Effect of heat treatment temperature on the performance of nano-TiO2 coating in protecting 316L stainless steel against corrosion under UV illumination and dark conditions

SCT-19608; No of Pages 10 Surface & Coatings Technology xxx (2014) xxx–xxx

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Effect of heat treatment temperature on the performance of nano-TiO2 coating in protecting 316L stainless steel against corrosion under UV illumination and dark conditions Mohammad Karimi Sahnesarayi, Hossein Sarpoolaky ⁎, Saeed Rastegari School of Metallurgy and Materials Engineering, Iran University of Science & Technology, Iran

a r t i c l e

i n f o

Article history: Received 13 October 2013 Accepted in revised form 24 July 2014 Available online xxxx Keywords: TiO2 coating Sol–gel Stainless steel Heat treatment Photocathodic protection Corrosion

a b s t r a c t In this study, the influence of heat treatment temperature on the corrosion protection performance of nano-TiO2 coated on 316L stainless steel has been investigated in ultraviolet (UV) illumination and dark conditions. Uniform crack-free nano-TiO2 coatings were prepared on 316L stainless steel using a sol–gel process. Surface morphology, thermal behavior, crystallographic structure and molecular bonds of the TiO2 coatings were analyzed by using SEM, AFM, STA, XRD, and FTIR, respectively. Photodegradation performances of the TiO2 coatings were investigated based on the degradation of an aqueous solution of methyl orange as a model. Furthermore, the influence of heat treatment temperature of the coatings on their protective properties was evaluated in both dark and UV illumination conditions based on the obtained polarization curves. The results demonstrated that the optimum corrosion protection performances of the coatings are achieved at heat treatment temperatures of 500 °C and 450 °C for dark and illumination conditions, respectively, while the sample heat treated at 450 °C showed the best photocatalytic activity. © 2014 Published by Elsevier B.V.

1. Introduction Stainless steels are widely used in various industrial fields due to their superior mechanical and corrosion properties. The outstanding corrosion resistance of stainless steels originates from a very thin passive film that naturally forms on the steel surface and is rich in oxyhydroxide or chromium oxide. Although well-known for their corrosion resistance, stainless steels are still susceptible to corrosion in certain environments, in the presence of halide ions, for example. The protection provided by such a natural passive film was thus found to be not sufficient for applications in aggressive environments [1]. Hence, researchers have sought remedies so as to correct this defect associated with stainless steels [2]. In this respect, the most effective technique in preserving stainless steels against corrosion was proposed to be changing the potential of the corroding stainless steel by pumping electron in which is widely known as cathodic protection and to be coating the stainless steel with protective layers. Due to their stability and corrosion resistance, ceramic coatings are utilized to ameliorate the corrosion resistance of stainless steels [2]. Among the ceramic coatings, TiO2 as multifunctional material has many potential applications, such as antireflection coating on the SLED ⁎ Corresponding author. E-mail addresses: [email protected] (M. Karimi Sahnesarayi), [email protected] (H. Sarpoolaky), [email protected] (S. Rastegari).

to suppress the lasing [3–5], dye-sensitized photovoltaic cells [6], gas sensors [7], electrochromic displays [8], as well as self-cleaning coatings on windows [9], owing to its favorable characteristics (physical and chemical stability and non-toxicity). Also TiO2 as an n-type semiconductor has attracted growing interests since the early 90s when a new corrosion protection mechanism, named photocathodic protection mechanism, was reported in addition to the conventional barrier corrosion protection mechanism [10–13]. The principle of this novel mechanism is known to be absorption of photons with certain energies (UV illuminated) by the TiO2 coating, followed by migration of electrons, generated from the TiO2 coating as a result of UV illumination, to the metal substrate, thereby making the electrode potential more negative than the corrosion potential (Fig. 1). Contrary to a sacrificial-type cathodic protection, TiO2 coating, in fact, functions as a non-sacrificial anode when it is exploited for the purpose of photocathodic protection of a substrate material, as the involved photoelectrochemical reaction is not TiO2 decomposition, but the oxidation of H2O by photogenerated holes. Several studies have elaborated on this process and the corresponding mechanisms [11,14–17]. Corrosion protection of a metal substrate by application of TiO2 can be achieved in two ways, either by connecting metal to the TiO2 photoanode or directly coating TiO2 onto the metal substrate. Many efforts have been made to use TiO2 coatings for photocathodic protection of various metal substrates such as copper [11,12], stainless steel [16,18–20], carbon steel [13,1,15,21] and weathering steel [22].

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isopropoxide (commonly referred to as titanium tetraisopropoxide or TTIP, Ti(OC3H7)4) as the starting material, and according to the following process: TTIP, diethanolamine (DEA, C4H11NO2) and three fifths of the total isopropanol (i-PrOH, C3H8O) volume were mixed at ambient temperature for 1 h in the first container, while the second container containing distilled water (W, H2O) and the rest of i-PrOH were added dropwise within 15 min to first container. In this solution, i-PrOH, as the solvent, provides a homogeneous media for the involved hydrolysis and condensation reactions and controls the reaction rate. Furthermore, DEA can stabilize the titanium ion in the alcoholic solution and suppress precipitation due to the strong dative nature [33]. Before conducting the coating on the substrate surface, the final solution was continuously stirred and aged for 2 h and 48 h, respectively. The relative molar ratio of each chemical in the sol was adjusted to 1:1:25:1 (TTIP:DEA:iPrOH:W). The flow chart of this method employed is presented in Fig. 2. Fig. 1. Photocathodic protection mechanism for TiO2 coated on a metal substrate.

2.2. Substrate preparation In photocathodic protection, various factors can influence the corrosion protection of the substrate, including the characteristics of the corrosive solution (such as being aerated or non-aerated [15], pH [16,18], concentration of the corrosive elements [16]), presence of an intermediate layer between the coating and the substrate [15,22] and UV light intensity [16]. Moreover, it cannot be overlooked the fact that the diffusion of elements from substrate to the TiO2 coating is another important factor which can play a leading role in TiO2 coating performance particularly in some coating methods such as sol–gel associated with heat treatment [15,23]. The physical properties of the TiO2 coatings strongly depend on the fabrication method which can be physical vapor deposition [24,25], chemical vapor deposition [26], plasma electrolytic deposition [27], plasma spray [28], electrochemical deposition [29], microwave activated chemical bath deposition [30] and sol–gel process [11,18,31,32]. Among these methods, sol–gel has extensively attracted the researches' attention due to its advantages including low processing temperature, simplicity, excellent homogeneity, possibility to control particle size and morphology, ability to coat large and complex shapes, and easy control of film thickness. In general, factors affecting the properties of the sol–gel derived coatings are classified into two main groups: the first is the chemical factors such as the type and amount of solvent, precursor and stabilizer, while the second is the technological parameters including type of the sol–gel technique (dip and spin coating), withdrawal speed, number of dipping and heat treatment temperature. Heat treatment temperature is considered as a key factor in sol–gel processing which can significantly influence the ultimate properties of the product. In particular, the corrosion properties of a substrate coated with a sol–gel derived material can be remarkably deteriorated or improved during heat treatment at different temperatures. Besides, depending on the condition, some materials such as TiO2 may exhibit unique behaviors, the photocathodic activity, for instance. The combined effects of these factors, however, have not yet been reported for TiO2 coated on a 316L stainless steel. The major aim of this study was to evaluate the influence of heat treatment temperature of the sol–gel derived TiO2 on the corrosion behavior of a 316L stainless steel under UV illumination and dark conditions. Beside corrosion protection, the photocatalytic activity and microstructural properties of the coatings were also closely investigated and discussed.

316L stainless steel sheet with a thickness of 2 mm was used for preparation of the substrate. The chemical composition of the steel is listed in Table 1. The substrates were cut into the size of 10 × 20 mm2. The testing sides of specimens were ground with No. 80 to 1500 emery papers and were polished to mirror finish using 0.05 μm- and 3 μm-Al2O3 particle-included polish solutions. Finally, the specimens were successively sonicated in acetone, ethanol and deionized water each for 10 min, followed by drying in air. 2.3. Sample preparation The TiO2 coatings were formed on the stainless steel substrate via dip-coating technique at a withdrawal speed of 3 cm/min. To achieve crack-free TiO2 coatings, the samples were dried for 2 min at the top region of the sol container before exposure to air [34]. The samples were finally heat treated at different temperatures of 400, 450, 500, 600 and 700 °C, each for 30 min. To reach each heat treatment temperature, a

2. Experimental 2.1. Sol preparation TiO2 polymeric sol for the deposition of films via coating technique was obtained through partial hydrolysis and condensation of titanium

Fig. 2. Schematic processing diagram showing the preparation method of the TiO2 polymeric sol.

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Table 1 Chemical composition of the316L stainless steel used in this study. Element %wt.

C

Si

Mn

P

S

Cr

Ni

Al

Mo

V

Cu

Nb

Ti

W

Co

Sn

Fe

0.03

0.387

1.48

0.072

0.083

16.868

9.144

0.004

1.953

0.046

0.069

0.01

0.006

0.012

0.022

0.003

Bal

heating rate of 1 °C/min was selected so as to avoid consequent cracks in the coatings. Fig. 3 portrays the schematic diagram of the process used for preparation of the sol–gel derived nano-TiO2 coatings.

2.4. Characterization To investigate the thermal behavior of TiO2 coatings, thermal analysis (TGA-DTA) was carried out using a PerkinElmer Pyris Diamond DSC. For the TGA-DTA analysis, the gel samples were prepared by drying the dip-coating solution in an oven at 60 °C for 12 h. TGA and DTA analyses were conducted under air atmosphere, with a ramp rate of 5 °C/min. Phase characterization was carried out based on the X-ray diffraction patterns obtained by means of a Philips-PW1800 diffractometer with Cu Kα radiation (λ = 1.5406 Å),at a scanning rate of 1° min−1 and with 2θ ranging from 10 to 90°. Fourier transform infrared (FTIR) spectra of the powders heat treated at 400 °C and 500 °C as well as the powders merely dried in the transmission mode were recorded within the wave number range of 400 to 4000 cm− 1 with a resolution of 4 cm−1 on a SHIMADZU FTIR model 8400 spectrophotometer. In order to study the optical behavior of the coatings, quartz samples with dimensions of 20 × 10 × 1 mm3 were coated and heat treated in the same way as the coated stainless steel substrates. The transmission and absorption spectra of these coatings were measured by a UV–vis Spectrophotometer Jenway 6705 UV/Vis. Photocatalytic activities of the prepared TiO2 coatings were investigated based on the photocatalytic degradation of methyl orange in an aqueous solution. For this measurement, the samples were placed in

10 cm distance from the UV source on a petri-box containing 30 ml of 10 ppm aqueous solution of methyl orange. The pH of the solution was adjusted to 3.0 by adding HNO3. The samples were then irradiated by a UV source (125 W, emission spectrum range of 360–415 nm) with a maximum intensity of 370 nm (i.e. close to the anatase band gap). Concentration of methyl orange was measured by using a UV–visible spectrophotometer at the wavelength of 505 nm corresponding to the maximum adsorption of methyl orange. Alteration in the concentration of methyl orange was measured in the presence of TiO2 coating as a function of time. The photoelectrochemical behaviors of the samples were evaluated in dark and UV illumination conditions using polarization tests by means of an Ivium Stat potentiostat. A custom made corrosion setup was consisted of a platinum counter electrode, a saturated calomel electrode (SCE) as the reference electrode and a bare or coated stainless steel specimen as the working electrode. In addition, corrosion setup was equipped with a quartz glass window on one side for the passage of UV radiation and samples were mounted onto a hole on the opposite sidewall of the electrochemical cell. In fact, UV radiation was then made to fall on the coated side of the samples after this radiation passed through a quartz window. The test solution was a 3.5 wt.% NaCl solution as well as the area of the specimen was exposed to test solution was 1 cm2. In order to simulate the ambient atmosphere the measurements were carried out under aerated conditions. Firstly, each sample was soaked in the solution for 30 min, and then the polarization curves were obtained by potential sweeps started from the rest potential with a scanning rate of 1 mV/s. Microstructures of the TiO2 coatings were examined using scanning electron microscope (SEM; WEGA\\TESCAN) and field emission scanning electron microscope (FESEM; Hitachi S-4160) an atomic force microscope (AFM) which was a NanoScope II from Digital Instruments, USA. Non-scraping Si3N4 tip was used throughout the examinations by AFM. Also the film thickness was measured using a profilometer (Dektak3, Veeco Inst.) It is noteworthy that, in this paper, the name of the samples is summarized on the basis of their heat treatment temperature; for example, the sample 700H specifies the coated 316L stainless steel heat treated at 700 °C.

3. Results and discussion 3.1. Gel characterization

Fig. 3. Diagram depicting the sample preparation method.

3.1.1. Thermal analysis Thermal behavior and weight loss of the TiO2 samples are shown in Fig. 4. The TGA curve which represents a total weight loss of 57% can be divided into four steps, namely below 100 °C, between 100 and 270 °C, from 270 to 510 °C and beyond 510 °C. In the first region, the weight loss associated with the endothermic peak is attributed to the evaporation of physically adsorbed water and alcohol from the powder. Weight loss and exothermic peak related to the removal of organic compounds such as DEA occur as temperature increases to about 270 °C in the second region. Between 270 and 510 °C, i.e. in the third stage, the approximate weight loss of 48% denoted probably further combustion of the remaining organic compounds. Except for the removal of OH groups characterized by strong bonds and consequently very little weight loss between 510 and 550 °C, no variation was observed in the weight loss curve beyond the temperature of 510 °C, as the whole organic materials

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Fig. 4. TGA and DTA curves of the TiO2 powder.

were so far removed at lower temperatures. Hence, as a deduction, weight loss of the powder was concluded at 550 °C. The exothermic peak at about 310 °C in the DTA curve reflects the transformation of amorphous TiO2 to anatase phase, while The exothermic peak at approximately 500 °C is attributed to not only combustion of organic compounds but also the anatase to rutile transformation of TiO2 which it will clearly prove by the results of XRD measurements in the phase analysis part; however, both of exothermic peaks are in accordance with other studies [35,36]. 3.1.2. Evaluation of the molecular bonds FTIR spectroscopy was used to investigate the molecular bonds. In this respect, the FTIR spectra of the sol–gel derived TiO2 nanostructured coatings are shown in Fig. 5. In all the three powder samples, i.e. dried and heat treated at 400 °C and 500 °C, the wide band observed in the frequency range of 400–800 cm− 1 corresponds to two overlapping components assigned to Ti\O and Ti\O\Ti groups, respectively [37, 38], indicating that titanium isopopoxide molecules underwent hydrolysis/polymerization reactions and formed a three-dimensional network. The broad bands in the ranges 3350–3450 cm−1 and 1620–1635 cm−1 are assigned to the stretching and bending (deformation) vibrations of the free and bounded O\H hydroxyl groups. Stretching and bending (deformation) vibrations of the N\H bond

also appear in the same wave number region. The bands around 2340 cm−1 correspond to CO2 at air [39]. The only difference between the above mentioned heat treated powders was the slight variations in the relative intensities of the bands. Contrary to the peaks related to the sol–gel derived TiO2 coating discussed above, the dried powder exhibited different behaviors. The peaks at 1452 and 1380 cm−1 signify C\H bending vibrations. The C\C stretching is clearly recognized based on the band at 960 cm−1. At 1078 cm− 1, a band characteristic of the C\O stretching or C\H bending (coupled) is identified [40]. In general, the C\O stretching in diethanolamine appears at 1111 cm−1 [41]. The shift in the position of the band, i.e. from 1111 to 1078 cm−1, indicates weakening of the C\O bond due to its chelation with the TiO6 octahedra. This observation emphasizes upon the fact that the stabilizing action of diethanolamine is an outcome of its chelate forming ability with the alkoxides [42]. This peak and the one specified in 1126 cm−1 represent the existence of unhydrolyzed alkoxy groups in the dried powder [38]. Accordingly, as the temperature increases, the bonds including elements such as N and C and organic materials are eliminated resulting in formation of a threedimensional network of titanium oxide. The reason for this fact is the existence of peaks related to C- and N-included bonds in the diagram of the dried sample, while no such peaks are observed for the heat treated samples. Therefore it is thus clearly determined by FTIR that a higher

Fig. 5. FTIR spectra of the sol–gel derived TiO2 powder in dried and in 400 and 500 °C-heat treated states.

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processing temperature is required in the case of the coatings deposited from the sol to ensure complete removal of the organic byproducts. 3.1.3. Phase analysis Fig. 6 presents the XRD patterns of the sol–gel derived TiO2 powders heat treated at 400, 500, 600 and 700 °C. The low intensity and broad peaks associated with anatase phase, which is assumed to form from amorphous TiO2 powder at 400 °C, reveal a polycrystalline structure with a main peak at 25.42° 2θ, a fingerprint peak of anatase (1 0 1). This is in agreement with the thermal analysis results. By increasing the temperature to 500 °C, the transformation of amorphous titanium oxide to anatase is completed and anatase peaks are intensified, suggesting that crystallization of anatase can be improved by raising the heat treatment temperature. Besides, observation of the fingerprint peak of rutile (110) at 27.62° 2θ, as a small peak after heat treatment at 500 °C, points out the formation of a small amount of rutile phase. In the powder heat treated at 600 °C, simultaneous existence of the two phases, rutile and anatase, is clearly distinguished, while the anatase to rutile transformation proved to be completed at 700 °C, after which no peak corresponding to anatase was detected in the XRD pattern. Weight fractions of the crystalline phases can be determined using Eq. (1) which is based on the intensity ratio of the rutile (110) peak to the anatase (101) peak, IR/IA [43]:   .  IR %wðAnataseÞ ¼ 1 þ 1:26 IA

ð1Þ

The results of these calculations are shown in Table 2. Due to anatase to rutile transformation, the weight fraction of rutile increases by raising the heat treatment temperature. Crystallite sizes can also be calculated using Scherrer equation as follows: D ¼ ð0=89  λÞ=ðβ  CosθÞ

ð2Þ

where D is the crystallites size, λ is the X-ray wavelength, β is the width of the diffraction line measured at half of its maximum intensity (FWHM) and θ represents the corresponding angle of this line. As observed in Table 3, by raising the heat treatment temperature from 400 °C to 700 °C, crystallites size is increased from 30 nm to 52 nm. This increase results from the fact that atoms prefer to occupy the energetically favorable sites in the crystal lattice. At higher temperatures, atomic vibrations and diffusion occur more easily, which in turn results ingrowth of the grains with lower surface energies [44].

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Table 2 Calculated values of the weight fractions of the crystalline phases available in specimens heat treated at various temperatures. Heat treatment temperature(°C)

400

500

600

700

wt.% (anatase) wt.%(rutile)

100 0

92 8

18 82

0 100

3.2. Characterization of the coated sample 3.2.1. Microstructure Fig. 7 illustrates the field emission SEM micrograph of the coating heat treated at 450 °C and well confirms the results presented in Table 3, particularly the nanostructure of the coating. It can be observed that the TiO2 coating is characterized by a uniform, smooth, crack-free and regular microstructure. AFM was employed to study the effect of heat treatment on the structural and textural characteristics of the sol–gel derived TiO2 coatings. AFM micrographs of the 450H, 600H and 700H samples are illustrated in Fig. 8a–c. Additionally, the nanoscale roughness values of the coatings as well as the average size of these domes are listed in Table 4. It is noted that the particle sizes observed by AFM are larger than the crystallite sizes estimated from line broadening of the XRD peak, suggesting multi-crystallite in the particle of as-prepared coatings. It is clear that the particle size rises with increasing the heat treatment temperature. Likewise, the roughness value of the 450H coating was about 8 nm and was found to increase to approximately 40 nm when the coating was heat treated at 700 °C. The 450H and 600H samples exhibited nearly uniform coatings with mono-dispersed regularly-shaped grains. By increasing the heat treatment temperature to 700 °C, the uniformity of the coating deteriorated and grains with poly-dispersed sizes formed. The sample 700H is composed of relatively large interconnected particles and pores, building up deep valleys and high mountains. In fact, increasing the heat treatment temperature has led to polydispersed particle sizes and formation of coatings with lower degree of uniformity. A narrow size distribution of the coating particles leads to relatively smooth coatings as were observed for the 450H and 600H samples, while a large poly-dispersed particle size leads to rougher and more textured coatings such as the 700H coating. Through the anatase to rutile transformation, the morphology and texture of the coatings alter from spherical to pyramid-like. All the samples, nevertheless, exhibited compact surface textures. In the case of the 450H coating, AFM micrograph confirms the formation of nanoscale particles.

Fig. 6. X-ray diffraction of the TiO2 heat treated at 400, 500, 600 and 700 °C.

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Table 3 Crystallite size of the samples heat treated at different temperatures. Heat treatment temperature (°C)

400

500

600

700

Crystallite size(nm)

30

35

41

52

3.2.2. Optical properties Fundamentally, investigation of the optical properties of a coating is on the basis of absorption and transmittance data. Fig. 9 shows the optical transmittance spectrum of the TiO2 coating heat treated at 450 °C. The coating was highly transparent in the visible range of the electromagnetic spectrum. Multiple reflections of the electromagnetic radiation between the quartz–TiO2 interface and the outer surface of the TiO2 coating causes optical interference in the transmittance curve. The appearance of these interference fringes in the transmission spectrum confirms the excellent surface quality, homogeneity as well as the uniform thickness of the coating [45,46]. The TiO2 coating also shows a sharp decrease in transmittance in the ultra-violet region which results from the fundamental absorption of light with energy the same as the band gap energy. The optical absorption coefficient can be calculated using the following equation [47]: ln α¼

  1 T d

ð3Þ

where T is the transmittance of the film at each wavelength and d is the thickness of the film. The band gap, Eg, can be calculated using the Tauc equation [47]:  n αhν ¼ A hν−Eg

ð4Þ

where A is a constant, hν is the photon energy (eV), and n can be 0.5 or 2 depending on whether it is a director an indirect transition. By extrapolating the straight line portion of the (αhν)0.5–hν diagram, the optical band gap can be calculated. The estimated band gap values are listed in Table 5 for TiO2 coatings heat treated at different temperatures. One can also remark that band gap of TiO2 coatings is decreasing with the heat treatment temperature. The coating heat treated at 400 °C is characterized by a high optical energy gap value of 3.39 eV. By heat treatment at 500 °C the crystalline structure of TiO2 coatings in the anatase phase is almost improved and band gap decreases to the value of 3.26 eV. Also increment of heat treatment temperature to 600 and 700 °C leads to reduction of TiO2 band gap to 3.05 and 3.02 eV respectively. This could be correlated to the high crystallinity

Fig. 8. AFM micrographs of the coatings heat treated at (a) 450,(b) 600 and (c) 700 °C.

Table 4 Surface roughness of the coatings heat treated at 450, 600 and 700 °C.

Fig. 7. FESEM micrograph of the coating heat treated at 450 °C.

Heat treatment temperature (°C)

450

600

700

Surface roughness (nm) Grain size (nm)

8.58 45

23.14 102

34.89 179

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Fig. 10. Methyl orange degradation percentage versus UV illumination exposure time. Fig. 9. Transmittance and absorption of TiO2 coating heat treated at 450 °C on a quartz substrate.

of these coatings and grain size increases which was confirmed by previous investigations [48–50]. 3.2.3. Photocatalytic activity Photocatalytic reactivity of the coatings was assessed based on the degradation of methyl orange as a model pollutant. In Fig. 10, percentage of methyl orange degradation, calculated using Eq. (5), is plotted as a function of UV illumination exposure time for all the samples: Degradation percentage ¼ ðC0 ‐CÞ=C0

ð5Þ

where C and C0 are the reactant concentration at time t = t and t = 0, respectively. For all the samples, methyl orange solution is removed as a result of the photocatalytic property of the TiO2 coatings. When exposed to UV illumination, TiO2 coating generates electron–hole pairs that in turn produce active agents such as the hydroxyl radical. These agents are able to decompose organic contaminations to inorganic materials, CO2 and H2O, for example. This process is well demonstrative of the photoactivity of the TiO2 coating. The results of photocatalytic degradation rate well follow the Langmuir–Hinshelwood kinetic model [51]:   C0 Ln ¼ Kt C

ð6Þ

where K is a constant relating to the apparent reaction rate and t stands for time. A plot of Ln(C0/C) as a function of time represents a straight line, the slope of which upon linear regression equals the value of K. Table 6 summarizes the K values for samples heat treated at different temperatures. It is clear that K increases as the heat treatment temperature rises up to 450 °C, followed by a decrease with a further increase in the heat treatment temperature. Thus heat treatment temperature of sample at 450 °C yields best results for photocatalytic activity. Surface roughness measurements of the TiO2 coatings suggested that the surface roughness increased by higher heat treatment temperature. The surface roughness enhancement will generally result in an increase in the surface area as confirmed by other researches [52,53]. Since the photocatalytic reaction takes places on the surface of a semiconductor, the surface area increment is expected to increase the Table 5 Band gap of TiO2 coatings heat treated at various temperatures determined using optical transmission spectra.

photocatalytic activity as a result of the rising in roughness. However the photacatalytic activity was descend by increasing heat treatment temperature from 450 to 700 °C. Kim et al. have ever reported that the reduction of photocatalytic activity by roughness increment was attributed to low light transmittance into the coating with increasing the surface roughness of TiO2 coating [52] but this explanation isn't in agreement with the result derived from light transmittance average of all TiO2 coatings heat treated at different temperatures in the spectral range 400–500 nm since similar light transmittance average of samples was observed. The decrease in photocatalytic degradation efficiency can be explained by three reasons: in the first place the coarsening of TiO2 particle increases volume recombination. In general, the photogenerated electrons and holes undergo volume recombination in large particle sizes before they can be trapped by the surface adsorbed molecules and form radicals which is important for the photocatalytic activity [54]. Secondly since anatase phase has more photocatalytic activity than rutile [55,56], with increasing temperature from 450 to 700 °C the rutile content increment decreases the number of active agents, followed by reduction of photodegradation efficiency. Furthermore, the higher photocatalytic activity of 450H than 400H can be attributed to the more anatase phase content, relying to transformation of amorphous phase to anatase phase. The third reason is attributed to band gap variation. The band gap increment doesn't change significantly photodegradation ability of anatase TiO2 but the photocatalytic activity of rutile TiO2 enhances with the increment of band gap [57]. Following the band gap reduction with increasing heat treatment temperature from 500 to 700 °C, the photocatalytic activity decreases. Furthermore, Guillard observed a decline in photocatalytic activity when the heat treatment temperature exceeded 450 °C [58]. The behavior is attributed to the reduction in coating thickness occurred as a result of increase in heat treatment temperature. Actually the greater number of electron–hole pair is generated on the surface of thicker TiO2 coating with increasing of coating thickness. Nevertheless this parameter has no significant effect on the photocatalytic degradation of methyl orange in this study because of the same coating thickness of samples, in the range of 190–200 nm.

Table 6 The constant value regarding the photocatalytic apparent reaction rate for samples heat treated at different temperatures.

Heat treatment temperature (°C)

400

450

500

600

700

Heat treatment temperature (°C)

400

450

500

600

700

Band gap (eV)

3.39

3.32

3.26

3.05

3.02

Rate constant (×10−5)

314

411

370

351

258

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Fig. 11. Polarization curves recorded for the bare 316L stainless steel and the TiO2-coated 316L stainless steels heat treated at different temperatures in dark condition (without UV illumination).

3.2.4. Corrosion protection Polarization tests were carried out in a 3.5 wt.% NaCl solution so as to determine the corrosion resistance performance of the coatings. Shown in Fig. 11 are the polarization curves associated with the specimens heat treated at various temperatures in the dark condition (without UV illumination) as well as the bare type 316L stainless steel. All coated samples presented a typical passivation behavior, in which the current density decreased and maintained a low value over a wide potential range. Generally, passive current density is an important parameter to assess the kinetics of corrosion reaction in specimens with passivation behavior. Table 7 summarizes the corresponding passive current density (Ipass) and corrosion potential (Ecorr) for each coated specimen and bare stainless steel. The bare stainless steel showed a corrosion potential and a passive current density of about −171 and 4/77 × 10−4, respectively. The passivation current density of the coated samples decreased about one to two orders of magnitude when compared to the bare steel. This result suggests that the presence of the TiO2 considerably increases the chemical resistance of the steel. The significant reduction in passive current density implies that the TiO2 coatings can help to decrease the corrosion rate of the stainless steel. Moreover, in comparison with the bare 316L stainless steel, the corrosion potentials for the coated samples were shifted to the noble direction indicating that the TiO2 coating as the protective coating may inhibit the anodic reaction and consequently keep the metal in a more stable state in dark conditions. An increase in the heat treatment temperature up to about 500 °C resulted in a decrease in the passive current density and an increase in the corrosion potential, while a further increase beyond 500 °C demonstrated a reversed trend, accompanied by a deterioration in the corrosion resistance. These results are well congruous with the results achieved by E. Szalkowska et al. [59], and confirm that raising the temperature up to the optimum heat treatment temperature of the TiO2

coating provides improved corrosion resistance, while the corrosion resistance declines at temperatures higher that the optimum temperature. The 500H specimen revealed the lowest corrosion rate; the corrosion rate proved to be lower for this sample by two orders of magnitude as compared to that for the bare 316L stainless steel. Due to the fact that the crack-free coatings with excellent surface quality and the uniform thickness were prepared, this coating can be considered as a suitable barrier to prevent the diffusion of corrosive element to the substrate. A typical SEM image of 500H specimen is given in Fig. 12. Moreover, particle sizes of the 450H and 500H samples were found to be

Table 7 Electrochemical corrosion parameters obtained from the polarization curves shown in Fig. 11.

Ecorr (mV) Ipass (μA cm−2)

Bare

400H

450H

500H

600H

700H

−171 477

−114 22.88

−98 25.11

−72 7.18

−89 8.63

−158 182

Fig. 12. SEM micrograph of the sample heat treated at 500 °C.

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M. Karimi Sahnesarayi et al. / Surface & Coatings Technology xxx (2014) xxx–xxx

smaller compared to the 700H sample, meaning that the formers included lower amount of defects which reduce the exposure of the substrate to the corrosive materials. Also by increasing heat treatment temperature from 400 °C to 500 °C, improvement of crystallization of TiO2 occurred, accompanied by the elimination of some structure defects in the amorphous coating, e.g. pinholes, pores, and increasing the compactness of TiO2 coating. As a result, coatings with nano-TiO2 particles are able to act as optimal barrier layers on the metallic substrates without a photo effect and reduce tendency and rate of dissolving substrate species into the electrolyte, NaCl solution in this case. Polarization curves of the heat treated specimens obtained after 30 min illumination are shown in Fig. 13. It is evident that the corrosion potentials of the samples examined under UV irradiation were shifted toward negative values and their corrosion-current densities increased. This phenomenon was observed in other papers. For example the photopotential of 500H is almost similar that reported by S. Li et al. [60] even though the materials and sample preparation process of the TiO2 coating used in their work were different from those used in the present work. In fact electron–hole pairs are generated in the TiO2 coating under UV irradiation. The excited electrons are injected into the stainless steel, thus increasing the anodic current of the TiO2-coated 316L stainless steel electrode under UV illumination so as to keep its potential more negative than its corrosion potential and to adjust it in the corrosion immunity region. Consequently, under UV irradiation, the nano-TiO2 coating plays the role of a nonsacrificial electrode so as to effectively protect the substrate against corrosion. From the viewpoint of photocathodic protection, more negative potentials or larger corrosion-current densities are of significance so as to achieve an appropriate protection [15,18]. The photopotentials of − 182, − 249, − 233, − 192 and − 179 mV (vs. SCE) were obtained for 400H, 450H, 500H, 600H and 700H samples, respectively, which are more negative than bare substrate potential. The 450H specimen demonstrated the least noble potential and the greatest corrosioncurrent density in the UV illumination test, indicating that this sample had the most photocathodic protection performance among all the samples tested in this condition. It can be attributed to the highest amount of anatase phase and the smallest grain size observed in the 450H sample compared to the other heat treated samples. According to the phase

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analysis results (Section 3.1.3), this sample was consisted only of anatase phase. Compared to rutile, anatase has more photoactivity; thus, anatase is capable of producing a greater number of electron–hole pairs, providing the transfer of more excited electrons to the 316L stainless steel substrate [56]. Besides, the small grain size of the 450H sample prevents volume recombination of generated electrons and holes. These reasons explain why the 450H sample showed the highest corrosion protection performance under UV irradiation. As mentioned before, the samples heat treated at higher temperature than 450 °C show higher surface area since the roughness of coating enhances with temperature increment. Thus photocathodic protection performance improvement is expected when the amount of photon absorption and the number of photogenerated electron grow with surface area increment. But it is completely contrary to trend exhibited in polarization test under UV illumination. Therefore, the roughness parameter isn't considered as a predominant factor in photocathodic protection. Moreover, as TiO2 coating thickness increases to light penetration depth, the absorption of photons can increases, followed by more electron–hole pair production [60]. However this parameter has insignificant influence in this study, according to small difference in TiO2 coating thickness. Good agreement is perceived between the corrosion results and the results of photocatalytic tests which follow the same mechanism, i.e. the generation of electron–hole pairs. All in all, for the sol–gel derived TiO2 coating on the 316L stainless steel substrate, the temperature of 450 °C proved to be the most efficient heat treatment temperature for the coating in order to impose photo-effect on the substrate. 4. Conclusions Heat treatment temperature of the TiO2 coatings deposited on 316L stainless steel via sol–gel process played a crucial role in determination of the morphology, structure, photocatalytic degradation and corrosion resistance protection of the TiO2 coating. Increasing the heat treatment temperature up to 450 °C resulted in increased amount of anatase phase formed from amorphous phase, while corrosion resistance of the 316L stainless steel in dark condition, photocathodic protection under UV illumination and the constant regarding the photocatalytic

Fig. 13. Polarization curves recorded for the bare 316L stainless steel and the TiO2-coated 316L stainless steel heat treated at different temperatures under UV illumination.

Please cite this article as: M. Karimi Sahnesarayi, et al., Surf. Coat. Technol. (2014), http://dx.doi.org/10.1016/j.surfcoat.2014.07.071

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Please cite this article as: M. Karimi Sahnesarayi, et al., Surf. Coat. Technol. (2014), http://dx.doi.org/10.1016/j.surfcoat.2014.07.071