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Vol. 9. Issue 1.
Pages 922-934 (January - February 2020)
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Vol. 9. Issue 1.
Pages 922-934 (January - February 2020)
Original Article
DOI: 10.1016/j.jmrt.2019.11.032
Open Access
Effect of nitrogen-doping on the surface chemistry and corrosion stability of TiO2 films
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Edvan Almeida de Souza Filho, Eurico Felix Pieretti, Rodrigo Teixeira Bento, Marina Fuser Pillis
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mfpillis@ipen.br

Corresponding author.
Materials Science and Technology Center, Nuclear and Energy Research Institute (CCTM-IPEN/CNEN), São Paulo, Brazil
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Tables (7)
Table 1. Chemical composition of the AISI 316 stainless steel (wt %).
Table 2. MOCVD process parameters.
Table 3. Surface parameters of uncoated and coated stainless steel.
Table 4. Atomic concentration of Ti2p, O1s, N1s and C1s elements.
Table 5. The binding energies (BE) of the bonds of N-doped films grown at 400° and 500°C.
Table 6. EIS fitting parameters adjusted using the ECs shown in Fig. 11 for uncoated and titanium-based coated AISI 316 austenitic stainless steel.
Table 7. Electrochemical parameters determined from the potentiodynamic polarization curves shown in Fig. 12.
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Abstract

TiO2 and N-doped TiO2 films were grown on AISI 316 stainless steel substrates and on Si (100) by metallorganic chemical vapor deposition (MOCVD) at 400°C and 500°C. X-ray photoelectron spectroscopy, scanning electron microscopy, and contact angle techniques were used to characterize de films. The corrosion behavior was assessed by monitoring the open circuit potential, electrochemical impedance spectroscopy and potentiodynamic polarization tests in 3.5wt% NaCl solution at room temperature. The results show that 6.18 at% of nitrogen was introduced in the films grown at 400°C and 8.23 at% at 500°C, and that besides TiO2, nitrogen phases were identified. All the films are hydrophilic and the contact angles varied from 48° to 72°. The films presented good homogeneity, low porosity and rounded grains in the range of 40–90nm. The RMS roughness varied between 5.5 and 18.5nm. Titanium dioxide films grown at 400°C showed better corrosion resistance than those grown at 500°C due to its compact morphology. Nitrogen-doping was not efficient to protect the substrate from corrosion.

Keywords:
N-doped TiO2films
TiO2
MOCVD
Corrosion resistance
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1Introduction

Titanium dioxide has attracted significant interest for many applications, especially due to its stability and high corrosion resistance [1,2], availability, and low cost of production. It presents high dielectric constant and electrical resistivity, high refractive index, and good optical transparency in a wide spectral range. It is extensively used as cosmetics, pigments, building materials, bactericidal surfaces, photocatalysis, biocompatible devices, and solar cells [3–6]. TiO2 is a transition metal oxide and presents three commonly polymorphs: TiO2 anatase (tetragonal), brookite (orthorhombic), and rutile (tetragonal) [2,7]. Anatase phase is more stable than rutile at 0K, but the difference in the enthalpy energy between these two phases is small (∼ 2–10kJ/mol) [8]. The anatase structure is preferred over other polymorphs for solar cell applications, due to the higher electron mobility, lower dielectric constant and lower density. The high photo reactivity is due to the higher Fermi level, the oxygen adsorption capacity and the higher degree of hydroxylation in the anatase phase [9]. The growth temperature of TiO2 films produced by the MOCVD technique directly affects the structure of these films, and this structure is closely related to their corrosion stability, and corrosion resistance in chlorine containing medium [10]. Several papers in the literature report on the use of TiO2 films in the corrosion protection of metallic substrates [10–13].

The good electrical, optical and mechanical properties, besides thermal stability presented for the coatings in the Ti-O-N system makes it interesting for several technological applications. Titanium oxynitride coatings have been used as absorber films, anti-reflective coatings, diffusion barriers in semiconductors and high hardness metallurgical coatings [14]. The use of N-doped TiO2 coatings as antibacterial surface on stainless steel [15], and as electrode in dye-sensitized solar cells [16] have been reported. Asahi et al. [17] proposed a method to elaborate N-doped TiO2 anatase films, replacing oxygen atoms by nitrogen atoms. The small difference between the ionic radii of oxygen and nitrogen species would facilitate the substitution of oxygen and would not cause great distortion in the TiO2 lattice [18]. The functional properties of TiO2 as the refraction index, electrical conductivity, as well as the mechanical properties are altered according to the nitrogen content incorporated in the structure [19].

When high contents of nitrogen are incorporated into the TiO2 film, titanium oxynitride TiOxNy may form, and the modification of the initial structure of the oxide occurs. On the other hand, for low nitrogen contents, the TiO2 structure is maintained [20]. The structure of the TiOxNy film may be of the amorphous type, since the nitrogen is known to be a crystallization inhibitor, or cubic when there is a ternary solid solution between TiN and TiO with FCC phase. Results obtained by Duminica et al. [21] in the production of titanium oxynitride films grown by the MOCVD process show that ranging the nitrogen content in the films significant changes in the morphological, structural and mechanical characteristics occur. When a large excess of the nitrogen precursor is used, the film contains about 17 % nitrogen, and forms TiOxNy with a dense and amorphous structure. Wang et al. [15] studied the corrosion resistance in Hanks’ solution of N-doped TiO2 films deposited on AISI 304 stainless steel by plasma alloying technique. The results showed that the film was effective to protect the substrate, because the coating retards the local pitting and crevice corrosion of the stainless steel.

Several techniques are available for the synthesis of nitrogen-doped TiO2 films, such as pulsed laser deposition [22], sputtering [23], sol gel [24], and CVD [18,20,21]. The CVD (chemical vapor deposition) technique consists of reacting compounds in the vapor phase, that are deposited on the surface of a heated substrate, forming a continuous and adherent film [25]. The metallorganic chemical vapor deposition (MOCVD) is a CVD process that uses organometallic compounds as precursors. This is an attractive process, since it needs lower temperatures of growth than the conventional CVD [25], presents high growth rates, besides allowing the covering of substrates with complex geometries. In this process the composition and structure of the films are determined by the chemical precursors used and by the deposition parameters [18,26,27]. This technique provides a good control of stoichiometry and thickness, uniformity of deposition and the possibility of covering large areas [28].

The aim of this work was to investigate the effect of nitrogen doping on the structure, morphology and electrochemical behavior of TiO2 films grown by metallorganic chemical vapour deposition at the temperatures of 400 and 500°C. The films were grown on AISI 316 substrates. For the characterisation atomic force microscopy, X-ray photoelectron spectroscopy (XPS), and field emission scanning electron microscopy (FE-SEM) analyses were done, besides electrochemical tests.

2Material and methods2.1Substrates

Samples of AISI 316 stainless steel of 20×20×5mm, and chemical composition (wt. %) presented in Table 1, were used as substrate. The samples were grounded in SiC papers, diamond polished until 3μm, degreased in acetone, subsequently in ethanol, rinsed in deionized water, and dried in nitrogen prior of being inserted into the MOCVD reactor. Silicon wafers substrates were also used for convenience in order to determine the growth rate of the films. The surface preparation consisted in the immersion of the samples in a 5 %v H2SO4 water solution for 3min, followed by rinsing in deionized water and drying in nitrogen.

Table 1.

Chemical composition of the AISI 316 stainless steel (wt %).

Elements
  Fe  Cr  Ni  Mo  Mn  Si 
Content  Bal.  17.08  10.29  2.27  1.57  0.72  0.35  0.06  0.10  0.12 
2.2Growth of the films

The 300 nm-thick films were produced in a conventional homemade MOCVD apparatus at the temperatures of 400°C and 500°C. For the production of TiO2 films, only TTiP was used as titanium and oxygen sources, and to obtain N-doped TiO2, ammonia (NH3) was added to the system. Nitrogen was used both as the carrier gas to transport the TTiP into the system, and as purge gas. The process parameters are given in Table 2.

Table 2.

MOCVD process parameters.

Parameters   
Nitrogen purge gas flow rate (L/min)  0.5 
Nitrogen carrier gas flow rate (L/min)  0.5 
Ammonia flow rate (mL/min)  0.5 
TTiP temperature (°C)  39 
Substrate temperature (°C)  400 and 500 
Growth pressure (mbar)  50 
2.3Characterisation2.3.1Scanning electron microscopy (SEM)

The cross section of the films grown on silicon substrates was evaluated by FE-SEM in a FEI Quanta 600 electronic microscope. The pits morphology on the surface after electrochemical tests was evaluated in a Hitachi table top microscope model TM3000.

2.3.2Wettability tests

The surface hydrophilicity of films was evaluated by contact angle measurements in a SEO Phoenix-I equipment. The substrate and the films were stored for 120h in a dark chamber before the tests to avoid the light interference. The sessile drop method was used by dropping 5μL of deionized water on the surface. The experiments were done in triplicate for each sample.

2.3.3Atomic force microscopy (AFM)

Surface topography, roughness measurements and mean grain size of the films were determined by atomic force microscopy operating in air in the tapping mode in a SPM Bruker NanoScope IIIA equipment with a silicon tip under ambient conditions. The AFM images were carried out on 2μm×2μm surface area.

2.3.4X-ray photoelectron spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS) analyses were carried out to determine the chemical state of the species close to the solid surface. A Thermo Scientific, K-Alpha model equipment and a spot beam of 400μm was provided. The high-resolution spectra of Ti2P, O1s and N1s were acquired, and the deconvolution was performed using the algorithm Smart in the software Avantage®. The binding energies were corrected based on the C1s reference peak at 284.8eV.

2.3.5Electrochemical behaviour

The electrochemical tests were carried out using a potentiostat-galvanostat EG&G - Princeton Applied Research, Model 273A, with a flat-cell of three electrodes composed of a working electrode with 1.0 cm² of exposed area, a counter-electrode of platinized platinum and a Saturated Calomel Electrode (SCE) (3M) as reference. The tests were conducted at room temperature in a 3.5wt. % NaCl, and the applied potential varied from −600mV to 2500mV. The scanning rate was 1.0mV/s. Before the potentiodynamic polarization tests the surfaces were evaluated by monitoring the open circuit potential vs. time and electrochemical impedance spectroscopy.

3Results and discussion3.1FE-SEM characterization

Fig. 1 shows the cross-section of TiO2 and N-TiO2 films grown at 400°C and 500°C on Si (100). It can be observed that the interface between the film and the substrate is flat, and that the films grow perpendicular to the surface of the substrate. In the figure it is also possible to observe significant morphological differences between the films. While films grown at 400°C independently of the doping present a densified morphology, films grown at 500°C show columnar morphology suggesting that these films present a high level of porosity. The growth rate was estimated by dividing the film thickness by the time of growth. The values found were of 13 and 8nm.min−1 for undoped and doped TiO2 grown at 400°C, respectively, and of 37 and 10nm.min−1 for TiO2 and N-TiO2 grown at 500°C, respectively. Based on these results 300 nm-thick films were grown on AISI 316 SS substrates.

Fig. 1.

Scanning electron microscopy images of the cross section of films grown for 60min on Si (100). (a) TiO2 grown at 400°C; (b) TiO2 grown at 500°C; (c) N-doped TiO2 at 400°C and (d) N-doped TiO2 grown at 500°C.

(0.5MB).
3.2Wettability tests

Fig. 2 exhibits the contact angle measurements of the undoped and N-doped TiO2 films, as well as for the AISI 316 substrate. It was found that the contact angle for water was 77° for the steel substrate (Fig. 2a). Undoped TiO2 film grown at 400°C (Fig. 2b) presented a contact angle value around 68°, while the measurement for the undoped film grown at 500°C (Fig. 2c) was of 72°. For the doped films the contact angle measurements decreased for 61° and 48°, respectively, for the N-doped films at 400°C (Fig. 2d) and 500°C (Fig. 2e).

Fig. 2.

Contact angle measurements. (a) uncoated 316 and coated (b) TiO2 film grown at 400°C; (c) TiO2 film grown at 500°C; (d) N-doped TiO2 film grown at 400°C; (e) N-doped TiO2 film grown at 500°C.

(0.26MB).

The results showed that all the films are hydrophilic and suggested that the N-doping process improves the hydrophilicity of TiO2 films together with the growth temperature, as it is indicated in Table 3. Surfaces that present contact angles ≤ 90° are referred as hydrophilic ones whereas contact angles greater than 90° corresponds to hydrophobic surfaces [29–31]. The main factors that influence the wettability of surfaces are the superficial microstructure and the chemical composition [32,33].

Table 3.

Surface parameters of uncoated and coated stainless steel.

Sample  Deposition temperature [ºC]  RMS roughness [nm]  Mean grain size [nm]  Mean contact angle [º] 
Substrate  –  9.6  –  77 
TiO2 film  400  12.2  75.8  68 
TiO2 film  500  18.5  87.7  72 
N-TiO2 film  400  5.5  43.6  61 
N-TiO2 film  500  14.7  59.7  48 
3.3AFM characterisation

Figs. 3 and 4 show the surface topography and the three-dimensional AFM images of the TiO2 films grown at 400°C and 500°C, respectively. Figs. 5 and 6 show the surface of the N-TiO2 films grown at 400°C and 500°C, respectively. The films presented good homogeneity, low porosity and rounded grains in the range of 40–90nm. The images show that TiO2 grains have grown following the marks left in the substrate by the polishing step. The insert in Figs. 3–6b reveals the columnar structure of the films, that grows perpendicularly to the substrate, as it was described in the literature [18,34], and shown previously by the SEM images. The increase in the growth temperature, promoted the increase of the roughness values and the mean grain size of the films, as it can be seen in Table 3, which suggests an increase on its crystallinity.

Fig. 3.

AFM surface images of the TiO2 film grown at 400°C. (a) topography; (b) 3D image. The insert shows the typical columnar structure of the film in details.

(0.61MB).
Fig. 4.

AFM surface images of the TiO2 film grown at 500°C. (a) topography; (b) 3D image. The insert shows the typical columnar structure of the film in details.

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Fig. 5.

AFM surface images of the N-doped TiO2 film grown at 400°C. (a) topography; (b) 3D image. The insert shows the typical columnar structure of the film in details.

(0.49MB).
Fig. 6.

AFM surface images of the N-doped-TiO2 film grown at 500°C. (a) topography; (b) 3D image. The insert shows the typical columnar structure of the film in details.

(0.54MB).

The nitrogen doping provoked a decrease in both the RMS roughness and grain size for the undoped and doped TiO2 films grown at 400°C and 500°C. Oja Acik et al. [35] suggested that the doping process stimulate the growth of individually grains. Rojviroon et al. [36] obtained similar trend. The authors deposited TiO2 and Fe-doped TiO2 films on 304 stainless steel substrates. The TiO2 films showed a surface roughness of 18.3nm, while the 0.3 % Fe-TiO2 and 0.7 % Fe-TiO2 films exhibited 11.92nm and 12.92nm, respectively. Duminica et al. [21] have grown N-doped TiO2 films on stainless steel by MOCVD. The films grown at 400°C presented roughness values around 10nm – close to the values obtained in the present study. The authors suggested that the presence of nitrogen at this temperature leads to the amorphization of the films.

3.4XPS characterisation

The XPS technique was used to analyse the composition and binding energies between the atoms that compose the N-doped TiO2 films obtained at 400°C and 500°C. Fig. 7 shows the XPS survey spectrum of the N-doped TiO2 films grown at both temperatures. The peak near 400eV shows that nitrogen was successfully incorporated into the films. The main components were quantified and the results are listed in Table 4. It is well known that the exposition of the sample to the air results in the contamination by carbon [37]. In addition, residual carbon due to the decomposition of the metallorganic precursor may also be incorporated into the film [37,38]. The carbon content decreases as the growth temperature increases [37].

Fig. 7.

XPS survey spectra of N-doped TiO2 films grown at 400 and 500°C.

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Table 4.

Atomic concentration of Ti2p, O1s, N1s and C1s elements.

Growth temperature (°C)  Ti2p (at.%)  O1s (at.%)  N1s (at.%)  C1s (at.%) 
400  26.70  48.79  6.18  18.33 
500  26.53  52.37  8.23  12.87 

Fig. 8a–f show the high-resolution XPS core-level spectra of the nitrogen-doped films grown at 400°C and 500°C. The chemical bonding states obtained by deconvoluting and fitting the curves are presented in Table 5. Fig. 8a and b show these spectra in the Ti 2p energy region. The doublets of Ti2p at 459.1eV (2p3/2) and 464.8eV (2p1/2) for the films grown at 400°C and at 458.8eV (2p3/2) and 464.5eV (2p1/2) for the films grown at 500°C are associated with anatase TiO2. Thus, the highest contribution is due to Ti+4, which corresponds to the contributions of Ti+4 2p1/2 and 2p3/2 of TiO2. These results are in good agreement to the literature [20,21,37,39,40]. The lower energy titanium peak values suggest the formation of Ti-N bonds, and when accompanied by a Ti 2p peak enlargement, confirms the contribution of Ti+3, originating from nitride or oxynitride [21]. The peaks found at 457.89eV and at 463eV are due to N-Ti-N and/or O-Ti-N bonds [21,41].

Fig. 8.

XPS core-level spectra of the N-doped TiO2 films grown at 400°C (a, c, e) and at 500°C (b, d, f).

(0.55MB).
Table 5.

The binding energies (BE) of the bonds of N-doped films grown at 400° and 500°C.

  400°C500°C
Component  BE (eV)  FWHM  at.%  BE (eV)  FWHM  at.% 
Ti4+ 2p3/2  459.1  1.45  47.35  458.8  1.63  50.81 
Ti4+ 2p1/2  464.8  2.32  26.58  464.5  2.47  27.45 
Ti3+ 2p3/2  457.8  2.34  9.03  457.2  1.89  6.17 
Ti3+ 2p1/2  461.2  4.71  17.04  461.2  4.42  15.57 
             
N1s  396.7  1.06  77.97  396.6  1.05  71.21 
  397.3  1.70  12.65  397.1  1.89  22.55 
  401.1  3.37  9.38  401.7  2.69  6.25 
             
O1s  530.8  1.30  74.42  530.6  1.33  64.57 
  531.6  2.57  25.58  531.7  2.39  35.43 

Fig. 8c and d show the high-resolution XPS core-level spectra of the nitrogen-doped films grown at 400°C and 500°C in the N1s energy region. Table 5 shows the chemical bonding states obtained by deconvoluting and fitting the curves. The peaks at 396.7eV and 396.6eV for films grown at 400°C and 500°C, respectively, can be attributed to substitutional nitrogen (β-N) replacing oxygen in the lattice in TiNxOy[17,20,39,42]. The peaks centered at 397.3eV and 397.1eV for films grown at 400°C and 500°C can be attributed to Ti-N [42,43]. A broad peak centered on 401eV was found in the spectra. According to Chen and Burda [44], a broadening peak centered at 401.3eV can be attributed to O-Ti-N bonds.

Fig. 8e and f show the high-resolution XPS core-level spectra of the nitrogen-doped films grown at 400°C and 500°C in the O1s energy region. The peaks at 530.8eV and 530.6eV for the films grown at 400°C and 500°C, respectively are related to Ti-O linkages in TiO2[21,38,39,42]. The peaks at around 531.6eV are attributed to adsorbed water, corresponding to oxygen in hydroxyl groups [21,42].

3.5Electrochemical behavior

The corrosion resistance of the AISI 316 stainless steel samples was evaluated by electrochemical methods analyzing an area corresponding to 1cm2. The results show the reproducible behavior obtained in 3 trials for each condition.

The open circuits potential vs. immersion time (OCP) curves for uncoated and 300 nm-thick coated AISI 316 stainless steel are presented in Fig. 9. The sample covered by N-doped TiO2 film grown at 400°C showed a slight increase in its potential in the initial minutes of the test and then a constant decrease of the potential as the immersion time increases. The sample covered by the N-doped TiO2 film grown at 500°C showed a sudden drop in the first minutes, and soon after, a tendency to stabilization. For the undoped film grown at 400°C the curve suggests the breakdown and recomposition of the passive layer in the first half of the test but shows a tendency to stabilize at a potential close to the substrate. The sample coated with TiO2 film grown at 500°C presented a more homogeneous behavior in the same immersion periods in 3.5 % p NaCl, in relation to its pairs, and shows greater stability of the film. Nitrogen doped films showed stabilization in OCP lower than those presented by the samples covered by undoped films. In addition, films grown at 400°C stabilize at higher potentials than those grown at 500°C.

Fig. 9.

Open circuit potential curves for the substrate, TiO2 and N-doped TiO2 films.

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The anti-corrosion properties of these coatings were evaluated via electrochemical impedance spectroscopy (EIS). For comparison reasons the substrate without coatings was also evaluated and the results are presented in Fig. 10. The tests were performed after OCP and prior to polarization, and all coatings showed inferior impedance values comparing to the substrate. Nyquist plots revealed that the films grown at 400°C presented higher impedance than films grown at 500°C for both conditions and N- doped TiO2 film grown at 500°C presented the minor value, corresponding to a lesser protective behavior.

Fig. 10.

Electrochemical impedance spectroscopy (Nyquist plots) for the substrate, TiO2 and N-doped TiO2 films grown at 400°C and 500°C.

(0.18MB).

Fig. 11 shows two separate electrical equivalent circuits (EEC) that were used to model the EIS data. This approach is commonly present in the literature [45–47]. The CPE is the constant phase element, C is the capacitance, R1 is the solution resistance, R2 is the titanium oxide based-film resistance and R3 is the passive layer resistance. The circuit shown in Fig. 11a was the one that best fits the uncoated sample. In Fig. 11b it can be seen the EEC for the coated specimens. The extracted values are presented in Table 6.

Fig. 11.

ECs used to simulate the EIS experimental data: (a) uncoated AISI 316 stainless steel; (b) coated AISI 316 stainless steel.

(0.05MB).
Table 6.

EIS fitting parameters adjusted using the ECs shown in Fig. 11 for uncoated and titanium-based coated AISI 316 austenitic stainless steel.

  R1 (Ω.cm²)  CPE2 (cm²s-nΩ)  n 2  R2 (Ω.cm²)  C3 (cm²s-nΩ)  R3 (Ω.cm²) 
Substrate  376.9  2.12×10−5  0.8579  1.20×106  –  – 
TiO2 400º  377.7  2.02×10−5  0.8661  1.67×105  1.76×10−6  9.28×105 
TiO2 500º  287.0  9.40×10−6  0.9154  1.49×106  7.74×10−6  8.68×105 
N-TiO2 400º  377.5  1.89×10−5  0.7259  983.8  1.82×10−5  1.23×106 
N-TiO2 500º  201.9  1.80×10−4  0.4534  498.8  5.95×10−5  4.23×105 

The lowest n2 values were found for N-TiO2 samples. The capacitance CPE2 is associated to the resistance R2. The values presented for N-TiO2 sample at 500°C are one order of magnitude higher than those obtained for the uncoated (substrate) sample. This result indicates the presence of a more defective film on this surface. The inner layer resistance R3, which is related to chromium oxides, showed values for N-TiO2 samples five and six orders of magnitude higher for the films grown at 500 and 400°C, respectively, than those obtained for the same films for R2.

Fig. 12 shows the linear polarization curves for the substrate, TiO2 and N-doped TiO2 films grown at 400°C and 500°C. It can be observed that the substrate presents well-defined passive layer formation with corrosion potential of −276mV and corrosion current of 0.163μA.cm−2. The breakdown of the passive film occurred at approximately +440mV. The TiO2 covered at 400°C sample presents values of corrosion potential a little nobler than the substrate, and corrosion current density about an order of magnitude lower. It presents a passive region with breaking potential of 482mV, very close to the uncoated steel. The sample covered by the N-doped TiO2 film grown at 400°C showed potential and corrosion current similar to the undoped film and presents a very discrete passive region. The samples coated at 500°C with TiO2 and N-doped TiO2 films showed no passive film formation, and the curves are representative of film dissolution. It is observed that the films produced at 500°C showed a tendency to dissolve, apart they were doped or not.

Fig. 12.

Linear polarization curves for the substrate, TiO2 and N-doped TiO2 films grown at 400°C and 500°C.

(0.16MB).

Values of corrosion potential (Ecorr), break potential (Ebreak), and corrosion current density (icorr) were determined from the polarization curves and are displayed in Table 7.

Table 7.

Electrochemical parameters determined from the potentiodynamic polarization curves shown in Fig. 12.

  Ecorr (mVSCEEbreak (mVSCEIcorr (μA.cm−2
Uncoated  −276  +440  0.163 
TiO2 400°C  −32  +482  0.342 
TiO2 500°C  −220  –  0.293 
N-TiO2 400°C  −182  78  0.117 
N-TiO2 500°C  −318  –  0.764 

The shift on the corrosion potential for TiO2 and N-doped TiO2 samples obtained at 400°C may be explained by the morphology obtained that diminish the access of the NaCl solution to the substrate. Similar results were found for AISI 316L SS and AISI 304L SS coated by titanium oxides in different media [48–50].

Fig. 13a and b show the surface of the sample covered by TiO2 at 400°C after potentiodynamic polarization test, evidencing corrosion pits on the film that exposes the substrate. The substrate dissolution under the TiO2 layer is evidenced in this figure, which corroborates the passive behavior presented by this sample. However, despite the passivation, the passive film breaking potential was in the same order that the substrate. Fig. 14 shows the surface of the stainless steel AISI 316 covered by an N-doped TiO2 film of grown at 400°C, after potentiodynamic polarization test in 3.5 % NaCl solution at room temperature and shows the delamination of the film.

Fig. 13.

(a) Surface of stainless steel AISI 316 covered by a TiO2 film grown at 400°C, after potentiodynamic polarization test in 3.5 % NaCl solution at room temperature; (b) higher magnification in a region of (a).

(0.58MB).
Fig. 14.

(a) Surface of the stainless steel AISI 316 covered by a of N-doped TiO2 film grown at 400°C, after potentiodynamic polarization test in 3.5 % NaCl solution at room temperature showing delamination of the film; (b) higher magnification in a region of (a).

(0.37MB).

TiO2 films grown at 400°C tend to offer better alloy protection than those grown at 500°C, probably due to the microstructural characteristics. Films grown at 500°C showed a columnar structure, which represents a high level of porosity, while the films grown at 400°C presented a denser structure. Similar results were reported by Antunes et al. [10]. In relation to the nitrogen-doping, the contents added not only do not protect the alloy, but also showed a deleterious effect on its corrosion resistance. A possible explanation is the formation of nitrogen-rich phases besides TiO2. Furthermore, the surface of the samples is hydrophilic, that is, has high affinity for water [51] and affects the corrosion behavior of the films [52].

The corrosion process in the films starts at the films surface that is in contact with the aggressive medium. A pit is formed in the film, which allows the corrosive medium to access the metallic substrate. The metal is attacked and dissolved under the film, and results in its delamination.

4Conclusions

Undoped and nitrogen-doped TiO2 films were grown on AISI 316 stainless steel substrates by the metallorganic chemical vapour deposition technique using titanium tetraisoproxide as titanium and oxygen precursors, and ammonia as nitrogen source. It was shown that the growth temperature influences the films morphology, the phases formed, as well as the nitrogen content incorporated into the films. All the films showed columnar morphology; however, at 500°C the microstructure is more porous.

Nitrogen was successfully incorporated into the TiO2 films in a content of 6.18 at% at 400°C and 8.23 at% at 500°C. Besides TiO2, nitrogen-rich phases were identified in the films. All the films presented hydrophilic characteristics and the contact angles varied from 48° to 72°.

The films presented good homogeneity, low porosity and rounded grains in the range of 40–90nm. The increase in the growth temperature, promoted the increase of the roughness values and the mean grain size of the films.

TiO2 films grown at 400°C showed better corrosion protection than those grown at 500°C, due to their microstructural differences. Films grown at 500°C showed a porous columnar structure, which represents a high level of porosity that favours the access of the electrolyte, while the films grown at 400°C presented a denser columnar structure. Nitrogen doping, in the incorporated contents, was not efficient to protect the substrate from corrosion possibly due to the formation of the nitrogen-rich phases.

Conflict of interest

The authors declare no conflict of interest.

Acknowledgements

The authors are thankful to the Brazilian Nanotechnology National Laboratory - LnNano for XPS analyses, and to the Brazilian agencies CAPES, CNPq (Proc. 168935/2018-0) and FAPESP (Proc. 05/55861-4) for the financial support.

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