The Corrosion Inhibition Effect of Triazinedithiol Inhibitors for Aluminum Alloy in a 1 M HCl Solution

Two environmental friendly triazinedithiol inhibitors 6-diallylamino-1,3,5-triazine-2,4dithiol monosodium (DAN) and 6-dibutylamino-1,3,5-triazine-2,4-dithiol monosodium (DBN) were synthesized and their corrosion inhibition for aluminum alloy in a 1 M HCl solution was studied using weight loss methods, electrochemical measurements, and scanning electron microscopy (SEM) techniques. The inhibition efficiency of both DAN and DBN improved with increases in inhibitor concentration but decreased with increases in temperature. Results from potentiodynamic polarization and EIS showed that the corrosion inhibition efficiency of DAN and DBN was excellent. The adsorption of inhibitors on the aluminum alloy surface followed Langmuir adsorption isotherms. Morphology observation revealed that the aluminum alloy was greatly protected by these triazinedithiol inhibitors. Further, density functional theory (DFT) was used to investigate the relationships between molecular structural and inhibition efficiency.


Introduction
Corrosion of aluminum and its alloys has attracted much attention from many researchers due to their high mechanical intensity, low cost, low density, and good machinability, and they have been widely used in industrial applications, especially in constructions, electronics, packing, storage, and transportation equipment and machinery [1][2][3][4][5][6].Corrosion is an electrochemical process and is often activated by industrial processes such as acid descaling, acid pickling, acid cleaning, and oil well acidizing [7].Efforts have been made to protect the integrity of the aluminum surface in an aggressive acid medium or other corrosive environment.In recent decades, the addition of inhibitors has been considered to be the most common approach to hinder the corrosion of aluminum [7][8][9][10].
Many organic compounds have been widely reported as corrosion inhibitors of aluminum in acid solution, such as aliphatic, aromatic amines, and nitrogen heterocyclic molecules [11][12][13][14][15].However, some of these compounds are costly and not easily biodegradable.As high reactive, low cost, high solubility, and environmentally friendly compounds, triazinedithiol, and its monosodium salt have been reported to prepare the effective corrosion inhibitive film on metal surfaces by electrochemical deposition [16][17][18][19].The special tautomer of thiol-thione with highly electronegative atoms like S and O, and the N-containing heterocyclic conjugate system, benefit the triazinedithiol molecules to adsorb on metallic surface.However, the research on triazinedithiol inhibitors for aluminum alloy is seldom reported.The purpose of present work is to investigate and compare the corrosion inhibition action of 6-diallylamino-1,3,5-triazine-2,4-dithiol monosodium (DAN) and 6-dibutylamino-1,3,5-triazine-2,4-dithiolmonosodium (DBN), and their protective performance for Metals 2017, 7, 44 2 of 11 aluminum alloy (AA5052) in 1 M HCl was studied utilizing a variety of electrochemical tests, weight loss methods, scanning electron microscopy (SEM) techniques, and quantum chemistry analysis.

Materials and Sample Preparation
The aluminum alloy sheet AA5052 (Cu: 0.1%, Si: 0.2%, Fe: 0.4%, Mn: 0.1%, Mg: 2.8%, Zn: 0.1%, Cr: 0.3%, others: 0.15%) was mechanically press-cut into specimens of dimension 30 mm × 50 mm × 0.3 mm.All test plates of AA5052 were ultrasonically degreased in the acetone for 15 min, and treated by the immersion in alkaline solution (15 g Na 2 CO 3 + 15 g Na 2 PO 4 per liter) at 60 • C for the debinding process [20].After that, the aluminum alloy specimens were washed thoroughly with distilled water and dried with nitrogen.The specimens with an exposed area of 1 cm 2 were used for potentiodynamic polarization and electrochemical impedance spectroscopy.AR (analytical reagent) grade hydrochloric acid and double distilled water were used to prepare the corrosive media.Molecular structures of DAN and DBN are displayed in Figure 1.In this paper, DAN was synthesized by the reaction between 6-N,N-diallylamino-1,3,5-triazine-2,4-dichloride and NaSH, while DBN was synthesized by the reaction between 6-N,N-dibutylamino-1,3,5-triazine-2,4-dichloride and NaSH according to the method in previous studies [19].

Materials and Sample Preparation
The aluminum alloy sheet AA5052 (Cu: 0.1%, Si: 0.2%, Fe: 0.4%, Mn: 0.1%, Mg: 2.8%, Zn: 0.1%, Cr: 0.3%, others: 0.15%) was mechanically press-cut into specimens of dimension 30 mm × 50 mm × 0.3 mm.All test plates of AA5052 were ultrasonically degreased in the acetone for 15 min, and treated by the immersion in alkaline solution (15 g Na2CO3 + 15 g Na2PO4 per liter) at 60 °C for the debinding process [20].After that, the aluminum alloy specimens were washed thoroughly with distilled water and dried with nitrogen.The specimens with an exposed area of 1 cm 2 were used for potentiodynamic polarization and electrochemical impedance spectroscopy.AR (analytical reagent) grade hydrochloric acid and double distilled water were used to prepare the corrosive media.Molecular structures of DAN and DBN are displayed in Figure 1.In this paper, DAN was synthesized by the reaction between 6-N,N-diallylamino-1,3,5-triazine-2,4-dichloride and NaSH, while DBN was synthesized by the reaction between 6-N,N-dibutylamino-1,3,5-triazine-2,4-dichloride and NaSH according to the method in previous studies [19].

Weight Loss Test
The aluminum alloy specimens in triplicate were immersed in a 1 M HCl solution with different concentration (0.01-1.00 mM) of inhibitors for 2 h.The temperature was controlled by a thermostat aqueous bath at 30 °C .Furthermore, the aluminum alloy specimens were immersed into a 1 M HCl solution with the presence of 1 mM inhibitors, and the experiments were conducted over the temperature range from 30 °C to 50 °C .For all weight loss tests, the volume of prepared solution is 200 mL, and buffer solutions of citric acid/sodium citrate were used to adjust the pH value of tested solution between 6 and 6.5.After the immersion, all specimens were brought out from the solution, scrubbed with bristle brush under running water, then washed thoroughly with distilled water, dried in acetone, and weighed accurately.At least three samples were tested and the average value was obtained.The corrosion rates (CR g•m −2 •h −1 ) and inhibition efficiency (ηw %) were determined from the following equations:

Weight Loss Test
The aluminum alloy specimens in triplicate were immersed in a 1 M HCl solution with different concentration (0.01-1.00 mM) of inhibitors for 2 h.The temperature was controlled by a thermostat aqueous bath at 30 • C. Furthermore, the aluminum alloy specimens were immersed into a 1 M HCl solution with the presence of 1 mM inhibitors, and the experiments were conducted over the temperature range from 30 • C to 50 • C. For all weight loss tests, the volume of prepared solution is 200 mL, and buffer solutions of citric acid/sodium citrate were used to adjust the pH value of tested solution between 6 and 6.5.After the immersion, all specimens were brought out from the solution, scrubbed with bristle brush under running water, then washed thoroughly with distilled water, dried in acetone, and weighed accurately.At least three samples were tested and the average value was obtained.The corrosion rates (C R g•m −2 •h −1 ) and inhibition efficiency (η w %) were determined from the following equations: where S is the area of aluminum alloy specimen (m 2 ); T is the exposure time (h); W 0 and W i are the weight loss value in the absence and presence of inhibitor.C 0 R and C i R are the corrosion rate (g•m −2 •h −1 ) in the absence and presence of inhibitor molecules, respectively.

Electrochemical Measurements
The potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements were carried out using CHI 660C electrochemical work station (CHI Instruments; Shanghai, China) in a three-electrode cell system with a saturated calomel electrode (SCE) as reference electrode and a rectangular piece of graphite as counter electrode.The working electrode was AA5052.Prior to any electrochemical measurements, the immersion in the solution for 1 h was necessary for the open circuit potential to reach a steady state.EIS was carried out at steady open circuit potential disturbed with amplitude of 10 mV alternative current sine wave in the frequency range of 100 mHz to 10 kHz.The polarization curves were obtained by changing potential from −250 mV to +250 mV versus OCP with a scan rate of 0.5 mV/s.

Scanning Electron Microscopy (SEM)
The surfaces morphologies of the aluminum alloy immersed in a 1 M HCl solution for 2 h with and without the triazinethiol inhibitors were observed via SEM (JSM-6360LV, JEOL, Tokyo, Japan) at an accelerating voltage of 20 kV, respectively.

Quantum Chemical Calculation
Theoretical calculations were performed using DFT (density functional theory) at B3LYP/6-31G (d, p) basis set level with Gaussian 03 program [20].Complete geometrical optimizations of DAN and DBN molecules structure, the highest occupied molecular orbital (HOMO), and the lowest unoccupied molecular orbital (LUMO) were obtained by using Gauss View.Some main quantum chemical indexes such as energy of HOMO (E HOMO ), energy of the LUMO (E LUMO ), and energy gap ∆E gap between E HOMO and E LUMO were calculated and discussed.

Effect of Concentration
The inhibition efficiency (η w ) and corrosion rate (C R ) of aluminum alloy acquired from the weight loss method in a 1 M HCl solution contained different concentrations of inhibitors (0.01-1.00 mM) at 30 • C are presented in Table 1.The results clearly show that the inhibition efficiency increases and the corrosion rate decrease by increasing the concentration of studied inhibitors.When concentration of inhibitors varies from 0.01 mM to 1 mM, there is an increase in efficiency from 78.41% to 98.56% for DAN, and an increase in efficiency from 43.79% to 99.21% for DBN (Table 1).This suggests that the presence of DAN or DBN acts as inhibitor retarding the corrosion of aluminum in a hydrochloric acid solution.When the concentration of inhibitor is low, more molecules are needed to adequately cover the aluminum surface.We also found that further enhancement in concentration did not bring any significant changes in the performance of inhibitors, indicating that the achievement of a limiting value.The effect is attributed to the amassing of inhibitor molecules onto the positively charged metal surface leading a decrease in direct contact with metal and corrosive environment.Similar results were found in other studies about different inhibitors for aluminum alloy in hydrochloric acid [21].
Table 1.Corrosion parameters for aluminum alloy in a 1 M HCl solution with inhibitors concentrations range from 0.01 mM to 1.00 mM at 30 • C.

Conc.
DAN DBN Temperature has great influence on the corrosion rate of metals.The effect of solution temperature (30-50 • C) on corrosion rate and inhibition efficiency is shown in Figure 2. Compared with blank solution, the corrosion rate was significantly decreased in presence of inhibitors.It means that the addition of DAN and DBN compounds greatly inhibits the corrosion of aluminum alloy in 1 M HCl.The decrease in inhibition efficiency and increase in corrosion rate was observed with increase in temperature from 30 to 50 • C in the presence of 1 mM inhibitor, which may be attributed to the fact that the adsorption processes were spontaneous and irreversible with heat evolution, and the increase in temperature was not beneficial to the adsorption.The influence may also come from the weakening of electrostatic adsorption on the metal surface and the aggravation of desorption of the inhibitor molecule from the metal surface when the temperature increases [20][21][22].In addition, the metallic corrosion in acidic environments is usually accompanied by the release of H 2 , and the adsorption process of the inhibitor could be affected by the agitation caused by the acceleration of H 2 evolution rates at higher temperature and lead to the decrease of corresponding inhibition efficiency [23].Temperature has great influence on the corrosion rate of metals.The effect of solution temperature (30-50 °C ) on corrosion rate and inhibition efficiency is shown in Figure 2. Compared with blank solution, the corrosion rate was significantly decreased in presence of inhibitors.It means that the addition of DAN and DBN compounds greatly inhibits the corrosion of aluminum alloy in 1 M HCl.The decrease in inhibition efficiency and increase in corrosion rate was observed with increase in temperature from 30 to 50 °C in the presence of 1 mM inhibitor, which may be attributed to the fact that the adsorption processes were spontaneous and irreversible with heat evolution, and the increase in temperature was not beneficial to the adsorption.The influence may also come from the weakening of electrostatic adsorption on the metal surface and the aggravation of desorption of the inhibitor molecule from the metal surface when the temperature increases [20][21][22].In addition, the metallic corrosion in acidic environments is usually accompanied by the release of H2, and the adsorption process of the inhibitor could be affected by the agitation caused by the acceleration of H2 evolution rates at higher temperature and lead to the decrease of corresponding inhibition efficiency [23].

OCP Measurement
Prior to each polarization or EIS experiment, the working electrodes were immersed in a 1 M HCl solution for 1 h to access the free corrosion potential or the quasi-stationary Eoc value.The plots of Eoc vs. time of AA5052 in the absence and presence of inhibitors for 1 h are given in Figure 3.For all test conditions, open circuit potential change (OCP) initially shifts fast towards a negative

OCP Measurement
Prior to each polarization or EIS experiment, the working electrodes were immersed in a 1 M HCl solution for 1 h to access the free corrosion potential or the quasi-stationary E oc value.The plots of E oc vs. time of AA5052 in the absence and presence of inhibitors for 1 h are given in Figure 3.For all test conditions, open circuit potential change (OCP) initially shifts fast towards a negative direction up to 200 s.However, it shows a very slow decline for the rest of the experiment time and finally attains a steady value in approximately 1 h.The slight oscillations in OCP curves revealed that destruction and formation of corrosion product layer occurred at the metal surface [24].The gradient shows that the surfaces of metals cannot achieve true steady states.In spite of this, the phenomenon indicates that there is a negligible change of OCP during the measurement period.
Metals 2017, 7, 44 5 of 11 direction up to 200 s.However, it shows a very slow decline for the rest of the experiment time and finally attains a steady value in approximately 1 h.The slight oscillations in OCP curves revealed that destruction and formation of corrosion product layer occurred at the metal surface [24].The gradient shows that the surfaces of metals cannot achieve true steady states.In spite of this, the phenomenon indicates that there is a negligible change of OCP during the measurement period.

Potentiodynamic Polarization
Potentiodynamic polarization profiles for AA5052 with different concentrations of DAN and DBN are presented in Figure 4.The corrosion kinetics parameters such as corrosion potential (Ecorr), corrosion current density (Icorr), and cathodic and anodic Tafel slopes (βa, βc) were given in Table 2, where the inhibition efficiency ηp (%) was calculated by the following equation: Compared with the blank solution, the cathodic currents were significantly decreased with the presence of inhibitors and the addition of these compounds made Ecorr shifted towards negative potentials (Figure 4), which suggested that DAN and DBN greatly reduced the hydrogen evolution reaction, but their inhibition effects on the anodic dissolution were unobvious.Besides, the addition of DAN and DBN shift the cathodic and anodic curves to lower values, while the concentration of inhibitors was increased.
From Table 2, it is clearly seen that, when more inhibitors were added into the corrosive solution, the corrosion current density decreased and the inhibition efficiency increased.When the concentration of DAN or DBN reached 1 mM, the lowest Icorr values of 63.6 μA•cm −2 and 33.5 μA•cm −2 were obtained, and the inhibition efficiency achieved 97.77% and 98.83%, respectively.Generally, a compound is considered to anodic or cathodic type when the displacement in Ecorr is greater than 85 mV; otherwise, inhibitor is considered as a mixed type [25].For DAN and DBN, the Ecorr values shift towards more a negative direction compared with the blank solution, but the change is not significant when the maximum displacement of Ecorr values is 10.9 mV, which indicated that DAN and DBN belonged to mixed-type inhibitors, mainly inhibiting the cathodic processes.

Potentiodynamic Polarization
Potentiodynamic polarization profiles for AA5052 with different concentrations of DAN and DBN are presented in Figure 4.The corrosion kinetics parameters such as corrosion potential (E corr ), corrosion current density (I corr ), and cathodic and anodic Tafel slopes (β a , β c ) were given in Table 2, where the inhibition efficiency η p (%) was calculated by the following equation: where I 0 corr and I i corr represent the corrosion current densities of AA5052 in the absence and the presence of inhibitor (DAN or DBN), respectively.Compared with the blank solution, the cathodic currents were significantly decreased with the presence of inhibitors and the addition of these compounds made E corr shifted towards negative potentials (Figure 4), which suggested that DAN and DBN greatly reduced the hydrogen evolution Metals 2017, 7, 44 6 of 11 reaction, but their inhibition effects on the anodic dissolution were unobvious.Besides, the addition of DAN and DBN shift the cathodic and anodic curves to lower values, while the concentration of inhibitors was increased.From Table 2, it is clearly seen that, when more inhibitors were added into the corrosive solution, the corrosion current density decreased and the inhibition efficiency increased.When the concentration of DAN or DBN reached 1 mM, the lowest I corr values of 63.6 µA•cm −2 and 33.5 µA•cm −2 were obtained, and the inhibition efficiency achieved 97.77% and 98.83%, respectively.Generally, a compound is considered to anodic or cathodic type when the displacement in E corr is greater than 85 mV; otherwise, inhibitor is considered as a mixed type [25].For DAN and DBN, the E corr values shift towards more a negative direction compared with the blank solution, but the change is not significant when the maximum displacement of E corr values is 10.9 mV, which indicated that DAN and DBN belonged to mixed-type inhibitors, mainly inhibiting the cathodic processes.

Electrochemical Impedance Spectroscopy (EIS)
Nyquist plots for aluminum alloy in the absence and presence of various concentrations of DAN and DBN are given in Figure 5.The impedance spectra are consisted of capacitive loops at higher frequency and inductive loops at lower frequency.The presence of depressed semicircle in Nyquist plot across the studied frequency range indicates that a charge transfer process mainly controls the corrosion of aluminum.In other literature, similar plots have been reported for the corrosion of aluminum alloys in HCl solutions [20].The inductive loop is generally attributed to the relaxation process in the oxide film covered on metal surface [26].The reasons behind the deviations from perfect semicircles are usually involved with the frequency dispersion of interfacial impedance, which can be attributed to various kinds of physical phenomena such as active sites, surface roughness, and non-homogeneity of the solids [27].The diameter of the capacitive loop is enlarging gradually with increasing concentrations of inhibitor, indicating that the charge transfer resistance is increased and the adsorbed inhibitor forms a more compact monolayer on metal surface with an increasing amount of inhibitor.

Surface Morphology
SEM technique was employed to further prove the corrosion resistance ability of DAN and DBN, and the surface observation images of aluminum alloy after a 2 h exposure in a HCl solution without and with inhibitors are shown in Figure 6.Before immersion, the bare aluminum plate looks very smooth (Figure 6a).In contrast, in the absence of inhibitor, the AA5052 presented a very rough surface covered with a huge amount of deep cracks and large holes, which suggests strong damage and a severe dissolution of aluminum alloy in contact with aggressive solution (Figure 6b).Nevertheless, in Figure 6c,d, the dissolution rate of aluminum alloy was substantially inhibited by DAN and DBN, exhibiting a comparative smooth surface with a few small pits.Therefore, it is concluded that the regular distribution of the DAN or DBN molecules adsorbed on AA5052 surface generates consistent protective layers, which effectively prevent HCl molecules from penetrating into the aluminum surface.

Surface Morphology
SEM technique was employed to further prove the corrosion resistance ability of DAN and DBN, and the surface observation images of aluminum alloy after a 2 h exposure in a HCl solution without and with inhibitors are shown in Figure 6.Before immersion, the bare aluminum plate looks very smooth (Figure 6a).In contrast, in the absence of inhibitor, the AA5052 presented a very rough surface covered with a huge amount of deep cracks and large holes, which suggests strong damage and a severe dissolution of aluminum alloy in contact with aggressive solution (Figure 6b).Nevertheless, in Figure 6c,d, the dissolution rate of aluminum alloy was substantially inhibited by DAN and DBN, exhibiting a comparative smooth surface with a few small pits.Therefore, it is concluded that the regular distribution of the DAN or DBN molecules adsorbed on AA5052 surface generates consistent protective layers, which effectively prevent HCl molecules from penetrating into the aluminum surface.

Surface Morphology
SEM technique was employed to further prove the corrosion resistance ability of DAN and DBN, and the surface observation images of aluminum alloy after a 2 h exposure in a HCl solution without and with inhibitors are shown in Figure 6.Before immersion, the bare aluminum plate looks very smooth (Figure 6a).In contrast, in the absence of inhibitor, the AA5052 presented a very rough surface covered with a huge amount of deep cracks and large holes, which suggests strong damage and a severe dissolution of aluminum alloy in contact with aggressive solution (Figure 6b).Nevertheless, in Figure 6c,d, the dissolution rate of aluminum alloy was substantially inhibited by DAN and DBN, exhibiting a comparative smooth surface with a few small pits.Therefore, it is concluded that the regular distribution of the DAN or DBN molecules adsorbed on AA5052 surface generates consistent protective layers, which effectively prevent HCl molecules from penetrating into the aluminum surface.

Adsorption Isotherm
The interactions between the inhibitor molecules and the aluminum alloy surface can be investigated by adsorption isotherm [28].To determine the adsorption mode, various isotherms including Langmuir, Frumkin, Temkin, and Freundlich were considered, and finally the Langmuir mode was found to be the most suitable adsorption isotherm.The linear relationships of C/θ versus C for DAN and DBN inhibitors are shown in Figure 7.

Adsorption Isotherm
The interactions between the inhibitor molecules and the aluminum alloy surface can be investigated by adsorption isotherm [28].To determine the adsorption mode, various isotherms including Langmuir, Frumkin, Temkin, and Freundlich were considered, and finally the Langmuir mode was found to be the most suitable adsorption isotherm.The linear relationships of C/θ versus C for DAN and DBN inhibitors are shown in Figure 7.The linear regression coefficients (R 2 ) are quite equal to 1, as shown in Figure 7, indicating that the adsorption of studied triazine derivatives on aluminum alloy surface obeys the Langmuir adsorption isotherm at 30 °C and 35 °C.The isotherm can be represented as follows: Kads is the adsorptive equilibrium constant, c is the corrosion inhibitor concentration, and the fractional surface coverage θ can be easily calculated from the weight loss results (Table 1).
The equilibrium constants Kads are related to the standard adsorption free energy (ΔG θ ) as shown in the following equation: where 55.5 (mol•L −1 ) is molar concentration of water in solution, T (K) is the absolute temperature, and R (J•mol −1 •K −1 ) is the molar gas constant.The Kads and ΔG θ values are given in Table 3.The negative values of ΔG θ indicate that the inhibitor molecules are spontaneously adsorbed on the AA5052 surface to form a stable layer.In addition, the high Kads value represents a strong adsorption of the inhibitor on aluminum alloy surface [29].The strong interaction of inhibitors with aluminum alloy can be attributed to the presence of electronegative elements like nitrogen and sulfur, plus the π electrons on the triazine heterocyclic ring [30].In general, the ΔG θ values of −40 kJ•mol −1 is accepted as threshold value between chemisorption and physisorption [30].The ΔG θ value more negative than −40 kJ•mol −1 may be indicative of the sharing or transfer of electrons from the inhibitor molecules to the metal surface, while the value less The linear regression coefficients (R 2 ) are quite equal to 1, as shown in Figure 7, indicating that the adsorption of studied triazine derivatives on aluminum alloy surface obeys the Langmuir adsorption isotherm at 30 • C and 35 • C. The isotherm can be represented as follows: K ads is the adsorptive equilibrium constant, c is the corrosion inhibitor concentration, and the fractional surface coverage θ can be easily calculated from the weight loss results (Table 1).The equilibrium constants K ads are related to the standard adsorption free energy (∆G θ ) as shown in the following equation: where 55.5 (mol•L −1 ) is molar concentration of water in solution, T (K) is the absolute temperature, and R (J•mol −1 •K −1 ) is the molar gas constant.The K ads and ∆G θ values are given in Table 3.
The negative values of ∆G θ indicate that the inhibitor molecules are spontaneously adsorbed on the AA5052 surface to form a stable layer.In addition, the high K ads value represents a strong adsorption of the inhibitor on aluminum alloy surface [29].The strong interaction of inhibitors with aluminum alloy can be attributed to the presence of electronegative elements like nitrogen and sulfur, plus the π electrons on the triazine heterocyclic ring [30].In general, the ∆G θ values of −40 kJ•mol −1 is accepted as threshold value between chemisorption and physisorption [30].The ∆G θ value more negative than −40 kJ•mol −1 may be indicative of the Metals 2017, 7, 44 9 of 11 sharing or transfer of electrons from the inhibitor molecules to the metal surface, while the value less negative than −20 kJ•mol −1 suggests a physisorption [31].As mentioned above, it was concluded that the chemical adsorption was most possible for DAN at 30 • C or 35 • C and DBN at 30 • C based on the calculated ∆G θ values of the present study displayed in Table 3.At 35 • C, the adsorption mechanism of DBN involves both physical and chemical processes with the calculated ∆G θ value of −36 kJ/mol.

Quantum Chemical Calculation
Quantum chemical calculations have proven to be a very powerful tool for studying the corrosion inhibition mechanism.The optimized geometry of DAN and DBN are shown in Figure 8, and the main quantum chemical parameters are displayed in Table 4.It is observed that the electron densities of HOMO location in studied molecules are mostly distributed near the triazine heterocyclic rings, thiols, and the double bond between two carbons.It is well established in the literature that the E HOMO often associated with the electron donating ability of a molecule.The high value of the HOMO energy is likely to indicate the tendency of a molecule to donate electrons to appropriate acceptor molecules with low-energy, empty molecular orbitals.Similarly, the value of E LUMO reflects the ability of the molecule to accept electrons [32].A molecule with a lower energy gap means a better inhibition ability, because the value of energy needed to remove an electron from the last occupied orbital is lower [33].From Table 4, a higher E HOMO value and lower energy gap were found in the DBN molecule, which demonstrated that DBN had an anti-corrosion capability superior to DAN.However, in the above experimental results, both DAN and DBN exhibited excellent corrosion performance, and the difference in inhibitive efficiency between them is not obvious.This may be related to the influence of their chemical structures.From Figure 8, we can find branched carbon chains in the DBN molecule structure that are longer than those of DAN.A larger space steric hindrance may influence the adsorption process of the inhibitor molecule [17], and the theoretical calculations that can only be a reference for the realities are usually more complex.
Metals 2017, 7, 44 9 of 11 negative than −20 kJ•mol −1 suggests a physisorption [31].As mentioned above, it was concluded that the chemical adsorption was most possible for DAN at 30 °C or 35 °C and DBN at 30 °C based on the calculated ΔG θ values of the present study displayed in Table 3.At 35 °C, the adsorption mechanism of DBN involves both physical and chemical processes with the calculated ΔG θ value of −36 kJ/mol.

Quantum Chemical Calculation
Quantum chemical calculations have proven to be a very powerful tool for studying the corrosion inhibition mechanism.The optimized geometry of DAN and DBN are shown in Figure 8, and the main quantum chemical parameters are displayed in Table 4.It is observed that the electron densities of HOMO location in studied molecules are mostly distributed near the triazine heterocyclic rings, thiols, and the double bond between two carbons.It is well established in the literature that the EHOMO often associated with the electron donating ability of a molecule.The high value of the HOMO energy is likely to indicate the tendency of a molecule to donate electrons to appropriate acceptor molecules with low-energy, empty molecular orbitals.Similarly, the value of ELUMO reflects the ability of the molecule to accept electrons [32].A molecule with a lower energy gap means a better inhibition ability, because the value of energy needed to remove an electron from the last occupied orbital is lower [33].From Table 4, a higher EHOMO value and lower energy gap were found in the DBN molecule, which demonstrated that DBN had an anti-corrosion capability superior to DAN.However, in the above experimental results, both DAN and DBN exhibited excellent corrosion performance, and the difference in inhibitive efficiency between them is not obvious.This may be related to the influence of their chemical structures.From Figure 8, we can find branched carbon chains in the DBN molecule structure that are longer than those of DAN.A larger space steric hindrance may influence the adsorption process of the inhibitor molecule [17], and the theoretical calculations that can only be a reference for the realities are usually more complex.

Conclusions
Two triazinedithiol compounds (DAN and DBN) as corrosion inhibitors for aluminum alloy in a 1 M HCl solution were investigated.For DAN and DBN, their inhibition efficiency increased with increases in inhibitor concentration and they belonged to mixed-type inhibitors predominantly retarding the cathodic reaction.The inhibiting efficiencies determined by weight loss methods, potentiodynamic polarization testing, and EIS measurements are all in good agreement.The adsorption processes of DAN and DBN molecules followed the Langmuir adsorption isotherm.The calculated ∆G θ values indicated that these inhibitors were spontaneously absorbed on the aluminum alloy surface and were more inclined to a chemisorption mechanism at 30 • C. The surface morphologies images were good proof for the reduction of dissolution of aluminum alloy ascribed to the formation of protective DAN/DBN film on the metal surface.In addition, the results were further verified by quantum chemical calculation, and these data support the good inhibition tendency of DAN and DBN.

Figure 2 .
Figure 2. Variation of (a) corrosion rate and (b) inhibition efficiency of aluminum alloy in 1 M HCl with 1 mM inhibitor under different temperature (30-50 °C ).

Figure 2 .
Figure 2. Variation of (a) corrosion rate and (b) inhibition efficiency of aluminum alloy in 1 M HCl with 1 mM inhibitor under different temperature (30-50 • C).

Figure 3 .
Figure 3. Open circuit potential change of AA5052 aluminum alloy in 1 M HCl without inhibitor (Blank) and with different inhibitors added (DAN or DBN), respectively.
corrosion current densities of AA5052 in the absence and the presence of inhibitor (DAN or DBN), respectively.

Figure 3 .
Figure 3. Open circuit potential change of AA5052 aluminum alloy in 1 M HCl without inhibitor (Blank) and with different inhibitors added (DAN or DBN), respectively.

Figure 4 .
Figure 4. Tafel plots for aluminum alloy in 1 M HCl containing different concentrations of (a) DAN and (b) DBN.

Figure 4 .
Figure 4. Tafel plots for aluminum alloy in 1 M HCl containing different concentrations of (a) DAN and (b) DBN.

Figure 5 .
Figure 5. Nyquist diagrams for aluminum alloy in 1 M HCl containing different concentrations of (a) DAN and (b) DBN.

Figure 6 .
Figure 6.SEM images of AA5052 surface before and after immersing in 1 M HCl for 2 h without and with triazinedithiol inhibitors.(a) Blank before immersion; (b) blank after immersion; (c) with DAN; (d) with DBN.

Figure 5 .
Figure 5. Nyquist diagrams for aluminum alloy in 1 M HCl containing different concentrations of (a) DAN and (b) DBN.

Figure 5 .
Figure 5. Nyquist diagrams for aluminum alloy in 1 M HCl containing different concentrations of (a) DAN and (b) DBN.

Figure 6 .
Figure 6.SEM images of AA5052 surface before and after immersing in 1 M HCl for 2 h without and with triazinedithiol inhibitors.(a) Blank before immersion; (b) blank after immersion; (c) with DAN; (d) with DBN.

Figure 6 .
Figure 6.SEM images of AA5052 surface before and after immersing in 1 M HCl for 2 h without and with triazinedithiol inhibitors.(a) Blank before immersion; (b) blank after immersion; (c) with DAN; (d) with DBN.

Table 1 .
Corrosion parameters for aluminum alloy in a 1 M HCl solution with inhibitors concentrations range from 0.01 mM to 1.00 mM at 30 °C .

Table 2 .
Tafel polarization parameters of the corrosion for aluminum alloy in 1 M HCl containing different concentrations of DAN and DBN.Inhibitor C (

Table 2 .
Tafel polarization parameters of the corrosion for aluminum alloy in 1 M HCl containing different concentrations of DAN and DBN.

Table 3 .
Values of Kads and ΔG θ of DAN and DBN adsorbed on AA5052 at 30 °C and 35 °C.

Table 3 .
Values of K ads and ∆G θ of DAN and DBN adsorbed on AA5052 at 30 • C and 35 • C.

Table 4 .
Quantum parameters for DAN and DBN a .HOMO is energy of HOMO, E LUMO is energy of LUMO, and ∆E gap = E HOMO − E LUMO .