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Electrochimica Acta 50 (2005) 50835089

Study on a hydrophobic nano-TiO2coating and its properties for

corrosion protection of metals

G.X. Shen a, Y.C. Chen a, L. Lin a, C.J. Lin a,, D. Scantlebury b

a State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen University, Xiamen 361005, Chinab Corrosion and Protection Center, UMIST, P.O. Box 88, Manchester M60 1OD, UK

Received 22 November 2004; received in revised form 1 April 2005; accepted 1 April 2005

Available online 11 July 2005

Abstract

A uniform TiO2 nanoparticle film has been coated on the surface of 316L stainless steel by using solgel and dip-coating technology.

A hydrothermal post-treatment method has been developed to eliminate the crack defects in the coatings, and to improve the structure and

property for the coating. A self-assembly of fluoroalkylsiane (FAS-13) has been conducted to enhance the surface hydrophobic property of

the nano-TiO2 coatings. The distribution of particle sizes of TiO2 sol has been analyzed by -potential analysis, and the surface morphology

and structure have been characterized by contract angle, XRD, and SEM measurements. The results indicate that the surface of coatings is

uniform and dense, with approximately 375 nm thickness. The diameter of particles of TiO2 anatase is in the range of 1518 nm. The contact

angle of the super-hydrophobic surface is 150 1. It shows, from the electrochemical tests, that the super-hydrophobic coatings on 316L

stainless steel exhibit an excellent corrosion resistance in chloride containing solution at the room temperature.

2005 Elsevier Ltd. All rights reserved.

Keywords: Hydrophobic coatings nano-TiO2; 316L stainless steel; Corrosion resistance

1. Introduction

Various surface techniques have been developed to

improve the corrosion resistance of metals. One of the most

effective methods is to deposit a protective ceramic coating

on the metal surface, e.g., nitrides, carbides, silicides or tran-

sition metal oxides [1]. The coatings with very low electronic

conductance, such as SiO2 [2], TiO2 [3], non-conducting

coatings Al2O3[4]or mix-oxides coating of TiO2, SiO2, and

Al2O3 [5] have been reported as the corrosion protections

in the literatures. Up to now, there are two kinds of meth-ods for preparing the coatings on metal surfaces, viz. from

gaseous phased (PVD[6] and CVD[7]) methods and from

the liquid phase (solgel method[8] and electrodeposition

process[9]) methods. And most of the ceramic coatings are

formed by solgel method, because the solgel techniques

require considerably less equipment, can be amenable to any

Corresponding author.

E-mail address:[email protected] (C.J. Lin).

accessories in a large and complex shape, and are appli-

cable to substrates that cannot withstand high temperature

[10].However, the coatings fabricated by solgel process are

encountered a technical problem, that is there always exists

pores and cracks in the coatings resulting from the sintering

in a high temperature, which may become a detrimental fac-

tor to cause delamination and corrosion at the interface of

coating and substrates[11].

Since the discovery of photo-electrochemical property of

TiO2 films, it has been noted that the TiO2 film, as a pho-

toanode, might provide metals cathodic protection underillumination[12]. It is believed that the TiO2 coating as a

ceramic coating might provide a good corrosion protective

property for the metals [13]. An attempt has been made in

this work to deposit a nano-TiO2composite coatings on 316L

stainless steel to enhance the barrier corrosion resistance

and hydrophobicity. The investigation on the modification

of nano-TiO2 composite coatings on metal for enhancing

and lasting its photogenerated cathodic protection will be

reported otherwhere.

http://www.paper.edu.cn


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5084 G.X. Shen et al. / Electrochimica Acta 50 (2005) 50835089

2. Experimental methods

2.1. Preparation of TiO2colloidal solution

In our experiments, all regents were purchased from Chi-

nese Chemical Regent Co.Using EAcAc as a chelating agent,

the TiO2 sol was prepared from tetra-n-butyl titanate (Ti(O-n-Bu)4) in the following process: firstly, 20 ml ethanol and

1 ml EAcAc without further dehydration were mixed, 2 ml

Ti(O-n-Bu)4 were added and the solution was continuously

stirred for 1 h. Within 30 min, 0.2 ml distilled water was care-

fully added to the solution for hydrolysis and kept stirring for

10 h. Theyellow transparent solution was aged for 24 h before

conducting coating on metal surface.

2.2. Fabrication of the hydrophobic nano-TiO2coatings

In the preparation of a uniform nano coating, the pre-

treatment of the substrate surfaces is important. The 316L

stainless steel sheet (0.5 cm 0.5 cm) were ground with

No. 3201500 emery papers gradually, polished with 1 and

0.3m Al2O3powder, and then ultrasonically cleaned in ace-

tone, ethanol, and distilled water for 10 min, respectively.

After hot air drying, it was coated with nanoparticle TiO2coatings by a dip-coating method. In this procedure, the

substrate was immersed in the sol solution for 5 min, and

withdrawn at a speed of 0.51 mm/s. After drying naturally

in the ambience, the specimens were heated in an oven at

150 C for 30 min. Such an operation as mentioned above

was repeated for four times to increase the coating thickness.

Then, the samples were heat-treated at 450 C for 30 min to

enable the oxide conversion and removal of solvent and resid-ual organics in the coating. However, during the procedure of

densification, crystallization of TiO2and removal of residual

hydroxyl and organic groups, the nano coatings were usually

prone to cracking. In order to eliminate the creak defects in

the coating and to optimize the coating structure and prop-

erties, the nano-TiO2 coated samples were immersed in a

boiling water for 1050 min, and then thermal-treated in the

muffle oven again at 450 C for 10 min.

FAS-13 was used as a water-repellent agent. A methanol

solution of 1% FAS-13 was hydrolyzed by adding a three-

fold-molar excess of water at room temperature and then kept

stirring for 3 h. Thenano-TiO2coatingswere immersed in the

solution for 3 h and dried at 140C for 1 h[14].

2.3. Electrochemical tests

The polarization curves and electrochemical impendence

spectroscopy (EIS) for the prepared coatings were carried out

in an oxygen-saturated Ringers solution (NaCl 8.6 g/dm3,

KCl 0.3 g/dm3, and CaCl2 0.48 g/dm3) by using Autolab

PGSTAT30 Electrochemical Measurement System. At the

same time, the effects of the hydrophobic post-treatment

on corrosion resistance of the nano-TiO2 coatings were

investigated. A three-electrode cell including a saturated

calomel reference electrode (SCE), a platinum auxiliary elec-

trode, and the samples as working electrodes was used in

the experiments. The working electrodes (samples) were

enveloped by epoxy resin andonly remained an exposurearea

(0.5 cm 0.5 cm) for testing. The Tafel polarization curves

were measured between15 mV at the open circuit potential

at the rate of 0.167 mV/s and started after 10 min immersionof samples in the solution. The EIS were performed at open

circuit potential. The applied frequencies were ranged from

105 to103 Hz using five points/decade. The impedance data

were analyzed by Autolab analysis systems[15].All experi-

ments were executed in a Faraday cage in order to minimize

the external electronic interference with the system at the

room temperature.

2.4. Surface composition analysis by XPS

The nano-TiO2 coating surface before and after being

immerged in Ringers solution for 1008 h was analyzed

by XPS (Physical Electrons Quantum 2000 Scanning Esca

Microprob). The X-ray in XPS was generated at 15 kV and

10 mA, using Al K radiation. The air pressure in the vac-

uum chamber was lower than 5 107 Pa and the analysis

area was 2 mm 1 mm. The depth profile were analyzed by

Ar+ sputter etching the thin film 200 and 400 nm using 5 keV

at a beam current 2040 mA. The XPS were corrected for

charge shifting by taking C 1s at 285.0 eV.

3. Results and discussion

3.1. Formation mechanism of the sol

In this work, ethyl acetoacetic (EAcAc) was used as a

catalyst and chelating agent. The form mechanism of ultra

fine particles of TiO2sol may be explained as the followings:

Firstly, the titanium alkoxide species react with EAcAc pro-

ducing hex-cyclic molecules, then the hex-cyclic molecules

form tri-dimensional network macromolecular in a polymer-

ization process. When adding H2O into the solution, the

hydrolysis rate of the macromolecules is slower than that of

titanium alkoxide, which may control effectively size parti-

cles of the sol. Besides, because of EAcAc partly hydrolysis,

on the surface of the sol particles hold the same property

charges, whichmay reduce aggregation of the particles.Fig. 1

shows a distribution of the sol particle diameter measured

by -potential analysis. The diameter of ultra fine particles

is mostly 1 nm in scale and no aggregation phenomenon is

observed. And the sol is stable and transparent and can be

kept for a long period.

3.2. Influence of hydrothermal post-treatment on

properties of nano-TiO2 coatings

Fig. 2 shows the SEM images of nano-TiO2 coatings

before (a and c) and after (b and d) hydrothermal post-



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G.X. Shen et al. / Electrochimica Acta 50 (2005) 50835089 5085

Fig. 1. Particle diameter distribution of the TiO2 sol.

treatment, indicating that there are some crack defects on

the coating surface due to the shrinkage occurring during the

thermal process. Though, these crack defects appear little

influence on the mechanical and adhesive properties of the

coatings, as a protective coating for the metals, these crack

defects may become a potential risk to cause local corrosion

of metals. In order to eliminate the cracks or other defects,

various methods, such as PEG[16],polyvinyl alcohol (PVA)

[17], and hydroxypropyl-cellulose (HPC) [18], etc. have

been reported to add into the colloidal solution to improve

the drying properties of the gel, adjust the viscosity of the

sol, and increase the strength of the material to prevent

crack formation [19]. However, those methods have made

a little progress in prevention from cracks defects in the

coatings, and the procedures are considerably complicated.

Comparing the SEM images in Fig. 2, it shows that after

being treated in boiling water for 10 min, the surfaceof coatings become smooth and uniform and the cracks

disappear, and the aggregation becomes less and diameter of

particles tends to uniform. The hydrothermal post-treatment

method developed in this work may not only eliminate the

cracks or other defects in the coatings successfully, but also

make the structure and morphology of the coatings more

perfect.

The influence of disposed time for the TiO2 coatings

exposed in boiling water on their morphology, hydrophobic

properties, and particle size of was deliberatively investi-

gated, and the results were listed inTable 1. It is interesting

to note that, as shown inTable 1, the coatings kept in boiling

water for 10 min appear free of crack defects, and its parti-

cles and pores are ranged in 1518 and 48 nm, respectively.

However, when the immersion time increases to 20 min, the

aggregation phenomenon of particles is observed and the

surface becomes coarse. And then if the immersion time

is longer than 30 min, the morphology and size of particles

becomeuniform again,similar to that of thesample treated for

Fig. 2. SEM images of the nano-TiO2 coated 316L before hydrothermal post-treatment (a and c) after post-treatment (b and d).



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Table 1

Influence of the immersion time in boiling water on morphology, size, and hydrophobic property of the titania coatings

No. Time (min) Configuration Diameter (nm) Thick (nm) Contact angle ()

1 0 Porous, aggregation, cracks 3021 375 41.5

2 10 Porous, evenly, no cracks 1518 375 104.4

3 20 Porous, aggregation, no cracks 3918 375 78.5

4 30 Porous, evenly, no cracks 1821 375 90.5

5 40 Porous, aggregation, no cracks 1825 372 64.86 50 Porous, evenly, no cracks 1821 370 78.5

10 min. Thestructure of thenano-TiO2coatings is varied with

the hydrothermal post-treatment time periodically for about

every 10 min. We believe that a process of the restructure

and recrystallization of titania might play an important role

in morphology, size and hydrophobic property of film dur-

ing the hydrothermal post-treatment process. The thickness

of the coatings does not change too much in the course of the

whole hydrothermal treatment, implying that the interaction

and bond strength between the coatings and the substratesare very firmly. Besides, the hydrophobic properties of the

coatings are closely associated with the morphology of the

nano coating, the contact angle of water on the fresh TiO2surface is nearly 0 1. It was found that the nano coatings

treated for 10 min were kept in dark for 2 months, the contact

angle of water increasedto 104.4 1. The mechanism of the

hydrophobic property of the nano-TiO2coatings is remained

unapprehended up to now. It could not be simply explained

by removing of the hydroxide radical and the contamination

of dusty on the coatings[20]. Tadanaga et al. studied on the

formation of super-water-repellent Al2O3 coatings, and pro-

posed that the hydrophobic property was resulted from the

nano-size particles and the existence of air in the pores on the

surface[21].

The XRD analytic results (inFig. 3) show that the strong

diffraction peaks were originated from (1 0 1), (0 0 4), (2 0 0),

(2 1 1), etc., which are the character peaks of anatase phase.

After conducting a self-assembly of FAS-13 on coatings, its

contact angle to water increased to 150 1, indicating that

the hydrophobic property of the coating is further strength-

ened.

Fig. 3. XRD spectrum of the TiO2 coatings.

3.3. Corrosion resistance of the hydrophobic coatings

According to the mechanism of corrosion protection of

metals, the hydrophobic coatings with low wettability are

possible to effectively prevent the water onto the substrate

surface, and exhibit an excellent corrosion resistance in the

wet environments. Fig. 4 shows the polarization curves of

bare 316L stainless steel, nano-TiO2 coated and FAS/nano-

TiO2 coated 316L stainless steel electrodes in Ringers solu-tion. The nano-TiO2 coated electrodes were placed in the

dark for more than 1 month before measurements, and all

the experiments were carried out in a dark room to avoid

from the photoelectrochemical influence. The nano-TiO2and

FAS/nano-TiO2coated 316L stainless steelelectrodes exhibit

relatively lower current densities,icorrdecreases two to three

order of magnitudes compared to thebare 316L stainlesssteel

electrode in the same condition. The anodic corrosion slope

of the FAS/nano-TiO2 coated electrode inclines to level and

current densities are clearly lower than that of the bare 316L

electrode. It is noted furthermore that the corrosion potential

of the nano-TiO2

and FAS/nano-TiO2

coated electrodes are

much noble than that of the 316L stainless steel, the corro-

sion potential shifting positively from0.21 to0.083 V and

0.04 V (versus SCE), respectively. The reason caused the

potentialpositive shift does notclear, which maybe explained

by the different effect of hydrophobicity of the coating. The

Fig. 4. Polarization curves for bare 316L and films in oxygen-saturated

Ringersolution.(a) Bare 316L; (b)TiO2/316L coatings; (c) FSA/TiO2/316L

coatings.



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Fig. 5. EIS spectra of the bare 316L and coated 316L electrodes in ringer solution, at the open circuit potential, the frequency range is 105103 Hz. (a) Bare

316L; (b) TiO2 coated 316L; (c) FSA/TiO2 coated 316L.

hydrophobicity of the porous coatings attributed to air trapped

in the nanopores limits water accessibility and concentration

of corrosive species in the stainless steel holes, and hence

cause a retardation of the anodic dissolution process. The

transportation of oxygen to the pores through the trapped gas

is usually not influenced or is even increased, thus resulting

in similar or even higher oxygen concentrations in the pores

and an enhancement of the cathodic process (oxygen reduc-tion). The different effect of hydrophobicity of the coating

on the position of the cathodic and anodic branches in the

polarization curves may then be explained why it is not only

the decrease in corrosion current but the shift in corrosion

potential when applied nano-TiO2 coatings.

The EIS spectra have also been obtained at the open circuit

potential from bare 316L, nano-TiO2 and FAS/nano-TiO2coated 316L stainless steel electrodes in Ringer solution

(Fig. 5).The Bode diagrams evidently show a clear differ-

ence of corrosion resistance between the bare 316L and the

nano-TiO2 coated 316L electrodes, noting the peak at the

high frequency in Fig. 5(b and c). In general, ac impedance at

high frequencies represents the responses of coatings in the

solution, and at low frequencies reflects the Faraday reac-

tion resistance Rt and double-layer capacitance. So in the

equivalent circuits, two electrochemical interface reactions

are used to the system of the coated 316L stainless steel. The

Nyquist plots presented here illustrate that the nano-TiO2and

FAS/nano-TiO2coated 316L stainless steel present an excel-

lent corrosion resistance compared to the bare 316L stainless

steel. The results are in accordance with those of polarization

curves.

The EIS spectra are analyzed with equivalent circuit in

Fig. 6, whereQ stands for constant phase element (CPE) in

circuit description code, which is commonly used to replace

capacitance, because it is hardly pure capacitance in the real

electrochemical process and it is defined by admittance Yand

power index number n, given by Y= Y0(j)n. Thisis a general

dispersion formula, for n = 0 it stands resistance, while it is

capacitance ifn = 1.In all cases ofstudy, n is close to 1, repre-

senting a capacitive characteristic of the interfaces. InFig. 6,

Rsdelegates the solution resistance and a pair of elements ofQc(CPE of coatings) andRcin parallel replace the dielectric

properties of the coatings.Qdl (CPE of double layer) and Rt(react resistance) are adopted to describe the charge trans-

fer process happening at the solution/steel interface in the

pinholes of coating. Using the equivalent circuits, the curve

fitting performed for both nano-TiO2 and FAS/nano-TiO2coating systems and for the bare 316L yields the values of

parameters for the circuit elements (Table 2).FromTable 2,

it is interesting to note that the apparent capacitance of the

coatings Qc is very low (approximately 108). This lower

value is consistent with the insulating nature of the coatings.

Rt (i.e., corrosion resistance) of the nano-TiO2coated and the

FAS/nano-TiO2coated electrodes have been improved nearly

15 and 1000 times, respectively. It is demonstrated that the

corrosion resistance for FAS/nano-TiO2 coating system is

much higher than that for the TiO2 coated system.

Fig. 6. Equivalent circuit for the bare 316L (a) and the coated 316L (b) in

ringer solution.



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Table 2

EIS parameters in the equivalent circuit for the bare and coated 316L systems

Electrodes Rs () Qdl Rc () Qdl Rt ()

Y0 (108) n Y0 (10

6) n

316L 30.9 6.08 0.863 1.32 105

TiO2/316L 45.8 4.4 0.705 2152 19.0 0.873 1.68 106

FAS/TiO2/316L 29.0 2.7 0.7213 884 1.25 0.845 1.74 108

3.4. The results of XPS analysis

Fig. 7 shows the XPS depth profile spectra of the

hydrophobic TiO2 coating before (a) and after immerged in

the solution for 1008 h (b). Because the organic FSA layers

is only one or two molecule layers, the surface of the coat-

ings is made up of TiO2 particles. The XPS spectra show

that the 400 nmdepth layer is 316L substrate. We did not

find and chloride element in the coatings, indicating that the

nano coating is possible to obstruct from Cl penetration.

There is a distinct difference for the elements distribution ofin the 200 nmdepth layer before and after immerged. After

immerged 1008 h, only some Fe and a little Cr elements, but

Fig. 7. XPS depth profile spectra of the hydrophobic nano-TiO 2 coatings

before (A) and after (B) being immerged in Ringers solution (a) 0 nm; (b)

200 nm; (c) 400 nm depth.

no metallic oxides is observed in the coatings, implying that

no corrosion has taken place during the immerging.

4. Conclusions

We have prepared nano-TiO2coatings on metal by solgel

method, and developed a hydrothermal post-treatment

method to effectively eliminate the cracks defects in the

coating to optimize structure and properties for the coat-

ings. Using self-assembly of FAS-13, organic and inorganic

hybrid nano coatings with high hydrophobic properties has

been fabricated. The results of electrochemical test indicate

that the hydrophobic coatings exhibit an excellent corro-

sion resistance in Ringer solution. The corrosion potentials

shift positively, icorrdecreases three order of magnitudes, and

the corrosion resistance Rt increases one to three order of

magnitudes. It is expected that the surface modification by

nano-TiO2 coatings may become a promising technique in

improvement of corrosion resistance of metals.

Acknowledgements

Financial support for the National Natural Science Foun-

dation of China (Grant numbers: 20127302, 20021002) is

gratefully acknowledged.

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