Performance evaluation of triethanolamine as corrosion inhibitor for magnesium alloy in 3.5 wt% NaCl solution

Abstract

The corrosion inhibition performance of triethanolamine (TEA) with AZ91D magnesium alloy in 3.5 wt% NaCl solution has been studied using an immersion experiment, electrochemical impedance spectroscopy (EIS), potentiodynamic polarization, surface analysis, and quantum chemical methods. The results revealed that TEA was a good corrosion inhibitor. The polarization plots indicated that TEA served as a mixed-type inhibitor. The adsorption of TEA molecules was found to follow the Langmuir adsorption isotherm, which was a spontaneous, exothermic process of increased entropy. TEA could considerably improve the corrosion resistance of the Mg alloy in 3.5 wt% NaCl.

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1. Introduction

As the lightest metal base material, magnesium alloy is a type of metal material that is frequently used in modern industry due to the advantages such as high chemical activity, high specific strength and specific stiffness, good machining stability, and recycling ability.¹ Although magnesium alloys have such outstanding advantages, their poor corrosion resistance, caused by the chemical activity of magnesium, has limited their application.^2–5

To promote the wider application of Mg alloy, an appropriate treatment can enhance its corrosion resistance, which has important practical significance. The use of a corrosion inhibitor is one of the most effective ways to slow the progress of the metal corrosion by adding it to the corrosive medium in small concentrations.^6,7 Organic compounds are often used as corrosion inhibitors to protect media against corrosion, especially those containing a heteroatom with lone-pair electrons (e.g., N, O, P, and S), a benzene ring, or conjugated functional groups.^8,9 The lone-pair electrons and π-electrons in these atoms and functional groups are the key to determining the corrosion inhibition performance of the compounds. Under the effects of electrostatic interactions or electrochemical action, these compounds can form a protective film on a metal surface, which can change the structure of the electric double layer and affect the process dynamics of metal corrosion, reducing the corrosion rate of the metal.¹⁰

The corrosion inhibitors applied to Mg alloy are mostly heavy metallic salts^11,12 and heterocycles.^13,14 Although these compounds are effective corrosion inhibitors for Mg alloys, their high toxicity and high cost has limited the production and applications of these types of corrosion inhibitors. Therefore, research on corrosion inhibitors for Mg alloys is very important.

As a common chemical reagent, triethanolamine (TEA) is economical and causes minimal pollution. TEA is a colorless transparent viscous liquid at room temperature. It can dissolve easily in water and ethanol. Besides, TEA has the properties of a tertiary amine and an alcohol, exhibiting significant coordination in synthetic applications. Furthermore, TEA compounds can also exhibit strong coordination ability.^15–17 Zhang¹⁸ et al. studied the corrosion inhibition performance of TEA with respect to LaFe_11.0Co_0.7Si_1.3 magnetic refrigeration materials in distilled water. It was considered that the TEA molecules were adsorbed on the metal surface by coordination bonding. Ding¹⁹ et al. studied the effects of the properties and microstructures of an anode oxide film for Mg alloy in an electrolyte containing TEA. It was indicated that the growth speed of the oxide film on the Mg alloy surface was rapid, due to the addition of TEA, and the film was composed of a surface layer and a transition layer. In a nutshell, these physical and chemical properties of TEA lead to its promising application for preventing corrosion in Mg alloys.

However, research on the corrosion inhibition performance of TEA for Mg alloys is limited, and the detailed study of the corrosion mechanism of TEA as an inhibitor for Mg alloy corrosion is still incomplete. In order to better understand the inhibition mechanism of TEA, this study has systematically studied the corrosion inhibition performance of TEA for Mg alloy in 3.5 wt% NaCl solution by a series of analytical techniques and methods. Electrochemical measurements were used to study the inhibition efficiency and adsorption behavior of the inhibitor. Immersion experiments were used to study the corrosion process with TEA of Mg alloy in 3.5 wt% NaCl solution. SEM and EDX analyses were used to demonstrate the formation of a TEA adsorption film on a Mg alloy surface. Finally, the theoretical parts of this study were carried out using quantum chemical calculations.

2. Experimental

2.1 Experimental material and sample preparation

The metal material used in the experiments was the AZ91D magnesium alloy. The Mg alloy was flaked before use, and the specification was 30 mm × 20 mm × 3 mm. The specific chemical compositions of the Mg alloy are listed in Table 1.

Table 1 AZ91D magnesium alloy chemical compositions and material analysis

Composition	Mg	Al	Zn	Cu	Si	Mn	Ni	Fe
Content (wt%)	90.8–89.12	8.5–9.5	0.45–0.9	<0.025	<0.05	0.17–0.4	<0.001	<0.004

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Before each experiment, the AZ91D magnesium alloy substrates were polished mechanically with 600#, 1000#, and 1400# emery paper until their surfaces were smooth and bright, and then they were degreased and cleaned with ethanol and distilled water using an ultrasonic cleaner, and dried at room temperature. Distilled water was used for washing in each process.

The treated Mg alloy was filled with epoxy resin, with an exposed area of 1 cm². After the drying step, the Mg alloy was immersed in 3.5 wt% NaCl solution containing different concentrations of TEA. After adsorption for a certain time, the Mg alloy was used for performance testing.

2.2 Performance testing

2.2.1 Electrochemical method. Electrochemical measurements were carried out using a CHI860 electrochemical workstation. The base electrolyte was 3.5 wt% NaCl solution without and with different concentrations of TEA. All experiments were conducted using a conventional three-electrode electrochemical cell with Pt as the counter electrode, a saturated calomel electrode (SCE) as the reference electrode, and the Mg alloy with an exposed area of 1 cm² as the working electrode.

Before starting the experiments, the open-circuit potential (OCP) was studied at 298 K. The working electrodes were immersed in the test solution for a certain period of time until a steady potential was reached. After the open-circuit potential was stabilized, polarization curves were acquired with a scan rate of 1 mV s⁻¹ in the potential range from E_OCP −200 mV to E_OCP +200 mV versus SCE at 298 K. Electrochemical impedance spectroscopy (EIS) was performed over a frequency range of 100 kHz to 0.1 Hz at the stable open-circuit potential for each test condition. The amplitude of the AC perturbation was 5 mV.

2.2.2 Surface morphology and component analysis. In order to better observe the TEA adsorption film on the Mg alloy surface, the immersion time was researched using polarization curves in 3.5 wt% NaCl containing 3 mL L⁻¹ TEA at 298 K.

The Mg alloy specimens were prepared as described earlier, and then immersed in the 3.5 wt% NaCl without and with 3 mL L⁻¹ TEA at 298 K for 1 h. Micromorphology analysis was used to demonstrate the formation of the TEA film on the Mg alloy surface with a JSM-5610LV scanning electron microscope. Component analysis of the TEA film was achieved by EDX.

2.2.3 Immersion experiment. The Mg alloy specimens were prepared as described earlier, and then immersed in the 3.5 wt% NaCl containing 3 mL L⁻¹ TEA for 1 to 5 days. After that, these specimens were analyzed by EIS to study the corrosion process of the Mg alloy in the TEA medium, and to set up a corrosion model. EIS was recorded over a frequency range of 100 kHz to 0.1 Hz at the stable open-circuit potential for each test condition. The amplitude of the AC perturbation was 5 mV. All the above tests were carried out at 298 K, and the samples with an exposed area of 10 mm × 10 mm were filled with epoxy resin.

2.2.4 Quantum chemical calculations. The quantum chemical calculations were carried out using the Gaussian 03 programs.²⁰ In this paper, the molecular optimization of TEA was performed with the density functional theory (DFT) method, B3LYP, combined with the 6-31G** (d, p) basis set for all atoms.²¹ There were several quantum chemical parameters, which are as follows: the energy of the highest occupied molecular orbital (E_HOMO), the energy of the lowest unoccupied molecular orbital (E_LUMO), ΔE = E_LUMO − E_HOMO, and the dipole moment (μ).

3. Results and discussions

3.1 Electrochemical testing

3.1.1 Potentiodynamic polarization measurements. Polarization measurements could supply information about the type of corrosion inhibitors and the kinetics of corrosion reactions.^22,23 The potentiodynamic polarization curves for Mg alloy in 3.5 wt% NaCl solutions in the absence and presence of TEA at different concentrations at 298 K are shown in Fig. 1. According to pertinent literature reports, the cathodic corrosion reaction of Mg alloy in NaCl solution was the hydrogen evolution reaction:²⁴

4H₂O + 4e⁻ → 4OH⁻ + 2H₂ (1)

Fig. 1 Polarization curves of AZ91D magnesium alloy in 3.5 wt% NaCl solutions in the absence and presence of TEA at different concentrations at 298 K.

The anodic corrosion reactions in NaCl solution represented the dissolution process of Mg:^25,26

At the same time, the possibility that magnesium could be transformed to Mg²⁺ and be dissolved in the solution cannot be ruled out:

Mg → Mg²⁺ + 2e⁻ (3)

From what has been discussed above, the total corrosion reaction for Mg alloy in NaCl solution was:²⁷

3Mg + 6H₂O → 2Mg²⁺ + 4OH⁻ + 2H₂ + Mg(OH)₂ (4)

As shown in Fig. 1, the cathodic polarization currents of the Mg alloy increased and no passivation happened in the anodic branch in 3.5 wt% NaCl solution due to the occurrence of the cathodic and anodic processes. It was clear that the cathodic and anodic currents were decreased after addition of TEA, showing that the rates of Mg alloy dissolution and hydrogen evolution were decreased due to the addition of TEA.

Table 2 lists the parameters obtained from Fig. 1. As shown in Table 2, the corrosion potential shifted toward the positive direction due to the addition of TEA, showing that the anti-corrosion tendency was improved. Besides, it was clear that the corrosion current densities of Mg alloy exhibited an extreme concentration phenomenon as the TEA concentration increased. The corrosion current density was gradually reduced as the TEA concentration increased from 1 mL L⁻¹ to 3 mL L⁻¹; the minimum corrosion current density was observed when the concentration of the TEA was 3 mL L⁻¹; the corrosion current density then increased with a further increase in the TEA concentration. In addition, the corrosion current density of the Mg alloy in the media without TEA was 9.039 × 10⁻⁶ A cm⁻², and the corrosion current density of the Mg alloy in 3 mL L⁻¹ TEA was 6.583 × 10⁻⁷ A cm⁻². Comparing the above values, it was found that the current density was reduced by 1 order of magnitude. This phenomenon indicated that TEA could suppress the dissolution of Mg alloy and exhibited a good corrosion-inhibiting performance for Mg alloy. On the other hand, it can be seen in Fig. 1 and Table 2 that the cathodic reaction of Mg alloy corrosion was significantly inhibited in the presence of TEA. Moreover, the cathodic current–potential curves were similar to the almost parallel Tafel lines. This meant that the addition of TEA did not change the mechanism of hydrogen evolution.²⁸ According to the anodic parts of the curves, the anodic reaction of Mg alloy corrosion was markedly suppressed in the presence of TEA. The inhibitor exhibited a stronger inhibitive effect on the anodic reaction than on the cathodic one, indicating that TEA acted as a mixed-type inhibitor to mainly inhibit the anodic reaction of Mg alloy in 3.5 wt% NaCl solution.^28,29

Table 2 Potentiodynamic polarization curves of different concentrations: data analysis

Concentration (mL L^−1)	Ecorr (V vs. SCE)	βc (V per decade)	βa (V per decade)	icorr (μA cm^−2)	η (%)
Bare	−1.534	7.141	10.164	9.039	—
1	−1.510	7.546	10.118	3.293	63.57
2	−1.508	7.481	9.966	1.290	85.73
3	−1.488	7.343	9.130	0.6583	92.71
4	−1.496	7.684	10.084	2.513	72.20
5	−1.516	7.720	10.117	3.970	56.08

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The inhibition efficiency, η (%), was calculated from eqn (5) and the results are listed in Table 2:

where i_c is the current density of the Mg alloy surface without the inhibitor, and i_i is the current density of the Mg alloy surface with the inhibitor. The obtained data revealed that the inhibition efficiency could exhibit an extreme phenomenon as the TEA concentration increased. Besides, the inhibition efficiency reached a maximum when the concentration of TEA was 3 mL L⁻¹, and this value was 92.71%. This illustrated that TEA could be classified as an adsorptive corrosion inhibitor for Mg alloy, because TEA was adsorbed on the Mg alloy to form a protective film and block the active corrosion sites. In addition, the extreme concentration phenomenon could be explained as follows: firstly, TEA could adsorb on the active sites on the surface of the Mg alloy to inhibit corrosion of the alloy to some extent when the TEA concentration was low (e.g., 1 mL L⁻¹, 2 mL L⁻¹). However, TEA might be unevenly distributed on the Mg alloy surface due to the small molecular volume of TEA, so the Mg alloy surface could form a small part of the active area. This was easy to corrode in the corrosive medium, so the corrosion current could be increased. Secondly, when the TEA concentration was 3 mL L⁻¹, TEA could evenly adsorb on the Mg alloy surface and cover its active sites. It could form a stable TEA adsorption film to effectively restrain the corrosion of Mg alloy. Therefore, the corrosion current was at a minimum. Thirdly, excess adsorption of TEA might occur, resulting in an agglomeration or shedding phenomenon under higher concentrations (e.g., 4 mL L⁻¹, 5 mL L⁻¹). The TEA film was unstable at that time, resulting in reduced protection for the Mg alloy. So, the corrosion current could also be increased.

3.1.2 Electrochemical impedance spectroscopy (EIS). The Nyquist plots for Mg alloy in 3.5 wt% NaCl solutions in the absence and presence of TEA at different concentrations at 298 K are shown in Fig. 2. Compared with the diagram, the EIS was markedly changed due to the adsorption of TEA. As shown in Fig. 2, the impedance diagram for the blank showed a capacitive arc at high frequency and an inductive arc at low frequency. The high-frequency semicircle was attributed to the charge transfer and double-layer capacitance. The inductive arc at low frequency was probably related to the solution of Mg alloy species due to local surface breakage or the low adsorption capacity of the surface film.

Fig. 2 Nyquist impedance spectra of AZ91D magnesium alloy in 3.5 wt% NaCl solutions in the absence and presence of TEA at different concentrations at 298 K.

The EIS curves for different TEA concentrations were composed of double capacitive arcs. The capacitive arc at high frequency was related to charge transfer, and reflected the corrosion resistance of the sample. The capacitive arc at low frequency was caused by the adsorbing–desorbing process of the corrosion inhibitor on the electrode surface.³⁰ In addition, compared with the blank sample, the semicircle diameter of the impedance arc at the high-frequency part was increased in the presence of TEA. This indicated that the charge-transfer resistance became dominant in the corrosion process due to the adsorption of TEA. Meanwhile, the capacitive arc at high frequency increased to a maximum when the concentration of the TEA was 3 mL L⁻¹, and then decreased as the additive TEA concentration increased. This showed that an extreme concentration phenomenon occurred, which was in accord with the obtained results of the polarization curve.

The equivalent-circuit model was used to fit the capacitance part of all the EIS (shown in the Nyquist spectra). All the EIS were interpreted by means of the equivalent circuits, which are shown in Fig. 3. The fitting results were obtained with ZView2 software, including the solution resistance (R_s), the film resistance (R_coat), the film capacitance (Q_coat), the charge-transfer resistance (R_ct), the interface capacitance (Q_dl), and the double-layer capacitance (CPE_dl). The CPE was defined by the values of Q and n, which were used to make up for the inhomogeneities in the electrode surface and described the nature of the Nyquist semicircle.³¹ The admittance and impedance of CPE were defined by eqn (6):

where Y₀ is the modulus, ω is the angular frequency, and n is the deviation parameter.

Fig. 3 Simulation equivalent fitting circuit diagrams: (a) bare, (b) different concentrations of TEA.

The inhibition efficiencies η (%) of different TEA concentrations were calculated by the charge-transfer resistance according to eqn (7):³²

where R⁰_ct and R_ct are the film resistances for the blank and different concentrations, respectively. The values of CPE capacitance were calculated by using eqn (8):³³

C_dl = Y₀^1/nR_ct^(1−n)/n (8)

where Y₀ and n are the magnitude of the CPE and deviation parameter.

These results are listed in Table 3. In Table 3, it can be seen that the R_coat values of all TEA concentrations were greater than the value for the blank, which indicated that TEA could adsorb on the Mg alloy surface to form a protective film, which provided better protection for Mg alloy. Besides, the values of the R_coat increased with the additive TEA concentrations, and then decreased. The value reached a maximum when the concentration of the TEA was 3 mL L⁻¹. The reasons for this result are as follows. At low concentrations (e.g., 1 mL L⁻¹), the TEA can still adsorb on the Mg alloy surface, but the formation of the protective film was flawed, so the corrosive ions could easily penetrate the protective film to corrode the Mg alloy. As the TEA concentration increased, the protective film became more complete, so the values increased (e.g., 2 mL L⁻¹). Moreover, at high concentrations (e.g., 4 mL L⁻¹, 5 mL L⁻¹), excess adsorption of TEA might occur, resulting in an agglomeration or shedding phenomenon. Thus, film-free or broken areas could exist.¹⁴ The corrosive ions could adsorb more easily to the film-free or broken areas to form soluble ionic metal compounds.^34,35 As the dissolution rates of the TEA film increased, the R_coat values decreased. Under the optimum concentration (e.g., 3 mL L⁻¹), TEA molecules could evenly adsorb on the Mg alloy surface to form a protective film, and continue to adsorb on the protective film to fill film-free or broken areas. So the value of the R_coat reached a maximum at 3 mL L⁻¹ TEA.

Table 3 Simulation parameters of equivalent circuit diagrams for different concentrations

Concentration (mL L^−1)	Rs (Ω cm^2)	Qcoat (μF cm^−2)	n	Rcoat (Ω cm^2)	Qdl (μF cm^−2)	Rct (Ω cm^2)	η (%)
Bare	15.53	16.738	0.86761	609.6	—	—	—
1	21.5	16.873	0.85516	3844	316.36	103.53	84.14
2	27.31	11.445	0.89351	4433	109.39	1746	86.25
3	27.53	7.3509	0.91128	7881	601.04	2471	92.26
4	28.69	10.672	0.90424	4147	1052.3	1464	85.30
5	39.57	16.022	0.8549	2585	2588.3	588.6	76.42

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The C_dl values of all TEA concentrations were smaller than the value for the blank. According to the Helmholtz model, the double-layer capacitance was inversely proportional to the surface changes:^32,36

where d is the film thickness, S is the electrode surface area, ε₀ is the permittivity of air, and ε is the local dielectric constant. Because of the formation of an effective TEA adsorption film, the active sites of the Mg alloy surface were replaced by TEA molecules. Meanwhile, the dielectric constant of TEA molecules was less than that of water. Thus, there were smaller double-layer capacitances in the TEA system. Another probable reason for the decrease in the C_dl values was that the surface area was reduced.³² At lower concentrations (e.g., 1 mL L⁻¹, 2 mL L⁻¹), the TEA protective film was flawed due to the lower TEA concentration, the alloy surface area exposed to the corrosive medium was increased, and reactive sites were also increased. The Mg alloy could therefore be corroded in the corrosive medium. However, the protective film became more even as the TEA concentration increased. Therefore, the C_dl values decreased. Similarly, at higher concentrations (e.g., 4 mL L⁻¹, 5 mL L⁻¹), the protective film might contain film-free or broken areas due to excess adsorption. The roughness of the film increased, and the film became unstable. The active areas of Mg alloy exposed to the corrosion medium were increased, so the C_dl values increased. Under the optimum concentration (e.g., 3 mL L⁻¹), the protective film was stable, and TEA could fill defects in the protective film by reabsorption.¹⁴ Therefore, the alloy surface area exposed to the corrosive medium was reduced, together with the reactive sites, so the C_dl value was at a minimum.

It could be seen that the inhibition efficiencies were closely related to the TEA concentrations.³⁷ There existed an optimal concentration of TEA, below which there was a decrease in the inhibition efficiency, and above which there was also a decrease in the inhibition efficiency. This was consistent with the results of the potentiodynamic polarization measurements. As shown in Table 3, the protection of the magnesium alloy was most effective at 3 mL L⁻¹ TEA. Therefore, 3 mL L⁻¹ TEA was the optimal concentration.

3.2 Surface morphology and component analysis

3.2.1 Effect of immersion time. In order to better observe the TEA adsorption film on the Mg alloy surface, the immersion time was researched by polarization curves in 3.5 wt% NaCl containing 3 mL L⁻¹ TEA at 298 K. The results are shown in Fig. 4. Table 4 lists the corresponding parameters obtained from Fig. 4. In addition, the inhibition efficiencies were calculated using eqn (5). The values of the coverage (θ) were calculated using eqn (10):³⁸where i_c is the current density of the Mg alloy surface without the inhibitor, and i_i is the current density of the Mg alloy surface with the inhibitor. It should be noted that the inhibition efficiency η (%) is a function of θ: η (%) = θ × 100%.

Fig. 4 Polarization curves of AZ91D magnesium alloy in 3.5 wt% NaCl solutions containing 3 mL L⁻¹ TEA with different immersion times at 298 K.

Table 4 Potentiodynamic polarization curves of Fig. 4: data analysis

Immersion time (h)	Ecorr (V vs. SCE)	icorr (μA cm^−2)	θ	η (%)
0.5	−1.488	0.9403	0.89597	89.60
1	−1.494	0.7257	0.91971	91.97
2	−1.478	1.223	0.86470	86.47
3	−1.471	2.324	0.74289	74.29
4	−1.472	4.141	0.54187	54.19

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It can be seen from Fig. 4 and Table 4 that the current density was reduced before immersing for 1 hour, which indicated the formation of a TEA protective film. The current density was then increased, which indicated that the TEA film could be destroyed due to multilayer adsorption of TEA or the diffusion of the corrosive ions. As regards the coverage, the values increased with the immersion time, and then decreased. Similarly, the inhibition efficiency behaved in a similar way. According to Table 4, both the coverage of TEA and the inhibition efficiency were greatest after immersing for 1 hour, so the optimal immersion time was 1 hour, and the inhibition efficiency was 91.97%.

3.2.2 Morphological studies. The Mg alloy was immersed in the different TEA systems at 298 K for 1 hour. SEM was used to characterize the surface morphology, as shown in Fig. 5. The different TEA systems were 3.5 wt% NaCl solution without TEA (a), distilled water with 3 mL L⁻¹ TEA (b), and 3.5 wt% NaCl solution with 3 mL L⁻¹ TEA (c), as shown in Fig. 5.

Fig. 5 SEM images of AZ91D magnesium alloy distilled in different TEA-systems for 1 h at 298 K: (a) 3.5 wt% NaCl solution without TEA, (b) distilled water with 3 mL L⁻¹ TEA, (c) 3.5 wt% NaCl solution with 3 mL L⁻¹ TEA.

Fig. 5(a) shows that NaCl adsorption could occur at the Mg alloy surface in the absence of inhibitor after immersion for 1 hour, and there may be corrosion products produced near the NaCl adsorption. It can be seen in Fig. 5(b) that a TEA adsorption film was produced on the Mg alloy surface and the surface was even with no obvious defects. It was inferred that TEA could effectively adsorb on the Mg alloy surface. The square area in Fig. 5(b) was magnified 5 times. It could be seen that the TEA particles had a slender shape, which ensured that TEA could adsorb in excess on the Mg alloy surface. As shown in Fig. 5(c), a TEA adsorption film was also observed on the Mg alloy surface, and the surface was even with no obvious defects. However, the white part of Fig. 5(c) may represent a reunion phenomenon of TEA, which indicated that TEA exhibited a slight reunion phenomenon at this concentration. This could also explain why the corrosion effect could be reduced under higher concentrations.

3.2.3 Component analysis. The components of the TEA adsorption film on the Mg alloy surface are shown in Fig. 6. It could be seen that, besides the matrix elements, Mg, Al, and Si, other elements, C, O, and N, were also presented on the surface. The elements C, O, and N belonged to the TEA. Besides, it can also be seen in Fig. 6 that each element was distributed evenly, which illustrated that the TEA was adsorbed homogeneously on the Mg alloy surface. According to the above, the TEA can be adsorbed on the Mg alloy surface, resulting in the formation of an effective passive film between the substrate and the corrosive medium.

Fig. 6 Surface elemental distribution of sample.

3.3 Corrosion behavior study

In order to study the corrosion process of Mg alloy in the TEA medium and set up a corrosion model, EIS was studied for different immersion times. The Nyquist impedance spectra with different immersion times are shown in Fig. 7. Three spectra were selected from all the EIS (including 1 hour, 3 days, and 5 days) and were interpreted by means of the equivalent circuit, as shown in Fig. 8. After being immersed for 1 hour, a TEA adsorption film was formed on the Mg alloy surface, which can increase the charge-transfer resistance and effectively prevent the aggressive ions from accessing the substrate surface. Besides, the electron transfer process has occurred. Meanwhile, TEA could also fill the defects in the protective film by readsorption.¹⁴ Thus, the alloy surface area exposed to the corrosive medium was reduced, and reactive sites were also reduced. Therefore, the protection of the Mg alloy was most effective at this time. It can be seen from Fig. 7 that the semicircle diameters of the impedance arcs were reduced as the immersion time increased, which indicated that the polarization resistance decreased gradually, and the TEA film was damaged in the corrosive medium. Moreover, it is also shown in Fig. 7 that the EIS curves were composed of double capacitive arcs from 1 hour to 4 hours. Then, the impedance diagrams showed a capacitive arc at high frequency and an inductive arc at low frequency from the first day to the fifth day. And at low frequency, the impedance diagrams indicated an inductance contraction phenomenon from the fourth day to the fifth day. This indicated that the interfacial properties and surface state had undergone significant changes, involving the Mg alloy surface/TEA adsorption film/corrosive medium interface. The reason for this phenomenon might be that the chloride ions adsorbed to the local active sites of the TEA film, and formed soluble complex ions with metal ions.^39,40 Therefore, the electrode surface was uneven and unstable, and the dissolution rate of the TEA film increased. Therefore, the polarization resistances decreased. Meanwhile, ion migration resistance in the film was increased.⁴⁰ Local areas of the TEA film constantly thinned, forming corrosion-induced holes. Thus, the appearance of the inductive arc at low frequency in the equivalent circuit was because of chloride ion adsorption to the TEA film, forming soluble complex ions.³⁵ Due to the autocatalytic process of pitting corrosion, thinning of the TEA film continued,⁴⁰ the corrosion current increased, and the polarization resistance decreased. Thus, impedance arcs were seen at low frequency, and the inductance contraction phenomenon and the impedance arcs were weakened as the immersion time increased. After immersion for 120 hours, the TEA film could be detached because the degree of chloride erosion was increased. Many corrosive products collected on the surface, but the film was loosely packed and was of a poor quality.⁴⁸

Fig. 7 Nyquist impedance spectra with different immersion times.

Fig. 8 Simulation equivalent fitting circuit diagram: (a) 1 hour, (b) 72 hours and 120 hours.

The equivalent circuit model was used to fit the capacitance part of all the EIS (shown in Nyquist spectra). The fitting results were obtained with ZView2 software, including the solution resistance (R_s), the film resistance (R_coat), the film capacitance (Q_coat), the charge-transfer resistance (R_ct), and the interface capacitance (Q_dl).

These results are listed in Table 5. From Table 5, it can be seen that the values of the R_coat gradually decreased as the immersion time increased from 1 hour to 120 hours, while the double-layer capacitance was changed remarkably. Several reasons for these results are listed as follows. First and foremost, the relationship between the immersion time and the coverage of TEA on the Mg alloy surface could be obtained according to the results of 3.2.1, and the fitting obtained through a function of two variables, as shown in Fig. 9. Fig. 9 shows that the correlation relation of the immersion time and the coverage coincided with the binary function because the correlation coefficient (R²) was very close to 1. So, it could be deduced from Fig. 9 that the coverage of TEA on the magnesium alloy surface was the greatest near 1 hour. In other words, the adsorption of TEA tended to a saturation state near 1 hour. So, the TEA adsorption film showed an excellent protection performance. Because of the formation of an effective TEA adsorption film, the active sites of the Mg alloy surface were replaced by TEA molecules. And the dielectric constant of the TEA molecules was less than that of the water. Thus, the film resistance was the largest and the double-layer capacitance was the smallest. Meanwhile, the TEA and corrosive ions still adsorbed constantly on the Mg alloy surface. The Mg alloy passed through a corrosion induction phase.

Table 5 Simulation parameters of equivalent circuit diagram with different immersion times

Immersion time (h)	Rs (Ω cm^2)	Qcoat (μF cm^−2)	n	Rcoat (Ω cm^2)	Qdl (μF cm^−2)	n	Rct (Ω cm^2)
1	25.55	8.3453	0.9242	12 420	5.24 × 10^−4	0.939	6695
72	33.96	47.218	0.7468	3464	—	—	—
120	24.27	21.59	0.8407	624.2	—	—	—

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Fig. 9 The relationship between the immersion times and the coverages of TEA.

In addition, the quality of the TEA film decreased as the immersion time increased, due to the multilayer adsorption of TEA. At that time, the corrosive ions more easily penetrated the TEA film to reach the Mg alloy surface, because the diffusion rate of the corrosive ions was faster than the formation of the TEA film. These factors were likely to lead to the loss of TEA film, so the film resistance was decreased. The alloy surface area exposed to the corrosive medium was increased due to the destruction of TEA film, so the film capacitance increased sharply. Then, the Mg alloy passed through a rapid local corrosion process.

Furthermore, after a period of corrosion, the corrosion products covered the Mg alloy surface and had little protection, so the film capacitance was decreased. However, the film was loosely packed, and was of a very poor quality. So, the corrosion reaction continued and the film resistance was further decreased. Then, the Mg alloy passed through a slow general corrosion process.

On the other hand, n is an apparent diffusion coefficient to characterize the electric double-layer capacitance of the electrolyte/Mg alloy interface. At the ideal capacitance, n = 1.^41,42 In general, the rough porous surface causes the electric double-layer capacitance to deviate from the pure ideal capacitance.⁴³ From Table 5, after immersion for 1 hour, n was most favorable, indicating that the surface was relatively uniform, while n was least favorable after immersion for 72 hours, possibly because the occurrence of pitting corrosion resulted in many corrosion holes on the Mg alloy surface. After immersion for 120 hours, n was increasing, rather than reducing, which could be because the corrosion products were formed at the defect.

3.4 Adsorption studies

3.4.1 Adsorption model. In order to investigate the interactions between the TEA and Mg alloy surface, the adsorption isotherms were studied. It was assumed that the adsorption of TEA molecules on the Mg alloy surface conformed to the Langmuir isotherm equation, eqn (11):where K_ads is the standard adsorption equilibrium constant, C is the inhibitor concentration, and θ is the surface coverage defined as η (%)/100 at different concentrations of TEA obtained from polarization measurements. Experientially, the applied assumption is that θ ≈ η (corrosion inhibition efficiency).⁴⁴ The corrosion inhibition efficiency and the surface coverage were calculated using eqn (5) and (12), respectively:where η is the corrosion inhibition efficiency with different concentrations, and η_m is the maximum corrosion inhibition efficiency.

The plots of against C yielded straight lines, as shown in Fig. 10. Both the linear correlation coefficient (R²) and other parameters are given in Table 6 at different temperatures. Fig. 10 and Table 6 show that the correlation relation of and C coincided with the linear relationship, because the linear correlation coefficient (R²) was very close to 1, indicating that the adsorption of TEA obeyed the Langmuir adsorption isotherm at different temperatures; in other words, the TEA molecules formed a single-molecule adsorbed layer on the Mg alloy surface. Generally, a higher value of K_ads is associated with strong adsorption.²⁰ This indicated that the corrosion inhibition efficiency of TEA was higher: 98.26% (20 °C, 3 mL L⁻¹) > 95.81% (30 °C, 3 mL L⁻¹) > 92.55% (40 °C, 3 mL L⁻¹) > 89.42% (50 °C, 3 mL L⁻¹) > 86.01% (60 °C, 3 mL L⁻¹). In this study, the value of K_ads obeyed the order at different temperatures: 293 K (20 °C) > 303 K (30 °C) > 313 K (40 °C) > 323 K (50 °C) > 333 K (60 °C), which is in accordance with the order of the corrosion inhibition efficiency.

Fig. 10 Relationship between and C at different temperatures.

Table 6 –C linear fitting parameters at different temperatures

Temperatures (T)	η (%)					Kads	R^2	ln Kads
	1 mL L^−1	2 mL L^−1	3 mL L^−1	4 mL L^−1	5 mL L^−1
293 K (20 °C)	91.06	93.30	98.26	96.01	95.34	3333.33	0.999	8.112
303 K (30 °C)	89.42	91.06	95.81	92.16	88.17	1666.67	0.9957	7.419
313 K (40 °C)	87.06	89.31	92.55	87.49	84.24	1000	0.9956	6.908
323 K (50 °C)	85.45	87.57	89.42	83.81	80.29	666.67	0.9948	6.502
333 K (60 °C)	83.28	85.12	86.01	79.43	74.09	416.67	0.9907	6.032

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3.4.2 Calculation of adsorption thermodynamic parameters. In order to further study the adsorption mechanism of TEA on the Mg alloy surface, the adsorption and thermodynamic behaviors were analyzed with the van't Hoff equation:⁴⁵

The plots of ln K_ads against yielded straight lines, as shown in Fig. 11. The adsorption enthalpy (ΔH_ads^θ) could be calculated from the straight slope. The standard Gibbs free energy of adsorption (ΔG_ads^θ) and the standard adsorption entropy (ΔS_ads^θ) could be calculated using eqn (14) and (15), respectively:^45,46

ΔG_ads^θ = −RT ln(55.5K_ads) (14)

ΔG_ads^θ = ΔH_ads^θ − TΔS_ads^θ (15)

where K_ads is the standard adsorption equilibrium constant, the value 55.5 is the molecular concentration of water in solution in mol L⁻¹, R is the gas equilibrium constant, and T is the temperature.

Fig. 11 The relationship between ln K and 1/T.

The adsorption thermodynamic parameters of TEA at different temperatures are listed in Table 7. Eqn (14), was used to calculate the values of ΔG_ads^θ from −29.544 to −27.820 kJ mol⁻¹. The obtained negative value of ΔG_ads^θ showed that the TEA was spontaneously adsorbed on the Mg alloy surface. Generally, for values of ΔG_ads^θ up to −20 kJ mol⁻¹, the adsorption is regarded as physisorption, while values around −40 kJ mol⁻¹ or smaller were associated with chemisorption.⁴⁷ So, TEA was adsorbed on the Mg alloy surface in 3.5 wt% NaCl solutions at different temperatures by physical adsorption and chemical adsorption, because of the electrostatic interactions between the electric charges of TEA and the Mg alloy surface, and the formation of coordination bonds at the interface between the TEA and the vacant d-orbitals of magnesium atoms.⁴⁵ Besides, the obtained negative value of ΔH_ads^θ (−41.308 kJ mol⁻¹) indicated that the adsorption process was exothermic on the Mg alloy surface in 3.5 wt% NaCl solutions, so the effect of the corrosion inhibitor decreased when the temperature increased. ΔS_ads^θ > 0 indicated that TEA adsorption on the Mg alloy surface was driven by an increase in entropy.

Table 7 The adsorption thermodynamic parameters of TEA at different temperatures

Temperatures, K	ΔHads^θ, kJ mol^−1	ΔGads^θ, kJ mol^−1	ΔSads^θ, J mol^−1 K^−1
293 (20 °C)	−41.308	−29.544	86.734
303 (30 °C)		−28.806	81.436
313 (40 °C)		−28.427	77.624
323 (50 °C)		−28.247	74.663
333 (60 °C)		−27.820	71.139

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3.5 Quantum chemical calculations

In order to clearly understand the adsorption mechanism of TEA on the Mg alloy surface, a quantum chemical calculation was considered to be an effective method, as this can provide several theoretical parameters. The structure of TEA was optimized, and it was ensured that the energy of the molecular system was minimum. The parameters of the geometry and Mulliken charge distribution of TEA were obtained after optimization, and are shown in Fig. 12. Fig. 12 shows that, after optimization, the TEA was a three-dimensional structure due to the existence of an induced effect on the TEA. From the Mulliken charge distribution, it could be seen that the negative charge resided mainly at the N and O atoms for TEA. Due to the existence of the induced effect on the TEA, the C atoms connecting with the N and O atoms had small amounts of negative charge.

Fig. 12 The parameters of the geometry and Mulliken charge distribution of GO and TEA.

The Frontier molecular orbital theory⁴⁸ considers that electron transfer can occur between the Frontier molecular orbitals when the reactants react with each other. Therefore, in order to analyze the adsorption process of TEA on the Mg alloy surface, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the TEA molecules must be considered, and these are shown in Fig. 13. It can be seen from Fig. 13 that the electron cloud of the HOMO orbit of the TEA is mainly distributed on the N and O atoms, which indicates that these atoms could be the major atoms to provide the electrons with the empty d-orbitals on the Mg alloy surface to form the coordinate bonds. The electron cloud of the LUMO orbital of the TEA focused on the C atoms, which indicated that the C atoms could accept electrons from the Mg alloy surface to form back-donating bonds.

Fig. 13 The Frontier molecular orbitals of TEA.

The calculated quantum chemical data were also reported, including the energy of the highest occupied molecular orbital (E_HOMO), the energy of the lowest unoccupied molecular orbital (E_LUMO), the energy gap (ΔE = E_LUMO − E_HOMO), the dipole moment (μ), the absolute electronegativity (χ), the global hardness (γ), and the fraction of the transferred electrons (ΔN). The Frontier molecular orbital energies, E_HOMO and E_LUMO, were associated with the ionization potential (I) and the electron affinity (A) of the magnesium atoms and the inhibitor molecules:⁴⁹

I = −E_HOMO and A = −E_LUMO (16)

The absolute electronegativity (χ) and the global hardness (γ) of the inhibitor molecule were approximated by eqn (17) and (18):⁵⁰

Therefore, the fraction of electrons transferred from the inhibitor to the metallic surface (ΔN) was calculated using eqn (19):^51,52

where χ_Mg and γ_Mg are the absolute electronegativity and global hardness of the Mg atom, and χ_inh and γ_inh are the absolute electronegativity and global hardness of the TEA molecules, respectively. In order to calculate the fraction of the electrons transferred, the theoretical values of I_Mg (7.661 eV), A_Mg (−1.717 eV), χ_Mg (2.972 eV), and γ_Mg (4.689 eV) were used.⁵³ According to eqn (16), the values of E_HOMO and E_LUMO of an Mg atom could be approximated to the negative values of the ionization potential and the electron affinity of an Mg atom, respectively. These results are presented in Table 8.

Table 8 Quantum chemical parameters calculated using the B3LYP method with a 6-31G** (d, p) basis set for TEA and Mg

	EHOMO (eV)	ELUMO (eV)	ΔE (eV)	μ (Debye)	χ	γ	ΔN
TEA	−0.21173	0.06911	0.28084	3.8676	0.07131	0.14042	0.300316
Mg	−7.661	1.717	9.378	1.31	2.972	4.689	—

---

E_HOMO usually describes the electron-donating capability of the molecule, and E_LUMO is related to the capability of the molecule to accept electrons.^54,55 As shown in Table 8, E_HOMO(TEA) > E_HOMO(Mg), and E_LUMO(Mg) > E_LUMO(TEA), which demonstrated that the electron-donating capability of TEA was better than that of Mg atoms, and the capability of TEA to accept electrons was also better than that of Mg atoms. The energy gap (ΔE = E_LUMO − E_HOMO) described the stability of the molecule.^56,57 The stability of the molecule was inversely proportional to the energy gap; in other words, the smaller the molecular energy gap, the easier it was to take part in a chemical reaction. This showed that it was easier for TEA to provide the electrons with the empty orbital of the Mg alloy and accept the electrons from the Mg alloy surface to form back-donating bonds and a relatively stable adsorption layer on the Mg alloy surface. Besides, μ_TEA > μ_Mg, and γ_TEA < γ_Mg, which illustrated that TEA was activated and was easier to polarize and react with the Mg alloy in 3.5 wt% NaCl solution.^58,59 Meanwhile, χ_TEA > χ_Mg, which illustrated that TEA could more easily provide the electron to form the chemical bond with the Mg alloy surface.⁶⁰ The ΔN value described the inhibition achieved from electron donation. If ΔN < 3.6, the inhibition efficiency was increased, with increasing ability to donate electrons to the metal surface.^61,62 It could be observed from Table 8 that ΔN_TEA > ΔN_GO, and their ΔN was less than 3.6, which showed that TEA had a stronger ability to donate electrons, and the TEA adsorption film could effectively inhibit the corrosion of Mg alloy.

In order to confirm the active sites, the Fukui function of the TEA molecule was analyzed.⁶³ The Mulliken charges and Fukui indices of selected atoms for the inhibitor TEA are listed in Table 9. The Fukui functions were calculated using eqn (20) and (21):^64,65

f⁺ = q_(N+1) − q_(N) (20)

f⁻ = q_(N) − q_(N−1) (21)

where q_(N+1), q_(N), and q_(N−1) are the charges of the atoms on the systems with N + 1, N, and N − 1 electrons, respectively; f⁺ is the electrophilic Fukui index, and f⁻ is the nucleophilic Fukui index.

Table 9 Mulliken charge distribution and Fukui indices of selected atoms for TEA inhibitor

	q(N)	f^+	f^−
N1	−0.444330	1.201373	−0.443282
C2	−0.045497	0.027492	−0.088348
C3	0.062546	−0.034712	0.024019
C8	−0.045589	0.027734	−0.089036
C9	0.062753	−0.034481	0.022357
C14	−0.045615	0.027539	−0.08862
C15	0.062735	−0.035405	0.023663
O20	−0.544190	0.557334	−0.520783
O22	−0.544205	0.557190	−0.520591
O24	−0.544267	0.557419	−0.520066

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From Table 9, it is clear that N1 had more negative charge, f⁺ was larger, and the electron cloud was mainly distributed on the N atom, so N1 showed clear nucleophilicity and could more easily provide the electrons with empty d-orbitals to form strong chemical bonds on the Mg alloy surface. O20, O22, and O24 also had a negative charge, while their nucleophilic Fukui indices were less than those of N1. They could form weak chemical bonds. Although the C atoms had small amounts of negative charge, their nucleophilic Fukui indices were low, so these C atoms were not the nucleophilic active sites. Although the heteroatom on the TEA could provide the electrons with empty d-orbitals to form the coordinate bonds on the Mg alloy surface, the Mg alloy could also provide electrons to the TEA to form back-donating bonds. From Table 9, it can be seen that the electrophilic Fukui indices of the C3, C9, and C15 atoms were larger than those of the others, and the electron cloud of the LUMO orbit of the TEA focused on these C atoms, so these could be the electrophilic activity sites, forming back-donating bonds with the Mg alloy. To sum up, the TEA inhibitor achieved polycentric adsorption on the Mg alloy surface via chemical bonds and back-donating bonds, which prevented the corrosive medium from transferring to the substrate surface.

4. Conclusion

According to the experimental results and analysis, the results obtained lead to the following conclusions:

(1) The inhibition efficiencies were closely related to the TEA concentrations. The value of 3 mL L⁻¹ TEA was the optimal concentration; lower concentrations caused a decrease in the corrosion current and the double-layer capacitance, and an increase in the film resistance with the TEA concentration; higher concentrations led to a decrease in the film resistance, and an increase in the corrosion current and the double-layer capacitance with the TEA concentration.

(2) The Langmuir isotherm equation could best describe the adsorption behavior of the TEA molecules on the Mg alloy surface, which was a spontaneous, exothermic process of increased entropy.

(3) The corrosion process with TEA of the Mg alloy went through four steps in 3.5 wt% NaCl solution, including a TEA adsorption process, a corrosion induction phase, a rapid local corrosion process, and a slow general corrosion process.

(4) TEA exhibited a good corrosion-inhibiting performance for the Mg alloy in 3.5 wt% NaCl solution by physical adsorption and chemical adsorption to form an evenly adsorbed film on the Mg alloy surface, because the TEA inhibitor could react with Mg alloy to form chemical bonds with impurity atoms, such as N and O atoms, and some C atoms could form back-donating bonds with the substrate.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No. 51664011 and No. 51665010) and the Guangxi Natural Science Foundation of China (No. 2014GXNSFAA118335).

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