Oxygen scavenging activity of smart colophony microcapsules containing nitrite corrosion inhibitors for steel reinforced concrete

In this study, the electrochemical performance of microcapsules (MCs) containing NaNO₂ corrosion inhibitors for steel reinforcements was studied in 0.6 M chloride contaminated simulated concrete pore solutions (SCPS) with varying pH for a period of 28 days. The corrosion inhibition was studied by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The polarization results for the MC samples showed improved corrosion resistance, with an i_corr of 2.54 ×10^–6 A/cm² after 28 days exposure in SCPS. Oxygen scavenging activity from the MCs was observed by reduced cathodic kinetics, displaying decreased cathodic exchange current densities of 2.66 ×10⁻⁸ A/cm², thus imparting cathodic inhibition in conjunction with anodic corrosion inhibitors. The microcapsules additionally displayed improvement over free NaNO₂ inhibitors after sufficient release time. The decrease in cathodic and anodic kinetics, along with the improved corrosion resistance, show a dual synergistic corrosion inhibition of the NaNO₂ loaded colophony MC.

Introduction

Reinforced concrete structures are widely used in global construction industry due to their high mechanical performance, however the corrosion of the reinforcements by aggressive environments lead to deterioration, spalling, loss of integrity, and eventual catastrophic failure (Bazant, 1979; Sánchez-Deza et al., 2018; Zheng et al., 2020). The alkaline concrete environment favors the development of a stable passive film, which impart protection to the reinforcement surface (Macdonald et al., 2020). Concrete reinforcements, however, are susceptible to environmental degradation that can lead to corrosion of the structure (Bastidas et al., 2008; Cao et al., 2022). The major forms of corrosion for rebars used in reinforced concrete are caused by chloride ingress and carbonation, both of which initiate the formation of unstable ferrous oxides (Bastidas et al., 2020; Al-Ameeri et al., 2021). Once chloride ions reach the steel surface and achieve the critical chloride concentration threshold, acid hydrolysis occurs at anodic sites of the reinforcement, initiating pitting corrosion (Kenny and Katz, 2020). Conversely, during the processes of carbonation, atmospheric CO₂ in contact with the concrete matrix reacts to form carbonates and bicarbonates, decreasing the pH of the concrete pore solution from around 12.6 to values near 9 (Andrade, 2020). Moreover, corrosion of the steel reinforcements causes the formation of unstable iron oxyhydroxides at the rebar-concrete interface, conferring increased pressure on the concrete cover, leading to cracking and catastrophic damage to the structure (Bastidas et al., 2020).

Encapsulation provides protection from the loss of corrosion inhibitors due to leaching and premature binding to the admixture in addition to controlled release mechanisms (Calegari et al., 2020). Encapsulation by emulsion via water-in-oil-in-water (W/O/W) double emulsion is an effective method due to its relative scalability and low cost (Ding et al., 2019). In the cement and concrete industry, microencapsulation presents an effective way to deliver corrosion inhibitors (Du et al., 2020). Microcapsules can be synthesized with smart materials to allow for a controlled release with a variety of triggering stimuli such as ionic interactions, temperature, and pH changes (Matsuda et al., 2019; Pan et al., 2022; Zhang et al., 2022). Smart corrosion inhibition can be especially helpful, as losses of corrosion inhibitors is minimized by selective release (Giuliani et al., 2020). Colophony microcapsules containing NaNO₂ corrosion inhibitors were previously developed by the authors, studying the physical characteristics and release properties (Ress et al., 2020). These microcapsules utilize a colophony shell due to its compatibility with concrete, enhancement of concrete’s mechanical properties, encapsulation capacity, and ability to control release (Cánovas et al., 1989; Panda et al., 2013).

Colophony is primarily composed of abietic acid along with its derivatives and other terpenes (Nong et al., 2013). The colophony microcapsules with alkaline release will allow for mitigation of corrosion inhibitor loss during the concrete mixing stage, due to leaching and premature reaction with the binder by controlling the diffusion of the inhibitors, allowing release once the concrete cures to a highly alkaline pH (∼12.6) (P. Kumar Mehta and Paulo J. M. Monteiro, 2014). Additionally, abietic is similar to tannins, which have been shown to decrease dissolved oxygen, in aqueous environments, thus acting as an oxygen scavenger (Scrivenand and Winter 1978).

Oxygen scavengers are molecules that readily oxidize or react with oxygen, thus reducing the availability of oxygen molecules to participate in the cathodic half reaction of the corrosion process. The use of oxygen scavengers to reduce corrosion has been present in industry in the application of boilers, pipelines, etc. (Lakshmi Priya et al., 2005). The use of sulfites removes dissolved oxygen by replacing the cathodic reaction of the corrosion process with oxidation of the sulfite into sulfate, thus reducing available oxygen (Rashid and Khadom, 2020). Similarly, organic molecules with alcohol functional groups react with dissolved oxygen, replacing the cathodic reaction of the corrosion process in favor of oxidation into ketone (Jafar and Fathi, 2015). The use of hydrazine also reacts in place of the metal, forming nitrogen gas (Schremp et al., 1961) Henna extract is made up of aromatic organic compounds, similar to colophony. The main lawsone constituent showed viability as a mixed inhibitor as well as cathodic inhibition via oxygen scavenging through the formation of Fe complexes (Ostovari et al., 2009).

In this study, pH triggered colophony microcapsules are used to deliver NaNO₂ corrosion inhibitors in simulated concrete pore solution (saturated calcium hydroxide, SCPS) and deionized water (DI) contaminated with 0.6 M NaCl. The anticorrosion performance of smart microcapsules with pH triggered release was studied over the course of 28 days in both test solutions with the presence of chloride ions. Both AC and DC methods were used, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP), respectively. The influence of the pH of the chloride containing solutions as well as the synergistic inhibition effect of the colophony based microcapsules were analyzed by studying the electrochemical reaction kinetic parameters of corrosion current density and cathodic exchange current density. The resistances of the passive film and the electrochemical double layer as well as their respective capacitance values were also analyzed to corroborate the DC results and study the changes in the electrode/electrolyte interfaces of the system.

Experimental

Colophony microcapsules (MCs) containing NaNO₂ corrosion inhibitors were prepared using the water-in-oil-in-water double emulsion protocol discussed in previous work by authors (Ress et al., 2020). A double emulsion synthetic route was used, in which colophony (Beantown Chemical) was dissolved using diethyl ether (99% Sigma Aldrich) and mixed with an overhead mechanical stirrer. Then, an aqueous solution of NaNO₂ and poly-vinyl alcohol (PVA, 99% hydrolized) was added dropwise while stirring to form the first emulsion for 5 min. The second emulsion was formed by the dropwise addition of a solution of PVA as a stabilizing agent to maintain the microcapsule structure before finally evaporating the ether solvent at 50 °C. The MCs were filtered and washed with DI before testing and exposure to test media. To prevent premature inhibitor release, microcapsules were stored in synthesis solution (pH < 7.2). The microcapsule size and morphology were verified by scanning electron microscope (SEM) using a Tescan Lyra three microscope using 30 kV excitation signal and a working distance of 7 μm. The microcapsules have been shown to release corrosion inhibitors in solution with pH > 7.2. This occurs due to the deprotonation of colophony (pK_a = 7.2), making the shell increase in solubility, thus increasing the shell porosity, and allowing diffusion of the encapsulated corrosion inhibitor. The release from the microcapsules were studied in-depth in the previous work by the authors (Ress et al., 2020).

The electrochemical tests were carried out in three solutions with varying pH: the near neutral solution of deionized water (pH 6.8) and SCPS (pH 12.6). Both electrolyte solutions were tested with contamination of 0.6 M NaCl, to simulate exposure to marine aggressive environments.

Electrochemical monitoring was performed using a Gamry 600 series potentiostat with a three-electrode setup. The reference electrode used was a saturated calomel electrode (SCE), the counter electrode used was a graphite rod, and the working electrode was a carbon steel rebar. The rebar was prepared by washing and sonicating in ethanol and dried with air. The working area was delimited using non-conductive lacquer to expose the corrugated outer edge of the rebar and connected with a copper wire. First, the open circuit potential (OCP) was conducted for 1 hour, until a steady-state potential was recorded. Secondly, electrochemical impedance spectroscopy (EIS) was performed at OCP using 10 mV r.m.s., a scan range of 10 mHz–100 kHz, and 10 points per decade following ASTM G106-89 (2015). Lastly, potentiodynamic polarization (PDP) scans were conducted from −200 mV_OCP to +200 mV_OCP and returning to −200 mV_OCP with a scan rate of 0.1667 mV/s according to ASTM G61-86 (2018).

The rebars were characterized by scanning electron microscopy (SEM) after exposure using a 30 kV excitation signal and a working distance of 17 μm to image the corrosion products formed. The surface of the rebars exposed to MCs were also analyzed by electron dispersive X-ray spectroscopy (EDX) to study the atomic composition of the surface corrosion products.

Results and discussion

Figure 1 shows an SEM micrograph of colophony microcapsules after filtration. Similar to those shown in the previous study, the microcapsules are spherical in morphology with the presence of pores on the outer shell. The diameter of the microcapsule is variable; however, the average diameter is around 80 µm. The encapsulation efficiency (EE) of the microcapsules was calculated by Eq. 1. The determined EE was determined to be 85% by induced coupled plasma optical emission spectroscopy.

$EE=1-\frac{{\left[{\mathrm{N}\mathrm{a}}^{+}\right]}_{\mathrm{S}\mathrm{u}\mathrm{p}\mathrm{e}\mathrm{r}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{t}}}{{\left[{\mathrm{N}\mathrm{a}}^{+}\right]}_{\mathrm{I}\mathrm{n}\mathrm{i}\mathrm{t}\mathrm{i}\mathrm{a}\mathrm{l}}}(1)$

Where [Na⁺]_Supernatant and [Na⁺]_Initial are the concentrations of Na in the filtered solution after microcapsule synthesis and the initial concentration of Na introduced from NaNO₂, respectively.

FIGURE 1

FIGURE 1. (A) SEM image of colophony microcapsules containing NaNO₂ corrosion inhibitors and (B) oxidation scheme for colophony (abietic acid).

Figure 2 shows the PDP curves obtained from the steel rebar samples exposed to 0.6 M NaCl in DI solution over the 28 days testing period and the E_corr and i_corr values obtained are shown in Table 1. The rebar samples in the presence of free NaNO₂ inhibitor show the most noble E_corr and lowest current density for the majority of the exposure times due to the inhibition mechanism of NO₂⁻ to form a passive oxide layer composed of maghemite (Fe₂O₃) and lepidocrocite (γ-FeOOH) (Dhouibi et al., 2003). The most noble samples, Day 0, measured the most noble E_corr value of –144 mV_SCE as the initial passive layer developed by the corrosion inhibitors has not been depleted from chloride exposure. However, as exposure time increases, the i_corr increases up to 4.24 ×10^–6 A/cm², denoting a loss of corrosion inhibition effectiveness while maintaining values that are lower than the control. The loss of corrosion inhibition of nitrites has been shown with exposure to chlorides, as the aggressive agents are able to cause passivity breakdown of the surface. The MC samples initially display values of E_corr and i_corr between the free inhibitor and blank samples. After 7 days of exposure, however, the MC exposed samples show both E_corr and i_corr values similar to the free inhibitor samples due to the release mechanism of the MC. The anodic kinetic parameters, shown in Table 2, highlight the inhibitor mechanism, as the anodic Tafel slope (β_a) increases in the presence of NaNO₂ compared to the control. The free inhibitor samples show βa of 0.518 V/dec, higher than the control with 0.033 V/dec, thus showing reduced anodic dissolution. Initially, the MC exposed rebar at day 0 shows low inhibition due to the low pH of 6.8 of the DI-water solution, below the pK_a of colophony (7.2), corresponding to a slow inhibitor release (Ress et al., 2020). As exposure time increases, β_a increases to 0.387 V/dec and the β_c increases to 0.156 V/dec. From the i_corr values, it can be seen that both the free NaNO₂ inhibitor and the MC samples are above the threshold for active corrosion (1 μA/cm²), yet they are close in value and nearly half the i_corr value of the control sample (9.21 ×10^–6 A/cm²).

FIGURE 2

FIGURE 2. Potentiodynamic polarization curves obtained for carbon steel rebars immersed in DI with 0.6 M NaCl for the control sample, free NaNO₂ corrosion inhibitor sample, and colophony MCs samples after different time exposure: (A) 0 days, (B) 7 days, (C) 14 days, (D) 21 days, and (E) 28 days.

TABLE 1

TABLE 1. Corrosion potential (E_corr) and corrosion current density (i_corr) values obtained from carbon steel rebar polarized in deionized water (DI) and simulated concrete pore solution (SCPS) contaminated with 0.6 M NaCl.

TABLE 2

TABLE 2. Anodic (β_a) and cathodic (β_c) Tafel slopes obtained from polarization tests in DI and SCPS and their corresponding charge transfer coefficients (α_a, α_c) calculated from the Tafel slopes.

Figure 3 shows the PDP curves for the carbon steel rebars exposed to 0.6 M NaCl in SCPS. The free inhibitor samples show more noble E_corr values (–548 mV_SCE) and current densities near 1 μA/cm² after the 28 days exposure time. The free inhibitor sample showed a similar E_corr of −613 mV_SCE with a higher i_corr than the MC sample with 4.26 ×10^–6 A/cm², nearly twice the MC value. By analyzing the i_corr values for the SCPS exposed samples, all samples initially show a lower risk of corrosion for the initial test, however, the samples exposed to the chlorinated solution for 7 days and longer show values above the 1 μA/cm² threshold. The MCs exposed samples show the most significant effect on β_c, showing a final value of 0.359 V/dec, thus the largest contribution from the colophony on the corrosion inhibition (Brocas et al., 2014). The MC samples also show higher potentials than the control in all the tested solutions due to the passive film formation in alkaline media as well as the larger burst release from the microcapsules. Additionally, the MCs exposed samples show the most significant effect on β_c, showing a final value of 0.359 V/dec, thus the largest contribution from the colophony on the corrosion inhibition, as nitrite acts only as an anodic inhibitor. The authors previously showed the oxidation of the colophony shell after exposure to aqueous solution, thus the oxidation of abietic acid consumes available dissolved oxygen, reducing the cathodic reaction kinetics and acting as an oxygen scavenger. Furthermore, the π-electrons of the abietic acid suggests it acts as an effective organic corrosion inhibitor for steel (Mohamed et al., 2021). Therefore, the improved corrosion protection afforded by the MCs can be attributed to a multi-inhibition system of the released NaNO₂ corrosion inhibitors facilitating passive film formation and oxygen scavenging by the colophony shell. The anodic kinetics also show significant increase to a β_a value of 0.972 mV/dec. This is caused by the pH triggered release, which is highest in the alkaline SCPS solution (Ress et al., 2020).

FIGURE 3

FIGURE 3. Potentiodynamic polarization curves obtained for carbon steel rebars immersed in SCPS with 0.6 M NaCl for the control sample, free NaNO₂ corrosion inhibitor sample, and colophony MCs samples after different time exposure: (A) 0 days, (B) 7 days, (C) 14 days, (D) 21 days, and (E) 28 days.

The i_corr values obtained from the curves through Tafel analysis were plotted in Figures 4A,B and the inhibition efficiency (IE) were calculated using Eq. 2 to analyze the polarization data and observe its trends over time more clearly, see Table 1.

$IE\left(\%\right)=\frac{i_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r},\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l}}-i_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r},\mathrm{i}\mathrm{n}\mathrm{h}\mathrm{i}\mathrm{b}\mathrm{i}\mathrm{t}\mathrm{o}\mathrm{r}}}{i_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r},\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{r}\mathrm{o}\mathrm{l}}}\times 100(2)$

Where i_corr, _control is the corrosion current density of the control sample with no inhibitor and i_corr, _inhibitor is the corrosion current density of the free inhibitor or MC sample. The MC samples show a significant decrease of the IE compared to the free inhibitor, 73% and 93%, respectively. However, after 7 days of exposure, the values were similar, ending with 47% and 42% for the free NaNO₂ and MC samples, respectively. For SCPS, all samples initially show a lower IE. For the 28 days of exposure, the release from the microcapsules was sufficient to cause improved IE values compared to the free NaNO₂ sample.

FIGURE 4

FIGURE 4. Corrosion current density (i_corr) of carbon steel rebars in the presence of NaNO₂ corrosion inhibitors and colophony microcapsules containing NaNO₂ in (A) DI water (pH 6.8) and (B) SCPS (pH 12.6), contaminated with 0.6 M NaCl, and cathodic exchange current densities (i_c) measurements in (C) DI and (B) SCPS.

The change in the cathodic kinetics observed in the polarization curves was further analyzed by calculating the cathodic exchange current density (i_c), which describes the kinetics of the oxygen reduction reaction (ORR) shown in Eq. 3:

${{\mathrm{O}}_2}_{\left(\mathrm{g}\right)}+{2\mathrm{H}}_2\mathrm{O}+4{\mathrm{e}}^{-}\to 4{\mathrm{O}\mathrm{H}}^{-}(3)$

The Butler-Volmer equation in Eq. 4 can be utilized to calculate i_c, see Table 3:

$i_{\mathrm{c}}=i_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}\exp \left[-\frac{{\alpha }_{\mathrm{c}}nF{\eta }_{\mathrm{c}}}{RT}\right](4)$

Where n is the number of electrons involved in the cathodic reaction, F is the Faraday constant, η_c is the activation overpotential (η_c = E_ORR–E_corr), α_c is the charge transfer coefficient of cathodic reaction, R and T are the ideal gas constant and temperature, respectively. The E_ORR values were obtained from literature to be +162 mV_SCE, and +581 mV_SCE for DI (pH 6.8) and SCPS (pH 12.6), respectively (Beverskog and Puigdomenech, 1996). The E_ORR is the electrochemical potential at which oxygen will be reduced in solution to hydroxide ions. Figures 4C,D shows the calculated i_c values for the control, free NaNO₂, and MC samples. The free NaNO₂ samples display an initial decrease in ic from 7.11 ×10⁻¹³ A/cm² to 1.68 ×10⁻¹⁵ A/cm² after 28 days of exposure, indicating the lack of cathodic inhibition, as the cathodic kinetics is similar to the control, thus the corrosion inhibition is strictly imparted from the anodic corrosion inhibitors. The MC samples, however, show lower values. This is caused by the addition of the colophony microcapsules and the oxidation effect of colophony, see Figure 1B (Ren et al., 2015). Tannins and other organic molecules have been shown to exhibit oxygen scavenging activity and have been used as such in the food processing industry and pharmaceutical industries (Dave and Shah, 1997). Lastly, the MC samples show the largest difference of i_c in this solution, with values of 2.64 ×10⁻⁹ A/cm², over two orders of magnitude lower than the control after 28 days of exposure.

TABLE 3

TABLE 3. Cathodic exchange current density (i_c) values for the exposed rebar samples.

The charge transfer coefficient, α, is a parameter depicting the symmetry of the energy barrier for the electrochemical corrosion reaction (anodic α_a and cathodic α_c) (Antonello and Maran, 1999). The transfer coefficient can be calculated from the relationship in Eq. (5) (Bauer, 1968):

$\alpha =\frac{2.303RT}{\beta nF}(5)$

where β is the Tafel coefficient obtained from the potentiodynamic polarization data. The α values are shown in Table 4. The changes in α, while highlighting the reaction symmetry of the energy barrier, is dependent upon both potential and surface coverage for organic species (Kornyshev and Schmickler, 1986). Additionally, the solvation of organic molecules can account for small changes in α, denoting outer-sphere reorganization (Savéant and Tessier, 1982). In the colophony microcapsule, dissolution of the shell, as well as changes in the charge transfer resistance and increased surface coverage of the rebar sample, lead to decreasing values in α_c, reaching values below 0.1, showing a non-reversible electron transfer (α = 0.5 for reversible reactions) (Bauer, 1968). This decrease in cathodic activity evidence the influence of colophony oxidation consuming dissolved oxygen and reducing the cathodic reaction of the rebar, thus acting as an oxygen scavenger. These values deviating from 0.5 highlight the asymmetric polarization of the rebar, while the control and free NaNO₂ inhibitor exposed samples have higher values close to 0.5. This variance is attributed to the cathodic inhibition and oxygen scavenging nature of the colophony shell, whose adsorption onto the rebar and depletion of dissolved oxygen influence the cathodic kinetics for exposed samples.

TABLE 4

TABLE 4. Anodic (β_a) and cathodic (β_c) Tafel slopes obtained from polarization tests in DI and SCPS and their corresponding charge transfer coefficients (α_a, α_c) calculated from the Tafel slopes.

The EIS results obtained in DI with 0.6 M NaCl shown in Figure 5 and Table 5 indicate poor inhibition of the MCs in DI water relative to free inhibitors, due to low release. In the obtained experimental spectra, two regions can be clearly distinguished: a high frequency region, where the ohmic drop is controlled by the electrolyte solution resistance, and a low-frequency region, in which the relaxation process is governed by the electrochemical behavior of the metal-electrolyte interface. The Nyquist spectra obtained during the initial period reveals that the experimental data obtained can be adequately fitted using the electrical equivalent circuit (EEC) shown in Figure 5. In this EEC, Rs represents the electrolyte solution resistance. The R_f is the resistance offered by the passive layer, and the constant phase element (CPE_f) corresponds to the pseudocapacitive behavior of this passive oxide film. The anodic dissolution process induced by Cl⁻localized corrosion attack is represented using R_ct for the charge transfer resistance inside the pits, and CPE_dl for pseudocapacitive behavior of the double layer. Each CPE accounts for the non-ideal capacitance response and depends on the capacitance and on a dimensionless parameter (n) that represents the nonideality of the CPE. The Nyquist plots show an increasing trend for both R_f and R_ct values for MC specimens, reaching 65 Ω cm² and 2.4 ×10⁴ Ω cm² for R_f and R_ct, respectively, thus promoting passive film formation, showing the best corrosion resistance after 28 days of release and exposure. The R_f of the MCs show values near 50 Ω cm², less than free inhibitor which shows decreasing values over time as the chloride is able to depassivate the rebar. Similarly, the R_ct values show the same trend until the final measurement, demonstrating a higher corrosion resistance than the free inhibitor. The Bode plots are shown in Figure 6 for 0 days and 28 days of exposure in solution including the fitting results with the proposed EEC. It can be seen that the plots show a good fitting with the proposed EEC and display a two-time constant behaviour even after prolonged exposure to the test solutions. The phase angle (θ) shows two inflection points, thus indicating two relaxion processes, i.e., two time constants. Additionally, the impedance modulus (Z_mod) corroborates the polarization results, showing the improved performance of the MC samples after 28 days of exposure due to higher Z_mod values in SCPS. Furthermore, the similarity in the Z_mod between the free NaNO₂ and MC samples are consistent with polarization results.

FIGURE 5

FIGURE 5. Nyquist plots for carbon steel rebars in the presence of NaNO₂ corrosion inhibitors and colophony microcapsules containing NaNO₂ in DI water (pH 6.8) contaminated with 0.6 M NaCl after different time exposure: (A) 0 days, (B) 7 days, (C) 14 days, (D) 21 days, and (E) 28 days. The equivalent circuit used for fitting the EIS is also shown.

TABLE 5

TABLE 5. EIS fitting parameters obtained for carbon steel rebar in DI water contaminated with 0.6 M NaCl.

FIGURE 6

FIGURE 6. Bode plots showing experimental and fitting data obtained from EIS analysis for carbon steel rebars in the presence of NaNO₂ corrosion inhibitors and colophony microcapsules containing NaNO₂: (A) 0 days in DI water, (B) 0 days in SCPS, (C) 28 days in DI water, and (D) 28 days in SCPS.

Additionally, the effective capacitance for the electrochemical double layer can be determined according to Eq. 6 (Brug et al., 1984):

$C_{\mathrm{e}\mathrm{f}\mathrm{f},\mathrm{d}\mathrm{l}}=Y_{\mathrm{d}\mathrm{l}}^{1/n_{\mathrm{d}\mathrm{l}}}{\left(\frac{1}{R_{\mathrm{s}}}+\frac{1}{R_{\mathrm{c}\mathrm{t}}}\right)}^{\left(n_{\mathrm{d}\mathrm{l}}-1\right)/n_{\mathrm{d}\mathrm{l}}}(6)$

where C_eff,dl is the effective double layer capacitance; Y_dl is the admittance; n_dl is the CPE exponents of the electrochemical double layer, R_s is the solution resistance, and R_ct is the charge transfer resistance. C_eff,dl was calculated (see Figure 7 and Table 5 and Table 6 for DI water and SCPS, respectively).

FIGURE 7

FIGURE 7. The calculated C_eff,dl values for (A) DI, and (B) SCPS contaminated with 0.6 M NaCl.

TABLE 6

TABLE 6. EIS fitting parameters obtained for carbon steel rebar in SCPS contaminated with 0.6 M NaCl.

The EIS results for SCPS are shown in Figure 8 and Table 6. Both the R_f and R_ct of the MC exposed specimens show enhanced values compared to the free NaNO₂, 2.7 ×10³ Ω cm² and 7.9 ×10³ Ω cm², respectively. These results validate the DC testing, indicating enhanced corrosion protection imparted by the MCs, as the MC exposed samples impedance is greater than the free inhibitor alone. Unlike the results obtained for DI, the protection of the MCs in the alkaline SCPS are exhibited immediately, as the alkaline release is triggered, the inhibitors are released. The R_f value for the MC specimens helps substantiate the development of the protective oxide passive film. The free inhibitor sample shows an initial increase from 588 Ω cm² to 3,611 Ω cm², showing a quick development, however, the MC sample surpasses the free inhibitor, a result from triggered release and shows a higher R_f for the remaining test periods. The C_eff,dl values are shown in Figure 7. In DI water, the C_eff,dl values obtained for the 28 days exposed samples in the presence of free NaNO₂ corrosion inhibitor displayed a C_eff,dl value of 1.51 ×10⁻⁴ F cm⁻², close to the MC sample with 1.09 ×10⁻⁴ F cm⁻², thus indicating similar anticorrosion performance. The MC exposed specimens in SCPS show the greatest difference with C_eff,dl values an order of magnitude lower than the free inhibitor for the first 14 days of exposure. Subsequently, the MC exposed specimens show lower capacitance than the inhibitor ending the test duration with a C_eff,dl value of 2.31 ×10⁻⁴ F cm⁻² compared to the free inhibitor with 8.59 ×10⁻⁴ F cm⁻², almost four times lower. More importantly the C_eff,dl shows for the MC shows almost one order of magnitude lower than the free inhibitors for all time periods, thus a lower susceptibility to corrosion. These results corroborate the improved corrosion inhibition efficiency for the MC in alkaline pH as further deprotonation of the colophony occurs and undergoes oxidation, thus more inhibitor molecules are released.

FIGURE 8

FIGURE 8. Nyquist plots for carbon steel rebars in the presence of NaNO₂ corrosion inhibitors and colophony microcapsules containing NaNO₂ in SCPS (pH 12.6) contaminated with 0.6 M NaCl after different time exposure: (A) 0 days, (B) 7 days, (C) 14 days, (D) 21 days, and (E) 28 days.

The SEM images in Figure 9 shows the free inhibitor and MC rebars exposed to SCPS contaminated with 0.6 M NaCl after 28 days of exposure. The MC specimens have visibly less corrosion products on the rebar surface compared to the free inhibitor. The corrosion products are most heavily concentrated on the rebar corrugations, where the geometry of the rebar and residual stresses created during manufacturing promote anodic dissolution (Bautista et al., 2019). Both rebar specimens show corrosion products near the corrugated section and some locations between the corrugations. Additionally, the MC exposed rebar showed microcapsules adhered to the surface of the rebar.

FIGURE 9

FIGURE 9. SEM images of carbon steel rebars exposed to SCPS with (A) 0.6 M NaCl and MCs, and (B) 0.6 M NaCl and NaNO₂ corrosion inhibitors.

In the images shown by Figures 10A,B MCs can be seen adsorbed to the rebar surface. This adsorption suggests that colophony shell of the MCs is indeed playing a more significant role in the inhibition process. Recently, researchers synthesized an imidazoline corrosion inhibitor using colophony, concluding that the new rosin-based inhibitors adsorbed to the metal surface and increased inhibition of imidazoline (Geng et al., 2022). EDX analysis was performed on the rebar exposed to MC in SCPS on locations 1) and (2). The elemental composition shows a large concentration of O, Fe, and Cl atoms. This underlines the large amount of oxidation of the MCs as well as chloride adsorption onto the MC shell. Smaller amounts of Na and N are seen from the NaNO₂ inhibitors encapsulated in the MCs. The SEM and EDX results corroborate the electrochemical tests, showing the ability of the MCs containing corrosion inhibitor to protect carbon steel.

FIGURE 10

FIGURE 10. SEM images, (A) and (B), of and MCs adsorbed to the rebar surface after 28 days exposure to SCPS contaminated with 0.6 M NaCl. The EDX spectra obtained from (C), point (1), and (D), point (2), from the SEM images for the MC surface elemental composition and the composition of the rust, respectively.

Conclusions

Colophony microcapsules containing NaNO₂ corrosion inhibitor were synthesized by W/O/W double emulsion method. The corrosion protection of the microcapsules was studied over 28 days in varying pH solutions containing chlorides. The following conclusions can be made:

• The colophony MCs releasing NaNO₂ corrosion inhibitor showed improved corrosion resistance in DI and SCPS solutions contaminated with 0.6 M NaCl. The MC exposed samples displayed improved i_corr values (2.54 ×10^–6 A/cm²) compared to free NaNO₂ inhibitors (5.96 ×10^–6 A/cm²).

• The cathodic Tafel slopes of the MC exposed samples show decreased cathodic kinetics from the addition of the colophony shell, as the abietic acid oxidizes and absorbs to the rebar surface, providing cathodic inhibition in addition to the release of nitrite inhibitors reducing the anodic kinetics. The change in cathodic kinetics is, in part, due to the oxygen scavenging of the abietic acid, oxidizing the abietic acid similar to mechanisms shown in literature for hydroquinone and henna extract.

• The analysis of the EIS data showed increased charge transfer resistance for the MC exposed samples in SCPS. Additionally, calculations of the C_eff,dl showed the MC exposed samples having improved corrosion resistance compared to the free NaNO₂ samples, especially after 28 days of exposure. (Gaikwad et al., 2020), (Keserovic and Birketveit, 2021), (Sastri and Malaiyandi, 1983), (Yan et al., 2013)

Data availability statement

The datasets presented in this article are not readily available because The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Requests to access the datasets should be directed to ZGJhc3RpZGFzQHVha3Jvbi5lZHU=.

Author contributions

Conceptualization, DMB; methodology, JR, and DMB; formal analysis, JR, UM and DMB; investigation, JR, UM and DMB; resources, DMB; data curation, JR and DMB; writing—original draft preparation, JR, and DMB; writing—review and editing, JR and DMB; visualization, DMB; supervision, DMB; project administration, DMB; funding acquisition, DMB. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by Firestone Research, grant number 639430, and The University of Akron Fellowship, grant numbers FRC-207160 and FRC-207865.

Acknowledgments

The authors acknowledge the technical support and facilities from The National Center for Education and Research on Corrosion and Materials Performance (NCERCAMP-UA), The College of Engineering and Polymer Science and The University of Akron.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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