Научная статья на тему 'Anti-Corrosion Coatings on SS 304 by Incorporation of Pr6O11-TiO2 in Siloxane Network'

Anti-Corrosion Coatings on SS 304 by Incorporation of Pr6O11-TiO2 in Siloxane Network Текст научной статьи по специальности «Химические науки»

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Ключевые слова
sol-gel process / anticorrosion / nanocomposites / EIS / золь-гель процесс / антикоррозия / нанокомпозиты / электрохимическая импедансная спектроскопия

Аннотация научной статьи по химическим наукам, автор научной работы — K. Jeeva Jothi, K. Palanivelu

This paper describes an attempt to develop anticorrosive siloxane coatings based on Pr6O11-TiO2 composite films for SS 304 substrate by sol-gel technique. We demonstrate for the use of praseodymium oxide doped Titanium oxide (Pr6O11-TiO2) nanocomposites loaded in a hybrid sol-gel layer, to effectively protect the underlying steel substrate from corrosion attack. The influence of Pr6O11-TiO2 gives the surprising aspects based on active anti-corrosion coatings. The silica sol was treated with Pr6O11-TiO2 to achieve different level of add-on i.e.) 0–1 wt% of nanocomposites. The influence of different weight percent of nanocomposites on silica matrix for anticorrosion performance was investigated by Electrochemical Impedance Spectroscopy (EIS). Pr6O11-TiO2 nanocomposites loaded in a hybrid sol-gel layer effectively protect the underlying substrate from corrosion attack. The results showed significant improvement in anticorrosion property for higher add-ons up to the optimized percent of nanocomposites. Furthermore, Transmission Electron Microscopy (TEM) and Scanning Electron Microscope Microscopy (SEM) were used to characterize the surface morphology of doped and undoped coatings. The studied showed a synergistic effect between Pr6O11-TiO2 and siloxane matrix has leads to a self-healing coatings.

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В статье описывается попытка разработки антикоррозионных покрытий для стали SS 304 на основе силоксановых композиционных пленок с Pr6O11-TiO2 наночастицами, получаемых золь-гель методом. Показано, что при использовании нанокомпозитов из оксида празеодима, легированного оксидом титана (Pr6O11-TiO2), внедренных в гибридный золь-гель слой, достигается эффективная защита от коррозии нижележащей стальной подложки. Влияние Pr6O11-TiO2 дает удивительные аспекты на основе активных антикоррозионных покрытий. Силиказоль обрабатывали Pr6O11-TiO2 для достижения содержания 0–1% мас. нанокомпозитов. Влияние различного содержания нанокомпозитов на матрицу из диоксида кремния для повышения антикоррозионной стойкости исследовали с помощью электрохимической импедансной спектроскопии (ЭИС). Нанокомпозиты Pr6O11-TiO2, внедренные в гибридной золь-гель слой, эффективно защищают подложку от коррозии. Результаты показали значительное улучшение антикоррозийных свойств с ростом содержания добавок вплоть до оптимизированной концентрации нанокомпозитов. Для характеризации морфологии поверхности легированных и нелегированных покрытий применяли просвечивающую электронную микроскопию (ПЭМ) и сканирующую электронную микроскопию (СЭМ). Исследования показали, что наличие синергетического эффекта между Pr6O11-TiO2 и силоксановой матрицей приводит к самовосстановлению покрытий.

Текст научной работы на тему «Anti-Corrosion Coatings on SS 304 by Incorporation of Pr6O11-TiO2 in Siloxane Network»

Anti-Corrosion Coatings on SS 304 by Incorporation of Pr6O11-TiO2 in Siloxane Network

K. Jeeva Jothi, K. Palanivelu*

Central Institute of Plastics Engineering and Technology (CIPET), Guindy, Chennai 600032, Tamil Nadu, India e-mail: kpalanivelucipet@gmail.com

This paper describes an attempt to develop anticorrosive siloxane coatings based on Pr6On-TiO2 composite films for SS 304 substrate by sol-gel technique. We demonstrate for the use of praseodymium oxide doped Titanium oxide (Pr6On-TiO2) nanocomposites loaded in a hybrid sol-gel layer, to effectively protect the underlying steel substrate from corrosion attack. The influence of Pr6On-TiO2 gives the surprising aspects based on active anti-corrosion coatings. The silica sol was treated with Pr6On-TiO2 to achieve different level of add-on i.e.) 0-1 wt% of nanocomposites. The influence of different weight percent of nanocomposites on silica matrix for anticorrosion performance was investigated by Electrochemical Impedance Spectroscopy (EIS). Pr6O11-TiO2 nanocomposites loaded in a hybrid sol-gel layer effectively protect the underlying substrate from corrosion attack. The results showed significant improvement in anticorrosion property for higher add-ons up to the optimized percent of nanocomposites. Furthermore, Transmission Electron Microscopy (TEM) and Scanning Electron Microscope Microscopy (SEM) were used to characterize the surface morphology of doped and undoped coatings. The studied showed a synergistic effect between Pr6O11-TiO2 and siloxane matrix has leads to a self-healing coatings.

Keywords: sol-gel process, anticorrosion, nanocomposites, EIS. УДК 621.373.826

Graphical abstract

INTRODUCTION

In recent years considerable work has been done on organic-inorganic hybrid materials for enhanced coating properties like thermal stability, corrosion resistance and scratch and abrasion resistance. The sol-gel route is the most commonly employed methods for the preparation of such hybrid coatings at macro/micro-scale. In the sol-gel process, for synthesis of inorganic part various organo metal precursors based on silicone, titanium, aluminium, and zirconium have been widely used [1]. Titanium oxide is a functional ceramic material that has attracted much attention in the last a few decades due to its unique properties [2-4]. In most cases, the material is used in the form of thin films or coatings on various substrates to study anticorrosive and light conduction properties [5]. In order to overcome the limitation of single component oxide layer on coatings, incorporation of nanocomposites may improve the surface protection [6].

Many attempts have been made to modify TiO2 coatings by doping it with metallic and nonmetallic ions. A well-known corrosion inhibitor, CeO2 is a promising choice to be incorporated into oxide films for a composite coating [7]. Synthetic ceramic coatings based on TiO2 have been used to modify

the surface of austenitic stainless steels and other biomedical metals. Titania coatings are demonstrated to be hard, corrosion resistant and biocompatible material with photo-induced hydrophilicity [8-10]. TiO2 has excellent chemical stability, heat resistance and low electron conductivity, making it an excellent anti-corrosion material. But the pure TiO2 films are mostly used in catalyst chemistry while very few have been reported for protective coatings on steel substrate [11]. Organic-inorganic multilayer coating containing organically modified silicates, epoxy resins and TiO2 nanocontainers loaded with 8-hydroxyquinoline was produced on AA2024-T3 substrates for anti-corrosion behavior [12].

One of the most important advantages of the solgel method is the possibility of using simple coating operations like immersion of the object in the solution followed by drying. The sol-gel deposition procedure requires relatively lower temperature when compared to chemical vapor deposition and therefore it is considerably less expensive. Perhaps the most important advantage of the sol-gel method is the possibility to control microstructure of the films [13].

In recent year's praseodymium compounds have attracted great attention as luminescent materials, catalysts, high-k gate dielectric materials, and opti-

© Jeeva Jothi K., Palanivelu K., Электронная обработка материалов, 2015, 51(6), 79-87.

cal filters [14]. This work reports the performance of active anti-corrosion coatings by forming a core-shell of SiO2 @ Pr6O11-TiO2 hybrid materials. The light rare earth elements Cerium, Praseodymium, and Neodymium have similar properties. The main reason for the choice praseodymium oxide is that it develops a green oxide coating when exposed air and thus may show corrosion resistance. The Pr6O11 modified with TiO2 could find a wider application in the area of paints and coatings. The anti-corrosion behavior of SiO2 @ Pr6O11-TiO2 hybrid coating on steel substrates was investigated in 0.5M of NaCl solution by electrochemical measurements. The results were utilized to quantify the corrosion protection efficiency and estimate the extent of coating degradation. To the best of our knowledge SiO2 @ Pr6O11-TiO2 hybrid materials have not been reported earlier. The synergistic effect of SiO2 @ Pr6O11-TiO2 renders a better anticorrosion behavior on the surface of coated substrate.

MATERIALS AND METHODS

Materials

The chemicals used for the preparation of silica sols using the precursors, such as 3-glycidoxypropyltrimethoxysilane (GPTMS), octyltriethoxysilane (OTES), N-[3-(trimethoxysilyl) propyl] aniline (TMPA) and zirconium (IV) propo-xide (TPOZ) from Aldrich®, 2-butoxy ethanol (C6H14O2), Titanium isopropoxide (Ti(OCH (CH3)2)4), isopropanol (C3H7OH) and hydrochloric acid (HCl), from MERCK, praseodymium oxide (Pr6On) from s. d. Fine CHEM. LIMITED, Mumbai and double distilled water were used throughout the experiments.

Substrate preparation

Stainless steel 304 (SS) with composition of Fe-Cr-Ni is 70-20-10 wt% had cut into 4 cm x 2 cm pieces. The SS substrates were polished with increasing grades of silicon carbide emery papers from 140 to 1200, were sonicated in acetone for 20 min, and were dried in oven.

Preparation of Pr6O11-TiO2

About 12.5 ml of Titanium isopropoxide and 80 ml of isopropanol were taken to form a homogeneous solution. To the latter solution, 0.243g of Pr6On (9 wt%) was dispersed to get a dark brown color solution. The above solution was stirred for 5h, while stirring 1.5 mL of water (for hydrolysis) has been added in drop wise and ensures the complete precipitation. Then the mixture was transferred into a teflon lined stainless steel autoclave, sealed and heated at 120°C for 12h at the pressure of 18 psi. Precipitated titanium hydroxide along with praseodymium hydro-

xide was filtered, washed with distilled water and ethanol, and dried in air oven at 90°C for 12h. The precipitate was sintered at 450°C for 4h in a muffle furnace to get Pr6On-TiO2.

Preparation of sol

The sol was prepared in two parts. In the first part a mixture of GPTMS/TMPA/OTES/2-butoxy ethanol in the molar ratio of 1:0.5:0.5:6 were taken and allowed to stir for 30 min at room temperature. The second part, TPOZ/2-butoxy ethanol/H2O mixture was taken in the molar ratio of 0.5:6:4 and HCl (0.1M) was used as a catalyst and stirred separately for 30 min in ice bath to control the exothermic reaction. The part I was added drop wise into part II with constant stirring (To avoid agglomeration it must be added in drop wise). The whole set was in ice bath to avoid exothermic reaction.

The final sol was then stirred for 24h at room temperature. The resultant sol was a clear and homogeneous solution and referred as GPZ. GPZ was divided into two parts. To the different portion of the first part 0.25, 0.50, 0.75, and 1% by weight of Pr6On was added. Then to the second part different concentration of Pr6On-TiO2 (0.25, 0.50, 0.75, and 1 wt%) was added. The above 8 compositions were stirred for 30 min and coated on the cleaned SS substrates by dip coating employing a withdrawal speed of 7 mm/s. The withdrawal speed above 7 mm/s, the coating gets peeled off while drying because of more thickness. So with the optimized withdrawal speed, films were dried at 150°C for 1h. The coated substrate (SS 304) was tested for corrosion and the composition was optimized. The sol with 0.75 wt% of Pr6O„ and 0.5 wt% of Pr6On-TiO2 shows best results. These two samples were characterized further and referred as GPZ-Pr and GPZ-Pr-Ti are shown schematically in Figure 1.

I GPZ I

I

[ Part GPZ

II Part UPZ

J__1J

0.25 || 0.50 | | 0.75 || 1.0 | 0.25 || 0.50 | | 0.75 || 1.0

1 ■

Corrosion Test, Kl S

Optimized

Corrosion Test, EIS

Optimized

^ Referred as | GPZ-Pr |

Referred as GPZ-Pr-Ti 1

Fig. 1. Preparation of GPZ-Pr and GPZ-Pr-Ti.

Characterization Methods

The morphology, size and structure of the synthesized particles were characterized by Transmis-

sion Electron Microscopy (TEM) (JEOL TEM 2010 UHR model) and Scanning Electron Microscope Microscopy (SEM, ZEISS, EVO MA15) were employed to analyze the distribution of nanocompo-sites in the silica network. The crystallographic patterns of coated films were analyzed by X-Ray Dif-fractometer (XRD) (Shimadzu, XD-D1 diffractome-ter). Fourier Transform Infrared Spectroscopy (FTIR) (Avatar 303) which gave the information about various chemical bonds such as O-H, Si-C, C-H and Si-O-Si of the coated films. X-ray Photoe-lectron Spectra (XPS) of the catalysts were recorded using an ESCA-3 Mark II spectrometer (VG Scientific Ltd, England) using Al Ka (1486.6 eV) radiation as the source. Thickness of the films was characterized by Mitutoyo absolute digimatic. Polarization tests were carried out for SS 304 substrates in 0.5M of NaCl solution. The potentiody-namic measurements were taken within the range of potential -0.5 to +2.5 mV. The salt spray technique was carried out using 3.5 wt% of NaCl solution at 35°C, in a Salt Spray Chamber supplied by ATLAS.

RESULTS AND DISCUSSION

FTIR analysis

FTIR spectra of GPZ, GPZ-Pr and GPZ-Pr-Ti are shown in Figure 2. From the figure it is clear that after modifying the surface with the nanocomposites adsorption band corresponding to O-H (around 3425 cm-1) increases gradually because of the hydrophilic nature of TiO2. The characteristic bands at 1090 and 750 correspond to the stretching, and bending of Si-O bonds respectively. The adsorption peaks at 2927 cm-1 and 2863 cm-1 are associated with C-H stretching modes which indicate the presence of long alkyl groups in the silica network. The bands at 2926 cm-1 and 2861 cm-1 show the C-H asymmetric and symmetric stretching frequencies, and that at 1462 cm-1 and 694 cm-1 correspond to C-H symmetric and asymmetric bending, respectively. The O-H band in GPZ-Pr-Ti got increased compared to GPZ and GPZ-Pr, which confirms the hydrophilic nature of TiO2. Because of the hydrophilic surface it's used for self-cleaning behavior. Peaks at 2926 and 2865 cm-1 correspond to C-H stretching mode also got increased compared to other peaks which confirms the good cross-linking between the nanocom-posites and the silica network. The strong band observed at 1107 cm-1 indicates the presence of Si-O-Si moiety. The wave number of 940-960 and 1080-1105 cm-1 in Fig. 1 indicate the band for Ti-O-Si and Si-O-Si bond, respectively. The band for Ti-O-Si vibration is observed due to the higher quantity of SiO2. The band intensity for Si-O-Si vibration increases with an increase of the silica con-

tent. This result suggests that the silica exists as a segregated amorphous phase in the anatase titania particles and some fraction of metal-O-metal bonding are Ti-O-Si. The broad absorption peak appearing near 3400 cm-1 relates to a stretching vibration of Ti-OH and Pr-OH group. Bands at 778 and 1276 cm-1 correspond to Si-CH3 groups where the peaks got increased from GPZ to GPZ-Pr-Ti due to increase in cross-linking.

Fig. 2. FTIR spectra of (a) GPZ; (b) GPZ-Pr and (c) GPZ-Pr-Ti.

Fig. 3. XRD patterns of (a) GPZ; (b) GPZ-Pr and (c) GPZ-Pr-Ti.

XRD analysis

The phase structures of different siloxane coatings were investigated by X-Ray Diffractogram (XRD) as shown in Figure 3. The SiO2 @ Pr6O11-TiO2 core-shell nanostructured present in GPZ-Pr-Ti are compared with GPZ and GPZ-Pr. In GPZ no diffraction regards nanocomposites, only one broad peak around 20° corresponds to the typical diffraction of amorphous SiO2 group [001]. In Fig. 3b except the broad peak around 20°, all the

other peaks can be attributed to the pure cubic phase of Pr6On. The 29 peaks at the values of 28.05, 32.62, 46.74 and 55.74° correspond to the diffraction planes of [111], [200], [220] and [311] of the crystalline Pr6On species respectively. The diffraction peaks in Fig. 3c show the mixed patterns of TiO2 and Pr6O„. For TiO2 at 25.22, 37.74, 47.93, 53.85, 55.13 and 62.73° corresponds to the diffraction planes of [101], [004], [200], [105], [211] and [204] of anatase TiO2, respectively. [(JCPDS Card No. 71-1169]. Few peaks of Pr6On absolutely match with Pr6On-TiO2@SiO2, but no intense peak of Pr6On is shown in GPZ-Pr-Ti, this is probably due to the low Pr6On doping content and the data may also imply that the Pr6On-TiO2 oxides are well dispersed within the SiO2 phase. There is no remarkable shift in the diffraction peaks, and the other crystalline impurities are not observed. After the addition of nanocomposites there is an increase in crystalline behavior of materials. It is important that the increase in crystalline is the most exaggerated when the concentration of nanocomposites increases beyond the optimized ratio and it may cause defects to the coating. Beside the crystallinity, stress and strain are other important parameters in coating characterization.

Fig. 4. TEM images of (a) Pr6O11-TiO2; (b) GPZ-Pr-Ti-0.2 ^m; (c) GPZ-Pr-Ti-100 nm and (d) SAED pattern.

TEM

To visualize the core-shell structure of SiO2 @ Pr6On-TiO2 particles TEM was performed. Representative TEM micrographs of the SiO2 particles coated by Pr6On-TiO2 shells are shown in Figure 4. The structural morphology of pure Pr6On-TiO2 nanocomposites in the absence of siloxane network has shown in Figure 4a. The core-shell structure of the SiO2 @ Pr6On-TiO2 particles can be seen clearly due to the different electron penetrability for the cores and shells (Fig. 4b,c). The cores are black spheres with an average thickness of 450 nm (SiO2) and the shells have gray color with an average thickness of 45 nm (Pr6On-TiO2). The electron diffraction rings with some disorder in Fig. 3d are shown, just to demonstrate the coexistence of crystalline phase (Pr6On-TiO2) and amorphous phase (SiO2) in the interface region of the core-shell particles.

Selected area electron diffraction pattern (SAED) from the SiO2 matrix along with Pr6On-TiO2 is also included in Figure 4d. The three rings that are clearly visible in the diffraction pattern correspond to the first three strongest reflections of Si that were formed by reflections from the {111}, {220} and {311} atomic planes with a spacing of 0.314, 0.192 and 1.64 nm respectively. The SAED pattern exhibits a number of bright spots arranged in concentric rings, which correspond to the {111}, {220} and {311} planes of SiO2.

SEM

The surface morphology of the prepared sol-gel composites was examined using FE-SEM analysis was presented in Figure 5. The morphology of GPZ shows the large particles of siloxane and GPZ-Pr shows the large particles of siloxane with layered flake structure due to Pr6On was shown in Fig. 5a,b, which are more loosely packed. It looks like a unique body shape of a Sperm Whale (given in inset Figure 5a). But in Fig. 5c,d, SiO2 @ Pr6On-TiO2 shows the more densely packed with the uniform distribution of nanocomposites throughout the silo-xane network. The surface morphology of GPZ-Pr-Ti indicates the advantageous effect of SiO2 functionalization on surface structure. There is an increased tendency for the presence of numerous primary particles of small diameter in Pr6On-TiO2 around SiO2. It is interesting to note that modification with silicone practically restricted the tendency for particle agglomeration. Energy dispersive spec-troscopy is generally accurate up to trace amounts of metal present in the surface of base materials. The EDS recorded from the selected area (Fig. 5e) reveals the presence of Si, Ti, Zr and O in the catalyst. Zirconium appears due to the presence of zirconium (IV) propoxide precursor present in the sol (Figure 5e).

run Safe 78M tBrusr -A23Q (<l os) liV

Fig. 5. SEM images of (a) GPZ; (b) GPZ-Pr; (c) GPZ-Pr-Ti (50 KX); (d) GPZ-Pr-Ti (100 KX) and (e) EDAX for GPZ-Pr-Ti.

80

60

S 40

£

20

0

OH

530.8'

548 544 540 536 532 538 524 Binding Enemy. eV

160

120

S 80

40

0

" Ti 2p (b)

Ti 2pv,

459.6 A

- Ti 2p,,: 1 1

464.8 t

—1—i i i

480 476 472 468 464 460 456 Rinding Energy, ¿V

a s -

960 952 944 Binding Energy. eV

102 104 106 108 Binding Energy, cV

Fig. 6. XPS analysis of SiO2 @ Pr6O11-TiO2 matrix (a) survey spectrum; (b) O 1s; (c) Ti 2p; (d) Pr 3d and (e) Si 2p. XPS analysis

XPS is a sensitive tool for the analysis of the chemical compositions of materials and hence XPS spectra of the SiO2 @ Pr6O11-TiO 2 composites were recorded in order to gain more structural information as shown in Figure 6. The O 1s profile is asymmetric and can be fitted to two symmetrical peaks a and P located at 530.8 and 532.8 eV, respectively, indicating two different kinds of O species in the sample (Figure 6a). The peaks a and P should be associated with the lattice oxygen (OL) of TiO2 and chemi-sorbed oxygen (OH) caused by the surface hydroxyl

[15] respectively. In Fig. 6b,c,d, the high-resolution binding energy spectra for Ti, Pr and Si species are shown, respectively. Two symmetric peaks at 459.6 and 464.8 eV in the high resolution XPS spectrum of Ti 2p are assigned to Ti 2p3/2 and Ti 2p1/2, indicating the existence of Ti2+ in the Pr6O11-TiO2 (Fig. 6b)

[16]. Two symmetric peaks at 933.9 and 954.2 eV, which represent the 3d3/2 and 3d5/2 electrons of Pr, (Fig. 6c) respectively. According to the results of He et al., we assign the signals at ca. 933.9 and 954.2 eV to Pr4+ [17]. There is an increase in the values of the binding energy of Ti 2p and Pr 3d but a slight decrease in the value of the binding energy of Si 2p. So interaction should exist among TiO2, Pr6O11 and SiO2 which cause the change of binding

energies of Ti, Pr and Si, i.e., it may confirm that SiO2 @ Pr6O11-TiO2 thereby leading the interaction to a certain extent among them.

Thickness measurements

Thickness of the silane/nanoparticles films was estimated using Mitutoyo absolute digimatic. Nano-particles has played an important role to increase the viscosity of the sol (thickness is directly prepositional to viscosity). The thickness of GPZ-Pr films is higher than GPZ films (GPZ - 2.02 ^m and GPZ-Pr - 2.76 ^m). Moreover the formation of core-shell (Pr6O11-TiO2) further increased the film thickness to 3.55 ^m. These may be attributed to increase in viscosity of the solution in presence of nanoparti-cles, thus giving a thick coating at the same withdrawal speed. Since no agglomeration of particles was observed in the silane coating and it may be concluded that, the coating is homogenous with uniform thick films and make the way easier to prevent it corroding.

Corrosion studies

EIS measurements were carried out to evaluate the coating performance and corrosion resistance of uncoated and coated SS 304 in 0.5M of NaCl solution. Nyquist plot for bare SS 304, GPZ, GPZ-Pr

¡00000 -Z', Ohm

Fig. 7. Nyquist plots for Uncoated, GPZ, GPZ-Pr and GPZ-Pr-Ti.

140 120 100

g SO

Oft tt

60

0» »

J 40 20 0 -20 -40

-

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- //

- /j^ //r A/

▼ ^ • Uncoated

▼ GPZ * GPZ-Pr

* GPZ-Pr-Ti

?= 1. -1 . J . -I: 1 1 1 t

.2

Log(Frcq. Hz)

(a)

1 2 3 Log(Fri:q, Hz)

(b)

Fig. 8. Bode plots showing the (a) absolute impedance and (b) phase angle.

and GPZ-Pr-Ti are shown in Figure 7. The bare SS 304 showed a depressed semicircle which represents the surface heterogeneity and interfacial (corrosion) process of the system, i.e., charge transfer resistance due to metal corrosion and the double layer capacitance of the liquid/metal interface [18]. Therefore, all coating systems have higher corrosion resistance than the control (bare SS 304) which demonstrates poor barrier properties resulting in insufficient corrosion resistance. The highest resistance value was obtained for the GPZ-Pr-Ti coating, followed by the lowest resistance values of uncoated and GPZ. The incorporation of nanocompo-sites allows homogeneous dispersion within the solgel matrix resulting in a better integrity and stability of the coating matrix. Furthermore, nanoparticles Pr6On (GPZ-Pr) provide larger interfacial area between the sol-gel matrix and the nanoparticles surface that allows the diffusion of aggressive electrolyte species towards the metal substrate. To make the diffusion path small the Pr6On were doped with TiO2 and make the core-shell around the SiO2 particle then there will be a less interfacial area between the sol-gel matrix and the nanoparticles surface.

Consequently, less diffusion paths are present in the coatings GPZ-Pr-Ti which restrict the diffusion of aggressive electrolyte species towards the metal substrate. The good barrier properties of the coatings with the nanocomposites (GPZ-Pr-Ti) give the good inhibiting effect. The positive effect of the inhibiting species relies on their release upon the onset of corrosion. The release inhibitor molecules form a layer on the metal surface, which terminate the corrosion processes and restore partially the coating barrier properties in the damaged coating area.

The impedance spectra of SS 304 coated with intact sol-gel coatings containing nanocomposites in 0.5M of NaCl solution are represented in the form of Bode plots in Figure 8. It could be visualized, GPZ-Pr-Ti showed increases in maximal phase angle value for better coating and barrier properties (Figure 8b). The high frequency time constant can be ascribed to the sol-gel coating capacitance and the low frequency time constant to the capacitance of the intermediate oxide layer at the metal-coating interface. As a result, sufficient coating barrier properties are revealed by good pore resistance resulting in higher |Z| values measured for the

GPZ-Pr-Ti coatings, which account for the better barrier properties than the other coatings (Figure 8a).

The potentiodynamic polarization curves for Bare SS 304, GPZ, GPZ-Pr and GPZ-Pr-Ti in 0.5M of NaCl solution are shown in Figure 9. These polarization curves showed the positive shift in the corrosion potential (Ecorr) and substantial reduction in corrosion current (Icorr) up to Pr6Oii-TiO2 @ SiO2. The values of Ecorr increase from Bare SS 304 to GPZ-Pr-Ti. If the concentrations of Pr6On-TiO2 increase beyond optimized weight percent, there is effect in the coatings due to the deterioration of the integrity and there is a possibility for agglomeration in nanocomposites at higher concentration. The protection efficiency (PE) of the coatings was calculated by using the formula (Equation 1):

PE (% ) =

1corr, bare ^corr, coated

100.

(1)

Where Ir,

, bare

and I,

corr, coated

are the corrosion current

density for uncoated and coated SS 304, respectively.

-5.0

-5.5 -, "60

<

-6.5

g

a -7.0 (J

1? -7.5

J

-8.0 -8.5 -9.0

-Uncoated -GPZ -GPZ-Pr -GPZ-Pr-Ti

-9.5

-0.8

-0.6

0.2

0.4

-0.4 -0.2 0.0 Potential, V

Fig. 9. Potentiodynamic polarization scans for Uncoated, GPZ, GPZ-Pr and GPZ-Pr-Ti.

Table. Electrochemical parameters obtained from the polarization measurements in 0.5M of NaCl solution.

Sample E ^corr (mV) Icorr (^A/cm2) Protection Corrosion rate

Efficiency (%) (mm/y)

Bare SS 304 -370 5.626 - 0.063

GPZ -229 4.987 11.35 0.055

GPZ-Pr -213 2.679 52.38 0.030

GPZ-Pr-Ti -132 0.126 97.76 1.414x10-3

The polarization curves and the calculated Tafel parameters of the samples after immersion in 0.5M of NaCl solutions are summarized in Table. Ecorr for bare and GPZ were determined as -370 mV and -229 mV, respectively. Ecorr is a measure of the tendency of the sample to corrode and the protection efficiency of coatings. As the value of Ecorr becomes

more positive, the efficiency of protection increases. The positive shift in Ecorr suggests an efficient protection of stainless steel by sol-gel coatings. Also, the measured corrosion for GPZ (i.e., 4.987 ^A-cm-2) is lower than the corrosion current for bare SS 304 (i.e., 5.626 ^A-cm-2). After the addition of Pr6O11-TiO2 nanocomposites, there is a positive shift in Ecorr value as well as a decrease in corrosion current. As shown in Table, the protection efficiency for GPZ-Pr-Ti is calculated as 97.76%, which is much higher than the protection efficiency of GPZ (11.35%). The corrosion rate is known to significantly decrease after the addition of the nanocomposites (Pr6On-TiO2). These results show the effect of preventing water from diffusing through the coating. The surfaces are deprived of oxygen and thus, extend at which the cathodic reaction happens is very limited. The protection efficiency and corrosion rate results reveal that the GPZ-Pr-Ti coated on SS 304 effectively protects the substrate and improves the corrosion resistance due to the homogeneous distribution of nanocomposites and relatively less porous surface of GPZ-Pr-Ti hindered the access of electrolyte to the substrate.

Fig. 10. Corrosion tested substrate after 480h in the salt spray chamber (a) Uncoated SS 304; (b) GPZ; (c) GPZ-Pr and (d) GPZ-Pr-Ti.

Salt spray test

The more traditional salt spray technique was used to investigate the corrosion performance of the SiO2 @ Pr6Oii-TiO2 films coated on SS 304. The coated plates were exposed in salt spray chamber for 480h (20 days) are shown in Fig. 10 (taped areas were cropped and specifically corrosion affected portions were taken). It can be clearly seen that GPZ-Pr-Ti exhibits less corrosion and retains their coating without any peel and crack on the surface. The bare SS 304 on contrast exhibits early signs of corrosion. The EIS results are in good agreement with visual observation. Therefore, the data confirm that SiO2 @ Pr6O11-TiO2 coatings have good anticorrosion behavior facilitating the corrosion inhibition comrade when compare to the other silane coatings.

corr, bare

CONCLUSIONS

This work illustrates the encapsulation of Pr6On-TiO2 by SiO2 layer to form core-shell microstructures. The superior performance can be attributed to the siloxane coatings combined with Pr6On-TiO2 to protect the substrate anodically. The fabrication process was easily controlled by varying the composition of nanocomposites. The results indicated that the optimal composition of Pr6On-TiO2 (0.5 wt%)/SiO2 showed an excellent anticorrosion effect for SS 304 in 0.5M NaCl solution. In comparison, GPZ and GPZ-Pr deteriorate the coating integrity by introducing diffusion paths for aggressive electrolyte species, which results in a loss of anti-corrosion efficiency. The best passive and active corrosion resistances were provided by the GPZ-Pr-Ti coating, as determined by EIS analysis. To the best of our knowledge, SiO2 @ Pr6On-TiO2 hybrid materials have not been reported earlier. It is believed that this offer an effective strategy and promising industrial application for fabricating anticorrosion coatings on other metallic materials.

ACKNOWLEDGEMENTS

The authors are grateful to the Defence Research and Development Organization (DRDO), Government of India, New Delhi for their financial support.

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Received 18.11.14 Accepted 06.02.15

Реферат

В статье описывается попытка разработки антикоррозионных покрытий для стали SS 304 на основе

силоксановых композиционных пленок с Pr6On-TiO2

наночастицами, получаемых золь-гель методом.

Показано, что при использовании нанокомпозитов из оксида празеодима, легированного оксидом титана (Рг60п-ТЮ2), внедренных в гибридный золь-гель слой, достигается эффективная защита от коррозии нижележащей стальной подложки. Влияние Рг60ц-ТЮ2 дает удивительные аспекты на основе активных антикоррозионных покрытий. Силиказоль обрабатывали Рг60ц-ТЮ2 для достижения содержания 0-1% мас. нанокомпозитов. Влияние различного содержания нанокомпозитов на матрицу из диоксида кремния для повышения антикоррозионной стойкости исследовали с помощью электрохимической импе-дансной спектроскопии (ЭИС). Нанокомпозиты Рг6011-ТЮ2, внедренные в гибридной золь-гель слой,

эффективно защищают подложку от коррозии. Результаты показали значительное улучшение антикоррозийных свойств с ростом содержания добавок вплоть до оптимизированной концентрации наноком-позитов. Для характеризации морфологии поверхности легированных и нелегированных покрытий применяли просвечивающую электронную микроскопию (ПЭМ) и сканирующую электронную микроскопию (СЭМ). Исследования показали, что наличие синерге-тического эффекта между Рг6011-ТЮ2 и силоксановой матрицей приводит к самовосстановлению покрытий.

Ключевые слова: золь-гель процесс, антикоррозия, нанокомпозиты, электрохимическая импедансная спектроскопия.

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