Effect of coating composition on the anticorrosion performance of a silane sol–gel layer on mild steel

Abstract

In this study, the effect of coating composition on the protective performance of an eco-friendly silane sol–gel film consisting of tetraethoxysilane (TEOS), methyltriethoxysilane (MTES) and glycidyloxypropyltrimethoxysilane (GPS) applied on a mild steel substrate was investigated using electrochemical impedance spectroscopy and surface analysis methods. Through taking advantage of coating resistance, charge transfer resistance and low frequency impedance parameters, the optimum formulation of silane molecules offering the most effective corrosion resistance to the mild steel in 0.1 M NaCl solution was determined. The reaction between glycidyl groups of GPS molecules with silanol groups as shown by FTIR spectra may result in a strong reticulated network. However, water contact angles revealed an increase in hydrophilicity of the hybrid coating containing a high concentration of GPS, probably due to the higher amount of –CH–OH, produced in the reaction between glycidyl and silanol groups. As a result, it is found that there might be an optimum point in which a more reticulated structure overcomes the silane layer hydrophilicity.

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1. Introduction

Organosilane films which are widely known as coupling agents between substrates and organic coatings can enhance corrosion resistance of metallic structures.¹ Providing an efficient physical barrier against aggressive species toward the coating/metal interface, they are gradually becoming a good alternative to carcinogenic chromate coatings.² Various strategies have been already proposed to improve anticorrosion properties of the hybrid sol–gel films. Literature reports that incorporation of a certain concentration of corrosion inhibitors such as cerium salts could offer an active protection to the sol–gel coating applied on various metal substrates.^2–5 It is believed that the inhibiting species, which form a protective layer at the coating/substrate interface, can restrict the electrochemical reactions.^2,3,6 An enhancement in barrier properties, due to the flake morphology of some nanoparticles such as cloisite and reticulation of silane layer, arising from interaction between the particles and silanol groups has been reported to be responsible for the improved corrosion resistance of silane films.^7,8 A structural rearrangement of the particles after sonication and addition to the silane-based matrix, which is connected to new and remarkable interactions between certain sites of the particles and Si atoms of the matrix, accounts for the improved corrosion protection.¹ Some studies clearly showed the importance of heat treatment in the silane coating performance, meaning any deviation from the optimum curing condition may lead to an insufficient condensation or developing defects/cracks in the overbaked films.^9–11 Effect of some parameters related to the silane solution was also studied in a number of papers.^12,13 For instance, the significant role of silane solution pH, affecting the rate of hydrolysis, in the hybrid layer performance was shown.¹⁴ In general, hybrid coatings composed of various silane molecules have been successfully examined as corrosion resistant pretreatments for different metallic substrates.^15–18 According to their chemical structure, silane molecules applicable for corrosion protection can be divided into “mono-silanes” (e.g. γ-mercaptopropyltrimethoxysilane and γ-ureidopropyltriethoxysilane) and “bis-silanes” (e.g. bis-1,2-[triethoxysilyl]ethane and bis-[triethoxysilylpropyl]tetrasulfide) categories.¹⁹ According to Cabral et al.,¹⁹ application of bis-silanes on aluminum leads to thicker and more homogeneous films.

As mentioned previously, the influence of a variety of parameters on the protective function of silane sol–gel films has been already studied. However, few reports can be found in the literature investigating the effect of silane solution composition on the protective performance of sol–gel coatings. This work intends to enhance the corrosion resistance of mild steel in a sodium chloride solution using hybrid coatings composed of different ratios of three silane molecules, namely tetraethoxysilane (TEOS), methyltriethoxysilane (MTES) and γ-glycidyloxypropyltrimethoxysilane (γ-GPS). The film structure is formed by a silica network of Si–O–Si bonds which is provided mainly by TEOS molecules, interrupted in some points by CH₃ groups attached to silicon atom from MTES.²⁰ γ-GPS is an organofunctionalized silane composed of a short carbon backbone with an epoxy functionalized tail and a Si atom substituted with (–O–CH₃) groups.¹² The eco-friendly silane coatings were characterized through electrochemical impedance spectroscopy (EIS), FTIR, FE-SEM and water contact angle measurements. Since the silane solution in this study was completely water based and no organic solvent was present, this technology can be considered eco-friendly.

2. Experimental

2.1. Materials

Mild steel panels²¹ were abraded with abrasive papers starting from 400 to 1000 grit size. The samples were rinsed with deionized water and dried in air, then followed by acetone degreasing. To chemically activate the sample surface, mild steel was dipped in 25 g l⁻¹ NaOH solution for 7 min at 55 °C. Finally, the substrate was rinsed in distilled water and blow-dried with compressed air. The alkaline treatment provides a high surface density of hydroxyl groups on the plate. The subsequent interaction between the metal surface and the hydrolyzed silane molecules is due to the presence of Fe–OH bonds.

The silane molecules employed in this study were tetraethoxysilane (TEOS), methyltriethoxysilane (MTES) and γ-glycidyloxypropyltrimethoxysilane (γ-GPS), purchased from Aldrich. In order to prepare the sol, 10% (w/w) of the silane mixture including four different weight percentages of the three components was dissolved in distilled water. The composition of different silane solutions is presented in Table 1.

Table 1 Composition of silane solutions

Sample	TEOS (wt%)	MTES (wt%)	γ-GPS (wt%)
S1	40	20	40
S2	50	—	50
S3	50	50	—
S4	40	40	20

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The value of pH was adjusted to 3.2 using acetic acid. The mixture was then magnetically stirred at room temperature for 24 h at a rate of 1000 rpm, resulting in a clear and homogenous solution.

The coating application was carried out through vertical dipping of the surface pretreated samples for 120 s in the prepared silane solutions. The silanized plates were then put in an oven at 150 °C for 30 min.

2.2. Methods

EIS measurements were executed in a conventional three-electrode cell, using a prepared sample as working electrode, a platinum counter electrode and a saturated calomel reference electrode. To perform the electrochemical tests, 1 × 1 cm² of the sample surface was exposed to 0.1 M NaCl solution at 25 °C. After 2 and 5 h of immersion in the electrolyte, electrochemical measurements were carried out employing a Solarton 1260 frequency response analyzer (FRA) and potentiostat-Galvanostat EG&G Model 273A. The EIS measurements were executed at open circuit potential (OCP) within the frequency domain 100 kHz to 0.01 Hz using a sine wave of 10 mV amplitude peak to peak. During the electrochemical measurements, the cell was placed in a Faradic cage to minimize possible external electromagnetic interference. Data analysis was made using ZSimpWin 3.22 software.

The FTIR measurements were performed with the use of a Bruker Equinox 55 model instrument in the range of 4000–400 cm⁻¹.To prepare the powder sample for FTIR, several little droplets of the silane solution were put on a Teflon plate and cured at 150 °C for 30 min. After the curing process, the dried flakes, which were easily removed from the sheet, were milled to obtain a fine powder.

In order to determine water contact angle, the samples were horizontally placed on a sample holder. Subsequently, a drop of DI water (about 3 μl) was put on the sample surface, and an image was taken from the water drop. The angle between the baseline of the drop and drop boundary was measured using ImageJ 1.32j software. The contact angle of the drop on the surface was measured at ambient temperature. Each test was carried out using three replicates to ensure repeatability.

3. Results and discussion

Taking advantage of electrochemical impedance spectroscopy, the protective performance of hybrid films formed from different silane solutions was evaluated. Fig. 1 and 2 present the Nyquist and Bode diagrams of different samples after 2 and 5 h of immersion in 0.1 M NaCl solution, respectively.

Fig. 1 EIS diagrams of mild steel plates coated through dipping in the silane solutions with different compositions after 2 h of exposure to the sodium chloride solution.

Fig. 2 EIS diagrams of mild steel plates coated through dipping in the silane solutions with different compositions after 5 h of exposure to the sodium chloride solution.

Several features can be inferred from the impedance spectra. One glimpse of the diagrams is enough to find a decline in protection over immersion time. This might indicate the continuous degradation of the silane coatings and underfilm corrosion progress since the silane coating is believed not to exhibit active protection and acts only as a physical barrier to aggressive species.²¹ In the plots, the impedance at the lowest frequencies can be correlated with the corrosion resistance of the system.²² The higher |Z|_0.01Hz values can be considered as an indication of more enhanced corrosion protection. It is worth noticing that the sample S3, which have no γ-GPS in its composition, revealed the lowest |Z|_0.01Hz and phase angle at high frequencies. The tendency of AC current to pass through the resistor in the circuit leads to a drop in the phase angle. Hence, the system with higher resistance can be characterized by higher phase angle.²³

The appearance of impedance spectra indicated the presence of three different time constants. The first, located in the high frequency domain, can be related to the sol–gel layer.^1,24 The second, at intermediate frequencies, accounts for the presence of the intermediate layer between the outermost silane film and the substrate^22,25 and the third one in the lowest frequency range can be attributed to the corrosion process occurring at the metal/solution interface.¹⁹ To model the spectra, the equivalent circuit which is shown in Fig. 3 was used, where R_s represents the solution resistance, R_ct the charge transfer resistance and CPE_dl the double layer constant phase element, R_sol–gel, CPE_sol–gel, R_int and CPE_int the parameters concerning the silane sol–gel coating and the intermediate layer, respectively. Fig. 4 shows the evolution of charge transfer resistance and coating resistance (the sum of R_sol–gel and R_int) as parameters extracted from the impedance spectra. It is simple to distinguish a good correlation between the bar diagrams in Fig. 4 and low-frequency impedance modulus data plotted as a function of immersion time in Fig. 5. The inferior performance of sample S3 is clearly visible from the figures.

Fig. 3 Equivalent circuit to model electrochemical behavior of silanized mild steels within 5 h of immersion in 0.1 M NaCl solution.

Fig. 4 Influence of the silane coating composition on coating resistance and charge transfer resistance as parameters obtained from the impedance spectra.

Fig. 5 Evolution of low-frequency impedance modulus of different samples.

According to the EIS data, the samples containing γ-GPS revealed superior corrosion protection in comparison with those with no γ-GPS. Of the samples containing γ-GPS, the most effective corrosion protection was observed for sample S1. In other words, the sample had the highest low frequency impedance and coating resistance and charge transfer resistance values. On the other hand, the worst performance was detected in the absence of γ-GPS. For instance, the coating resistance of sample S1 was approximately ten times higher than that of sample S3 after 2 h of exposure to 0.1 M NaCl solution. Since the R_ct, as the resistance against charge transfer between a metal and the adjacent electrolyte, is inversely proportional to the corrosion rate,²⁶ it is found that the sample S1 provided the highest degree of protection for the steel substrate during the whole exposure.²¹ Furthermore, although the diagrams in Fig. 4 and 5 showed a declining trend for all the samples as immersion time increased probably due to continuous degradation of the silane films and progression of corrosion beneath the coating, sample S1 provided the most effective protection. The superior performance of the hybrid coatings in the presence of γ-GPS molecules might be attributed to the enhanced barrier properties. The strong reticulation can be arose from the ability of glycidyl groups of γ-GPS molecules to react with silanol groups of other molecules as shown in Fig. 6 and to bond with the rest of the linkages in the network. Thus, it seems that higher percentage of γ-GPS may lead to more crosslink density in accordance with the reaction depicted in Fig. 6. However, the best anticorrosion performance was not observed for sample S2 with the highest content of γ-GPS in its composition. This might be attributed to an increase in the coating hydrophilicity because of higher amounts of hydroxyl group, produced during the epoxide ring-opening reaction (Fig. 6). Then, there might be an optimum point in which more reticulated structure overcomes the film hydrophilicity. Sample S1 containing 40% γ-GPS may be such point among the other samples in this study.

Fig. 6 Reaction between silanol and glycidyl groups.²⁷

For all samples, the two resistances decreased steadily as time elapsed, probably due to the coating deterioration allowing further corrosion progress on the substrate surface.²¹

Fig. 7 shows the FTIR spectra of powder of the silane coating with the highest concentration of γ-GPS. From the figure, it is obvious that the diagram lacks the bands near 3050 and 3000 cm⁻¹, which are related to the CH stretching of the epoxide groups of γ-GPS molecules.²⁴ The epoxide group signal also appears near 1200, 910 and 840 cm⁻¹ due to asymmetric and symmetric C–O–C stretching (ring breathing). Moreover, a new band appeared near 1730 cm⁻¹ for C=O stretching could be associated with the oxidation of the epoxide ring. Therefore, the FTIR spectra might indicate the ability of glycidyl groups to engage in a reaction with silanol groups.

Fig. 7 FTIR spectrum of powder of silane coating with the formulation of S2.

Fig. 8 illustrates the water contact angles on the hybrid films with different concentrations of γ-GPS to determine the degree of surface hydrophilicity. From the figure, the water contact angle values obtained for samples S1, S2, S3 and S4 were 75.68, 72.49, 67.16 and 73.10, respectively. In the case of sample S3 with no γ-GPS, the lowest water contact angle could be an indication of the most hydrophilic surface. In other words, the coating with low crosslink density might be more prone to water uptake.²⁸ Furthermore, higher amount of –CH–OH groups in case of sample S2, resulting from decomposition of glycidyl groups, could be responsible for higher surface hydrophilicity.²⁹

Fig. 8 Water contact angles on the surface of silane coatings with different compositions.

The cross-section view and surface morphology of the sample S1 are presented in Fig. 9. The FE-SEM micrographs indicated that the silane sol–gel coating with an average thickness of approximately 690 nm, offering the best corrosion protection, is relatively uniform, crack-free and adhered to the substrate.

Fig. 9 FE-SEM images of the silane coating with the formulation of S1.

4. Conclusion

In the aid of electrochemical measurements and surface analysis, the effect of composition of an eco-friendly hybrid silane coating on the corrosion behavior of mild steel in a sodium chloride electrolyte was studied. According to the magnitude and trend of parameters extracted from the AC impedance spectra, the silane composed of 40% GPS, 40% TEOS, and 20% MTES could offer superior corrosion resistance to the mild steel in 0.1 M NaCl solution. This might be connected to the strong network reticulation as a result of the reaction between glycidyl groups of GPS molecules with hydrolysed silanes, confirmed by FTIR spectra. On the other hand, the results of water contact angle measurements indicated an increase in hydrophilicity of the film with high amount of GPS, probably due to higher amount of –CH–OH, produced in the reaction between glycidyl and silanol groups. Consequently, the GPS concentration in formulation S1 could be considered as the optimum point in which more reticulated structure overcomes the film hydrophilicity.

Notes and references

1. M. Fedel, E. Callone, S. Dire, F. Deflorian, M. Olivier and M. Poelman, Electrochim. Acta, 2014, 124, 90.
2. D. Wang and G. P. Bierwagen, Prog. Org. Coat., 2009, 64, 327.
3. R. Naderi, M. Fedel, F. Deflorian, M. Poelman and M. Olivier, Surf. Coat. Technol., 2013, 224, 93.
4. F. Ansari, R. Naderi and C. Dehghanian, RSC Adv., 2015, 5, 706.
5. M. Schem, T. Schmidt, J. Gerwann, M. Wittmar, M. Veith, G. E. Thompson, I. S. Molchan, T. Hashimoto, P. Skeldon, A. R. Phani, S. Santucci and M. L. Zheludkevich, Corros. Sci., 2009, 51, 2304.
6. A. Pepe, M. Aparicio, A. Duran and S. Cere, J. Sol-Gel Sci. Technol., 2006, 39, 131.
7. R. Naderi, M. Fedel, T. Urios, M. Poelman, M. Olivier and F. Deflorian, Surf. Interface Anal., 2013, 45, 1457.
8. E. Huttunen-Saarivirta, G. V. Vaganov, V. E. Yudin and J. Vuorinen, Prog. Org. Coat., 2013, 76, 757.
9. A. Phanasganokar and V. S. Raja, Surf. Coat. Technol., 2009, 203, 2260.
10. A. Franquet, C. le Pen, H. Terryn and J. Vereecken, Electrochim. Acta, 2003, 48, 1245.
11. B. Chico, J. C. Galvan, D. de la Fuente and M. Morcillo, Prog. Org. Coat., 2007, 60, 45.
12. A. Romano, M. Fedel, F. Deflorian and M. Olivier, Prog. Org. Coat., 2011, 72, 695.
13. W. J. van Ooij, D. Zhu, M. Stacy, A. Seth, T. Mugada, J. Gandhi and P. Puomi, Tsinghua Sci. Technol., 2005, 10, 639.
14. N. Asadi, R. Naderi, M. Saremi, S. Y. Arman, M. Fedel and F. Deflorian, J. Sol-Gel Sci. Technol., 2014, 70, 329.
15. W. J. van Ooij and D. Zhu, Corros. Sci., 2001, 57, 413.
16. R. Tremont, H. de Jesus-Cardona, J. Garcia-Orozco, J. Castro and C. R. Cabrera, J. Appl. Electrochem., 2000, 30, 737.
17. M. G. S. Ferreira, R. G. Duarte, M. F. Montemor and A. M. P. Simoes, Electrochim. Acta, 2004, 49, 2927.
18. V. Subramanian and W. J. van Ooij, Corros. Sci., 1998, 54, 204.
19. A. M. Cabral, R. G. Duarte, M. F. Montemor and M. G. S. Ferreira, Prog. Org. Coat., 2005, 54, 322.
20. A. Pepe, P. Galliano, M. Aparicio, A. Duran and S. Cere, Surf. Coat. Technol., 2006, 200, 3486.
21. N. Asadi, R. Naderi and M. Saremi, Appl. Clay Sci., 2014, 95, 243.
22. A. Cabral, R. G. Duarte, M. F. Montemor, M. L. Zheludkevich and M. G. S. Ferreira, Corros. Sci., 2005, 47, 689.
23. R. Naderi and M. M. Attar, Prog. Org. Coat., 2014, 77, 830.
24. C. M. Bertelsen and F. J. Boerio, Prog. Org. Coat., 2001, 41, 239.
25. L. M. Palomino, P. H. Suegama, I. V. Aoki, M. F. Montemor and H. G. de Melo, Corros. Sci., 2008, 50, 1258.
26. F. Deflorian, S. Rossi, M. Fedel and C. Motte, Prog. Org. Coat., 2010, 69, 158.
27. P. R. Underhill, G. Goring and D. L. DuQuesnay, Appl. Surf. Sci., 1998, 134, 247.
28. L. M. Palomino, P. H. Suegama, I. V. Aoki, M. F. Montemor and H. G. de Melo, Corros. Sci., 2008, 50, 1258.
29. T. H. Muster and C. A. Prestidge, J. Pharm. Sci., 2005, 94, 861.

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