ISSN 20702051, Protection of Metals and Physical Chemistry of Surfaces, 2014, Vol. 50, No. 3, pp. 371–377. © Pleiades Publishing, Ltd., 2014.

NANOSCALE AND NANOSTRUCTURED MATERIALS AND COATINGS

Titania Nanostructured Coating for Corrosion Mitigation of Stainless Steel1 Nastaran Baratia, Mohammad Ali Faghihi Sania, Zahra Sadeghianb, and Hadi Ghasemic a

Department of Materials Science and Engineering, Sharif University of Technology, Azadi Street, P.O. Box 113659466, Tehran, Iran bDepartment of Gas, Research Institute of Petroleum Industry (RIPI), P.O. Box, 187454163, Tehran, Iran c Department of Mechanical Engineering, Massachusetts Institute of Technology Massachusetts Avenue, Cambridge email: [email protected], [email protected] Received March 07, 2013

Abstract—Anatase nanostructured coating has been prepared on 316 L stainless steel by sol–gel dip coating. The topography of the coatings surface has been analyzed using atomic force microscopy. The anticorrosion performance of the coatings has been evaluated using polarization curves. Effects of calcination temperature, withdrawal speed and times of coating on corrosion protection have been studied. The results showed calci nation temperature of 400°C and withdrawal speed of 10 cm/min are desirable conditions to achieve high corrosion protection of 316 L stainless steel in chloride containing environments. Coatings with 3 times exhibit better resistance against corrosion in 0.5 molar NaCl solutions. This protection against corrosion arises from photocatalytic properties of anatase nanoparticles. DOI: 10.1134/S2070205114030034 1

1. INTRODUCTION

Corrosion resistance is one of the most important material properties. Corrosion is always the major rea son of energy and material losses. It was reported that 1/5 of energy globally and average of 4.2% of gross national product (GNP) are lost each year due to cor rosion [1, 2], thereby corrosion resistance improve ment is among the priorities and challenges that scien tists are devoted to achieve. Steels and stainless steels are widely used in differ ent industrial fields because of their mechanical and chemical properties. However, they still tend to cor rode in the presence of halide ions. There are different ways to improve the corrosion property of materials from the viewpoint of surface engineering. One of the most effective corrosion con trol techniques is the electrical isolation of the anode from the cathode [3–5]. Another method to protect metals against corrosion includes introducing extrinsi cally a protective coating and applying protective films or coatings [6]. Various organic coatings have been investigated for corrosion protection [7–9]. Further more, oxide coatings have been applied to protect stainless steel against corrosion [10–13]. Surface protective coatings can be prepared by dif ferent techniques. One of the most important methods which has been used to prepare protective coatings is sol–gel method. The use of sol–gel process as a mod ern surface modification technique offers the potential 1 The article is published in the original.

for preparing ultra thin coatings with welldefined sur face chemistry. Ten years ago, Guglielmi [14] has already discussed the potential of sol–gel coatings as a corrosion inhibiting system for metal substrates. Since then, a great deal of work has been done to make vari ous sol–gel based protective coatings. Several oxides including SiO2, ZrO2, Al2O3, TiO2 and CeO2 show very good chemical stability and can provide effective pro tection to metal substrates [15]. Among various semiconductor photocatalysts, titania (TiO2) has been proved to be the most suitable for widespread environmental applications because of its biological and chemical inertness, strong oxidizing power, cost effectiveness, and longterm stability against both photo and chemical corrosion [16–18]. TiO2 has excellent chemical stability, heat resistance and low electron conductivity, making it an excellent anticorrosion material. Titanium dioxide has three kinds of phase structures, i.e. anatase, rutile and broo kite. The photocatalytic activity of titania is phase dependent. Anatase as a metastable phase usually exhibits the best photocatalytic activity due to a low recombination rate of photogenerated electrons and holes. On the contrary, the most stable rutile phase is least active or not active at all [19]. Many research papers [20– 25] have reported that the surface properties such as porosity, specific surface area and crystallinity influence the photocatalytic activity of titania. However, it is very difficult to estimate the exact contribution of each factor on the photocatalytic activity [29].

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Among various techniques for the preparation of TiO2 films such as sputtering, chemical vapor deposi tion, and electronbeam evaporation, the sol–gel technique is one of the most promising methods because the microstructure of the film can be easily controlled with changing the solution composition and deposition conditions. In addition, it provides uniform TiO2 films with large specific surface area, which is desirable for achieving good photo activity. The applications of sol–gel derived TiO2 coatings for the cathodic protection of metals and steels under ultraviolet (UV) illumination have been reported [26– 28]. The mechanism of corrosion protection of metal lic substrates using a semiconductor coating (mostly TiO2) under UV illumination is well known and described clearly in some previous references [26–28]. Under illumination with light having a wavelength equal to the band gap of TiO2 (3.2 eV), ehole pairs are created in the TiO2 layer. These ehole pair’s migration to underlying metal substrate and to the electrolyte led to corrosion protection of substrate in a similar way to the cathodic protection of metals by sacrificial anodes [26]. This study aimed at obtaining anatase nanostruc ture coating on 316 L stainless steel substrates using sol gel alkoxide route under different experimental condi tions and to examine its corrosion properties in NaCl solution. The Investigation of the effects of Solgel parameters (withdrawal speed, time of coating and calcination temperature) on corrosion properties and photocathodic protection of coated samples are the main novelty of this research work. The corrosion characteristics have been evaluated through potentio dynamic polarization curves and electrochemical parameters. Atomic Force Microscopy (AFM) has been used to investigate the topography of the coated samples. 2. EXPERIMENTAL Titania nanostructured layers were prepared via the sol–gel dip coating method on 316L stainless steel. In this process TetraηButyle ortho Titanat (TBT) was used as starting alkoxide. Precursor solutions for TiO2 films were prepared by the following method: Tetraη Butyle ortho Titanat (TBT) (Merck 99.99%) was added to ethanol (solvent) and Ethyl Aceto Acetate (EAcAc) as catalyst with a defined ratio while the solu tion was stirred at room temperature. A Slight amount of ammonium polyacrylate was added to prevent agglomeration of particles in sol. The solution was kept standing for hydrolysis reaction for 24 h, resulting in the formation of TiO2 stable sol. More details of the sol prep aration can be found in our previous paper [30]. 316L stainless steels were used as substrates for thin films deposition. TiO2 films prepared from the men tioned TiO2 sol by dippingwithdrawing cycles at ambient using a dipcoater. After that, the coated sam ples were dried at 150°C.

To investigate the effect of calcination temperature of coated samples on corrosion protection of coatings, the coated substrates were heattreated at different temperatures from 350 to 450°C for 1 hour in air. Coatings took place on steel surface by DipCoat ing method with various withdrawal speeds Using a dipcoater, in this case each substrate was dipped into the sol with a special speed in the range of 3– 25 cm/min, and then withdrawn at approximately the same speed. The coatings were airdried for approxi mately 10 min and placed into a drier at 150°C for 30 min, then the samples were heattreated at 400°C for approximately 1 hour at a fixed heating and cooling rate of 5°C/min. The thickness of TiO2 films was adjusted by repeat ing the cycle from dipping to heat treatment for 1–4 times to study the effect of times of coating on corro sion protection properties. To investigate the effect of mentioned parameters on corrosion protection of316 L stainless steel, corro sion analysis of the coatings was performed using EG and G Electrochemical measurement system con nected to a corrosion analysis software program (EG and G Princeton Applied Research). Polarization measurements were carried out potentiostatically in a threeelectrode cell at room temperature using a satu rated calomel reference electrode (SCE), a coated sample as working electrode and a bear 316L Stainless Steel electrode. The potentiodynamic measurements were taken within the range of –400 mV to 400 mV versus SCE at a rate of 2 mV/s prior to the measure ments; each sample was immersed in 0.5 molar NaCl solution (pH = 4.5) for at least 15 min. The surface topography of the coatings was charac terized using Atomic Force Microscopy (AFM) with 0.1nm accuracy in Zaxis direction. The MultiMode Nanoscope SPM from Digital Instruments (Veeco Metrology Group) in contract model was used. 3. RESULTS AND DISCUSSION Using the mentioned procedure, we could achieve a stable sol of titania nanoparticles which was stable more than 3 months. Figure 1 shows tafel polarization curves for three samples with different calcination temperatures. The electrochemical parameters obtained from Tafel curves through the analytical program of autolab sys tem are given in Table 1. From these data it could be understood that corrosion potential in sample C2 is higher than samples C1 and C3, therefore sample C2 is more stable from thermodynamic viewpoint. Also, comparing current densities of samples, C2 shows lower current density therefore presents lower corro sion rate which means better corrosion protection kinetically. In the polarization behavior of samples C1 and C2, a passivation region is present, but in sample C3 which

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(a)

0.5 0.3

–0.1 –0.3

C2

–0.5 –0.7 –14

50 nm/Div

E, V

0.1

C3 –12

–10

C1

–6 –8 –4 logI [A cm–2]

–2

0

was calcined at 450°C, this region is not clear, suggest ing most probably the existence of cracks and holes, direct contact of sample with electrolyte (NaCl solu tion) chloride ions lead to corrosion of the coated sample. The Investigation of the effect of calcination tem perature on corrosion property of coated samples indi cated that sample C2 which was calcined at 400°C, shows better corrosion protection in 0.5 mol L–1 NaCl solution. XRD patterns of these samples as previously reported [30] revealed that all the prepared samples are crystalline with anatase structure; known to offer much higher photocatalytic properties than the other phases [31, 32]. The texture and morphology of TiO2 thin films after calcination at 400 and 450°C are shown in Figs. 2a and 2b, respectively. Figure 2a clearly shows parti cles with uniform size in the calcined TiO2 thin film. Another important observation is that TiO2 crystallites grown on the stainless steel substrate tend to arrange into uniform structures with minimum distance between them. However by increasing calcination temperature to 450°C, aggregation and grain growth of TiO2 crystallites occur. This can be clearly seen from Fig. 2b. Thus, an enhancement of the crystallinity goes in parallel with the aggregation of TiO2 crystallites as the calcination temperature is increased. Therefore, 400°C is considered to be the optimum temperature for obtaining TiO2 thin films with uniform morphol

0

2 (b)

4 µm

0

2

4 µm

50 nm/Div

Fig. 1. Tafel curves for the coatings with different calcina tion temperatures in the 0.5 mol L–1 NaCl (pH = 4.6) solution (C1) 350°C; (C2) 400°C; (C3) 450°C.

Fig. 2. Topview AFM image of TiO2 thin film on stainless steel substrate calcined at different temperatures. (a) 400°C, (b) 450°C.

ogy and particle size distribution. Moreover, it is observed that by increasing calcination temperature the crystallite size increases thereby decreasing surface area, which is destructive for photocatalytic property, then corrosion protection would be badly affected by grain growth. Therefore to obtain the best photocata lytic property and effective corrosion protection, avoiding grain growth is inevitable; our purpose is to achieve anatase nanostructure coating simultaneously

Table 1. Electrochemical parameters from Tafel curves for the coatings calcined at different temperatures Sample code

Calcination temperature, °C

Ecorr, mV

Icorr, A/cm2

Corr. Rate, mpy

Epas, mV

C1

350

–127

28.61 × 10–9

0.0124

–50

C2

400

–48

1.58 × 10–9

–10

C3

450

–255

12.42 × 10–9

6.85 × 10–4 0.005

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–

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NASTARAN BARATI et al. (a) 0.1

E, V

0 –0.1 –0.2 –0.3 –0.4 –14

–12

–10 –8 logI [A cm–2]

–6

–4

(b)

0.5

E, V

0.3 0.1

–0.1 –0.3 –10

–8

–6 –4 logI [A cm–2]

–2

(c)

0.5 0.4 0.3

E, V

0.2 0.1 0 –0.1 –0.2 –0.3 –0.4 –0.5 –11

–9

–7 –5 –3 logI [A cm–2]

–1

Fig. 3. Tafel curves for the coatings with different with drawal speeds in the 0.5 mol L–1 NaCl (pH = 4.6) solution (a) 3; (b) 10; (c) 15; (d) 25 cm/min.

limiting grain growth. Altogether, the above characters can be seen in sample C2. In Fig. 3 the effect of withdrawal speed on tafel polarization curves for four samples with different withdrawal speeds have been reported. Electrochemi cal parameters extracted from tafel curves are given in Table 2. The results show that increasing withdrawal

speed up to 10 cm/min leads to the increase of corro sion potential. The effect of withdrawal speed on coat ing thickness has been studied in our recent paper [30]. Results showed that by increasing withdrawal speed, the thicknesses of coating increases as given in Table 3. On the other hand, the variation of withdrawal speed affects the solvent evaporation and coating morphology. From electrochemical parameters given in Table 2, increasing withdrawal speed up to 10 cm/min leads to an increase in the corrosion potential as observed in sample R2. In this sample increasing thickness of coat ing culminate in quality improvement of the coating by hindering the cracks and holes. This phenomenon arises from the fact that higher withdrawal speeds retard solvent evaporation that has positive effect on decreasing cracks formation. So, increasing the thick ness of coating until improvement of coating quality has positive effect on corrosion potential increase. It is found that higher withdrawal speeds cause thicker coatings leading to coarser particles, which result in the enlargement of the existing holes [33]. In samples R3 and R4 coated by withdrawal speed of 15 and 25 cm/min respectively, the phenomenon of holes enlargement will occur and then corrosion protection property of coatings will be lost, i.e. enlarged holes produce penetration sites for chlorine ions and so cor rosion incensement. On the other hand, noneffec tiveness of corrosion protection in thicker coatings (coated by withdrawal speeds higher than 10 cm/min) is attributed to depth of light penetration in coated samples, the thickness of coating is higher than the depth of light penetration. Generally speaking, it can be concluded that increasing withdrawal speed up to an optimum value can be effective for corrosion pro tection but higher than that it becomes destructive. Figure 4 shows polarization curves to evaluate the influence of times of coating on corrosion protection of coated samples. Electrochemical parameters of these samples have been reported in Table 4. The obtained results indicate that increasing times of coat ing causes an increase in the electrochemical potential and thereby improvement of corrosion protection thermodynamically. On the other hand, from kinetics viewpoint, this phenomenon leads to an increase in the corrosion rate, but in this case corrosion rate increase is more insignificant compared to the ther modynamic effect and totally with increasing times of coating where coating will be more efficient against corrosion. These results are valid under specific times of coating, afterward coating layer will be destructive on corrosion protection from thermodynamic and kinetics viewpoints. In this case, 3 times of coating was the optimum condition. In samples n1, n2, n3 the pas sivation region is available but in sample n4, which was coated 4 times, this region is vanished. In this case increasing electrical potential leads to an increase in the current density which is an indication of an active electrochemical reaction. In former samples, the pas sivation region is increased by more times of coating

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Table 2. Electrochemical parameters from Tafel curves for the samples coated with different withdrawal speeds Sample code

Withdrawal Speed, cm/min

R1 R2 R3 R4

3 10 15 25

Ecorr, mV

Icorr, A/cm2

Corr. Rate, mpy

–48 157.9 31 –51

1.58 × 10–9 7.8 × 10–8 7.9 × 10–9 5.0 × 10–9

6.85 × 10–4 3.3 × 10–2 3.4 × 10–3 2.17 × 10–3

up to 3 times. In this region, coating acts as a barrier against corrosion, so with increasing electrical poten tial, current density remains constant. This phenome non is attributed to the improvement of surface quality by increasing the times of coating; hence the obtained surface will be perfect and free from defects. When sample with 1 times of coating is exposed to electro lyte, and because of the presence of holes and cracks, corrosive ions such as chlorine would be able to pene trate and attack steel substrate. By applying more number of coating layers, at each stage, the prepared coating can seal the holes and cracks of former layer, thereby the penetration space for corrosive ions will be inhibited. This mechanism could be effective to improve corrosion protection initially and only up to a

Table 3. Variation of coating thickness with withdrawal speed Withdrawal speed, cm/min

Thickness, nm

3 10 15 25 30

–300 –400 –500 –1100 –2900

specific number of applied coating layers, after that it has a reverse effect. This means that applying layers more than the optimum critical number, will result in

(a)

(b) 0.1

0

0

–0.1

–0.1

E, V

E, V

0.1

–0.2

–0.2

–0.3

–0.3

–0.4 –14

–12

–10 –8 –6 logI [A cm–2]

–0.4 –14

–4

–4

(d)

0.4 0.2 0 E, V

E, V

–10 –8 –6 logI [A cm–2]

0.6

(c) 0.3 0.2 0.1 0 –0.1 –0.2 –0.3 –0.4 –0.5 –11 –10 –9

–12

–0.2 –0.4 –0.6 –0.8

–8 –7 –6 logI [A cm–2]

–5

–4

–1.0 –8

–7

–6

–5 –4 logI [A cm–2]

–3

–2

Fig. 4. Tafel curves for coatings with different times of coating in the 0.5 mol L–1 NaCl (pH = 4.6) solution (a) 1; (b) 2; (c) 3; (d) 4 times. PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 50 No. 3 2014

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Table 4. Electrochemical parameters from Tafel curves for the samples with different times of coating Icorr, A/cm2

Sample code

Times of coating

n1

1

–48

1.58 × 10–9

0.685 × 10–3

n2

2

–29

2.51 × 10–8

1.08 × 10–3

n3

3

39

2.4 × 10–9

1.04 × 10–3

n4

4

–453

5.01 × 10–9

2.17 × 10–3

Ecorr, mV

tremendously accelerating corrosion of substrates. This phenomenon arises from mechanical effects that each coated layer exerts on one another, because each layer is exposed to tension stresses along different directions. On the other hand, the defects formed within the coated layers are exposed to extensive stresses; thereby the increase of stress within the coated layers leads to a continuous increase of the defects within the layers. As a result, applying further number of layers results in lower corrosion protection of coated samples. Another reason for improving corrosion protection of steel substrates by applying specific number of coat ing layers is related to the possibility of preparing thicker coatings with further coated layers. It is gener ally recognized that thicker layers of titania nanoparti cles will be able to absorb further photons which can induce positive effect up to a saturation point, after that increasing thickness of coated layer don’t cause any further photon absorption, in this case the thick ness of coated layer is larger than that of the depth of light penetration. 4. CONCLUSIONS A corrosion protective layer of nanostructured tita nia has been prepared on 316 L stainless steel using alkoxide sol–gel method. It is indicated that the nano structure titania coatings exhibit an excellent corro sion resistance in 0.5 mol L–1 NaCl solution due to the photocathodic protection and photocatalytic property of titania nanoparticles. The influences of calcination temperature, with drawal speed and times of coating on corrosion behav ior of the coated samples have been examined, it is indicated that 400°C is the optimum temperature to achieve a nanostructure anatase coating with the best corrosion protection. Layer coated with withdrawal speed of about 10 cm/min exhibits the best corrosion resistance, thickness increases with increasing withdrawal speed which led to perfect microstructure of coating. Sam ples coated with lower and higher speeds than the mentioned value exhibit lower corrosion resistance because of lower thickness and holes enlargement, respectively. Increasing the coating times, results in more pro tection against corrosion due to the improvement in

Corr. Rate, mpy

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