Comparative Studies on Steel Corrosion Resistance of Different Inhibitors in Chloride Environment: The Effects of Multi-Functional Protective Film

Abstract

A corrosion inhibitor was widely used to improve corrosion resistance of steel bar in reinforcement concrete structure. A kind of multi-component corrosion inhibitor, which is composed of organic and inorganic substances, was developed in this research. This corrosion inhibitor was comparatively studied with various other inhibitors by using open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV) methods. The results show that the OCP values and charge transfer resistance (calculated by EIS curves) of the multi-component corrosion inhibitor remain, respectively, as high as −0.45 V and 932.19 kΩ·cm⁻² after 60 days immersion, which are significantly better than other groups. Wide passivation interval and various peaks in cyclic voltammograms (CV) were applied to analyze the mechanism of adsorption (organic substance) and oxidation–reduction reactions (inorganic substance). The functional groups -OH in triethanolamine (TEA) and tri-isopropanolamine (TIPA) bond to the steel bar surface quickly, behaving as an adsorbent of organic substance in early age. An additional protective precipitate related to the reactions of Fe³⁺ was formed by inorganic substances (Fe₂(MoO₄)₃ and FePO₄), which is consistent with the EIS results and equivalent electrochemical circuits. As an eco-friendly substitute, multi-component corrosion inhibitors possess similar or even better protecting effects on steel bars in comparison to calcium nitrite. In addition, the concept of a “multi-functional protective film” was proposed, providing a new insight to achieve modified anti-corrosion capacity of inhibitors.

1. Introduction

Steel reinforced concrete is recognized as an outstanding artificial construction material, being widely used in construction engineering [1,2] due to its excellent durability and compatibility. Steel bars work durably because of the protection effect of increasingly developed passive film in a high alkaline concrete pore solution [3]. Steel bar corrosion resistance primarily depends on the existence of the passive film, which is a natural barrier between the steel bars and introduced ions [4]. Despite this, the steel bar remains corroded in most of the actual structure [5]. This phenomenon is mainly ascribed to the destruction of concrete by the invasion of sulfate ions, concrete neutralization caused by carbonation, and chloride attacks and their coupling effects [6]. Finally, it results in a lot of engineering damage and huge economic loss [7]. The chloride-ion-introduced corrosion of steel bars has attracted attention of researchers and practitioners [8,9,10].

Among many methods for preventing corrosion, inhibitors have been widely used in construction buildings due to their outstanding corrosion prevention effects [10]. Since the 1950s, nitrites were used in reinforcement concrete as corrosion inhibitors, e.g., calcium nitrites and sodium nitrites [11,12], and were regarded as extremely useful inhibitors. Nitrites can be hydrolyzed into NO²⁻, which then adhere to passive film, promoting the formation process of passive film [13]. Yet, the dissolved NO²⁻ exhibits potential toxicity and environmental issues, thus delaying the utilization of nitrites [12]. Searching for a safe and environmentally friendly corrosion inhibitor is necessary for corrosion inhibition [10]. Organic inhibitors, such as amino-based and imidazoline substance inhibitors, are utilized to mitigate steel bar corrosion in simulated concrete pore solutions [14], based on their great adsorption abilities. In the case of amines, such as TEA and TIPA, adsorption occurs in the form of covalent bonds. It is evident in the functional group (Figure 1), where the polar functional group -OH shares its lone pair of electrons with Fe²⁺ to form the covalent bond.

The formation velocity and the long-term stability of the protective film are among the most important indicators to evaluate the efficiency of inhibitors. However, organic substances can disintegrate the form of the steel bar surface due to the desorption effect, leading to a lack of long-term stability [16,17]. On the other hand, the inorganic inhibitors become non-effective in high alkaline conditions because the formation velocity of inorganic inhibitors on steel bars is lower than the corrosion reaction introduced by chloride. In addition, the microstructure of passive film can be changed, owing to the pH change or chloride attack [18]. Consequently, even though corrosion inhibitors were equipped, pitting corrosion can still occur in high chloride contamination [19]. The facts proved that using organic or inorganic inhibitors alone on steel bars is not sufficient for mitigating corrosion.

A reasonable choice of experimental method is a significant factor for exploring the reaction mechanism of inhibitors on a passive film. Studies for corrosion inhibitors subjected to chloride attack have been typically summarized by means of various electrochemical methods, e.g., electrochemical impedance spectroscopy (EIS) and polarization curve or cyclic voltammetry (CV) [20]. As a transient electrochemical measurement, the EIS test is widely used in the choice of corrosion inhibitor [21]. However, the use of the EIS test is merely incommensurate with the long-term monitoring of steel bar corrosion in concrete. The long-term observation of a steel bar surface contributes to knowing the formation process of a passive film [22,23]. Moreover, EIS and polarization curves in the chloride-containing solutions are occasionally affected by crevice corrosion; this phenomenon could reduce by the utilization of a non-stationary technique such as the CV test [24]. The CV test covers the overall process of steel bar corrosion including oxygen absorption and hydrogen evolutional corrosion in high alkaline solutions [25]. Nevertheless, the extensive potential range (test in the −1.5–1 V) alters the specimen (steel bar surface) and changes the specimen before each test. In addition, cyclic voltammetry is suitable for transient measurements because electronic reactions that occur on steel bar surfaces do not achieve equilibrium conditions [26]. Interface complexity is considered to be a significant factor in the study of steel bar surface properties. For complex mortar, hydration products at the metal–hydrated cement paste interface may affect the onset of localized corrosion, complicating the analysis of the mechanism. Hence, the simplified simulated pore solution can be used in the mechanism investigations of passive film formation in well-controlled environment.

In our previous research, a multi-component corrosion inhibitor was successfully prepared, and its inhibition properties and synergistic mechanisms were investigated [27]. Multi-component corrosion inhibitors are a new trend in the field of electromigration inhibitor research, which could combine the advantages of the quick absorption of the organic and the excellent long-term performance of the inorganic [11,28]. However, whether the long-term performance of multi-component corrosion inhibitors could achieve the inhibition level as traditional inhibitors (such as calcium nitrite) is still unclear, especially considering stability. Thus, the main purpose of this research was to compare the inhibition effect of various corrosion inhibitors by electrochemical measurements (open circuit potentials, EIS, and CV) and SEM over as long as 60 days. We also explored the mechanism of different corrosion inhibitors acting on steel bars when exposed to a simulated concrete pore solution with a high concentration of chlorides. Based on the experimental results, a protective film that can serve in alkaline environments over the long term was developed. According to the results of the experiments, a new concept of “multi-functional protective film” was proposed and provided a new insight into strengthening the microstructures of passive film.

2. Experimental Programs

2.1. Materials and Mix Proportions

The HPB300 steel bars were used as the working electrodes, which conforms to Chinese National Standard GB/T 1449.1–2017 [29]. The low-carbon steel bars were sliced into cylindrical specimens with lengths of 10 mm. The chemical compounds of steel bars are listed in

Table 1. Chemical compounds of used steel bar.

Element	C	S	P	Mn	Si	Fe	Other
Content (%)	0.192	0.024	0.012	0.580	0.355	98.370	0.467

.

A simulated concrete pore solution was prepared by dissolving 0.6 mol/L KOH, 0.2 mol/L NaOH, and 0.04 mol/L Ca(OH)₂ (analytical regent) associated with 2% NaCl [30]. Different types of substance, such as calcium nitrite, multi-component corrosion inhibitor, and inorganic or organic compounds were prepared as corrosion inhibitors. Typical organic substances, e.g., Triethanolamine (TEA) and tri-isopropanolamine (TIPA) were studied in this research. Thus, as were the inorganic substances monofluorophosphate (MFP) and sodium molybdate (T-4). Organic and inorganic corrosion inhibitors were composed of TEA TIPA and MFP T-4, respectively, as illustrated by Gao [27].

2.2. Sample Preparation

Figure 2 shows the machining process of steel bars (working electrode), the preparation of SCP solutions, and the experimental apparatus. Weld steel bars were, firstly, polished with emery sand paper from 240# to 1500# and diamond polishing fluid successively [31]. The polished lateral was 0.785 mm². Steel bars were, secondly, fixed in the PVC tube by epoxy resin. A one-dimensional area of steel bar was exposed to the SCP solutions, thus securing the corrosion reaction, according to the chloride reacting constantly. Before testing, acetone, alcohol, and deionized water were used to clean the steel bars and remove impurities. Standard Ca(OH)₂ solutions were prepared and soaked the welded steel bar electrode for 10 days as strong oxidizing agents and received the passivated steel bar [32] before the corrosion test. The steel bar corrosion test was performed by soaking the welded steel bars in the chloride-containing SCP solutions with different corrosion inhibitors at 22 ± 2 °C for 5, 30, and 60 days, respectively.

2.3. Test Methods

Steel bar corrosion was recorded by open circuit potential (OCP). The OCP test was measured for 400 s to ensure the diffusion of irons in solutions, achieving an equilibrium. The CHI604E electrochemical workstation produced by Chenhua Instrument Co. Ltd. (Shanghai, China) was used to collect electrochemical impedance spectroscopy (EIS) data. A three-electrode system was measured in this research. The steel bar electrode and platinum plate were plugged into the electronic workstation. The saturated calomel electrode (SCE) was also plugged into the electronic workstation to work as the reference electrode in order to calibrate the potential. In the EIS test, the AC signal disturbance amplitude corresponded to 10 mV, and sinusoidal voltage frequency ranged from 10⁻² to 10⁵ Hz. A cyclic voltammetry (CV) measurement was used to evaluate the oxidation–reduction reactions of passivated steel bars associated with various corrosion inhibitors in the SCP solutions [33]. The potential scan was cycled from −1.5 V to 0.8 V vs. SCE, and the scan rate was 50 mV/s, scanning for five times. The microstructure of passive film was scanned by scanning electron microscopy (SEM, Carl Zeiss AG, Merlin Compact, Cambridge, UK) in the second electron detection mode [34]. All tests were repeated 3 times to sustain reproducibility.

3. Results and Discussion

3.1. Open Circuit Potentials

The value of open circuit potentials could represent the potentials of steel bars out of the circuit state as well as reflect the degree of formation or the corrosiveness of the passivation film. A passive film was perfectly formed and performed a higher resistance, while the OCP was about −0.35 V, so that steel bars could be protected and the corrosion probability could lower [35]. Once the open circuit potential became more positive, the steel bar approached corrosion, indicating the incomplete passive film developed on the steel bar. The OCP values of the samples with different kinds of corrosion inhibitors, together with the samples soaked in the simulated pore solutions without corrosion inhibitors, are listed in

Table 2. OCP values of steel bars immersed in different corrosion inhibitors.

Specimen	OCP (V vs. SCE)
	5 d	30 d	60 d
Ref.	−0.23	−0.22	0.12
Calcium nitrite	−0.28	−0.31	−0.38
Multi-component corrosion inhibitor	−0.37	−0.36	−0.38
Organic corrosion inhibitor	−0.22	−0.32	−0.33
Inorganic corrosion inhibitor	−0.39	−0.4	−0.45

.

Compared with original steel bars, OCP values of steel bars without any corrosion inhibitor become positive after soaking in the chloride-containing solution for 5 days (5 d). After 30 and 60 days, the OCP value becomes more positive. Especially after 60 days soaking, the potential has already exceeded the index of 0 V and reached 0.12 V, indicating the occurrence of corrosion on the steel bar surface. As for the calcium nitrite, the OCP value decreased slightly at 5 d and then rose again after 60 days of calcium nitrite protection. In the early stage (5 d), little calcium nitrite was absorbed on steel bar surface, leading to the shortage of the protecting effect on the passive film, while the protecting effect enhanced with the precipitate process of calcium nitrite in the late stage (60 d) of chloride erosion. The OCP of the multi-component corrosion inhibitor enhanced quickly and remained at a high level, reaching −0.37 V and −0.38 V at 5 days and 60 days, respectively. This phenomenon demonstrates the excellent corrosion resistance of multi-component corrosion inhibitor compared with any other groups. Regarding the organic and inorganic corrosion inhibitors, the OCP of all the samples changes to a more negative direction over time in different degrees. Molybdates could reduce the current density sufficiently and change the OCP to more negative values [36]. The OCP value of the organic corrosion inhibitor sample is slightly lower compared with that of the inorganic corrosion inhibitor, which may relate to the different corrosion resistance mechanisms of corrosion inhibitors.

3.2. Evolution of Electrochemical Impedance Spectroscopy (EIS)

The EIS results (Nyquist plot) for steel bars with or without corrosion inhibitors at different immersion times (5 days, 30 days, and 60 days) are observed in Figure 3, Figure 4 and Figure 5. The diameter of the capacitive loop in the Nyquist plots represent the corrosion degree of steel bars [37]. It is evident that the capacitive loop of the reference group reduces over time, which reflects the corrosion caused by chloride occurred on the steel bar surface without the corrosion inhibitor protection.

The group of multi-component corrosion inhibitors displays a larger diameter capacitive loop than other groups for the early corrosion stage (5 d). In contrast, steel bars with the addition of calcium nitrite have a smaller capacitive loop diameter than the multi-component corrosion inhibitors. The development of the incomplete passive film on steel bars is due to the lack of protection effect from organic or inorganic corrosion inhibitors.

The EIS results of steel bars after soaking for 30 days and 60 days with different corrosion inhibitors are shown in Figure 4 and Figure 5. After being immersed for 30 days, the multi-component corrosion inhibitor exhibits excellent corrosion resistance, which is primarily characterized by a large diameter capacitive loop. Simultaneously, calcium nitrite and organic corrosion inhibitors are involved in the formation of passive film. Nevertheless, the capacitive loop of those two groups becomes smaller than the multi-component corrosion inhibitors. It may be attributed to passive film degradation and steel bar corrosion caused by chloride. After 60 days soaking, the low-frequency capacitive reactance diameter of the inorganic corrosion inhibitor group decreases sharply. The capacitive reactance diameter of the multi-component corrosion inhibitor is larger than that of the reference group, as evidenced by the results shown in Section 3.1.

3.3. Equivalent Electrochemical Circuits Analysis

Different equivalent electrochemical circuits are used to fit the Nyquist plots for the analysis of the EIS results, which are distinguished based on the real conditions on adsorption mechanism of different corrosion inhibitors. The change transfer resistance of passive film on different groups is fitted by following equivalent electrochemical circuits, as shown in Figure 6. All of the parameters fitted by the following equivalent electrochemical circuits were accurate to 97%. R_s represents the resistance of solution, C_dl represents the ideal double-layer capacitance, the coefficient n is close to 1, and Q reflects the constant phase element (0 < n < 1, being equal to the value of C_dl in real conditions). The function of Q_CPE can be calculated by Equation (1) [38]. Significantly, n reflects the phase shift regarding to the surface homogeneous degree. Therefore, homogeneous and smooth surface can be described as C_dl, and inhomogeneous and rough surface can be described as Q. R represents the resistances of passive film formed by different corrosion inhibitors. R1 and R2 represent the resistance of organic components or organic components, and Rct represents the charge transfer resistance.

$$Q_{CPE}=Y_0^{-1}{\left(j\omega \right)}^{-n}$$

(1)

It was evident in Figure 6b that the simple equivalent electrochemical circuit could reflect the adsorption characteristics of calcium nitrite. It mainly contained the charge transfer resistance (Rct) in parallel with C_dl. This proves that the additional contribution provided by calcium nitrite contributes to the formation of passive film [39]. As shown in Figure 6e, the equivalent electrochemical circuit of inorganic corrosion inhibitor is similar to that of calcium nitrite [40]. R1 is in parallel with Q in equivalent electrochemical circuits. The composition and characteristics of passive film are accurately represented by many equivalent circuit elements, such as Q and C. Along with the converse phenomenon of Q to C, more inhomogeneous and rougher passive film are formed [38]. Compared with calcium nitrite and inorganic corrosion inhibitors, passive film forms in more complicated structures with organic or multi-component corrosion inhibitors. The R1 and R2 display on the fitting circuit are in parallel with capacitor C_dl and element Q, respectively. It corroborates that the double-layer structure of passive film could form on the steel bars with the soaking process.

Table 3. Fitting parameter of EIS results prepared with different corrosion inhibitors.

Specimen	Rct (kΩ·cm2)
	5 d	30 d	60 d
Ref.	93.85	18.21	5.63
Calcium nitrite	69.54	532.22	465.17
Multi-component corrosion inhibitor	730.51	1000.01	932.19
Organic corrosion inhibitor	119.26	691.15	723.38
Inorganic corrosion inhibitor	123.41	52.23	48.75

illustrates the EIS fitting parameters in terms of different Nyquist plots (Figure 3, Figure 4 and Figure 5) and equivalent electrochemical circuits (Figure 6). The value of Rct is analyzed in detail, so that the protective efficiency of corrosion inhibitors on steel bars are reflected [38]. A great Rct value confirms a thick passive film grows on steel bar lateral. Moreover, the thickness and stabilization of passive film directly determines the steel bar protecting effect. With the attack of chloride, steel bars begin to corrode. The resistance of reference group changes from 93.85 kΩ·cm² at 5 days to 5.63 kΩ·cm² at 60 days, indicating the destruction of the passive film. The addition of all kinds of corrosion inhibitors could improve the Rct reference group. Thus, all the corrosion inhibitors are effective in corrosion resistance.

For calcium nitrite, the Rct value was 69.54 kΩ·cm² at 5 days, which is quite close to the reference group. Absolutely, during 60 days’ immersion, the resistance of the steel bar raises to 465.17 kΩ·cm². In other words, the anti-corrosion efficiency of calcium nitrite appears in the late period. In the multi-component corrosion inhibitor group, maximum Rct reached 1000 kΩ·cm² at 30 days and maintained a high level after 60 days soaking. The greater Rct value prevents the steel bar from receiving a chloride attack. The Rct values of organic and inorganic corrosion inhibitors were 119.26 kΩ·cm² and 123.41 kΩ·cm², respectively. The resistance is still a little more than the reference group, and corrosion resistance cannot improve, even though long-term immersion [3]. Multi-component corrosion inhibitors could generate protectable films and protect steel bars from chloride ions attack in the long term.

3.4. Discussion on Corrosion Inhibitors Comparison

Corrosion resistance of different types of corrosion inhibitor are listed in

Table 4. Comparison of multi-component corrosion inhibitors over other (including organic and inorganic) corrosion inhibitors.

No.	Corrosion Inhibitor	Classification	Cl− Concentration	Test Methods	IE/Best Values	Refs.
1	N-(n-octyl)-3-methylpyridinium bromide (Py8) and N-(n-dodecyl)-3-methylpyridinium bromide (Py12)	Organic inhibitor	3.5% NaCl, pH 1.5	OCP, Tafel Polarization, EIS	85.1% (IE)	[41]
2	Imidazoline	Organic inhibitor	32.107 g/L NaCl, pH 6.3	Polarization curve, EIS	67% (IE)	[42]
3	3-sulphinylalkyl-5-amino-1H-1,2,4-triazoles	Organic inhibitor	1% HCl solution	EIS, Accelerated Corrosion Test, Quantum-Chemical Simulation	115.01 kΩ·cm2 (Rp, EIS)	[43]
4	Migratory corrosion inhibitors (MCIs)	Organic inhibitor	3.5% NaCl, pH 12.3	EIS, Tafel curve	9.94 kΩ·cm2	[44]
5	Organic compound-based corrosion inhibitor	Organic inhibitor	brackish water	EIS, Tafel curve	99.3% (IE)	[45,46]
6	Mg-Al-LDH	Inorganic inhibitor	0.05 mol/L NaCl + 0.1 mol/L Na2SO4	FT-IR, OCP, EIS	832 kΩ·cm2	[47]
7	Zinc Molybdate (ZM) encapsulated NPs Stearic	Inorganic inhibitor	3.5 wt.% NaCl	EIS, PDP	−0.4 V (OCP)	[48]
8	Na2HPO4–MBT	composite rust inhibitors	3.5 wt.% NaCl	OCP, linear polarization resistance (LPR), potentiodynamic polarization (PP), XRD	−0.45 V (OCP)	[49]
9	sodium chromate (Na2CrO4) + benzotriazole (BTA)	Composite rust inhibitors	8.42 g/L NaCl	XPS, XRD	96.62% (IE)	[22]
10	Compound Na2MoO4 + BTA	Composite rust inhibitors	0.01 mol L−1 NaCl + 0.1 mol L−1 NaHCO3	OCP, Tafel Polarization, EIS, XPS	97% (IE)	[50]

. The inhibitors are classified as organic, inorganic, or composite corrosion inhibitors. The inhibition efficiency of our studied multi-component corrosion inhibitors reached 95.3% [27], and the OCP and Rct values reached −0.38 V and 932.19 kΩ·cm², respectively. The test results given in these studies are compared to the relevant values correlated with multi-component corrosion inhibitors because different test methodologies are utilized to detect steel corrosion in different papers. It can be found that the inhibition efficiency and change transfer resistance of these organic inhibitors are generally lower than that of used in this research. The inorganic inhibitors listed in the table lead to −0.4 V of OCP value, which is still not up to the level of inorganic corrosion inhibitor used in this work. The means of selecting MFP and T-4 as inorganic components in this research could provide better protecting effects. The composite corrosion inhibitors illustrated in Table 4 exhibit excellent anti-corrosion efficiency, and the IE value of these inhibitors reaches up to 95%. Compared with the composite inhibitors listed in the table, our studied corrosion inhibitor also reached the same level, which is reflected in the high IE values. As for the testing methods, both common methods, such as OCP and EIS, and unique methods including potentiodynamic polarization and XPS were used to detect the steel bar interface characteristic and molecular structures of the corrosion inhibitors. However, these methods explored the mechanism in the perspective of electrochemistry, which is a static state reaction process that occurs on steel bar surfaces that cannot be analyzed. As a dynamic non-destruction measurement, CV is usually used to detect oxidation–reduction reactions among the interface of metal. Thus, we attempt to explore the mechanism of multi-component corrosion acts on steel bar surfaces by cyclic voltammetry (CV).

3.5. Cyclic Voltammetry (CV) Test Results

Figure 7 and Figure 8 show the cyclic voltammograms (CV) for samples immersed in simulated pore solutions with and without chloride, at a scan rate of 20 mV·s⁻¹. The arrows in Figure 9, Figure 10, Figure 11 and Figure 12 show the direction of the scans. In the voltammogram, it is noticeable that Fe is electroactive by the appearance of the three cathodic (or four cathodic peaks) and two anodic peaks.

In the scanning process, from the reduction reaction to the oxidation reaction, three cathodic potential peaks (marked as A1, A2, and A3) are identified. In the reverse scan process, two reduction peaks (marked as C1 and C2) are also displayed in the reverse scan procedure.

At approximately −0.9 V, oxidation peak A1 is formed, which is related with the transformation of Fe to Fe(OH)₂, according to Equation (2):

$${\mathrm{Fe}+2\mathrm{OH}}^{-}\leftrightarrow {\mathrm{Fe}\left(\mathrm{OH}\right)}_2{+2\mathrm{e}}^{-}$$

(2)

As the passive film forms progressively with the process of the oxidizing reaction, the peak associate with the formation of Fe₃O₄ appears in A2, according to Equations (3) and (4) [40]:

$$3\mathrm{Fe}{\left(\mathrm{OH}\right)}_2{+2\mathrm{OH}}^{-}\leftrightarrow {\mathrm{Fe}}_3{\mathrm{O}}_4{+2\mathrm{e}}^{-}$$

(3)

$${3\mathrm{FeO}+2\mathrm{OH}}^{-}\leftrightarrow {\mathrm{Fe}}_3{\mathrm{O}}_4{+2\mathrm{e}}^{-}$$

(4)

Oxidation peak A3 is formed at 0.3 V, which is related with the transformation of magnetite (Fe₃O₄) to lepidocrocite (FeOOH) (Equation (5)) and γ-Fe₂O₃ (Equation (6)):

$${\mathrm{Fe}}_3{\mathrm{O}}_4{+\mathrm{OH}}^{-}{+\mathrm{H}}_2{\mathrm{O}+2\mathrm{e}}^{-}\leftrightarrow {3\mathsf{\gamma }-\mathrm{FeOOH}+\mathrm{e}}^{-}$$

(5)

$${2\mathrm{Fe}}_3{\mathrm{O}}_4{+2\mathrm{OH}}^{-}{+2\mathrm{H}}_2\mathrm{O}\leftrightarrow {3\mathsf{\gamma }-\mathrm{Fe}}_2{\mathrm{O}}_3{+2\mathrm{e}}^{-}$$

(6)

Beyond peak A3, oxygen absorption corrosion occurred. In the reverse scan, the reduction peaks C1 and C2 correspond to the reaction of Fe³⁺ to Fe²⁺ [51]. A wide passivate interval located between C1 and C2 is captured, following the process of adsorption of Ca²⁺ ions as a calcium oxide gel in the presence of calcium hydroxide [20]. This wide passivation interval is advantageous to the formation of the passive film. The passive film was broken over the C2 peak, and hydrogen evolutional corrosion occurred. Similar results can be found in Ref. [20].

Chloride ions can exert ruinous effects on passive films, generating much more porous corrosion product, leading to the formation of non-protective passive films on steel bar surfaces. CV curves of steel bars with chloride also demonstrate three oxidation peaks (defined as A1, A2, and A3) in the oxide scan and two reduction peaks (defined as C1 and C2) in the reverse scan. As shown in Figure 8, the passivate interval between A1, A2, and A3 contracts sharply, and the reaction intensity of this area is significantly increased.

The chloride ions can be adsorbed on the oxide layer and produce a complex loose interface to the oxide matrix. The mechanism of pitting initiation is the competitive chemisorption of chloride and OH^- over passive film. When chlorides substitute OH⁻ in a sufficient number of adjacent sites, the rupture of the passive film occurred with the formation of nascent pits. Finally, it leads to an enhancement of anodic dissolution, and even the protective film is still unable to be produced and sustained [40].

$$\mathrm{Fe}{\left(\mathrm{OH}\right)}_2{+\mathrm{Cl}}^{-}\to {\mathrm{FeOCl}+\mathrm{OH}}^{-}{+\mathrm{e}}^{-}$$

(7)

$${\mathrm{FeOCl}+\mathrm{OH}}^{-}\to \mathrm{Fe}{\left(\mathrm{OH}\right)}_2{+\mathrm{Cl}}^{-}$$

(8)

In the eroded areas, OH⁻ and dissolved oxygen restore the passive film on the inner surface of the primitive pit again. Consequently, the substitution between chloride and OH⁻ is a dynamic state process.

The corrosion resistance effect of calcium nitrite for blocking chloride attacks is verified by voltammograms and shown in Figure 9. The CV curves in Figure 9 are similar to Figure 10, which consist of three oxidation peaks and two reduction peaks, being close to the passivation status. This result proves that the addition of calcium nitrite is effective for corrosion resistance. In addition, calcium nitrite hydrolysis produced NO^2- and reacted with Fe(OH)₂, forming Fe₃O₄ and promoting the formation of inner passive film [2], as shown in Equation (9). The presence of calcium nitrite accelerates the development of a homogenous layer and swarm with Fe₃O₄ (magnetite), which could block the chloride ions and steel to exacerbate the corrosion [52].

$$6\mathrm{Fe}{\left(\mathrm{OH}\right)}_2+2{\mathrm{NO}}^{2-}\to 2{\mathrm{Fe}}_3{\mathrm{O}}_4+{\mathrm{N}}_2\mathrm{O}+2{\mathrm{OH}}^{-}+5{\mathrm{H}}_2\mathrm{O}$$

(9)

Figure 9. Cyclic voltammograms (1st and 5th cycles) in SCP solution with calcium nitrite.

Figure 9. Cyclic voltammograms (1st and 5th cycles) in SCP solution with calcium nitrite.

The cyclic voltammograms of calcium nitrite group are exhibited in Figure 9. In comparison with Figure 8 and Figure 10, chloride introduced corrosion reductions, which was demonstrated by the re-appearance of the wide passivation interval between A1 and A2. This result proves that organic corrosion inhibitor was conducive to passive film generation. It was mainly induced by the continuous adsorption process of TEA and the formation of protective film, blocking the contact of chloride ions with steel bars. TIPA had the same effect, to some extent. Moreover, the TIPA could combine with chloride and lead to the reduction in free chloride in the SCP solution, thereby decreasing the attack risk on passive film by chloride irons [53]. It was also reported that the organic corrosion inhibitor has a more excellent anti-corrosion effect compared with calcium nitrite [20].

Figure 10. Cyclic voltammograms (1st and 5th cycles) in SCP solution with organic corrosion inhibitor.

Figure 10. Cyclic voltammograms (1st and 5th cycles) in SCP solution with organic corrosion inhibitor.

The voltammogram for the inorganic corrosion inhibitor group is demonstrated in Figure 11. It was shown that cathodic potential peaks A1, A2, and A3 appeared in the voltammograms, as well as reduction peaks C1 and C2, due to the generation of Fe³⁺. Significantly, a new cathodic potential peak appeared on the voltammograms, coded as A4, reflecting further formation of Fe. It is an additional reaction to form extra protective film after the formation of passive film, which is associated with the formation of FePO₄ [54] and Fe(MoO₄)₃ [50,55], according to Equations (10) and (11). Nevertheless, this protection did not work for a long time because the thin layer is easy to be attacked [19].

$${2\mathrm{Fe}}^{3+}{+3\mathrm{MoO}}_4{}^{2-}\to {\mathrm{Fe}}_2{\left({\mathrm{MoO}}_4\right)}_3$$

(10)

$${\mathrm{PO}}_4{}^{3-}{+\mathrm{Fe}}^{3+}{+\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{FePO}}_4{·\mathrm{H}}_2\mathrm{O}$$

(11)

Figure 11. Cyclic voltammograms (1st and 5th cycles) in SCP solution with inorganic corrosion inhibitor.

Figure 11. Cyclic voltammograms (1st and 5th cycles) in SCP solution with inorganic corrosion inhibitor.

The voltammogram for the multi-component corrosion inhibitor group is shown in Figure 12. The compound substance integrates the advantages of organic and inorganic compounds, which was the re-appearance of the wide passivated interval and the appearance of cathodic potential peak A4 [56]. Moreover, the current density of corresponding curves between cathodic potential peaks A1 and A2 were lower than that of same position shown in Figure 8. It proves that passive film developed continuously due to the compound effect, overcoming the insufficient protection capacity of the protective film formed by inorganic corrosion inhibitors [57,58]. Meanwhile, according to the results illustrated in Section 3.2, the addition of inorganic components is also a benefit for the promotion of change transfer resistance. In summary, the anti-corrosion effectiveness of the multi-component corrosion inhibitor was confirmed, and its mechanism has also been interpreted by various experimental techniques.

Figure 12. Cyclic voltammograms (1st and 5th cycles) in SCP solution with multi-component corrosion inhibitor.

Figure 12. Cyclic voltammograms (1st and 5th cycles) in SCP solution with multi-component corrosion inhibitor.

3.6. Micromorphology Observation

The microphotography and EDS results of steel bars immersed in SCP solutions with different corrosion inhibitors are observed in Figure 13. Before photographing, the immersed steel bar was removed from the solution and dried. The polished steel bar showed a metallic luster, and there were polishing marks on the bars surface. A large number of yellow-brown corrosion products appeared on steel bar surface after the chloride attack. Corrosion products and chloride were also detected in the images. In the images, the passive film became incomplete and the corrosion occurred on most areas of steel bar surface, consequently, hindering the passive film formation in abundant chloride environment [59,60]. Under the protection of calcium nitrite, the corrosion phenomenon becomes milder, which is embodied by disappeared corrosion products and visible polished marks. However, the Cl element can still be detected in the SEM and EDS results. When the organic corrosion inhibitor is adsorbed on the steel bar, corrosion pits and chloride are not detected, and the corrosion is effectively inhibited, owing to the formation of a protective film layer. In addition, consistent with the results mentioned above, a small number of corrosion pits was pictured on steel bar’s surface with the protection of the inorganic corrosion inhibitor. The reaction products of the inorganic group, as described above, were detected in the SEM images and accumulated on the steel bar’s surface. Moreover, no corrosion product or corrosion pit could be found in Figure 13f, proving that steel bar could be protected by multi-component corrosion inhibitors before the induction period of pitting corrosion.

4. Conclusions

In this research, the anti-corrosion efficiency of multi-component corrosion inhibitors were comparatively studied with other inhibitors. The OCP, EIS, and CV methods were experimentally determined and compared. The mechanism of passive film enhancement was discussed. Based on the experimental results, the following conclusions can be drawn:

(1)

Multi-component corrosion inhibitors significantly improved the corrosion resistances of steel bars in an SCP solution over a long period (60 days). The open circuit potential reached −0.37 V after 5 days and remained above −0.45 V at 60 days, and its electrochemical impedance spectroscopy reached 730.51 kΩ·cm² and remained 932.19 kΩ·cm². As a contrast, the OCP and Rct values of calcium nitrite after 5 days of immersion merely reached −0.28 V and 69.54 kΩ·cm².

(2)

The adsorption and oxidation–reduction reactions that occurred on the steel bar’s surface were comparatively analyzed and experimentally determined by the cathodic potential and the reduction peaks in cyclic voltammetry for the first time. The organic compounds could adhere on steel bar surface, thereby reducing the corrosion current densities in the wide passivation interval of the voltammograms. Additional protective precipitates (FePO₄ and Fe₂(MoO₄)₃) formed by the inorganic substance were detected, which contributed to corrosion resistance.

(3)

A “multi-functional protective film” was generated by the synergy of organic and inorganic substances, possessing a superiority inhibition effect and serving as a thick barrier. Considering their ideal inhibition efficiency and eco-friendly characteristics, multi-component component corrosion inhibitors could be an ideal substitute and have great application potential for the reinforcement of concrete in a chloride environment.