Unraveling the anti-corrosion mechanisms of a novel hydrazone derivative on steel in contaminated concrete pore solutions: An integrated study

Graphical abstract

Highlights

• •A new hydrazone derivative inhibits steel rebar corrosion in chloride-contaminated SCPS.
• •The tested inhibitor exhibits an excellent corrosion protection for 720 h of immersion.
• •XPS and other characterization techniques confirm the effective adsorption of inhibitor molecules on steel rebar.
• •SCC-DFTB simulation reveals the formation of chemical bonds between inhibitor molecules and iron surface.

Abstract

Introduction

Corrosion-induced deterioration of infrastructure is a growing global concern. The development and application of corrosion inhibitors are one of the most effective approaches to protect steel rebar from corrosion. Hence, this study focuses on a novel hydrazone derivative, (E)-N′-(4-(dimethylamino)benzylidene)-2-(5-methoxy-2-methyl-1H-indol-3-yl)aceto-hydrazide (HIND), and its potential application to mitigate corrosion in steel rebar exposed to chloride-contaminated concrete pore solutions (ClSCPS).

Objectives

The research aims to evaluate the anti-corrosion capabilities of HIND on steel rebar within a simulated corrosive environment, focusing on the mechanisms of its inhibitory effect.

Methods

The corrosion of steel rebar exposed to the ClSCPS was studied through weight loss and electrochemical methods. The surface morphology of steel rebar surface was characterized by FE-SEM-EDS, AFM; oxidation states of the steel rebar and crystal structures were examined using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) methods. Further, experimental findings were complemented by theoretical studies using self-consistent-charge density-functional tight-binding (SCC-DFTB) simulations. The performance of HIND was monitored at an optimal concentration over a period of 30 days.

Results

The results indicated a significant reduction in steel rebar corrosion upon introducing HIND. The inhibitor molecules adhered to the steel surface, preventing further deterioration and achieving an inhibition efficiency of 88.4% at 0.5 mmol/L concentration. The surface morphology analysis confirmed the positive effect of HIND on the rebar surface, showing a decrease in the surface roughness of the steel rebar from 183.5 in uninhibited to 50 nm in inhibited solutions. Furthermore, SCC-DFTB simulations revealed the presence of coordination between iron atoms and HIND active sites.

Conclusion

The findings demonstrate the potential of HIND as an effective anti-corrosion agent in chloride-contaminated environments. Its primary adsorption mechanism involves charge transfer from the inhibitor molecules to iron atoms. Therefore, applying HIND could be an effective strategy to address corrosion-related challenges in reinforced infrastructure.

Introduction

Despite advancements in construction technologies, corrosion-induced damage in reinforced concrete structures remains a significant issue, threatening infrastructure durability worldwide [1], [2]. This corrosion leads to expensive repairs, reduced service life, and in severe cases, catastrophic failures [3], [4]. Reinforced concrete, a critical material in construction industries, is utilized globally thanks to the wide availability and affordability of steel rebar [5], [6].

The Pourbaix diagram for the Fe/H₂O system indicates a protective passive film on reinforcing steel for pH values between 12 and 14, suggesting that the high pH of the concrete pore solution should protect the steel rebar against corrosion [7], [8]. However, maintenance neglect often leads to reinforced concrete structure failures, resulting in substantial economic and safety issues [9]. In environments like marine conditions, reinforced concrete structures become susceptible to chloride-induced and carbonation corrosion mechanisms [7]. The diffusion of CO₂ within concrete typically results in a drop in pH, attributable to an increased rate of carbonation. This shift creates ideal conditions for the breakdown of the passivity of the protective film on the steel surface. Similarly, the incursion of chloride ions from external sources into the concrete triggers the depassivation of the protective film on the metal. This depassivation tends to occur locally on the most vulnerable surfaces, leading to the formation of pits. These pits become small anodes, producing active corrosion cells characterized by very low pH [10], [11].

The ongoing pursuit of effective preventive measures has presented various methods, such as cathodic protection, corrosion-resistant alloys, protective coatings, optimized concrete chemistry, and corrosion inhibitors [12], [13], [14]. Among these, corrosion inhibitors have shown to be cost-effective and easy to apply, providing a promising solution for corrosion issues [1], [15], [16]. Inorganic corrosion inhibitors such as chromates, molybdates, nitrates, phosphates, etc., are well-recognized for their effectiveness in protecting metals against corrosion [17], [18]. However, they have been criticized for their toxic nature, creating a need for non-toxic, eco-friendly alternatives [1], [19].

A strategic approach to enhancing the effectiveness and non-toxicity of inhibitors involves designing and synthesizing green organic compounds. These compounds often act as mixed inhibitors, acting on both anodic and cathodic corrosion reactions, forming a protective hydrophobic film on the metal surface. The presence of heteroatoms and functional groups aids the adsorption process. Organic compounds can function as physical barriers against chlorides or offer electrostatic repulsion for compounds with negatively charged chain ends [20]. They also enable other inhibition mechanisms, such as chelating mechanism [20]. Numerous organic materials, including organic core–shell [21], polyacrylamide [22], and dimethyl-ethanol-amine [23], have been utilized as steel rebar corrosion inhibitors [24], [25].

Hydrazones, a unique class of organic compounds, exhibit several biological and clinical applications due to their distinctive chemical characteristics [26], [27]. Our research team has developed functionalized hydrazones from non-steroidal anti-inflammatory drugs (NSAIDs) like Mefenamic acid, Naproxen, and Ibuprofen, which have proven effective against acid corrosion of steels in HCl solutions [28], [29], [30]. Despite their strong adsorption ability, their application in corrosion mitigation in simulated concrete pore solutions is understudied.

As the pharmaceutical industry expands globally, managing the increase in impurities and waste is challenging. The impurity 5-methoxy-2-methyl-3-indoleacetic acid (MMIAA), found in the NSAID Indomethacin (IND), can be repurposed to develop effective corrosion inhibitors [31]. The mechanism by which the IND is functionalized applies to the MMIAA compound, which signifies that a similar path can be followed for its valorization as a starting material to develop effective corrosion inhibitors.

On the other hand, the computational evaluation of adsorption properties and bonding mechanisms of corrosion inhibitors has emerged as a crucial tool for a deeper understanding of the corrosion inhibition attributes of organic compounds [32]. Specifically, ab initio and semi-empirical-based DFT methods are highly accurate theoretical instruments for predicting reactivity and electronic structure in inhibitor-metal systems [33]. For example, the self-consistent-charge density-functional tight-binding (SCC-DFTB) method can accurately simulate the bonding of inhibitor molecules to metal atoms in a reasonably short time, even for larger adsorption systems [34], [35], [36]. This approach significantly surpasses traditional tools, such as global reactivity descriptors and classical molecular dynamics, in terms of accuracy and predictive power.

Motivated by the success of our previous research on hydrazone derivatives [37] and the need for efficient, non-toxic corrosion inhibitors, we introduce the synthesis and application of a new hydrazone derivative, specifically E-N′-(4-(dimethylamino)benzylidene)-2-(5-methoxy-2-methyl-1H-indol-3-yl)acetohydrazide (henceforth referred to as HIND), originating from indomethacin (IND). This is done to protect steel rebar within concrete environments. The protective capacity of this compound for steel rebar is assessed in simulated chloride-contaminated concrete pore solutions (ClSCPS), using chemical (weight loss) and electrochemical (electrochemical impedance spectroscopy and potentiodynamic polarization curves) methodologies. To gain in-depth knowledge of the chemical states and morphologies of the corroded and inhibited steel surfaces, we employed X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), SEM/EDS, and atomic force microscopy (AFM). Moreover, we utilized SCC-DFTB simulations and PDOS analysis to provide a deeper physical understanding of the bonding mechanisms of HIND-Fe(1 1 0) interactions.

Materials and methods

Chemicals

The extra pure-grade chemicals of Sodium hydroxide (NaOH), Potassium hydroxide (KOH), calcium hydroxide (Ca(OH)₂), and sodium chloride (NaCl) were used in this study. For the current investigation, all chemicals utilized were procured from Duksan Pure Chemicals in South Korea, without additional purification.

Synthesis of hydrazone compound

The title compound (E)-N′-(4-(dimethylamino)benzylidene)-2-(5-methoxy-2-methyl-1H-indol-3-yl)acetohydrazide (5) was synthesized according to scheme 1 (given in the supplementary material file) and procedures represented in the supplementary material. The selected compound can be synthesized in high yields by three-steps reaction using easily available impurities compounds such as MMIAA, thereby substantially lowering the synthetic cost. Previously, in reference [38], the synthesis and characterization of compound 2 were reported using the single-crystal X-ray diffraction method. This compound has served as the initial material for the successful creation of various hydrazones [39], [40], [41]. This indicates the high precision and reproducibility of the chosen synthetic pathway.

Preparation of steel rebar and solutions

In this study, thermomechanical treated (TMT) steel rebar was used as the working electrode and its chemical composition is (wt.%): 98.632 % Fe, 0.193% C, 0.125% Cr, 0.281% Si, 0.540% Mn, 0.024% P, 0.125% Cu, and 0.080% S. 13 mm diameter and 120 mm length of the TMT steel rebars were taken and the oxide film on their surface was cleaned by soaking it in pickling solution (ASTM G1-03) [42] for 3–5 min. Then, the steel rebars were thoroughly cleaned using distilled water and isopropanol. After that, it was dried at 60 °C and then the 11.304 cm² active area of the steel rebars was chosen for electrochemical studies, whereas the other portions of the steel rebars were sealed with epoxy.

The simulative concrete pore solution (SCPS) was prepared as per the procedure reported elsewhere [43], [44] and a detailed procedure is given as follows. For every liter of saturated Ca(OH)₂ solutions, the SCPS contains 3.36 g/L of KOH and 8.33 g/L of NaOH. Then, this solution was filtered by Whatman paper to remove insoluble CaO precipitate. Thereafter, 35 g of NaCl was dissolved in 1 L of SCPS to make the chloride-contaminated SCPS (ClSCPS), designated herein as a blank solution. The alkalinity of the ClSCPS was determined by a digital portable pH meter (±0.01) and found to be 12.55 ± 0.22 at the room temperature of 25 °C. Different concentrations of inhibitors, i.e., 0.1, 0.25, 0.5, 0.75, and 1.0 mmol/L of inhibitor were added to the SCPS. The abbreviations and alkalinity of prepared solutions are shown in Table S1 (supplementary material file). All experiments were performed three times. Studying corrosion in concrete requires simulated solutions like cement extract and synthetic concrete pore solution. These substitutes accurately replicate the complex chemical environment inside concrete. Simulated solutions are practical and cost-effective, allowing long-term research and consistent testing under ASTM or ISO standards. Utilizing these simulated solutions fosters cooperation among scientists and leads to improved strategies for enhancing concrete structures’ durability.

Evaluation of corrosion mitigation properties

Weight loss experiments

The gravimetric experiments were conducted as per the ASTM G1-03 standard [42]. The oxide layer of the steel rebar surface was cleaned then the initial weight and surface area were noted. A known weight of the steel rebars was exposed to different solutions as ascribed in Table S1 (supplementary material file). After the end of exposure (720 h), the steel rebars were taken out and acid pickling (ASTM G1-03) was used for 3–5 min to remove any corrosion products that had formed on the steel rebar surface. Consequently, the steel rebar surfaces were washed using distilled water and isopropanol followed by 30 min drying at 60 °C. The final weight of the steel rebars was noted by the Mettler Toledo balance (Torunska 5, 26–600, Radom, Poland) with an accuracy of 0.0001 g.

To determine the corrosion rate of steel rebars exposed to various solutions, the following equation was employed for calculation purposes [42].

$$\mathit{Corrosion}rate\left(mmpy\right)=\frac{87.6(W_i-W_f)}{\mathit{DAt}}$$ (1)

where W_f and W_i stand for steel rebar’s final and initial weights in milligrams, respectively. ‘D’ and ‘A’ stand for the steel rebar’s density (g/cm³) and exposure area (cm²), respectively, while ‘t’ denotes the exposure time (h).

To determine the surface area coverage (θ) and inhibitor efficiency (ƞ%) based on corrosion rate values obtained from the weight loss method, the following equations were utilized [44]:

$$\theta =1-\frac{{\mathit{CR}}_{\mathit{inh}}}{{\mathit{CR}}_0}$$ (2)

$$\mathrm{\eta }\left(\%\right)=1-\frac{{\mathit{CR}}_{\mathit{inh}}}{{\mathit{CR}}_0}\times 100$$ (3)

where CR₀ and CR_inh denote the corrosion rate in the absence and the presence of HIND, respectively.

Electrochemical studies

A three-electrode experiment setup was used for electrochemical studies, where the area of 11.304 cm² steel rebar acted as a working electrode, 316L Stainless steel (SS), and saturated calomel electrode (SCE) used as counter and reference electrode, respectively.

For this study, the CE electrode area was 3 times higher than the working electrode area and the distance between WE, RE, and CE was maintained at 2 cm (an illustration of the used cell is reported in supplementary material; Fig. S1). Open circuit potential (OCP) of the steel rebar immersed in ClSCPS with and without HIND inhibitors was monitored at different immersion periods. The electrochemical impedance spectroscopy (EIS) studies were performed at various exposure times (1 h to 720 h) by changing the frequency range of 10 mV sinusoidal voltage from 100 kHz to 0.01 Hz. The potentiodynamic polarization of steel rebars after the exposure period of 720 h was carried out using the potential range of ± 200 mV from the half-cell potential at a 0.2 mV/s potential scan rate. Electrochemical testing was conducted using VersaSTAT (Princeton Applied Research, Oak Ridge, TN) potentiostat, and Metrohm Autolab Nova 1.10′s system software was used for monitoring and fitting the graph of the experimental data to find the electrochemical parameters of EIS and potentiodynamic polarization graphs.

The following relationships were used to measure the inhibitor efficiency (ƞ %) and surface coverage (θ) by i_corr values obtained from potentiodynamic polarization studies [44].

$$\mathrm{\eta }\left(\%\right)=\frac{i_{\mathit{corr}}^0-i_{\mathit{corr}}^{\mathit{inh}}}{i_{\mathit{corr}}^0}\times 100$$ (4)

$$\mathit{Surface}coverage(\theta )\left(\%\right)=\frac{i_{\mathit{corr}}^0-i_{\mathit{corr}}^{\mathit{inh}}}{i_{\mathit{corr}}^0}$$ (5)

where $i_{\mathit{corr}}^0$ and $i_{\mathit{corr}}^{\mathit{inh}}$ are the corrosion current density in the absence and presence of inhibited medium, respectively.

The R_ct values obtained from EIS studies enable the determination of inhibition efficiencies and surface coverage using the following equations.

$$\mathrm{\eta }\left(\%\right)=\frac{R_{\mathit{ct}}^{\mathit{inh}}-R_{\mathit{ct}}^0}{R_{\mathit{ct}}^{\mathit{inh}}}\times 100$$ (6)

$$\mathit{Surface}coverage(\theta )=\frac{R_{\mathit{ct}}^{\mathit{inh}}-R_{\mathit{ct}}^0}{R_{\mathit{ct}}^{\mathit{inh}}}$$ (7)

where $R_{\mathit{ct}}^0$ and $R_{\mathit{ct}}^{\mathit{inh}}$ represent the charge transfer resistance in the blank and inhibited medium, respectively.

Characterization of steel rebar surface

For the characterization purpose, a steel rebar coupon with 0.75 cm × 0.75 cm immersed in the ClSCPS with and without HIND inhibitor was utilized to characterize the steel rebar’s surface. The schematic diagram for the preparation of steel rebar coupon for surface characterization is shown in Fig. S2 (supplementary material file). Different analytical instruments were used to characterize the surface of steel rebar exposed to ClSCPS with the absence and presence of HIND at the exposure period of 720 h. To investigate the crystal phase structure of the steel rebar surface, computer-controlled X-ray diffraction spectroscopy was employed using CuαK radiation (λ = 1.54059 Å) generated at 40 kV and 20 A. The D/MAX-2500 instrument from Rigaku, Japan was utilized for this purpose. X’pert high score plus (version 2.0, PA Analytical) software was used for peak analysis. In order to examine the surface roughness and morphology of the steel rebar surface, both atomic force microscopy (AFM) and scanning electron microscopy (SEM) were employed. The absence and presence of tested inhibitors were taken into account during the investigation. The AFM analysis was conducted using an XE-100 instrument, with a scan range of 10 μm × 10 μm, to capture the surface topography. SEM coupled with energy-dispersive X-ray spectroscopy (EDS); (TESCAN MIRA3, Brno, Czech Republic) was used to examine (SEM analysis) the surface morphology of the steel rebar. The inhibitor adsorption and chemical states of steel rebar surface in inhibited and uninhibited solutions were characterized by X-ray photoelectron spectroscopy (XPS, Scienta Omicron R3000, USA) with Al-Ka radiation of 1486.6 eV.

Computational assessment: SCC-DFTB simulation

The methodology employed in this study hinges on the utilization of the self-consistent-charge density-functional tight-binding (SCC-DFTB) technique, an approximation technique derived from expanding the Kohn-Sham Density Functional Theory to the second order. This technique delivers similar accuracy levels as primary calculations in determining structural and electronic attributes while providing expedited computational speed, particularly in extensive systems. In this study, we employed a spin-polarized SCC-DFTB approach with the dispersion interaction to optimize the interactions between the inhibitor and iron. We utilized Slater-Koster trans3d and performed the calculations using the DFTB + code. [45]. We accomplished comprehensive optimization of the adsorption systems by setting an SCC tolerance of 10⁻⁸ atomic units, implementing thermal smearing, and using the Broyden mixing scheme. Simultaneously, we performed optimization of bulk lattice parameters through an (8x8x8) k-point grid. The optimized parameters resulted in a value of 2.878 Å, closely mirroring the experimental value of 2.862 Å, which substantiated our chosen parameters. The electronic energy of the HIND-Fe(1 1 0) surface converged on a (2x2x1) k-point grid. We constructed adsorption models by creating a Fe(1 1 0) iron surface from a (5x5) supercell and a 20 Å vacuum spacing in the z-direction to separate periodic images in every direction. We positioned the neutral and protonated variants of the HIND molecule on the slab's upper side. With the exception of the two bottom-most atomic layers, relaxation of all atoms was permitted. For all simulations, we adopted a surface coverage of 1/25 monolayer (ML). We performed SCC-DFTB optimization of individual molecules by creating a 30 Å cubic box. The primary parameter in estimating the adsorption strength of molecules, the interaction energy, was calculated using the following equation:

$$E_{\mathit{inter}}=E_{\mathit{mol}/surf}-\left(E_{\mathit{mol}}+E_{\mathit{surf}}\right)$$ (8)

In the equation, E_mol, E_surf, and E_mol/surf denote the total energies of isolated molecules, the Fe(1 1 0) iron surface, and the molecule/Fe(1 1 0) adsorption systems, respectively.

Results and discussion

Weight loss measurements

The weight loss method is a primary corrosion assessment test to estimate the corrosion rate of metals in a corrosive environment. Herein, the corrosion prevention capacities of the investigated hydrazone compound are evaluated using mass loss tests for steel rebar exposed to ClSCPS without and with different concentrations of HIND for 720 h immersion time. Table 1 lists mass loss parameters along with surface coverage, inhibition efficiency, and pH values at the initial and final exposure periods.

Table 1.

Weight loss method for steel rebar after an exposure period of 720 h.

System	Weight loss (g)	Corrosion rate (mmpy)	Surface area coverage (θ)	IE (%)	Alkalinity of the SCPS
					Initial	Final
ClSCPS	0.3146	0.4301 ± 0.0028	–		12.55	11.85
ClSCPS0.1	0.1036	0.1419 ± 0.0035	0.670	67.0	12.64	11.88
ClSCPS0.25	0.0597	0.0817 ± 0.0077	0.787	78.7	12.65	11.90
ClSCPS0.5	0.0366	0.0501 ± 0.0082	0.884	88.4	12.66	12.01
ClSCPS0.75	0.0568	0.0778 ± 0.0073	0.819	81.9	12.66	11.94
ClSCPS1.0	0.0862	0.1181 ± 0.0048	0.725	72.5	12.66	11.89

The corrosion rate of steel rebar in blank solution is around 0.4301 mmpy. Unsurprisingly, adding an increasing amount of HIND up to 0.5 mmol/L to the corrosive solution results in a significant decrease in corrosion rate. At 0.5 mmol/L of HIND, the corrosion rate is 0.0501 mmpy, with an equivalent corrosion inhibition efficiency of 88.4%. However, a further increase in inhibitor concentration has a negative effect on its inhibition performance. At concentrations higher than 0.5 mmol/L, the corrosion rate decreases to 0.0778 mmpy (0.75 mmol/L) and 0.1181 mmpy (1.0 mmol/L). Consequently, the inhibition efficiency decreased to 81.9% and 72.5%, respectively. This could be the result of the solution being congested at a higher concentration, which could limit the inhibitors' ability to move around, lowering the formation of the passive film. Therefore, the chloride ion could access the steel rebar surface more easily, breaking the passive film, and increasing the corrosion rate. Furthermore, alkalinity measurement results show that, at optimum conditions, a slight change is observed in the alkalinity of the inhibited solutions. The pH drops slightly at 0.5 mmol/L of HIND from 12.66 to 12.01. This decrease, however, is more significant in the blank solution. This finding demonstrates that the inhibitor molecules had no impact on the SCPS’s alkalinity when compared to the blank solution.

Although, the inhibitor maintains a high corrosion inhibition performance after 720 h of immersion. The observations indicate that the investigated corrosion inhibitor operates by adsorbing onto the steel surface, thereby creating a protective barrier that shields against corrosive ions [44]. This adsorption is supposed to occur due to several reactive sites in HIND’s molecule, such as heteroatoms, which are rich in free-electron pairs and π-electron of aromatic rings. However, weight loss can only provide initial insights, while a deep understanding of the adsorption mechanism is unachievable from this method. Further analyses of the HIND adsorption process over steel rebar are conducted in the following sections.

OCP measurements

Open circuit potential (OCP) measurements are a valuable tool in corrosion studies as they provide crucial insights into the electrochemical behavior of metals in different environments, particularly in identifying the onset and progression of corrosion [46], [47]. Fig. S3 (in the supplementary material file) displays the time variation of steel rebar’s OCP in ClSCPS at 25 °C with varying concentrations of HIND. As seen from Figure S3, no significant variation is observed in both blank and inhibited systems during the 1-hour immersion period. For instance, the OCP value is −521 mV for blank systems and is −520, −508, −502, −507, and −515 mV for 0.10, 0.25, 0.50, 0.75, and 1.0 mmol/L of HIND, respectively, suggesting that the HIND compound functions as a mixed-type inhibitor. Furthermore, extending the immersion time in ClSCPS causes the OCP values to shift more negatively. This negative shift is notably more prominent for blank and inhibited systems at 48 h, then slows down slightly up to 240 h and 720 h for blank and inhibited systems, respectively. This behavior suggests that a protective oxide layer forms on the steel rebar surface due to the highly alkaline pH. However, significant changes have been observed in the OCP values within the blank systems in the 240 h to 720 h range, implying that the productive oxide layer is unstable and weak due to the continuous attack of chloride ions. In contrast, the rate of negative shift is lower for inhibited systems, spanning from 48 to 720 h, indicating the stability of the oxide layer and only minor decomposition. Furthermore, the OCP values of the steel rebar in ClSCPS with 0.5 mmol/L of HIND range from −502 to −570 mV for 1 to 720 h, significantly lower than all other systems. This suggests minimal corrosion processes at the steel rebar/solution interface. The OCP values in ClSCPS with 0.75 and 1.0 mmol/L of HIND are lower than those with 0.5 mmol/L of HIND, implying that a higher concentration of HIND inhibitor molecules in the solutions may cause congestion, thereby reducing the adsorption rate of inhibitor molecules [48]. This finding will be further confirmed by EIS and potentiodynamic polarization studies.

Electrochemical impedance spectroscopy (EIS) tests

In order to characterize the corrosion prevention processes at the metal/solution interface, EIS is a helpful non-destructive technique. Herein, EIS tests were conducted to get accurate information about the electrical double layer at steel/ClSCPS and the passive layer behavior in the presence and absence of investigated hydrazone. Fig. 1 displays the Nyquist plots of EIS results for steel rebar submerged in ClSCPS with varying concentrations of HIND inhibitor. EIS experiments were performed at multiple immersion durations, spanning from 1 h to 720 h.

Fig. 1.

Nyquist plots for the steel rebar exposed to ClSCPS with different concentrations of HIND at the exposure period of 1 h (a); 48 h (b); 120 h (c); 240 h (d); 480 h (e) and 720 h (f). The Chi-square values for fitting range between 5 × 10⁻³ and 8 × 10⁻³.

A general observation reveals that the capacitive loop radii diminish as immersion time increases, indicating a rising corrosion rate over time. Conversely, when comparing typical solutions, a notable difference between arc radii can be seen in the presence and absence of inhibitors. Furthermore, the arc radius of the capacitive loops substantially grows with an increase in inhibitor concentration up to 0.5 mmol/L. The formation and expansion of an inhibitor film on the steel rebar surface can explain the concentration-dependent nature of the capacitive loops' size [11], [49]. The decrease of capacitive loop diameter with immersion time in both inhibited and uninhibited solutions is mainly due to the increase of passive film permeability, therefore. a less dense or defective protective layer is formed as a result of continuous chloride ions attack [11].

The Bode phase angle and Bode magnitude plots can provide further details regarding mechanisms of corrosion and corrosion inhibition. As shown in Fig. 2, at the intermediate frequency region, the phase angle of the blank substrate decreases from −64° to −44° after 1 h and 720 h immersion time, respectively, and then decreases further with a reduction in frequency, demonstrating the fewer protective nature of the inherent oxide layer on steel rebar [50], [51]. In the presence of 0.5 mmol/L of HIND inhibitor, the phase angle values show a significant shift towards higher values in the mid-frequency region at all immersion periods and then decrease to lower values in the low-frequency region because of ingress of electrolyte and aggressive ions into the formed inhibitor layer [50], [51], [52]. Overall, Bode phase angle plots can be classified into three regions at high, medium, and low frequencies [49]. The first region at very high frequencies characterizes the electrolyte resistance. At medium frequencies (10³ kHz to 10⁻¹ Hz), the Bode phase angle responses are ascribed to forming the passive layer and the adsorbed inhibitor layer. At these frequencies, the greater slope of diagrams signifies increased protection of steel rebar against corrosion [52]. Such protection is due to the formation of a robust molecular layer on the steel rebar that works as a physical barrier to corrosive particles [53], [54]. This explains that increasing the concentration of inhibitors leads to an increase in Bode phase angle peaks at this region. The low-frequency region, on the other hand, exhibits a classic double-layer feature reflecting the transport process at the passive film/electrolyte interface [11], [49].

Fig. 2.

Bode phase angle and Bode magnitude plots for the steel rebar exposed to ClSCPS with different concentrations of HIND at the exposure period of 1 h (a); 48 h (b); 120 h (c); 240 h (d); 480 h (e) and 720 h (f).

According to Nyquist representation, Bode magnitude plots show a high |Z| value after adding an increased inhibitor dosage. It also reflects the excellent protective properties of the formed inhibitor layer. Furthermore, Bode modulus plots confirm the negative influence of longer immersion time on the corrosion resistance capacity of the inhibitor layer and the passive oxide layer in the case of blank solutions. With a prolonged immersion time, chloride ions could break down the oxide and inhibitor film layers [49].

According to the abovementioned conclusions, the equivalent circuits (EECs) given in Fig. S4 (supplementary material file) were used to fit the EIS data. Figs. S4 (a) and S4(b) illustrate the employed EEC fitting for the samples immersed in the blank and inhibited solutions, with Fig. S4(a) corresponding to a 1 h immersion period and Fig. S4(b) representing immersion durations longer than 1 h.

In these EECs, R_s represents the electrolyte resistance, R_f and CPE_f describe the first-time constant parameters and refer to passivation/adsorbed oxide film resistance and constant phase element, respectively. The second time constant is characterized by R_ct and CPE_ct, denoting the charge transfer resistance and constant phase element related to double-layer capacitance. Incorporating the CPE instead of pure capacitance is due to the deviation from the ideal behavior as confirmed by flattening capacitive reactance arcs and maximum Bode phase angles, which do not reach the −90°, that is the maximum possible phase shift between the voltage and current [52]. This non-ideal behavior is related to the heterogeneous nature of the steel surface [55]. The impedance of CPE is described by its two parameters, Q, and n, as follows [56]:

$$Z_{\mathit{CPE}}=Q^{-1}{\left(i\omega \right)}^{-n}$$ (9)

where n represents the heterogeneity measure and Q denotes the CPE constant.

Electrochemical parameters obtained by fitting Nyquist plots (Fig. 1 and Fig. 2) data are listed in Table S2 (supplementary material file). Regarding the charge transfer resistance (R_ct) of blank, one can notice that its values are greatly influenced by immersion time, decreasing from an initial value of 630 Ω.cm² to 72 Ω.cm² after 720 h. When the SCPS is contaminated by chlorides, its ionic conductibility increases, and the passive layer's breakdown is provoked, promoting localized and in-depth corrosion progress [57]. At this stage, chloride ions will migrate to anodic sites for further chloride attack while ferrous iron ions will precipitate in oxygen-rich cathodic regions [2]. As a result, still dissolution will be accelerated and pH in the pit is decreased, where the corrosion process becomes an autocatalytic process [57], [2]. The R_ct values for steel rebar in ClSCPS with varying concentrations of HIND (0.1, 0.25, 0.5, 0.75, and 1.0 mmol/L) registered at 639.9, 842.46, 1092.8, 897.4, and 755.61 Ω.cm², respectively after a 1 h immersion period. These values surpassed those of the blank systems, suggesting robust adsorption of HIND molecules on the steel rebar surface, thereby reducing the corrosion process. A detailed analysis of the results shows that the R_ct values for the 0.75 and 1.0 mmol/L concentrations of HIND are 1.22 and 1.45 times lower than that of 0.5 mmol/L of HIND, respectively. This observation indicates that an agglomeration phenomenon occurs due to the high concentration of HIND molecules, consequently lowering the adsorption rate of HIND molecules on the steel rebar surface and enhancing the charge transfer [48]. As the immersion periods lengthen, the R_ct values decrease progressively. For instance, the R_ct values for the blank and 0.5 mmol/L of HIND were 295.6, 174.18, 164.27, 158.14, and 72.0 Ω.cm², and 421.17, 406.44, 398.72, 379.46, and 340.61 Ω.cm² at immersion durations of 48, 120, 240, 480, and 720 h, respectively. The R_ct values for the blank and 0.5 mmol/L of HIND at 720 h are 8.76 and 3.21 times lower than those after 1 h of immersion, respectively. The reduction in R_ct value in the 0.5 mmol/L system is less than in the blank system. This evidence implies that HIND molecules strongly adsorb onto the steel rebar surface, thus reducing the charge transfer process between the steel rebar surface and solution interfaces.

The R_f values of the blank sample dropped from 94.1 Ω.cm² at 48 h to 21.1 Ω.cm² at 720 h of immersion. The same can be said for samples immersed in inhibited solutions; however, in this case, film resistance is much higher at all immersion periods than that of the blank test. Typically, it reaches 133 Ω. cm² at 0.5 mmol/L of inhibitor and 48 h of immersion. This is mainly attributed to the accumulation of adsorbed inhibitor molecules, forming a protective barrier on steel rebar against corrosive ions. It should be noted that the solution resistance shows no significant changes, suggesting that the inhibitor does not affect the conductivity of solutions [58].

Also, one can notice that the inhomogeneity measure parameter n values are similar in both blank and inhibited solutions. The results indicate that the adsorption of inhibitor molecules on the steel rebar surface did not lead to significant alterations in surface inhomogeneity. Furthermore, the increase of charge transfer resistance and the decrease of constant phase element (CPE_dl) is consequent with the increase of inhibitor concentration. Further, Brug's equation is used to determine the values of the effective double-layer capacitance (C_eff.dl) [59], [60], [55].

$$C\_(eff^\text{'}dl)=Q\_dl^(1/n)\times (1/R\_s+1/R\_CT)^\left(\right(n-1)⁄n)$$ (10)

The calculated C_eff.dl values for steel rebar in ClSCPS with various concentrations of HIND inhibitors are shown in Fig. S5 in the supplementary material file. As depicted in Fig. S5, the C_eff.dl values for steel rebar in SCPS are 3.14 × 10⁻⁴F.cm⁻² for the blank system and 2.86, 2.78, 1.67, 1.71, and 2.74 × 10⁻⁴F.cm⁻² for 0.1, 0.25, 0.5, 0.75, 1.0 mmol/L of HIND after 1-hour immersion, respectively. However, as the immersion period extends, the C_eff.dl values gradually increase in both the blank and inhibited systems, with the blank systems recording significantly higher values than the inhibited systems. For example, at the 720-hour immersion mark, the C_eff.dl values were 96.7 × 10⁻⁴F.cm⁻² for the blank system (uninhibited), and 18.89, 17.99, 7.52, 17.81, 18.2 × 10⁻⁴F.cm⁻² for systems with 0.1, 0.25, 0.5, 0.75, 1.0 mmol/L of HIND added, respectively. At the Helmholtz level, inhibitor molecules are anticipated to displace water molecules due to their higher dielectric constant, increasing the thickness of the adsorption layer, which consequently lowers the dielectric constant, hence reducing the C_eff.dl [61]. However, as immersion times lengthen, the passive/inhibitor layers start to weaken due to attacks from chloride ions, thereby decreasing film and charge transfer resistances [62]. The C_eff.dl values in the 0.5 mmol/L of HIND are relatively lower (by an order of 10⁻¹) than those in all other systems, signifying the optimal HIND concentration required to form a superior protective layer, thus curtailing the corrosion process of steel rebar in ClSCPS.

The calculated inhibition efficiency (ƞ %) is presented in Table S3 in the supplementary information file. Table S4 reveals that the inhibition efficiency of 0.5 mmol/L of HIND was 42.3%, 43.3%, 57.1%, 58.8%, 58.9%, and 78.9% at immersion periods of 1, 48, 120, 240, 480, and 720 h, respectively. A decrease in inhibitor efficiency at concentrations higher than optimal conditions (0.5 mmol/L) - i.e., 0.75 and 1.0 mmol/L - is likely attributable to the congestion of inhibitor molecules in the solution [44], [63]. Furthermore, the inhibitor's efficiency rise over longer immersion periods (720 h) verifies that inhibitor molecules effectively impede corrosion initiation by forming a protective barrier against chloride ions.

Together, it becomes evident from EIS data that the duration of immersion plays a crucial role in affecting the longevity of the protective layers on the steel rebar—particularly the inherent oxide layer and the additional layer formed by the HIND inhibitor. Over time, as the steel is continuously exposed to ClSCPS, these layers undergo a degradation process instigated by the persistent attack from chloride ions. The corrosion resistance of the steel rebar inevitably decreases as these protective layers weaken, a fact demonstrated by the noticeable reduction in the values of R_CT and R_f over prolonged immersion times.

Nonetheless, it is significance to observe that even under these extended immersion conditions, the samples inhibited with the HIND inhibitor manage to sustain comparatively higher R_CT and R_f values than their uninhibited counterparts. This key observation suggests that despite the continuous chloride attack, the HIND inhibitor effectively contributes to a significant delay in the onset of corrosion. This emphasizes the potential of HIND as a valuable compound in the combat against corrosion, particularly in chloride-rich environments.

Potentiodynamic polarization curves

The potentiodynamic polarization curves (PPC) were used to evaluate the anodic and cathodic corrosion reactions of steel rebar immersed in ClSCPS and the effect of the tested organic corrosion inhibitor on their responses. Fig. 3(a) depicts the PPC of steel rebar immersed in blank and inhibited solutions for 720 h. Analyzing reported curves leads to extracting the corrosion kinetics data listed in Table S3 (supplementary material file). A visual inspection of PP curves for inhibited systems shows a wide passive range in the anodic branches. This passivity would remain stable until it reaches a critical pitting potential, mainly at more positive potentials [62]. Besides, the corrosion potential (E_corr) shows a considerable shift towards a nobler region after adding an increased dosage of HIND inhibitor. To be more precise, the corrosion potential of the steel rebar immersed in the blank solution is positioned at −725 mV/SCE. When increasing the inhibitor concentration from 0.1 to 0.5 mmol/L, the corrosion potential shifts to −657 mV/SCE. This shift in corrosion potential is associated with a remarkable decrease in current density (i_corr). Numerically, the corrosion current density drops from 22.7 × 10⁻⁶ A/cm² for the blank test to 3.85 × 10⁻⁶ A/cm² in the presence of 0.5 mmol/L of the HIND compound. The corrosion current density (i_corr) for steel rebar in ClSCPS with 0.5 mmol/L of HIND is 5.89 times lower than in the blank (uninhibited) system. However, when the concentration of the inhibitor exceeds 0.5 mmol/L, there is a slight increase in the i_corr values. To illustrate, the i_corr values for steel rebar in ClSCPS with 0.75 and 1.0 mmol/L of HIND are 4.691 and 6.335 × 10⁻⁶ A/cm², respectively. These values are 1.22 and 1.64 times higher than for the system with 0.5 mmol/L of HIND. This again suggests that as the concentration of HIND escalates, a clustering effect occurs [48], which reduces the adsorption of the inhibitor on the steel rebar, resulting in an increase in i_corr values.

Fig. 3.

(a) Potentiodynamic polarization curves for the steel rebar exposed to ClSCPS with various HIND concentrations at 720 h; (b) same curves with E_corr values shifted to zero.

Shifting the corrosion potentials of all PP curves to zero would make it easier to explain how inhibitor molecules affect anodic and cathodic corrosion reactions. As illustrated in Fig. 3(b) (with E_corr shifted to zero), the inhibitor's concentration results in a considerable reduction of both anodic and cathodic current densities compared to the blank system. This indicates that the inhibitor primarily hinders both anodic and cathodic corrosion processes, leading to a combined inhibition impact [64], [65]. Additionally, both the anodic (β_a) and cathodic slopes (β_c) exhibit a slight decrease in the presence of the HIND inhibitor, indicative of its role in mixed-type corrosion inhibition [66]. Within the system containing 0.5 mmol/L of HIND, the βa and βc values show a minor reduction compared to the blank sample, attributable to the inhibition of the O₂ depolarization process [16]. The anodic slope is also affected by the addition of the inhibitor to ClSCPS, further illustrating that the inhibitor molecules concurrently influence both cathodic and anodic corrosion reactions.

Numerically speaking, the results in Table S3 (supplementary material file) confirm the considerable decreasing shift in corrosion current densities that occurred in the presence of HIND compared with the blank test, which resulted in a substantial decline in corrosion rate values. The corrosion inhibition efficiency peaks at 83% with 0.5 mmol/L of the inhibitor, marking this as the optimal condition for the prolonged protection of steel rebar in ClSCPS. Aligning with findings from earlier methods, the inhibitory capabilities of the tested inhibitor decline when concentrations exceed 0.5 mmol/L.

Adsorption isotherm

The adsorption on metal surfaces is the primary corrosion inhibition mechanism by organic compounds [17], [67]. It is noticed from weight loss results that there is a strong correlation between the inhibitor's concentration and the rate of metal surface area covered by inhibitor molecules. The surface area coverage of steel rebar is increased with inhibitors’ concentration up to 0.5 mmol/L. In this section, several adsorption isotherm models are evaluated to establish a simple relationship between experimental data and theoretical adsorption models (supplementary material file; Fig. S6). Among tested models, excellent results are found using the Langmuir adsorption isotherm model (Fig. 4). The surface coverage (θ) values obtained from weight loss, EIS, and potentiodynamic polarization studies can be used in adsorption isotherm fitting, utilizing the Langmuir isotherm model, which is represented by the following equation [44]:

$$\frac{C_{\mathit{inh}}}{\theta }=\frac{1}{K_{\mathit{ads}}}+C_{\mathit{inh}}$$ (11)

Fig. 4.

Langmuir adsorption isotherm model for HIND adsorption on the steel rebar from weight loss (a), EIS (b) and potentiodynamic polarization methods (c) (after 720 h immersion periods).

C_inh denotes the inhibitor's concentration, θ represents the inhibitor molecules’ surface area coverage on the steel rebar surface. K_ads is the equilibrium constant.

The following equation represents the relation between standard adsorption (Gibbs) free energy (ΔG⁰_ads) and K_ads [44].

$${\mathrm{\Delta }G}_{\mathit{ads}}^0=-RTln(55.5K_{\mathit{ads}})$$ (12)

ΔG⁰_ads denotes the standard adsorption (Gibbs) free energy, ‘T’ and ‘R’ are absolute temperature (298.15 K) and gas constant (8.314), respectively. Thermodynamic parameters are grouped in Table S4 (supplementary material file).

The reported parameters show that the adsorption-free energy (ΔG⁰_ads) value obtained from the Langmuir isotherm model is negative, suggesting spontaneous adsorption of inhibitor molecules over the steel rebar and making a stable passive layer [68], [69]. Besides, ${\mathrm{\Delta }G}_{\mathit{ads}}^0$ values from the Langmuir model are −35.77, −38.07 and −37.50 kJ/mol from weight loss, EIS and potentiodynamic polarization studies, respectively. The value in the range of −20 and −40 kJ/mol, signifying an adsorption process that mainly taken place through a blend of physical and chemical interactions [70], [71]. In the case of organic corrosion inhibitors, in most cases, physical adsorption is the first essential step in inhibiting metal corrosion [72], [73]. When inhibitor molecules approach the steel surface, a charge transfer can occur between free electron pairs on heteroatoms and/or π-electrons in aromatic rings with vacant d-orbitals of Fe-atoms, creating therefore, a protective barrier against corrosive species [44]. Nonetheless, it is important to recognize that satisfying every criterion of an isotherm is not a readily attainable objective [74]. Consequently, this examination of the isotherm and its associated parameters is presented as semi-quantitative indicators of the isotherm's shape. However, it is important to note that definitive conclusions cannot be drawn based solely on this analysis [32], [74], [75], [76].

X-ray photoelectron spectroscopy analysis

The XPS is a sufficiently sensitive method to analyze the chemical state of steel rebar in ClSCPS without and with HIND inhibitors. In this part of our research, XPS is conducted to figure out the underlying mechanism controlling the corrosion and corrosion inhibition of steel rebar by the investigated hydrazone in a chloride-contaminated concrete environment.

XPS survey spectrum for steel rebars exposed to ClSCPS without and with 0.5 mmol/L of HIND is shown in Fig. S7 (supplementary material file). In blank and inhibited solutions, intensive peaks related to Fe 2p, O 1s, and C 1s are detected for steel rebars. However, an exception is the detection of an additional peak related to N 1s for the steel rebar in an inhibited solution and a small decrease in Fe 2p signal due to the inhibitor’s adsorption effect.

In Fe 2p spectra of the blank test (Fig. 5(a)), three distinguishable peaks are observed at 706.93, 708.29, and 710.55 eV. The peak at 706.93 is mainly associated with the Fe⁰ valence state, while those located at 708.29 and 710.55 eV are ascribed to iron oxides that may exist in the form of Fe₂O₃ and Fe₃O₄ and hydroxyl groups (FeOOH, or Fe(OH)₂) [77], [78], [79]. The XPS analysis of steel rebar immersed in inhibited conditions (Fig. 6(a)) reveals the same chemical states with a slight shift in binding energy values. This shift in binding energy values confirms the chemical action between the iron surface and adsorbed inhibitor molecules [80].

Fig. 5.

XPS spectra for steel rebar exposed to ClSCPS with 0 mmol/L of HIND (blank) after 720 h exposure Fe2p (a); O1s (b) and C1s (c).

Fig. 6.

XPS spectra for steel rebar exposed to the ClSCPS with 0.5 mmol/L of HIND after 720 h exposure Fe2p (a); O1s (b) C1s (c) and N1s (d).

The O 1s XPS spectra are decomposed into three prominent peaks in both uninhibited and inhibited mediums, as shown in Fig. 5(b) and 6(b). The O 1s photoelectron peaks of steel rebar in blank solution are located at 528.78, 530.35, and 532.28 eV, while those obtained at inhibited conditions showed a small shift to 528.37, 530.10, and 531.67 eV. Peaks detected at 528.78/528.37 eV and 530.35/530.10 eV are, respectively, associated with O²⁻ in iron oxides [81], [82], and OH⁻ in iron hydroxides [82], [81], while those located at 532.28 eV (in blank solution) and 531.67 eV (in inhibited solution) are mainly ascribed to OH⁻ in Ca(OH)₂ [81], and oxygen double bonded to carbon (C = O) of the inhibitor molecule [83], [84]. Most of these results provide compelling proof that passive film forms on the steel rebar surface with the absence and presence of the inhibitor. So, C 1s and N 1s XPS spectra would provide useful information about the corrosion protection abilities of the investigated hydrazone.

The curve-fitting of the C 1s spectrum of steel rebar in inhibited solution is depicted in Fig. 6(c), and it is decomposed into three peaks at 285.05, 285.95, and 288.44 eV. The first peak at 285.05 eV is related to different carbon bonds such as C–C, C = C, and C–H present in the inhibitor molecule [85]. The second peak at 285.95 eV corresponds to C = O of the carbonyl group of the HIND molecule [86]. The third and last peak at 288.44 eV is attributed to C-O and/or C = O [87], [88].

The N 1s XPS spectrum can be split into three peaks at 399.98, 401.05, and 403.48 eV (Fig. 6d). The peak located at 399.98 eV can be assigned to non-bonded nitrogen, while that at 401.05 eV is due to the coordination of nitrogen atoms of inhibitor molecules with Fe atoms (N-Fe) [89], [90], [91]. The third peak detected at 403.48 eV can be assigned to molecular N₂ of the hydrazone functional group [92], [93].

XPS results indicate that, under certain conditions, iron oxides and hydroxides form as corrosion products on the steel rebar's surface. There is strong evidence of chemical HIND-iron interactions, leading to a non-permeable inhibitor layer. This layer interacts with a pre-existing iron oxide layer, creating a protective barrier against chloride ions and enhancing steel rebar protection. The presence of –C = O, C–C, C–N, –C–H, and –C = C on the steel surface confirms that the HIND molecules' unpaired electrons in their functional group can be transferred to vacant d-orbitals of iron atoms. The steel rebar has a positive charge in chloride-contaminated alkaline solution, suggesting that chloride ions would initially adsorb on the steel surface and then act as adsorption intermediates as the first physical adsorption step [94]. Besides, it should be observed that these findings are obtained from samples immersed for 720 h, which are supposed to be less promising as demonstrated by electrochemical and weight loss tests. It signifies inhibitor molecules' strong adsorption and affinity for engaging with the steel rebar surface and subsequently forming a protective layer.

Further, the peak area obtained from XPS spectra was used to calculate the ratio of the Fe²⁺/Fe³⁺, and the corrosion prevention of HIND molecules on the steel rebar surface was investigated, as listed in Table S5 (supplementary material file). It can be noticed from Table S5 that the Fe²⁺/Fe³⁺ ratio for the steel rebar surface immersed in the ClSCPS with 0.5 mmol/L of HIND is obviously higher than the blank samples, indicating the passive layer’s corrosion resistance behavior. This might be the result of HIND molecules inhibiting corrosion reactions (anodic and cathodic reactions) on the steel rebar surface. For instance, the Fe²⁺/Fe³⁺ ratio is 1.02 in the blank and 1.96 in the presence of 0 0.5 mmol/L of HIND, with the blank value being lower. This is due to the transformation of Fe²⁺ to Fe³⁺ oxide being higher in the blank samples, which contributes to its highest corrosion reactions [62].

SEM/EDS analysis

SEM analysis can provide an overall view of how the inhibitor affects the surface morphology of steel rebar when it is added to ClSCPS. SEM images of steel rebar immersed in ClSCPS and after adding the optimum concentration of tested inhibitor are represented in Fig. 7a-f. Figure labels from (a) to (c) and from (d) to (f) refer to an increased magnification from 1 kx to 10 kx of steel rebar in ClSCPS and inhibited ClSCPS, respectively. In the case of steel rebar immersed in blank conditions (Fig. 7a-c), it can be noticed that there are many unstructured corrosion products spread over the steel rebar surface. A close zoom of the steel rebar surface shows that corrosion products have an unstructured morphology, mostly attributed to an oxide layer composed of iron oxides and iron hydroxide products. For a further examination of the unprotected steel rebar morphology, elemental analysis is carried out as shown in Fig. S8a (supplementary material file). At a specific point, elemental analysis shows that the unstructured layer observed on the surface the of rebar contains mainly iron with 81.14 wt%, followed by O having 12.01 wt%, and then 2.37 wt% of chloride ions and 2.83 wt% of carbon atoms. These confirm the visual-based conclusion that the observed layer is constituted mainly of iron oxide and iron hydroxide products.

Fig. 7.

SEM surface morphology for steel rebar exposed to the ClSCPS with 0 (blank) (a-c) and 0.5 mmol/L of HIND (d-f) at various magnifications of 1 k (a&d); 5 k (b&e); and 10 k (c&f), after 720 h exposure.

Moving to the steel rebar morphology in the inhibited solution shown in Fig. 7d-f, one can notice a wide smooth surface area along with randomly adsorbed inhibitor molecules on the steel rebar surface. It is well-known that organic corrosion inhibitors tend to be adsorbed in a tree or a flower-like architecture, which is obvious from higher magnification images. However, quantitative analysis by the EDS technique is required for more definite conclusions. As seen in Fig. S8b (supplementary material file), elemental analysis reveals a considerable increase in wt.% of C, and O atoms while showing a big drop in wt.% of Fe, in addition to the newly detected nitrogen atom. Compared to blank conditions, the wt.% of C and O increases from 2.83 wt% and 12.01 wt% (blank) to 16.78 wt% and 24.42 wt% (with inhibitor), respectively. This was combined with a drop in wt.% of Fe from 81.14 in blank to 27.37 wt%. The same can be mentioned for Cl, which dropped from 2.37 wt% in blank to 1.53 wt% in inhibited solution. These remarks confirm and reinforce the above conclusions that inhibitor molecules are adsorbed on the surface of steel rebar, forming a barrier layer that hinders the corrosion process. The presence of a small percentage of Fe is due to the pre-existed iron oxide layer and/or to defects in the formed protective layer at a longer immersion time.

XRD analysis

X-ray diffraction technique can offer useful complementary insights for understanding the chemical state of steel rebar surfaces in both uninhibited and inhibited conditions. Although a more sensitive method like XPS would be sufficient to achieve this goal, drawing conclusions based on several analyses will be useful. Fig. S9 (supplementary material file) depicts the XRD results of steel rebar exposed to ClSCPS without (a) and optimum inhibitor concentration (b) after 720 h of exposure time. The analysis of XRD peaks of steel rebar in blank solution identified in the recorded range shows that iron, iron oxides, and iron hydroxides dominate the steel rebar surface. For example, the diffraction peaks appeared at 2θ = 44.47° and 82.16° correspond to (1 1 1) and (2 1 1) crystal plane of iron (Fe), which is matched with standard JCPDS No: 98–000-0259. The diffraction peaks at 2θ = 36.06° and 40.34° attributed to the (1 1 1) and (2 1 0) crystal plane of FeO (JCPDS: 00–006-0615) and FeOOH (JCPDS: 01–084-8277), respectively. The peaks identified at 2θ = 39.00° and 66.31° are related to the (0 0 2) and (2 0 0) crystal plane structure of Fe(OH)₂ (Amakinite), which coincides with JCPDS No: 01–081-8012. It can be noticed from Fig. S9a that the peaks observed at 2θ = 58.49° and 64.87° correspond to the (0 1 8) and (3 0 0) crystal plane structure of Fe₂O₃. This indicates the formation of FeO/FeOOH is unstable due to its porous nature and perpetual attack of chloride ions: hence it can form Fe₂O₃ (corrosion products). When the inhibitor is incorporated in a chloride-contaminated solution, peaks related to iron become more prominent while those associated with iron oxides significantly decrease and some disappear (Fig. S9b). For instance, the diffraction peaks at 2θ = 44.66, 64.95 and 82.30 ascribes to (1 1 0) (2 0 0) and (2 1 1) crystal plane structure of Fe (JCPDS: 998–000-0259). Further, the peaks at 31.71 are corresponding to the (1 0 0) crystal plane structure of Fe(OH)₂. These results prove that the steel rebar surface is covered by a protective inhibitor layer that serves as a barrier against corrosive particles, thus preventing, or more precisely delaying the metal dissolution process.

AFM analysis

Surface characteristics of steel rebar surfaces can be further analyzed by atomic force microscopy. Particularly, information about total roughness and a specific surface area can be determined, which would be useful in comparing the morphology of the steel rebar surface before and after adding the inhibitor to the chloride-contaminated solutions. Fig. 8(a) and 8(b) show 2D and 3D graphical representations of AFM results for steel rebar in blank solution, while Fig. 8(c) and 8(d) show the same dimensions of the surface state of steel rebar in inhibited solution.

Fig. 8.

AFM image (2D (a &C) and 3D (b&D)) for steel rebar exposed to the ClSCPS with 0 (blank) (a & b) and 0.5 mmol/L of HIND c & d after 720 h exposure.

The calculated surface roughness of the entire scan range is 183.5 and 50 nm for uninhibited and inhibited steel rebar, respectively. A decrease in surface roughness suggests low corrosion products and the potential formation of a smooth inhibitor layer over the steel rebar surface [95], [96]. This conclusion can also be drawn from visual analysis of AFM images of steel rebar in uninhibited and inhibited conditions. Very distinguishable surface characteristics can be observed from these images, signifying that a compact and more homogenous inhibitor layer or a combination of inhibitor and iron oxide layer is created over the steel rebar surface after adding the optimum concentration of the investigated hydrazone molecule.

SCC-DFTB simulations

Geometries and energies

Computational calculations based on ab initio DFT are very accurate prediction tools of molecular properties [32]. Their predictive accuracy allows the investigation of molecules’ reactivity and their bonding characteristics with other species. The DFTB, as a semi-empirical quantum mechanical method, provides the same accuracy with much faster time than ab initio DFT, which makes it a perfect choice for large-size systems [34]. The most stable adsorption geometries of the HIND molecule are investigated by DFTB simulation, as shown in Fig. 9, aiming to provide a computational explanation of the bonding characteristics of the tested compound.

Fig. 9.

The DFTB-optimized adsorption geometries of HIND molecules on the Fe(1 1 0) surface illustrated in (a) for the first orientation and (b) for the second orientation.

As can be seen from Fig. 9a, the first orientation of the HIND molecule tends to parallelly adsorb on the metal surface through its entire molecular structure. After adsorption, the HIND molecule establishes multiple bonds with iron atoms through the carbon atoms of the phenyl rings, specifically the indole and dimethylaminophenyl components. However, no bonding is observed involving the nitrogen and oxygen atoms. Phenyl rings are electron-rich sites with several π-systems that can coordinate with electron-deficient d-orbitals of iron, whereas nitrogen atoms are mainly involved in physical interactions [37]. The carbon atoms of the indole moiety form three bonds with iron atoms with length distances of 2.32, 2.28, and 2.27 Å. On the other hand, carbon atoms of the dimethylaminophenyl moiety form two Fe-C bonds with distances of 2.33 and 2.26 Å. When the initial orientation of the HIND molecule changed, its adsorption geometry is significantly different. As observed from Fig. 9b, the second orientation of the molecule bonds with the iron surface only through carbon atoms of the dimethylaminophenyl moiety. Upon adsorption, the molecule forms four Fe-C bonds with length distances between 2.26 and 2.30 Å. It has been informed that the sum of the covalent radii for Fe-C (r_C + r_Fe) is 2.08 [97]. The optimized adsorption geometries reveal that the Fe-C bond lengths closely match the sum of their corresponding covalent radii compared to the predicted bond distances. This indicates that HIND molecules likely form strong chemical interactions with the iron surface.

The interactive force and thermodynamical stability of both adsorption configurations can be assessed by calculating the interaction energy of each adsorption system [98]. The calculated interaction energies of the first and second geometries of the HIND@Fe(1 1 0) surface are −2.34 and −0.54 eV, respectively. The adsorption of the HIND molecule is favorable and thermodynamically stable, considering the obtained negative interaction energies [98]. Recently, a DFTB simulation of a hydrazone derivative (noted FMAH) with a furan moiety instead of dimethylaminophenyl has been reported under similar experimental and theoretical conditions [37]. It has been found that the FMAH@Fe(1 1 0) had interaction energy of −1.27 eV for the parallel adsorption configuration, while other configurations had interaction energies of −0.53 and −0.61 eV. The stronger negative interaction energy of HIND may explain its superior corrosion inhibition performance compared with the FMAH compound. Besides, simulation results reveal that the investigated compound has excellent bonding characteristics with iron atoms, which explains its good corrosion inhibition performance. The substitution of furan by dimethylaminophenyl moiety seems to have a positive impact on the bonding properties of the tested hydrazone.

Projected density of states

The DFT-optimized adsorption geometries of HIND molecules on Fe(1 1 0) surface can be analyzed through the projected density of states to investigate their electronic structure [99]. PDOS is a useful theoretical tool to assess the nature of bonding in adsorption systems [100], [101]. Figs. S10a-c (supplementary material file) represents PDOS plots of HIND molecules before and after their adsorption on the iron surface. Before the adsorption, where molecules are placed far from the iron surface, several s,p-peaks of HIND molecule can be observed within the energy range of −5/5, which is the same energy range of 3d iron orbitals [102]. The presence of such peaks within that energy range means that there will be a high chance for hybridization between molecule and iron orbitals upon adsorption. By inspecting results in Figs. S10b-c, one can notice that all peaks related to HIND molecule shown in Fig. S10a disappear and decrease in intensity after the adsorption of molecules on the iron surface. Besides, a slight shift in energy to lower values can be noticed after the adsorption. These significant changes are likely attributed to hybridization interactions [102]. Charge transfer causes the inhibitor's molecular orbitals, notably its p-orbitals, to hybridize with metal d-states, generating hybrid orbitals. As a result, notable alterations occur in the peaks of the projected density of states (PDOS), with a substantial redistribution observed within these peaks [103], [104]. These results suggest that HIND molecules can chemically bond to iron atoms through more than one adsorption configuration.

Mechanism of corrosion inhibition

The corrosion mechanism of steel rebar embedded in concrete under a marine environment is mainly influenced by the presence of chloride ions. In the chloride environment, the oxygen reduction reaction takes place at the cathodic site (equation (13)), being the electron acceptor reaction from the oxidation of the steel rebar (equation (14)).

$$2H_{2}O+O_2+4e^{-}\to {4OH}^{-}$$ (13)

$$\mathit{Fe}\to {\mathit{Fe}}^{2+}+{2e}^{-}$$ (14)

Dissolution of the iron matrix, in a chloride environment, occurs through a series of auto-catalytic reactions producing a Fe₃O₄ (corrosion product) [105]. The result of these reactions suggests that the corrosion products occupy a higher volume than the volume of the steel rebar. It creates the internal stress of concrete, which leads to cracking or spalling on the concrete surface: thus unexpected failure happens, reducing its service life. Therefore, preventing or hindering either the cathodic or anodic, or both reactions, is essential for effective inhibition.

In this investigation, the addition of HIND to the contaminated concrete pore solutions causes a substantial reduction in both cathodic and anodic current densities compared to the control test, as demonstrated by PPC. This observation implies that the inhibitor molecules primarily hinder both anodic and cathodic corrosion processes, resulting in a combined-type inhibitory action. The HIND compounds under alkaline condition get decomposed to form R₁R₂+, =N-, and –NH₂- and becomes highly polar. The polar end of R₁R₂ + tends to adsorb on the cathodic site of the steel rebar surface, which slows down the cathodic reaction. Moreover, the presence of = N-, and –NH₂-, –OCH₃, etc. in the HIND compounds can adsorb on the anodic site (XPS results confirm the presence of N on the steel rebar surface) of the steel rebar surface via electrostatic attraction (physisorption). Then, free electron pairs on heteroatoms and/or π-electrons in aromatic rings can coordinate with Fe-atoms (vacant d-orbitals) by chemisorption as confirmed from SCC-DFTB simulations. These anodic and cathodic adsorptions of HIND molecules improve the passive layer and thus reduce the formation of corrosion products on the steel rebar surface. The diagram of the proposed inhibition mechanism is given in Fig. S11 (supplementary material file).

Comparison with other organic inhibitors

The most typical dosage of organic corrosion inhibitors in a simulated concrete environment is provided in Table S6 (supplementary material file) for comparison purposes. Table S6 shows the efficiency of organic corrosion inhibitors and most of the experiments were carried out in saturated Ca(OH)₂, and SCPS mediums. Reported data demonstrate that the examined HIND molecule had significantly higher corrosion effectiveness compared to other inhibitors, demonstrating its superior corrosion inhibition performance. The HIND compound demonstrated an inhibitory efficiency of 88.40% in the ClSCPS at a very low dose of 0.5 mmol/L. The proposed HIND compound has remarkable anti-corrosive capabilities, outperforming most previously reported data when compared to specific previously published results and taking into account the relationship between concentration and efficiency. These encouraging findings highlight the significance of these compounds' nitrogenous molecular structures in preventing steel rebar corrosion in SCPS with chloride contamination.

Conclusions

The effect of a new hydrazone derivative; (E)-N'-(4-(dimethylamino)benzylidene)-2-(5-methoxy-2-methyl-1H-indol-3-yl)aceto- hydrazide (HIND), on the corrosion mitigation of steel rebar exposed to ClSCPS solution has been investigated. A maximum concentration of HIND at 0.5 mmol/L prohibited 88.4% of corrosion as observed from weight loss methods. Electrochemical impedance spectroscopy tests indicated that increasing the concentration of HIND up to 0.5 mmol/L significantly increased R_CT values of steel rebar compared to all other systems, confirming the effective hindering of corrosion reactions by the HIND molecules in the corrosive medium. PPC revealed that the presence of molecules in the corrosive medium exerted a mutual inhibition effect on anodic and cathodic corrosion reactions. The corrosion inhibition efficiency was 83% at 0.5 mmol/L of inhibitor, as observed from PPC results, which is considered optimum concentration for the long-term protection of steel rebar in ClSCPS. Strong evidence for the formation of the oxide layer and adsorbed inhibitor film on steel rebar of blank and inhibited tests, respectively was achieved from XPS results, forming a non-permeable inhibitor layer that was probably combined and interacted with a pre-existing iron oxide layer. SEM/EDS, AFM, and XRD results were consistent with XPS analysis and confirmed the formation of a protective barrier consisting of randomly adsorbed inhibitor molecules on the steel rebar surface. Theoretical simulations by SCC-DFTB revealed that coordination between iron vacant orbitals and HIND’s reactive atoms could be the main interacting mechanism. Research on corrosion inhibitors could enhance infrastructure durability, minimize environmental harm, and boost concrete performance, providing cost-effective solutions for the construction industry. This understanding extends the lifespan of critical structures like bridges and buildings, resulting in lower maintenance costs and increased public safety. Overall, it fosters the development of more efficient, sustainable infrastructure, contributing positively to society.

Compliance with Ethics Requirements

This article does not contain any studies with human or animal subjects.

CRediT authorship contribution statement

Karthick Subbiah: Formal analysis, Data curation, Methodology, Conceptualization, Writing – original draft. Han-Seung Lee: Conceptualization, Project administration, Supervision, Funding acquisition, Validation, Writing – review & editing. Mustafa R. Al-Hadeethi: Investigation, Writing – review & editing. Taejoon Park: Conceptualization, Project administration, Supervision, Funding acquisition, Validation, Writing – review & editing. Hassane Lgaz: Formal analysis, Data curation, Methodology, Conceptualization, Writing – original draft.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

The following are the Supplementary data to this article: