Comparative Study of Volatile Corrosion Inhibitors in Various Electrochemical Setups

Abstract

Volatile corrosion inhibitors (VCIs) are increasingly used in closed systems affected by atmospheric corrosion. In order to achieve a satisfactory level of protection, an inhibitor must be present in a sufficient concentration that should be determined experimentally. Electrochemical measurements are indispensable in corrosion studies examining the protection efficiency of corrosion inhibitors. Volatile corrosion inhibitors are often examined by electrochemical measurements conducted in a bulk of electrolyte, although they protect metal surfaces from atmospheric corrosion where a thin film of electrolyte is present. The aim of this work is to study the protection of carbon steel by two VCIs on different types of electrodes that allow electrochemical tests in a thin electrolyte film and to compare the obtained results with those obtained in a larger volume in a classical electrochemical cell. For this purpose, disc and comb-like electrodes are used. The investigations are carried out in two corrosion media simulating either a marine or urban polluted atmosphere. Studies are performed on low-carbon S235JR steel, which is typically used for crude oil tank bottoms that often suffer from atmospheric corrosion and are increasingly protected by VCIs. Two benzoate-based VCIs recommended for such application are selected for this study.

1. Introduction

The corrosion of metallic materials is a huge problem for modern industry, as it causes considerable material losses and safety endangerment. To prevent corrosion, or slow it down to acceptable corrosion rates (usually below 0.025 mm/year [1]), various corrosion protection methods are used [2]. The main methods for improving the corrosion resistance of metals are the use of various coatings [3,4,5], cathodic protection [3,4,6], and corrosion inhibitors [3,7,8]. For closed and difficult to access surfaces, the use of corrosion inhibitors is often the best solution, as the application of coatings and their repair on inaccessible surfaces is difficult. Similarly, the design and installation of cathodic protection for closed systems can be complicated, whereas the application of corrosion inhibitors is relatively simple, and a high level of corrosion protection can be achieved. In cases where the metal surface is not immersed in a bulk of electrolyte but is rather covered with a thin layer of electrolyte, volatile corrosion inhibitors (VCIs) are increasingly applied [9,10,11,12,13,14,15,16,17,18,19,20]. These inhibitors are organic substances with vapor pressures of around 10⁻³–10⁻² Pa. VCIs are carried via the gas phase and adsorbed on the metal in a few monolayers, thereby preventing corrosion. VCIs do not affect the physical properties or functionality of the metal, as they act by forming an invisible protective film on the metal surface. The resulting adsorbed monolayer can influence the rate of electrochemical reactions such as metal dissolution or oxygen reduction. Sodium carboxylates, aliphatic amines, cyclohexylamines, benzoates, and nitrobenzoates are the most commonly used VCIs [9,10,11]. They are particularly advantageous for the protection of metals in cavities and other hard-to-reach areas. The commercial use of VCIs (packaging, coatings, and water treatment) began in the 1970s, and nowadays, their use is widespread in various industries [21]. A detailed overview of the development of VCIs and the areas of application, as well as of the different forms of VCI products (powders, papers, and coatings), can be found in the article by Gangopadhyay et al. [12].

Despite the widespread use of VCIs, there are insufficient studies to evaluate their effectiveness. In general, such studies involve the exposure of metallic samples to various corrosive environments (temperatures, humidity levels, corrosive salt solutions, or gases) with or without the presence of VCIs. Usually, a visual assessment of the extent of corroded surfaces on the control and inhibited samples is taken as an indicator of the inhibitor’s effectiveness [21]. One of such experimental methods is the vapor-inhibiting ability (VIA) test, in which samples are exposed to a humid environment with or without preconditioning in an inhibitor atmosphere [14,15]. Unfortunately, such a test cannot be used to quantitatively assess the effectiveness of individual inhibitors, and in particular, it is not possible to determine the inhibition mechanisms. Field testing of VCI effectiveness is usually performed using electrical resistance probes, which determine the rate of corrosion by measuring the change in resistance of a wire as it corrodes and loses the cross-section [22]. Although this method is very helpful for following the extent of corrosion protection over time, it requires long-term measurements and cannot provide information about the inhibitor mechanism.

The review works of Subramanian et al. [9] and Ansari et al. [20] summarize other methods used in the research of volatile corrosion inhibitors, including gravimetric measurements, determination of the inhibitor vapor pressure, ellipsometric determination of the thickness of the inhibitor layer, measurement of the mass of adsorbed inhibitors using an electrochemical quartz crystal microbalance (QCM), and various electrochemical methods. In the gravimetric measurements, the metal samples are stored for several days in closed containers in a humid atmosphere with or without VCI vapors (similar to the VIA test). Inhibitor effectiveness can be calculated by the comparison of corrosion rates in inhibited and corrosive atmospheres [14,15,16,18]. However, gravimetric measurements provide little information about the protective mechanism. A quartz crystal microbalance can be used to determine the mass of inhibitors adsorbed on a metal surface by measuring the shift in resonance frequency of a quartz crystal [9]. Still, the limitation of this technique is that the difference between the mass increase due to the adsorption of the inhibitor and the mass increase due to the formation of corrosion products cannot be distinguished, which limits its applicability for corrosion studies. The adsorption of inhibitors is often studied by various spectroscopic techniques, such as X-ray photoelectron spectroscopy [13,17] or Fourier-transform infrared spectroscopy [15,17]. The evaluation of the inhibitor effect on metal corrosion is also carried out using various surface techniques, such as atomic force microscopy [13,17] or scanning electron microscopy [13,17,18]. Electrochemical methods are the most common methods for corrosion measurements in laboratory studies and are important for deducing the corrosion inhibition mechanism. They mainly include polarization measurements [16] and electrochemical impedance spectroscopy (EIS) [13,17,19,23,24]. However, such tests are mainly carried out in a larger volume of electrolyte, which does not correspond to the real application conditions of volatile inhibitors used for atmospheric corrosion protection.

It is known that atmospheric corrosion takes place in a thin film of electrolyte, which has a considerable influence on the concentration of oxygen and certain ionic species in the vicinity of the metal surface. For this reason, specially designed electrodes are used in the research of atmospheric corrosion, which enable measurements in such conditions. However, they are rarely used in the research of volatile corrosion inhibitors. Examples of such electrodes are disc [25,26] or comb electrodes [27] made of the same materials that build both the working and counter electrodes used for conducting electrochemical measurements in a thin electrolyte film. Since there are very few papers in the scientific literature that use these types of electrodes for the study of volatile inhibitors [11,13,14,15], there is a need for a thorough analysis and comparison of individual experimental systems in order to determine which of them are the most suitable for the study of VCIs and to what extent the results obtained in different systems are consistent. The aim of this work is to investigate different types of electrodes that enable electrochemical tests in a thin electrolyte film and to compare the obtained results with those obtained in a larger volume. The comparative studies were carried out by electrochemical impedance spectroscopy, as it is one of the most commonly used electrochemical methods in corrosion research. It is used both for measurements in the bulk of the electrolyte, as well as in the thin film of the electrolyte. Research is conducted in two corrosion media simulating either a marine or urban polluted atmosphere, with two corrosion inhibitors at different electrolyte thicknesses. The investigations are carried out on low-carbon EN 10025 S235JR steel, which is typically used for crude oil tank bottoms that often suffer from atmospheric corrosion and are increasingly protected by VCIs. Model VCIs are selected from the commercially available inhibitors for this application.

2. Materials and Methods

In this work, the behavior of volatile corrosion inhibitors is studied on different types of electrodes. All electrodes were made of low-carbon steel EN 10025 S235JR (Strojopromet d.d., Zagreb, Hrvatska).

The studies were carried out by using different electrochemical setups:

• A classical three-electrode electrochemical cell in which carbon steel plates (S235JR) were used as working electrodes with an exposed area of 4.91 cm². In this arrangement, a saturated calomel electrode served as a reference and a carbon rod as a counter electrode.
• Two-electrode disc system. A pair of identical metal plates (1 cm × 1 cm) [25] were embedded in parallel in epoxy resin with a 1 mm distance between the plates. In this setup, one steel plate EN 10025 S235JR served as the working electrode and the other as a counter and reference electrode.
• Comb-like electrode [15]. It was made of carbon steel plates (EN 10025 S235JR) with an exposed area of 2 × 0.1 cm, where 8 identical plates were separated from each other with 0.5 mm thick foils before being fixed in epoxy resin [9]. Four of them served as counter electrodes and the other four as working electrodes. A schematic representation of all three experimental setups is shown in Figure 1.

All electrodes were ground with 800, 1200, and 2400 grid paper; degreased with ethanol; and rinsed with distilled water before the experiments.

The studies were carried out in two corrosive media simulating atmospheric corrosion conditions: artificial acid rain solution (0.2 g/L NaNO₃, 0.2 g/L NaHCO₃, and 0.2 g/L Na₂SO₄, pH = 6.5) and 1% NaCl solution (simulating marine atmosphere). Two commercial powder volatile inhibitors (Inhibitors A and B) were examined in various concentrations. The studied model inhibitors are products from two different companies in the USA. Both are based on benzoates and are primarily intended for corrosion protection of the soil side of aboveground storage tank bottom plates. They were dissolved in one of the above-mentioned corrosive electrolytes in selected concentrations. Studies were performed either in cells with 50 mL of corrosive solution (classical three-electrode electrochemical cell setup described above) or under the thin film of electrolyte (setups 2 and 3 described above). The thickness of the thin electrolyte film was controlled by controlling the volume of the electrolyte dripped on the defined electrode surface area by using a micropipette. The electrolyte was then gently spread over the electrode with a metallic syringe such that the thickness of the electrolyte film was approximately 0.5 mm. For the thin film method studies, a closed chamber (3 L) with an electrolyte layer at the bottom of the chamber was used in order to ensure high humidity in the chamber and to prevent evaporation of the thin film.

Corrosion measurements were carried out on all electrode types by using electrochemical impedance spectroscopy. The EIS measurements were performed at an open circuit potential (EOC) with an amplitude of 10 mV and a frequency range of 100 kHz–10 mHz. All investigations were conducted after 1 h of corrosion potential stabilization. The ZsimpWin program was used to fit the experimental data to the selected electrical equivalent circuits.

For the classical electrochemical cell setup, in addition to the EIS, polarization measurements were performed in the potential range from −150 mV vs. EOC to +150 mV vs. EOC, with a scan rate of 0.166 mV/s. The Tafel extrapolation method was used to determine the corrosion parameters from the polarization curves. All electrochemical measurements were performed with BioLogic SP-300 potentiostat (Biologic, Seyssinet-Pariset France).

3. Results and Discussion

3.1. Studies in a Classical Three-Electrode Electrochemical Cell

The initial studies were carried out in a classical three-electrode electrochemical cell with the aim to examine the behavior of two selected VCIs in a bulk of corrosive solution. For this purpose, polarization measurements were performed in artificial acid rain solution and in 1% NaCl solution. Both media are interesting for atmospheric corrosion studies, one simulating an electrolyte covering surface in polluted urban areas and the other simulating the electrolyte in a marine atmosphere.

The polarization measurements were performed in a wide potential range at different concentrations of inhibitors A and B. The aim was to determine at which inhibitor concentrations a high corrosion inhibition is observed.

Figure 2 shows the polarization curves of carbon steel in a blank acid rain solution and with the addition of inhibitor A. The measurements were performed in a wider range of inhibitor concentrations, but the inhibiting effect was observed only above the concentration of 10 g/L. The significant shift of the polarization curves towards more positive potentials and lower current densities at higher inhibitor concentrations indicates that inhibitor A is a predominantly anodic inhibitor. Although the composition of the inhibitor formulation is not fully known, both studied inhibitors are benzoate-based. It is well recognized that benzoates are dominantly anodic inhibitors [28,29,30] that enhance the passivation of steel. For anodic inhibitors, it is very important to determine in which concentration range they provide inhibition, as they can otherwise increase the metal corrosion rate if present in too-low concentrations. Other studies have also shown that relatively high concentrations of benzoate are required to achieve a satisfactory level of corrosion protection and avoid an increase in the corrosion rate [29,30].

The Tafel extrapolation method was used to determine the corrosion current densities at different inhibitor concentrations. The obtained corrosion current densities (j_corr) and corrosion potentials (E_corr), as well as the anodic (β_A) and cathodic (β_C) Tafel slopes, are presented in Table 1. In addition, the inhibition efficiency was calculated according to Equation (1):

$$IE(\%)={\displaystyle \frac{\left(j_{corr}^0-j_{corr}^{inh}\right)}{j_{corr}^0}}\times 100\%,$$

(1)

• j⁰_corr—corrosion current density in a blank solution;
• j^inh_corr—corrosion current density in a solution with an inhibitor;
• IE—inhibition efficiency.

Table 1. Corrosion parameters obtained from polarization curves for carbon steel in acid rain and with different concentrations of inhibitors A and B after 2 h of immersion.

Concentration of Inhibitor (g/L)	Inhibitor	Ecorr (mV)	jcorr (µA/cm2)	βA (mV/dec)	−βC (mV/dec)	IE (%)
BLANK		−750.2	3.485	123.0	182.5
10	A	−782.7	16.576	183.6	229.6	-
20	A	−213.6	0.124	334.9	62.6	96.44
40	A	−118.1	0.131	219.1	164.4	96.24
4	B	−161.2	0.130	380.5	71.9	96.27
10	B	−151.4	0.224	681.5	86.3	93.57
20	B	−171.7	0.316	744.5	86.6	90.93
40	B	−251.9	0.537	196.5	129.5	84.59

Table 1. Corrosion parameters obtained from polarization curves for carbon steel in acid rain and with different concentrations of inhibitors A and B after 2 h of immersion.

Concentration of Inhibitor (g/L)	Inhibitor	Ecorr (mV)	jcorr (µA/cm2)	βA (mV/dec)	−βC (mV/dec)	IE (%)
BLANK		−750.2	3.485	123.0	182.5
10	A	−782.7	16.576	183.6	229.6	-
20	A	−213.6	0.124	334.9	62.6	96.44
40	A	−118.1	0.131	219.1	164.4	96.24
4	B	−161.2	0.130	380.5	71.9	96.27
10	B	−151.4	0.224	681.5	86.3	93.57
20	B	−171.7	0.316	744.5	86.6	90.93
40	B	−251.9	0.537	196.5	129.5	84.59

The data in Table 1 show that a high efficiency of 96.44% was achieved for inhibitor A at a concentration of 20 g/L. When the concentration was further increased to 40 g/L, the efficiency of the inhibitor was not improved. The passivating effect of inhibitor A is visible from the increase in β_A in solutions containing the inhibitor compared to that observed in the blank solution, which indicates the formation of a protective film on the steel surface.

Polarization measurements were also carried out for inhibitor B (Figure 3) under the same conditions. For this inhibitor, a significant shift of the polarization curves towards lower current densities and more positive potentials was observed at lower concentrations than for inhibitor A. The lowest current densities were obtained for a concentration of 4 g/L, while a further increase in concentration did not lead to an improvement in inhibition. Early research on benzoate corrosion inhibitors has attributed this decrease in corrosion inhibition at high concentrations to the difference in solubility of various forms of ferric benzoate complexes [29]. The corrosion parameters shown in Table 1, indicate that inhibitor B achieved the same efficiency as inhibitor A at 10 times lower the concentration. The passivation effect of inhibitor B is observed for all four concentrations, which is evident from the increase in β_A values and more positive corrosion potentials compared to the blank.

The polarization measurements were also carried out in a 1% NaCl solution. The polarization curves obtained in the blank solution and in solutions containing different concentrations of inhibitor A are shown in Figure 4. It can be seen that the highest efficiency is achieved at a concentration of 20 g/L, while a negative effect on steel protection is observed for the lower concentration of 10 g/L. The polarization curves obtained in a 10 g/L (or lower) solution of inhibitor A are shifted towards higher currents and more negative potentials compared to the blank solution. Inhibitor A at a concentration of 20 g/L shows inhibiting properties and induces steel passivation, visible from current densities reduction and a positive shift in corrosion potential. However, at 40 g/L, no further reduction in current densities is observed; rather, the polarization curves are shifted toward more negative potentials compared to 20 g/L inhibitor solution. The effectiveness of the inhibitor (

Table 2. Corrosion parameters obtained from polarization curves for carbon steel in 1% NaCl solution with different concentrations of inhibitors A and B after 2 h of immersion.

Concentration of Inhibitor (g/L)	Inhibitor	Ecorr (mV)	jcorr (µA/cm2)	βA (mV/dec)	−βC (mV/dec)	IE (%)
BLANK		−733.0	4.429	74.5	259.4
10	A	−797.8	9.834	150.4	127.9	-
20	A	−299.4	0.504	85.6	83.7	88.62
40	A	−431.4	1.037	122.4	215.3	76.59
10	B	−682.0	4.943	74.0	255.9	-
20	B	−141.6	0.278	657.3	80.2	93.72
40	B	−605.2	1.604	87.9	318.3	63.78

) is somewhat lower than in the simulated rain solution, which can be related to the fact that chloride ions have a negative influence on the stability of oxide layers.

Figure 5 shows the polarization curves obtained in the presence of inhibitor B in 1% NaCl. At an inhibitor concentration of 10 g/L or less, the inhibition of steel corrosion is not observed. The highest inhibition of steel corrosion is obtained at an inhibitor concentration of 20 g/L, which shows significantly lower currents and a more positive potential in the polarization curves, with an inhibition efficiency of 92% (Table 2). A concentration of 20 g/L is considered optimal for these conditions, while a further increase in concentration decreases the efficiency.

By comparing the results from Table 1 and Table 2 obtained in different media (acid rain and 1% NaCl solution), it can be seen that the studied inhibitors are generally less effective in the 1% NaCl solution than in acid rain, as their efficiencies are lower. This is probably due to the fact that chloride ions destroy the passive layer on carbon steel. The obtained results show that inhibitor A efficiently inhibits the corrosion of steel at concentrations of 20 g/L and 40 g/L in both media. In contrast, inhibitor B shows better effectiveness in acid rain at lower concentrations, while, in the 1% NaCl solution, the highest inhibiting efficiency is observed at a concentration of 20 g/L.

Based on the results obtained, a concentration of 20 g/L was selected for further studies on both inhibitors. These studies were carried out using electrochemical impedance measurements (EIS).

The first electrochemical setup on which EIS measurements were conducted was a classic three-electrode electrochemical cell.

Figure 6 shows Nyquist plots obtained after 1 h of immersion in acid rain solution, with or without the studied inhibitors. It can be seen that the diameters of the impedance loops are much larger in the presence of inhibitors than for the unprotected sample. In other words, the corrosion resistance of carbon steel in the solutions containing inhibitors is much greater than in the blank solution. In accordance with the polarization measurements for the 20 g/L concentration (Table 1), the impedance is higher in the presence of inhibitor A than in the presence of inhibitor B. The experimental data were analyzed by fitting to the equivalent electrical circuit shown in Figure 7. It was necessary to use an electrical model with two time constants to achieve good agreement between the experimental and fitted data. In this model, R_s represents the electrolyte resistance, and the first pair (Q₁ and R₁) describes the capacitance and resistance of an oxide layer on the steel surface, while Q₂ and R₂ describe the double-layer capacitance and the charge transfer resistance of the bare steel surface. Since real electrochemical systems do not exhibit ideal capacitive behavior, the capacitor element was replaced by a constant phase element (CPE). The impedance of the CPE is ZCPE = 1/[Q(jω)ⁿ], where Q is the CPE’s frequency-independent parameter, which represents the pure capacitance when n = 1.

The efficiencies of the inhibitors were calculated according to Equation (2), where R_p = R₁ + R₂.

$$IE(\%)={\displaystyle \frac{R_p^{inh}-R_p^0}{R_p^{inh}}}\times 100\%,$$

(2)

• R_p^inh—the resistance of the steel in a solution with an inhibitor;
• R_p⁰—the resistance of the steel in the blank solution;
• IE—inhibition efficiency for the measured concentration.

The impedance parameters presented in

Table 3. Impedance parameters for carbon steel in the three-electrode system after 1 h of immersion in acidic rain solution and with the addition of 20 g/L of inhibitors.

Solution	Q1 (µS sncm−2)	n1	R1 (kΩ cm2)	Q2 (µS sncm−2)	n2	R2 (kΩ cm2)	IE (%)
BLANK	560	0.98	1.70	350	0.81	1.84
Inhibitor A	52.83	0.91	62.34	29.37	0.74	65.43	97.23
Inhibitor B	84.55	0.90	38.32	51.92	0.67	66.28	96.61

clearly show that the addition of inhibitors to the acid rain solution increases the R₁, i.e., the oxide resistance, which indicates the formation of a protective oxide layer on the surface. In the presence of inhibitor A, the resistance R₁ increases by more than 36 times compared to the blank, while the resistance of inhibitor B is 22 times higher. The charge transfer resistance (R₂) in the presence of both inhibitors is about 36 times higher than in the blank acid rain solution. Both inhibitors significantly reduce the Q₂ values, related to the electrochemical double-layer capacitance, confirming the inhibitor adsorption on the steel surface. This is also reflected in the high inhibition efficiency, which is over 95% for both inhibitors.

The impedance spectra obtained in 1% NaCl solution, with and without the addition of the investigated inhibitors in the 20 g/L concentration, are shown in Figure 8. The diameter of the impedance loop for the blank solution is much smaller than for the EIS spectra in both inhibited solutions. In accordance with the polarization measurements, a higher impedance was observed for inhibitor B. The EIS spectra were fitted to the model presented in Figure 7, while the obtained impedance parameters are listed in

Table 4. Impedance parameters for carbon steel in the three-electrode system after 1 h of immersion in 1% NaCl solution and with the addition of 20 g/L of inhibitors.

Solution	Q1 (µS sncm−2)	n1	R1 (kΩ cm2)	Q2 (µS sncm−2)	n2	R2 (kΩ cm2)	IE (%)
BLANK	399.10	0.81	0.26	169	0.80	3.29
Inhibitor A	301.70	0.83	0.72	63.65	0.50	9.34	64.71
Inhibitor B	217.40	0.85	1.83	96.03	0.57	13.12	76.25

.

The comparison of the impedance parameters for inhibitors A and B with those obtained in the blank solution shows that the addition of inhibitors results in an increased R₁, or oxide resistance. This points towards the formation of a protective oxide layer on the carbon steel surface; however, the increase in oxide film resistance is less pronounced than in the case of the artificial rain solution, which can be ascribed to the corrosive effect of chloride ions present in this solution. The charge transfer resistance (R₂) of inhibitor A has increased threefold compared to the blank, while the resistance of inhibitor B is almost four times higher. The inhibitor adsorption on the metal surface is also confirmed by the decrease in Q₂ values. From the comparison of the inhibiting efficiencies obtained by EIS and Tafel measurements, for the 20 g/L inhibitor concentration in the acid rain solution, it can be concluded that the IEs obtained from EIS for inhibitor A are similar to those obtained by the Tafel method, while, for inhibitor B, they are slightly higher for the EIS measurement. In the 1% NaCl solution, the Ies determined from EIS are somewhat lower than those obtained from the Tafel method, but the same trend is observed, i.e., at this concentration, inhibitor B shows a higher inhibiting efficiency than inhibitor A.

3.2. Studies on the Two-Electrode Disc System

The second electrochemical setup, used for the EIS measurements, was a two-electrode disc system with two identical carbon steel plates (S235JR). Such electrodes are often used for atmospheric corrosion studies in a thin electrolyte layer. The electrode was covered with a thin layer of acid rain solution, with or without the investigated inhibitors, and EIS measurements were performed after one hour of exposure. The EIS spectra obtained are shown in Figure 9.

From the spectra obtained in the thin layer, it is evident that the diameter of the impedance loop is significantly larger for inhibitor A compared to inhibitor B and, especially, compared to the EIS spectrum measured in the blank solution. Although the smaller impedance observed on the EIS spectra in the solution with inhibitor B indicates its lower corrosion protection compared to inhibitor A, it still provides a good protection to the steel surface.

The research of Shi et al. [25] showed that the current distribution on such electrodes below the thin electrolyte layer depends on the ratio of the solution resistance over the electrode per unit length and metal–thin electrolyte interfacial impedance per unit length. A non-uniform current distribution is expected, when, for n = 1, the phase angle values do not go above −45°, and in such a case, modeling of EIS spectra with a one-dimensionally distributed constant circuit, so-called transmission line-type equivalent circuit, is recommended (Figure 9b). In this model, R_gs^* represents the solution resistance at the gap between electrodes per unit length, R_s^* is the solution resistance per unit length, R_ct^* is the charge transfer resistance per unit length, Q_dl^* is the constant phase element representing the double layer capacitance per unit length, X_g is the gap distance between the electrodes, and X_w is the width of the electrode.

$$Z_{total}=Z_w\coth \left(\gamma X_w\right)+{\displaystyle \frac{R_{sg}}{2}},$$

(3)

$$\gamma =\sqrt{{\displaystyle \frac{R_s^{*}}{Z^{*}}}},$$

(4)

$$Z_w=\sqrt{R_s^{*}{Z}^{*}},$$

(5)

The EIS spectra in Figure 9a were fitted to this model, and the impedance parameters obtained are shown in

Table 5. Impedance parameters of the carbon steel disc electrode in the acid rain thin film solution after 1 h.

Solution	Rct* (kΩ cm)	Qdl* (μF cm−1 sn−1)	n	IE (%)
BLANK	5.50	12.4	0.66
Inhibitor A	142.4	83.0	0.76	96.14
Inhibitor B	27.61	43.8	0.70	80.08

. The table of electrochemical tests shows the resistance values under the influence of the acid rain medium in a thin layer at room temperature. In addition, the table also shows the inhibitor efficiency for the thin layer system, which was calculated according to Equation (2).

The comparison of the values of the impedance parameters obtained in the inhibited and blank thin solution layers (Table 5) shows that the addition of inhibitors to the corrosive thin electrolyte layer increases the R_ct values, especially in the case of inhibitor A. Actually, this inhibitor showed a similar inhibiting effect as in previous measurements carried out in the bulk of the solution (Table 3). On the other hand, the IE for inhibitor B was lower.

Impedance measurements were also conducted in a thin layer of 1% NaCl solution on a carbon steel disc electrode (Figure 10). The EIS spectra obtained in the blank solution exhibit a smaller semicircle, whereas the semicircles for the inhibitors were significantly larger, particularly for inhibitor A. The opposite effect was observed in measurements in the bulk of a solution where inhibitor B exhibited slightly better performance (Figure 8). These EIS spectra were much less depressed than those obtained in the thin layer of acid rain, which is a consequence of the higher electrical conductivity of 1% NaCl solution. For that reason, it was possible to analyze the spectra with electrical equivalent circuits presented in Figure 7. The obtained impedance parameters are shown in

Table 6. Impedance parameters of the carbon steel disc electrode covered by a thin film of 1% NaCl solution after 1 h.

Solution	Q1 (µS sncm−2)	n1	R1 (kΩ cm2)	Q2 (µS sncm−2)	n2	R2 (kΩ cm2)	IE (%)
BLANK	41.76	0.58	0.89	72.91	0.85	1.20
Inhibitor A	42.41	0.85	0.03	13.12	0.96	270.3	99.23
Inhibitor B	136.70	0.58	0.82	18.27	1.00	9.84	80.39

.

For the blank and inhibitor B thin film solution, slightly lower charge transfer resistance values are obtained than for the bulk measurements (Table 4). On the other hand, the EIS spectrum for inhibitor A exhibits much higher impedance values. The very high R₂ value for this inhibitor could be explained by the surface passivation. For metals covered by a passive film, the assignment of the elements of the electrical equivalent circuit is usually different than for freely corroding metals covered by a less protective oxide layer. In the case of surfaces covered by a passive film, R₁ and Q₁ describe the resistance and dielectric properties of the porous outer oxide layer, while R₂ and Q₂ are associated with the inner compact layer resistance and capacitance. For this reason, the R₁ for carbon steel in inhibitor A solution seems to be low in comparison to the blank and inhibitor B solution, but it actually represents only the resistance of the outer oxide layer.

The better performance of inhibitor A in the thin layer of the solution than in the bulk of the solution could be the result of the higher concentration of oxygen in the thin electrolyte film than in the bulk of the solution, due to the shorter diffusion path from the air. The same effect would be expected for inhibitor B, which is also benzoate-based, but its behavior was similar to that in the bulk of the solution.

3.3. Studies in the Comb-like Electrode System

The third electrochemical setup was based on planar comb-like electrodes with eight identical plates covered with a thin layer of electrolyte, either acid rain or 1% NaCl solution.

Figure 11 shows the impedance spectra obtained on the comb-like electrode covered by a thin layer of simulated acid rain solution and with the addition of inhibitors A and B. In the presence of both inhibitors, the corrosion resistance of carbon steel significantly increases compared to the blank solution, indicating a high level of corrosion protection.

All EIS spectra obtained in this experimental setup exhibited phase angle maxima above 45° and could be modeled by the electrical equivalent circuit presented in Figure 7. Such models were also used by other authors using comb-like electrodes [31].

The impedance parameters shown in

Table 7. Impedance parameters for a carbon steel comb-like electrode after 1 h of immersion in the simulated rain solution and with the addition of 20 g/L of inhibitors.

Solution	Q1 (µS sncm−2)	n1	R1 (kΩ cm2)	Q2 (µS sncm−2)	n2	R2 (kΩ cm2)	IE (%)
BLANK	39.77	0.76	0.64	19.33	0.76	4.6
Inhibitor A	18.21	0.84	4.34	11.0	0.59	295.0	98.25
Inhibitor B	13.85	0.86	87.04	13.58	0.62	216.9	98.27

reveal that the addition of the studied inhibitors to the corrosive thin layer solution increases the oxide film resistance, R₁, for inhibitor A by more than sevenfold compared to the oxide resistance in the blank solution, while it is 136 times higher for inhibitor B. The charge transfer resistance also increases in the presence of both inhibitors, namely 64 times for inhibitor A and 47 times for inhibitor B. This results in an inhibiting efficiency exceeding 98% for both inhibitors.

By comparing the impedance results obtained in the bulk of a solution and in a thin layer for acid rain, it is interesting to notice that the performance of inhibitor B in this experimental setup is better than in the bulk of the solution (Table 3) or on the disc electrode (Table 5).

The impedance spectra obtained in a thin film of 1% NaCl solution on an eight-plate comb-like electrode are shown in Figure 12. The same trend of the impedance semicircle diameter is observed as for the measurements conducted in the bulk of the solution (Figure 8), where inhibitor B shows slightly higher impedance values than inhibitor A, which is in contradiction to the results obtained on the disc electrode where inhibitor A provided significantly higher corrosion protection.

The impedance parameters shown in

Table 8. Impedance parameters for the carbon steel comb-like electrode in 1% NaCl solution and with the addition of 20 g/L of inhibitors.

Solution	Q1/ (µS sncm−2)	n1	R1/ (kΩ cm2)	Q2/ (µS sncm−2)	n2	R2/ (kΩ cm2)	IE/(%)
BLANK	108.0	0.74	0.28	49.05	0.50	2.64
Inhibitor A	32.04	0.84	3.25	136.60	0.53	39.67	93.20
Inhibitor B	32.34	0.84	4.68	115.50	0.62	38.33	93.21

were obtained by fitting the EIS spectra presented in Figure 12 to an equivalent electrical circuit in Figure 7. It can be seen that the addition of inhibitors to the thin electrolyte layer increases both the R₁ and R₂ values. The oxide film resistance (R₁) increased significantly in the presence of both inhibitors, which indicates that the protective surface film was formed. The charge transfer resistance (R₂) increased by 10 times in both inhibited solutions compared to the resistance observed in the blank chloride solution. This ultimately reflected an inhibition efficiency of over 93% for both inhibitors.

It is interesting to compare the impedance parameters obtained by measurements in the classical three-electrode setup in the bulk of the solution (Table 4) with those shown in Table 8. While the R₁ and R₂ values for the blank solution are similar in both experimental setups, they are quite different for the inhibited solutions. The polarization resistance of carbon steel (R₁ + R₂) observed in the chloride thin layer with inhibitor A was five times higher than the resistance in the chloride bulk solution of inhibitor A, while the resistance of inhibitor B in the thin layer was three times higher than in the bulk of the solution. As in the case of measurements in the thin film on the disc electrode, this increase in inhibiting efficiency compared to measurements in the bulk of the solution can be attributed to the higher oxygen supply in the thin film solution.

Table 9. Summary of the inhibiting efficiencies for all the studied types of electrodes and electrolytes.

Conditions	IE (%)
	Acid Rain		1% NaCl
	Inh_A	Inh_B	Inh_A	Inh_B
Three-electrode electrochemical cell	97.23	96.61	64.71	76.25
Thin layer disc el.	96.14	80.08	99.23	80.39
Thin layer comb el.	98.25	98.27	93.20	93.21

summarizes the inhibiting efficiencies for three experimental systems and for each inhibitor, A and B. The results are categorized based on two corrosive solutions (acid rain and 1% NaCl), clearly demonstrating the inhibiting effect of the selected inhibitors at a concentration of 20 g/L. In most of the cases, a higher inhibiting effect is observed in the acid rain solution than in the chloride solution, which can be explained by taking into account that both studied inhibitors are passivating inhibitors while chloride ions negatively affect the stability of passive films.

The comparison of the results obtained in the different electrochemical setups shows that a higher inhibiting effect is generally observed for the thin film measurement, particularly on the comb-like electrode, which could be ascribed to a higher O₂ content in the thin electrolyte layer than in the bulk of the solution, as O₂ promotes the formation of a passive film. The only exception is inhibitor B in a thin film of acid rain solution on the disc electrode, where a lower inhibiting effect was observed compared to the measurements in the bulk of a solution. The uneven distribution of the current densities on the disc electrode could lead to uneven passivation of the surface. It should also be considered that the passive film formation is influenced by the actual electrode potential, which is more difficult to control in a two electrode system. Since the EIS spectra obtained on the comb-like electrode, with alternate working and counter electrodes, exhibit a much less depressed feature than the spectra recorded on the disc electrode, it can be assumed that the former provides a more uniform current distribution, which is then reflected in lower differences in the obtained inhibiting efficiencies of the studied inhibitors. It should be noted that a more uniform current distribution could be also achieved on a disc electrode by increasing the thickness of the electrolyte film or by using the electrolyte with higher conductivity [25]. However, our results show that, under the similar experimental conditions (i.e., electrolyte thickness), uniform current distribution is more easily achieved on a comb-like electrode. For that reason, this electrode is a better choice for electrochemical measurements carried out under the thin film of the electrolyte.

4. Conclusions

In this work, two benzoate-based corrosion inhibitors were investigated in three different electrochemical setups. Polarization measurements, carried out in a classical three-electrode electrochemical cell, revealed that both inhibitor mixtures have a passivating effect on carbon steel when added in sufficient concentrations. However, the overall inhibiting effect is higher in the acid rain solution than in the chloride solution.

The measurements on a thin film of electrolyte, both on the disc and comb-like electrode, showed higher corrosion inhibition values than in the case of the classical three-electrode cell with the bulk of an electrolyte. This is attributed to a higher oxygen content in the thin film of the electrolyte, which enhances the surface passivation. Therefore, studies on passivating VCIs, such as benzoates, carried out in the bulk of the electrolyte, can lead to underestimation of the corrosion protection that will be achieved under the atmospheric corrosion conditions, i.e., under the thin film of electrolyte.

The only exemption was observed on a disc electrode system with acid rain solution containing inhibitor B. This is ascribed to the uneven current distribution on the electrode for this particular case, as deduced from the strongly depressed feature of the EIS semicircle.

The difference in inhibiting efficiency of the studied inhibitors is the smallest in the case of the comb-like electrode system. For this experimental setup, EIS spectra exhibit a much less flattened semicircle, which is an indication of a more uniform current distribution than on the disc electrode under a similar electrolyte thickness. Based on these observations, it can be concluded that, among the investigated experimental setups, the comb-like electrode is the preferred system for studying the inhibiting properties of volatile corrosion inhibitors.