condensation

reactions rates [8,11,12].

However, hydrolysis and condensation are equilibrium

reactions and can proceed simultaneously once hydrolysis

reactions have been initiated. The structure and properties of the

final product depend on the sequence of these steps, which in

turn are dramatically affected by the initial reaction conditions

such as pH, molar ratios of reactants, solvent and temperature.

The prepared sol-gel systems can be applied on metallic

substrates using different techniques including dip-coating,

Routes to extrinsic and intrinsic self-healing corrosion protective sol-gel coatings: a review

Novel Aerospace Materials Group, Department of Aerospace Materials and Manufacturing, Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS, Delft, The Netherlands

M. Abdolah Zadeh*, S. van der Zwaag,

S.J. Garcia

Received 18 January 2013 Accepted 07 April 2013

AbstractSol-gel pre-treatments and coatings are an important class of passive protective coatings, which can effectively prevent corrosion of various metallic substrates through adhesion improvement and barrier protection. Recently, sol-gel chemistry has been proposed as an appropriate method for implementation of self-healing functionality in coatings via extrinsic concepts. In this review we will analyze the most relevant existing works on self-healing sol-gel coatings, including new work done in the direction of implementing intrinsic healing capabilities to sol-gels. The development of active sol-gel coatings is due to the broad chemical versatility of precursors and low processing temperature of this type of chemistry.

KeywordsCorrosion protection • Sol-gel coatings • Self-healing

© Versita Sp. z o.o.

M. Abdolah Zadeh et al.

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healing of epoxies with encapsulated liquid agents. Since then,

a growing number of books, reviews and research papers have

been published on different self-healing approaches [16-18].

Following the recent classification proposed by Garcia et al [15] for

self-healing coatings, self-healing materials can be classified into

two main classes namely (i) extrinsic and (ii) intrinsic self-healing

systems. In extrinsic self-healing systems such as capsule-based

and vascular systems, the healing agents are added as a separate

phase into the matrix, while intrinsic systems such as ionomers,

hydrogen-bonded systems, etc., possess a latent self-healing

functionality due to the architecture of the molecules themselves.

The extrinsic and intrinsic self-healing materials can be further

subcategorized into several groups considering their architecture,

healing mechanism, and healable damage size-scale. In

addition, self-healing materials, and specifically coatings, can

be categorized based on the functionality to be recovered such

as corrosion protection, barrier functionality and hydrophobicity.

Extensive information about self-healing coatings can be found

elsewhere [6,7,15,17,19,20].

2. Passive corrosion protection by sol-gel coatings

Sol-gel coatings, in particular silane-based coatings, have been

successfully tested as corrosion protective pre-treatments

(thickness < 1 µm) and coatings (thickness > 5 µm) on different

spin-coating, spray-coating and electrodeposition, followed by

a drying/curing step [6-8,11,12].

In 1997, Guglielmi reviewed the potential application of sol-

gel coatings (mainly ceramic coatings) as corrosion protective

coatings and discussed the associated advantages and

disadvantages [13]. Since then, numerous papers have been

published on corrosion protective sol-gel coatings, revealing

quite promising results regarding to the improvement of organic

coatings adhesion to substrate and protection of various metallic

substrates against corrosion.

Ideally, protective coatings prevent corrosion of the

underlying substrate during its service time. Nevertheless,

despite the substantial advances in coating technology, coatings

fail to fulfill their functionalities over time due to damage and/

or degradation resulting from mechanical and chemical attacks,

as well as from thermal cycles. Damages in (organic) coatings

can occur at three different levels: macro, micro and nanoscales

affecting interfaces, networks and even molecular structure of

the coating by dissociation or bond breakage [14,15].

The need of high-value materials in sophisticated coatings

for improved reliability has motivated researchers to work on a

new class of coatings known as self-healing coatings, which are

coatings with the capability of reacting on external stimulus to

heal damages. The field of self-healing engineering materials was

first introduced in the nineties by Dry and Sottos in cementitious

materials and boosted in 2001 by White’s work on autonomous

Figure 1. Different steps in preparation of hydrolytic sol-gel coatings.

Routes to extrinsic and intrinsic self-healing corrosion protective sol-gel coatings: a review

3

which can be hydrophobic. This protection aims at reducing the

rate at which water and other electrolytes can reach the metal/

coating interface, delaying the corrosion process.

Unfortunately, even in the most hydrophobic films,

prolonged exposure to water/electrolyte will eventually lead to

moisture penetration to the metal/coating interface. Considering

the reversible nature of hydrolysis and condensation reactions,

water penetration can result in hydrolysis of bonds formed during

condensation reaction. If the coating is not dried fast enough to

promote condensation reactions, water/moisture ingress can end

up with coating failure/delamination [6,18,21,22]. Modification of

formulation parameters can nonetheless lead to an improvement

of general passive protective properties. The effect of different

parameters such as the nature of organic components, ratio

of organic/inorganic components, etc., will be analyzed in the

following sections.

2.1. Critical formulation parameters2.1.1. Organic-inorganic componentsHybrid sol-gel coatings can be prepared over a wide

compositional range. Evidently, the nature of the organic

components has a crucial effect on corrosion properties of hybrid

coatings. Different hybrid sol-gel materials can be classified into

5 groups, based on the type of organic component to be added

to the inorganic network, as well as on the interactions between

organic and inorganic counterparts (Figure 2).

Hybrid organic-inorganic sol-gel coatings can be prepared

by addition of either organic oligomers/polymers (Types I & II) or

monomers (Type III) into inorganic sols. There are no chemical

bonds between organic and inorganic components of Types

metallic substrates such as aluminum alloys, steel, copper,

magnesium and galvanized steel. However, highly corrosion

resistant coatings can only be achieved through (i) proper

surface preparation of the substrate, as it plays a crucial role in

formation of covalent bonds between sol-gel coating and metal

surface and (ii) a fine balance between application conditions

and the ingredients used.

The final properties of sol-gel coatings dramatically depend

on the starting materials and processing conditions, e.g. pH and

temperature. Despite the fact that the sol-gel processing on

metallic surfaces has experienced a number of improvements,

preparation of thick and defect-free inorganic sol-gel coatings

is still a challenge. In fact, this is the main drawback of sol-gel

coatings from corrosion protection point of view.

In the last decades, the introduction of organic groups into

inorganic sol-gel networks has facilitated the preparation of

thick and crack-free hybrid coatings, with properties of both

organic and inorganic materials. While the inorganic component

of hybrid coatings contributes to enhancement of mechanical

properties, the organic component imparts an increased

density and flexibility, as well as a desired functionality such

as hydrophobicity. The application of hybrid sol-gel coatings

significantly improves the corrosion performance by providing a

relatively thick and crack-free barrier on the substrate.

Nonetheless, silanes are electrochemically inactive in

solution or in the solid state, so they cannot be reduced or

oxidized unless they carry electrochemically active functional

groups. As a consequence, silane-based hybrid sol-gel coatings

do not offer active protection by themselves but passive

protection through formation of a well adherent barrier layer

Figure 2. The five classes of hybrid sol-gel materials/coatings [adapted from 9].

M. Abdolah Zadeh et al.

4

branched polyethylene imine (PEI) and DETA as cross-linker. In

addition, Vreugdenhil et al [30] have employed three different

curing agents to conclude that it is not the hydrophobicity but

instead the number of reactive sites per curing agent molecule

that plays a significant role in the electrochemical properties of

GLYMO-TMOS hybrid coatings.

Hybrid sol-gel coatings can be prepared over a continuous

compositional range from almost organic to almost inorganic.

An increase in organic content leads to formation of less porous

and thicker films, appropriate for barrier protection of metals.

Nevertheless, high concentrations of organic component can

lower adhesion and mechanical properties of the final coating.

In other words, although hybrid coatings potentially do exhibit

higher corrosion resistance than their inorganic or organic

counterparts, there is an optimum ratio for the components to

deliver maximum corrosion resistance. The optimum organic/

inorganic ratio (OOIR) varies depending on the system, as it

has been reported to be 0.7, 3 and 0.8 for GLYMO/Boehmite,

MTEOS/TEOS and VTMS/TEOS, respectively. OOIR can also

vary for a single system depending on the coating application

method. Metroke et al have reported 11% and 67% as optimum

values of [mol GLYMO/(mol GLYMO + mol TEOS)] for corrosion

protective GLYMO-TEOS hybrid coating applied using spray

and dip-coating methods, respectively [22,31]. Moreover, the

coating application method not only affects the OOIR but also

the maximum achievable thickness of the hybrid sol-gel coatings

[21,22,32,33].

2.1.2. Curing temperatureThe drying/curing of sol-gel coatings is an important stage in

the sol-gel process which dramatically affects the coating’s final

properties. The measurement of linear shrinkage and weight

loss of sol-gel derived materials as a function of temperature

provides valuable information on the effect of heat-treatment.

Plotting the measured parameters versus temperature gives

rise to a curve with three distinct regions (Figure 3), as shown

(I) and (III) hybrids. The organic and inorganic counterparts of

such systems are normally connected to each other through

physical bonds. Despite the presence of weak dispersion forces

and van der Waals interactions between organic and inorganic

components, these hybrid coatings are not stable enough to bear

weathering. The formation of strong chemical bonds between

organic and inorganic components can significantly improve

barrier properties of hybrid coatings. This goal can be achieved

through end-capping of oligomers/polymers with functional

groups capable of reacting with inorganic network (Type II), or

through application of organically modified metal/metalloid

alkoxides with general formula of R’x-M(OR)n-x as starting

material or end-capping groups in sol-gel process (Types IV and

V). R’ can be either a non-functional group such as methyl, ethyl

(Type V) or a functional group such as epoxy, vinyl, methacrylic,

etc. (Type IV), which can undergo further polymerization. Hybrid

sol-gel coatings containing functional groups outperform pure

sol-gel or polymeric coatings due to higher cross-linking density

and better mechanical properties, respectively [6,18,23-25].

Different functional groups impart different protective

properties to hybrid coatings as illustrated by Liu et al [24], where

electrochemical properties of several modified tetraethoxy

silane (TEOS) sol-gel coatings were compared. In their work

three different precursors i.e. vinyltrimethoxysilane (VTMS),

[3-(methacryloxy)propyl] trimethoxysilane (MAPTMS) and

(3-glycidoxyproyl)trimethoxysilane (GLYMO) were used to modify

TEOS coatings. The results revealed that corrosion resistance of

the coatings decreases in the order VTMS>MAPMS>GLYMO.

However, GLYMO has attracted considerably more interest than

the other (aforementioned) organically modified precursors.

The anti-corrosion properties of GLYMO-derived hybrid

sol-gel coatings dramatically depend on absence or presence

of a curing agent and its chemical composition. According to

Metroke’s work, the incorporation of primary aliphatic amines

as cross-linking agents into GLYMO-trimethoxy silane (TMOS)

coating formulation not only reduced the curing time of the

deposited film from almost 7 days to approximately 30 min,

but also led to formation of hard hybrid films, which were highly

adherent to aluminum alloys [26]. Croes et al [27] have used

three different isomers of phenylenediamine (PDA) to show

the superior corrosion protection properties of GLYMO-TMOS

hybrid coating cross-linked, using an aromatic amine rather than

aliphatic ones. This can be attributed to oxidative polymerization

of PDA cross-linking agents to form electro-active oligomers and

polymers of PDA that are structurally similar to polyaniline.

Khramov et al [28] have studied the effect of different

curing agents on the electrochemical properties of GLYMO-

TMOS coatings using different electrochemical techniques.

The obtained results showed that hybrid coatings cured using

di- and tri-amino-alkoxy silanes outperformed those cured by

di-ethylene-tri-amine (DETA), as amino-silanes have the ability to

react with epoxy hybrid coatings through both amino- and alkoxy-

functionalities. In another work Roussi et al [29] have shown

the undeniable effect of cross-linking density on the corrosion

resistance of GLYMO-TEOS hybrid coatings by using a hyper- Figure 3. Stages of drying process [adapted from 9].

Routes to extrinsic and intrinsic self-healing corrosion protective sol-gel coatings: a review

5

has been implemented by the incorporation of corrosion

inhibitors into sol-gel coatings. The role of corrosion inhibitors

is to be released when needed to protect exposed areas of the

underlying metal from the surrounding corrosive media. The

protection conferred by corrosion inhibitors occurs by chemical

or physical interactions of inhibitors with the metallic surface,

forming dense but thin inorganic layers or thin layers of adsorbed

organic molecules that reduce the corrosion rate (i.e. adsorption

and chemisorption mechanisms between inhibitor and metallic

surface).

Sol-gel coatings containing corrosion inhibitors can be

described as an extrinsic self-healing coating approach [13,15,16]

due to the restoration (i.e. healing) of the original corrosion

protection of the underlying metal after damage has occurred.

In this classification, although not described in this paper, sol-

gel coatings containing micro encapsulated liquid healing agents

would also be included. The use of encapsulated healing agents

in epoxy organic coatings has already been explored and it is

only a matter of time until the concept is implemented in sol-gel

coatings [42,43].

In this kind of systems, the active agent (i.e. corrosion

inhibitor) can be added to the sol-gel coating formulation either

(i) directly into the sol-gel matrix, as traditionally has been done

with protective coatings and will be discussed in section 3.1.1,

or (ii) immobilized in a carrier to reduce interactions with the

matrix and to control the release of inhibitor. Although both

approaches have advantages and disadvantages, the second

approach is the one that is leading to major developments, as

will be explained in section 3.1.2.

3.1.1. Direct addition of inhibitor The most common way of incorporating corrosion inhibitors into

sol-gel systems is simple mixing with the coating formulation.

The most important factor to be taken into account in such

systems is the solubility of inhibitor in the corrosive media. On

the one hand, low solubility of inhibitor can lead to weak self-

healing effect due to low concentration of active agents at the

damaged site. On the other hand, high solubility of corrosion

inhibitors in aggressive media will limit prolonged healing effect

as a result of rapid leach out of active agents from the coating.

Furthermore, the high inhibitor solubility can lead to coating

degradation by blistering and delamination processes due to

increased osmotic pressure, which can force water ingress into

coating/substrate interface. Despite the potential drawback of

this class of extrinsic self-healing sol-gel coatings, they have

been widely studied due to easiness of preparation and are used

for protection of different metallic substrates. The corrosion

inhibitors under use can be divided according to their intrinsic

nature into (i) inorganic and (ii) organic inhibitors. Both inorganic

and organic inhibitors have been added either directly to the sol-

gel coating or in carriers, leading to different results as will be

discussed here on. Hybrid organic-inorganic inhibitors are not

further discussed in this section as they have not been explored

yet in sol-gel coatings, in spite of having been successfully

introduced into organic protective coatings [44].

elsewhere for high temperature curing of sol-gels [9]. Region (I) (T

< 200°C) is associated with a sharp decrease in material weight

due to solvent evaporation/desorption. Region (II) shows a linear

shrinkage and weight loss at intermediate temperatures (200-

500°C), which can be attributed to further condensation reactions

and decomposition of organic components, respectively. Finally,

region (III) shows the collapse of pores formed as a result of

solvent evaporation and loss of organic compounds leading to

fast shrinkage. Therefore, an increase of the curing temperature

up to 500-700°C can result in a less corrosion resistant sol-gel

coating, unless the heating rate is small enough to avoid cracks

formation [6,12].

Also, sol-gel coatings can be cured at or near room

temperature. Low temperature drying is normally employed

for curing of hybrid sol-gel coatings entrapping organic

compounds [6,12]. Although compact crack-free films can be

obtained depending on the film composition and thickness, Du

et al have shown that room temperature cured sol-gel coatings

incorporated with different functional groups are water sensitive

in comparison to the ones cured at 80oC [34], which limits

their application as corrosion protective systems. Recently, a

relatively new approach has been proposed for curing of sol-

gel coatings using UV radiation, reporting a 2.5 fold increase

in corrosion resistance of sol-gel films treated by UV radiation

at room temperature, in comparison with conventional films

dried at 300°C [6]. This new methodology for curing of sol-gel

coatings further facilitates the implementation of this technology

on temperature sensitive substrates.

2.1.3. Micro/nanoparticlesIn order to improve mechanical and barrier properties of sol-gel

coatings, micro and nanoparticles can be added to the coating

formulation, facilitating preparation of thick and defect-free

coatings. The degree of improvement of physical/mechanical

properties is a function of particle size and shape. However,

the critical pigment volume concentration (CPVC), beyond

which coating physical/mechanical properties start to degrade,

must be taken into account. Still, achieving desired properties

even below the CPVC requires a strong interaction between

particle and matrix interfaces [18,22]. Sol-gel process can be

beneficially employed for surface functionalization of particles to

be added into coating formulations [35-41]. In addition, micro/

nanoparticles may either be added to sol-gel coatings or be

formed in-situ within hybrid coatings, eliminating the challenges

associated with CPVC and strong interfacial forces between

matrix and particles.

3. Active corrosion protection by sol-gel coatings

3.1. Extrinsic self-healing sol-gel coatings Although sol-gel coatings offer good passive protective

properties against corrosion of underlying metals, these systems

are prone to failure under many circumstances, with the water

ingress being just a matter of time. In order to improve the

protective properties of sol-gel coatings, active protection

M. Abdolah Zadeh et al.

6

of these coatings were not only affected by the presence of

ions but also by silane molecules. The resistance of Ce-doped

BTESPT coatings was more than two orders of magnitude higher

than that of unmodified ones, whereas in case of La-doped

coatings the difference was only about one order of magnitude.

For BTSE films, the resistance values were closer, being slightly

lower than those for Ce-containing films [47].

Lian-Kui Wu et al [48] have employed zinc to impart active

protection properties into BTSE sol-gel coatings on cold rolled

steel (CRS). In the mentioned coating, the silane film was

responsible for barrier protection of CRS while zinc provided

cathodic protection arising from its sacrificial dissolution.

However, although incorporation of zinc into the sol-gel coating

improved its corrosion performance, no healing was observed as

the resistance of the coating decreased over time.

The amount of active species to be added into a sol-gel

system is an important issue in modification of sol-gel coatings

using active ions that cannot be overlooked. On the one hand,

low concentration of active species can result in insufficient

inhibition of corrosion. On the other hand, high concentration

of the active species can induce formation of defects in the

sol-gel coatings, lowering their barrier properties. Trabelsi et

al [49] have studied the effect of Ce(III) ion concentration on

the electrochemical properties of galvanized steel coated with

Ce(III)-modified BTESPT. They have reported 1×10-3 M (Figure 5)

as the optimum value for Ce(III) ion concentration in solution.

In another work carried out by Garcia-Heras et al [50] based

on the study of structure and properties of Ce-doped TMOS-

MAPTS coatings on zinc plates, the optimum concentration of

the Ce(III) ions was found to be in the range of 0.2–0.6 (wt.%).

Zandi Zand and co-workers [51] have evaluated the corrosion

protection of GLYMO- coated AISI 304L in a marine environment.

PP and EIS tests showed a significant improvement in the

corrosion protection properties of the Ce(III) ion doped coatings,

in spite of the corrosion resistance of the coating being strongly

influenced by presence or absence of bisphenol A as cross-linker.

Moreover, the interest in corrosion inhibitors has moved

towards the development and use of environmentally friendly

ones that can replace chromate-based inhibitors. Accordingly,

new inhibitor-doped sol-gel coatings have been generally

developed using (new) environmentally friendly corrosion

inhibitors.

i) Inorganic inhibitorsActive ions with well-known corrosion inhibition ability such as

rare earth metal (REM) ions, e.g. cerium and lanthanum, have

attracted considerable interest as a replacement for toxic Cr(VI).

Although the inhibition mechanism of REM ions is not thoroughly

understood, the processes by which REM, and particularly Ce

ions, protect the substrate are generally accepted. Pirhady-

Tavandashti and co-workers [45] studied the healing effect of

Ce(III) ions on the protection properties of GYLMO-TEOS coated

AA2024 using potentiodynamic polarization (PP) test. The

restoration of coating protective properties was confirmed by a

sharp decrease in icorr after 168 h and a shift of EOCP towards more

positive values (Figure 4).

Trabelsi et al [46] have studied bis-[3-(triethoxysilyl)-propyl]

tetrasulfide (BTESPT) coatings modified with Ce(NO3)3 or Zr(NO3)3

on galvanized steel. Although incorporation of active ions into

coating formulation led to formation of thick and defect free

coatings on galvanized steel improving the barrier properties, the

coating performance depended dramatically on the incorporated

ion and NaCl concentration. At early stages of immersion, the Zr-

doped coatings provided very good barrier, but under prolonged

immersion conditions, they could not retard corrosion activity

as effectively as the Ce-doped coatings. Moreover, no active

protection was observed, so that after a week corrosion signs

were visible in all the samples.

In another work, the effect of addition of cerium and

lanthanum nitrates on the electrochemical properties of BTESPT

and BTSE coatings applied onto galvanized steel was studied.

The obtained results showed that the electrochemical properties

Figure 4. Comparison of corrosion behavior of the GLYMO-TEOS coatings after different immersion times in 3% NaCl solution using saturated calomel electrode (SCE) as reference [adapted from 45].

Figure 5. Coating resistance of Ce(III)-modified BTESPT after 150 h immersion in NaCl solution using SCE as reference [adapted from 50].

Routes to extrinsic and intrinsic self-healing corrosion protective sol-gel coatings: a review

7

improvement in corrosion resistance of bis-amino/VTMS and

BTESPT coated AA2024 samples by incorporation of Ce(III) ions.

The selective leaching of Ce(III) ions at scribes made manually on

coated samples was confirmed by the EDS technique. In another

work, Cabral et al [57] compared electrochemical impedance

spectroscopy (EIS) spectra of two sets of AA2024 samples

coated with Ce(III)-modified BTESPT and treated with Cr(VI),

reporting higher impedance of Ce-modified coatings, which was

twice that of Cr(VI) treated samples.

In another work, Lakshmi et al [58] have employed SEM

complemented by PP and EIS, to show the positive effect of

silica particles and Ce(NO3)3 on the preparation of thick and

defect-free methyltriethoxysilane films on AA2024. Although

the corrosion resistance was negatively influenced by the Ce

concentration in the coating, the active protection of coating

systems was confirmed by a considerable decrease in corrosion

current upon prolonged immersion in NaCl solution.

Comparison of corrosion protection properties of Ce(III)-,

VO3-, and MoO4- doped epoxy/zirconia sol-gel coatings on

AA2024 revealed that sol-gel coatings modified with NaVO3 and

Na2MoO4 failed to provide adequate corrosion protection due to

decrease in sol-gel network stability. However, Ce-doped sol-gel

coatings were as good as the undoped epoxy/zirconia films from

barrier protection point of view [59].

Although Ce ion can induce active protection mechanism

into sol-gel coatings, the final properties of the coating will be

significantly influenced by the salt used as the source for Ce ions.

While nitrate salts impart active corrosion protection to the sol-

gel coating, chloride and sulphate salts can lower the corrosion

resistance of the coating as a result of release of aggressive ions

(Cl-) and decrease of network stability [60,61].

Rosero-Navarro et al [62] developed an active sol-gel

coating by sandwiching a Ce(III)-doped layer between two

undoped layers. The coating formulation, abbreviated as TMEG,

consisted of TEOS, MAPTS, ethylene glycol dimethacrylate

(EGDMA) and glycidyl methacrylate (GMA). The comparison

of corrosion performance between the active system with a

passive one consisting of two layers of undoped sol-gel film

(Figure 6) revealed lower corrosion current densities and much

higher impedance values for TMEG/TMEG-5CeN/TMEG coated

AA2024. Although the impedance decreased with increase

of immersion time, the rate of decrease was much lower for

The positive effect of Ce(III) ions on the corrosion protection

of stainless steels was further investigated by Certhoux et al [52].

The thickness of TEOS- MAPTS coatings on AISI 430 was tuned

through addition of the polyethylene glycol (PEG 35000), used

as plasticizer. The corrosion protective properties of AISI 430

coated with the developed coating could be raised to reach that

of uncoated AISI 304L through controlled deposition and drying.

Also, martensitic steels can be effectively protected by

Ce(III) ion doped GLYMO- tri-sec-butoxide Al(OsBu)3 coatings.

Nevertheless, beyond an optimal Ce(III) concentration of 0.01

M the barrier and corrosion protective properties lowered, as

a result of a change in chemical structure of sol–gel network,

which was confirmed by 29Si NMR [53].

Despite the beneficial effects of Ce ions on corrosion

protection of sol-gel coatings, it should be noted that the

inhibition mechanism of Ce(III) and Ce(IV) ions is different, leading

to differences in their inhibition performance. Pepe et al [54]

have investigated the effect of addition of three and four valent

cerium ions on the structure and properties of TEOS-MTES

hybrid sol-gel coatings. SEM images of AISI 304 samples coated

with the prepared solutions revealed the formation of dense and

crack-free coatings on the substrates. Contrarily to homogenous

Ce(IV) doped coatings, in Ce(III) modified coatings some plate-

like inhomogeneities rich in Ce were observed. Electrochemical

studies disclosed that although coatings containing Ce(III)

ions enhanced the corrosion behavior via a barrier effect, they

experienced a slight weakening in time. In contrast, those

doped with Ce(IV) ions evidenced an improvement in the coating

performance over time. The authors attributed this to the plugging

of defective areas of the film by corrosion products (mainly

Ce(OH)3), which were probably produced through a chemical/

electrochemical mechanism involving a redox reaction between

Cr and Ce(IV) ions. In another work, Suegama and co-workers

[55] suggested a catalytic role for Ce(IV) ions in condensation

reactions of bis-[triethoxysilyl]ethane (BTSE) based on FT-IR

and 29Si NMR results. A higher degree of poly-condensation of

Ce(IV)-modified BTSE, together with the formation of uniform

and thicker coatings on carbon steel substrate enhanced the

barrier properties compared to unmodified silane film.

The beneficial effects of Ce ion on the corrosion behavior

of sol-gel coated Al alloys have been documented in various

studies. Palanivel et al [56] employed PP tests to show the

Figure 6. Structure scheme for the (A) non-inhibited and (B) inhibited systems deposited on AA2024 [Adapted from 62].

M. Abdolah Zadeh et al.

8

hydrophobic characteristic of its surfaces, and (5) precipitation

of a passive cerium oxide film on the steel. For coatings with

thickness of about 8.5 µm the useful lifetime of steel in a salt-

fog chamber extended from only ~10 h to ~768 h. In the case

of aluminum panels no corrosion signs were observed on the

samples even after 1440 h exposure to salt-fog [65].

Ce(III) ions have also been successfully used as a proper

inhibitor for magnesium alloys. Montemor et al incorporated

cerium and lanthanum ions into BTESPT sol-gel coatings. The

electrochemical properties of undoped, as well as La(III)- and

Ce(III)-doped BTESPT coatings, applied on AZ31 magnesium

alloy, were studied using EIS and SVET in 0.005 M NaCl solution.

The EIS spectra indicated higher coating resistance in both high-

and low-frequency ranges for La- and Ce(III)-doped coatings.

However, Ce(III)-doped coatings outperformed La-doped ones.

Further characterization of developed systems pre-immersed in

electrolyte for 24 h using SVET, revealed that La(III) was not a

proper inhibitor as it failed to stop corrosion on the scratched

zone due to high solubility of La(III) hydroxides (Figure 7). In Ce-

doped coatings no corrosion activity was detected after 24 h

immersion in electrolyte. In other words, incorporation of Ce

into the coating formulation improved the barrier properties of

the film as illustrated by the EIS, introducing active corrosion

inhibition properties in the films as demonstrated by SVET [66].

ii) Organic InhibitorsOrganic inhibitors which are usually designated as film forming

agents, limit corrosion by adsorption on the metal surface,

forming a hydrophobic film on it. Organic inhibitors prevent

corrosion by either increasing the anodic or cathodic polarization

resistance of the corrosion cell or retarding diffusion of corrosive

agents to the metallic surface. However, their inhibition efficiency

depends on the chemical composition, molecular structure, and

affinity for the metal surface.

Organic inhibitors have been successfully incorporated into

sol-gel systems to improve their corrosion protection properties

inhibitor-doped coating because, as a result of concentration

gradient, Ce(III) ions can migrate to metal surface and slow down

corrosion processes.

Also, Andreatta et al [63] developed active ZrO2 based pre-

treatments through multi-layer approach using both organic

(Zr(OBun)4) and inorganic (ZrO(NO3)2) precursors. In the case

of organic precursor, a layer with small pore size was applied

on AA2024 substrate, which was subsequently covered by

a layer with larger pores. The later layer was loaded with

inhibitor by immersion in a 0.3 M Ce(NO3)3 solution. In order

to entrap inhibitor in the sol-gel coating derived from inorganic

precursors, first the substrate was dipped into Ce(NO3)3 solution

and then two layers of ZrO2 were deposited on top of it. Both

of the above mentioned pre-treatments were then coated with

a layer of water-based epoxy and their adhesion and corrosion

performance evaluated. The obtained results showed that the

developed system using inorganic precursor outperformed the

one obtained using organic precursor on barrier protection.

In another work, L. Paussa et al [64] reported that the addition

of Ce(III) ions to ZrO2 sol-gel coatings derived from organic

precursors lowers the barrier properties of the sol-gel coatings

due to the increased viscosity of the sol, which ultimately resulted

in thicker coating with higher density of defects. The ZrO2-based

sol-gel coatings were not able to compensate the decrease in

coating resistance over time, unless loaded with Ce ions.

The beneficial effects of Ce ion on corrosion performance

of sol-gel coatings are not only affected by the ion source but

also by its concentration in the coating formulation. Sugama has

shown that incorporation of 0.2-0.3 wt% cerium acetate hydrate

into hybrid sol-gel coatings derived from aminopropylsilane-triol

(APST) led to formation of coatings with the highest thermal

resistance, hydrophobicity and corrosion resistance. The

improved corrosion resistance of these samples can be attributed

to (1) minimum content of water-soluble non-reacted APST

monomers and Ce-acetate dopant, (2) moderate densification

of siloxane linkages, (3) good adherence to steel surfaces, (4)

Figure 7. Evolution of the high and low frequency resistance for the silane pre-treated samples in 0.005M NaCl using SCE as reference [66].

Routes to extrinsic and intrinsic self-healing corrosion protective sol-gel coatings: a review

9

Similarly to inorganic inhibitors, there is an optimum

value for the amount of organic inhibitor to be added to

coating formulations [73]. Quinet et al incorporated different

concentrations of tetrachloro-p-benzoquinone (chloranil)

into a hybrid organosiloxane/zirconia sol-gel coating. EIS

measurements indicated that high content of chloranil led to

disorganization of the sol-gel matrix, which in turn caused a

decrease in the coating protective properties. Nonetheless,

the incorporation of lower concentrations of chloranil resulted

in homogeneous structures, thereby increasing the protective

properties of the sol-gel coatings [73].

On the other hand, Palanivel et al reported no self-healing for

bis-amino:VTMS organosilane coatings doped with tolyltriazole

and benzotriazole inhibitors, despite the known efficiency of

these inhibitors. Evaluation of the aforementioned coatings

using scratch cell test showed that, although organic inhibitors

improved the overall corrosion resistance of the AA2024-T3

substrate, no selective leaching of inhibitors was observed [56].

3.1.2. Indirect addition of inhibitorAlthough incorporation of corrosion inhibitors into sol-gel

coatings is a promising route to the development of active

corrosion protective sol-gel coatings, there are inevitable

drawbacks associated with the direct addition of active agents

to coating formulation. Firstly, it is quite difficult to control the

leaching of entrapped inhibitors, especially when they are poorly

soluble within the coating matrix. Secondly, inhibitors can

chemically interact with the coating matrix losing their own activity

and lowering barrier properties of the matrix. The mentioned

drawbacks of such systems have motivated researchers to think

of new ways of inhibitor introduction, enabling isolation of active

agents from coating components. This can be achieved either

by encapsulation of active species or complexation with other

chemicals.

i) Cyclodextrin-inhibitor complexesA quite simple approach for inhibitor entrapment/immobilization

is based on the complexation of organic molecules with

β-cyclodextrin. Cyclodextrins are cyclic oligosaccharides

that possess a unique molecular cup-shaped structure with a

hydrophilic exterior and a hydrophobic interior cavity. These

compounds are known complexation agents. They are capable

of forming inclusion complexes with various organic guest

molecules which fit within their cavities. Organic aromatic and

heterocyclic compounds are normally the main candidates

for the inclusion complexation reaction. Most of the known

organic inhibitors for various metallic substrates are heterocyclic

compounds. Therefore, cyclodextrins can be effectively used for

the immobilization of these species.

As mentioned in the previous section, Khramov et al

have shown how non-ionizable organic inhibitors such

2-mercaptobenzothiazole (MBT) and 2-mercaptobenzimidazole

(MBI) can efficiently reduce the corrosion rate of AA2024

alloy in aggressive environments. They also introduced the

aforementioned corrosion inhibitors as inclusion host/guest

by inducing active protection. In several cases, the release of

organic molecular species from hybrid sol-gel matrix is based

on the pH-triggered release mechanism [67]. One of the

advantageous features of pH-triggered desorption processes is

that they can provide an intelligent release of corrosion inhibitors

only in damaged areas, which normally experience local pH

changes originated from localized corrosion processes.

The beneficial effects of phosphonic acid on corrosion

resistance of sol-gel coated magnesium alloys have already

been shown by incorporation of phosphonic acid modified silane

precursors in the coating formulation [68,69]. Recently, Viviane

Dalmoro et al [70] reported an improvement in barrier properties of

TEOS-derived sol-gel coatings on AA2024 through incorporation

of an optimum amount of aminotrimethyllenephosphonic acid.

The increased resistance of the coating can be explained

by a strong chemical bonding of phosphonate groups to the

substrate.

Although organic inhibitors can improve the corrosion

resistance of sol-gel coatings, the degree of improvement will

change as a function of coating nature and possible interactions

between inhibitor and coating.

Sheffer and coworkers [32,33] studied the inhibition effect

revealed by introduction of phenylphosphonic acid into two

hybrid sol-gel matrixes with different hydrophobicity. The more

hydrophobic coatings based on phenyltrimethoxysilane (PTMOS)

showed the best corrosion protection, while the other one made

of TEOS provided only minor protection to the Al substrate. An

additional inhibition effect was observed for phenylphosphonic

acid doped sol-gel coatings containing phenyl groups. This can

be attributed to the entrapment of phenylphosphonate inside the

matrix due to π-π interactions.

In another work the effect of adding organic corrosion

inhibitors to epoxy-based hybrid sol-gel coatings (GLYMO-

TEOS), cross-linked by Diethylenetriamine (DETA) and a

hyperbranched Polyetherimine (PEI), was studied. PEI cross-

linked coatings, either with or without inhibitors, showed lower

corrosion current densities in comparison to the ones cross-

linked with DETA. As mentioned in previous sections, higher

cross-linking density, resulting from more reactive sites of PEI,

plays a role in raising corrosion resistance. Additionally, PEI

deposited on Al surfaces was only partially oxidized. Therefore,

it could undergo further oxidation upon application of oxidative

stresses and act as a corrosion inhibitor itself. Furthermore,

higher solubility of inhibitors in PEI, which led to more effective

incorporation of the corrosion inhibitor into the coating, improved

coating performance [29].

The nature of inhibitor itself also has a significant effect

on its corrosion efficiency. Khramov et al have studied the

electrochemical properties of AA2024 samples coated with

TMOS-GLYMO hybrid coatings incorporated with ionizable and

non-ionizable corrosion inhibitors. Current density distribution

maps acquired over scribed coatings revealed that ionizable

inhibitors show a significantly weaker effect than non-ionisable

ones due to the strong interaction of the former with the sol-gel

matrix, which lowers the inhibitor release rate [71,72].

M. Abdolah Zadeh et al.

10

Although particles can improve corrosion properties of

sol-gel coatings due to enhanced thickness and low crack

sensitivity, they are normally incapable of imparting self-

healing properties to the coatings. Still, they can play the role

of container and entrap corrosion inhibitors. Zheludkevich et al

[77,78] immobilized Ce(III) ions on the surface of zirconia nano-

particles through controlled hydrolysis of tetra-n-propyl zirconate

(TPOZ) in a Ce(NO3)3 containing aqueous solution. They have

reported an improvement in long-term corrosion performance

of GLYMO-TEOS hybrid coatings doped with inhibitor-loaded

ZrO2 nanoparticles, compared to coatings directly doped with

the same inhibitor, due to more stable structure of the sol-gel

coating. The increased stability can be explained by the fact that

oxide nanocarriers can prevent the negative effect of Ce(III) ions

on the hydrolytic stability of the hybrid sol-gel coating.

In another work, Tavandashti et al immobilized Ce(III) ions

on boehmite nanoparticles through hydrolysis of aluminium

isopropoxide in a solution containing cerium nitrate. The

evaluation of electrochemical properties of undoped GLYMO-

TEOS coatings, coatings doped with boehmite nanoparticles and

coatings doped with inhibitor-loaded boehmite nanoparticles,

revealed that the corrosion protection properties of hybrid sol-gel

coatings were significantly improved upon addition of boehmite

nanoparticles. Moreover, addition of Ce(III) ions to coating

formulation led to an enhancement of coating performance over

time, indicated by a decrease in the reduction rate of impedance

values [79].

Also, the inhibiting ions can be immobilized on the surface

of commercially available nanoparticles by immersion of the

so-called particles in an inhibitor-containing solution. Although

addition of ceria or silica nanoparticles in certain amounts

(100-250 ppm and 250 ppm for ceria and silica nanoparticles,

respectively) into coating formulation improved corrosion

resistance of BTESPT coated galvanized steel substrates,

it did not render self-healing ability to the coatings. However,

the activation of the aforementioned nanoparticles with Ce(III)

ions not only imparted active corrosion properties in the

coating but also reduced the agglomeration of nanoparticles

complexes with β-cyclodextrin into TMOS-GLYMO hybrid

coatings. The obtained PP curves  illustrated the reduction

in corrosion rate for both inhibitors. Comparison of EIS

measurements and optical micrographs of the samples revealed

that coatings containing β-cyclodextrin/inhibitor complexes

outperformed those containing inhibitor alone. Even after four

weeks of immersion in dilute Harrison’s solution, the scratch

remained shiny without any visible corrosion products in the

case of β-cyclodextrin/inhibitor-containing coatings, while

corrosion products covered the defect in hybrid coatings directly

doped with corrosion inhibitors. These observations can be

attributed to the complexation equilibrium which slows down

the inhibitor release rate. However, the complexation process

retards the immediate response to corrosion in contrast to

stimuli-responsive self-healing systems [71,72].

ii) Micro-nanocontainersThe first approaches proposed for modification of sol-gel

coatings consisted of addition of ceramic particles such as

alumina and silica to improve the mechanical properties of the

coatings. However, not only mechanical properties but also

corrosion resistance of aluminum alloys and carbon steel seemed

to increase by addition of a controlled amount of particles. As

reported by Palanivel et al, the addition of silica particles with

average diameter of 1 µm to BTESPT films, in the range of 5-15

ppm, increased corrosion protection properties of these films, as

a small amount of silica can react with cathodically generated

OH− ions, and form a passive Al-silicate compound on cathodic

areas of AA2024 surface [74]. Nevertheless, agglomeration of

the embedded particles, which can be facilitated by gelation

process, may lead to coating rupture and deterioration of the

coating barrier properties [75]. Further reduction in corrosion

resistance of bis-[trimethoxysilylpropyl]amine (BTSPA) coated

carbon steel has been reported for silica particles volume

concentration (PVC) > 300 ppm due to agglomeration of the

embedded silica particles [76]. In this case, even application of

a second layer could not cover the defected coating structure

resulting from the particles agglomeration.

Figure 8. 2D and 3D Schematics of β-cyclodextrin.

Routes to extrinsic and intrinsic self-healing corrosion protective sol-gel coatings: a review

11

loaded with MBT and vanadate, respectively. They reported

a considerable increase in coating performance as a result of

fast protection offered by stabilization of oxide layer with MBT,

followed by a long-term protection due to release of vanadate

anions from the LDH nanoparticles added to the primer [85].

Kartsonakis et al [89] have encapsulated MBT in cerium

molybdate containers. Incorporation of the MBT-loaded

containers into epoxy-silane coatings increased the corrosion

resistance of magnesium alloys ZK10 in long-term exposure to

0.5 M NaCl solution. Additionally, immersion of scribed coatings

in 1 mM NaCl solution for 73 h revealed partial self-healing due

to the increase of the charge transfer resistance.

Mekeridis and coworkers [90] showed a significant

improvement (three orders of magnitude) in corrosion resistance

of multilayer hybrid sol-gel coated AA2024 through incorporation

of 8-hydroxyquinoline-loaded TiO2 nanoparticles within coating

formulation. In spite of non-uniform coating thickness resulting

from uneven distribution of nano-particles in the coating, an

increase in the coating barrier properties was observed. In

addition, prolonged release of the inhibitor led to long term

corrosion protection of the AA2024-T3 substrate.

Borisova et al [91] have recently developed an active

protective sol-gel coating through incorporation of

1H-benzotriazole (BTA) loaded mesoporous silica nanoparticles

into SiOx/ZrOx matrix. High surface area (≈1000 m2.g-1) alongside

with large pore volume (≈1 mL.g-1) of mesoporous nanoparticles

led to high loading capacity of these nanocontainers (409 mg

BTA/1 g SiO2). Characterization of the developed system using

SVET in 0.1 M NaCl solution revealed a significant improvement

of its electrochemical properties compared to both un-doped

coatings and the one directly loaded with the same corrosion

inhibitor. Investigation of corrosion inhibitor release kinetics

by fluorimetrical analysis showed that the release process is

triggered by pH changes, with the highest release rate reported

for pH = 10, followed by pH 2. The proposed release mechanism is

based on the electrostatic repulsion between silica nanoparticles

and inhibitor molecules. According to the authors, at pH values

different from neutral, both silica particles and inhibitor molecules

due to stabilization of particle surface charge. Studies on

electrochemical properties of the prepared systems revealed

that, from the barrier properties point of view, coatings can be

ranked as CeO2+Ce(III) ions > SiO2+Ce(III) ions ~ SiO2 ~ CeO2>

unmodified silane coating. This observation can be explained as

follows: upon formation of a defect in the coating, the underlying

metal will be exposed to oxidants such as O2, and occurring

reduction reactions which mainly release OH- will lead to an

increase of local pH to values as high as 11. This high pH can

decompose and delaminate the silane-based film. Similarly to

the silane-based film, silica nanoparticles are susceptible to

alkaline decomposition but ceria nanoparticles are quite stable

under alkaline conditions, so during decomposition of silane film

they will precipitate with zinc corrosion products on the surface,

reinforcing the protective properties of the corrosion products

[80,81].

Motte and co-workers [82] introduced another approach

for immobilization of Ce(III) ion, not only on the surface but also

inside montmorillonite clay (MMT) through cation exchange.

Although Na-MMT clay particles improved the barrier properties

of GLYMO-TEOS-MTEOS hybrid coating more effectively than

Ce(III)-MMT particles, selective leaching of Ce(III) ions, which

might reinforce the formed oxide layer, provided a long term

protection for the galvanized steel substrate.

Snihirova et al [83] loaded Ce(III) ions in pH-sensitive micron-

sized CaCO3 carriers. Upon exposure to acidic environment the

carriers dissolve, releasing the corrosion inhibitor, which can

suppresses the corrosion activity of the bare metal.

Poznyak et al developed Zn-Al Layered double hydroxides

(LDHs) which can entrap several anionic inhibitors such as

vanadate, phosphate, and MBT via anion-exchange [84-88]. In

contrast to cation-exchange hosts, LHDs not only can release

the inhibitor by anion-exchange trigger, but also can entrap

aggressive anions such as chloride ions. Tedim and co-workers

studied electrochemical properties of a three-layer coating

system for AA2024-T3, consisting of hybrid sol-gel, a water-

based epoxy primer and a water-based epoxy topcoat, in which

the sol-gel and primer layers were doped with 10 wt% of LDHs

Figure 9. (a) SEM and (b) TEM micrographs of mesoporous silica nanoparticles [92].

M. Abdolah Zadeh et al.

12

SiO2 sol-gel coatings [94-96]. Halloysites are two-layered

aluminosilicate nanotubes with internal diameter, external

diameter and length of 15, 50 and 300-800 nm, respectively.

Halloysites can be employed as an entrapment system or

a reservoir for loading, storage, and controlled release of

corrosion inhibitors and biocides. Fundamental research is

being undertaken to enable control over release rates by

variation of internal fluidic properties, formation of nanoshells

over the nanotubes and by creation of smart caps at tube ends.

The LbL self-assembly method was successfully employed for

development of multi-layer polyelectrolyte nanoshells on the

surface of halloysite nanotubes as well as smart end-caps [97].

Anodic current density maps of undoped and inhibitor-loaded

halloysites-doped sol-gel coatings (Figure 10) indicated superior

corrosion resistance for coatings doped with inhibitor-loaded

nanotubes in comparison to undoped ZrO2/SiO2 films. The self-

healing mechanism is analogous to previous systems.

In another work, Ekaterina et al [98] have developed another

self-healing system with high corrosion inhibitor loading capacity

incorporating 2-(benzothiazol-2-ylsulfanyl)-succinic acid-loaded

mesoporous silica nanoparticles in SiOx-ZrOx sol-gel coatings. In

order to prevent the spontaneous release of corrosion inhibitor,

the inhibitor loaded nanoparticles were coated with four

subsequent layers of PEI/PSS/PEI/PSS. In analogy to the two

systems described above, the electrochemical characterization

of AA2024 coupons coated with the developed system using

SVET revealed enhanced corrosion resistance and healing

performance for inhibitor doped coatings in comparison to

undoped sol-gel coatings.

Lamaka et al [99] have presented another approach for

inhibitor entrapment. They used a porous layer of TiOx obtained

by template-based synthesis as a nanostructured reservoir

with high loading capacity for the organic corrosion inhibitor.

The nanoporous TiOx layer was obtained through controlled

hydrolysis of tetra isopropyl orthotitanate in the presence of the

nonionic block copolymer Pluronic F127. During immersion of

AA2024 samples in the prepared solution titania nanoparticles

were self-assembled forming a cellular network that replicated

the surface structure of the etched alloy. Then, titania coated

samples were loaded with inhibitor by immersion in an alcoholic

solution of n-benzotriazole and finally coated with SiO2/

ZrO2hybrid so-gel coating.

Electrochemical characterization of prepared samples

using EIS revealed a well-defined self-healing ability leading

to effective long-term active corrosion protection. In other

words, after about 100 h of immersion in 0.05 M NaCl, there

was only a slight decrease of the Al2O3 resistance. Although the

first breakdown of the oxide film started only after 330 h, the

resistance increased again, almost immediately achieving the

values before breakdown. Breakdown of the oxide layer occurred

several times with consequent healing revealing development of

a system capable of multiple healing. The obtained results have

been confirmed by SVET measurements [99].

Alvarez et al [100] have developed hybrid TPOZ-GLYMO

sol-gel coatings incorporated with different concentrations of

had the same charge (positive at pH < 6 and negative at pH >

6). Not only BTA but also 2-mercaptobenzothiazole (MBT) can

be effectively loaded into mesoporous silica nanoparticles (1.42

gMBT/1 g SiO2). However, as mentioned earlier, the optimum

barrier properties of sol-gel coatings can be achieved at

certain dosage of embedded nanoparticles due to compromise

between delivering sufficient corrosion inhibitor and preserving

the coating barrier properties. The highest passive and active

corrosion protection of modified SiOx/ZrOx sol-gel coating was

achieved with 0.7 wt% MBT-loaded silica nanoparticles [92].

iii) Entrapment of inhibitors in nanocontainers in a LBL configuration

The application of Layer-by-Layer (LbL) assembled shells on

the surface of micro/nanocontainers facilitates development of

containers with regulated storage/release of the inhibitor. The LbL

method is based on the adsorption of oppositely charged layers

on the surface of a template material. Entrapment of corrosion

inhibitors in polyelectrolyte multi-layer systems via LbL method

has several advantages: it will isolate the inhibitor avoiding its

negative effect on the integrity of the coating; additionally, such

a system can provide an intelligent release of the corrosion

inhibitor, as permeability of the polyelectrolyte assemblies can

be regulated by changes in pH and humidity. The change of

pH together with ion-exchange are among the most preferable

stimulus to initiate the release of corrosion inhibitors [93].

Zheludkevich et al [19] used silica nanoparticles with

average diameter of 70 nm as template for entrapment of MBT

inhibitor through LbL method. Negatively charged SiO2 particles

were coated with successive layers of poly(ethyleneimine) (PEI) /

poly(styrene sulfonate) (PSS)/MBT/PSS/MBT. MBT content in the

final SiO2-based nanoreservoirs was estimated to be about 95

mg per gram of SiO2 nanoparticles. The incorporation of inhibitor-

loaded nanoparticles in hybrid epoxy-functionalized ZrO2/SiO2

sol-gel coatings improved corrosion protection properties of

the coating over both undoped sol-gel film and a film doped

directly with benzotriazole. The electrochemical characterization

of nanocontainer-loaded coatings using SVET revealed that

even though the cathodic corrosion activity appeared over the

manually made defect, it was deactivated after 2 hours. This was

not the case for the sol-gel coating doped directly with the same

inhibitor. According to the authors, local corrosion processes will

induce pH changes in the neighboring area. The pH changes

can increase the permeability of polyelectrolyte shells, with the

consequent release of benzotriazole suppressing the corrosion

activity. Eventually, the pH value will recover closing the

polyelectrolyte shell of nanocontainers and terminating further

release of the inhibitor.

Although inhibitor loaded nanocontainers coated with

polyelectrolytes provided quite promising results, still they

have a limited inhibitor loading capacity. Porous structures and

nanocontainers with higher aspect ratio are more promising

from inhibitor loading capacity point of view. Recently, a self-

healing system has been prepared by incorporation of MBT-

or 8-hydroxyquinoline-loaded halloysite nanotubes into ZrO2/

Routes to extrinsic and intrinsic self-healing corrosion protective sol-gel coatings: a review

13

3.2. Intrinsic self-healing sol-gel coatingsIntrinsic self-healing coatings not requiring the addition of discrete

healing agents, but relying on a suitable generic modification

of the polymer architecture to achieve complete healing even

in the case of multiple damage events at specific locations, are

ultimately the most promising type of self-healing coatings even if

they need some controlled supply of an external stimulus. Unlike

extrinsic self-healing systems, intrinsic self-healing approaches

rely on local temporary mobility of the network (re-flow) leading

to damage closure as shown schematically in Figure 11.

Intrinsic self-healing polymers (potentially applicable to

coatings technology) developed to date are mainly based on

shape-memory effect in polymeric materials, reversible physical

bonds such as hydrogen bonds or reversible chemical bonds

such as Diels-Alder groups and sulphur chemistry [15]. The

versatility of sulphur chemistry, alongside with relatively low bond

strength of S-S bridge, make it a suitable candidate to induce

hydrotalcite. Pull-off test results indicated a significant increase

in adhesion for low concentrations of hydrotalcite. According

to the authors, at low dosages the hydrotalcite particles were

preferably deposited on substrate surface increasing adhesion

through formation of hydrogen bonds between their hydroxyl

groups and metal hydroxide. But at higher concentrations, the

fraction of particles inside the sol-gel coating increased more

than at the interface. In contrast to the adhesion performance,

EIS results revealed higher impedance values in the low

frequency range for coatings loaded with the highest amount

(10%) of hydrotalcite. The effect is more pronounced over

time. Although the impedance values for all the other systems

showed a decrease with increase in immersion time, those

corresponding to the coating loaded with 10% hydrotalcite

remained unchanged. This can be attributed to the possible

uptake of aggressive chloride ions by hydrotalcite due its anion-

exchange properties.

Figure 10. Current density observations of samples immersed for 0, 4.5, and 10 h in aqueous 0.1 M NaCl. (top) sol-gel coated AA2024, (middle) AA2024 coated with sol–gel layer doped with benzotriazole-loaded halloysite nanotubes and (bottom) AA2024 coated with sol–gel layer doped with hydroxyquinoline -loaded halloysite nanotubes [adopted from [94-96].

M. Abdolah Zadeh et al.

14

by direct or indirect (i.e. via carriers) incorporation of active

species such as corrosion inhibitors into the coating formulation.

Although the beneficial effects of inhibitor addition has been

confirmed by various researchers, the direct addition of these

compounds can lead to network instability and loss of inhibitor

efficiency, as a result of chemical interaction with the sol-

gel network. These challenges have been overcome through

immobilization of corrosion inhibitors into micro/nanocontainers

or via complexation with cyclodextrins. The inhibitor release

rate can be effectively controlled either through application of

polyelectrolyte multi-layers on containers or ion exchange in the

case of ionic species. Although active corrosion inhibitors lead

to sufficient temporary protection of the underlying metal, in the

case of local damage of the protective coating, to reach an even

more extended lifetime protection or to have repeatable healing

at a specific location, an additional functionality of damage

closure is required. As highlighted in this review, the development

of a combined extrinsic-intrinsic self-healing sol-gel coating is a

feasible concept leading to a multifunctional coating that could

significantly extend the service lifetime of protected metals.

multiple healing properties in a desired system, facilitating the

development of various self-healing polymeric systems in which

different stimuli such as elevated temperatures [101,102], shear

forces [103], reduction reactions [104-106] and UV-irradiation

[107] can trigger healing phenomenon.

While for other types of coatings early versions of intrinsically

self-healing variants have been reported, no intrinsic self-healing

sol-gel coating has been yet reported in literature. In order to fill

this lack in self-healing sol-gel coatings, recently we presented a

temperature-pressure triggered intrinsic self-healing hybrid sol-

gel system capable of healing at moderate temperatures [108].

Our newly developed intrinsic self-healing hybrid sol-gel

systems consists of diglycidyl ether of bisphenol A (DGEBA)

epoxy resin and organically modified alkoxy silanes (OMAS),

aiming at implementing sulphur bonds through sol-gel process

into a rigid cross-linked network [86]. This sol-gel derived hybrid

system showed the possibility of filling a gap with an approximate

width of 400 µm in relatively short times (below 15 min) at

moderate temperatures (60-90°C) and pressures. The healing

kinetics significantly depended on the chemical composition,

processing conditions, pressure and healing temperature. The

healing mechanism is believed to be based on the local mobility

of the network upon temperature trigger. The increased mobility

of the system can be attributed to temporary breaking of di-

sulphide bonds with ulterior bond restoration after the trigger

has been removed, process known as reshuffling of di-sulphide

bonds. Such temperature-pressure triggered intrinsic healing

sol-gel systems provide a valuable first step to fully autonomous

self-healing sol-gel coatings which, combined with extrinsic

healing approaches, should lead to substantial lifetime extension

of coated materials.

4. Conclusions

Sol-gel coatings have lately attracted considerable interest as

pre-treatment and corrosion protective coatings due to their

advantageous features such as low processing temperature,

high chemical versatility, easiness of application and

environmentally friendly nature of the process. Extrinsic self-

healing corrosion protective sol-gel coatings can be prepared

Figure 11. Schematic representation of an intrinsic self-healing system.

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