Improvement in the corrosion resistance of stainless steel 304L in sodium chloride solution by a nanoclay incorporated silane coating

Abstract

An eco-friendly silane sol–gel coating incorporating nanoclay was formulated to provide an effective corrosion protection for stainless steel 304L in a NaCl solution. Electrochemical measurements were used to determine the nanoparticle content in which the most efficient protection was provided. In consistency with the EIS data, electrochemical noise analysis indicated the significant impact of nanoclay concentration on the coating barrier properties. According to the surface analysis including XRD, FTIR, FESEM, AFM and water contact angle measurements, the behavior might be attributed to the dispersion of nanoparticles, chemical analysis of the films, surface hydrophobicity, thickness, roughness and morphology of the coatings.

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1. Introduction

Various approaches have been proposed to enhance the protective performance of silane coatings functioning as a physical barrier against the penetration of aggressive species toward the coating/metal interface.^1–7 The corrosion inhibitors appear to provide an active protection when electrolyte reaches the substrate.^1,4,5,8–17 The inhibiting species incorporated in the silane film might migrate and produce a passivating deposit where a defect originates.^14,16 Rare earth elements, particularly cerium, have been frequently reported as efficient cathodic corrosion inhibitors.^{7,13,14,16–20} According to Trabelsi et al.,²¹ pre-treatments for galvanised steel based on the use of bis-[triethoxysilylpropyl]tetrasulfide silane doped with cerium nitrate show very good anti-corrosion properties. Zandi zand et al.¹⁸ showed that the addition of Ce component to a silane sol–gel coating on 304L stainless steel can lead to an improvement in the corrosion protection. The cerium produces insoluble hydroxides when it reacts with hydroxyl groups from cathodic half-reactions in corrosion. These hydroxides together with corrosion products decrease the cathodic current and, therefore, reduce the overall corrosion rate. The results of Wang's study have also demonstrated that a high performance anti-corrosion coating, based upon a silane sol–gel system with an incorporated cerium nitrate inhibitor, can be produced and successfully applied to mild steel substrates.¹⁹ In general, the sol–gel coatings contain micro-pores, cracks and areas of low cross-link density that provide pathways for diffusion of corrosive species to the interface. The crack ability and porosity of the sol–gel films can be decreased by incorporation of nanoparticles into the hybrid matrix.¹³ To enhance the corrosion protection of silane films, a variety of oxide nanoparticles, such as ceria, silica, titania and zirconia, has been examined.^{7,13,14,17,22–29} In addition to formation of a crack-free film, the pore plugging effect of nanoparticles can improve the barrier properties.²⁴ The incorporation of commercial SiO₂ nanoparticles to a mixture of tetraethylorthosilicate and methyltriethoxysilane has been carried out by Castro et al. who obtained uniform crack-free sol–gel films on AISI 304 stainless steel substrate.^22,23 Nevertheless, when the optimum concentration is exceeded, an increase in the film porosity may facilitate penetration of electrolyte into the coating.²⁴ Liu et al.²⁷ prepared silica nanoparticle-filled silane films by electrodeposition technique. Experimental results showed that even tiny amounts of silica nanoparticles could enhance the protectiveness of silane-treated aluminum, while higher amounts of silica nanoparticles would lead to the deterioration of silane film protectiveness. Moreover, Liu and coworkers²⁸ showed that BTSPS/TiO₂ hybrid films not only offer good photocathodic protection under UV irradiation but also offer excellent barrier protection in the dark.

According to the literature, another way to modify the protective function of silanes is to use the combination of nanoparticles and corrosion inhibitors.^7,14,22–25 In previous publication, the synergistic effect of insertion of clay nanoparticles along with cerium nitrate as a corrosion inhibitor into an eco-friendly silane coating deposited on pure Al was studied.⁴ Rosero-Navarro et al.¹⁴ used a combination of SiO₂ nanoparticles and cerium nitrate, as a source of Ce(III), to improve the mechanical and barrier properties of a silane sol–gel film. Moreover, silane films formed using bis-[triethoxysilylpropyl]-tetrasulfide silane filled with of SiO₂ and CeO₂ nanoparticles revealed improved barrier properties compared with the blank silane films.³⁰ In the field of protective coatings, nanoclay has gained many attentions as an effective anticorrosion additive.^1,3,4,8 The presence of clay nanoparticles inside a matrix can enhance the barrier properties through filling of the micro voids and crevices. Because the nanoparticles increase the length of diffusion paths of the aggressive species through the film, they can delay the corrosion process beneath the coating.^{1,2,10,31–33} Some reports have already demonstrated the influence of nanoclay on the corrosion performance of silane coatings.^{3,8,11,32,34–36} Jothi and Palanivelu⁸ reported the modified corrosion resistance of stainless steel 304 by a silane coating designed by dispersing nanocomposites (Cloisite 15A, multiwalled carbon nanotubes and cerium chloride) which act as a corrosion inhibitor. Fedel et al.³ showed that the sonication treatment applied to an aqueous suspension of sodium montmorillonite nanoparticles can affect the corrosion protection properties of the sol–gel coatings on hot dip galvanized (HDG) steel plates. Deflorian et al.³⁷ successfully applied high-performance silane sol–gel films on galvanized steel by employing Cloisite and cerium oxides grafted montmorillonite nanoparticles. The impact of Cloisite nanoparticles concentration on the protective performance of an eco-friendly silane sol–gel coating applied on mild steel was investigated previously. According to the literature, no report can be found determining the optimum nanoclay content in a silane sol–gel coating which provides the most effective corrosion resistance for stainless steel in an electrolyte containing chloride ions. Although stainless steels are widely used in different industrial applications because of their mechanical and corrosion inhibition properties, they are susceptible to localized corrosion in the presence of chlorides or other aggressive ions.^8,17,18

The aim of this work is to modify the behavior of stainless steel 304L in a sodium chloride solution by an eco-friendly silane sol–gel film containing clay nanoparticles. Electrochemical impedance spectroscopy (EIS) and electrochemical noise (EN) were used as powerful electrochemical tools to study the influence of nanoclay concentration on the corrosion protection performance of the hybrid coating synthesized by mixing of gamma-glycidyoxypropyltrimethoxysilane (γ-GPS), tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES). Determination of the optimum concentration of nanoclay was achieved with the aid of the electrochemical techniques and structure analysis methods including X-ray diffraction (XRD), Fourier-transformed infrared spectroscopy (FTIR), field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM) and contact angle (CA) measurements.

2. Experimental

2.1. Materials

Stainless steel panels with the chemical composition presented in Table 1 were abraded with abrasive papers starting from 600 to 1200 grit size. The samples were rinsed with distilled water and blow-dried with compressed air, then followed by acetone degreasing. The alkaline surface treatment was performed through 15 min dipping in 2.5 M NaOH solution at 90 °C. This treatment can provide an alkaline etching and a chemical activation of the surface. The alkaline treatment leads to the formation of a high surface density of hydroxyl groups on the metal sample. The presence of Fe–OH bonds is responsible for the subsequent interaction between the metal surface and the hydrolyzed silane molecules.^38,39 The substrates were then rinsed in distilled water and blow-dried with compressed air.

Table 1 Chemical composition of the stainless steel 304L panels

Elements	Fe	C	Si	Mn	P	S	Cr	Ni	Mo	Al	Cu
wt%	Base	0.0317	0.591	1.18	0.0211	<0.00020	18.65	9.17	0.174	0.00052	0.138

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The silane molecules employed in this study were tetraethoxysilane (TEOS), methyltriethoxysilane (MTES) and gamma-glycidoxypropyltrimethoxysilane (γ-GPS), purchased from Aldrich. In order to prepare the neat sol, 10% (w/w) of the silane mixture including an equal weight percentage of the three components was dissolved in distilled water. The value of pH was adjusted to 2.1 using HCl. The mixture was then magnetically stirred at ambient temperature for 24 h at a rate of 1000 rpm, resulting in a clear and homogenous solution.

Nanoparticle incorporation was performed using ultrasonic dispersion of different concentrations of sodium montmorillonite particles (NaMt), 500, 1000, 2000 and 5000 ppm into the acidified water prior to inclusion of the three-component silane mixture. The sodium montmorillonite particles were Cloisite®Na supplied by Southern Clay Products U.S.A. The coating application process was carried out through vertical dipping of the alkaline surface pretreated samples for 10 s in the prepared silane solution. After drawing of the samples, a heat treatment in an oven was executed at 150 °C for 30 min.

2.2. Electrochemical tests

To perform EIS test, a three electrode cell including the prepared stainless steel specimen as working electrode, a platinum counter electrode and a saturated calomel reference electrode (SCE) was used. The measurements were performed using a Solarton 1260 frequency response analyzer (FRA) and potentiostat–galvanostat EG&G Model 273A. The impedance spectra were obtained at the open circuit potential (OCP) within the frequency domain 100 kHz to 0.01 Hz using a sine wave of 10 mV amplitude. Data analysis was made using Zview 3.1c software.

Electrochemical noise measurements were carried out using an Autolab instrument model PGstat 302 N. Electrochemical potential and current noises were simultaneously measured in a freely corroding system employing two nominally identical working electrodes of the same area and a saturated calomel reference electrode. The area of each electrode exposed to the solution was about 1.0 cm². The reference electrode was placed in the middle of 1 cm distance between the two working electrodes. During the electrochemical measurements, the cell was placed in a Faraday cage to minimize possible external electromagnetic interference. The noise data were recorded for 1024 s at a sampling rate of 1 s. Before statistical data analysis, DC trend removal was accomplished through moving average removal (MAR) method. The potential and current noise data collected in the time domain were transformed in the frequency domain through the fast Fourier transform (FFT) method. Hann window function was used to present power spectral density (PSD) plots.

The electrochemical tests were performed after 2, 7, 14 and 21 days of immersion in a 3.5 wt% NaCl solution at ambient temperature. To seal the edges and back sides of the panels, they were covered with a beeswax–colophony mixture, leaving an area of 1 cm² unmasked.

2.3. Structure analysis

The XRD analysis on the silane powder were performed by Philips XPert pro X-ray diffractometer using Cu K(α) radiation (λ = 0.15406 nm) and step size of 0.02 over the 2 theta range 10–110 degree. To prepare the powder, a few drops of the sol–gel solution were applied on a Teflon sheet and cured at 150 °C for 30 minutes. The drops were small enough to allow a complete curing of the silanols. After the heat treatment, the sol–gel flakes which can be easily removed from the sheet were milled to obtain a powder. The procedure was repeated to get enough material to perform XRD measurements.

The FTIR measurements were performed with the use of a Bruker Equinox 55 model instrument in the range of 600–4000 cm⁻¹. The morphology and thickness of the hybrid films was studied using a ZEISS σIGMA VP (Germany) model field emission type scanning electron microscopy (FE-SEM). For the cross-section analysis, samples with size 1 cm × 1 cm were cut from the silane coated panel using a guillotine. The procedure was performed to ensure a sharp cut of the film. In order to determine water contact angle, the samples were horizontally placed on a sample holder. Subsequently, a drop of water was put on the sample surface, and an image was taken from the water drop at room temperature. The angle between the baseline of the drop and drop boundary was measured using image analysis software. To ensure repeatability, the surface analysis was performed on three replicates. Investigations for surface topography of the coatings were carried using atomic force microscopy (AFM) model Dual Scope C-26.

3. Results and discussion

Fig. 1 presents the typical Nyquist and Bode diagrams of the samples after 2 and 21 days of immersion in 3.5 wt% NaCl solution. From the figure, the Nyquist diagrams featured two capacitive loops, with the high-frequency loop and the low frequency one which are attributed to the silane sol–gel film and response of the interface, respectively.^36,40 Since the corresponding semicircles are overlapped, particularly after 21 days of immersion, it might be easy to detect the second time constant through the Bode plots. After 2 days of exposure, the curves in Fig. 4 indicated an enhancement in the barrier properties of the hybrid coatings in the presence of nanoparticles. Moreover, the influence of nanoclay concentration is obvious from the impedance spectra. At the end of immersion period, the diameters of high frequency semicircles indicated that incorporation of 1000 ppm of nanoclay can lead to the most effective barrier to movement of the electrolyte through the coating network.

Fig. 1 Typical EIS diagrams of stainless steel plates coated with silane films containing different nanoclay concentrations after (a and b) 2 days and (c and d) 21 days of exposure to 3.5% NaCl solution.

Because of the improvement of the coating resistance in the presence of 1000 ppm of nanoclay, the values of impedance modulus at low frequency range remained rather high for long immersion times. The Bode plots showed a change in |Z|_{0.01 Hz} from 1 × 10^6.195 Ω cm² after 2 days to 1 × 10^6.135 Ω cm² at the end of immersion. Further, the protective performance was significantly failed by increasing the nanoparticle content over 1000 ppm for long immersion time. It was found that the |Z|_{0.01 Hz} of the sample with 5000 ppm of nanoclay deteriorated from about 1 × 10⁶ Ω cm² at 2 days of immersion to 1 × 10^5.49 Ω cm² at the end of exposure. The electrochemical system behaves similar to an electronic circuit and thus can be numerically fitted to an equivalent circuit model so that the impedances of the different elements can be estimated. Fig. 2a depicts the equivalent circuit to model electrochemical behavior of the samples during 21 days of immersion in the electrolyte, where R_s represents the solution resistance, R_ct the charge transfer resistance, CPE_dl non ideal double layer capacitance, R_C and CPE_C parameters concerning the silane coating.^10,41 The resistors and capacitors in the equivalent circuit can be correlated with the physical features of the electrochemical system. In the circuit, CPE has been introduced account for the frequency depression phenomenon. According to eqn (1), the parameter consists of Y₀ and n which are the magnitude of the CPE and exponent of CPE, respectively.^41,42 In the equation, j represents the imaginary number and ω the angular frequency.

Fig. 2 (a) Equivalent circuit to model the behavior of stainless steels coated with the silane films incorporating different concentrations of NaMt within 21 days of exposure to 3.5 wt% NaCl solution and (b) Bode diagrams for the measured data points along with the fitted curves (solid line), which is equivalent to the electrical circuit with two time constant.

The CPE exponent ranging from 0 to 1 is a good indication of the surface condition. The constant phase elements correspond to a capacitor when the CPE exponent is one. To calculate the coating capacitance, eqn (2) was used.⁴⁰

C_c = (Y₀R¹⁻ⁿ)^1/n (2)

A typical fitting result of EIS diagrams with the equivalent circuit having two time constants is shown in Fig. 2b.

The evolution of coating resistance and coating capacitance as parameters extracted from the impedance spectra is presented in Fig. 3. The capacitance of coating is a measure of water permeation into the coating. When a coating is submerged in a corrosive electrolyte, the C_c usually increases, particularly at the initial stage of exposure.⁴³ The results from the silane coatings with different nanoclay concentrations indicated an increasing trend for the C_c parameter during 21 days of immersion in 3.5 wt% NaCl solution, whereas the coating resistance encountered a drop. The data is in agreement with the results of previous works on aluminum alloys and mild steel with the eco-friendly silane coatings filled with NaMt nanoparticles.^4,36 The magnitude and trend of R_C clearly denoted the noticeable effect of 1000 ppm of nanoclay on the barrier properties of the hybrid film during the whole immersion period. For instance, the resistance of the silane coating with 1000 ppm of nanoclay was 6 times larger than that of the coating bearing the maximum concentration of the nanoparticles after 21 days of dipping in the electrolyte. This means that the rate of coating degradation was effectively retarded by 1000 ppm of the nanoclay. Moreover, the data in the presence of 2000 and particularly 5000 ppm of NaMt showed that the concentrations exceeding from the optimum value have an adverse effect on the coating barrier properties. As it can be seen from Fig. 3, there is a gradual increase in the C_c values over 21 days of exposure for the sample with 1000 ppm of the nanoparticles, associated with the electrolyte uptake.⁴⁴ However, the coating capacitance in the presence of 5000 ppm of nanoclay drastically increased at the end of dipping time. After 21 days, the C_c value of the sample with the maximum nanoclay content was two times higher than that of the sample with 1000 ppm of the nanoparticles. Therefore, analysis of the resistance and capacitance of the silane films showed a remarkable drop in the barrier properties when high concentrations of clay nanoparticles are entered to the silane matrix.

Fig. 3 Influence of NaMt concentration on the coating resistance and coating capacitance.

The low frequency impedance is also reported to be a good indication for monitoring the protective function of coatings.¹⁰ The effect of incorporation of NaMt on the corrosion protection properties of the silane layer is visible from Fig. 4a, showing variation of low frequency impedance values during 21 days of immersion in 3.5 wt% NaCl solution. It is clearly denoted from the impedance values at low frequency range that the sample with 1000 ppm of nanoclay demonstrates the best corrosion protection after 21 days of exposure in the corrosive environment as its value is the highest compared to the rest samples. Moreover, the |Z|_{0.01 Hz} values were found to depend upon the concentration of nanoparticles. Although the samples revealed an almost similar behavior during the first days of immersion, a big drop was occurred after 7 days for the silane film with maximum amount of nanoparticles (5000 ppm). Within the whole immersion period, the hybrid film with 5000 ppm of nanoclay showed the lowest values of the low frequency impedance even compared to the neat silane. Furthermore, a pronounced drop in the low frequency impedance values is observed for all samples except the coating with 1000 ppm of nanoclay at the end of immersion. As time elapsed, the corrosion protection failure can be associated with the electrolyte diffusion through the pores, and/or defects in the coatings.⁴⁴ In the presence of 1000 ppm of nanoclay, the impedance values at low frequency remained approximately constant during 21 days exposure to the sodium chloride solution. At the end of immersion period, the |Z|_{0.01 Hz} values of the stainless steels covered by the silane layers incorporating 1000 ppm of nanoclay (∼1364 kΩ cm²) were about two times more than those of the samples with 0, 500 and 2000 ppm of the nanoparticles, showing approximately the same low frequency impedance values (∼600 kΩ cm²). According to Fig. 4a, 1000 ppm of clay nanoparticles could be consequently considered as the optimum concentration in which the most effective protection was offered. The time variation of |Z|_{0.01 Hz} values indicated that a negligible protection is obtained in the presence of 500 ppm of the nanoparticles. On the other hand, the silane coating with the maximum amount of the nanoparticle had the lowest value of low frequency impedance (∼300 kΩ cm²). The incorporation of 5000 ppm of NaMt facilitated the coating deterioration. It is well-known that the nanoparticles can modify the barrier properties of silane coatings.^1,3,4,8,36 However, in the presence of 5000 ppm of nanoclay exceeding the optimum concentration, the material may become brittle and lose some of its cohesive capability.¹⁰

Fig. 4 Evolution of (a) low-frequency impedance modulus and (b) noise resistance for stainless steels covered by the silane coatings with different concentrations of nanoclay.

To provide better understanding of the role of clay nanoparticles in the protective function of the three component silane coatings, the corresponding electrochemical noise data were analyzed. One of the most useful parameter to assess the coating performance is the noise resistance (R_n) obtained by dividing the standard deviation of potential by the standard deviation of current (σ_V/σ_I).^36,45,46 Several workers have analyzed the relationship between the R_n and the low frequency limit of the electrochemical impedance.^47–50 Fig. 4b displays the variation of R_n for all samples as function of exposure time to 3.5 wt% NaCl solution. In agreement with the other publications,^36,40,51 a good trend correlation is observed between the R_n values and the data obtained from EIS. Unlike the stainless steel specimens with the silane sol–gel coatings incorporating 0, 500, 2000 and 5000 ppm of NaMt, the stainless steel coated with the hybrid film bearing 1000 ppm of nanoclay revealed a gradual decrease in the noise resistance within 21 day immersion. In addition to introduce 1000 ppm of nanoclay as the optimum concentration, the noise resistance indicated the adverse effect of incorporation of higher contents on the protective performance of the hybrid coating. It is clear from Fig. 4b that the R_n data is in consistency with the EIS results.

The time records of electrochemical current noise for the stainless steels coated with silanes including different concentrations of the nanoparticles after 21 days of immersion in the electrolyte (Fig. 5) could confirm the results of noise resistance measurements. The level of electrochemical current noise can be described as being associated with the corrosion events beneath the coating. An increase in electrochemical current noise may imply an increase in the rate of electrochemical reactions which occur at the interface. The highest level of electrochemical current noise was observed in the presence of 5000 ppm of the nanoparticles.

Fig. 5 Time records of electrochemical current noise for stainless steels covered by the silane coatings with (a) 0, (b) 500, (c) 1000, (d) 2000 and (e) 5000 ppm of nanoclay after 21 days immersion in 3.5% NaCl solution (the noise data were recorded for 1024 s at a sampling rate of 1 s. DC trend was removed through moving average removal method).

In consistency with EIS and EN data, the current noise fluctuations in Fig. 5 indicated the significant impact of nanoclay concentration on the protective performance of the eco-friendly silane sol–gel coating. In comparison with the stainless steel samples with silane coating containing 500, 1000 and 2000 ppm of nanoclay, those with neat silane layer generated current fluctuations with higher amplitude. In the presence of 1000 ppm of NaMt, a significant decrease in the noise level could indicate lower electrochemical activity and be a reflection of superior corrosion protection.^36,40,51 In addition to the time records, the EN data in frequency domain could reveal the superiority of the hybrid coating with 1000 ppm of clay nanoparticles. Fig. 6 shows the effect of nanoclay incorporation on the PSD plots at the end of exposure. The enhanced corrosion resistance of the stainless steel covered by silane with 1000 ppm of NaMt may be found from the figure, where its PSD (I) plot appeared lower than that of the stainless steel coated with the neat silane.

Fig. 6 PSD (I) plots of stainless steels covered by silane with 0 and 1000 ppm of nanoclay after 21 days immersion in 3.5% NaCl solution (Hann window function was used to present PSD plots).

On a basis of comparatively electrochemical studies, the silane coating with 1000 ppm of NaMt was demonstrated to offer an enhanced corrosion resistance to stainless steel 304L in a sodium chloride solution. To provide an insight into the mechanism of protection, the coating structure was studied using some surface analysis methods. Some possible mechanisms might contribute to the improved protective function of the hybrid coating with the optimum nanoclay content. The nanoparticles could improve the quality of the cured silane film, reduce the porosity of the coating matrix and zigzagge the diffusion path available by deleterious species, leading to enhanced barrier performance of the silane coating.²

Furthermore, the hydrolysed silane molecules can graft to the Na-montmorillonite nanoparticles, leading to embedment of the clay into the hybrid network.⁵² This might be attributed to the exfoliation of the clay particles in the coating which acts as filler for micro voids and crevices that are exist in the film. Thus, the diffusion of the sodium chloride solution through the silane layer can be limited.¹⁰ The nanolayered silicates such as montmorillonite have gained attentions due to their high performance at low filler loadings, rich intercalation chemistry, high surface area, high strength and stiffness, high aspect ratio of individual platelets, abundance in nature, and low cost.⁵³ The layers are held together by relatively weak electrostatic forces and interlayer cations. Therefore, water and other polar molecules can enter between the unit layers leading to expansion of the lattice structure. To obtain exfoliated or delaminated structures, the clay layers should be completely and uniformly dispersed in a continuous matrix.⁵³ The exfoliation configuration is of particular interest because it can maximize the matrix–clay interactions. This should lead to the most significant changes in the coating properties.⁵⁴ The XRD spectra of the NaMt powder, neat silane and silane with 1000 ppm of nanoclay powders are shown in Fig. 7. The crystalline plot relates to the Na-montmorillonite. There is no obvious peak in the XRD spectra of the neat silane and silane incorporating nanoclay. The absence of diffraction peak is indicative of either an exfoliated morphology or highly intercalated structure for the prepared nanocomposites.⁵⁵ Therefore, it could be concluded that the nanoclay platelets in the silane film are intercalated. The complete dispersion of clay nanoparticles in the matrix led to the enhanced corrosion protective behavior.

Fig. 7 XRD spectra for nanoclay, neat silane and silane coating with 1000 ppm of nanoclay.

Evidence provided by FTIR confirmed the results of XRD analysis relating to reaction of the silanol groups with the NaMt nanoparticles. FTIR spectra of the 304L stainless steel substrates coated with the silane coatings incorporating 0, 500 and 1000 ppm of nanoclay are present in Fig. 8. The siloxane peak, in the range of 900 to 1100 cm⁻¹, intensified by increasing the NaMt content whereas a decrease was observed in the intensity of the broad dip at 3200–3500 cm⁻¹ identified with the SiO–H stretching signal as well as the corresponding silanol band around 920 cm⁻¹.^3,36,56,57 This could indicate that the pre-hydrolyzed silane has been successfully grafted to the clay nanoparticles, which is consistent with some reports in the literature.^36,58 These FTIR features revealed that the condensation reactions among the silanol groups could be promoted in the presence of the nanoparticles. The most effective barrier properties in the presence of 1000 ppm of nanoclay, which was shown in the electrochemical investigation, might be attributed to a strong reticulated network. The enhancement in coating effectiveness may be also arised from filling of the micro voids and crevices in the film.^10,59 Furthermore, clay nanoparticles can increase the length of the diffusion pathways of the corroding agent through the film due to their unique plate-like structure and high aspect ratio. Thus, the performance may make it more difficult for the deleterious species to seep through the coating towards the interface.

Fig. 8 FTIR spectra of the 304L stainless steel substrates coated with the silane containing different concentrations of nanoclay.

The decay of the protective function of hybrid films with increasing the nanoclay content over the optimum concentration has been reported in the literature for different metallic substrates.^4,41,60 It is suggested that an excessive nanoparticles can decrease the degree of polycondensation reactions, resulting in a lot of silanol groups still remained uncondensed.^36,60,61 Some similar results have been reported in the case of incorporation of zirconia and silica nanoparticles in silane films.⁶⁰ Water contact angles were measured to determine the degree of surface hydrophilicity of the hybrid films. The effect of nanoclay concentration on the θ_water of the silane sol–gel films is depicted in Fig. 9. In comparison to the neat silane, the nanoclay inserted coatings appeared less hydrophilic. Of them, the least surface hydrophilicity was observed for the silane with 1000 ppm of NaMt, meaning the surface is less prone to water. As a result of more condensation reactions between SiOH groups making the silane network denser, the surface hydrophilicity decreased.⁵² By increasing the nanoclay content over 1000 ppm, water contact angle met a drop. This might be connected to the higher amount of SiOH groups remaining without reaction; probably due to this fact the excessive nanoparticles can hinder the condensation reactions. Moreover, defects can appear when a silane film is heavily loaded with nanoparticles. Cracks and low cross-linking can lead to a porous film, facilitating the electrolyte diffusion.^5,60 In the presence of nanoclay over the optimum concentration, the decrease in water contact angle may also show increased surface roughness.

Fig. 9 Water contact angles on the surface of the silane films with different amounts of nanoclay.

In order to assess the effect of the nanoparticles on the hybrid coating structure, the surface morphology of the sample with 1000 ppm of NaMt was analyzed by means of FESEM (Fig. 10). The micrograph showed a uniform, defect and crack-free coating in case of the addition of the nanoclay content in which the most efficient protection was provided.

Fig. 10 Top-view FESEM image of the silane sol–gel coating with 1000 ppm of nanoclay.

Atomic Force Microscopy was employed to further investigate the nanoclay content on the structure of the hybrid coatings. Fig. 11, illustrating AFM images of the silane sol–gel films with 1000 and 5000 ppm of clay nanoparticles applied on stainless steel specimens revealed that the surface morphology depends upon the nanoparticle content. A relatively smooth and uniform surface morphology can be seen in the presence of 1000 ppm of nanoclay, whereas the addition of higher concentration made the film heterogeneous. The surface roughness of the coating with 1000 ppm of nanoclay was 30.3 nm (RMS), while that of the coating modified with the maximum nanoclay concentration was 109 nm (RMS). These heterogeneities might be due to the presence of agglomerates of the nanoparticles, micropores or defects in the coating.⁶² The surface topography of the coating can affect the inferior corrosion protection of the coating with 5000 ppm of nanoclay. In other words, the introduction of a large amount of nanoclay is likely to result in shrinkage of the hybrid matrix.⁴⁴ The properties can be explained by possible cracks that were formed on the coatings after their exposure to corrosive environment. The defects may permit the diffusion of water molecules and chloride ions into the films.⁴¹

Fig. 11 AFM images of silane coatings incorporated with (a) 1000 and (b) 5000 ppm of nanoclay.

To measure the coating thickness, cross-sections of the hybrid coatings with different amounts of nanoclay were observed by FESEM (Fig. 12). It can be observed from the figure that the coatings are relatively homogeneous and crack-free and adhered to the substrate. The thickness of the films with 0, 500 and 1000 ppm of the nanoparticles was about 350, 417 and 873 nm, respectively. According to the literature,⁵ an increase in the coatings thickness was seen as the nanoclay concentration increased. As mentioned previously, the anticorrosion performance of silane pretreatments is dependent on the coating barrier properties. Therefore, the FESEM results demonstrated that nanoclay-inserted films are likely to be more protective due to their increased thickness. The increase of thickness may retard access of the electrolyte to the substrate/coating interface.^60,63 In the correlation with electrochemical data, the silane with 1000 ppm of nanoclay having the highest thickness could play the role of an effective barrier to aggressive species.

Fig. 12 Influence of different concentrations of NaMt (a) 0, (b) 500 and (c) 1000 ppm on the cross-section morphology and coating thickness.

4. Conclusion

In the aid of electrochemical measurements and surface analysis, an eco-friendly hybrid silane coating with clay nanoparticles was developed to enhance the corrosion resistance of stainless steel 304L in a sodium chloride electrolyte. The results could be summarized as follow:

(1) In addition to confirming the low frequency impedance data, the noise resistance values indicated the significant impact of nanoclay content on the protective performance of the silane sol–gel coatings.

(2) As shown by electrochemical measurements, the silane with 1000 ppm of clay nanoparticles could offer superior corrosion resistance to the stainless steel 304L in 3.5% NaCl solution.

(3) The results of FTIR, XRD, FESEM, AFM and water contact angle measurements revealed that this superiority could be arised from the modified barrier properties, resulting from the film uniformity, coating thickness, strong reticulated network and complete dispersion of the nanoparticles.

(4) In agreement with the results of EIS, EN data revealed the inferior protective performance of the hybrid film incorporating 5000 ppm of nanoclay, the concentration exceeding from the optimum value. According to the surface analysis, the nanoparticle aggregation, film cracking that occurred during curing of the sol–gel films due to volume shrinkage and low cross-linking could provide easy paths for diffusion of electrolyte to the interface.

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