Synthesis of a Novel Gemini Cationic Surfactant and Its Inhibition Behaviour and Mechanism Study on 2024 Al-Cu-Mg Alloy in Acid Solution

Isopropylamine was taken as a raw material to synthesize a new multi-alkyl multiple quaternary-ammonium salts gemini surfactant bis[2-hydroxy-3-(dodecyldimethylammonio)propyl]-isopropylamine dichloride.The structure of the synthetic product was characterized by HNMR and FTIR.The surface activity was investigated; the inhibition efficiencies and inhibitionmechanism of the synthetic product were studied by weight loss method, electrochemical method, microscopic morphology observation, and adsorption model calculation. The results indicate that cmc of synthetic product was 9.204 ×10−4mol/L; when the concentrations were lower than cmc, the inhibition efficiencies rose substantially, which was up to 89.3% with the concentration of 9.204 × 10−4mol/L; when they were higher than cmc, inhibition efficiencies were basically unchanged; polarization tests showed that the synthesis product could restrain both anodic and cathodic reactions; when the concentrations were lower than cmc, the adsorption of the synthetic product conformed to the Langmuir model, which formed monolayer on the 2024 Al-Cu-Mg alloy surface; when they were higher than cmc, it formed bilayer, so the adsorption of the synthetic product did not conform to the Langmuir model anymore.


Introduction
Aluminium and aluminium alloys have been widely used in aerospace, transportation, and construction industries because of their good mechanical properties, low density, and high corrosion resistance [1].In the process of metal acid pickling, corrosion inhibitors are usually added to reduce the corrosion of acid to metallic matrix.During this process, surfactants are often selected as the corrosion inhibitors because of their good corrosion inhibition performance [2][3][4][5].Unlike traditional surfactants, gemini surfactant has novel structure and excellent performance: using the spacer; it can connect two hydrophilic groups by chemical bonds to form a structure with double hydrophilic groups and double hydrophobic chains.This structure owns more advantages, such as higher surface tension, higher micelle forming ability, and better water solubility [6][7][8].At present, gemini surfactant has been widely used in metal corrosion inhibition, transportation, medicine, and other industries, which plays a huge role in improving production process, enhancing product quality, reducing the cost, and increasing the added value [8][9][10][11][12][13].Therefore, it is of great significance to study the synthesis process of gemini surfactant with novel structure and excellent properties and to apply it to pickling process of aluminium alloy for aviation.
In recent times, the studies of the corrosion inhibition performance of gemini surfactants are getting more and more; in particular, multiple quaternary-ammonium salts gemini surfactants have been reported [14][15][16][17][18][19][20][21][22].Kim et al. [15] synthesized a series of new alkyl based quaternaryammonium cationic surfactants by inserting a methylimino group (>NCH 3 ), a dodecylimino group (>NC 12 H 25 ), a dimethylammonium group (>N + (CH 3 ) 2 ), and a dodecylmethylimmonio group (>N + (CH 3 )C 12 H 25 ) into two dodecyldimethyl quaternary-ammonium salts, respectively, and their properties were studied.The result showed that the multi-alkyl hydrophobic group was the main reason leading to the decrease of the critical micelle concentration; even with long carbon chains, spacers did not play the role of hydrophobic groups.Lim et al. [17] synthesized single, double, triple quaternary-ammonium salts gemini surfactant by using condensation reactions.Surface tension test showed that the lowest surfactant tension was 28.88 mN/m, which meant these gemini surfactants could be used to reduce the surface tension of the aqueous solution and to act as emulsifiers; foaming performance tests showed that the foam volume's decline rate increased when the number of ammonium and hydroxyl groups increased.In recent times, the studies of the corrosion inhibition performance of gemini surfactants are getting more and more [23][24][25][26][27][28][29]: Xu et al. [23] synthesized a new amide gemini inhibitor (OPDO) and studied its corrosion inhibition performance to Q235 carbon steel in hydrochloric acid solution.The results showed that OPDO had excellent corrosion inhibition effect for carbon steel, and it was a mixed inhibitor; its maximum inhibition rate was up to 99%; the results also showed that the adsorption on the surface of carbon steel conformed to the Langmuir isothermal adsorption model.Hegazy et al. [24] synthesized three kinds of gemini surfactants with long chains containing benzene rings as spacers; results showed that, in acidic solution, those three kinds of surfactants could be used as mixed-type inhibitors with both physical and chemical adsorption models and its corrosion inhibition rate for carbon steel decreased firstly and then increased with temperature's increase.The studies on corrosion inhibition performance for carbon steel are quite a lot now, while there are still few studies on corrosion inhibition performance of gemini surfactants for aviation used aluminium alloys; at the same time, the corrosion inhibition studies of multi-alkyl multiple quaternary-ammonium salts cationic gemini surfactants with imino groups as spacers are more focused on using straight chain imino groups as spacers, those using branched chain imino groups as spacers are rarely reported.In this paper, isopropylimino (>NCH(CH 3 ) 2 ) is used as the spacer to connect two dodecyldimethyl quaternary-ammonium salts to finally synthesize a novel multi-alkyl bisquaternary ammonium cationic gemini surfactant; then its surface properties and its corrosion inhibition efficiency and the inhibition mechanism for aviation used Al-Cu-Mg 2024 alloy in HCl solution (1 mol/L) are studied.

Reagents and Instruments
Reagents.The reagents were isopropylamine, epichlorohydrin, dodecyldimethylamine, hexane, acetone, absolute ethanol, and hydrochloric acid (mass fraction: 36%-38%).All the chemicals were of analytical reagent grade and were used without further purification.

Synthesis of Bis
Preparation of Middle Product.In a beaker of 250 ml, epichlorohydrin (9.3 ml) was added at the speed of 10 seconds International Journal of Corrosion 3 per drop to a mixture of absolute ethanol and isopropamide (5 ml) under ice water bath.After that, the mixture was stirred at indoor temperature for 10 h.Then, rotatory evaporator was used to remove solvent and redundant epichlorohydrin in the 40 ∘ C water bath for 1 h from which the transparent viscous liquid bis(2-hydroxy-3-chloropropyl) isopropylamine was obtained as the middle product.
Preparation of Final Product.The middle product (12.2g), dodecyldimethylamine (27 ml), and absolute ethanol were mixed and heated to reflux for 10 h.From reduced pressure distillation at 60 ∘ C for 1 h and being washed three times by hexane, the white waxy product (32.6 g) bis[2-hydroxy-3-(dodecyldimethylammonio)propyl]-isopropylamine dichloride was finally extracted.

Surface Tension Measurement.
At 25 ∘ C, the DSA25 Drop Shape Analyzer was applied to measure surface tension curves of the mixed solution of 1 mol/L HCl and different concentrations of synthetic product.In addition, the critical micelle concentration (cmc) and its corresponding surface tension can be ensured by the intersection point of the curves.

Weight Loss Measurement.
The 2024 Al-Cu-Mg alloy specimens were applied with dimensions 50 mm × 25 mm × 2 mm.The surface was abraded in sequence by abrasive papers ranging from #800 to #2000 and then washed by deionized water, acetone, and absolute ethanol in order to remove the grease and impurities of the surface.Cold-blast air was used to dry the specimens and then initial weights of specimens were measured and recorded.After that, specimens were immersed in different corrosion solutions for 4 h and then a soft brush was dipped in acetone and absolute ethanol to remove the corrosion products on the metal surface.Finally the specimens were cleaned ultrasonically in deionized water and ultimate weights were measured and recorded again.
The average corrosion rate () and the inhibition efficiency (IE, %) the synthetic product were calculated as follows: where  0 and  1 are, respectively, the initial and ultimate weight;  is the immersed surface;  is the corrosion duration;  and  0 are, respectively, the average corrosion rate in presence and absence on synthetic product.was about 12 cm, and the distance between work electrode and reference electrode was about 2 cm.
Before the impedance test, open circuit potential (OCP) test was carried out to ensure that the follow-up experiments were tested in a stable state.The OCP plot of 2024 Al-Cu-Mg alloy in 1 mol/L HCl solution with 9 × 10 −4 mol/L synthetic product was shown in Figure 2.
It can be seen that, after soaking for about 15 minutes, the curve reaches a stable stage, and the OCP is basically stable at −0.48 V. Therefore, impedance tests were carried out after 15 minutes of immersion in the specimens.The results obtained by adding other concentrations of the synthetic product are basically the same.
In order to make an approximate equilibrium of the system, potentiodynamic polarization curves were measured under scanning range of ±250 mV relative to the OCP, and scanning rate of 0.166 mV/s.The following equation was used to compute the inhibition efficiency as where  0 and  are, respectively, the corrosion current intensity in absence and presence of synthetic product.The impedance measurements were carried out under frequency range of 10 mHz∼100 kHz and amplitude of 10 mV.Equivalent circuits were simulated from ZsimpWin software.The inhibition efficiency of synthetic product was calculated as follows: where    and   are, respectively, the polarization resistance in absence and presence of synthetic product.

Corrosion Surface Observation. HITACHI S-3400N
scanning electronic microscope (SEM) was applied to observe the corrode surface in absence and presence of synthetic product in order to determine their corrosion state and degree.
International Journal of Corrosion 2.7.Adsorption Isothermal Model.The adsorption mechanism of inhibitor can be justified by the adsorption isothermal model where a classic Langmuir adsorption model obeys the following assumption: every adsorption site on the metal surface shares the same properties and can only adsorb one molecule at best, the molecules onside exert no acting force on each other.The adsorption isothermal formula is where  is the concentration of inhibitor;  is the adsorption equilibrium constant;  is the fraction of coverage which can be attained from where IE is the inhibition efficiency of corresponding inhibitor concentration; IE  is the maximum inhibition efficiency.Besides, the Langmuir model can be also presented as follows [30]: where  is the corrosion rate;   is the reciprocal of the uninhibited corrosion rate;   is a constant, which resulted from the weight loss method.

Adsorption Thermodynamics Parameters.
The adsorption equilibrium constant  could be concluded from Langmuir simulated curve.The relation between  and the adsorption free energy Δ 0 ads can be expressed as where  is molar gas constant, 8.314 J⋅mol −1 ⋅K −1 ; T is the environmental temperature, 298 K; Δ 0 ads is used to justify furthermore the adsorption mechanism.
2.9.FTIR Tests.The absorption peaks in FTIR curves can be used to determine whether a new chemical bond occurs or disappears on the metal surface.FTIR curves of corrosion products on the surface of 2024 Al-Cu-Mg alloy in 1 mol/L HCl solution with/without the synthetic product have been plotted to determine the adsorption type of the synthetic product.

Structure Characterization of Bis[2-hydroxy-3-(dodecyldimethylammonio)propyl]-isopropylamine Dichloride.
The FTIR test result is shown in Figure 3.It can be seen that 3371 cm −1 is the vibration peak of -OH; the two adsorption peaks at 2923 cm −1 and 2853 cm −1 are characteristic vibration peaks of long carbon chains [31]; 1462 cm −1 is the flexural vibration peak of C-H [32]; peaks between 1050∼1200 cm −1 and 500∼700 cm −1 are the characteristic peaks of C-C, C-N, and C-Cl [32].Based on the analysis, it can be concluded that the synthetic product is the target product. .By analyzing the peak positions of relative hydrogen atoms and the relative areas of absorption peaks, it can be concluded again that the synthetic product is the target product.

Surface Tension
Test.Surface tension () was plotted versus the logarithm of concentration of synthetic product, as shown in Figure 5.As seen in Figure 5, at low concentrations,  drops sharply with the increase of log ; while, at higher concentrations,  keeps essentially constant.The surface tensions of these two states are fitted linearly, respectively, and the intersection point is approximately the critical micelle concentration (cmc) of the synthetic product [33].Thus it can be seen that the cmc of the synthetic product is 9.204 × 10 −4 mol/L, which is 1∼2 orders of magnitude lower than that of conventional single-chain quaternary-ammonium salt surfactant.cmc of a range of conventional and gemini surfactants are given in Table 1.
The cause of the decrease of cmc may be that the synthetic product connects two long carbon chains replacedammonium cations by using its spacer group, thus greatly increases the number of carbon atoms of the hydrophobic group [15], and finally reduces the cmc.Meanwhile, the hydrophilic group is connected with the spacer group by a chemical bond, thus could arrange closely in adsorption layer of the solution surface, and the hydrophobic groups arrange vertically in solid-liquid interface, thereby, the surface activity is increased and it is easier to form micelles.Combined with surface tension test result, it can be analyzed that corrosion rate first decreases rapidly with the increase of the concentration of synthetic product; when the concentration is near to cmc (9.204 × 10 −4 mol/L), the corrosion rate decreases from a rapid decline to a slow decline, and eventually almost no change occurs; the rule of inhibition efficiency is just the opposite.The reason may be that when the concentration is low, the ammonium cation of the synthetic product is easy to be adsorbed to the surface of 2024 Al-Cu-Mg alloy; at the same time, the long carbon chains arrange orientated and orderly, which form a protective film on the surface of the alloy and play a role in corrosion inhibition; when the concentration reaches to cmc, the adsorption of the surfactant molecules on the metal surface is approximately saturated; therefore, with the increase of concentration, the corrosion rate and corrosion efficiency change slowly or almost remain unchanged.2. It could be observed that  corr decreases with the increase of synthetic product's concentration.When the concentration is 9 × 10 −4 mol/L,  corr has reduced by an order of magnitude over the blank solution and reaches 7.07 × 10 −4 A. Meanwhile, both cathodic and anodic curves move in the direction of decreasing the current density;  corr changes little compared with blank solution; it suggests that both cathodic and anodic reactions are suppressed with the addition of synthetic product, besides; the synthetic product is a mixed-type inhibitor which reduces anodic dissolution and also retards the hydrogen evolution reaction.The corrosion efficiencies (IE) are calculated by using (2); results show that IE increases monotonously with the increase of synthetic product and is up to 88.6%, which means the synthetic product has a good inhibiting effect on metal corrosion.This result is consistent with the weight loss method.frequency zone, which are related to double-layer capacitance as well as the charge transfer resistance.The capacitive arcs are depressed in nature; it is due to the micro roughness of the surface of metal formed during corrosion.The proposed equivalent circuit by ZsimpWin software is shown in Figure 9; related parameters are listed in Table 3.It can be analyzed that   increases and  0 decreases with increasing in the concentration of the synthetic product, which indicates the formation of adsorption layer on the surface of the alloy.It can be calculated through (3) that the inhibition efficiency increases with increasing in the concentration of the synthetic product and is up to 94.6% at 9 × 10 −4 mol/L.Compared with the weight loss test results, the inhibition efficiencies obtained by impedance test are slightly higher, but the overall trend is the same, so the results are still consistent.10 demonstrates the damage caused by exposure of 2024 Al-Cu-Mg alloy to 1 mol/L HCl in absence and presence of the synthetic product for 4 hours.It is evident that the surface becomes very rough and a large amount of corrosion pits can be found after being corroded in 1 mol/L HCl without any synthetic product, which means that a severe corrosion has happened; when the synthetic product is added, the surface becomes smoother and no big corrosion pits could be found.Figure 10(b) clearly shows that the synthetic product could form a smooth adsorption layer on the surface to inhibit the corrosion of 2024 Al-Cu-Mg alloy.

Adsorption Isotherms and Thermodynamic Calculations.
Figure 11 shows the adsorption isotherms of the synthetic product on Al-Cu-Mg alloy in 1 mol/L HCl solution.In this case, the line has a slope of 0.936 and the correlation coefficient ( 2 ) is 0.9998.The linear relationship of c/ versus c shows that the synthetic product obeys Langmuir adsorption isotherm, which means that the synthetic product forms a dense monomolecular film on the surface of 2024 Al-Cu-Mg alloy [37], effectively blocks corrosion ions near the metal surface, and slows down the corrosion of alloy surface.
The adsorption equilibrium constant  could be calculated by using the fitted line above and K = 1.789 × 10 4 L/mol.Through (7), adsorption free energy Δ 0 ads can be  calculated and equals −34.2 kJ/mol.Δ 0 ads < 0 illustrates the adsorption process is spontaneous.A value of Δ 0 ads = −40 kJ/mol is usually adopted as the threshold value between chemisorption and physisorption [38].Thus, it indicates that the adsorption of the synthetic product is more inclined to physisorption, where ammonium cations are attracted to the negative charge of the metal surface by electrostatic interaction and are adsorbed to the metal surface, as shown in Figure 12.
Fit again the weight loss test results through (6), fitted lines are shown in Figure 13, where the ratio of the concentration of synthetic product to the concentration of the critical micelle concentration cmc was set up as abscissa in order to explain the influence of cmc on the corrosion rate.It shows that the two fitted lines intersect when C/cmc nearly equals 1.Thus it can be deduced that when the concentration is higher than cmc, the adsorption behaviour of synthetic product on the metal surface changes.

Study on Corrosion Inhibition Mechanism. FTIR results
of corrosion products on the surface of 2024 Al-Cu-Mg alloy in 1 mol/L HCl solution with/without the synthetic product are shown in Figure 14.It can be analyzed that 3401 cm −1 is the vibration peak of -OH [31]; 1626 cm −1 is the vibration peak of AlCl 3 [39]; 1397 cm −1 and 946 cm −1 are the vibration peaks of Al 2 O 3 and MgO [40]; 618 cm −1 is the vibration peak In order to better explain the corrosion inhibition process and mechanism of the synthetic product on the surface of the alloy, the schematic diagram of the adsorption and desorption process is shown in Figure 15.
As seen in Figure 15, there might be two adsorption modes when the concentration of synthetic product is lower than cmc, as shown in (a) and (b).At low concentration, two hydrophilic groups are adsorbed on the metal surface; that is, each hydrophilic group occupies one adsorption site, as shown in (a); at higher concentration, one of two hydrophilic groups of gemini surfactant may be adsorbed on the metal surface adsorption site, while another cannot adsorb onto it, as shown in (b).In actual situation, the adsorption mode is more inclined to (a) (b) coexistence, and this adsorption mode still meets the Langmuir adsorption model, that is, monolayer adsorption.
When the concentration of synthetic product is higher than cmc, its adsorption on the metal surface is saturated.When keeping increasing the concentration, similar to the conventional surfactants which form micelles with inner hydrophobic groups and outer hydrophilic groups, the synthetic product molecules will form a bilayer adsorption film with hydrophobic groups at the middle and hydrophilic groups at both ends, as shown in (c).Therefore, the adsorption pattern does not accord with the monolayer adsorption film of Langmuir model assumed, which is a good explanation of the intersection point in Figure 13.
As a new environmental friendly inhibitor, the synthetic product has many advantages, one of which is its physisorption on metal surface.The synthetic product can be desorbed naturally without any extra process when the metal surface is dried.Hence, compared with chemical absorption/desorption, physical absorption/desorption is more concise, efficient, and environmentally friendly.
(2) Critical micelle concentration cmc of the synthetic product is 9.204 × 10 −4 mol/L; when the concentration is lower than cmc, inhibition efficiency of the synthetic product to 2024 Al-Cu-Mg alloy increases monotonously and is up to 89.3% at 9 × 10 −4 mol/L; when it is higher than cmc, inhibition efficiency of the synthetic product keeps basically unchanged.
(3) The Tafel polarization studies indicate that the synthetic product is of mixed anodic-cathodic type; each Nyquist plots is composed of a depressed capacitive arc in the high frequency zone; the electrochemical test results are consistent with the weight loss method.
(4) The adsorption type of synthetic product is physisorption.When the concentration is lower than cmc, the adsorption mode of the synthetic product conforms to the Langmuir adsorption model and forms a monolayer adsorption film on 2024 Al-Cu-Mg alloy surface; when it is higher than cmc, it forms International Journal of Corrosion 11 bilayer adsorption film on metal surface and thus does not conform to the Langmuir adsorption model anymore.
Figure 7 represents the cathodic and anodic curves measured by adding different concentrations of synthetic product (0.5∼9 × 10 −4 mol/L) into 1 mol/L HCl; related parameters of polarization measurements are shown in Table

Figure 7 :
Figure 7: Tafel polarization curves with different concentrations of synthetic product added.

Figure 8 :Figure 9 :
Figure 8: Nyquist diagrams with different concentrations of synthetic product added.

Figure 14 :
Figure 14: FTIR plots of corrosion products on metal surface in absence/presence of synthetic product.

Figure 15 :
Figure 15: Adsorption principle of synthetic product on 2024 Al-Cu-Mg alloy surface.