SHIB protects low carbon steel in hydrochloric acid through adsorption controlled corrosion inhibition

Abstract

Sodium (E)-4-hydroxy-3-(((3-hydroxyphenazin-2-yl)imino)methyl)benzene sulfonate (SHIB) was synthesized and evaluated as a structurally distinctive phenazine salicylaldimine sulfonate inhibitor for low-carbon steel (LCS) in 0.5 mol L^-1 HCl. The study addresses the need for efficient acid-phase inhibitors that combine extended aromatic adsorption domains with heteroatom-rich donor sites and aqueous-phase compatibility. Weight-loss measurements, open-circuit potential monitoring, potentiodynamic polarization, electrochemical impedance spectroscopy, SEM, AFM, DFT calculations and Monte Carlo simulation were used to relate molecular structure to corrosion protection. SHIB showed a concentration-dependent response and reached 91.95% inhibition efficiency by weight loss, 92.25% by polarization and 92.80% by impedance at 300 ppm. The protective effect remained significant up to 328 K, although efficiency decreased at higher temperature, indicating partial thermal weakening of the adsorbed layer. Adsorption data were most consistent with a Langmuir-type relationship, and adsorption free energies in the range of − 27.76 to − 29.89 kJ mol^-1 were interpreted cautiously as evidence for spontaneous adsorption with dominant electrostatic contribution and partial donor-acceptor interaction. Polarization and impedance analyses indicate mixed-type corrosion suppression through surface blocking, increased charge-transfer resistance and decreased apparent double-layer capacitance. Surface microscopy confirmed reduced acid-induced damage, while theoretical calculations supported adsorption through the nitrogen, oxygen and aromatic π centers of SHIB. The results position SHIB as a promising laboratory-scale inhibitor candidate, while further work is required to assess wider temperature limits, long-term safety and field applicability.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-026-57768-9.

Introduction

Corrosion of low-carbon steel in acidic media remains a persistent problem in acid cleaning, pickling, stimulation and fluid-handling operations because the alloy combines low cost and attractive mechanical properties with a pronounced susceptibility to rapid dissolution in hydrochloric acid and related aggressive environments. In oil and gas service, the problem is further intensified by the simultaneous presence of chloride ions, dissolved gases such as CO₂ and H₂S, free water and temperature fluctuations, all of which can accelerate interfacial reactions and compromise asset integrity^1–3.

Among the available mitigation strategies, organic corrosion inhibitors remain especially attractive because they can be deployed at relatively low dosage and can suppress corrosion through adsorption-driven formation of a protective interfacial layer. Molecules containing aromatic π systems together with nitrogen- and oxygen-bearing functional groups are particularly important in this context because they can combine electrostatic interaction, donor–acceptor bonding and surface-film formation to reduce both anodic metal dissolution and cathodic hydrogen evolution^4–6.

The performance of such inhibitors is commonly interpreted through a combination of gravimetric, electrochemical, surface-analytical and theoretical methods. Weight-loss measurements provide direct evidence of total material loss, potentiodynamic polarization reveals the effect of the inhibitor on anodic and cathodic partial reactions, and electrochemical impedance spectroscopy probes the evolution of interfacial resistance and double-layer properties. When these methods are complemented by microscopy and molecular modeling, a more reliable picture emerges of adsorption strength, film persistence and the molecular origins of protection⁵.

In this work, attention is focused on sodium (E)-4-hydroxy-3-(((3-hydroxyphenazin-2-yl)imino)methyl)benzene sulfonate (SHIB), a phenazine salicylaldimine sulfonate Schiff base synthesized from 2-amino-3-hydroxyphenazine and sodium 2-hydroxybenzaldehyde-5-sulfonate. Unlike many conventional Schiff base inhibitors based mainly on simple aromatic imine frameworks, SHIB combines several adsorption-relevant structural features in one molecule. These include an extended fused phenazine π system, an azomethine linkage, phenolic oxygen atoms, nitrogen-containing sites and a sulfonate functionality. This molecular combination is expected to promote steel-surface interaction through π-electron participation, heteroatom-assisted donor-acceptor interaction, electrostatic contribution in acidic chloride medium and improved compatibility with the aqueous acid phase. Therefore, SHIB represents a structurally distinctive inhibitor candidate rather than a simple extension of common salicylaldimine-type Schiff bases^6,7.

Recent studies on aromatic and heteroatom-rich organic inhibitors have shown that efficient steel protection in acidic media is commonly associated with strong adsorption, extensive surface coverage and simultaneous suppression of anodic and cathodic corrosion reactions. The inhibitors selected for comparison in Table S1 were chosen because they provide structurally relevant benchmarks for adsorption-controlled protection. AHPC represents a heteroatom-rich aromatic inhibitor, cinnamaldehyde-derived systems illustrate the role of conjugated imine-type structures, and MPO provides an example of a π-conjugated organic inhibitor containing adsorption-active donor sites. These systems therefore help position SHIB within the broader class of aromatic and heteroatom-containing inhibitors used for acid corrosion protection⁸.

Accordingly, the novelty of the present study lies in the first systematic evaluation of SHIB as an adsorption-controlled corrosion inhibitor for low-carbon steel in 0.5 mol L^-1 HCl. The work integrates NMR-supported structural confirmation, weight-loss measurements, open-circuit potential monitoring, potentiodynamic polarization, electrochemical impedance spectroscopy, SEM, AFM, DFT calculations and Monte Carlo adsorption simulation to establish the relationship between SHIB molecular structure, interfacial adsorption, electrochemical response and surface protection. This integrated approach clarifies whether the phenazine salicylaldimine sulfonate scaffold can provide effective acid-phase protection and explains the molecular origin of its inhibition performance.

Experimental

Materials and solutions

Low-carbon steel (LCS) coupons were sourced and used with nominal composition C 0.20 wt%, Mn 0.60 wt%, Si 0.03 wt%, P 0.04 wt% and balance Fe. The specimens were mechanically abraded using successive SiC emery papers up to 1200 grit, rinsed thoroughly with double-distilled water, degreased with ethanol and air-dried before immersion. The same preparation protocol was applied before weight-loss, electrochemical and surface-characterization measurements to reduce surface-preparation variability.

All reagents were of analytical grade. The corrosive electrolyte was 0.5 mol L^-1 HCl freshly prepared from concentrated acid and double-distilled water. This acid concentration was selected as a moderately aggressive chloride medium relevant to acid cleaning, pickling and acidizing-related laboratory screening, while avoiding the excessively rapid attack that can occur at higher acid strengths and obscure adsorption-dependent inhibitor trends. Electrochemical tests employed a classical three-electrode configuration consisting of LCS as working electrode, platinum as counter electrode and a saturated calomel electrode as reference.

Synthesis of SHIB

The synthetic route to SHIB is summarized in Scheme 1.

Scheme 1.

Synthetic route to SHIB from 2-amino-3-hydroxyphenazine and sodium 2-hydroxybenzaldehyde-5-sulfonate in ethanol under reflux for 12 h.

Sodium (E)-4-hydroxy-3-(((3-hydroxyphenazin-2-yl)imino)methyl)benzene sulfonate (SHIB) was synthesized by condensing 2-amino-3-hydroxyphenazine (0.21 g, 1.0 mmol) with sodium 2-hydroxybenzaldehyde-5-sulfonate (0.23 g, 1.0 mmol) in absolute ethanol (30 mL) under reflux for 12 h. After cooling to room temperature, the formed precipitate was collected by filtration and washed with small portions of cold ethanol to remove unreacted starting materials and soluble impurities. The crude solid was then recrystallized from hot ethanol and dried to afford white crystals in 78% yield. Thus, ethanol served first as a washing solvent for the isolated precipitate and subsequently as a recrystallization solvent for purification.

Weight-loss method

Weight-loss (WL) experiments were performed from 298 to 328 K with inhibitor concentrations ranging from 50 to 300 ppm. Polished steel specimens were immersed in 100 mL portions of the acid solution for a fixed exposure period of 6 h, then removed, cleaned, dried and reweighed. Corrosion rate was calculated from k = ΔW/At, where ΔW is the weight loss, A is the exposed area and t is the immersion time. Each determination was carried out in triplicate, and the reported values represent mean results.

Inhibition efficiency was calculated from the relative change in weight loss between the blank and inhibited systems. Each measurement was repeated in triplicate and reported as the mean of the replicate determinations.

Electrochemical testing

Potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) were performed using a Gamry Reference 3000 potentiostat/galvanostat/ZRA, Gamry Instruments, USA, in a conventional three-electrode cell consisting of LCS as the working electrode, platinum as the counter electrode and a saturated calomel electrode as the reference electrode. Before each PDP or EIS measurement, the working electrode was allowed to stabilize at open circuit for 1800 s, as shown by the OCP-time curves in the Supplementary Information. The recorded OCP response was used to confirm sufficient potential stabilization before polarization and impedance testing. PDP scans were recorded at 298 K using a scan rate of 0.5 mV s^-1 over the selected potential range, and corrosion current densities were obtained by Tafel extrapolation of the linear anodic and cathodic regions.

EIS spectra were recorded at open circuit using a 5 mV sinusoidal perturbation over the frequency window 100 kHz to 50 Hz. The resulting spectra were represented in both Nyquist and Bode formats, and equivalent-circuit fitting was used to extract the interfacial parameters. The lower frequency limit is acknowledged as a methodological limitation because slower interfacial relaxation or diffusion-related features below 50 Hz may not be fully resolved.

Surface analysis

LCS specimens were mechanically polished, rinsed and then immersed in 0.5 mol L^-1 HCl for 48 h with and without inhibitor before SEM and AFM analysis. Surface morphology was examined using a JEOL JSM-6510LV scanning electron microscope, JEOL Ltd., Japan to visualize the extent of acid attack and the degree of surface coverage produced by SHIB. Atomic force microscopy was performed using a Bruker Dimension Icon atomic force microscope, Bruker, USA, and the resulting roughness descriptors were interpreted according to ISO 25178-2. Surface analysis was restricted to morphology and roughness; surface-sensitive chemical spectroscopy of the adsorbed film was not available for the present revision and is identified below as a limitation.

Quantum-chemical calculations

Quantum-chemical descriptors were obtained by DFT using the B3LYP/6–31 + G(d′,p′) level, and the resulting structures and orbitals were visualized in GaussView. Molecular-mechanics and Monte Carlo adsorption simulations were performed using BIOVIA Materials Studio. The SHIB molecule was first constructed and geometry-optimized using the Forcite module with the COMPASS force field. Atomic charges were assigned using the force-field-assigned charge scheme, and both electrostatic and van der Waals interactions were treated using the atom-based summation method. The calculation quality was set to Fine.

For adsorption simulation, an Fe(110) surface was constructed by cleaving the bulk iron crystal along the (110) plane using the Cleave Surface tool. The Fe(110) plane was selected as a stable and commonly used model surface for iron-based corrosion-inhibition studies. Monte Carlo simulations were then carried out using the Adsorption Locator module with the COMPASS force field to identify the most energetically favorable adsorption configuration of SHIB on the Fe(110) surface. The adsorption energy, rigid adsorption energy, deformation energy and equilibrium adsorption geometry were obtained from the lowest-energy configuration. This simplified Fe(110) model was used to support molecular-level interpretation of adsorption rather than to reproduce the full chemical complexity of industrial low-carbon steel.

Results and discussion

Structural characterization of SHIB

The successful formation of SHIB is supported by the synthetic route and by the combined NMR and FT-IR characterization data. Fig. S1 presents the experimental ¹H and ¹³C NMR spectra of SHIB, while the FT-IR spectrum of pure SHIB is provided in Fig. S2 in the Supplementary Information.

In the ¹H NMR spectrum, a deshielded phenolic resonance is observed near δ 9.43 ppm, while the azomethine proton appears near δ 8.81 ppm. The remaining aromatic resonances are distributed mainly across the δ 7.69–8.28 ppm region, consistent with the fused phenazine core and the substituted salicylaldimine fragment. In the ¹³C NMR spectrum, highly deshielded signals near δ 158.03–164.17 ppm are consistent with oxygenated and imine-adjacent carbons, whereas the remaining aromatic and conjugated carbons appear between approximately δ 111.13 and 143.43 ppm. These NMR features support successful Schiff-base formation rather than physical mixing of the starting materials.

The FT-IR spectrum of pure SHIB further supports the proposed structure. The broad absorption in the 3200–3500 cm^-1 region is assigned to phenolic O–H stretching. The band near 1618 cm^-1 is consistent with the azomethine C = N vibration together with aromatic skeletal contribution, while the bands around 1508 and 1477 cm^-1 are attributed to aromatic C = C/C = N vibrations. The absorptions near 1171 and 1039 cm^-1 are consistent with sulfonate S = O/S–O vibrations, supporting the presence of the sulfonate functionality. Additional bands below 900 cm^-1, including those near 827, 754 and 695 cm^-1, are assigned to aromatic C–H out-of-plane bending modes.

NMR and FT-IR data are consistent with the expected SHIB framework and support the formation of sodium (E)-4-hydroxy-3-(((3-hydroxyphenazin-2-yl)imino)methyl)benzene sulfonate.

Weight-loss measurements

Effect of SHIB concentration

The corrosion of LCS in hydrochloric acid proceeds through coupled anodic iron dissolution and cathodic hydrogen evolution, while dissolved oxygen in aerated solution may also contribute to cathodic reduction. These processes may be represented mainly as:

The presence of chloride ions significantly enhances the aggressiveness of the medium by facilitating adsorption onto the steel surface, disrupting any nascent oxide layer, and promoting localized corrosion through pit initiation. Consequently, the introduction of an effective inhibitor is essential to suppress these processes via surface adsorption and film formation^9–11.

Gravimetric (weight-loss) measurements were employed to quantify the corrosion behavior of LCS in 0.5 mol L^-1 HCl both in the absence and presence of SHIB at concentrations ranging from 50 to 300 ppm. This method provides a direct evaluation of material degradation over time and is widely regarded as a reliable approach for assessing inhibitor performance under conditions relevant to practical applications.

The corrosion rate (k), inhibition efficiency based on weight loss (IE w), and surface coverage (θ) were calculated using the following expressions:

1

where ΔW is the measured weight loss of the steel specimen, A is the exposed surface area, and t represents the immersion duration.

2

Wcorr and Winh denote the weight loss values recorded in uninhibited and inhibited solutions, respectively. The experimental data (Tables S2 and S3) indicate a strong dependence of corrosion inhibition performance on SHIB concentration. At 298 K, the addition of 50 ppm of SHIB results in an inhibition efficiency of 62.37%, reflecting partial surface coverage. Increasing the inhibitor concentration progressively enhances the protective effect, reaching a maximum efficiency of 91.95% at 300 ppm. This trend is accompanied by a corresponding decrease in corrosion rate, confirming the effectiveness of SHIB in mitigating steel dissolution in acidic media^12–15.

The improvement in inhibition performance with increasing concentration can be attributed to enhanced adsorption of SHIB molecules on the steel surface. At lower concentrations, adsorption occurs at isolated active sites, leaving portions of the surface exposed to the corrosive environment. As the concentration increases, a greater number of inhibitor molecules become available, leading to more extensive surface coverage and the formation of a coherent protective layer that limits both anodic and cathodic reactions.

To gain further insight into the adsorption behavior, the surface coverage values derived from weight-loss data were analyzed using the Langmuir adsorption isotherm. The linear relationship obtained by plotting C/θ against C (Fig. S3) suggests that the adsorption of SHIB follows Langmuir-type behavior, implying monolayer adsorption on energetically equivalent sites without significant interaction between adsorbed species. The near-unity slope and high correlation coefficient support the applicability of this model.

The inhibition performance of SHIB can be rationalized based on its molecular structure, which contains multiple adsorption-active centers, including heteroatoms (such as nitrogen and oxygen) and conjugated π-electron systems. These features facilitate strong interactions with the steel surface through donor–acceptor mechanisms, leading to the formation of a protective adsorbed film that acts as a barrier against corrosive species.

The monotonic rise in protection with concentration indicates progressive surface occupation by SHIB molecules. The corresponding reduction in corrosion rate from 155.56 × 10^-3 mg cm^-1 min^-1 for the blank to 12.52 × 10^-3 mg cm^-2 min^-1 at 300 ppm shows that the inhibitor does not merely delay attack but substantially suppresses total dissolution over the measurement window. Within the investigated range, 300 ppm can be considered the optimum tested concentration because it gives the highest efficiency in all three evaluation methods. Concentrations above 300 ppm were not examined; therefore, further addition could either produce only marginal improvement after near-saturation of adsorption sites or, at excessive dosage, alter interfacial packing and solution properties. The optimum concentration should therefore be considered valid only within the present 50–300 ppm screening window.

The effect of temperature

The influence of temperature on the corrosion behavior of LCS was examined over the 298–328 K range, as summarized in Table S2. In both the blank and SHIB-containing systems, the corrosion rate increased with increasing temperature, which is expected in strongly acidic media because higher thermal energy enhances the mobility of aggressive species, especially H⁺, and facilitates metal dissolution together with hydrogen evolution. Even so, SHIB maintained a pronounced inhibitive effect across the full temperature window. At 300 ppm, the inhibition efficiency decreased from 91.95% at 298 K to 81.94% at 328 K, showing only a moderate loss of performance despite the elevated thermal stress.

This gradual decline in efficiency suggests that the adsorbed SHIB layer is partly weakened at higher temperature, most likely because of partial desorption from the metal surface. At the same time, the inhibitor still preserved more than 80% protection at 328 K, which indicates that surface coverage remained substantial and that the adsorbed film was not completely disrupted. The present temperature-stability evidence is therefore operational rather than thermal-analytical: it shows that SHIB remains effective up to 328 K under the tested immersion conditions, but it does not define the decomposition temperature of the isolated inhibitor. Overall, the temperature response is consistent with an adsorption process in which electrostatic attachment contributes to initial surface coverage, while partial donor-acceptor interaction helps sustain interfacial protection as the system is heated^16–20.

Corrosion kinetics

The temperature dependence of the corrosion process was further analyzed using Arrhenius and transition-state formalisms in order to extract the relevant activation parameters governing steel dissolution in the absence and presence of SHIB. These approaches provide quantitative insight into the kinetic barrier associated with the corrosion reaction and the influence of inhibitor adsorption on the reaction pathway.

The Arrhenius relationship is expressed as:

3

while the transition-state equation is given by:

4

where k is the corrosion rate, A is the pre-exponential factor, E_a^* is the apparent activation energy, R is the universal gas constant, T is the absolute temperature, N is Avogadro’s number, and h is Planck’s constant. The thermodynamic activation parameters, namely the enthalpy of activation ΔH^* and entropy of activation ΔS^*, were obtained from the slope and intercept of the transition-state plots.

The Arrhenius plots (ln k vs. 1/T) for LCS in uninhibited and SHIB-containing solutions are presented in Fig. S5, exhibiting excellent linearity, which confirms the validity of the applied kinetic model. The calculated activation energy increases from 12.90 kJ mol^-1 for the blank solution to 24.70 kJ mol^-1 at 300 ppm SHIB, indicating that the presence of the inhibitor introduces an additional energy barrier to the corrosion process. This increase in Ea* reflects the adsorption of SHIB molecules on the steel surface, which hinders charge transfer and reduces the rate of metal dissolution^21–25.

Further insight is provided by the transition-state analysis (ln k/T vs. 1/T), from which ΔH^* and ΔS^* values were derived (Table S4). The positive values of ΔH^* confirm that the dissolution of steel in acidic media is an endothermic process, and the observed increase in ΔH^* in the presence of SHIB is consistent with the formation of an energy barrier associated with the adsorbed inhibitor layer. In parallel, the negative values of ΔS^* indicate a decrease in disorder during the formation of the activated complex, suggesting a more organized interfacial structure. This behavior is characteristic of the formation of an adsorbed inhibitor–metal ensemble, where the adsorption of SHIB molecules leads to increased ordering at the metal/solution interface²⁶.

These kinetic and thermodynamic findings are in good agreement with the weight-loss results and the observed temperature dependence of inhibition efficiency. The increase in activation energy and the nature of the thermodynamic parameters support an adsorption-controlled inhibition mechanism, in which SHIB forms a protective layer that impedes both anodic and cathodic reactions.

To complement the kinetic analysis, open-circuit potential (OCP) measurements were performed for LCS in 0.5 mol L^-1 HCl with and without SHIB. As illustrated in Fig. S9, the OCP values shift toward more positive potentials with increasing inhibitor concentration, indicating a reduction in corrosion activity and enhanced surface protection in the presence of SHIB.

Adsorption isotherm

The adsorption characteristics of SHIB on the surface of low-carbon steel (LCS) were evaluated using the surface coverage values (θ) derived from weight-loss measurements. Several adsorption models can be considered to describe inhibitor-metal interactions. Langmuir behavior was selected because the C/θ versus C plots provided the most direct linear relationship, with near-unity slopes and high correlation coefficients across the investigated temperature range, whereas alternative Freundlich and Temkin linearizations gave less direct physical interpretation and comparatively less consistent fitting behavior. The use of the Langmuir model therefore does not imply a perfectly ideal surface, but indicates that the experimental data are best represented by predominant monolayer-type adsorption within the concentration range studied.

The Langmuir isotherm is expressed as:

5

where C represents the inhibitor concentration, θ is the surface coverage and Kads is the equilibrium adsorption constant. The plots of C/θ versus C (Fig. S3) exhibit excellent linearity with correlation coefficients approaching unity across the investigated temperature range. The slopes are close to unity, with only slight deviations, which may be attributed to weak lateral interactions between adsorbed SHIB molecules on the steel surface. To verify this selection, Temkin and Freundlich linearizations were also examined. Langmuir gave the highest and most consistent correlation coefficients, with R² = 0.999 at 298, 308, 318 and 328 K, whereas Temkin gave R² = 0.988–0.992 and Freundlich gave R² = 0.989–0.993. Therefore, Langmuir was retained for the quantitative adsorption analysis(Fig. S4) (Table S11).

The adsorption equilibrium constant was determined from the intercepts of the linear plots and the corresponding values are summarized in Table S5. The relatively high values indicate appreciable interaction between SHIB molecules and the LCS surface, leading to the formation of a protective layer. The temperature-dependent variation in Kads reflects the balance between adsorption and desorption under heated conditions. The persistence of appreciable Kads values even at elevated temperature confirms that adsorption remains significant under these conditions²⁷.

The standard free energy of adsorption was calculated using the relation:

6

where R is the universal gas constant and T is the absolute temperature. The calculated ΔG°ads values for SHIB fall within the range of − 27.76 to − 29.89 kJ mol^-1 (Table S5). The negative sign confirms that the adsorption process is spontaneous and thermodynamically favorable over the entire temperature range studied.

The magnitude of ΔG°ads provides insight into the nature of the adsorption mechanism. Values around − 20 kJ mol^-1 or less negative are typically associated with physisorption, while values more negative than − 40 kJ mol^-1 indicate stronger chemisorption involving electronic interaction. The intermediate values obtained for SHIB should therefore be interpreted cautiously as being consistent with mixed adsorption, rather than as direct proof of a purely chemical bond. Under acidic chloride conditions, electrostatic attraction involving protonated SHIB and adsorbed chloride can support physical adsorption, while the nitrogen, oxygen and π-electron centers may contribute partial donor-acceptor interaction with iron sites.

Accordingly, the adsorption of SHIB on LCS can be described as a cooperative process in which electrostatic physisorption contributes to initial surface attachment, while partial chemisorptive interaction enhances the stability of the adsorbed layer. This interpretation is consistent with the high inhibition efficiencies observed, the intermediate ΔG°ads values and the ability of SHIB to maintain substantial surface coverage at 328 K, although further surface-sensitive spectroscopic evidence would be required to quantify the relative contribution of each interaction mode²⁸.

Potentiodynamic polarization (PDP) measurements

The influence of SHIB on the anodic and cathodic kinetics of LCS in 0.5 mol L^-1 HCl was examined by potentiodynamic polarization at 25 °C. The overlaid curves in Fig. S6 show a general decrease in current density as the inhibitor concentration increases from 50 to 300 ppm, demonstrating progressive suppression of interfacial dissolution and the cathodic reaction. The visual separation of the branches at 50 ppm is less distinct than at higher concentrations; therefore, the low-concentration PDP efficiency should be interpreted cautiously and mainly as part of the overall concentration-dependent trend confirmed by weight-loss and EIS data.

Fig. S6 shows that the polarization branches shift to lower current values with increasing SHIB concentration, while the displacement in corrosion potential remains within the commonly accepted ± 85 mV range. This behavior indicates that SHIB functions as a mixed-type inhibitor rather than acting predominantly on only one of the partial reactions. In other words, the inhibitor modifies both the anodic iron-dissolution pathway and the cathodic reduction process to a comparable extent.

The inhibition efficiency from the PDP measurements was calculated using:

7

where Icorr and Icorr, inh are the corrosion current densities in the absence and presence of SHIB, respectively. The values summarized in Table S6 show a steady improvement in protection with increasing concentration, reaching 92.25 ± 0.17% at 300 ppm. The agreement between PDP, weight-loss and EIS efficiencies supports the reliability of the high-dose inhibition trend, whereas the 50 ppm PDP value is treated with greater caution because of the weaker visual branch definition at this concentration.

The modest changes in the anodic and cathodic Tafel slopes suggest that SHIB does not fundamentally alter the corrosion mechanism itself. Instead, the inhibitor appears to work mainly by reducing the number of accessible reaction sites and restricting the transport of corrosive species to the steel surface. This interpretation is consistent with the formation of an adsorbed protective film, in which the aromatic framework and heteroatom-containing groups of SHIB contribute to surface attachment through donor–acceptor interaction and electrostatic attraction under acidic conditions²⁹.

Electrochemical impedance spectroscopy (EIS)

Electrochemical impedance spectroscopy was used to further evaluate the interfacial response of LCS in 0.5 mol L^-1 HCl and to verify the trends observed from the gravimetric and polarization measurements. The Nyquist spectra show that the impedance response becomes progressively larger as the SHIB concentration increases from 50 to 300 ppm. In the uninhibited solution, the impedance diagram exhibits a much smaller capacitive response, whereas the addition of SHIB produces a marked expansion in the depressed semicircular arc, reflecting a substantial rise in the resistance to charge transfer across the steel/solution interface. Consistent with this behavior, the fitted Rct values increase from 81.0 to 1132.0 Ω cm², while the apparent Cdl decreases from 144.0 to 25.0 µF cm^-2 as the inhibitor concentration is raised³⁰.

Fig. S7 shows the Nyquist plots for LCS in blank acid and in the presence of SHIB at the investigated concentrations. The gradual enlargement of the capacitive loop with inhibitor dosage indicates that SHIB increasingly blocks the active metal surface and suppresses interfacial electron transfer. The depressed shape of the semicircles indicates non-ideal capacitive behavior, which is commonly associated with surface roughness, heterogeneity, adsorption-film nonuniformity and a distribution of relaxation times. In practical terms, a larger Nyquist diameter corresponds to a slower corrosion process, since it signifies a higher barrier to charge migration between the electrolyte and the steel substrate.

The Bode plots in Fig. S8 support the same conclusion but require cautious interpretation because the magnitude plot is most reliable when compared within the same frequency window and scale. The inhibited systems show higher low-frequency impedance than the blank, consistent with stronger interfacial resistance. The phase-angle response becomes broader and shifts toward a more capacitive character after SHIB addition, indicating formation of an adsorbed organic layer. Any apparent inversion or overlap in the high-frequency region is not interpreted as improved corrosion activity of the blank; rather, it reflects the solution-resistance-dominated part of the spectrum and the limited low-frequency range of the measurement.

To quantify the interfacial behavior, the EIS data were fitted using the equivalent circuit shown in Fig. S10, consisting of solution resistance (Rs) in series with a parallel combination of charge-transfer resistance (Rct) and a constant phase element (CPE), i.e., Rs–(Rct‖CPE). This model was selected because the Nyquist plots are dominated by one depressed capacitive loop and do not show a clearly resolved diffusion tail or a second time constant within the measured frequency range. Therefore, more complex circuits were not used to avoid over-parameterization. The use of CPE rather than an ideal capacitor accounts for non-ideal interfacial behavior caused by surface roughness, adsorption-film heterogeneity and distributed relaxation times. The CPE impedance is expressed as ZCPE = [Q(jω)ⁿ]^-1, where Q is the CPE constant, ω is the angular frequency and n is the CPE exponent. The apparent double-layer capacitance values were calculated from the CPE parameters using Cdl, app = (Q Rct⁽¹⁻ⁿ⁾)^(1/n). Accordingly, the capacitance values reported in Table S7 are apparent capacitance values derived from CPE-based fitting rather than directly measured ideal capacitances.

The inhibition efficiency calculated from the impedance data is given by:

8

where (Rct) and (Rct, inh) correspond to the charge-transfer resistance values in the absence and presence of SHIB, respectively. The results show a clear concentration-dependent increase in efficiency, reaching 92.80 ± 0.15% at 300 ppm. This agrees closely with the weight-loss and polarization results, confirming that SHIB offers strong interfacial protection through adsorption-driven surface blocking.

Overall, the Nyquist and Bode responses, together with the equivalent-circuit fitting, demonstrate that SHIB suppresses corrosion by building an adsorbed film on the steel surface. This film increases the resistance to charge transfer, lowers the apparent double-layer capacitance and slows the electrochemical reactions responsible for metal dissolution in acidic chloride media. The EIS interpretation has been limited to the main charge-transfer process resolved in the measured frequency range; slower interfacial processes below 50 Hz require future measurements over an extended low-frequency window^32,33.

Surface morphology

Scanning electron microscope

The surface morphology of LCS after immersion in 0.5 mol L^-1 HCl, in the absence and presence of SHIB, was examined using SEM to provide direct visual evidence of the corrosion and inhibition processes. The micrographs reveal a clear distinction between the uninhibited and inhibited surfaces, consistent with the electrochemical and gravimetric findings.

In the blank solution (Fig. S11 a), the steel surface exhibits severe degradation, characterized by pronounced roughness, heterogeneous topography and extensive localized attack. Features such as pits, micro-cracks and irregular surface damage are clearly visible, reflecting the aggressive nature of the acidic chloride environment. These observations are in agreement with the low charge-transfer resistance and high corrosion rate obtained from EIS and weight-loss measurements, indicating the absence of any effective protective barrier.

In contrast, the specimen exposed to 300 ppm SHIB (Fig. S11 b) shows a markedly improved surface condition. The morphology appears significantly smoother and more uniform, with a substantial reduction in the density and severity of corrosion features. Only minor traces of surface irregularities are observed, suggesting that the majority of active sites have been effectively shielded from the corrosive medium. This visual improvement correlates well with the high inhibition efficiency, increased Rct and decreased corrosion rate measured in the presence of SHIB³¹.

At intermediate concentrations (not shown), partial surface protection is expected, where the inhibitor film is less continuous, leading to moderate surface damage. This behavior aligns with the concentration-dependent inhibition performance observed across all experimental techniques.

Overall, the SEM results provide compelling morphological confirmation of the protective action of SHIB. The formation of a relatively smooth and homogeneous surface in the inhibited system supports the presence of an adsorbed film that acts as a physical barrier, reducing the accessibility of aggressive species and suppressing both anodic and cathodic corrosion reactions.

Atomic force microscopy

Atomic force microscopy was employed to quantitatively evaluate the surface topography of LCS before and after immersion in 0.5 mol L^-1 HCl, in the absence and presence of SHIB. The three-dimensional surface profiles and the associated roughness parameters provide a more precise assessment of the extent of corrosion damage and the effectiveness of surface protection Table S8.

The polished control surface exhibits a relatively smooth morphology, with low roughness values, reflecting the uniform finish obtained prior to exposure. Upon immersion in the uninhibited acidic solution, the surface undergoes significant deterioration, as evidenced by the sharp increase in roughness parameters. The arithmetic mean height (Sa) rises from 58.2 μm for the polished specimen to 395.4 μm in the blank solution, while the root-mean-square roughness (Sq) increases from 73.1 to 410.8 μm. Similarly, the peak-to-valley descriptors (Sp, Sv, and Sz) show substantial increases, confirming the development of deep pits and pronounced surface irregularities resulting from aggressive acid attack.

In contrast, the sample treated with 300 ppm SHIB shows a marked reduction in surface roughness. The Sa and Sq values decrease to 72.6 and 89.3 μm, respectively, approaching those of the polished control. The maximum height parameters (Sp, Sv and Sz) are also significantly lower compared to the uninhibited case, indicating that both the depth and extent of surface damage have been effectively suppressed. These results demonstrate that SHIB significantly mitigates both large-scale and fine-scale topographical degradation (Figs. S12-S14).

The substantial decrease in roughness parameters in the presence of SHIB can be attributed to the formation of an adsorbed protective layer on the steel surface. This layer acts as a barrier that limits direct contact between the aggressive electrolyte and the metal substrate, thereby reducing dissolution and inhibiting pit formation. The improved surface uniformity observed in the AFM analysis is in excellent agreement with the SEM observations and supports the electrochemical findings, particularly the increase in charge-transfer resistance and decrease in corrosion rate.

DFT calculations

To provide a molecular-level interpretation of the inhibition behavior, density functional theory (DFT) calculations were performed to evaluate the electronic structure and reactivity descriptors of SHIB. Geometry optimization and electronic-property calculations were carried out using the B3LYP/6–31 + G(d′,p′) basis set to obtain the most stable molecular configuration and its associated frontier molecular orbital characteristics. The optimized structure along with the HOMO and LUMO distributions is presented in Fig. S15.

A set of global reactivity descriptors relevant to adsorption and charge transfer at the metal interface—including ionization potential (I), electron affinity (A), electronegativity (χ), chemical potential (µ), hardness (η), softness (σ), and the energy gap (ΔE)—was derived from the frontier orbital energies using the following relations:

10

11

12

13

14

I = -EHOMO; A = -ELUMO; χ = (I + A)/2; η = (I - A)/2; σ = 1/η; µ = -χ; ω = µ²/(2η).

The calculated frontier orbital energies for SHIB are E_HOMO = -8.16 eV and E_LUMO = -6.26 eV, yielding an energy gap ΔE = 1.90 eV. This relatively small gap indicates that the molecule possesses a favorable degree of electronic softness and polarizability, facilitating interaction with the metal surface. In addition, the dipole moment (µ = 3.20 D) reflects a moderate polarity, which can enhance electrostatic interaction and influence adsorption orientation under acidic conditions.

Analysis of the spatial distribution of the frontier orbitals reveals that electron density is distributed over the heteroatom-containing and aromatic regions of the molecule. SHIB contains three nitrogen atoms, five oxygen atoms and four aromatic rings that can participate directly or indirectly in adsorption. The nitrogen atoms include the azomethine nitrogen and the phenazine nitrogens, while the oxygen atoms include two phenolic oxygen atoms and the sulfonate oxygen atoms. These centers, together with the extended aromatic π framework, provide the principal adsorption-active sites for electron donation, electrostatic interaction and possible donor-acceptor stabilization at the steel surface.

The combination of a relatively low energy gap, moderate dipole moment, and localized electron density on heteroatoms suggests that SHIB is well suited to participate in donor–acceptor interactions with the steel surface. These electronic features are consistent with the formation of a stable adsorbed layer, as inferred from the experimental results.

Overall, the DFT analysis supports the proposed adsorption mechanism and aligns well with the electrochemical and gravimetric findings. The calculated descriptors provide a coherent explanation for the high inhibition efficiency of SHIB, linking its molecular electronic structure to its observed performance in acidic chloride media.

Monte Carlo simulation

Monte Carlo (MC) simulations were employed to further elucidate the adsorption behavior of SHIB at the metal/solution interface and to complement the quantum-chemical and experimental findings. This molecular-mechanics approach enables efficient exploration of possible adsorption configurations and provides quantitative descriptors of interaction strength between the inhibitor and the metallic surface.

The optimized adsorption geometries of SHIB on the model LCS surface are presented in Fig. S16 (side and top views). The results indicate that SHIB adopts a configuration that allows close contact with the metal surface, maximizing interaction through its heteroatom-containing functional groups and conjugated framework. Such an orientation favors effective surface coverage and supports the formation of a coherent protective layer (Table S10).

The adsorption energy, which reflects the strength and spontaneity of the interaction, was calculated using:

9

where E_total (surface+inhibitor) is the total energy of the adsorbed inhibitor-metal system, Esurface is the energy of the isolated model surface and Einhibitor is the energy of the isolated inhibitor molecule.

As summarized in Table S10, the calculated adsorption energy for SHIB is − 175.28 kcal mol^-1, while the rigid adsorption energy reaches − 180.67 kcal mol^-1. These highly negative values indicate a strongly favorable adsorption process and confirm the high affinity of SHIB for the steel surface. The relatively small deformation energy (5.38 kcal mol^-1) suggests that the molecule can adapt to the surface geometry without significant structural distortion, which is advantageous for maintaining stable surface coverage.

The optimized configurations also reveal that adsorption is primarily facilitated through the heteroatom-rich regions of the molecule, particularly the three nitrogen atoms and five oxygen atoms, along with contributions from the four aromatic rings and the π-electron system. These features enable interaction with the metal surface through electrostatic and donor-acceptor contributions, consistent with the trends identified in the DFT analysis.

Overall, the MC simulation results provide strong support for the proposed adsorption mechanism. The combination of high adsorption energy, favorable orientation, and low deformation cost indicates that SHIB forms a stable and persistent adsorbed layer on the LCS surface. These findings are in excellent agreement with the electrochemical, gravimetric, and surface-characterization results, reinforcing the conclusion that SHIB acts as an efficient corrosion inhibitor through adsorption-driven film formation.

Mechanism of action

The combined experimental and theoretical results consistently indicate that SHIB inhibits the corrosion of low-carbon steel in acidic chloride media through an adsorption mechanism involving both physical and partial chemical contributions at the metal/solution interface.

From a molecular standpoint, SHIB contains multiple adsorption-active centers, including three nitrogen atoms, five oxygen atoms and four aromatic rings. These structural features enable interaction with the steel surface and play a central role in the formation of a protective interfacial film. The effectiveness of inhibition is governed by several factors, including the electronic structure of the molecule, the spatial distribution of active sites and the physicochemical state of the metal surface under acidic conditions.

In solution, SHIB can exist in neutral and partially protonated forms. The neutral species may interact directly with the metal surface through lone-pair donation from nitrogen and oxygen centers to vacant iron orbitals. In addition, the π-electron system of the phenazine and phenyl rings can contribute to surface interaction, further stabilizing the adsorbed layer through donor-acceptor mechanisms.

At the same time, under strongly acidic conditions, a fraction of SHIB molecules becomes protonated. Although direct adsorption of positively charged species onto a positively polarized metal surface may be less favorable, the presence of chloride ions at the interface alters this behavior. Chloride ions tend to adsorb onto the steel surface, creating localized negatively charged regions that can attract protonated SHIB species through electrostatic interaction. This process facilitates physisorption and enhances overall surface coverage.

The simultaneous operation of these two adsorption pathways leads to the formation of a compact and adherent protective film on the steel surface. This film acts as an effective barrier, limiting the access of aggressive species to the metal and reducing both anodic dissolution and cathodic hydrogen evolution. The reduction in charge-transfer processes, decrease in double-layer capacitance, and improvement in surface morphology observed experimentally all support this mechanism.

Overall, the inhibition behavior of SHIB can be attributed to a cooperative adsorption process in which electrostatic physisorption and partial chemisorptive interaction act together to produce a protective interfacial layer. Because film FT-IR or XPS evidence was not available in the present revision, this mechanism should be regarded as a supported interpretation based on thermodynamics, electrochemistry, microscopy and molecular modeling rather than as a direct spectroscopic proof of surface bonding.

Comparative discussion with recent literature

The inhibition performance of SHIB can be meaningfully positioned within the broader context of recently reported organic corrosion inhibitors for steel in acidic chloride media. The maximum inhibition efficiency achieved in the present study, together with the sustained performance at elevated temperatures, places SHIB among the more effective inhibitor systems reported for comparable conditions.

In particular, SHIB exhibits performance characteristics that are consistent with, and in some cases comparable to, those reported for heteroatom-rich organic inhibitors containing nitrogen, oxygen, or sulfur functionalities. Such compounds are widely recognized for their ability to adsorb strongly onto metal surfaces through a combination of electrostatic interaction and donor–acceptor bonding, leading to the formation of protective films that effectively suppress both anodic and cathodic reactions.

A notable aspect of SHIB performance is its ability to maintain high inhibition efficiency as temperature increases within the studied range, which is often a limiting factor for many organic inhibitors due to desorption effects. The retention of more than 80% protection at 328 K suggests that SHIB forms a relatively stable interfacial layer, supported by physical adsorption and partial donor-acceptor interaction. This behavior aligns with trends reported in the literature, where inhibitors exhibiting mixed adsorption contributions tend to demonstrate sustained surface coverage over moderate temperature ranges.

More broadly, the results obtained in this study are in agreement with established structure–activity relationships for corrosion inhibitors, which emphasize the importance of conjugated systems, electron-donating heteroatoms, and favorable molecular electronic properties. The convergence of evidence from weight-loss measurements, electrochemical techniques (PDP and EIS), surface analyses (SEM and AFM), and theoretical calculations (DFT and Monte Carlo) reinforces the conclusion that SHIB operates through an adsorption-driven mechanism consistent with contemporary understanding in the field.

Overall, the comparative analysis indicates that SHIB performs competitively within the current landscape of organic corrosion inhibitors and follows well-established mechanistic principles that underpin effective corrosion protection in acidic environments. This statement is limited to the laboratory conditions tested here and should not be interpreted as proof of industrial readiness without additional long-duration, flow-loop, compatibility and safety evaluation.

Limitations sustainability and practical considerations

The present work provides an integrated laboratory evaluation of SHIB, but several limitations should be clearly recognized. First, the EIS frequency window ended at 50 Hz; therefore, slower low-frequency processes could not be fully resolved. Second, the temperature study was limited to 328 K. Additional experiments at 333 K and 338 K would be valuable for a more complete thermodynamic profile, but they were not available in the present dataset and are therefore proposed as future work rather than inferred from extrapolation.

Third, the inhibitor concentration was optimized only within the tested 50–300 ppm range. The plateau or possible change in adsorption behavior above 300 ppm cannot be established from the present results. Future work should include higher concentrations to determine whether the efficiency reaches saturation, remains stable or decreases because of aggregation or changes in interfacial packing.

Fourth, the surface mechanism was inferred from electrochemical behavior, SEM, AFM, DFT and Monte Carlo calculations. FT-IR spectra of the pure SHIB and the film scraped from the LCS surface would provide valuable additional confirmation of surface functional groups; however, scraped-film FT-IR data were not available for the current revision. Therefore, claims regarding donor-acceptor interaction and mixed adsorption have been worded cautiously.

From a sustainability and safety perspective, SHIB is effective at relatively low ppm dosage and operates in aqueous acidic medium, which is favorable for laboratory-scale acid-corrosion control. Nevertheless, practical deployment requires additional evaluation of toxicity, biodegradability, persistence, synthesis scalability, compatibility with field additives and performance under flow and multiphase conditions. SHIB should therefore be considered a promising inhibitor candidate rather than a fully validated industrial product at this stage.

Conclusions

SHIB is an effective laboratory-scale corrosion inhibitor for low-carbon steel in 0.5 mol L^-1 HCl, delivering 91.95% inhibition efficiency by weight loss, 92.25% by polarization and 92.80% by impedance spectroscopy at 300 ppm.

The structural features of SHIB, including three nitrogen atoms, five oxygen atoms, four aromatic rings, an azomethine linkage and a sulfonate functionality, provide multiple adsorption-relevant sites that distinguish it from simpler Schiff base inhibitors.

Weight-loss data are best represented by Langmuir-type adsorption, and ΔG°ads values of − 27.76 to − 29.89 kJ mol^-1 indicate spontaneous adsorption with mixed physical and partial chemical contributions.

Electrochemical measurements show mixed-type inhibition, increased Rct from 81.0 to 1132.0 Ω cm² and decreased apparent Cdl from 144.0 to 25.0 µF cm^-2 at 300 ppm, supporting adsorption-driven interfacial blocking.

SEM and AFM confirm that SHIB strongly reduces acid-induced surface damage, with Sa decreasing from 395.4 μm in blank acid to 72.6 μm in the protected sample.

The main limitations are the restricted EIS low-frequency window, the maximum tested temperature of 328 K, the absence of concentration data above 300 ppm and the lack of scraped-film FT-IR evidence. Further thermal, spectroscopic, safety and field-simulation studies are required before industrial extrapolation.

Supplementary Information

Below is the link to the electronic supplementary material.

Author contributions

Mostafa A. A. Mahmoud conceived the study, supervised the work, analyzed the results and wrote the main manuscript draft. Sherin A. M. Ali performed electrochemical methodology, validation and visualization. Mostafa E. Salem and Mohamed Abdel-Megid carried out the theoretical calculations and post-processing. Mansour Alsarrani contributed resources and supervision. Ahmed Z. Ibrahim contributed industrial context, resources and validation. All authors reviewed the manuscript.

Funding

This work was supported and funded by the Deanship of Scientific Research at Imam Mohammad Ibn Saud Islamic University (IMSIU) (grant Number IMSIU-DDRSP2602).

Data availability

The data supporting this study are included in the manuscript and in the Supplementary Information.

Declarations

Competing interests

The authors declare no competing interests.