Study on the Performance and Mechanism of Morpholine Salt Volatile Corrosion Inhibitors on Carbon Steel

Abstract

A series of morpholine salt volatile corrosion inhibitors (VCIs) were synthesized via solid-phase chemical reactions. The corrosion inhibition performance was assessed using evaporation weight loss, VCI capability, and corrosion weight loss tests. The corrosion inhibition mechanisms of the morpholine salt VCIs for carbon steel in atmospheric conditions were explored through electrochemical testing under thin film electrolytes, X-ray photoelectron spectroscopy (XPS), and computational simulations. Morpholine carbonate exhibited higher volatility. Corrosion weight loss tests showed an >85% reduction for steel treated with morpholine benzoate or morpholine carbonate. The inhibitors’ inhibition mechanism, elucidated through X-ray photoelectron spectroscopy (XPS) and computational simulations, revealed that morpholine carbonate and benzoate form protective layers via physical and chemical adsorption on the steel surface, coordinating with iron atoms through nitrogen and oxygen atoms. Quantum chemical calculations demonstrated that morpholine carbonate had stronger adsorption energy and electron transfer capabilities, indicating superior corrosion inhibition performance over morpholine benzoate.

1. Introduction

Atmospheric corrosion is ubiquitous and impacts all aspects of human production and life [1,2]. Therefore, effectively preventing and mitigating atmospheric corrosion has always been a significant research topic in the field of corrosion and protection. Studies [3,4,5] have shown that vapor phase rust prevention materials can effectively address the issue of atmospheric corrosion, and volatile corrosion inhibitors (VCIs) play a pivotal role in these materials. VCIs form a protective film on the metal surface through volatilization and diffusion, thereby preventing the contact between the corrosion medium and the metal, achieving the purpose of corrosion prevention [6,7].

The effectiveness of VCIs is influenced by various factors [8,9,10,11,12,13]. The volatility of VCIs directly affects their ability to form a protective film on the metal surface. The stronger the volatility, the faster the protective film forms, and the better the corrosion prevention effect. The adsorption performance of VCIs determines the coverage on the metal surface and the stability of the protective film. The stronger the adsorption performance, the more stable the protective film, and the better the corrosion prevention effect. Additionally, environmental conditions, such as temperature, humidity, the corrosion medium, etc., will affect the corrosion inhibition performance of VCIs.

Organic amine salt VCIs have nitrogen atoms in their molecules that can be firmly adsorbed on the metal surface, thereby playing a corrosion inhibition role. Due to their good volatility and inhibition efficiency, they are currently commonly used VCIs to inhibit atmospheric corrosion under thin liquid films of carbon steel [14]. One of the classes of organic amine salt VCIs are morpholine derivatives. Morpholine derivatives are one of the hotspots in the research of vapor-phase corrosion inhibitors due to their low toxicity, high safety, and effective metal corrosion inhibition. Wang et al. [15] found that new morpholine Gemini surfactants exhibit superior corrosion inhibition and antibacterial properties compared to their monomer, due to the stronger adsorption capacity and effective isolation of corrosive mediums. Nnaji et al. [16] studied the corrosion inhibition effects of morpholine’s carboxamide derivatives on mild steel in hydrochloric acid medium, finding that they effectively inhibited corrosion through a physical adsorption mechanism. Chegeni et al. [17] Found the excellent corrosion inhibition performance of three new morpholine-based inhibitors for P460N steel in a 3.5 wt.% NaCl solution, with the reduced form (I3) showing the highest efficiency.

They are usually prepared by neutralization reactions between acids and bases, but conventional preparation methods use a large number of organic solvents, which not only pollute the environment but also pose safety hazards. Considering the environmental safety issues, many environmentally friendly heterocyclic molecules such as drugs, natural extracts, biomolecules, and ionic liquids, have been investigated as corrosion inhibitors [18,19,20,21].

This paper prepares a series of morpholine salt VCIs through acid–base neutralization solid phase chemical reactions, reducing the use of a large number of organic solvents. Using volatilization weight loss tests, VCI ability tests, corrosion weight loss tests, and macro–micro morphology analysis, two of the five synthesized morpholine salt VCIs are screened out. Then, the two screened morpholine salt VCIs are used to explore the mechanism of morpholine salt VCIs’ inhibiting the atmospheric corrosion of carbon steel through thin liquid film electrochemical testing, XPS testing, and simulation calculations.

2. Experiments

2.1. Materials

The materials used in the experiment were made of 20# steel, and their chemical composition is shown in

Table 1. Chemical Composition of 20# Steel.

Element	C	Si	Mn	S	P	Ni	Cr	Cu	Fe
Content (wt.%)	0.20~0.22	0.29~0.31	0.48~0.52	≤0.035	≤0.035	≤0.30	≤0.25	≤0.25	Balence

. The content of each element was measured using an atomic absorption spectrometer. All chemical reagents used were of an analytical grade, including hydrochloric acid, nitric acid, absolute ethanol, sodium chloride, glycerol, formic acid, acetic acid, propionic acid, benzoic acid, and morpholine. Hydrochloric acid and nitric acid are used for the pretreatment of samples to remove surface oxide layers and other impurities. Ethanol is used to clean the samples after handling, removing any residual acids and impurities. Formic acid, acetic acid, propionic acid, and benzoic acid react with morpholine to synthesize different morpholine salt corrosion inhibitors.

2.2. Synthesis of Morpholine Salt VCIs

The synthesis process is shown in molecular formula (1)–(5). One mole of formic acid (or acetic acid, propionic acid, or benzoic acid) was placed in a beaker, and one mole of morpholine was gradually added at room temperature with constant stirring until solid morpholine salts (morpholine formate, morpholine acetate, morpholine propionate, morpholine benzoate) formed. The solid was aged for 24 h. The synthesis of morpholine carbonate VCIs differed: 100 mL of morpholine was placed in a beaker within a 1 L high-pressure reactor, 2 MPa of CO₂ was introduced, and the reaction proceeded at room temperature for 24 h.

Morpholine formate (MF):		(1)
Morpholine acetate (MA):		(2)
Morpholine propionate (MP):		(3)
Morpholine benzoate (MB):		(4)
Morpholine carbonate (MC):		(5)

2.3. Experimental Procedures and Characterization Methods

2.3.1. Evaporation Weight Loss Test

Two grams of a morpholine salt VCI were evenly spread in a container with a 24 mm radius and heated in an oven at 30 °C for 7 days. The mass was measured daily, and weight loss and vapor pressure were calculated.

2.3.2. VCI Capability Test

Before the experiment, the surface of the samples was polished step by step using waterproof abrasive paper. First, 800# abrasive paper was used for mechanical grinding. Once the surface scratches were uniform and consistent, a finer grade of abrasive paper was used. The samples were rotated 90° to ensure that the scratches from the previous paper were completely removed. The sequence of abrasive papers used was 800#, 1200#, 1500#, and 2000#. The polished samples were then etched using a 2% nitric acid alcohol solution, followed by cleaning and drying. As shown in Figure 1, 20 mL of 35% glycerol solution was added to a 1 L wide-mouth bottle, and 1 g of a morpholine salt VCI was spread evenly and placed at the bottom. The treated samples were inserted into rubber stoppers, ensuring the experimental surface was within 3 mm of the stopper. The assembly was heated at 30 °C for 66 h, after which ice water was added to the aluminum tube for 3 h. The samples were examined for rust, with a blank control test under the same conditions. The blank experiment had all the experimental conditions identical to those of the experimental group, except for the absence of a corrosion inhibitor.

2.3.3. Corrosion Weight Loss Test

The test specimens were 20# steel coupons, each measuring 50 mm × 10 mm × 3 mm. The surface of the specimens was first polished with 800# sandpaper to achieve a smooth finish. They were then cleaned, dried, and weighed. As shown in Figure 2, 25 mL of 3.5% NaCl solution was added to a 1 L wide-mouth bottle, and a container with a morpholine salt VCI was placed at the bottom. The prepared specimens were suspended in the bottle. The setup was then placed in an oven maintained at 30 °C and heated for 7 days. After the 7-day period, the steel coupons were removed from the bottle. After recording the surface corrosion and the adherence of the corrosion scale, the specimens were immediately rinsed with clean water and dried with filter paper, followed by photography. Subsequently, the recorded specimens were placed in acetone, and surface dirt was removed with degreasing cotton. They were then soaked in anhydrous ethanol for 5 min for further degreasing and dehydration. The specimens were then taken out and placed in a prepared acid cleaning solution for ultrasonic cleaning, The acid washing solution used in the weight loss experiment is a 50% hydrochloric acid aqueous solution with the addition of 0.35 wt.% hexamethylenetetramine. After the corrosion products were removed, any residual acid was rinsed off with tap water, and then the specimens were soaked in anhydrous ethanol for about 5 min, undergoing two cleaning and dehydration cycles. The specimens were then removed, placed on filter paper, dried with cold air, and left to sit for 1 h before the final weight was measured, accurate to 0.1 mg. The corrosion rate is calculated using the following formula:

$$r_{corr}={\displaystyle \frac{8.76\times 10^4\times \left(m_0-m_t\right)}{S\cdot t\cdot \rho }}$$

(1)

where $r_{corr}$ is the corrosion rate in mm/year, $m_0$ is the initial mass of the sample in grams (g), $m_t$ is the mass of the sample after time t in grams (g), S is the surface area of the sample in square centimeters (cm²), t is the exposure time in hours (h), and ρ is the density of the material in grams per cubic centimeter (g/cm³).

Additionally, the microstructure of the corrosion product film on the samples will be observed using scanning electron microscopy (SEM) before acid cleaning after the experiment.

2.3.4. EIS Testing

Electrochemical impedance spectroscopy (EIS) tests were conducted on the samples using an electrochemical workstation (CHI660D). As shown in Figure 3, the experiments were carried out with a conventional three-electrode system: 20# steel served as the working electrode, a saturated calomel electrode was used as the reference electrode, and a platinum sheet functioned as the counter electrode. A layer of three filter papers soaked in 3.5% NaCl solution was laid on the working electrode. The auxiliary and reference electrodes were connected on top of the three filter papers to simulate atmospheric corrosion under a thin liquid film. The tests covered a frequency range from 100 kHz to 0.01 Hz, with an amplitude of 10 mV. EIS data fitting was performed using Zsimpwin 3.60 software.

2.3.5. XPS Testing

Samples of 20# steel were cut into dimensions of 10 mm × 10 mm × 3 mm and placed in a beaker containing 2 g of an organic amine salt vapor-phase corrosion inhibitor. The beaker was sealed and placed in an oven at a constant temperature of 30 °C for 7 days to allow for pre-filming. Subsequently, X-ray photoelectron spectroscopy (XPS) was used to investigate the adsorption of the organic amine salt vapor-phase corrosion inhibitor on the surface of the 20# steel. The survey scans were conducted over a binding energy range of 0–1200 eV to identify the elements present on the surface. High-resolution spectra were obtained for the core levels of elements of interest, such as C 1 s, O 1 s, N 1 s, and Fe 2p, with a pass energy of 20 eV.

2.3.6. Computational Simulation Method

All simulations in this study were performed using the Material Studio software suite [22,23,24]. The specific steps are as follows: First, an interface model of the organic amine salt vapor-phase corrosion inhibitor molecules with the Fe-H₂O-O₂ surface was established. Second, the structure of the organic amine salt vapor-phase corrosion inhibitor molecules was optimized using the DMol3 module. Third, the adsorption configurations of the organic amine salt vapor-phase corrosion inhibitor molecules on the Fe-H₂O-O₂ surface were simulated through molecular dynamics. Fourth, the adsorption energy of the organic amine salt vapor-phase corrosion inhibitor molecules on the iron surface was calculated using DFTB+. The B3LYP hybrid density functional method was employed, with convergence criteria set as follows: an energy gradient of 0.002 Ha/Å, a total energy convergence of 1 × 10⁻⁵ Ha, a self-consistent field (SCF) convergence of 10⁻⁶, and a maximum atomic displacement of 0.005 Å.

3. Results and Discussion

3.1. Screening of Morpholine Salt VCIs

3.1.1. Evaporation Weight Loss Test Results

An appropriate vapor pressure is crucial for ensuring that VCIs can volatilize onto metal surfaces at specific concentrations and temperatures to exert their corrosion inhibition effect.

Table 2. Vapor pressure of morpholine salt volatile corrosion inhibitors.

Corrosion Inhibitor	Weight Loss (g)	Vapor Pressure (Pa)
Morpholine formate	1.09	3.14 × 10−3
Morpholine acetate	1.49	4.09 × 10−3
Morpholine propionate	1.54	4.05 × 10−3
Morpholine benzoate	0.20	4.59 × 10−4
Morpholine carbonate	2.00	4.45 × 10−3

shows the weight loss and vapor pressure of five morpholine salt VCIs. The weight losses of morpholine formate, morpholine acetate, and morpholine propionate VCIs are similar, indicating moderate volatility; morpholine benzoate has the least weight loss and lowest volatility, while morpholine carbonate shows the highest weight loss and volatility, almost completely volatilizing. It is noteworthy that the increase in weight loss of morpholine salt VCIs corresponds with an increase in gas pressure. Morpholine benzoate shows the least weight loss, with a corresponding vapor pressure of 4.59 × 10⁻⁴. The weight loss for the other morpholine salts exceeds 1 g, with corresponding vapor pressures also reaching the 10⁻³ magnitude.

3.1.2. VCI Capability Test Results

Figure 4 shows the macroscopic photographs of 20# steel surfaces after the VCI capability test. Before the corrosion experiment, the surface of the sample was bright with a metallic luster (Figure 4a). After the corrosion experiment, without the addition of corrosion inhibitors, the surface was covered with reddish-brown rust, indicating severe corrosion (Figure 4b). Under all conditions with added corrosion inhibitors, the surface of the samples appeared blue, which is related to the adsorption and film formation of the inhibitors. Among them, the samples with morpholine formate, morpholine acetate, and morpholine propionate corrosion inhibitors showed slight yellow rust spots on the surface, while the samples with morpholine benzoate and morpholine carbonate corrosion inhibitors showed almost no yellow rust spots, indicating that morpholine benzoate and carbonate have better VCI capabilities.

3.1.3. Corrosion Weight Loss Test Results

Figure 5 displays the corrosion rates and corrosion inhibition efficiencies of five morpholine salt VCIs in a 3.5% NaCl solution with a concentration of 10 g/L. The corrosion rate of 20# steel without any corrosion inhibitor can reach 0.075 mm/a. With the addition of morpholine formate, morpholine acetate, and morpholine propionate, the corrosion rate slightly decreases, achieving an inhibition efficiency below 30%, indicating the poor performance of these inhibitors. However, after the addition of morpholine benzoate and morpholine carbonate, the corrosion rate rapidly decreases to below 0.01 mm/a, with an inhibition efficiency exceeding 85%, demonstrating the excellent performance of these inhibitors. The corrosion inhibition ranking is as follows: morpholine carbonate > morpholine benzoate > morpholine propionate > morpholine formate > morpholine acetate.

3.1.4. Corrosion Morphology Analysis

Figure 6 presents the macro and micro corrosion morphologies of 20# steel samples after corrosion weight loss tests in a 3.5% sodium chloride solution containing 10 g/L of five different morpholine-based VCIs. The samples treated with morpholine benzoate and morpholine carbonate exhibited smooth surfaces without any visible corrosion product film deposition, and the mechanical polishing marks were clearly visible, indicating minimal corrosion. In contrast, the samples treated with other morpholine salts showed severe localized corrosion, with the surface covered by discontinuous, loose corrosion products. This further confirms that morpholine benzoate and morpholine carbonate can adsorb tightly onto the steel surface, providing effective corrosion protection.

In summary, morpholine benzoate has weaker volatility, resulting in a longer induction period and thus longer-term corrosion prevention, while morpholine carbonate has stronger volatility, resulting in a shorter induction period but rapid consumption, necessitating timely replenishment for long-term protection.

3.2. Corrosion Inhibition Mechanism of Morpholine Salt VCIs on 20# Steel

The density functional tight-binding (DFTB+) method allows for the accurate calculation of crucial parameters of complex molecules at a low cost, providing a systematic approach to studying the interactions between VCIs and metal surfaces. This study uses electrochemical testing in a thin film electrolyte, XPS analysis, and computational simulations to discuss the corrosion inhibition mechanisms of two morpholine salt VCIs on 20# steel.

Figure 7 shows the EIS results of 20# steel electrodes in a 3.5% NaCl thin liquid film with five morpholine salt VCIs. Compared to the impedance arc of the untreated steel, the addition of morpholine salt VCIs significantly increases the radius of the impedance arc, indicating good corrosion inhibition.

The EIS test results were fitted using an equivalent circuit (Figure 8). R_s represents solution resistance, R_f corresponds to the gas phase passivation film resistance formed on the surface of the No. 20 steel electrode, and R_ct stands for charge transfer resistance, while CPE_dl and CPE_f denote constant phase angle elements. The fitting error for each parameter in EIS is within 10%.

The corrosion inhibition efficiency of the five morpholine salt VCIs was calculated using Equation (2):

$$\mathsf{\eta }={\displaystyle \frac{{\mathrm{R}}_{\mathrm{ct}}^{\mathrm{i}}-{\mathrm{R}}_{\mathrm{ct}}^0}{{\mathrm{R}}_{\mathrm{ct}}^{\mathrm{i}}}}\times 100$$

(2)

where ${\mathrm{R}}_{\mathrm{ct}}^0$ is the charge transfer resistance of the untreated steel, and ${\mathrm{R}}_{\mathrm{ct}}^{\mathrm{i}}$ is the charge transfer resistance of the steel treated with the morpholine salt VCI.

As shown in

Table 3. Impedance spectrum parameters of morpholine salt volatile corrosion inhibitors in 3.5% NaCl thin liquid film.

Corrosion Inhibitor	Rct (ohm·cm2)	H (%)
Blank	1013	-
Morpholine formate	3166	68.00
Morpholine acetate	2331	56.54
Morpholine propionate	3715	72.73
Morpholine benzoate	5507	81.61
Morpholine carbonate	8442	88.00

, the inhibition efficiency ranking is morpholine carbonate > morpholine benzoate > morpholine propionate > morpholine formate > morpholine acetate. This ranking is consistent with the results from the corrosion weight loss tests.

The EIS results indicate that morpholine benzoate and morpholine carbonate form a more effective adsorptive film on the steel surface compared to the other three morpholine derivatives. This enhances the resistance to electrochemical corrosion reactions, thereby reducing the corrosion rate.

To further investigate the action mechanism of the two morpholine salt VCIs on 20# steel surfaces, XPS tests were conducted to characterize the steel surface treated with these inhibitors. Figure 9 presents the XPS full spectrum of 20# steel covered with the morpholine benzoate and morpholine carbonate inhibitors. In comparison to the uncoated samples, the coated samples showed an increase in the peak intensities of N, O, and C elements, with the N element exclusively sourced from the morpholine salt inhibitors. Concurrently, the Fe peak intensity markedly decreased, indicating that a protective film was formed on the surface of the 20# steel by both morpholine salt volatile corrosion inhibitors.

The high resolution of C 1 s, N 1 s, O 1 s, and Fe 2p spectra of morpholine benzoate and morpholine carbonate are represented in Figure 9. The XPS peaks were fitted with reference to the literature [15,25,26].

Figure 10a shows the N1s spectrum of morpholine benzoate adsorbed on the surface of the 20# steel. The coated surface exhibits a significant N1s peak, indicating that the morpholine benzoate volatile corrosion inhibitor (VCI) is adsorbed on the 20# steel surface. The peak at 401.4 eV corresponds to the uncoordinated C-N characteristic peak in the morpholine benzoate VCI molecule, and the peak at 398.7 eV corresponds to the N atoms coordinated with Fe. Figure 10b shows the O1s spectrum of morpholine benzoate adsorbed on the surface of the 20# steel. The peak at 529.7 eV corresponds to lattice oxygen in metal oxides, while the peaks between 531 and 533 eV correspond to the -COOH and C-O bonds in the morpholine benzoate VCI molecule. The increased binding energy at 531.7 eV is due to the coordination of O atoms with Fe atoms. Figure 10c shows the C1s spectrum of morpholine benzoate adsorbed on the surface of the 20# steel. The C elements exhibit three chemical states: the peaks at 284.8 eV, 285.9 eV, and 288.3 eV correspond to the C-C bond, the C-O-C bond, and the carboxylate group in the morpholine benzoate VCI molecule, respectively. Figure 10d shows the Fe2p spectra of the 20# steel surface before and after the adsorption of morpholine benzoate. Before the adsorption of morpholine benzoate, the spectra can be deconvoluted into the peaks at 706.4 eV, 710.6 eV, 719.3 eV, and 724.1 eV, which could be attributed to Fe, Fe₂O₃, Fe³⁺, and FeO, respectively. After the adsorption of morpholine benzoate, it decreases the Fe content on the steel surface, resulting in weakened peak intensities. Additionally, the binding energy of Fe, Fe₂O₃, Fe³⁺, and FeO decreases to 706.3 eV, 710.4 eV, 719.2 eV, and 724.0 eV, respectively.

Figure 10e shows the N1s spectrum of morpholine carbonate adsorbed on the surface of the 20# steel. The peak at 400.9 eV corresponds to the uncoordinated C-N characteristic peak in the morpholine carbonate VCI molecule, while the peak at 398.6 eV corresponds to the N atoms coordinated with Fe, resulting in an increased binding energy. Figure 10f shows the O1s spectrum of morpholine carbonate adsorbed on the surface of the 20# steel. The peak at 529.3 eV corresponds to the lattice oxygen in the metal oxides. The peaks between 531 and 533 eV correspond to the -COOH and C-O bonds in the morpholine carbonate VCI molecule, with the increased binding energy at 531.5 eV due to the coordination of O atoms with Fe atoms. Figure 10g shows the C1s spectrum of morpholine carbonate adsorbed on the surface of 20# steel. The C elements exhibit three chemical states: the peaks at 284.8 eV, 285.4 eV, and 288 eV correspond to the C-C bond, the C-O-C bond, and the carboxylate group in the morpholine carbonate VCI molecule, respectively. Figure 10h shows the Fe2p spectra of the 20# steel surface before and after the adsorption of morpholine carbonate. Before the adsorption of morpholine carbonate, the spectra can be deconvoluted into the peaks at 706.4 eV, 710.6 eV, 719.3 eV, and 724.1 eV, which could be attributed to Fe, Fe₂O₃, Fe³⁺, and FeO, respectively. After the adsorption of morpholine benzoate, it decreases the Fe content on the steel surface, resulting in weakened peak intensities. Additionally, the binding energy of Fe, Fe₂O₃, Fe³⁺, and FeO decreases to 704.8 eV, 709.2 eV, 717.9 eV, and 722.9 eV, respectively.

Figure 11 shows the optimized geometrical structures of the two morpholine salt volatile corrosion inhibitor molecules. In the morpholine benzoate molecule, the benzene ring is flat, and the morpholine six-membered ring adopts a chair conformation. In the morpholine carbonate molecule, the carbonate group is positioned between two morpholine six-membered rings, both of which also adopt a chair conformation. The relevant parameters of the morpholine salt volatile corrosion inhibitors were calculated using Koopmans’ theorem, with the related formulas as follows [27,28]:

$$\mathrm{I}=-{\mathrm{E}}_{\mathrm{HOMO}}$$

(3)

$$\mathrm{A}=-{\mathrm{E}}_{\mathrm{LUMO}}$$

(4)

$$\chi =-{\displaystyle \frac{1}{2}}({\mathrm{E}}_{\mathrm{HOMO}}+{\mathrm{E}}_{\mathrm{LUMO}})$$

(5)

$$\mathsf{\eta }=-{\displaystyle \frac{1}{2}}\left({\mathrm{E}}_{\mathrm{HOMO}}-{\mathrm{E}}_{\mathrm{LUMO}}\right)$$

(6)

$$\mathsf{\sigma }={\displaystyle \frac{1}{\mathsf{\eta }}}$$

(7)

Table 4. Quantum chemical parameters of two morpholine salt volatile corrosion inhibitors.

Corrosion Inhibitor	Morpholine Benzoate	Morpholine Carbonate
HOMO	−5.396	−5.022
LUMO	−2.330	−2.503
ΔE	3.066	2.519
I	5.396	5.022
A	2.330	2.503
η	1.533	1.260
σ	0.652	0.794
χ	3.863	3.763

lists the relevant parameters of the two morpholine salt volatile corrosion inhibitor molecules. A higher E_HOMO value indicates that the morpholine salt VCI molecule more readily donates electrons, while a lower E_LUMO value indicates that the molecule more readily accepts electrons. As the E_HOMO increases and the E_LUMO decreases, the interaction between the morpholine salt VCI and the metal substrate strengthens [29,30,31,32]. The energy gap ΔE relates to the charge transfer capability of the morpholine salt VCI molecules; a larger ΔE suggests that the molecule can more easily undergo charge transfer and form coordination bonds with Fe atoms, enhancing corrosion inhibition performance. Table 4 shows that morpholine carbonate has a higher ΔE value than morpholine benzoate. Hard morpholine salt VCI molecules have a larger energy gap, while soft morpholine salt VCI molecules have a smaller energy gap, making them more reactive and more likely to donate electrons to the metal. As shown in Table 4, the hardness value η is lower for morpholine carbonate than for morpholine benzoate, while the softness value σ is higher for morpholine carbonate. These calculations indicate that the corrosion inhibition performance of the two morpholine salt VCIs is in the following order: morpholine carbonate > morpholine benzoate. This is consistent with the results from the weight loss corrosion tests and electrochemical tests.

The adsorption capacity of morpholine salt volatile corrosion inhibitors on metal surfaces is directly related to the adsorption energy. A smaller adsorption energy value indicates that the morpholine salt VCI molecules release more energy upon adsorption onto the metal surface, resulting in stronger interactions and easier adsorption.

The adsorption states of morpholine benzoate and morpholine carbonate VCIs on metal substrates were simulated using the density functional tight-binding method (DFTB+). The simulations were conducted at a temperature of 40 °C, in an H₂O/O₂ environment, with an Fe substrate. The adsorption states of the two morpholine salt VCI molecules are shown in Figure 12.

As shown in Figure 12, the morpholine benzoate VCI molecules exhibit a flat adsorption state on the iron substrate surface. The oxygen and nitrogen atoms in the morpholine quaternary ammonium cation adsorb tightly onto the iron substrate surface, as do the double-bonded oxygen atoms in the benzoate anion. The benzene ring also lies flat on the iron substrate surface. In comparison, the morpholine carbonate VCI molecules show a more flattened adsorption state than their ground state. The oxygen and nitrogen atoms in the morpholine quaternary ammonium cation adsorb tightly onto the iron substrate surface, as do the double-bonded oxygen atoms in the carbonate anion.

The adsorption energy (${\mathrm{E}}_{\mathrm{interaction}}$) can be calculated using the following equation [33,34,35,36,37,38,39,40,41]:

$${\mathrm{E}}_{\mathrm{interaction}}={\mathrm{E}}_{\mathrm{total}}-{\mathrm{E}}_{\mathrm{Fe}}-{\mathrm{E}}_{\mathrm{inhibitor}}-{\mathrm{E}}_{\mathrm{water}}$$

(8)

where ${\mathrm{E}}_{\mathrm{total}}$ represents the total energy of the system (kJ/mol); ${\mathrm{E}}_{\mathrm{Fe}}$represents the energy of the Fe (001) surface (kJ/mol); ${\mathrm{E}}_{\mathrm{inhibitor}}$ represents the energy of the morpholine salt VCI molecule (kJ/mol); ${\mathrm{E}}_{\mathrm{water}}$ represents the energy of the corrosion medium (kJ/mol).

The adsorption energies of the two morpholine salt volatile corrosion inhibitors at 40 °C were calculated using the density functional tight-binding method (DFTB+). The results show that the adsorption energies of morpholine benzoate and morpholine carbonate are −64,081 kJ/mol and −64,408 kJ/mol, respectively. Therefore, the corrosion inhibition performance of the two types of amine salt gas phase corrosion inhibitors follows the sequence of carbamic acid amine > benzylamine. This aligns with the results obtained from corrosion weight loss experiments and electrochemical tests.

Through XPS analysis and computational simulations, it can be concluded that both morpholine benzoate and morpholine carbonate VCIs adsorb onto the steel surface through physical or chemical adsorption centered around N and O atoms. The specific adsorption states are related to the different functional groups present in the VCI molecules.

Morpholine benzoate VCI inhibits corrosion by volatilizing and dissolving in the thin liquid film on the 20# steel surface, ionizing to produce benzoate anions and morpholine cations. Raji et al. [42] and Wang et al. [15] showed that both benzoic acid anions and morpholine cations can adsorb on metal surfaces and thus act as corrosion inhibitors.

As shown in Figure 13, the benzoate anions adsorb onto the anode region of the corrosion cell, while the morpholine cations adsorb onto the cathode region, inhibiting both anodic and cathodic reactions. The adsorption relies on chemical adsorption of the O atoms in the carboxyl group and the physical adsorption of the salt anions, as well as the chemical adsorption of N and O atoms and the physical adsorption of the salt cations. The benzene ring and six-membered ring structures in the VCI molecule form a hydrophobic film that blocks water and oxygen from reaching the steel surface, thereby inhibiting corrosion.

Morpholine carbonate VCI inhibits corrosion by volatilizing and dissolving in the thin liquid film on the 20# steel surface, ionizing to produce carbonate anions and morpholine cations. As shown in Figure 13, the carbonate anions adsorb onto the anode region of the corrosion cell, while the morpholine cations adsorb onto the cathode region, inhibiting both anodic and cathodic reactions. The adsorption relies on chemical adsorption of the O atoms in the carboxyl group and the physical adsorption of the salt anions, as well as the chemical adsorption of N and O atoms and the physical adsorption of the salt cations. The two six-membered ring structures in the VCI molecule form a hydrophobic film that blocks water and oxygen from reaching the steel surface, thereby inhibiting corrosion.

4. Conclusions

This study synthesized five morpholine salt VCIs (morpholine formate, acetate, propionate, benzoate, and carbonate) through acid–base neutralization and solid-phase chemical reactions, significantly reducing the use of organic solvents. Two morpholine salts with superior corrosion inhibition performance were selected. The conclusions are as follows:

(1)

Volatility and Inhibition Efficiency: Morpholine carbonate showed stronger volatility compared to morpholine benzoate, which had weaker volatility due to the presence of a benzene ring. The corrosion inhibition efficiency ranking was morpholine carbonate > morpholine benzoate > morpholine propionate > morpholine formate > morpholine acetate. Morpholine benzoate and carbonate exhibited better performance due to their higher content of N and O atoms, which form coordination bonds with Fe.

(2)

Inhibition Mechanism: Based on XPS analysis, morpholine benzoate and morpholine carbonate inhibitors can coordinate with Fe through N and O atoms, forming a protective layer on the steel surface via physical and chemical adsorption, thereby exerting corrosion inhibition effects. Quantum chemical calculations and adsorption energy analysis revealed that morpholine carbonate exhibits higher adsorption energy and a smaller band gap, indicating a stronger electron transfer capability and an easier formation of coordination bonds with Fe atoms. Therefore, morpholine carbonate demonstrates superior corrosion inhibition performance compared to morpholine benzoate.