Mesoporous silica based reservoir for the active protection of mild steel in an aggressive chloride ion environment

Abstract

Spherical mesoporous silica (m-SiO₂) with well-ordered pores was synthesized by a modified Stöber method using CTAB micelles. The as-prepared silica was used as a reservoir to load a corrosion inhibitor. Steel samples that were coated with the inhibitor loaded silica reservoir exhibited improved corrosion resistance compared to the samples without the silica reservoir in an aggressive chloride environment. Localised electrochemical impedance spectroscopy (LEIS) and scanning vibrating electrode technique (SVET) data suggest that inhibitor release is triggered by the local corrosion phenomena, which leads to active protection of the metal.

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Introduction

Performance improvement in corrosion control methods is highly needed, thus eventually all process industries try incremental research to achieve the same. Organic coatings, which are a major and proven domain of corrosion control, have seen multi-directional advancements in the last decade. Avoiding the use of carcinogenic/toxic ingredients in corrosion protective coatings is a yet another milestone. Bio-mimetic coatings, intelligent/smart coatings, and self-healing coatings use borrowed knowledge from other frontier areas such as drug delivery and nature. Intelligent/smart coatings have a built in mechanism to release the required corrosion inhibitor while ensuring the barrier effect of the coating. This greatly reduces the degradation/interference of the erstwhile pre mixed inhibitor. Micro/nano reservoir/self healing materials incorporated in conventional coatings can sense surrounding environmental changes such as pH,^1,2 temperature,³ light,^4,5 aggressive ionic concentration,⁶ mechanical action,⁷ electric field,⁸ humidity,⁹ and pressure,¹⁰ and ensures the timely release of corrosion inhibitors or healing materials to compensate the defect areas, thus providing both excellent physical barrier effect and self-healing functionality. This development utilizes high end synthetic chemistry for the design of reservoirs and also for the release kinetics associated therein.^11–24 Porous materials with a well-defined size, shape and geometry in the nano to micro-scale have fascinated researchers and have a great impact in various applications. These materials possess unique properties, such as fibrous surface morphology, good thermal and hydrothermal stability, and high mechanical stability, which enable a large increase in surface area and access by guest species to their internal pores. The Stöber method is a facile and effective approach for the synthesis of mesoporous silica nanoparticles in an aqueous ethanol solution.^25–29 Template aided synthesis has been a focus of materials science as it provides a unique way to prepare a variety of ordered mesostructured materials with high surface areas and large uniform pores.^30–32 However, it is difficult to obtain materials with desired pore sizes and shapes because of the structural freedom of the long hydrophobic chains of the surfactants that are normally used as templates. Aqueous cetyltrimethyl ammonium bromide (CTAB), which has a relatively long alkyl chain (n = 16), gives spherical-like micelles at wider mixing compositions and allows precise control of pore geometry and pore size. Poly aspartic acid salts are a type of biodegradable, innocuous and environmental friendly bioorganic polymer, which are recognized as green material and are widely applied in areas such as agriculture, medicine, commodities, water treatment, and petroleum.^33–36 The pioneering work by Shchukin et al.^37–40 utilizes polyelectrolyte layer-by-layer assembly to build the pH sensitive shells of nano-containers in order to control the release of encapsulated corrosion inhibitors and achieve significant progress in feedback anticorrosion coatings. In this study, a modified Stöber method using CTAB as the template has been attempted in order to synthesize spherical mesoporous SiO₂ for use as a reservoir. The reservoir was loaded with poly aspartic acid (PAA) with subsequent multilayer coverage of polyelectrolytes and was used as an active corrosion protection coating. More insight into the smartness of the coating was established for the first time using LEIS studies.

Experimental

Synthesis of mesoporous SiO₂ (m-SiO₂)

In a typical procedure, 3 g CTAB was first dissolved in 100 mL of distilled water, which is called as micelle solution. 8.5 mL of TEOS, 24 mL of ethanol and 10 mL of distilled water were added to form a silanol mixture. The micelle and silanol solutions were mixed followed by the dropwise addition of 5 mL (2 M) NH₄OH with continuous stirring for 15 minutes. This solution was then transferred into an autoclave and was maintained at 120 °C for 3 h. Furthermore, silica was precipitated, separated by centrifugation and dried at room temperature. The surfactant was removed by calcination at 550 °C for 5 h. The mechanism for the formation of the m-SiO₂ spheres is described as follows (Fig. 1a).

Fig. 1 (a) Synthesis of mesoporous SiO₂. (b) Inhibitor loading and polyelectrolyte covering of SiO₂. (c) Zeta potential measurements of SiO₂-r.

First, the TEOS molecules approached the catalyst in the aqueous phase, followed by hydrolysis of the TEOS. The hydrolyzed TEOS molecules then condensed at the aqueous–oil interface. Finally, the resultant negatively charged silicates and the cationic surfactant, CTAB, steadily self-assembled at the aqueous–oil interface to form a silica sphere.

Synthesis of silica reservoir (SiO₂-r)

The as-synthesized silica water suspension (5 mg³ mL⁻¹) was mixed with an aqueous 3 M solution of poly aspartic acid (PAA, MW ∼155.01, 5 mg³ mL⁻¹) in a volume ratio of 1 : 1 and stirred for 2 h.

The loaded reservoirs were separated by centrifugation and dried overnight. Reservoir coverage by the PEI/PSS complex shell was done by the LBL technique in three subsequent steps, as reported by Shchukin (Fig. 1b). In the first stage, deposition of the positively charged PEI on the negatively charged SiO₂ nanoparticles was performed by mixing 20 mL of 5 wt% SiO₂ suspension with 5 mL of 2 mg mL⁻¹ PEI solution for 10 min with constant stirring. The SiO₂/PEI sample was thus obtained and it was washed three times by Milli-Q water. This washing procedure was performed after each deposition step. Deposition of the negative PSS layer was carried out using 5 mg mL⁻¹ PSS solution and the resulting (Fig. 1c) nano reservoir has a SiO₂ (PAA)/PEI/PSS structure.

Characterization of mesoporous SiO₂

FTIR spectra of the mesoporous of SiO₂ were recorded using a Nicolet 380 (Thermo, USA) FTIR instrument having ATR attachment. In situ FTIR measurements were obtained using an FTIR microscope (Nicolet Centraurus). The morphology of the reservoir was analysed using a Tescan (model vega3 SB-Easyprobe) Scanning Electron Microscope, a ZEISS (model Supra-VP55) Field Emission Scanning Electron Microscope (FESEM) and a Transmission electron microscope (Tecnai-G2 20) (TEM).

Zeta potential measurements were performed using a zetasizer (MALVERAN Nano Series, Malvern Instruments Ltd, Enigma Business Park, Grovewood Road, Malvern, Worcestershire, UK. WR14 1XZ).

X-ray diffraction patterns were obtained using a PAN Analytical (Model PW 3040/60) X-ray diffractometer using Cu Kα radiation in the 2θ range of 0°–90°and 0°–5° at the scan rate of 0.017°/2θ.

Corrosion protection evaluation of steel with/without the reservoir coating

EIS method. The PowerSuite software was used for the impedance data acquisition using a PARSTAT 2273 electrochemical workstation with an AC signal of 20 mV amplitude for a range of frequencies from 10 kHz to 0.1 Hz, which were impressed sequentially to the coated steel at the open circuit potential. From the Bode plots the solution resistance (R_s), coating resistance (R_c), charge transfer resistance (R_ct), double layer capacitance (C_dl) and coating capacitance (C_c) values were calculated by fitting the data in the equivalent circuits shown in Fig. 2a and (b) using the Zsimpwin software. The impedance data were analysed using the simple Randles circuit (Fig. 2a) in the case of the uncoated steel and the circuit shown in Fig. 2b for the coated steel.

Fig. 2 (a) Equivalent circuit for uncoated specimen. (b) Equivalent circuit for coated specimen.

From the measured charge transfer resistance value, the protection efficiency (PE) of the coatings was obtained from the relationship:

where R^*_ct and R_ct are the charge transfer resistance values in the presence and absence of the coating, respectively.

LEIS method. LEIS measurements were performed on a coated steel electrode, which had an artificial defect, using an SCV Model 470 Scanning Electrochemical Workstation that comprised a 470 scanning control unit, a 3300 potentiostat, a lock-in amplifier and a video camera system. The relative location of the microprobe to the WE was monitored by the video camera system. The scanning micro probe was operated in two modes. LEIS was operated in two different modes. The first mode was used for the complete spectrum of LEIS measurements. A microprobe with a 10 μm tip was set directly above the defect to measure the typical impedance response at that individual point. The distance between the probe tip and the WE surface was 100 μm, which was adjusted and monitored by the video camera. During the LEIS measurements, an AC disturbance signal of 10 mV was applied to the electrode at the open circuit potential. The measuring frequency ranged from 1 MHz to 1 μHz. The second mode was used for LEIS area mapping. The microprobe was stepped over a designated area of the electrode surface. The scanning took the form of a raster in X–Y plane. The step size was controlled to obtain a plot of 31 points × 31 points. The ac disturbance signal was 10 mV, and the excitation frequency for impedance measurements was fixed at 50 Hz.

Active corrosion protection property evaluation

The scanning vibrating electrode technique (SVET) instrumentation used in these experiments was from Princeton Applied Research (SCV 370 control unit). The samples prepared for SVET measurement were 1 × 1 cm squares with a scan area of 4000 × 4000 μm and 64 × 48 points X and Y axis. The coated sample was scribed to introduce an artificial defect of size ranging from 0.1 to 0.3 mm². The sample was mounted in a Teflon holder and 1% NaCl solution was added. Scans were initiated within 5 min of immersion and data were collected for various durations. Each scan consisted of 400 data points on a 20 × 20 grid with an integration time of 1 s per point. A complete scan required 10 min, followed by a 5 min rest period prior to the next scan. Current density maps were plotted in the 3D format over the scan area, with positive and negative current densities representing the anodic and cathodic regions, respectively. The measurements were taken at the open-circuit potential.

Results and discussion

Scanning electron microscopy (SEM) images (Fig. 3a and S1†) show spherical SiO₂ particles with a narrow size distribution of around 100 nm in diameter. The surface of SiO₂ was roughened due to the removal of the surfactant upon calcination (Fig. 3b). TEM images (Fig. 3c–h) indicate the formation of a well-defined mesoporo us structure. The nitrogen adsorption/desorption isotherm of the as-prepared SiO₂ is shown in Fig. 4a. The sample exhibits a type I isotherm, which clearly indicates its microporous nature. The narrow/low hysteresis appears at the desorption section, which indicates the occurrence of capillary condensation in the mesopores. The nitrogen sorption results for the SiO₂ spheres, which were calcined at 550 °C, show the average pore size of 3.1 nm (BJH pore size distributions are given in Fig. S2†), and BET surface area of 458 m² g⁻¹; moreover, the total pore volume of the mesoporous silica is 0.363 cm³ g⁻¹. The analysis of the poly aspartic acid loaded m-SiO₂, which was estimated using the nitrogen adsorption/desorption isotherm results, indicates the average pore size of 1.9 nm and BET surface area 8.3 m² g⁻¹; the total pore volume of the poly aspartic acid in m-silica is 0.181 cm³ g⁻¹. The long range ordering of the as-prepared SiO₂ was investigated using XRD. The sample shows a typical broad peak at 2θ = 21°, which corresponds to amorphous silica (inset of Fig. 4b). The broad peak may be due to the small size and incomplete inner structure of the particles. Fig. 4b shows the low angle XRD pattern of SiO₂. Three Bragg diffraction peaks could be observed at the 2θ low angles of 0.5°, 2.6° and 3.4°, in the range of 0 to 5, which could be assigned to the (100) (110) and (200) reflections of a hexagonal symmetry structure, respectively. No other impurity peaks are present, which indicates the purity of the silica nanoparticles. The FTIR data (Fig. S3†) of SiO₂ confirms the removal of CTAB upon calcination.

Fig. 3 SEM and TEM images of mesoporous SiO₂. Captions (a)–(f) indicate different magnifications of the mesoporous silica.

Fig. 4 (a) N₂ adsorption/desorption isotherm and XRD (b) pattern of the mesoporous SiO₂.

Sol gel solution preparation

The organosiloxane sol was prepared by hydrolyzing (3-glycidyloxypropyl) trimethoxysilane (GPTMS) in propan-2-ol by the addition of acidified water in a molar ratio (GPTMS–propan-2-ol–water) of 1 : 3 : 2. The final sol–gel was stirred under ultrasonic agitation for 60 min and then aged overnight at room temperature.

Evaluation of the corrosion protection ability of the coating

EIS analysis. The sol–gel system was homogeneous and transparent in nature. Before coating, the metal substrates were pre-treated in a pickling solution. The pre-treated substrates were coated with sol–gel and sol–gel with the silica reservoir (SiO₂-r). The sol–gel films were produced by the dip-coating method. After coating, the samples were cured at 110 °C for 24 h. The corrosion behavior of without/with SiO₂-r incorporated sol gel coated steel in 1% NaCl was evaluated by the EIS method. Fig. 5a and b show the Bode plot of the impedance behavior of the without/with SiO₂-r incorporated sol gel coated steel (active corrosion protection coating). The impedance parameters, such as coating resistance (R_c), coating capacitance (C_c), charge transfer resistance (R_ct), and double layer capacitance (C_dl), that were derived from these curves are given in Table 1. It can be clearly seen from Fig. 5a and Table 1 that the increased number of dippings of the steel surface in the sol gel coating increases the protection efficiency. The charge transfer resistance is increased from 25 Ω cm², which corresponds to the bare steel surface, to 142 Ω cm² after 3 dippings, which result in 82% protection efficiency. The impedance behavior of the SiO₂-r incorporated sol gel coated steel in 1% NaCl is shown in Fig. 5b. In this case, the R_ct value is increased drastically to 130 Ω cm² for the SiO₂-r incorporated coating in a single dip thus offering 82% protection efficiency and with further dippings the R_ct values increases marginally and shows 92% protection efficiency after the 3^rd dip. Moreover, the coating capacitance values of the SiO₂-r coated steel are found to be much less, which indicates the compact nature of the coating. On comparing the coating resistance values, the coating resistance for the reservoir incorporated coated steel is 159 Ω cm², which is one time higher than that of the sol gel coated steel. The higher coating resistance values in the case of the reservoir incorporated coated steel indicates the formation of a more compact coating. These results show that reservoir incorporation has improved the protective nature of the coating.

Fig. 5 (a and b) EIS plot of without/with SiO₂-r coating.

Table 1 EIS analysis of sol gel and active corrosion protection coating on mild steel in 1% NaCl

No. dipping	EIS-sol–gel coating					EIS-active corrosion protection coating
	Rc Ω cm^2	Cc F cm^−2	Rct Ω cm^2	Cdl F cm^−2	P.E %	Rc Ω cm^2	Cc F cm^−2	Rct Ω cm^2	Cdl F cm^−2	P.E %
Blank	—	—	25	7.4 × 10^−5	—	—	—	25	7.4 × 10^−5	—
1	37	3.4 × 10^−3	101	9.4 × 10^−5	75	112	1.1 × 10^−5	130	7.0 × 10^−5	80
2	59	3.3 × 10^−3	110	6.7 × 10^−5	77	133	1.3 × 10^−5	291	2.7 × 10^−5	91
3	72	2.7 × 10^−5	142	3.7 × 10^−5	82	159	7.6 × 10^−5	408	9.4 × 10^−6	93

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LEIS analysis. Fig. 6a shows the Bode diagrams of the pure sol gel coating, which was measured directly above the defect area at various immersion times, i.e., 0, 30, 60 and 1200 minutes. From the LEIS plot, it was seen that after the initial immersion, the impedance magnitude at low frequency decreased remarkably from 30 to 60 minutes of immersion, and then decreased continuously with the immersion time. The low frequency impedance at 0.01 Hz was approximately 2.87 × 10² Ω cm², which was lower than that measured at the defect area in the initial immersion (4.61 × 10² Ω cm²) (Fig. 6a). Fig. 6b shows the Bode diagrams of the SiO₂-r incorporated sol gel coating, which were was measured directly above the defect area at different immersion times such as 0, 30, 60 and 1200 minutes. It was seen that the impedance magnitude at low frequency increased remarkably from 30 to 60 minutes of immersion, and then increased continuously with the immersion time (i.e. 1200 minutes). The low frequency impedance at 0.01 Hz was approximately 1.15 × 10³ Ω cm², which was higher than that measured at the defect area in the initial immersion (3.61 × 10² Ω cm²) (Fig. 6b).

Fig. 6 (a and b) LEIS plot of without/with SiO₂-r coating.

Fig. 7a–d show the maps of localized impedance over the specimen from 0 to 1200 minutes of exposure in 1% NaCl. The defect area in the coating corresponds identically to the lowest value of the impedance map. Obviously, the mapping technique demonstrates the lowest resistance in the artificial defect surface. After the initial immersion, such as 30 minutes, the defect area on the impedance map (Fig. 7b) is reflected in by the large decrease in localized impedance at the defect.

Fig. 7 (a–d) LEIS area plot of the pure sol gel coating.

After 60 minutes, under film corrosion significantly increased (Fig. 7c). After 1200 minutes of immersion, the original area of under film corrosion further increased (Fig. 7d) and this coincides with the substantial decrease in localized impedance at the defect that is visible in the area map. Fig. 8a–d show the maps of localized impedance over the SiO₂-r incorporated sol gel coating specimen with exposure to 1% NaCl for 0 to 1200 minutes. Initially, the active corrosion protection coating on the defect area has a low value on the impendence map. After 30 minutes, the impedance map of the SiO₂-r incorporated sol gel coating (Fig. 8b) increased at the defect area. After a further 60 minutes of immersion, the impedance value further increased (Fig. 8c). Finally, after 1200 minutes of immersion, the defect area was completely passivated by poly aspartic acid. Furthermore, there was an increase in the localized impedance value at the defect area map (Fig. 8d).

Fig. 8 (a–d) LEIS area plot of the active corrosion protection coating.

In situ FTIR analysis. As can be seen in Fig. 9(a and b), the IR spectra from the bottom to the top correspond to the two samples.

Fig. 9 (a and b) FTIR analysis of the local spot without/with the reservoir coating.

The broad intense peak at 1240 cm⁻¹, which is due to the symmetrical stretching or ring breathing frequency of the epoxide ring, confirms the presence of Si–O–Si bridging sequences. The peak at 948 cm⁻¹ is due to the residual organic group and results in the asymmetric vibration of the Si–OH bonding. The vibration of Si–O occurs at 798 cm⁻¹. The band at 817 cm⁻¹ confirms the symmetric stretching vibrations of the Si–O–Si bonds that belong to the ring structure. The peaks at 1649 and 1400 cm⁻¹ are due to the presence of the stretching vibration of the amide group and free carboxylic acid in poly aspartic acid.

Active corrosion protection analysis. SVET was used to study the active corrosion protection of a steel surface without/with the SiO₂-r incorporated sol gel coating in 1% NaCl for different durations of exposure. Fig. 10a depicts the local current maps over the surface of the pure sol gel coated mild steel, which was recorded immediately after exposure to 1% NaCl (i.e. after 5 min). A steep anodic current flow in the flaw area indicates the occurrence of accelerated corrosion. With continued exposure, the corrosion activity at the defect areas spreads laterally as evidenced by the increase in width of the current flow pattern in 30 minutes exposure (Fig. 10a). Fig. 10b shows the SVET current mapping for the SiO₂-r incorporated sol gel coating on mild steel. As shown, there is an anodic current flow at the defect area in the initial instant. Fig. 10b shows the current density map for the coated steel after 30 minutes immersion in 1% NaCl. The anodic current flow area decreases dramatically at the defect area. We observed that the anodic current flow at the defect area becomes suppressed and is maintained at a very low current flow. Visually it has been observed that the defect is covered with a dark passive film. This clearly demonstrates the smart property of the SiO₂-r incorporated sol gel coating over a steel surface.

Fig. 10 (a) SVET plot of the pure sol gel coating. (b) SVET plot of the active corrosion protection coating.

In the case of the sample with the coating containing SiO₂-r, the corrosion protection is triggered by the corrosion process itself. It is well known that the corrosion process is accomplished by cathodic oxygen reduction, which results in the formation of OH– ions. These accumulated OH– ions increases the local pH, which causes morphological deformations in the polyelectrolyte layers and hence the inhibitor is released from the SiO₂-r network in a sufficient amount, without undesired leakage, to stop corrosion. The anticorrosive effect of the organic inhibitor poly aspartic acid (PAA) is based on the formation of a film on the metal surface, which acts as a physical barrier for the aggressive medium.^41–48

Based on these observations, a suitable mechanism has been proposed, as presented in Fig. 11, which is self-explanatory. In summary, a new route, which is a slightly improved version, combining a micro emulsion followed by hydrothermal processing has been attempted for the preparation of m-SiO₂ spheres. CTAB was used as the surfactant and the silica shells were self-assembled at the interface via ammonia catalyzed hydrolysis and condensation of TEOS. The excess amount of organic components was removed from the m-SiO₂ spheres by the treatment with distilled water, in which the mesoporous spheres could maintain their completely spherical structure. The prepared spheres were porous and an abundance of pores with the average pore size of 3.1 nm was formed in the shell. The smartness of the coating containing the inhibitor loaded mesoporous silica reservoir was evaluated by spot and area mapping LEIS, which is supported by SVET studies.

Fig. 11 Mechanism of active corrosion protection coating.

Conclusions

Modified synthetic strategies will become the general method to systematically manipulate the expected shapes and sizes of inorganic compounds in the nanoscale range. The corrosion efficiency of a new active coating system consisting of a sol–gel coating with and without a PAA loaded mesoporous silica nano reservoir has been evaluated. Furthermore, its well-pronounced active corrosion protection properties were confirmed by EIS, LEIS and LEIS mapping, FTIR and SVET measurements.

Acknowledgements

The authors wish to express their sincere thanks to the Director, CECRI, Karaikudi for his encouragement. The authors thank CSIR, New Delhi for financial support through the 12th Five Year Plan network project IntelCOAT (CSC0114).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: SEM, FTIR data of porous silica. See DOI: 10.1039/c5ra04670j

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