Corrosion resistance and adsorption behavior of bis-(γ-triethoxysilylpropyl)-tetrasulfide self-assembled membrane on 6061 aluminum alloy

Abstract

A bis-(γ-triethoxysilylpropyl)-tetrasulfide (BTESPT) self-assembled membrane (SAM) was prepared by self-assembly membrane technology on 6061 aluminum alloy. The SAM was evaluated using electrochemical techniques (potentiodynamic polarization and electrochemical impedance) in 3.5 wt% NaCl solution. In addition, molecular dynamics calculations showed a high binding energy between the self-assembled molecule and aluminum alloy surface. The formation of the self-assembled molecule was believed to be achieved by a chemical bond between the silicon oxide group and the metal surface atoms. Finally, X-ray photoelectron spectroscopy and scanning electron microscopy were carried out to confirm that BTESPT could form a membrane on 6061 aluminum alloy.

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1 Introduction

Aluminum alloys have many advantages, such as low density, good ductility, high strength, good electrical conductivity, easy processing, etc., and have been widely used in many fields such as electronics, aerospace, national defense and so on. However, the surface of the aluminum alloy surface often has localized corrosion because the surface oxide film was destroyed in a medium of corrosive ions. This leads to a reduction of the life of the aluminum alloy component. Nowadays, chromate conversion coating is one of the most used adhesion promoters for aluminum and its alloys due to its characteristics such as easy application and effectiveness.¹ Recently, environmental requirements are prompting many surface suppliers to develop new technologies based on environmentally friendly processes.² Therefore, it is significant to study the corrosion inhibition technology of aluminum alloys in corrosive media. Self-assembled membrane technology (self-assembled membrane, SAM) is one of the most effective methods for improving the corrosion resistance of an aluminum alloy. In addition, SAM technology has the unique advantages of no pollution, low cost, etc.

Nowadays, self-assembled membranes are used in metal anticorrosion research mainly focused on the relatively uniform surface of pure metal aspects e.g. Au, Ag, Cu, Fe, and Al,^3–8 and there are very few reports about the applications of self-assembled membranes on alloy surfaces.^9–14

In this work, a SAM of bis-(γ-triethoxysilylpropyl)-tetrasulfide (BTESPT) was prepared by the self-assembly (SA) method. Its corrosion resistance was assessed by electrochemical techniques (potentiodynamic polarization and electrochemical impedance). And the mechanism of BTESPT self-assembled membrane formation was studied by using the molecular dynamics simulation method.^15,16 The results of theoretical calculations were verified by SEM and XPS tests.

2 Experimental details

2.1 Materials

The working electrodes (with dimensions 30 mm × 40 mm × 3 mm) were prepared from an Al alloy 6061 sheet with a chemical composition (wt%) of: Si (0.3), Fe (0.7), Cu (0.25), Mn (1.0–1.5), Mg (0.8–1.3), Zn (0.25) and Al (balance). Before SAM treatment, the working electrodes were successively polished with a series of abrasive papers and degreased in an alkaline solution, then sealed with epoxy resin leaving a polished square surface with an area of 1 cm² and ultrasonically cleaned with anhydrous ethanol and distilled water. Then the working electrodes were immersed in a freshly prepared bis-(γ-triethoxysilylpropyl)-tetrasulfide (BTESPT) mixed solution with methanol and distilled water (3 h), followed by washing with ultra-pure water to remove the rest of the BTESPT solution (2 min), and finally dried in 100 °C air.

2.2 Measurement methods

The surface morphology of the SAM was observed with a JSM-6360LV scanning electron microscopy (SEM). All electrochemical experiments were performed using an electrochemical workstation (CHI660C, Shanghai Chenhua Co., China). A three-electrode cell was used for the electrochemical measurements. The working electrode was a SAM modified Al sheet, the counter and the reference electrodes were a large platinum foil (about 3 cm²) and a saturated calomel electrode (SCE), respectively. A 3.5 wt% NaCl solution was used as the electrolyte. Dynamic measurements of polarization curves were acquired at a scan rate of 0.5 mV s⁻¹ in Tafel mode when the open circuit potential (OCP) became stable. The corrosion current (i_corr) and corrosion potential (E_corr) were obtained automatically from the Tafel plots using the Electrochemical Workstation analysis software. EIS measurement was operated in the frequency range of 10⁵ to 10⁻² Hz at the OCP. The ac signal amplitude was 10 mV. The corrosion resistance of the SAM sample was compared with the bare aluminum alloy substrate.

2.3 Molecular dynamics calculation details

The Discover molecular dynamics module in Materials Studio 6.0 software from Accelrys Inc.¹⁷ allows the selection of a thermodynamic ensemble and the associated parameters, defining simulation time, temperature and pressure and initiating a dynamics calculation. The molecular dynamics simulation procedures have been described elsewhere.¹⁸ The first step to calculate the interaction energy between the self-assembled molecule and the metal surface was to build the aluminum (1 1 0) surface by importing the aluminum crystal and then cleaving its surface through the cleavage plane (1 1 0). To get an accurate result, the thickness of the surface must be more than the non-bond cut-off distances in the force field. After building the aluminum (1 1 0) surface, the surface must be relaxed by minimizing its energy using molecular mechanics. The next step was to increase the surface area of the aluminum (1 1 0) and change its periodicity by constructing a super cell and building a vacuum slab with zero thickness. The BTESPT was built using the sketching tools in Materials Visualizer and their geometries were optimized. In the third step, the supercell which contains 400 water molecules and a BTESPT molecule was created with size of 40.49 Å, 28.63 Å, 11.50 Å. Subsequently, another supercell which contains 200 water molecules were created with size of 40.49 Å, 28.63 Å, 7.73 Å. Finally, all atoms were fixed in the process of simulation layers, the entire model size was 40.49 Å, 28.63 Å, 36.25 Å (Fig. 1). The whole system was performed at 298 K controlled by the Andersen thermostat, NVT ensemble, with a time step of 1.0 fs and simulation time of 500 ps, using the COMPASS force field. The MD simulation was carried out in a simulation box (40.49 Å × 28.63 Å × 36.25 Å) with periodic boundary conditions. The box included an Al slab and a self-assembled molecule solution layer.

Fig. 1 The adsorption model of the BTESPT in solution.

The interaction energy E_Al–SAM of the Al surface with the BTESPT was calculated according to the following equation eqn (1):

E_Al–SAM = E_complex − (E_Al + E_SAM) (1)

where E_complex is the total energy of the Al crystal together with the adsorbed self-assembly molecule, E_Al and E_SAM are the total energy of the Al crystal and free self-assembly molecule, respectively. The binding energy of the inhibitor molecule is the negative value of the interaction energy eqn (2).¹⁹

E_binding = E_Al–SAM (2)

3 Results and discussion

3.1 Potentiodynamic polarization curves

The polarization curves of the BTESPT SAM and aluminum alloy matrix in a 3.5 wt% NaCl solution are shown in Fig. 2. The samples with the SAM showed higher corrosion resistance than the bare aluminum alloy in Fig. 2. The parameters of the pitting corrosion potential (E_pit) and corrosion potential (E_corr), corrosion current density (i_corr), and the anodic/cathode Tafel constant (β_a and β_c) were derived directly from the polarization curves by Tafel region extrapolation. The corrosion resistance (R_p) was calculated on the basis of the following eqn (3).²⁰

Fig. 2 Potentiodynamic polarization curves of samples in 3.5% NaCl solutions.

The results are summarized in Table 1. As shown in Fig. 2 and Table 1, the SAM on aluminum alloy obviously reduced i_corr and caused the E_corr and E_pit shift (about 0.3316 V). The corrosion potential shifted positively contributes to increase the difficulty of metal corrosion. The higher E_pit was considered to be affected by the denser and less pitting surface. The lower i_corr and higher R_p mean that the SAM could effectively block aluminum alloy from the anodic dissolution of Al³⁺. And from the Fig. 1, it was showed that the effect of the membranes on the anodic reaction was more observable than that on the cathode reaction. These means that the SAM act as a mixed type of inhibitor.

Table 1 Results of potentiodynamic polarization tests of two samples in 3.5 wt% NaCl solution

Sample	Ecorr/V (vs. SCE)	Epit/V (vs. SCE)	Epit − Ecorr/V (vs. SCE)	βa/V per decade	βc/V per decade	icorr/A cm^−2	Rp/Ω
Bare Al alloy	−1.0609	−0.8201	0.2408	0.00524	0.005468	4.806 × 10^−6	242.07
SAM	−0.7293	−0.3987	0.3306	0.020511	0.004374	8.764 × 10^−8	17 885.36

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3.2 Electrochemical impedance spectroscopy (EIS)

The electrochemical impedance spectroscopy (EIS) measurements were carried out to estimate the healing effect. The results of the EIS measurements are presented in Fig. 3 as Bode plots. Impedance measurements were conducted in 3.5 wt% NaCl solution. It was clearly shown that the samples with a SAM had a higher impedance value than the bare Al alloy in Fig. 3. The impedance of the bare Al alloy was in the range of 10³ Ω cm² at low frequencies. In the same frequency range, the impedance of the SAM was increased to the range of 10⁵ Ω cm², an increase of two orders of magnitude. This indicates that the SAM was an effective membrane formed on the aluminum alloy. It could be inferred that the membrane acts as a blocking barrier to chloride ions attracted to the surface.

Fig. 3 Impedance plots of samples in 3.5% wt NaCl solution.

3.3 Scanning electron microscopy

To establish whether inhibition was due to the formation of the self-assembled membrane on the 6061 aluminum alloy surface via adsorption, scanning electron photographs were taken (Fig. 4). The Fig. 4A shows the surface of the 6061 aluminum alloy. Polishing scratches are visible. The morphology of the BTESPT self-assembled membrane surface is shown in Fig. 4B. It can be clearly seen that the self-assembled membrane network covered the surface of Al alloy by comparison with the aluminum alloy substrate. And the BTESPT self-assembled membrane was compact. It could be inferred that the special network structure was conducive to the protection of the Al alloy. This could be verified from the electrochemical performance test results. So, the SAM could had better corrosion resistance than the aluminum alloy substrate in air.

Fig. 4 Surface morphology of the aluminum substrate (A) and the SAM on aluminum alloy 6061 (B).

3.4 Molecular dynamics calculations

Molecular dynamic simulations were performed to study the behavior of the BTESPT self-assembled on the Al (1 1 0) surface. In this simulation, the Al (1 1 0) surface was chosen from the three kinds of Al surfaces (1 1 0, 1 0 0, 1 1 1), because the Al (1 1 0) surface is the most active form.²¹ The calculation details described above were used. It is usually considered that a system has reached equilibrium when the temperature and energy have reached balance. The geometry optimization of the studied system was carried out using an iterative process, in which the atomic coordinates were adjusted until the total energy of a structure was minimized. Fig. 5 shows the temperature fluctuation curves. Fig. 6 shows the energy fluctuation curves. It can be seen that the system tends to equilibrium by the end of the simulation process in Fig. 5 and 6. And the calculated value of E_binding according to eqn (4) was 564.21 kJ mol⁻¹. The large negative value could be attributed to the strong adsorption between BTESPT molecules and the aluminum alloy surface.

E_binding = −E_adsorption = E_total − (E_{surface+solution} + E_SAM+solution) + E_solution (4)

where E_total is the total potential energy of the system; E_{surface+solution} and E_SAM+solution are the potential energies of the system without the SAM and the system without the aluminum alloy surface, respectively; E_solution is the potential energy of all the water molecules.

Fig. 5 Temperature equilibrium curve of BTESPT adsorbed on Al (1 1 0) surface.

Fig. 6 Energy equilibrium curve of BTESPT adsorbed on Al (1 1 0) surface.

Balance graphs of the system before and after calculation are shown in Fig. 7. The organic molecule was surrounded by water, and only the four S atoms were close to the aluminum alloy surface in Fig. 7A. While the two silicon oxygen groups were far from the metal surface (Fig. 7A). When the calculation was over, the two silicon oxygen groups were close to the aluminum alloy surface atoms, even closer to the metal surface than the four S atoms (Fig. 7B). It can be deduced that the formation of the self-assembled membrane was mainly dependent on interaction between the silicon oxygen groups and the metal surface atoms by a comparison of the model before and after the dynamics simulation. The role of the self-assembled membrane was to act as a barrier between the corrosive medium and the metal surface.

Fig. 7 Comparison of configuration between the initial (A) and the final (B) adsorption model on the Al (1 1 0) surface.

The radial distribution function (or pair correlation function) g(r) in a system of particles (atoms, molecules, colloids, etc.), describes how density varies as a function of distance from a reference particle. It is a characteristic physical quantity which reflects the microstructure of the material. The radial distribution function can provide information of the degree of order of the simulation system.²² When a peak appears at greater than 3 Å, it indicates that the molecular chain with long-range order belongs to a crystallization system. When a peak appears at less than 0.3 Å, it indicates that the molecular chain with short-range disorder belongs to an amorphous structure. The relative correlation function could be obtained by analyzing the trajectory file of the dynamics simulation. The radial distribution function of the atomic and metal surface atoms in the computational system can also be analyzed (Fig. 8). It is generally thought that the formation of a chemical bond will be shown by a peak at less than 3.5 Å, a peak at more than 3.5 Å is in the scope of a van der Waals force or Coulomb force in the radial distribution function curve.²³ From Fig. 8, it was clear that the first peaks of S 3.42 Å (Fig. 8B), O 2.97 Å (Fig. 8C) and H 2.99 Å (Fig. 8D) appeared at less than 3.5 Å in the radial distribution curve. These results indicate that the adsorption of BTESPT on the aluminum alloy surface depends on the interaction between S, O and H and the metal surface atoms. It can be deduced that this was a van der Waals force or Coulomb force between C, Si and metal surface atoms, because their first peaks of the atomic radial distribution function were greater than 3.5 Å (C (3.79 Å) Fig. 8E and Si (3.99 Å) Fig. 8F). So, it was inferred that the formation of the BTESPT self-assembled membrane depended mainly on the chemical adsorption of S and O with the metal surface atoms on the 6061 aluminum alloy.

Fig. 8 The pair correlation function of sulphur (B), oxygen (C), hydrogen (D), carbon (E) and silicon (F) atoms from BTESPT with Al atoms from Al (1 1 0) surface in solution.

3.5 X-ray photoelectron spectroscopy (XPS)

XPS measurements were performed to investigate the composition of the self-assembled membrane formed on the 6061 aluminum alloy surface in order to verify the results of the dynamic calculation. It displayed five main elements of C, O, Si, S and Al (Fig. 9). Peaks for Al appeared, which indicate that the thickness of the SAM may be beyond the detection depth of XPS. The elements C, O, and S formed the main composition of the SAM. The C 1s core-level XPS spectra were deconvoluted into the following four types of carbon bands: 284.82 eV (C–C), 286.48 eV (C–O), 288.78 eV (C=O), and 163.7 eV (S–C) (Fig. 10A and C). The Si 2p and O 1s peaks were relatively simple, the respective bands were at 102.47 eV and 532.14 eV (Fig. 10B and D). It could be determined that the self-assembled membranes were successfully prepared on the surface of aluminum alloy. The results were in agreement with the theoretical calculations.

Fig. 9 XPS spectra for SAMs of BTESPT.

Fig. 10 High resolution XPS spectra of the atoms Si (A), O (B), C (C) and S (D).

4 Conclusions

In this study, BTESPT was investigated as a self-assembled membrane for 6061 aluminum alloy in 3.5 wt% NaCl solution. Electrochemical tests, SEM, XPS, and dynamic simulation were used to study the mechanism of the formation of the BTESPT self-assembled membrane. The results obtained lead to the following conclusions:

(1) The potentiodynamic polarization study showed that BTESPT was a mixed type inhibitor by forming the self-assembled membrane on the aluminum alloy. And the current density values of a BTESPT SAM working electrode was two orders of magnitude smaller than that of the blank aluminum alloy electrodes, and reached up to 8.764 × 10⁻⁸ A cm². And the corrosion resistance of SAM was more 70 times larger than the aluminum alloy blank electrode. In addition, the samples with the SAM showed higher impedance value than bare Al alloy by EIS. This indicated that the SAM was an effective membrane formed on the aluminum alloy. So it may be concluded that the membrane acts as a blocking barrier for chloride ions attracted to the surface.

(2) The theoretical study of the dynamic simulation indicated that the BTESPT SAM could be adsorbed on the aluminum alloy surface by chemisorption, and the binding energies of BTESPT were 564.21 kJ mol⁻¹. Radial distribution function analysis of the dynamic simulation confirmed that the formation of the BTESPT self-assembled membrane depended mainly on the chemical adsorption of S and O with the metal surface atoms on the aluminum alloy. The results of the theoretical calculations were verified by SEM and XPS tests. The corrosion resistance of the self-assembled membrane was mainly dependent on the interaction between the silicon oxygen functional group and the aluminum alloy surface.

Acknowledgements

This work was financially supported by the Project of Guangxi Natural Science Foundation of China (No. 2014GXNSFAA118335).

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