Quaternium-22 as a High-Performance Corrosion Inhibitor for Carbon Steel in Acidic Media: Experimental and Theoretical Insights

Abstract

This work provides an integrated experimental and computational evaluation of the cationic surfactant Quaternium-22 (Q-22) as a potentially eco-compatible corrosion inhibitor for carbon steel (CS) in 1 M hydrochloric acid. Gravimetric analysis and electrochemical techniques, including electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP), were employed over a temperature range of 20–50 °C. Q-22 exhibited mixed-type inhibition behavior, with efficiency rising to 97% at an optimal concentration of 277 μmol L⁻¹. Performance was concentration-dependent but diminished with increasing temperature, indicating partial inhibitor desorption at elevated temperatures. Thermodynamic evaluation confirmed a spontaneous adsorption process consistent with the Langmuir isotherm, involving a combined physisorption and chemisorption mechanism. Surface characterization via scanning electron microscopy (SEM), atomic force microscopy (AFM), contact angle (CA) measurement, and X-ray photoelectron spectroscopy (XPS) confirmed the formation of a coherent, hydrophobic inhibitor layer that substantially reduced surface roughness and corrosion damage. Theoretical investigations using density functional theory (DFT), natural bond orbital (NBO) analysis, and molecular dynamics (MD) simulations revealed strong adsorption energies and favorable electronic properties consistent with the inhibitor’s high experimental efficacy. Overall, the results demonstrate that Q-22 is a highly effective, eco-compatible corrosion inhibitor for CS in acidic environments, operating through a stable adsorptive film-forming mechanism.

1. Introduction

The corrosion of metals and alloys represents a pervasive and costly global challenge, with profound implications for industrial safety, economic stability, and environmental integrity [1,2]. Over USD 2.5 trillion is spent on corrosion worldwide each year, much of which can be avoided with effective management techniques [3]. Nowhere is this more evident than in the petroleum industry, where aggressive environments, characterized by the presence of acids, brines, and corrosive gases, relentlessly attack infrastructure. Carbon steel (CS), the workhorse material used for pipelines, tanks, and reactors due to its mechanical strength and cost-effectiveness, is highly susceptible to degradation, particularly in acidic media used for pickling, descaling, and oil well acidizing [4,5]. The transfer of crude oil, which often contains corrosive species like chlorides, carbon dioxide, and hydrogen sulfide, accelerates this degradation, leading to equipment failure, production downtime, environmental contamination, and substantial financial losses [6].

Within the suite of corrosion mitigation strategies, such as protective coatings, cathodic protection, and alloy selection, the application of corrosion inhibitors stands out as a particularly efficient and cost-effective approach, especially for enclosed systems and acidic media [7,8]. These compounds function by introducing a small concentration of chemical species into the aggressive environment. An effective inhibitor adsorbs onto the metal substrate, forming a protective interfacial layer that significantly retards the corrosion rate [9]. In this context, organic molecules featuring heteroatoms (e.g., N, O, S), π-electron systems, and polar functional groups have emerged as highly promising candidates. Their efficacy predominantly stems from their strong adsorption affinity for metal surfaces. Upon adsorption, these molecules block electrochemically active sites and create a physical barrier that isolates the metal from the corrosive electrolyte [10,11,12].

The amphiphilic structure of surfactants, which consists of a hydrophilic head group coupled with a hydrophobic tail, makes them an effective class of organic corrosion inhibitors [13]. While the hydrophobic tail creates a water-repellent barrier that prevents corrosive species from entering acidic media, the hydrophilic head—typically a quaternary ammonium group in cationic surfactants—promotes electrostatic adsorption onto the metal surface [14]. Molecular characteristics such as chain length, head group type, and the presence of specific functional groups that increase adsorption strength have a significant impact on the inhibition efficiency [15].

Driven by growing environmental and health concerns, the field of corrosion science is increasingly shifting towards “green” or eco-friendly inhibitors [16,17]. These are characterized by low toxicity, reported biodegradability, and relatively well-documented toxicological profiles. While many traditional inhibitors are effective, their ecological unfriendliness has spurred the search for safer alternatives. In this context, quaternary ammonium compounds (Quats), which are widely used as biocides and disinfectants, are being re-evaluated for their corrosion inhibition potential. Their inherent biodegradability and well-understood toxicological profiles make them attractive candidates for developing effective yet environmentally acceptable inhibitors [18,19].

Quaternium-22 (Q-22), chemically defined as N-(2-hydroxyethyl)-N,N-dimethyl-3-(gluconoylamino)propane ammonium chloride, is a cationic surfactant that fits this profile of potentially less hazardous inhibitor. Its molecular structure (C₁₃H₂₉ClN₂O₇) is particularly noteworthy, as it integrates multiple functional groups synergistic to corrosion inhibition within a single molecule: a quaternary ammonium center (N⁺) for strong electrostatic/physisorption to the negatively charged steel surface, a hydrophobic alkyl chain for surface coverage, multiple ether and hydroxyl groups (-O-) from the gluconate moiety for potential chemisorption via lone-pair electrons, and an amide linkage (-NH-CO-) for additional anchoring points [20]. This unique architecture, combining a cationic head with a polar, hydroxy-rich tail, is hypothesized to enable a multimodal adsorption mechanism that could lead to superior inhibition efficiency and film stability. The gluconate moiety is itself known to be benign and biodegradable, enhancing the compound’s green credentials [21].

However, despite its commercial availability and multi-functional structure, the corrosion inhibition mechanism of Quaternium-22 for CS in acidic media remains insufficiently understood, mostly concerning the synergistic roles of its quaternary ammonium and polyhydroxy gluconate moieties. This unique structure offers the potential for multi-dentate surface binding through both nitrogen and oxygen donor atoms, a feature not present in simpler quaternary ammonium surfactants such as Q-15 [9]. Furthermore, the systematic comparison between Q-15 and Q-22 under identical experimental conditions provides unprecedented insight into structure–property relationships governing inhibitor performance.

Therefore, this study addresses this gap through a comprehensive evaluation of Quaternium-22 as an efficient and environmentally acceptable corrosion inhibitor for CS in 1.0 M HCl. Inhibition performance and mechanism were evaluated using gravimetric and electrochemical techniques, complemented by thermodynamic and adsorption analyses. Its mixed adsorption mode and high efficiency at low concentrations were explained and mechanistically elucidated using surface characterization and multiscale computational insights to further correlate molecule structure with inhibitory activity.

2. Materials and Methods

2.1. Materials

ASTM Grade A35 carbon steel, which is commonly used in industrial settings, was used to prepare the working electrode.

Table 1. Chemical composition of carbon steel.

Elements	Carbon (C)	Silicon (Si)	Manganese (Mn)	Sulfur (S)	Copper (Cu)	Iron (Fe)
Content (wt.%)	0.26	0.28	1.05	0.003	0.2	Balance

summarizes its chemical composition as verified by optical emission spectrometry. Before each experiment, coupons were mechanically abraded with increasing grades of silicon carbide paper (up to 1200 grit), ultrasonically cleaned in acetone for 10 min, thoroughly rinsed with double-distilled water, and dried under a warm-air stream to ensure a reproducible surface condition.

2.2. Inhibitor and Solution Preparation

The corrosion inhibitor, Quaternium-22 (Q-22), was acquired from UFC Bio-technology Co., Ltd., Niagara Falls, NY, USA. Q-22 is a cationic surfactant recognized for its eco-friendly characteristics, commonly used in cosmetic and antistatic formulations. Its molecular structure, IUPAC name, chemical formula, and molecular weight are listed in

Table 2. Structure formula of the used inhibitor.

Organic Compound
Quaternium-22 (Q-22)
Structure
IUPAC Name & Chemical Formula	N-(2-hydroxyethyl)-N,N-dimethyl-3-(gluconoylamino)propanaminium chloride C13H29N2ClO7
Molecular Weight	360.83 g/mol

. An accurately weighed amount of Q-22 was dissolved in 1.0 M hydrochloric acid (HCl) to create a stock solution (1.0 × 10⁻² M). This stock was then appropriately diluted with the 1.0 M HCl electrolyte to generate all functioning inhibitor solutions. Analytical-grade concentrated HCl (BDH) was diluted with double-distilled water to create an aggressive electrolyte, 1.0 M HCl.

2.3. Gravimetric (Weight Loss) Measurements

Gravimetric measurements were conducted to quantitatively assess the corrosion rate and inhibition efficiency of Q-22 over time. The tests were performed in a 250 mL glass vessel with an inner diameter of 6 cm, containing 100 mL of the test solution (1.0 M HCl with and without the inhibitor) and maintained at 25 ± 1 °C. CS coupons, with dimensions of 2.0 cm × 2.0 cm × 0.1 cm, were prepared by sequential abrasion with progressively finer grades of silicon carbide paper (from coarse to 1200 grit), followed by ultrasonic cleaning in acetone for 10 min, rinsing with double-distilled water, and complete drying before being accurately weighed.

Each coupon was suspended approximately 1 cm below the solution surface using a glass hook. After a specified immersion period (e.g., 6 h), the coupons were removed, carefully rinsed with double-distilled water to remove loose corrosion products, dried, and reweighed. The experiment was performed in triplicate for each condition to ensure reproducibility, and the average values were used for all calculations.

The average weight loss (ΔW), corrosion rate (CR), and inhibition efficiency (IE%) were calculated as follows (Equations (1), (2), and (4)):

The weight loss for each coupon was determined using:

W_o = W_B − W_A

(1)

where:

• W_o = weight loss of metal in the corrosive solution;
• W_B = weight of metal before exposure to the corrosive solution;
• W_A = weight of metal after exposure to the corrosive solution.

The corrosion rate (CR) was then calculated from the average weight loss:

$$\mathrm{C}\mathrm{o}\mathrm{r}\mathrm{r}\mathrm{o}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{e}(\mathrm{m}\mathrm{p}\mathrm{y})={\displaystyle \frac{K\mathsf{\Delta }W}{\rho At}}$$

(2)

where:

• K is a constant (8.76 × 10⁴ for the rate in mpy);
• A is the total surface area of the coupon (cm²);
• t is the immersion time (hours);
• ρ is the density of the CS (g cm⁻³).

The surface coverage (θ) and inhibition efficiency (IE%) were determined as:

$$\theta ={\displaystyle \frac{{\mathsf{\Delta }W}_0-{\mathsf{\Delta }W}_{inh}}{\mathsf{\Delta }{W}_0}}$$

(3)

$$IE\%=\mathrm{\theta }\times 100={\displaystyle \frac{W_0-W_{inh}}{W_0}}\times 100\%$$

(4)

where ΔW₀ and ΔW_inh are the average weight losses for the blank and inhibited solutions.

2.4. Studies on Electrochemistry

A Gamry Reference 3000^TM potentiostat/galvanostat/ZRA, Warminster, PA, USA was used to perform electrochemical experiments. A typical double-jacketed glass cell with three electrodes was used. The CS rod with a 0.5 cm² exposed geometric area was used as the working electrode; a graphite rod was used as the counter electrode; and a saturated calomel electrode (SCE) was used as the reference. A 1.0 M HCl solution with and without different doses of the Q-22 inhibitor (41, 69, 138, and 277 µmol L⁻¹) served as the electrolyte. A Julabo thermostatic water circulator was used to keep the cell temperature at the specified level (20, 30, 40, or 50 °C). The working electrode was submerged before every test until a stable open-circuit potential (OCP) was reached, which required approximately 20 min.

At the OCP, electrochemical impedance spectroscopy (EIS) was carried out using a sinusoidal potential perturbation with an amplitude of 10 mV RMS throughout a frequency range of 100 kHz to 0.1 Hz (10 points per decade). Following the EIS measurements, Potentiodynamic Polarization (PDP) scans were obtained. With a scan rate of 0.167 mV s⁻¹, the potential was swept from −250 mV to +250 mV vs. OCP. To guarantee reproducibility, each experiment was carried out in triplicate.

2.5. Surface Characterization

Surface characterization was conducted to provide direct visual, topographical, and chemical evidence for the protective film formation indicated by the electrochemical and gravimetric data. The CS specimens were examined after 24 h of immersion in 1.0 M HCl, both in the absence (blank) and presence of the optimal concentration of Q-22. Identical sample preparation protocols were followed for all analyses to ensure a direct and consistent comparison.

2.5.1. X-Ray Photoelectron Spectroscopy (XPS)

Surface chemical analysis was conducted via X-ray photoelectron spectroscopy (XPS) using a Kratos Axis Ultra DLD spectrometer, Kratos Analytical Ltd., Wharfside, Manchester, UK.

This technique was employed to identify the elemental composition and chemical bonding states, specifically to verify the adsorption of the Q-22 inhibitor on the CS surface. All XPS spectra were charge-corrected by referencing the adventitious carbon C 1s peak to 284.8 eV.

2.5.2. SEM and AFM

The surface morphological and chemical composition of the CS specimens were examined after 24 h of immersion in 1.0 M HCl solution, both without inhibitor (blank) and with the optimal concentration of Q-22. The analysis was performed to visually corroborate the protective film formation indicated by the electrochemical and weight-loss data. High-resolution micrographs were obtained using a Field Emission Scanning Electron Microscope (FE-SEM, NOVA NANOSEM 450, FEI, Hillsboro, OR, USA). Furthermore, the surface topography and roughness at the nanoscale were quantitatively assessed using an Atomic Force Microscope (AFM, MFP-3D, Asylum Research, CA, USA) operating in tapping mode.

2.5.3. Surface Wettability (Contact Angle) Measurements:

Surface wettability, an indicator of hydrophobicity, was evaluated using an automatic tensiometer (Easy Drop D04010, KRÜSS GmbH, Hamburg, Germany) at 20 °C. The static water contact angle was measured to quantify the change in surface hydrophobicity induced by the inhibitor film.

2.6. Quantum Chemical Studies

2.6.1. Theoretical Computations

Density Functional Theory (DFT) calculations were performed to determine the molecular parameters of Q-22. Structural visualization and vibrational frequency analyses were carried out using GaussView 5.0.9. All quantum chemical calculations were conducted with the B3LYP hybrid functional and the 6–311++G(d,p) basis set using the Gaussian 09 package [22]. Global reactivity descriptors, including E_HOMO, E_LUMO, energy gap (ΔE), dipole moment (μ), ionization potential (I), electron affinity (χ), electronegativity (ϕ), chemical hardness (ψ), softness (S), electrophilicity (ω), nucleophilicity (ε), electro-accepting (ω⁺) and electro-donating (ω⁻) powers, back-donation energy (ΔE), fraction of electron transfer (ΔN), and inhibitor–metal interaction energy (ΔEFe/Q-22), were calculated. Local reactivity was evaluated using Fukui functions [23,24,25], while Natural Bond Orbital (NBO) analysis was employed to examine charge distribution, orbital interactions, and their influence on molecular stability.

2.6.2. Molecular Simulation (MD) Dynamics

The adsorption behavior of Q-22 on the Fe (111) surface was investigated using molecular dynamics (MD) simulations. The metal surface was modeled as a solvent-free vacuum slab under periodic boundary conditions, with a ten-layer Fe (111) supercell and a 30 Å vacuum to prevent interactions between periodic images (simulation box: 24.32 × 24.32 × 9.10 ų). After energy minimization of both the Q-22 molecule and the Fe surface, the inhibitor was adsorbed, and simulations were performed using the COMPASS force field within the Discover module. Adsorption strength was evaluated through interaction and binding energies, while radial distribution function (RDF) analysis was applied to characterize the nature of Q-22–Fe interactions.

2.6.3. NBO Analysis

The intra- and intermolecular interactions of Q-22 were examined using Natural Bond Orbital (NBO) analysis, specifically donor-acceptor interactions between filled (bonding or lone-pair) and empty (antibonding) orbitals. Second-order perturbation theory was used to determine the stabilization energies of these interactions (Equation (5)):

E⁽²⁾ = ΔE_ij = q_i (F(ij) 2/ε_j − ε_i)

(5)

where $q_i$ is the occupancy of the donor orbital, $F_{ij}$ is the off-diagonal Fock matrix element, and ${ϵ}_i$ and ${ϵ}_j$ are the orbital energies of the donor and acceptor, respectively. This analysis provides insight into the electron delocalization and the strength of interactions that govern Q-22 adsorption on the steel surface.

3. Results and Discussion

3.1. Gravimetric (Weight Loss) Measurements

The corrosion behavior of CS in 1.0 M HCl solution, in the absence and presence of varying concentrations of Q-22, was assessed using the weight loss technique. As depicted in Figure 1, the mass loss decreases progressively with increasing concentrations of Q-22, demonstrating the inhibitor’s effectiveness in retarding metal dissolution. This decline in weight loss reflects the ability of Q-22 molecules to adsorb onto the steel surface, thereby reducing the number of active sites available for the corrosion reaction and forming a protective barrier against the acidic medium.

The linear correlation observed between weight loss and immersion time for both inhibited and uninhibited systems suggests that the corrosion process proceeds uniformly and that no insoluble corrosion products accumulate on the surface during immersion. In such cases, inhibition occurs primarily through the adsorption of inhibitor molecules on the metal surface. These adsorbed species can either physically block anodic and cathodic reaction sites or modify the kinetics of the charge-transfer reactions occurring at the interface.

Data presented in

Table 3. Surface coverage (θ) and inhibition efficiency (IE%) for CS in 1.0 M HCl at 20 °C as a function of immersion time and varying concentrations of Q-22 inhibitor.

Concentration (µmol L−1)	Parameter	30 min	60 min	90 min	120 min	150 min	180 min
0	θ	—	—	—	—	—	—
	IE%	0	0	0	0	0	0
41	θ	0.7985 ± 0.02	0.7972 ± 0.02	0.7675 ± 0.02	0.7259 ± 0.01	0.7175 ± 0.01	0.7062 ± 0.01
	IE%	79.85 ± 1.6	79.72 ± 1.6	76.75 ± 1.5	72.59 ± 1.5	71.75 ± 1.4	70.61 ± 1.4
69	θ	0.8917 ± 0.01	0.8797 ± 0.01	0.8770 ± 0.01	0.8614 ± 0.01	0.8472 ± 0.01	0.8436 ± 0.01
	IE%	89.17 ± 1.3	87.97 ± 1.3	87.70 ± 1.3	86.14 ± 1.3	84.72 ± 1.3	84.36 ± 1.3
138	θ	0.9477 ± 0.01	0.9209 ± 0.01	0.9108 ± 0.01	0.9006 ± 0.01	0.8997 ± 0.01	0.8981 ± 0.01
	IE%	94.77 ± 0.95	92.09 ± 0.92	91.08 ± 0.91	90.06 ± 0.90	89.97 ± 0.90	89.81 ± 0.90
277	θ	0.9552 ± 0.005	0.9450 ± 0.005	0.9394 ± 0.005	0.9367 ± 0.005	0.9308 ± 0.005	0.9299 ± 0.005
	IE%	95.52 ± 0.48	94.50 ± 0.47	93.94 ± 0.47	93.67 ± 0.47	93.08 ± 0.47	92.99 ± 0.46

for surface coverage (θ) and inhibition efficiency (IE%) indicate that the fraction of the surface covered by the inhibitor increases with rising inhibitor concentration, and consequently, the inhibition efficiency (IE%) shows a corresponding enhancement.

As the Q-22 concentration increases from 41 to 277 μmol L⁻¹, the corrosion rate decreases significantly, while inhibition efficiency increases markedly. This trend is attributed to the cationic surfactant nature of Q-22, which promotes molecular adsorption through electrostatic interactions between the positively charged quaternary ammonium headgroups and the negatively charged steel surface. Additionally, the polar functional groups and hydrophilic moieties of Q-22 facilitate stronger anchoring and more uniform surface film formation. The adsorbed inhibitor film minimizes metal dissolution by blocking electrolyte access and suppressing both anodic and cathodic reactions [26].

Overall, the gravimetric results confirm that adsorption at the metal–solution interface is the predominant mechanism of corrosion inhibition, with efficiency directly linked to the degree of surface coverage by Q-22 molecules.

3.2. Electrochemical Measurements

3.2.1. Electrochemical Impedance Spectroscopy (EIS) Measurements

Nyquist and Bode plots for CS in the absence and presence of various Q-22 concentrations (41–277 µmol L⁻¹) at temperatures ranging from 20 to 50 °C are presented in Figure 2 and Figure 3. The Nyquist plots (Figure 2a–d) consistently display a single, depressed capacitive loop, the diameter of which increases significantly with increasing Q-22 concentration. The depression of the semicircle, a common feature attributed to surface inhomogeneity and roughness [27], was accounted for in the analysis by replacing an ideal capacitor with a constant phase element (CPE). The impedance spectra consistently feature a prominent capacitive loop, accompanied by a low-frequency inductive dispersion (inductive arc). This inductive feature is commonly ascribed to the relaxation of adsorbed intermediates (e.g., FeCl_ads or FeOH_ads) involved in the anodic dissolution process [28,29]. As is frequently noted for adsorbed intermediates in acidic corrosion systems, the low-frequency inductive loop was not included in the equivalent circuit fitting because of its weak contribution and instability, and to focus on the dominant charge-transfer parameters.

The corresponding Bode plots (Figure 3a–d) show a single, well-defined time constant, confirming that the corrosion process is governed by a single charge-transfer mechanism.

The experimental impedance data were fitted to the equivalent circuit model shown in Figure 4. This model represents the physical electrode–electrolyte interface, where the charge-transfer resistance (R_ct), the resistance between the metal and the outer Helmholtz plane that governs the corrosion rate, is connected in parallel with a CPE representing the non-ideal double layer, and both are in series with the solution resistance (R_s). The primary parameters derived from this fitting, R_ct and CPE, are summarized in

Table 4. Fitted EIS parameters, and inhibition efficiency for CS in 1.0 M HCl solution in the absence and presence of different concentrations of Q-22 inhibitor at (a) 20, (b) 30, (c) 40, and (d) 50 °C.

T (°C)	Cinh (µmol L−1)	Rct (Ω cm2)	Yο × 10−6 µΩ−1 sn cm−2	Cdl, µF	n	ϴ	%IE
20°	0 (Blank)	5.96 × 101	5.70 × 102	2.67 × 102	0.918	----	----
	41	2.71 × 102	3.48 × 101	5.86 × 101	0.884	0.7814	78.14
	69	4.59 × 102	8.67 × 101	3.46 × 101	0.806	0.8701	87.01
	138	9.34 × 102	5.68 × 101	1.70 × 101	0.808	0.9360	93.60
	277	2.10 × 103	5.84 × 101	7.57× 100	0.770	0.9710	97.00
30°	0 (Blank)	4.57 × 101	6.49 × 102	3.48 × 102	0.897	----	----
	41	1.92 × 102	2.57 × 102	8.25 × 101	0.861	0.7629	76.29
	69	3.16 × 102	1.06 × 102	5.03 × 101	0.832	0.8550	85.55
	138	6.40 × 102	9.82 × 101	2.48 × 101	0.799	0.9228	92.28
	277	1.25 × 103	6.50 × 101	1.26 × 101	0.808	0.9663	96.63
40°	0 (Blank)	3.07 × 101	7.09 × 102	5.18 × 102	0.893	----	----
	41	9.22 × 101	1.46 × 102	1.72 × 102	0.767	0.6672	66.72
	69	1.42 × 102	1.02 × 102	1.11 × 102	0.847	0.7845	78.45
	138	2.97 × 102	4.59 × 102	5.34 × 101	0.854	0.8967	89.67
	277	3.99 × 102	8.76 × 101	3.98 × 101	0.702	0.9223	92.23
50°	0 (Blank)	1.89 × 101	8.86 × 102	8.42 × 102	0.871	----	----
	41	4.86 × 101	1.81 × 102	3.27 × 102	0.843	0.6111	61.11
	69	6.68 × 101	2.62 × 102	2.38 × 102	0.826	0.7174	71.74
	138	8.96 × 101	2.14 × 102	1.77 × 102	0.800	0.7890	78.90
	277	1.27 × 102	1.54 × 102	1.25 × 102	0.821	0.8512	85.12

.

The surface coverage (θ) and the inhibition efficiency (IE%) were calculated from the charge-transfer resistance values using the following Equations (6) and (7):

$$\theta =\left(1-{\displaystyle \frac{R_{\mathrm{ct}}^o}{R_{\mathrm{ct}}}}\right)$$

(6)

$$\%\mathit{IE}=\left(1-{\displaystyle \frac{R_{\mathrm{ct}}^o}{R_{\mathrm{ct}}}}\right)\times 100$$

(7)

where $R_{\mathrm{ct}}^o$ and R_ct are the charge transfer resistances for uninhibited and inhibited solutions, respectively.

Analysis of the data reveals a clear trend: the value of R_ct increases markedly with increasing Q-22 concentration, while the effective double-layer capacitance (C_dl) decreases. The increase in R_ct signifies a higher energy barrier for the corrosion reaction, directly leading to the observed enhancement in inhibition efficiency (up to 97% at 277 µmol L⁻¹). The concurrent decrease in C_dl suggests the displacement of water molecules and aggressive ions from the interface by adsorbed Q-22 inhibitor molecules, leading to either an increase in the thickness of the protective layer or a decrease in the local dielectric constant [30]. This behavior is consistent with the adsorption of Q-22 molecules, forming an insulating barrier that effectively shields the CS surface from the corrosive HCl medium.

The high inhibition performance is attributed to the multifunctional molecular structure of Q-22, which features a quaternary ammonium center (N⁺) for strong electrostatic adsorption and a polar gluconate tail with multiple electron-donating oxygen atoms, facilitating robust surface coverage.

3.2.2. Potentiodynamic Polarization

Potentiodynamic polarization (PDP) curves for CS immersed in 1.0 M HCl solution, both without and with varying concentrations of the surfactant Q-22, measured at different temperatures (20, 30, 40, and 50 °C), are presented in Figure 5a–d. Key corrosion parameters derived from these curves, such as corrosion current density (i_corr), corrosion potential (E_corr), anodic and cathodic Tafel slopes (β_c and β_α), degree of surface coverage (θ), and inhibition efficiency (%IE), were all evaluated as a function of Q-22 concentration and solution ambient temperature, as summarized in

Table 5. Electrochemical parameters from potentiodynamic polarization for CS corrosion in 1.0 M HCl with varying Q-22 concentrations and temperatures.

T (°C)	Cinh µmol L−1	−Ecorr (mV) SCE	icorr (µA cm−2)	βa, mV decade−1	βc, mV decade−1	Rp, Ω cm2	CR, mpy	θ	IE%
20	Blank	407	439	170.9	224.8	96.15	401.4	----	----
	41	409	95.1	107.6	164.1	296.73	43.47	0.781	78.14
	69	453	50.2	80.5	110.3	402.53	22.93	0.890	89.01
	138	449	29	45.6	79.3	433.49	13.24	0.936	93.62
	277	487	13.1	34.8	60.8	733.61	7.381	0.972	97.20
30	Blank	405	780	229.7	283.7	70.75	713.2	----	----
	41	441	209	77.6	110	94.53	95.41	0.732	73.29
	69	408	111	50.8	120.7	139.86	50.5	0.865	86.55
	138	409	76.1	87.8	146.1	312.92	34.78	0.902	90.28
	277	411	36.1	59.4	111.5	466.14	16.51	0.956	95.63
40	Blank	396	1187	362.8	332.5	63.54	1085	----	----
	41	427	401.0	65.3	126.7	46.66	183.4	0.667	66.72
	69	441	265	86.7	142.8	88.39	121.1	0.764	76.45
	138	433	175	99.3	97.1	121.81	80.07	0.856	85.67
	277	408	101	50	109.0	147.36	46.11	0.912	91.23
50	Blank	389	1443	371.5	361.1	55.17	1319	----	----
	41	482	600	128.9	237.6	60.48	274.3	0.581	58.11
	69	425	430	119.6	238.6	80.45	196.3	0.707	70.74
	138	426	384	124.2	220.6	89.85	175.6	0.739	73.90
	277	425	232	71.9	109.1	81.11	105.9	0.841	84.12

.

The polarization resistance ($R_p$) was calculated using the Stern–Geary Equation (8):

$$R_p={\displaystyle \frac{{\beta }_a{\beta }_c}{2.303i_{corr}({\beta }_a+{\beta }_c)}}$$

(8)

The inhibition efficiency (IE%) and surface coverage (θ) were determined from the corrosion current density ($i_{corr}$) (9):

$$IE\%=\mathrm{\theta }\times 100=={\displaystyle \frac{{\mathrm{i}}_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}1}-{\mathrm{i}}_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}2}}{{\mathrm{i}}_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}1}}}\times 100$$

(9)

where $i_{corr1}$ and $i_{corr2}$ are the corrosion current densities in the absence and presence of the inhibitor, respectively.

Analysis of the polarization curves and Table 5 reveals several key findings:

• The obtained cathodic and anodic polarization curves exhibit well-defined Tafel behavior. Addition of the investigated inhibitors to the corrosive acid medium decreases the anodic polarization current more than the cathodic one at all temperatures (mixed-type control) with a concomitant shift of E_corr toward more positive values. The pronounced positive shift in E_corr values and the greater suppression of the anodic branch classify Q-22 as a mixed-type inhibitor with predominant anodic behavior [31,32], which preferentially restrains the anodic reaction of CS corrosion in HCl solution [33]. A shift in E_corr greater than 85 mV is commonly used as a benchmark to categorize inhibitors as either anodic or cathodic [34]. Therefore, despite the anodic predominance, both anodic and cathodic reactions are inhibited.
• The corrosion current density (i_corr) for CS in 1.0 M HCl solution decreases progressively with increasing inhibitor concentration (Q-22), which indicates that the presence of these compounds retards the dissolution of CS in 1.0 M HCl solution, and the degree of inhibition depends on both concentration and temperature of the corrosive medium. A maximum inhibition efficiency of 97.21% was achieved at the optimal concentration of 277 µmol L⁻¹. This concentration-dependent behavior is characteristic of adsorption-controlled inhibitors forming a protective layer on the metal surface [35]. However, a temperature-dependent decrease in efficiency at fixed concentrations suggests partial thermal desorption of the inhibitor, compromising the protective layer [36]. The inhibition efficiencies calculated from potentiodynamic polarization closely correlate with gravimetric measurements.
• The slopes of the cathodic and anodic Tafel curves (βc and βa) are only slightly affected by increasing the concentration of the tested compound. This indicates that the fundamental corrosion reaction mechanism remains essentially unchanged. The fact that the βc values are slightly higher than βa suggests that the inhibitive action of this inhibitor is by blocking the active sites on the CS surface, forming a barrier film without changing the reaction kinetics, thereby reducing the effective surface area available for electrochemical reactions [37,38].

3.3. Adsorption Studies and Thermodynamic Isotherm

Adsorption isotherms describe the relationship between the concentration of an inhibitor in solution and the extent of its adsorption on a metal surface at constant temperature. These equations are generally expressed as (10) [39]:

f(θ,x) exp(−a,θ) = KC_inh

(10)

where f(θ, x) is the configurational factor depending on the physical model and assumptions of the isotherm, a is a molecular interaction constant independent of surface coverage (θ), and K is the equilibrium constant for the adsorption–desorption process. The standard free energy of adsorption (ΔG°_ads.) is related to K_ads through the following expression (11) [40]:

K_ads = 1/55.5 exp (−ΔG°_ads./RT)

(11)

Here, 55.5 represents the molar concentration of water in the solution, R (8.314 J mol⁻¹ K⁻¹) is the universal gas constant, and T is the absolute temperature in Kelvin.

To understand the adsorption mechanism of Q-22 molecules on the CS surface, adsorption isotherm calculations were conducted. The surface coverage (θ), calculated as θ = %IE/100 from polarization data, was used to fit the most appropriate adsorption model.

Various isotherms, Temkin, Freundlich, Frumkin, and Langmuir, were tested to determine the adsorption behavior. The experimental data showed the highest correlation coefficient (R²) with the Langmuir isotherm, indicating monolayer adsorption of Q-22 molecules on the CS surface. The Langmuir model is expressed as in Equation (12) [41]:

$$\mathrm{C}/\theta =1/K_{\mathrm{ads}}+\mathrm{C}$$

(12)

where C is the equilibrium inhibitor concentration (mol L⁻¹), θ is the fractional surface coverage, and K_ads is the adsorption equilibrium constant (L mol⁻¹). The parameter K_ads reflects the strength of interaction between the inhibitor and metal surface; a higher value signifies stronger adsorption and higher inhibition efficiency. For ideal Langmuir behavior characterized by monolayer adsorption on a homogeneous surface with no lateral interactions between adsorbed molecules, the slope of this plot should be unity (1.0) [42].

Figure 6 displays a Langmuir adsorption plot of C/θ vs. C_inh for the adsorbed organic chemical (Q-22) on the surface of CS in a 1.0 M HCl acid solution at 20, 30, 40, and 50 °C exhibit excellent linearity (R² > 0.99). Figure 6 and Figure 7 provide a linear relationship showing that the adsorption of the tested chemicals (Q-22) on the CS surface obeys Langmuir’s adsorption isotherm with minimal deviations. The slope value is equal to unity, and the intercept value is equal to 1/K_ads. This indicates that Q-22 forms a compact monolayer on the carbon steel surface with minimal lateral interactions between adsorbed molecules; a characteristic consistent with its high inhibition efficiency at relatively low concentrations.

Table 6. Parameters of the linear regression.

Temperature (K)	Kads (L mol−1)	(1/Kads L mol−1) (Intercept)	R2
293	84.3	1.88	0.999
303	74.1	1.93	0.999
313	50.4	1.95	0.999
323	41.4	2.02	0.996

demonstrates that K_ads values decrease with increasing temperature. This finding suggests that certain adsorbed inhibitor molecules are desorbed from the surface as a result of the temperature increase. In fact, it aligns with the suggested physisorption process [43]. The strength between the adsorbent and adsorbate is commonly indicated by K_ads. Higher K_ads values indicate more effective adsorption and, hence, more effective protection.

The standard free energy of adsorption (ΔG°_ads) and the equilibrium constant of adsorption (K_ads) are connected, as may be determined using Equation (11). However, the Van’t Hoff Equation (13) [44] could be used to compute the standard enthalpy of adsorption (ΔH⁰_ads):

$$ln{K}_{ads}=-{\displaystyle \frac{\Delta H_{ads}^o}{RT}}+{\displaystyle \frac{\Delta S_{ads}^o}{R}}$$

(13)

or,

$${\displaystyle \frac{dln{K}_{ads}}{dT}}={\displaystyle \frac{\Delta H_{ads}^o}{R{T}^2}}$$

A plot of lnK_ads versus 1/T (Figure 7) provides a linear fit, from which $\Delta H_{ads}^{°}$ can be determined from the slope (−$\Delta H_{ads}^{°}$/R). The intercept yields the entropy term ($\Delta S_{ads}^{°}$/R). Using these values listed in

Table 7. Standard thermodynamic parameters for the adsorption of Q-22 molecules on the carbon steel surface in 1.0 M HCl solution.

Temp, (K)	Langmuir Isotherm
	ΔG°ads. (kJ mol−1)	ΔH°ads. (kJ mol−1)	ΔS°ads. (J mol−1 K−1)
293	−37.4	−8.32	99.3
303	−38.5		99.2
313	−38.9		97.5
323	−39.8		98.4

, the standard entropy of adsorption ($\Delta S_{ads}^{°}$) was calculated from the Gibbs–Helmholtz relation (Equation (14)):

$$\Delta G_{ads}^o=\Delta H_{ads}^o-T\Delta S_{ads}^o$$

(14)

It is widely acknowledged that adsorption is classified as physisorption when $\Delta G_{ads}^{°}$ are up to −20 kJ mol⁻¹, indicating that the inhibitory effect is caused by the electrostatic interaction between charged molecules and charged metal. Chemisorption, associated with charge sharing or transfer between the inhibitor molecules and the metal surface to create a coordinated type of bond, is identified by values around −40 kJ mol⁻¹ or more negative [45]. In the present study, the calculated $\Delta G_{ads}^{°}$ are negative, ranging from −37.4 to −39.8 kJ mol⁻¹, indicating strong and spontaneous adsorption of Q-22. Mixed adsorption, also known as chemi/physisorption, has recently been found to have free energy change values between −28 and −38 kJ mol⁻¹ [46,47]. Therefore, the present results are consistent with a mixed adsorption mechanism [48,49,50]. Furthermore, the fact that %IE decreases as temperature rises indicates that physical adsorption plays a dominant role [51].

The negative sign of $\Delta H_{ads}^{°}$ (Table 7) confirms that the adsorption process is exothermic [44,52], while the positive $\Delta S_{ads}^{°}$ values indicate increased randomness at the metal–solution interface during adsorption. This entropy increase can be attributed to the displacement of water molecules from the steel surface by inhibitor molecules, leading to a more disordered interfacial region [53]. Consequently, the combined thermodynamic results affirm that Q-22 adsorption on carbon steel in 1.0 M HCl occurs spontaneously through a mixed physisorption–chemisorption mechanism, leading to the formation of a stable and protective adsorbed layer.

3.4. Corrosion Kinetic and Thermodynamic Studies

To better understand the inhibition mechanism of Q-22 on CS in 1.0 M HCl, kinetic and thermodynamic analyses were carried out. These evaluations provide insight into the energy barriers and molecular interactions involved in the corrosion and inhibition processes.

The apparent activation energy (E_a), enthalpy of activation (ΔH*), and entropy of activation (ΔS*) for carbon steel dissolution were determined in both uninhibited and inhibited media using the Arrhenius and transition-state models [54]. The temperature dependence of the corrosion rate, expressed through the corrosion current density (i_corr), was fitted to the Arrhenius relationship (15) and the transition-state Equation (16):

$$\log CR=\mathrm{l}\mathrm{o}\mathrm{g}A-{\displaystyle \frac{E_{\mathrm{a}}}{2.303RT}}$$

(15)

$$CR={\displaystyle \frac{RT}{Nh}}{exp}^{{\displaystyle \frac{∆S^{\ast }}{R}}}{\mathrm{e}\mathrm{x}\mathrm{p}}^{{\displaystyle \frac{-∆H^{\ast }}{RT}}}$$

(16)

where A is the frequency factor, h is Planck’s constant, N is Avogadro’s number, R is the universal gas constant, and T is the absolute temperature in K.

Linear plots of log i_corr vs. 1/T, Figure 8, and log (i_corr /T) vs. 1/T, Figure 9, produced straight lines whose slopes correspond to −E_a/2.303 R, and −ΔH*/2.303 R, respectively. The intercepts of these plots yield (2.303 log A) and (log (R/Nh) + ΔS*/2.303 R) for Arrhenius and transition-state equations. The resulting activation parameters (the apparent activation energy, E_a, activation entropies, ΔS*, and activation enthalpies, ΔH*) are summarized in

Table 8. Activation Energy (E_a), Enthalpy of activation (ΔH*), and Entropy of activation (ΔS*) for CS in 1.0 M HCl solution in the absence and presence of different concentrations of the Q-22 corrosion inhibitor.

Cinh (µmol L−1)	Ea (kJ mol−1)	ΔH* (kJ Mol−1)	ΔS* (J mol−1 K−1)
0	31.5	29.01	−209.354
41	48.8	46.23	−163.387
69	57.6	55.12	−138.662
138	67.6	65.05	−109.429
277	76.01	73.46	−87.493

. The values of activation energy (E_a) were consistently higher in the presence of surfactant Q-22 than in its absence, and E_a increased with inhibitor concentration. This trend indicates that the surfactant molecules create an energetic barrier by adsorbing onto the steel surface and blocking active corrosion sites, thereby impeding charge-transfer reactions [55].

The activation parameters ΔH* and ΔS* were further evaluated using the transition-state formulation, an alternative form of the Arrhenius equation [56].

In this expression, the corrosion rate (CR) depends on N (Avogadro’s number), R (the universal gas constant), h (Planck’s constant), and T (temperature in Kelvin). The parameters ΔH* and ΔS* represent the enthalpy and entropy of activation, respectively.

All systems exhibited positive ΔH* values, confirming that the corrosion of carbon steel in HCl is an endothermic process. Moreover, both ΔH* and ΔS* values were lower than the corresponding E_a, suggesting that the transition from reactants to the activated complex requires energy input and involves a more ordered configuration. Notably, the relatively high and negative values of ΔS* suggest that the transition from reactants to the activated complex involves a decrease in system randomness. This behavior implies that during the rate-determining step, the transition complex is more ordered, representing an association rather than a dissociation process [57,58].

The magnitude and sign of ΔS* further suggest that the corrosion process, especially in the presence of Q-22, involves the formation of a structured interface where inhibitor and water molecules organize around the carbon steel surface. Such ordering facilitates the adsorption of Q-22 molecules onto the metal surface, stabilizing the protective layer and minimizing the availability of active corrosion sites.

Moreover, as the concentration of Q-22 increases, both E_a and ΔH* rise, signifying that the inhibitor elevates the energy threshold required for corrosion reactions to occur. Concurrently, the increasingly negative ΔS* values indicate a greater degree of surface organization, corresponding to the development of a compact, adsorbed inhibitor film that effectively hinders both the anodic and cathodic processes.

Overall, the kinetic and thermodynamic analyses confirm that Q-22 acts as an efficient corrosion inhibitor for CS in 1.0 M HCl. It suppresses metal dissolution primarily through adsorption, forming a dense, thermally stable film that reduces reaction kinetics and alters the energetic landscape to favor surface passivation.

3.5. Surface Characterization

3.5.1. Scanning Electron Microscopy (SEM) Analysis

The surface morphology of CS was examined using scanning electron microscopy (SEM) to obtain direct visual evidence of corrosion damage and the protective effect of the inhibitor. Samples were analyzed in three states: pristine (unexposed), after 24 h immersion in uninhibited 1.0 M HCl, and after immersion in HCl containing 277 µmol L⁻¹ Q-22. Scanning Electron Microscopy (SEM) works by directing a focused beam of electrons over the surface of a sample. The interaction between the electrons and the atoms of the sample generates signals from secondary and backscattered electrons. These signals are then collected to create high-resolution images that offer detailed insights into the surface topography, microstructural changes, and the presence of corrosion products or protective films [59,60].

The SEM micrographs are presented in Figure 10A,B, which depict scanning electron micrographs of an unexposed specimen of CS, alongside a CS sample that was exposed for 24 h to a blank 1.0 M HCl solution. Figure 10C displays scanning electron micrographs of CS specimens that were exposed for 24 h to a 1.0 M HCl solution containing 277 µmol L⁻¹ of Q-22.

Figure 10A illustrates that the surface of the pristine carbon steel (CS) metal is entirely free from any pits or cracks. In contrast, Figure 10B shows significant damage across the entire CS surface after being immersed in a 1.0 M HCl solution, characterized by uniform corrosion and extensive surface roughness and etching. Figure 10C indicates that, in the presence of 277 µmol L⁻¹ of the Q-22 inhibitor, the metal surface is largely covered by an adsorbed inhibitor layer. This treatment results in much less corrosion compared to the specimen exposed to the acid solution without the inhibitor, effectively reducing the formation of corrosion pits and impeding direct acid attack on the steel substrate.

Better results can be seen from the micrograph with the presence of 277 µmol L⁻¹ Q-22 depicted in Figure 10C. The improved surface morphology observed in the inhibited system corroborates the electrochemical and thermodynamic findings, providing direct microstructural evidence of the high inhibition efficiency of Q-22 through surface adsorption and barrier film formation.

3.5.2. AFM Analysis

Atomic Force Microscopy (AFM) was employed to quantitatively evaluate the surface topography and roughness of CS samples after 24 h of immersion at 20 °C, both in uninhibited 1.0 M HCl and in 1.0 M HCl containing 277 µmol L⁻¹ Q-22. AFM provides three-dimensional surface images and nanoscale roughness measurements, offering detailed insight into the microstructural changes caused by corrosion and the protective effect of inhibitors [59,61].

The root mean square (RMS) roughness was used as a key parameter to evaluate the extent of surface degradation and inhibitor performance. RMS values reflect surface irregularity: higher RMS indicates more severe corrosion, whereas lower RMS indicates a smoother, better-protected surface. The RMS values for the different samples are summarized in

Table 9. Surface roughness parameters derived from atomic force microscopy (AFM) analysis of carbon steel samples after 24 h immersion at 20 °C under different conditions.

No.	CS Sample	Roughness (Ra)/nm
A	Blank (Polished CS)	10
B	1.0 M HCl (Corroded CS)	200
C	1.0 M HCl + 277 µmol L−1 Q-22 (Inhibited CS)	30

, and representative AFM micrographs are shown in Figure 11A–C.

Figure 11A illustrates the three-dimensional surface topography of a blank CS specimen, whereas Figure 11B shows the atomic force micrograph of a CS specimen after immersion in 1.0 M HCl solution. Figure 11C shows the atomic force micrographs of a CS specimen in 1.0 M HCl solution containing (277 µmol L-1 Q-22) corrosion inhibitor, after 24 h immersion at 20 °C.

The surface morphology of the CS was severely corroded when exposed to an HCl solution without an inhibitor. This can be qualitatively seen from AFM micrographs, as there is the formation of deep holes and pits due to the aggressive chloride environment, giving a high RMS value of approximately 200 nm. In contrast, in the presence of 277 µmol L⁻¹ Q-22, the RMS value decreases dramatically to ~30 nm, reflecting a smoother, more homogeneous surface. This improvement is attributed to the formation of a compact protective film on the metal surface, effectively mitigating corrosion.

The observed reduction in RMS roughness, compared to the uninhibited CS surface, provides direct quantitative evidence for the adsorption of Q-22 molecules and the formation of a protective layer. This organic film acts as a physical barrier, impeding the diffusion of corrosive ions and preventing localized acid attack on the substrate [62,63].

These results confirm that Q-22 molecules strongly adsorb onto the CS surface, forming a compact and highly protective film. The significant decrease in RMS roughness from 200 nm to 30 nm provides direct, quantitative proof of the film’s efficacy in preventing corrosion.

3.5.3. XPS Analysis

High-resolution X-ray photoelectron spectroscopy (XPS) was employed to investigate the chemical composition and bonding characteristics of the protective film formed on the CS surface after immersion in 1.0 M HCl containing the Q-22 inhibitor. The survey spectra revealed the presence of characteristic peaks corresponding to Fe 2p, O 1s, C 1s, and N 1s, confirming the coexistence of metallic and organic constituents on the surface.

Figure 12 presents the XPS survey spectra of CS specimens immersed for 24 h in 1.0 M HCl, both without and with 277 µmol L⁻¹ Q-22. The C 1s spectrum (Figure 13a) of the Q-22–treated surface exhibits a dominant photoemission peak centered around 284.8 eV, which corresponds to C–C/C–H bonds commonly associated with the long alkyl chains of quaternary ammonium surfactants. Additionally, a distinct secondary peak appearing at approximately 286–287 eV is assigned to C–N bonding, confirming the incorporation of nitrogen-containing groups on the steel surface.

These observations demonstrate that the organic film detected on the surface originates from the adsorption of Q-22 molecules, verifying the formation of an inhibitor-derived protective layer rather than contamination from adventitious carbon species [63].

The N 1s region (Figure 13b) provides further insight into the adsorption mechanism. The Q-22 spectrum shows multiple components: a strong quaternary N⁺ peak near ~400 eV, together with a lower-binding-energy feature at ~398–399 eV assigned to Fe–N coordination, indicating nitrogen–metal interaction [64]. The presence and relative intensity of this low-BE N component suggest that, in addition to electrostatic adsorption via its positively charged headgroup, Q-22 undergoes chemisorptive bonding with the carbon–steel surface through partial electron donation from nitrogen (and possibly oxygen) atoms to vacant Fe d-orbitals. Similar Fe–N bonding behavior has been widely observed in studies of quaternary ammonium and nitrogen-based inhibitors [62,65,66].

This interpretation is further supported by the Fe 2p spectra presented in Figure 13c, which shows a significant reduction in the intensity of metallic iron peaks for the inhibited sample compared to the blank surface. The Fe 2p₃/₂ and Fe 2p₁/₂ components display diminished intensity and slight positive shifts in binding energy, indicating the formation of a Q-22–metal interfacial complex. These spectral changes suggest that the inhibitor molecules strongly interact with surface Fe atoms, modifying their electronic environment. The marked suppression of the Fe⁰ signal, together with the reduced contribution of iron oxide species, implies the development of a compact organic protective layer that effectively isolates the metal substrate from the aggressive acidic medium. This observation confirms that Q-22 promotes efficient surface passivation through the formation of a stable and adherent film [65,67].

Overall, the XPS results confirm that Q-22 adsorbs onto the carbon–steel surface via a mixed physical–chemical mechanism. The quaternary ammonium headgroup anchors electrostatically to the negatively charged, chloride-covered surface, while the nitrogen atoms form coordinate (Fe–N) bonds, enhancing the film’s stability. Simultaneously, the long hydrophobic alkyl chains orient outward, generating a compact, low-permeability barrier against acid attack. This dual mechanism—electrostatic attraction followed by chemisorptive stabilization—results in a strongly bound, uniform, and corrosion-resistant film [61,62].

These XPS surface-chemical insights are fully consistent with the electrochemical and weight-loss measurements. The formation of a thicker and more cohesive film of Q-22 enhances the charge-transfer resistance (R_ct) observed in EIS, enhances the polarization resistance (R_p), decreases the corrosion current density (i_corr) in Tafel measurement, and reduces weight loss, collectively confirming the inhibitor’s superior performance. Furthermore, analysis of the adsorption behavior using the Langmuir isotherm reveals that Q-22 has a high adsorption equilibrium constant (K_ads) and a more negative ΔG°_ads, indicating strong and spontaneous adsorption, consistent with the chemisorption characteristics inferred from the N 1s spectrum [59,65].

3.5.4. Surface Wettability Analysis (Contact Angle)

The protective film formed by Q-22 was further evaluated by investigating its effect on the CS surface’s wettability through contact angle measurements. Surface wettability is a key factor in corrosion resistance, as a hydrophobic surface can act as an effective barrier against an aqueous corrosive medium by reducing electrolyte contact [68].

Figure 14a shows the water contact angle for a freshly polished CS sample, which was ~63°, indicating a moderately hydrophilic surface. After immersion in the aggressive 1.0 M HCl solution without an inhibitor, the surface became highly hydrophilic, with the contact angle drastically decreasing to ~34° Figure 14b. This change is attributed to surface roughening and the formation of corrosion products, which increase the surface’s affinity for water and facilitate wetting.

In contrast, the surface inhibited with 277 µmol L⁻¹ of Q-22 exhibited a strongly hydrophobic character, with a contact angle of ~101° Figure 14c. This remarkable increase provides direct, quantitative evidence that Q-22 molecules form a uniform and hydrophobic film on the steel surface [69,70]. Effective surface hydrophobization is confirmed by this value of ~101°, which is comparable to or exceeds those reported for effective hydrophobic inhibitor coatings on carbon steel (~85–100°). The adsorption of Q-22, with its long alkyl chain, displaces water molecules and creates a non-polar barrier. This hydrophobic layer physically impedes contact and penetration by the aqueous corrosive electrolyte, thereby significantly enhancing the corrosion-inhibition efficiency [71]. Additionally, similar contact angle values have been reported for steel surfaces protected by effective organic inhibitors and surfactants in acidic environments [70], suggesting the formation of a compact, water-repellent protective coating [72]. This result is consistent with the protective film observed in SEM and AFM micrographs, which provides a functional property (hydrophobicity) that directly accounts for the superior electrochemical performance of Q-22.

3.6. DFT Studies

Density Functional Theory (DFT) was employed to investigate the electronic reactivity and inhibition performance of Q-22 and its protonated form (Q-22H⁺). Quantum chemical descriptors, including E_HOMO, E_LUMO, energy gap (ΔE), dipole moment (μ), ionization potential (I), electron affinity (A), electronegativity (χ), hardness (η), softness (S), electrophilicity (ω), nucleophilicity (ε), electro-accepting (ω⁺) and electro-donating (ω⁻) powers, back-donation energy (ΔE_back), fraction of electron transfer (ΔN), and inhibitor–metal interaction energy, were calculated and summarized in

Table 10. Computed quantum chemical descriptors for the studied neutral Q-22 and protonated Q-22 H⁺ inhibitor in the gas phase.

Descriptors	Equations	Q-22	Q-22 H+
Energy of highest occupied molecular orbital (EHOMO), (eV)		−6.465	−6.370
Energy of lowest unoccupied molecular orbital (ELUMO), (eV)		−1.052	−0.8587
Energy Gap ΔE	(LUMO-HOMO)	5.413	5.511
Dipole moment, (µ), (Debye)		16.401	17.146
Ionization energy (I) (ev)	$I={-E}_{HOMO}$	6.465	6.370
$\mathrm{Electron}\mathrm{affinity}(Y$) (ev)	$Y={-E}_{LUMO}$	1.052	0.8587
Electronegativity (ϕ)	$\varphi ={\displaystyle \frac{I+Y}{2}}$	3.758	3.614
Global hardness ψ	$\psi ={\displaystyle \frac{I-Y}{2}}$	2.706	2.756
Global softness (S)	$\mathrm{s}={\displaystyle \frac{1}{\psi }}$	0.370	0.363
Global electrophilicity (ω)	ω = ϕ2/2ψ	2.609	2.370
Global nucleophilicity (ε)	$\mathsf{\epsilon }={\displaystyle \frac{1}{\mathsf{\omega }}}$	0.383	0.422
Electroaccepting (ω+) power	${\omega }^{+}={\displaystyle \frac{{\left(I+3A\right)}^2}{16\left(I-A\right)}}$	1.068	0.9076
Electrodonating (ω−) power	${\omega }^{-}={\displaystyle \frac{{\left(A+3I\right)}^2}{16\left(I-A\right)}}$	4.827	4.522
Net electrophilicity (Δω± = ω+ + ω−)	(Δω± = ω+ + ω−)	5.895	5.430
Fraction of transferred electrons (ΔN)	$∆N={\displaystyle \frac{{\varphi }_{\mathrm{F}\mathrm{e}}-{\varphi }_{\mathrm{i}\mathrm{n}\mathrm{h}}}{2({\psi }_{\mathrm{F}\mathrm{e}}+{\psi }_{\mathrm{i}\mathrm{n}\mathrm{h}})}}$	−0.1009	−0.1197
Back-donation energy ΔE back-donation (ev)	${\mathsf{\Delta }\mathrm{E}}_{\mathrm{b}\mathrm{a}\mathrm{c}\mathrm{k}-\mathrm{d}\mathrm{o}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}=-{\displaystyle \frac{\psi }{4}}$	−0.6765	−0.6890
Metal/inhibitor interaction energy ΔEMetal/inhibitor (ev)	${\mathsf{\Delta }\mathrm{E}}_{steel/\mathrm{Q}-22}={\displaystyle \frac{{({\chi }_{Fe}-{\chi }_{inh)}}^2}{4({\eta }_{Fe}+{\eta }_{inh})}}$	0.0357	0.0509

and

Table 11. Fukui indices of the studied Q-22 and Q-22 H⁺.

Q-22				Q-22H+
Atoms	${\mathit{f}}_{\mathit{k}}^{+}$	${\mathit{f}}_{\mathit{k}}^{-}$	$∆{\mathit{f}}_{\mathit{k}}$	Atoms	${\mathit{f}}_{\mathit{k}}^{+}$	${\mathit{f}}_{\mathit{k}}^{-}$	$∆{\mathit{f}}_{\mathit{k}}$
Cl (1)	0.022	0.003	0.019	Cl (1)	0.01	0.002	0.008
O (2)	0.028	0.001	0.027	O (2)	0.024	0.016	0.008
N (4)	0.003	0.008	−0.005	N (4)	0	0.001	−0.001
C (5)	0.014	0.005	0.009	C (5)	0.016	0.01	0.006
C (6)	0.015	0.001	0.014	C (6)	0.014	−0.001	0.015
C (7)	0.01	−0.01	0.02	C (7)	0.004	0.007	−0.003
N (9)	0.015	−0.002	0.017	N (9)	0.004	0	0.004
C (10)	0.024	−0.001	0.025	C (10)	0.023	0.008	0.015
C (11)	0.012	0.004	0.008	C (11)	0.004	0.006	−0.002
C (13)	0.021	0.003	0.018	C (13)	0.019	0	0.019
O (14)	0.01	0.015	−0.005	O (14)	0.014	0.004	0.01
O (15)	0.032	−0.002	0.034	O (15)	0.033	0.003	0.03
C (16)	−0.002	0.011	−0.013	C (16)	−0.004	−0.004	0
O (17)	−0.002	0.089	−0.091	O (17)	−0.001	−0.002	0.001
O (18)	−0.003	0.065	−0.068	O (18)	0.007	−0.009	0.016
C (19)	0.005	0.041	−0.036	C (19)	0.014	0	0.014
O (22)	0.001	0.061	−0.06	O (22)	0.01	0.005	0.005
O (23)	0.005	0.038	−0.033	O (23)	0.009	0.005	0.004
C (27)	0.001	0.004	−0.003	C (27)	−0.007	−0.004	−0.003
C (28)	0.001	0.005	−0.004	C (28)	0	0	0
C (34)	−0.001	0.01	−0.011	C (34)	−0.007	−0.002	−0.005
C (35)	0.002	0.028	−0.026	C (35)	0	−0.002	0.002
C (36)	0.001	0.022	−0.021	C (36)	0.004	−0.001	0.005
C (38)	0.019	0.009	0.01	C (38)	0.013	0.001	0.012
C (39)	0.01	−0.001	0.011	C (39)	−0.003	0.023	−0.026
C (40)	0.019	0.001	0.018	C (40)	0.004	0.014	−0.01
C (41)	0.022	0.002	0.02	C (41)	0.005	0.012	−0.007
C (42)	0.019	0	0.019	C (42)	0.003	0.013	−0.01
C (43)	0.022	0.003	0.019	C (43)	0.003	0.013	−0.01
C (45)	0.037	0.005	0.032	C (45)	0.012	0.027	−0.015
C (46)	0.003	−0.003	0.006	C (46)	−0.004	0.029	−0.033
C (47)	0.006	0.004	0.002	C (47)	0.004	0.014	−0.01
C (48)	0.01	−0.001	0.011	C (48)	0.003	0.016	−0.013
C (49)	0.009	0.008	0.001	C (49)	0.002	0.015	−0.013
C (50)	0.012	0.004	0.008	C (50)	0.003	0.015	−0.012
C (51)	0.018	0.01	0.008	C (51)	0.008	0.032	−0.024

(see Figure 15 and Figure 16). The HOMO is primarily localized on aromatic rings and heteroatoms, identifying these as preferred electron-donating sites, which is consistent with the enhanced inhibition efficiency and experimental observations [73,74,75,76,77]. Lower ΔE values, reduced dipole moment (μ), and lower electronegativity (χ) indicate higher chemical reactivity, while an increased softness suggests effective electron donation to the carbon steel (CS) surface. Protonation further increases softness and surface affinity, indicating that Q-22H⁺ provides enhanced surface adherence and protection compared to the neutral Q-22 molecule [78,79].

Local reactivity was evaluated using Fukui functions (Equations (17)–(19)) [24,25], with nucleophilic (f⁺) and electrophilic (f⁻) attack sites identified in Table 11 and

Table 12. Second-order perturbation theory analysis of Fock Matrix in NBO Basis of Q-22 compound at B3LYP/6-311++G (d, p) level in the gas phase.

Compound	Donor	Acceptor	E(2) a (kcal/mol)	Occupancy
Q-22	πC3–O13	π*C25–O11	23.52	1.77
	πC4–O15	π*C26–O12	25.13	1.65
	πC5–O14	π*C5–O14	33.52	1.67
	πC10–O11	π*C4–O15	35.13	1.66
	πC9–O12	π*C9–O20	44.52	1.68
	πC1–O20	π*C1–O28	36.13	1.75
	πC25–O28	π*C27–O13	55.05	1.55
	LP (1) N22	π*Cl51–N24	26.74	1.87
	LP (1) N24	π*C16–N22	65.21	1.45
	πC6–N22	π*C24–N24	36.58	1.82
	πC26–N24	π*O14–C6	16.47	1.94
	LP (1) O11	π*O23–O15	31.04	1.84
	LP (1) O12	π*C13–O26	59.83	1.75
	LP (1) O13	π*C32–O15	18.81	1.88
	LP (1) O14	π*C10–O24	60.05	1.70
	LP (1) O15	π*O14–N22	26.74	1.57
	LP (3) Cl51	π*Cl51–H50	45.21	1.35
	LP (2) O20	π*C10–C7	36.58	1.82
	LP (2) O28	π*O15–C6	26.47	1.94

^a Stabilization energy.

.

f⁺_k = q_k(N + 1) − q_k(N)   (nucleophilic attack)

(17)

f⁻_k = q_k(N) − q_k(N − 1)   (electrophilic attack)

(18)

Δf_k = f⁺_k − f⁻_k

(19)

where q_k(N),q_k(N + 1), and q_k(N − 1) are the charge values of atoms k for neutral, anionic, and cationic, respectively. Δfk is the dual descriptor that Morell et al. presented, which is calculated as the difference between the nucleophilic and electrophilic Fukui functions [80,81].

In neutral Q-22, O15, C41, and C45 act as nucleophilic centers, whereas O17, O18, C19, O22, O23, and C41 function as electrophilic sites. Protonation significantly alters the active sites, generating O15 and Cl1 as nucleophilic sites and C49–C51 as electrophilic centers in Q-22H⁺. Molecular electrostatic potential (MEP) and electrostatic potential (ESP) maps (Figure 17) confirm that regions surrounding heteroatoms (N, Cl, O) are electron-rich (red/yellow, nucleophilic). Conversely, aromatic rings exhibit electron-deficient character (blue, electrophilic), supporting the proposed adsorption mechanism on the CS surface [82,83].

Molecular electrostatic potential (MESP) analysis can be utilized to identify electrophilic (electron-rich region) and nucleophilic (electron-poor region) reactive sites. In the MESP maps, red and blue regions correspond to electron-rich and electron-poor areas, respectively, while green regions indicate near-neutral electrostatic potential. Since binding sites generally arise from regions of opposite electrostatic potential, the electrostatic potential distribution plays a critical role in molecular adsorption at receptor or metal surfaces. Figure 17 presents the MESP map of the investigated compound (neutral Q-22 and protonated Q-22 H⁺), generated at optimized geometries using GaussView software, with potential values ranging from −6.346 × 10⁻² (red) to +6.346 × 10⁻² (blue). The most pronounced negative potential region, local around the oxygen atom and nitrogen atom are clearly visible in the MESP of the molecule as a yellowish-red zone, indicating preferred sites for electrophilic interaction. Hydrogen atoms H21, H19, H23, and H32 exhibit the highest positive potential charge, whereas the remaining molecular framework displays near-neutral electrostatic potential, suggesting a balanced charge distribution favorable for surface adsorption.

3.6.1. Natural Bond Orbital (NBO) Analysis

Natural Bond Orbital (NBO) analysis was performed to investigate charge transfer and electronic stabilization in Q-22. Second-order perturbation energies (E²) quantify electron delocalization from filled orbitals, including bonding (BD) and lone pairs (LP), to empty antibonding (BD*) or Rydberg (RY*) orbitals, providing insight into hyperconjugation, resonance, and non-covalent interactions that stabilize the molecule [84,85,86,87,88]. The most significant contributions (Table 12) arise from LP(1) N24 → πN22–C16 (65.21 kcal·mol⁻¹), LP(1) O14 → πC10–O24 (60.05 kcal·mol⁻¹), LP(1) O12 → πC13–O26 (59.83 kcal·mol⁻¹), and πC25–O28 → πC27–O13 (55.05 kcal·mol⁻¹), with additional stabilization from N24 → πCl51–N22 (13.19 kcal·mol⁻¹). These results highlight the key role of nitrogen, oxygen, and chlorine lone pairs in hyper conjugation and electron delocalization, supporting strong inhibitor–metal interactions. Overall, the high E² values indicate substantial electronic stabilization of Q-22, consistent with its observed corrosion inhibition efficiency and experimental PDP, EIS, and surface analysis results.

3.6.2. Molecular Simulation Dynamics

The adsorption of Q-22 on the carbon steel (CS) surface was investigated using molecular dynamics (MD) simulations. The inhibitor was positioned on a solvent-free Fe (111) surface, and the interaction (E_interaction) and binding (E_binding) energies were calculated using Equations (20)–(21) [86], yielding −732.67 kcal·mol⁻¹ for both energies, indicating strong adsorption.

E_interaction = E_total − (E_{surface+solution} − E_inhibitor)

(20)

E_binding = −E_interaction

(21)

As shown in Figure 18, Q-22 adopts a nearly parallel orientation on the Fe surface, maximizing coverage through interactions involving aromatic π-electrons, nitrogen, oxygen, and chlorine atoms. Equilibrium parameters for Q-22 and Q-22H⁺ are listed in

Table 13. Molecular dynamic simulation outputs and descriptors of the studied neutral and protonated Q-22 molecule on the iron (111) surface.

Structures	Q-22	Q-22-H+
Total energy	157.26	360.54
Adsorption energy	−543.38	−269.28
Rigid adsorption energy	−452.06	−8.14
Deformation energy	−121.32	−253.14
dEad/dNi	−543.38	−269.28

, while adsorption energies are depicted in Figure 19. Radial distribution function (RDF) analysis (Figure 20) shows a dominant peak below 3.5 Å, confirming chemisorption, with additional peaks beyond 3.5 Å reflecting physisorption. These results demonstrate that Q-22 achieves strong and stable adsorption, facilitating effective electron donation and acceptance at active sites, consistent with theoretical predictions, MD simulations, and experimental surface analyses [79].

4. Conclusions

This comprehensive study demonstrates that Q-22 is an effective and potentially eco-benign corrosion inhibitor for CS in a 1.0 M HCl environment. Unlike previously reported quaternary ammonium inhibitors, Q-22’s oxygen-dense gluconate tail enables multi-dentate surface coordination, resulting in exceptional inhibition efficiency (97.2%) at a remarkably low concentration (277 µmol L⁻¹). The systematic comparison with Q-15 under identical conditions establishes clear structure–property relationships, demonstrating that strategic incorporation of oxygenated functional groups significantly enhances adsorption strength, film stability, and overall inhibition performance. The integration of gravimetric, electrochemical, surface morphological, and theoretical analyses leads to the following key conclusions:

• Q-22 functions as an efficient mixed-type corrosion inhibitor, achieving a maximum inhibition efficiency of 97.2% at an optimal concentration of 277 µmol L⁻¹. Its performance is concentration-dependent, with efficiency increasing as the inhibitor dosage rises.
• Electrochemical analyses confirm strong adsorption of Q-22 onto the CS surface, resulting in a substantial increase in charge transfer resistance (R_ct) and a marked reduction in double-layer capacitance (C_dl). These findings indicate the formation of a dense, adherent, and resistive protective film that effectively impedes ionic and molecular transport to the metal surface.
• Adsorption studies reveal that Q-22 adsorbs spontaneously in a monolayer, following the Langmuir adsorption isotherm. The calculated standard free energy of adsorption (ΔG° ≈ −37 kJ mol⁻¹) suggests that both physisorption and chemisorption contribute to the inhibition mechanism.
• Thermodynamic analysis reveals that the presence of Q-22 significantly increases the apparent activation energy (E_a) of the corrosion process, indicating that the inhibitor raises the energy barrier for metal dissolution, primarily through a physisorption–chemisorption adsorption mechanism.
• Direct visual evidence from SEM, AFM, and XPS analyses corroborates the electrochemical findings. The inhibited surface remains smooth and uniform, with a drastically reduced roughness (RMS = 30 nm) compared to the severely corroded, pitted surface in the blank acid (RMS = 200 nm), signifying the formation of a continuous and stable protective layer.
• Quantum chemical (DFT) and molecular modeling studies, including NBO and MD simulations, demonstrate that Q-22 possesses strong electron-donating and accepting capabilities, facilitating efficient adsorption on the metal surface and supporting the suggested inhibition mechanism.

In summary, Q-22 exhibits excellent inhibition efficiency for carbon steel corrosion in acidic environments through a spontaneous adsorption mechanism that produces a robust and adherent surface film. Its molecular characteristics and performance make it a promising green corrosion inhibitor in industrial applications.