Halogen-Substituted Acridines as Highly Effective Corrosion Inhibitors

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Halogen-Substituted Acridines as Highly Effective Corrosion Inhibitors for Mild Steel in Acid Medium Weiwei Zhang, Hui-Jing Li, Meirong Wang, Li-Juan Wang, Fei Shang, and Yan-Chao Wu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07015 • Publication Date (Web): 22 Oct 2018 Downloaded from http://pubs.acs.org on October 27, 2018

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The Journal of Physical Chemistry

Halogen-Substituted Acridines as Highly Effective Corrosion Inhibitors for Mild Steel in Acid Medium Weiwei Zhang,† Hui-Jing Li,*,† Meirong Wang,‡ Li-Juan Wang,*,‡ Fei Shang,† and Yan-Chao Wu*,†,§ †School

of Marine Science and Technology, Harbin Institute of Technology, Weihai 264209, P. R. China of Materials Science and Engineering, Harbin Institute of Technology, Weihai 264209, P. R. China §Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry Chinese Academy of Sciences, Beijing 100190, P. R. China ‡School

ABSTRACT: A series of acridines were designed and synthesized for the development of effective inhibitors for mild steel corrosion in 1 M HCl solution, in which the halogen-substituted acridines showed better inhibitive performance than the non-halogen-substituted acridines. The corrosion protection properties of the halogen-substituted acridines, including 2-chloro-9-phenylacridine (CPA), 2-chloro-9(2-fluorophenyl)acridine (CFPA) and 2-bromo-9-(2-fluorophenyl)acridine (BFPA), were further investigated using weight loss test and electrochemical techniques. The results indicated the halogensubstituted acridines have excellent inhibitiion performance, and these acridines act as mixed type inhibitors with predominant cathodic effectiveness. Adsorption of acridines on a mild steel surface obeyed the Langmuir adsorption isotherm. The adsorption of the inhibitor molecules on steel surface was further supported by scanning electron microscope (SEM), scanning electrochemical microscope (SECM) and FTIR spectroscopy. The inhibition mechanism of the investigated halogen-substituted acridines was derived using DFT based quantum chemical calculations for their neutral as well as protonated forms. Both experimental and DFT studies suggested that the inhibition efficiency of three halogen-substituted acridines followed the order of η(BFPA) > η(CFPA) > η(CPA).

1. INTRODUCTION Mild steel has been extensively applied in construction and other industrial fields owing to its remarkable mechanic performance. One of the greatest challenges with mild steel in industrial pickling, acid descaling and oil well acidizing process is that mild steel is susceptible to corrosion under these acidic conditions,1-3 which could lead to huge economic losses and potential environmental problems. A practical and effective solution to this problem is to use corrosion _________________ * Corresponding author. Tel.: +86-0631-5687230；

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E-mail address: [email protected] (H.-J. Li), [email protected] (L.-J. Wang), [email protected] (Y.-C. Wu) inhibitors due to their easy synthesis, remarkable inhibition effect and economic advantages. The reported corrosion inhibitors against steel corrosion in acidic medium are usually polar organic heterocyclic compounds bearing larger electronegativity atoms (i.e., N, O, S, and P), polar functional groups, and conjugated double bonds.4-7 For example, azoles,8 Schiff bases,9 quinolones,10 acridines11 and pyrimidines12 have been reported as one of the most effective acidic corrosion inhibitors. The polar units of these corrosion inhibitors are regarded as the reaction centers to promote their adsorption on the steel surface, forming a protective layer to prevent the steel from undertaking corrosion attacks. Various inhibitor platforms are needed to form more effective protective layers for steel protection in hydrochloric acid solution, which makes the exploitation of late-model chemical class of corrosion inhibitors for mild steel protection a high priority. Acridine derivatives, as a kind of N-heterocyclic organic compounds, are easy to be synthesized with perfect inhibition ability and have been under intensive study in metal corrosion inhibition.13-15 Granese et al.11 have studied the interactions of acridine inhibitors with Fe and steel surfaces in HCl media and reported that acridine derivatives had good anticorrosion properties. We envisioned that acridine might be such a corrosion inhibitor platform for steel protection as it contains both a polar unit of nitrogen atom and a large conjugation system. Indeed, unsubstituted acridine had been used as an effective corrosion inhibitor for the hot dipped Zn-Al alloy coatings in diluted HCl medium.16 On the other hand, it is known that the inhibition efficiency of some corrosion inhibitors could be obviously improved in the presence of halogen-substitution.17 Qiang et al. studied three halogeno-indazole compounds as corrosion inhibitors of copper in a neutral chloride solution, and found that the halogenation effect can markedly improve the inhibitive performance of indazole.17 Recently, Verma et al. reported that the contribution of substituent groups to corrosion inhibition potentials has some direct relationships with Hammett substituent constants (σ), which reflects the total electronic effect on the reaction center.18 In general, an inhibition with negative value of σ associated with stronger adsorption tendency as compared to the inhibitor with either positive or less negative value of σ. Although halogens have a large positive value of σ (–Br: σp = +0.232; –Cl: σp = +0.227), they seem to be more efficient than –CH3 (σp = -0.170), –H (σp = 0.0) and –NO2 (σp = +0.778) groups because halogens as highly electronegative atoms can withdraw electrons inductively, and have the ability to provide nonbonding electron pairs via the π-resonance effect. In this context, herein we report that halogen-substituted acridines could be used as highly effective inhibitors for mild steel corrosion in HCl medium.


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In this paper, a series of acridines were designed and smoothly synthesized by using a modified synthetic procedure for acridine synthesis. For the first time acridines were found to be potential inhibitors for mild steel corrosion in HCl solution, in which halogen-substituted acridines, such as 2chloro-9-phenylacridine (CPA), 2-chloro-9-(2-fluorophenyl)acridine (CFPA) and 2-bromo-9-(2fluorophenyl)acridine (BFPA), showed better inhibitive performance than the non-halogen-substituted acridines. The corrosion protection properties of the halogen-substituted acridines for mild steel in 1 M HCl solution were studied using weight loss test, potentiodynamic polarization measurement and electrochemical impedance spectroscopy (EIS) techniques. Surface morphology characteristic was tested by SEM, SECM and FTIR spectroscopy technology. By calculating the electronic structure property of inhibitor molecules as well as protonated forms, the inhibition mechanism of corrosion inhibitor and mild steel was discussed using quantum chemical calculation method. Our efforts will conduce to better comprehend the elements controlled by their structural units, and provide new possible considerations in the design of practical acridine-type inhibitors for mild steel in HCl solution.

2. EXPERIMENTS 2.1. Materials and Solutions. Components of mild steel materials used for experiments are as follows (wt.%): 0.046 C, 0.021 Si, 0.16 Mn, 0.024 P, 0.005 S and balance Fe. The steel materials for electrochemical experiments and surface analysis have sizes of 1.8 cm × 0.6 cm × 0.4 cm and 1.0 cm × 1.0 cm × 0.1 cm, respectively. For electrochemical experiments, the electrodes retained a working area of 0.2 cm2, and then the non-working surface was encapsulated with epoxy resin. The electrode working surface was polished with different grades of SiC abrasive paper, then washed with ethanol and dried at room temperature before using them for experiments. Analytical grade 37% HCl and ultra-pure water were used to prepare 1 M HCl corrosion test solution. The concentration of halogen-substituted acridine inhibitors in corrosive solution ranged from 0.1 mM to 0.4 mM. Acridines were synthesized by oxidation of cyclohexanones and 2-aminophenyl ketones, in which molecular oxygen is the only oxidant. The modified acridine synthetic procedure has more strength, including non-hazardous oxidants, readily available raw materials, strong regioselectivity, and water as sole by-product. The mixture of 2-aminobenzophenone (197.2 mg, 1.0 mmol), cyclohexanone (124 uL, 1.2 mM), citric acid (105 mg, 0.5 mM), Pd(TFA)2 (26.5 mg, 10 mol %), and 1,10-phenanthroline (14.4 mg, 8 mol %) in 1-methyl-2-pyrrolidinone (5.0 mL) was stirred at 120 °C for 20 h under oxygen atmosphere. The molecular structures of acridines were shown in Figure 1. The 1H NMR, 13C NMR, IR and HRMS of acridines were given in Supplementary Information.



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2.2. Weight Loss Experiments. Weight loss tests were carried out in a constant temperature water bath bottle, which containing 100 mL 1 M HCl within and without a series of concentrations of acridine inhibitors at room temperature [(30±0.1) °C] for 72 h. After the corrosion test was completed, carefully rinsed the specimens surface with ultrapure water, dried and weighed. Each test was performed in triplicate under the same conditions, and the standard deviation values of the weight loss results were less than 6% (Figures. 2 and 3), indicating that the reproducibility of the experiment is acceptable. The corrosion rate (v) was obtained from the average of three parallel experimental data.5 2.3. Electrochemical Measurement. Electrochemical tests were measured using CHI 760E workstation with a standard three-electrode system: mild steel as the working electrode, a large platinum plate as auxiliary electrode and saturated calomel electrode as reference electrode. Prior to electrochemical testing, the working electrode needs to be immersed in the electrolyte solution (100 mL 1 M HCl with different concentrations of inhibitors) for a period of time to obtain a stable open circuit potential (OCP). The polarization curves were measured in the potential rang of ±250 mV vs. EOCP with a scan rate of 1 mV s-1. EIS measurement was performed at EOCP in a frequency range from 100 kHz to 10 mHz with a sinusoidal potential perturbation of 5 mV. The EIS data were fitted with ZSimpWin software to obtain the corresponding impedance parameters and equivalent circuit. All electrochemical tests were measured in triplicate at room temperature and the average values were presented. 2.4. Surface Morphology: SEM and SECM. The surface morphologies of steel specimens after immersion in 1 M HCl solution without and with 0.4 mM studied inhibitors were observed by SEM (100 mL electrolyte solution) and SECM (10 mL electrolyte solution). SEM study was conducted on a SUPRTM55 instrument (Zeiss, Germany) at 2 k× magnification. SECM (Uniscan Instruments Ltd., UK) was performed with a four-electrode system: 25 μm platinum ultra-microelectrode as the probe, mild steel as the working electrode, platinum plate as counter electrode and saturated calomel electrode as reference electrode. The potentiostat/galvanostat was used to control the probe tip potential to 0.6 V (vs. SCE). 2.5. FTIR spectra experiments. FT-IR spectrum was performed with Nicolet 700 FT-IR spectrometer with ranging from 4000 to 400 cm-1. The absorption spectrum of 1 M HCl with three studied inhibitors were investigated before and after 24 h of mild steel immersion at ambient temperature (~25 °C). 2.6. Quantum Chemical Calculations. Quantum chemical calculations were performed by density function theory (DFT) with B3LYP/6-31G(d, p) level within Gaussian 09 program package in


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gas and aqueous phases.19 Generally, the calculated quantum chemical parameters such as the energy of lowest unoccupied molecular orbital (ELUMO), energy of highest occupied molecular orbital (EHOMO), energy gap (ΔE = ELUMO – EHOMO), fraction of electrons transferred (ΔN), Mülliken charges and natural bond orbital (NBO) charges, which can be used to analyze the relationship between the adsorption activity of corrosion inhibitor and the corrosion resistance of steel.

3. RESULTS AND DISCUSSION 3.1. Weight Loss Study. Inhibitive Behavior of Five Acridines. Weightlessness was one of the simple monitoring methods for evaluating the inhibition performance of corrosion inhibitors, and has high reliability. To investigated the inhibitive performance of non-halogen-substituted and halogensubstituted acridines for mild steel in HCl solution, five acridines (MA, PA, MPA, CPA, CMPA) were selected here as the target corrosion inhibitors. The concentration of the target acridines inhibitors in corrosive solution were fixed at 0.2 mM and 0.3 mM, and the inhibition efficiency of mild steel after immersion in 1 M HCl with acridines was shown in Figure 2. As expected, the halogen-substituted acridines CPA and CMPA exhibit better corrosion inhibitive performance, and the corrosion efficiencies of these acridines follows the order of CMPA > CPA > MPA > PA > MA. Considering the different electronic structures of these compounds, quantum chemical calculations were performed to reconfirm the important role of halogens in corrosion. The values of some quantum chemical parameters (EHOMO, ELUMO and ΔE) were calculated. With the theory of frontier molecular orbital, the adsorption process of inhibitor molecules is affected by ΔE. Usually, a lower ΔE will lead to stronger adsorption capacity of molecules, then enhance the inhibition efficiency.4,6 The calculated ΔE values of five acridines obey the order of ΔECMPA (3.26) < ΔECPA (3.29) < ΔEMPA (3.38) < ΔEPA (3.52) < ΔEMA (3.89), which is consistent with the weight loss test results. The above research shows that halogen atoms play an important role in anticorrosion process. Therefore, only halogen-substituted acridines were used as inhibitors for the following corrosion experiments. Inhibitive Behavior of CPA, CFPA and BFPA. Figure 3 displayed the corrosion rate (v) and inhibition efficiency (ηw) of steel in 1 M HCl with different concentrations of FPA, CFPA and BFPA. The v and ηw were calculated as Eqs. (1) and (2): 𝑣= 𝜂𝑤 =


𝑊1 ― 𝑊2 𝑠× t

𝑣𝑜 ― 𝑣 𝑣𝑜

× 100%

(1) (2)


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where v is corrosion rate, W1 and W2 are the steel weight before and after immersion, s is exposed area of specimens, t is immersion time, vo and v represent corrosion rate without and with inhibitors, respectively. As seen in Figure 3, the values of v are significantly reduced and the ηw increases after adding any inhibitors for the study. The maximum ηw values of CPA, CFPA and BFPA were 92.8%, 94.2% and 98.2%, respectively, and the v value follows the order of v(BFPA) < v(CFPA) < v(CPA). The results indicated that the studied halogen-substituted acridines could efficiently restrain the steel corrode in HCl solution, and the halogenation effect can remarkably improve the inhibition performance of acridines compared with other halogen-free inhibitors previously reported.20 The adsorption mechanism of acridine inhibitors and steel surface was further elaborated in Section 3.8, Quantum chemical calculations. 3.2. Open circuit potential (EOCP) curves. Figure 4 shows the variation of open circuit potential (EOCP) of mild steel electrode with time (s) in 1 M HCl solution in absence and presence of inhibitor at 25 °C. It could be observed from OCP curves that the EOCP shifted toward negative potentials with addition of different concentration of inhibitor, which can be explained by the adsorption of inhibitor on the mild steel.4 By increasing the concentration of the inhibitor, shifting of the steady state potentials (EOCP) toward more negative direction in the presence of studied inhibitors suggests that the cathodic reactions are relatively more affected as compared to anodic reactions. It took about 20 min to reach the steady state, so we chose the immersion time of 30 min for electrochemical measurements. 3.3. Potentiodynamic Polarization Curves. Figure 5 compares the polarization curves for mild steel in 1M HCl solution without and with investigated inhibitors. The calculated inhibition efficiency (η) and related electrochemical parameters obtained by Tafel extrapolation method were listed in Table 1. The inhibition efficiency was calculated by Eq. (3) based on the measured icorr: 𝜂(%) =

𝑖0corr ― 𝑖corr 𝑖0corr

× 100

(3)

herein, i0corr and icorr denote the corrosion current densities of unprotected and protected steel in acidic solution, respectively. As showed in Figure 5, all polarization curves showed a trend to low current density direction when CPA, CFPA and BFPA inhibitors were added to the system. The icorr values in Table 1 decreased gradually as the inhibitor concentration increased, which also makes the inhibition efficiency (η) increased. These results indicated that the three studied inhibitors have an inhibitive effect on the anodic and cathodic reactions of the electrode. Moreover, the cathodic polarization curves of the test were nearly parallel and did not change noticeably, indicating that the mechanism of the hydrogen evolution reaction


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was not affected by halogen-substituted acridine inhibitors. The reduction of H+ ions available on steel surface was active sites of the hydrogen evolution reaction being inhibited by the inhibitor molecules attached to metal surface. In anodic region, the anode curves of CPA and CFPA became steep, overlapping and intersect with the blank hydrochloric curve at a -0.26 V vs. SCE. According to Zhang,4 this phenomenon on anodic region means the dissolution rate of steel is faster than the formation of protective film, causing the inhibitor molecules desorption on steel surface. For BFPA at a concentration above 0.2 mM, the anodic curves also became steep, overlapping but not intersect with the blank hydrochloric curve, demonstrating that BFPA adsorption is more stable on mild steel surfaces than CPA and CFPA. Moreover, it is noteworthy in Table 1 that the η of three halogen-substituted acridines follows the order of η(BFPA) > η(CFPA) > η(CPA), with the maximum inhibition efficiency of 98.08%, 96.54% and 90.95%, respectively, and the experimental results are in accordance with those obtained by the weightlessness method. Simultaneously, the potential of studied inhibitors at any concentration was negative shift and less than 51 mV compared with blank hydrochloric acid, showing that the three inhibitors studied mainly restrained the cathodic reaction and belonged to the mixed-type inhibitor. The similar results of other N-heterocyclic compounds as inhibitors of mild steel in 1 M HCl have been reported.20-22 Another result in Table 1 shows that the βa and βc values changed after adding inhibitor, indicating that the studied inhibitors have the effect of inhibiting anode and cathode reactions. It can be reasonably explained, the lone pair of electrons possessed by halogen, nitrogen atoms and unsaturated bonds in the acridines formed Fe-In complex with Fe on steel surface, thus affecting the cathode and anode corrosion process of iron. These results show that the studied acridines can be used as excellent inhibitors for mild steel corrosion in 1 M HCl, and BFPA has the best inhibition effect. 3.4. The EIS results. To further study the interfacial properties and kinetic process of electrode, the EIS of steel in hydrochloric acid corrosion was conducted. The Nyquist plots of three halogen-substituted acridine inhibitors were displayed in Figure 6. As showed in Figure 6, the diameters of the capacitive loop with either studied inhibitor were much larger than the blank hydrochloric acid, and the inhibition strength was enhanced with adding concentration, indicating that the inhibitor film gradually became compact and eventually leading to an improvement in protection. Besides, the Nyquist plots exhibit an approximately compressed semi-circle capacitive loop, which is usually considered to be caused by “dispersion effect”.23,24 Ordinarily, such phenomenon can be traced to the frequency dispersion of the interfacial impedance and the inhomogeneous steel surface because of the microscopic roughness and



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inhibitor adsorption.24 And according to Bode plots in Figure 7, it can be noted that the phase angle remains constant over a wide frequency range as the inhibitor concentration increases, which means there are two phase constants of these compounds and the double-layer impedance does not show perfect capacitance behavior. Besides, the absolute impedance increases significantly with the constant addition of inhibitors, which are ascribed to the fact that the steel surface was adsorbed more molecules at high concentrations, making the adsorption film thicker and causing the charge transport process difficult. Therefore, constant phase element (CPE) was commonly applied to replace the double-layer capacitance to obtain the ideal electrochemical process.25 The impedance of a CPE(ZCPE) was represented as below: 𝑍CPE = [𝑌0(𝑗𝑤)n]

―1

(4)

where Y0 is a proportionality coefficient(Ω–1·sn·cm–2), j is the imaginary number, ω and n represent the angular frequency and phase shift, respectively. Usually, for n=0, 1 and –1, CPE represents a resistance, a capacitance and an inductance, respectively. The corresponding two equivalent circuits in Figure 8a and b were used to fit the impedance data. The fitted solid lines in Figure 6 were fitted by the electric circuit Figure 8, which is consistent with the EIS experimental data, indicating that the equivalent circuits of the selection conform to the experimental requirements. For example, the EIS result of HCl and CPA was simulated by an equivalent circuit revealed in Figure 8, and the obtained fitting curve was illustrated in Figure 9. The small values of χ2 (6.41 × 10-3 and 6.24 × 10-4) mean better anastomosis. The electrochemical parameters in Table 2: Rs is electrolyte resistance, Rp is polarization resistance, Rf is film resistance, Cd represents double-layer capacitance and Cf is film capacitance. Besides, CPE1 element was substituted for the double-layer capacitance (Cd) which contained H2O and other ions adsorbed on steel surface, and CPE2 reflecting inhibitor membrane (Cf).26,27 The inhibition efficiency (ηz) can be expressed by the polarization resistance Rp (Rp = Rf + Rct) in Eq. (5): 𝜂z(%) =

(𝑅p ― 𝑅0p) 𝑅p

× 100

(5)

where R0p and Rp represent the sum of Rf and Rct of mild steel electrode without and with acridine inhibitors, respectively. According to Table 2, the Rf and Rp values increased dramatically after adding studied inhibitors, indicating that these compounds adsorbed on the metal surface and thus effectively inhibit the charge transfer behavior. While, the values of Cf and Cdl were gradually reduced with the constant addition of inhibitors, which is usually caused by a decrease in the local dielectric constant or the increase of the thickness of the electric double layer capacitor.28-30 It may be known that the water


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molecules on the steel surface were gradually replaced by inhibitor molecules, forming a layer of dense protective film to prevent contact between metal and acidic medium, thus prevent the occurrence of corrosion. Furthermore ，another interesting discovery in Table 2 was that the value of n was ranged from 0 to 1, and these values depended on the deviation of the ideal capacitance behavior,24 revealing that the interface behaves closed to the capacitive and the dissolution mechanism of the steel was controlled by charge transfer. Finally, it should be pointed out that with the addition of inhibitors, the calculated ηz value of EIS increases, and the maximum ηw reached 93.21% for CPA, 95.95% for CFPA and 97.54% for BFPA at 0.4 mM, respectively, which is in accordance with the results of weight loss and polarization curves. Compared with other halogen-substituted inhibitors by EIS method in 1 M HCl was shown in Table 3, CPA, CFPA and BFPA exhibited better corrosion inhibition behaviour. This phenomenon is related to the halogenation and steric hindrance effect in the structure. 3.5. Adsorption Isotherms. Generally, the adsorption isotherms can give basic information for the adsorption mechanism of inhibitors by fitting the relation function between the surface coverage and concentration. In this research, it was found that the Langmuir adsorption calculated by Eq. (6) best fits the data of weightlessness test, with a linear regression coefficient (R2) close to 1. 𝑐 1 = +𝑐 𝜃 𝐾𝑎𝑑𝑠

(6)

where c is inhibitor concentration, θ represents surface coverage which was calculated according to Eq. (2), and Kads is adsorption equilibrium constant. The plots of c/θ versus c as shown in Figure 10, and the linear regression parameters were displayed in Table 4. Then the values of Kads can be obtained from the intercepts of the plots, and the standard adsorption free energy (ΔG0ads) was calculated by Eq. (7): ∆𝐺0𝑎𝑑𝑠 = ―𝑅𝑇ln(55.5𝐾𝑎𝑑𝑠)

(7)

Herein, T and R represent the thermodynamic temperature and gas constant, respectively. The ∆G0ads of three inhibitors calculated in Table 4 were -35.92, -36.47 and -36.70 kJ/mol, respectively. Customarily, the values of ∆G0ads around −20 kJ/mol or less are related to the electrostatic interaction of positively charged inhibitor molecules and negatively charged metal surfaces, which are belonging to physisorption. Instead, if the value of |∆G0ads| exceeds 40 kJ/mol is chemisorption owing to the charge sharing or transfer between the inhibitor molecules and iron atoms.3,22,31 As mentioned above, the actual value of ∆G0ads for the inhibitors in this work (–35 ― –37 kJ/mol), showing that the adsorption of corrosion inhibitors on steel surface consists of physical adsorption and chemical adsorption interaction rather than a single



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adsorption action. Besides, the values of Kads follow the order of Kads(BFPA) > Kads(CFPA) > Kads(CPA), and BFPA shows the maximum negative value of ∆G0ads, further confirming that BFPA has strong adsorption ability on mild steel surface and shows excellent inhibition performance. 3.6. SEM and SECM Experiments. Figure 11 presents SEM images of mild steel samples immersion in 1 M HCl solution without and with 0.4 mM acridines for 36 h. Before soaking, the steel samples surface (Figure 11a) was flat and smooth with only a few scratches produced by the polishing process. However, the uninhibited steel sample (Figure 11b) was seriously corroded with large pits and cracks due to acid erosion. Moreover, in contrast, the surface of specimen (Figure 11c-e) was relatively smooth and only a few shallow pitting corrosions occurred after adding three inhibitors. Especially, the surface of steel protected by BFPA was the flattest and close to the sample polished surface. It can be deduced from these results that the three studied inhibitors can be stably adsorbed on metal surface to form a dense protective film, and the corrosion resistance of BFPA is stronger than other two inhibitors. SECM can directly characterize the surface morphology and electrochemical activity of substrate through the Faraday current distribution obtained by the scanning of micro-probe at different positions on the substrate. The Fe2+ ions produced by the substrate under open circuit potential will be oxidized to Fe3+ ions at a 0.60 V potential of the probe as detected in generator–collector mode.32 The SECM images of steel samples in test solutions without adding and adding 0.4 mM inhibitors were displayed in Figure 12. Figure 12a shows a typical SECM image of the acid corrosion of steel, in which the amplitude of the current varies greatly on the scanned map with a maximum value of 1470 pA, due to the conductive properties of the metal surface, and the current oscillations were produced by anodic dissolution of the substrate. However, the surface current density of the protected steel (Figure 12b-d) has a great decrease and small amplitude fluctuation. This is due to a stable insulating protective film formed on the electrode surface, which prevented the diffusion process around the tip, and thus reduced the tip current. The results revealed that a film of inhibitors was formed on metal surface, which is consistent with the SEM experiment. The current maximum value for CPA, CFPA and BFPA was respectively 263, 161 and 112 pA, which again indicated that BFPA has better inhibitive performance. The inhibition efficiency (ηSECM) at 0.4 mM inhibitor can be calculated using Eq. (8):33 𝜂𝑆𝐸𝐶𝑀(%) =

(𝑖tip(max) ― 𝑖′tip(max)) 𝑖tip(max)

× 100 (8)

where itip(max) and i'tip(max) represent the maximum current density for the unprotected and protected steel, respectively. As shown in Table 5, it can be observed that at 0.4 mM of inhibitors, the ηSECM of three


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studied acridines increased in the preferential order of η(BFPA) > η(CFPA) > η(CPA), which is in accordance with those obtained by the weight loss and electrochemical measurements. In addition, the inhibition efficiency measured by SECM is significantly lower than that of weight loss and electrochemical tests. The difference in values of inhibition efficiency may be due to the short time of the current measurement at the microelectrode, however, the weight loss test and electrochemical methods measure the average corrosion current density. 3.7. FTIR spectroscopy analysis. In order to further research the inhibition mechanism of inhibitors for mild steel in corrosion media, the pure inhibitor and the sample scratched from steel surface were analyzed by FTIR, and the related spectrum was shown in Figure 13a and b, respectively. As shown in Figure 13b, the FTIR of the adsorbed film and the pure inhibitor molecules have similar characteristic peaks in the same region, indicating that the inhibitor molecules were adsorbed on mild steel surface. And as the molecular vibration characteristic peak shifts toward a higher or lower wave numbers, possibly due to the formation of inhibitor and Fe2+ complex on steel surface.34 Comparison of Figure 13a with Figure 13b shows that the peaks of stretching vibration at C-H (CPA: from 2924 cm-1 to 2943 cm-1; CFPA: from 3063 cm-1 to 3035 cm-1; BFPA: from 3062 cm-1 to 3021 cm-1) and C=N (CPA: from 1621 cm-1 to 1606 cm-1; CFPA: from 1625 cm-1 to 1617 cm-1; BFPA: from 1627 cm-1 to 1613 cm-1) were shifted significantly, which shows that they were involved in the formation of coordination bonds with Fe atoms, supporting the protective film formation via chemical adsorption. However, the C = C peaks at 1443 cm-1 and 1468 cm-1 for TP, 1472 cm-1 and 1496 cm-1 for CFPA, and 1448 cm-1 and 1470 cm-1 for BFPP were merged at 1452 cm-1, 1486 cm-1 and 1458 cm-1, respectively, suggesting that the π electrons of phenyl and acridines ring were involved in the adsorption process of the inhibitor. The above studies clearly demonstrate that there is a chemical interaction between inhibitor and mild steel. 3.8. Quantum Chemical Calculations. 3.8.1. Frontier Molecular Orbitals and Parameters. The electron distributions of the HOMO and LUMO of the frontier molecular orbitals were favorable for investigating the adsorption activity of inhibitor molecules, which is related to the molecular donor electrons and accept electrons ability, respectively.24,34,35 The frontier molecule orbital electron distributions of neutral and protonated inhibitors were investigated in both gas phase and aqueous phase, as shown in Figures 14 and 15. All the quantum chemical parameters were summarized in Tables 6 and 7. It can be seen from Table 6 that the values of different quantum chemical calculation parameters for CPA, CFPA and BFPA using different basic sets namely 6-31G, 6-31++G, 6-311G and 6-311++G did not show any regular trend. As 6-31G basis set provided accurate geometry and electronic properties for



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a wide range of organic compounds, therefore in the present study 6-31G basic set was employed. The HOMO and LUMO diagrams of three neutral form inhibitors presented in Figures 14 and 15 all showed that the electron densities were mainly concentrated on the acridine ring including the halogen atoms, which mean that they are both an electron donor and an acceptor for electrons. This means, electron transfer occurs when acridine is interacting with the metal surface. For the protonated molecules, the HOMO electron density is almost distributed among the entire molecule and the substituted phenyl segment can accept electrons. While, their LUMO electron density is similar to neutral inhibitors. As reported previously,36-38 the molecules with high EHOMO values give greater electron donation capabilities, and conversely, the lower the ELUMO values, the stronger the acceptance of electronic ability. It is apparent from the data in Tables 6 and 7 that there is no significant difference in the values of EHOMO and ELUMO computed in both phases of the neutral form. Meanwhile, the EHOMO and ELUMO of the protonated molecules are lower than those of the neutral molecules. It just reveals that the neutral form has a stronger ability to donate electrons compared to the protonated form, but the ability of accepting electrons is the opposite. The EHOMO values obey the order: EHOMO,BFPA > EHOMO,CFPA > EHOMO,CPA, which is in the same order as the inhibition efficiency of η(BFPA) > η(CFPA) > η(CPA). This shows that there is a good correlation between η and EHOMO. While, the values of ELUMO obey the order: ELUMO,CPA < ELUMO,BFPA < ELUMO,CFPA, which is inconsistent with the results obtained by experiment, and may be affected by the complex interaction of the adsorption process. Moreover, separation energy (ΔE = ELUMO – EHOMO) is usually used to determine the adsorption activity of inhibitor molecules on the metal surface. The lower the values of ΔE are, the stronger the reactivity of corrosion inhibitor will be. According to Tables 6 and 7, the ΔE values of different inhibitors decreased in the order: ΔEBFPA < ΔECFPA < ΔECPA, which illustrated the adsorption capacity of BFPA among the studied inhibitors is the most stable and possess higher anticorrosion ability. In addition, the protonated form shows a lower value of ΔE than the neutral form, which may be due to the enhanced attraction of excess electrons on metal surface after protonation, making the inhibitor molecules easily adsorbed on metal surface. The relationship between corrosion inhibition and molecular structure can also be analyzed and predicted using the fraction of transfer electrons (ΔN). A perusal of the literature,39,40 revealed that the work function (Φ) of the metal surface is an appropriate measure of its electronegativity and the fraction of electrons (ΔN) can be calculated by Eq. (11): ∆𝑁 =


𝛷 ― 𝜒inh 2(𝜂Fe + 𝜂inh)

(11)

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where χ and η are the absolute electronegativity and global hardness, respectively, as indicated by Eqs. (12) and (13): 𝜒=

―𝐸𝐻𝑂𝑀𝑂 ― 𝐸𝐿𝑈𝑀𝑂

𝜂=

2 𝐸𝐿𝑈𝑀𝑂 ― 𝐸𝐻𝑂𝑀𝑂 2

(12) (13)

In Eq. (11), the value of ηFe is 0 eV and Φ obtained from DFT calculations was 4.82 eV for the Fe (110) plane. As reported previously,39,40 if ΔN > 0, the electrons transfer from the inhibitor to the metal and vice versa if ΔN < 0. In this study, it can be clearly found in Tables 6 and 7 that the neutral form has positive ΔN value in both phases, but were negative for the interaction between the protonated form and Fe (110). Accordingly, the acridines inhibitor molecule generates coordination bonds that donate electrons to empty d-orbital of Fe atom mainly in neutral form, and forms back-donating bond to accept electrons from Fe atom with its vacant π* orbital mainly in protonated form, and hence strengthen the adsorption of inhibitor molecule on steel surface. It can be seen from Figures 14 and 15 that the solvent effect did not significantly change the frontier molecule orbital electron distributions of the neutral and protonated inhibitors, however, a slight modification was usually obtained for the calculated parameters. Therefore, calculations in the gas phase are acceptable, and sections 3.8.2 and 3.8.3 are calculated under gas phase because it reduces the time and has no significant differences. 3.8.2. Mülliken Atomic Population. Generally, Mülliken charges was commonly used to analyze the adsorption-activated sites and the donor-acceptor electron interaction between inhibitor molecules and iron atoms.41,42 The optimized geometry (Figure 16) and Mulliken charges of the neutral and protonated inhibitors atoms were investigated in gas phase. In this study, the C, Cl, Br, F and N atomic charges obtained by Mülliken charges were listed in Table 8. It can be seen that atoms with a larger negative change are N30 for CPA, N30 and F32 for CFPA, and N30, F32 and Br33 for BFPA, which are the main active atoms that are capable of forming coordinate bonds through the lone pair electrons and the empty d-orbital of iron atoms. Besides, for the protonated inhibitors, some carbon atoms also have a large negative charge, such as C2, C10, C23 and C24, which may also act as active sites. Thus, acridine ring and halogen are the main adsorption centers of the inhibitor molecule on steel surface. 3.8.3. NBO Charge Distributions and 3D MEP Plots. The NBO atomic charges and 3D Molecular electrostatic potential (MEP) plots for neutral and protonated inhibitors are displayed in Figure 16. NBO



Page 14 of 49

charges analysis has also been used to forecast the adsorption activity center of inhibitor, and the higher negative charge of a heteroatom means the more easily adsorbed on metal surface. It can be clearly found from Figure 16 that the nitrogen, fluorine and carbon atoms for all studied neutral and protonated inhibitors have larger negative charges as the main adsorption center which could donate electrons to steel surface form coordination bonds with Fe atom. In addition, it could be found that the neutral structure has more negative charge centers than the protonated structure. Additionally, the MEP can provide a visual method to define the relative polarity, which facilitates understanding the corrosion process. The electrostatic potential negative region is prone to undertake an electrophilic attack (shades of red), while the positive potential region (shades of blue) for nucleophilic attack. As shown in Figure 16a, for neutral form the electrostatic potential negative regions are mainly located in the halogen group and acridine ring, indicating the possible sites for electrophilic attack. However, positive potential regions are located around hydrogen atoms and phenyl ring, which are the possible sites for nucleophilic attack. Thus, it can be rationally concluded that these compounds have three or four major adsorption activity sites, bearing nitrogen, halogen atoms and phenyl ring. While, it is expected that the protonated structure has excess positive potential regions (Figure 16b), which means the protonated form is more likely to undergo nucleophilic attack and has a stronger ability to accept electrons from steel surface. Identical conclusions were obtained from the frontier molecular orbitals (EHOMO and ELUMO) and ΔN analyses (Tables 6 and 7). It is noteworthy that the quantum chemical calculation depictions are in good agreement with the inhibition efficiency in the experimental chapters, which demonstrated that the presence of halogen atoms enhanced the donor-acceptor interaction, made their adsorption on steel surfaces more stable. Substituent effects on the inhibition efficiency of CPA, CFPA and BFPA on mild steel in 1 M HCl showed a trend of η(BFPA) > η(CFPA) > η(CPA), and was also consistent with the strength of electron withdrawing abilities of the substituents as quantified by Hammett constants. The value of σp for –Br (+0.232) is large than that of –Cl (+0.227) and –F (+0.062), which indicates that –Br is a stronger electron withdrawal group than –Cl, resulting in a stronger donor-acceptor interaction between inhibitors and metals.

4. CONCLUSIONS For the first time acridines were found to be potential corrosion inhibitors for mild steel in HCl solution, and the halogen-substituted acridines showed better inhibitive performance than the non-halogensubstituted acridines. The inhibition efficiencies of the investigated halogen-substituted acridines


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obtained by weight loss test and electrochemical methods follow the order of η(BFPA) > η(CFPA) > η(CPA). Potentiodynamic polarization curve results showed that three compounds belonged to mixedtype inhibitors with predominantly cathodic inhibitive effects, and the adsorption on steel surface followed Langmuir adsorption isotherm with both physisorption and chemisorption. Surface morphology by SEM, SECM and FTIR showed that the adsorption film of acridines on steel surface effectively inhibited the corrosion process. DFT based quantum chemical parameters proved that the inhibitor molecule forms coordination bond with Fe atom mainly in neutral form, and the formation of backdonating bond is mainly in protonated form, which also proved that the adsorption activity centers are mainly the acridine ring and halogen atoms. Besides, the solvent effect did not significantly change the frontier molecule orbital electron distributions for the neutral and protonated inhibitors. This research gives an inspiration to design reasonable routes to modify the studied inhibitor to improve the inhibitive ability.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (H. J. Li), [email protected] (L. J. Wang), [email protected] (Y. C. Wu). Tel.: +86-0631-5687230 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was funded by the National Natural Science Foundation of China (21672046, 21372054, 21503056), and the Fundamental Research Funds for the Central Univercities (HIT.NSRIF.201701).

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Figure captions Figure 1. Molecular structures of acridines. Figure 2. Inhibition efficiency obtained from weight loss of five acridines. Figure 3. Variation of corrosion rate and inhibition efficiency with different concentrations of CPA, CFPA and BFPA by weight loss. Figure 4. EOCP – t curves for mild steel in 1 M HCl solutions with different concentrations of CPA(a), CFPA(b) and BFPA(c). Figure 5. Potentiodynamic polarization curves of mild steel in 1 M HCl solution without and with different concentrations of CPA(a), CFPA(b) and BFPA(c). Figure 6. Nyquist plots for mild steel in 1 M HCl solution without and with different concentrations of CPA(a), CFPA(b) and BFPA(c). Figure 7. Bode plots for mild steel in 1 M HCl solution without and with different concentrations of CPA(a), CFPA(b) and BFPA(c). Figure 8. Equivalent circuit used to fit the EIS experiment data: (a) without inhibitor and (b) the presence of inhibitor. Figure 9. Equivalent circuit fit of Nyquist plot for mild steel in the presence of (a) 1M HCl and (b) 0.40 mM CPA. Figure 10. Langmuir adsorption isotherm of the inhibitors in 1 M HCl solution with different concentrations of inhibitors. Figure 11. SEM images of mild steel surfaces: (a) polished metal, (b) 1 M HCl, (c) CPA, (d) CFPA, (e) BFPA. Figure 12. SECM images of mild steel after immersion in 1 M HCl solution. Figure 13. FTIR spectra of pure substance (a) and the adsorption film on mild steel surface (b). Figure 14. HOMO and LUMO orbitals of the neutral and protonated inhibitors in gas phase. Figure 15. HOMO and LUMO orbitals of the neutral and protonated inhibitors in aqueous phase. Figure 16. The optimized structures, NBO charges population and 3D MEP plot of inhibitor molecules.


Page 21 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Polarization curve parameters (± SD) for the corrosion of mild steel in 1 M HCl solution without and with different concentrations of inhibitors. Inhibitors

c (mM)

Ecorr (mV vs.SCE)

icorr (µA cm-2)

βa (mV dec-1)

-βc (mV dec-1)

η (%)

-

Blank

-481 ± 1.9

451.92 ± 2.7

117.2 ± 0.45

197.4 ± 0.36

-

CPA

0.10

-491 ± 1.1

130.71 ± 1.1

97.8 ± 0.36

171.1 ± 0.21

71.08

0.20

-498 ± 3.

85.20 ± 1.5

99.0 ± 0.31

178.1 ± 0.15

81.15

0.30

-501 ± 2.2

40.92 ± 1.7

100.9 ± 0.28

180.0 ± 0.17

90.95

0.40

-504 ± 2.6

35.19 ± 1.4

107.1 ± 0.16

182.2 ± 0.18

92.21

0.10

-519 ± 3.2

109.65 ± 2.0

116.0 ± 0.38

179.1 ± 0.22

75.74

0.20

-527 ± 3.1

71.98 ± 1.7

122.8 ± 0.23

193.7 ± 0.19

84.07

0.30

-525 ± 2.6

24.26 ± 1.6

132.9 ± 0.21

186.6 ± 0.16

94.63

0.40

-532 ± 2.3

15.64 ± 1.2

128.9 ± 0.18

183.8 ± 0.10

96.54

0.10

-480 ± 4.2

96.84 ± 2.2

111.2 ± 0.43

177.9 ± 0.20

78.57

0.20

-488 ± 3.4

54.63 ± 1.4

117.9 ± 0.32

170.4 ± 0.17

87.91

0.30

-491 ± 3.1

17.16 ± 1.8

125.4 ± 0.25

180.1 ± 0.19

96.20

0.40

-497 ± 1.9

8.66 ± 1.1

119.2 ± 0.19

168.4 ± 0.09

98.08

CFPA

BFPA



Page 22 of 49

Table 2. EIS parameters (± SD) for the corrosion of mild steel in 1 M HCl solution without and with different concentrations of inhibitors. Inhibitors

c (mM)

Rf (Ω

cm2)

Rct (Ω

cm2)

-

Blank

-

53.8

CPA

0.10

7.9

190.6

0.20

11.3

0.30 CFPA

BFPA

Cf (n1) (μF

cm-2) -

Cd (n2) (μF

cm-2)

Rp (Ω

ηz (%)

cm2)

595.1 (0.89)

53.8 ± 0.25

-

37.5 (1)

128.0 (0.67)

198.5 ± 0.43

72.90 ± 0.15

275.3

32.6 (0.99)

101.2 (0.67)

286.6 ± 0.46

81.23 ± 0.12

14.5

607.2

23.3 (1)

86.1 (0.64)

621.7 ± 0.52

91.35 ± 0.08

0.40

15.5

776.4

21.5 (0.98)

65.0 (0.53)

791.9 ± 0.61

93.21 ± 0.03

0.10

9.5

204.3

33.8 (1)

117.8 (0.67)

213.8 ± 0.45

74.84 ± 0.16

0.20

16.7

336.9

29.1 (0.94)

92.1 (0.68)

353.6 ± 0.39

84.79 ± 0.11

0.30

22.4

990.1

22.6 (0.96)

53.2 (0.67)

1012.5 ± 0.71

94.69 ± 0.06

0.40

23.8

1306.3

18.6 (1)

45.7 (0.62)

1330.1 ± 0.93

95.95 ± 0.07

0.10

10.6

261.4

26.6 (1)

102.4 (0.66)

272.0 ± 0.42

80.22 ± 0.13

0.20

22.1

442.5

17.4 (0.95)

84.7 (0.64)

464.6 ± 0.51

88.42 ± 0.09

0.30

25.2

1420.1

14.3 (0.98)

36.2 (0.71)

1445.3 ± 0.83

96.28 ± 0.12

0.40

29.1

2162.2

10.2 (0.99)

22.7 (0.65)

2191.3 ± 1.21

97.54 ± 0.07


Page 23 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3. Comparison of the inhibition efficiency of acridines with the literature data as corrosion inhibitors for mild steel in 1 M HCl solution. Inhibitor

c (mM)

η (%)

Ref.

2-chloro-9-phenylacridine

0.4

93.21

This paper

2-chloro-9-(2-fluorophenyl)acridine

0.4

95.95

This paper

2-bromo-9-(2-fluorophenyl)acridine

0.4

97.54

This paper

(4-chloro-benzylidene)-pyridine-2-yl-amine

1.0

93.25

[1]

(3-bromo-4-fluoro-benzylidene)-[1,2,4]triazol-4-yl-amine

0.8

73.72

[5]

(2-fluoro-4-nitro-benzylidene)-[1,2,4]triazol-4-yl-amine

0.8

62.42

[5]

2,6-Bis-(chloro)pyridine

0.5

78.25

[7]

2-(2-trifluoromethyl-4,5-dihydro-imidazol-1-yl)-ethylamine

0.5

59.1

[20]

2-(2-trichloromethyl-4,5-dihydro-imidazol-1-yl)-ethylamine

0.5

84.5

[20]

3-bromo-2-phenylimidazol[1,2- α] pyridine

1.0

86.18

[29]

4-(N,N,N-dimethyldodecylammonium bromide)benzylidene-

0.5

89.2

[31]

1-vinyl-3-methylimidazolium iodide

0.5

80.6

[36]

4-chloro-acetophenone-O-1′-(1′,3′,4′-triazolyl)-metheneoxime

≈0.4

85.3

[38]

4-fluoro-acetophenone-O-1′-(1′,3′,4′-triazolyl)-metheneoxime

≈0.4

72.3

[38]

(E)-ethyl 2-(4-(3-(4-fluorophenyl)acryloyl)phenoxy)acetate

0.5

73

[39]

(E)-ethyl 2-(4-(3-(3,4-

0.5

80

[39]

4-chlorobenzene-2-yl-amine

dichlorophenyl)acryloyl)phenoxy)acetate



Page 24 of 49

Table 4. Thermodynamic parameters of inhibitors on mild steel surface in 1 M HCl solution. Compounds

R2

Slope

Kads (104 M-1)

ΔG0ads (kJ mol-1)

CPA

0.9998

1.008

3.57

-35.92

CFPA

0.9999

1.007

4.44

-36.47

BFPA

0.9998

0.967

4.88

-36.70


Page 25 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 5. SECM date in the absence and presence of 0.4 M inhibitors. Compounds

itipmin (pA)

itipmax (pA)

η (%)

Blank

218

1470

-

CPA

65

263

82.11

CFPA

73

165

88.78

BFPA

24

112

92.38



Page 26 of 49

Table 6. Quantum chemical parameters for neutral as well as protonated inhibitors in gas phase using different basic sets. Basic set

Inhibitor

EHOMO (eV)

ELUMO (eV)

ΔE (eV)

ΔN

6-31G

CPA

-5.78

-2.49

3.29

0.21

CFPA

-5.49

-2.36

3.13

0.28

BFPA

-5.47

-2.41

3.06

0.29

CPA

-6.03

-2.68

3.35

0.14

CFPA

-5.79

-2.54

3.25

0.20

BFPA

-5.76

-2.62

3.14

0.20

CPA

-6.03

-2.63

3.40

0.14

CFPA

-5.77

-2.48

3.29

0.21

BFPA

-5.75

-2.57

3.18

0.21

CPA

-6.10

-2.72

3.39

0.12

CFPA

-5.85

-2.58

3.27

0.19

BFPA

-5.84

-2.69

3.15

0.18

CPA-H+

-9.83

-6.74

3.09

-1.12

CFPA-H+

-9.55

-6.60

2.95

-1.10

BFPA-H+

-9.47

-6.67

2.80

-1.16

6-31++G

6-311G

6-311++G

6-31G

26


Page 27 of 49 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 7. Quantum chemical parameters for neutral and protonated inhibitors in aqueous phase at the B3LYP/6-31G (d, p) basis set. Inhibitor

EHOMO (eV)

ELUMO (eV)

ΔE (eV)

ΔN

CPA

-5.87

-2.51

3.36

0.19

CPA-H+

-6.78

-3.66

3.12

-0.13

CFPA

-5.63

-2.39

3.24

0.25

CFPA-H+

-6.55

-3.53

3.02

-0.07

BFPA

-5.58

-2.44

3.14

0.26

BFPA-H+

-6.50

-3.63

2.87

-0.09

27



Page 28 of 49

Table 8. Mulliken atomic charges of neutral and protonated inhibitors in gas phase. Compounds

Atoms

Mulliken charge neutral

CPA

CFPA

BFPA

C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C12 C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C12 C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C12

protonate -0.1067 d -0.1104

-0.1111 0.0084 0.0763 0.0989 charge 0.3397 0.2338 -0.0926 0.0288 -0.0567 0.0379 0.0693 -0.0159 0.3401 0.2301 0.0683 0.0972 -0.1208 -0.0271 -0.0025 -0.0813 -0.1066 -0.1110 -0.1078 0.0081 0.0783 0.1005 0.3410 0.2341 -0.0915 0.0305 -0.0563 0.0386 0.0929 0.0120 0.3399 0.2296 0.0811 0.1088 -0.1074 -0.0231 -0.0022 -0.0823 0.0465 0.0405 -0.1307 -0.0168 0.0765 0.0979 0.3426 0.2362 -0.0941 0.0276 0.0198 -0.0729 0.0134 0.0934 0.2302 0.3399 0.1080 0.0807 -0.0229 -0.1075 -0.0023 -0.0825

Atoms

Mulliken charge neutral

C13 C14 C21 C22 C23 C24 C25 C27 N30 Cl32

0.0013 0.0180 -0.0081 0.0043 0.0035 0.0052 0.0067 0.0121 -0.6008 -0.1021

C13 C14 C21 C22 C23 C24 C25 C27 N30 F32 Cl33 C13 C14 C21 C22 C23 C24 C25 C27 N30 F32 Br33

0.0023 0.0195 -0.0928 0.3401 0.0108 -0.0283 0.0091 0.0174 -0.5999 -0.2847 -0.0194 0.0024 0.0193 -0.0919 -0.09198 0.3399 0.0111 -0.0283 0.0091 0.009083 0.0174 -0.6006 -0.2848 -0.28482 -0.1251 -0.12511 -0.28482

protonated -0.0726 -0.0985 -0.0168 -0.1011 -0.1029 -0.0843 -0.0836 -0.0692 -0.7089 -0.1069 -0.0721 -0.0973 -0.1056 0.3443 -0.1016 -0.1295 -0.0831 -0.0742 -0.7086 -0.2786 -0.0712 -0.0722 -0.0975 -0.1043 0.3439 -0.1013 -0.1296 -0.0832 -0.0742 -0.7092 0.10278 -0.2787 -0.1031

28


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Figure 1

MA: R = CH3; PA: R = Ph

MPA: R = H; CMPA: R = Cl

CPA: R1 = Cl; R2 = H CFPA: R1 = Cl; R2 = F BFPA: R1 = Br; R2 = F

29



Page 30 of 49

Figure 2

30


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Figure 3

31



Page 32 of 49

Figure 4 (a)

(b)

(c)

32


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33



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Figure 5 (a)

(b)

(c)

34


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35



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Figure 6 (a)

(b)

(c)

36


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37



Page 38 of 49

Figure 7 (a)

(b)

(c)

38


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39



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Figure 8 (a)

(b)

40


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Figure 9 (a)

(b)

41



Page 42 of 49

Figure 10

42


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Figure 11

43



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Figure 12

44


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Figure 13

45



Page 46 of 49

Figure 14

46


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Figure 15

47



Page 48 of 49

Figure 16 (a)

(b)

48


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TOC Graphic

49


Halogen-Substituted Acridines as Highly Effective Corrosion Inhibitors

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