Evaluation of Corrosion Resistance of Corrosion Inhibitors for Concrete Structures by Electrochemical Testing in Saturated Ca(OH)2 Solutions with NaCl and Na2SO4

Reinforcing steel maintains passivity in an alkaline concrete environment. However, the passive film on the steel can be destroyed as the concrete becomes acidic, which could induce the corrosion of reinforcing steel. Carbonates and sulfates destroy the concrete matrix and accelerate the penetration of hazardous ions, thereby deteriorating the structure. To alleviate the corrosion of internal reinforcing steel within concrete, corrosion inhibitors are most widely used. In this study, the effects of inorganic nitrite (lithium nitrite) and amino acid corrosion inhibitors (diethanolamine, methyl diethanolamine, and 4-aminobutyric) on corrosion resistance and the pH of the solution with various concentrations of Na2SO4 (0.89, 1.77 g/L) in saturated Ca(OH)2 that contained NaCl (0.98 g/L), which simulated the concrete environment, were investigated. +e corrosion resistance of inhibitors was evaluated by corrosion potential, electrochemical impedance spectroscopy, and potentiodynamic techniques. +e results indicated improvement of corrosion resistance by the addition of amino acid corrosion inhibitors. It was confirmed that the inhibitor adsorbed on the surface of the specimen and passivated to reduce the corrosion reaction. In addition, the 4-aminobutyric acid corrosion inhibitors had the corrosion protection efficiency of 67.87–77.80%, which is a higher value than that of the inorganic nitrite corrosion inhibitor (lithium nitrite: 69.36–75.93%) and other amino acid corrosion inhibitors (diethanolamine: 35.69–39.91%; methyl diethanolamine: 66.07–69.09%).


Introduction
e corrosion of steel reinforcements within concrete occurs as the passive films, formed on the steel surfaces in the alkaline environment of concrete, are destroyed. e passivity of reinforcing steel is promoted by the alkalinity of concrete, arising from the high pH of the concrete pore solution. However, protective oxides and passive films can be destroyed by the weakening of concrete and the ingress of harmful ions caused by the carbonation, sulfates attack, and acidic substances in industrial areas. e corrosion of steel can also be alleviated by using corrosion-resistant steels, cathodic protection, fusion-bonded epoxy coatings, corrosion inhibitors, and admixtures. Among these methods, corrosion inhibitors are the most widely used, with high cost-effectiveness and convenient usage. Inhibitors prevent the onset of corrosion by increasing the concrete pH or by fixating harmful ions that can cause the corrosion of steel [1,2]. Frequently used corrosion inhibitors include inorganic nitrites [3][4][5], inorganic phosphate [6] and molybdenum [7], monofluorophosphate that is applied to the surface of the concrete [8], and organic inhibitors, such as alkanolamine and amine groups [9][10][11][12][13][14][15].
Inorganic corrosion inhibitors are mostly nitrites, which have been used since the 1940s. Nitrite-based corrosion inhibitors are environmentally unfriendly with hazardous biological effects [16]. To replace nitritebased corrosion inhibitors, organic-based inhibitors have been developed and used since the 1990s. Organic corrosion inhibitors show chelating effects with iron, which forms covalent bonds with lone pairs of electrons of the heteroatoms in the organic compound. e heteroatoms act as nucleophiles of iron, which is an electrophile. e lone pairs of the heteroatoms are donated to the vacant dorbital of iron atoms, forming very strong covalent bonds. In this way, the organic inhibitor is adsorbed on the metal surface, yielding a very protective inert film [17,18]. Organic inhibitors, especially amine-and alkanolaminecontaining effectors, are frequently used as corrosion inhibitors owing to their high solubilities in aqueous solutions. Because of the functional groups of organic inhibitors, nitrogen and oxygen atoms are adsorbed on the metal surface with their lone pairs of electrons; the iron ions act as Lewis acids, accepting electrons from the nitrogen and oxygen donors. Very strong covalent bonds are formed between nitrogen/oxygen and iron metal, and considerable amounts of nitrogen and oxygen are adsorbed on the surface of the reinforcing steel [19]. Such organic inhibitors reduce or impede the corrosion of reinforcing steel through the adsorption of polar heteroatom groups, which form very thin protective layers. erefore, inhibitors containing amino groups are commonly used to decrease the corrosion of reinforcing steel in contaminated concrete.
Before corrosion begins or at relatively low corrosion levels, amino alcohol inhibitors are more effective [20,21]. Martin and Miksic reported that dimethylethanolamine blocks active sites and acts as a cathodic inhibitor. e oxygen absorbs electrons, thereby being reduced to OH − and adsorbing on the anode. Amino alcohol inhibitors act more effectively than nitrite inhibitors, i.e., lithium nitrite, in both acceleration tests and saturated Ca(OH) 2 solutions with different concentrations of chloride ions [22].
Studies about corrosion of reinforcing steel-simulating concrete environments using saturated Ca(OH) 2 solutions have been reported elsewhere [23][24][25]. However, only a few corrosion studies have been performed for amine-based corrosion inhibitors in saturated Ca(OH) 2 solutions bearing sulfate and chloride ions via electrochemical methods [26,27]. Sulfate attack on concrete is a chemical deterioration mechanism where sulfate ions attack components of the cement paste to form expansive crystalline products called ettringite. Expansion due to ettringite formation results in stresses, and cracks develop in the concrete; therefore, it can accelerate corrosion by destroying the passive state of the rebar through lowering of the pH in the concrete pore environment [28].
In this study, the effects of inorganic nitrite and amino alcohol inhibitors on corrosion resistance and the pH of the solution with various concentrations of Na 2 SO 4 in saturated Ca(OH) 2 that contained NaCl, which simulated the concrete environment, were investigated. e corrosion resistance of inhibitors was evaluated by various electrochemical tests, such as potential times, electrochemical impedance spectroscopy, and potentiodynamic techniques. e chemical composition of the steel specimens used in the test is shown in Table 2.

Materials and Methods
In this study, the effects of the inhibitors on corrosion resistance and the pH of the solution with various concentrations of chloride and sulfate ions, which simulated the concrete environment, were investigated. For this, the chloride ion concentration was fixed in the Ca(OH) 2 solution, and by adjusting the admixing amount of sulfate ions, electrochemical tests were conducted to evaluate the performance of four different corrosion inhibitors. Table 3 shows the mixtures of each tested solution.

Electrochemical
Testing. Electrochemical testing was conducted with a three-electrode system, in which the steel specimen acted as the work electrode (WE), platinum wire as the counterelectrode (CE), and silver chloride as the reference electrode (RE). e areas of the working electrodes were 0.78 cm 2 , held constant for all specimens. e steel specimens were exposed to the solutions, and the potential was stabilized with a potentiostat before the test.
Electrochemical impedance spectroscopy (EIS) was performed by changing the frequency of a 10 mV sinusoidal voltage from 0.1 Hz to 100 kHz. e DC polarization was performed from −0.3 V to +0.3 V vs. the opencircuit potential with a scanning rate of 1 mV/s. e potentiostat was a VersaSTAT system (Princeton Applied Research, Oak Ridge, TN, USA), and data analysis was conducted by fitting the test data to the constant phase element (CPE) model using Metrohm Autolab Nova 1.10 software.

Results and Discussion
3.1. pH Measurement. Cement consists of calcium silicate phases termed C 2 S and C 3 S and calcium aluminate phases termed C 3 A and C 4 AF in cement chemistry notation. Among these compounds, C 2 S (belite) and C 3 S (alite) account for >70% of cement, and these two phases produce C-S-H and calcium hydroxide through hydration. In general, when the cement is completely hydrated, calcium hydroxides corresponding to ∼20% of the prehydration weight are formed, thereby maintaining a high pH [29].
When sulfates penetrate the concrete, calcium hydroxide reacts with them to produce dihydrate gypsum. rough this process, an in ation pressure accumulates inside the concrete during the phase transition from hexagonal plates of calcium hydroxide to columns of dihydrate gypsum. Above a certain level of dihydrate gypsum generation, or upon localized concentration of dihydrate gypsum under continued sulfate attack, cracks form in the concrete. Furthermore, a pH decreases because the calcium hydroxide, which maintains the high pH of the concrete, is changed to plaster; therefore, damage is expected to occur in the passive lm of reinforcing steel in the acidifying environment. Figure 1 shows the measured pH for each aqueous solution. e pH of the Ca(OH) 2 solution was 12.78; with admixing of Na 2 SO 4 , the pH was dropped to ∼11.7. Following the admixing of the corrosion inhibitors, pH recovery was con rmed; the nitrite and amino acid corrosion inhibitors show the pH recovery functions of 6% and 3-6%, respectively. 4-Aminobutyric acid showed the lowest recovery rate of 3%. It is con rmed that the pH is reduced due to the reaction of calcium hydroxide with sulfate ion to produce dihydrate gypsum. It is considered that the pH can be recovered due to the immobilization reaction of sulfate ion by incorporating inhibitors.

Corrosion Potential.
Corrosion potential was measured for the steel specimens in saturated Ca(OH) 2 solutions containing di erent concentrations of NaCl, Na 2 SO 4 , and corrosion inhibitors. e changes in the corrosion potential are shown in Figure 2, which provides information about the initiation of corrosion at the surface of the reinforcing steel.    Advances in Materials Science and Engineering e corrosion potential (absolute value) increases actively as the concentration of NaCl is increased. e steel surface is deteriorated more easily by NaCl in the Ca(OH) 2 solution, and corrosion of the steel specimen is increased [30]. Figure 2 shows a reduction in corrosion potential as the concentration of Na 2 SO 4 increases because the reduction in pH could accelerate the corrosion at the surface of the steel. However, it is apparent that the corrosion potential increases when the inhibitors are mixed in the solution. is indicates the surface of the steel specimen is less active than the specimen without the inhibitor. e inhibitor could be adsorbed on the surface of the specimens and passivated them, thereby decreasing the corrosion reaction.
e specimen with lithium nitrite shows similar corrosion potential to that without inhibitors at 1.77 g/L Na 2 SO 4 . A similar trend is observed in the activation of the corrosion potential for diethanolamine and methyl diethanolamine. e specimen with 4-aminobutyric acid has the best corrosion resistance with the lowest corrosion potential regardless of the concentration of Na 2 SO 4 .

Electrochemical Impedance Spectroscopy.
In the saturated Ca(OH) 2 solutions containing 0.98 g/L NaCl and 0.89, 1.77 g/L Na 2 SO 4 , EIS was conducted for steel specimens with various corrosion inhibitors. e size of the semicircular loop in the EIS is decreased as the concentration of sulfate ions increases (Figure 3). is can promote the corrosion of the reinforcing steel by increasing the concentration of anions in the solutions [31]. Figure 4 shows the impedance-frequency plots of reinforcing steels in the Ca(OH) 2 solutions mixed with various concentrations of Na 2 SO 4 . As the concentration of sulfate ions is increased, the pH decreases and the resistance of the passive lm decreases due to the lm conductivity. Because chloride ions are present in the solution along with the sulfate ions, a reaction can occur at the activated region of the metal/solution interface [32]: For all reactions, it can be written as follows: Fe Fe(OH) 2 is unstable; by reacting with chloride ions in the solution, acidic ferrous chloride is formed:   FeCl 2 (acidic ferrous chloride) and OH − at the anode and cathode parts induce the anodic dissolution of the reinforcing steel [33].
Reinforcing steel is in the passive state in alkaline concrete environments. When the concentration of chloride ions increases or the pH decreases below 12.5, the passivity can be destroyed. e pH can be decreased by the carbonation of concrete via acidic substances present in solution. is reduction in pH causes destruction of the passive lm, and in the presence of anions such as Cl − , the steel rebars produce corrosion products by the same reaction as in equations (4) and (5). In this study, the sodium sulfate admixed in the saturated Ca(OH) 2 solution decreases the pH; because chloride ions are present, the passive lm on the steel specimen surface becomes hydrated.
Nyquist plots and impedance-frequency bode plots of the steel specimens in the solution with various inhibitors are shown in Figures 5 and 6, respectively. Figure 5 shows the Nyquist plots; when the concentration of corrosion inhibitor is increased, the size of the semicircular loop increases. It was con rmed that the capacitance increases with the incorporation of the corrosion inhibitor.
is result indicated that the corrosion inhibitor is adsorbed on the surface of the steel specimen to form a protective inactive lm. e concentration of the inhibitor is important in strengthening the passive lm. In this process, the corrosion inhibitor provides a passive lm and homogeneity. is result implies that the inhibitor substitutes chloride ions on the steel surface and forms a protective inert lm [34]. Figure 6 shows the total impedance at a low frequency (0.1 Hz). e impedance values are higher in the presence of inhibitors compared to that without an inhibitor, and the highest impedance value occurs with 4-aminobutyric acid. e corrosion inhibitor is adsorbed on the steel surface, thereby homogenizing the passive lm. It is con rmed that the corrosion inhibition performance of 4-aminobutyric acid is the highest. Figure 7 shows a graph of the phase frequencies in the electrochemical corrosion measurements of each aqueous solution.
In the case of 0.98 g/L NaCl and 1.77 g/L NaSO 4 mixed in the saturated Ca(OH) 2 solutions, the inhibitor e ects are con rmed with 0.6 mol lithium nitrite, diethanolamine, methyl diethanolamine, and 4-aminobutyric acid, as shown in Figures 8 and 9. e semicircle loop sizes for the steel specimens are increased in the Ca(OH) 2 solution mixed with 0.6 mol inhibitor compared to the case of 1.2 mol inhibitor ( Figure 8). is is because of the capacitive characteristic of the passive lm formed by the inhibitor. e passive lm on the steel specimen becomes weaker in the Ca(OH) 2 solution containing 1.77 g/L Na 2 SO 4 compared to that containing 0.89 g/L Na 2 SO 4 . With this result, it is determined that not only the decrease of pH but also the concentration increase of sulfate ions hinders the passivating e ect of the inhibitor. Figure 10 shows a graph of the phase-frequencies in the electrochemical corrosion measurements of each aqueous solution.
Compared to the case without an inhibitor, the increase of the semicircle loop is larger with the organic diethanolamine, methyl diethanolamine, and 4-aminobutyric acid corrosion inhibitors than that with the inorganic lithium nitrite corrosion inhibitor. With equal concentrations of chloride ions, the loop increase of methyl diethanolamine is high for low concentrations of sulfate ions and that of 4-aminobutyric acid is high for high concentrations of sulfate ions. In Figure 9, it can be con rmed that the impedance value is decreased more at the low frequency (0.1 Hz) in the aqueous solution containing a high concentration of sulfate ions because of the hindrance of sulfate ions, compared to the case of low concentration. With 4aminobutyric acid, the high impedance value is also maintained in the aqueous solution bearing highly concentrated sulfate ions.
For 0.98 g/L NaCl and 0.89 or 1.77 g/L Na 2 SO 4 mixed in the saturated Ca(OH) 2 solution, polarization resistance of the steel specimen was measured following the addition of the corrosion inhibitors. e polarization resistance    Table 4 shows the polarization resistance values for each solution. According to the admixing of the corrosion inhibitor, the polarization resistance increases, and as the mixing amount of sodium sulfate increases, the polarization resistance decreases. In the case of 0.89 g/L Na 2 SO 4 , the corrosion inhibitor becomes homogenized, thereby strengthening the inert characteristic of the lm, but at a high concentration (1.77 g/L) of Na 2 SO 4 , the formation of the inert layer between the steel interface and solution is hindered. e e ciency of each corrosion inhibitor can be calculated with the following equation [35]: where R pore and R 0 pore are the polarization resistance with and without the inhibitor, respectively. As the amount of NaSO 4 increases, the e ciency of the corrosion inhibitor decreases. In the solution mixed with a low concentration of 0.89 g/L Na 2 SO 4 , the lithium nitrite inhibitor had 69.36% e ciency, diethanolamine inhibitor 39.91%, methyl diethanolamine inhibitor 69.09%, and 4-aminobutyric acid 77.80%. Furthermore, in the solution mixed with a high concentration of 1.77 g/L Na 2 SO 4 , the lithium nitrite inhibitor had 75.93% e ciency, diethanolamine inhibitor 35.69%, methyl diethanolamine inhibitor 66.07%, and 4aminobutyric acid 67.87%. Overall, the 4-aminobutyric acid inhibitor has the highest e ciency.

Potentiodynamic Test.
Potentiodynamic tests were performed for steel specimens with various corrosion inhibitors in the saturated Ca(OH) 2 solutions containing 0.98 g/L NaCl and 0.89 or 1.77 g/L Na 2 SO 4 . Figure 11 shows the potentiodynamic plots according to the admixed amount of sulfate ions. When chloride ions are present, pitting corrosion is induced on the steel specimen in the saturated Ca(OH) 2 solution; as the sulfate ions are admixed, the corrosion current density (I corr ) increases and the corrosion potential (E corr ) is changed more signi cantly. Figure 12 shows the potentiodynamic plots for the cases of 0.98 g/L NaCl and 0.89 g/L Na 2 SO 4 mixed in the saturated Ca(OH) 2 solutions. In Figure 12, the corrosion current density decreases and the corrosion potential moves in the positive direction with the addition of corrosion inhibitor.

Conclusions
is study evaluated the performances of inorganic nitrite and amino acid corrosion inhibitors and their effects on the pH with various concentrations of Na 2 SO 4 in saturated Ca(OH) 2 solutions that contained NaCl, which simulated the concrete environment. e results are as follows: (1) In the saturated Ca(OH) 2 solution bearing a certain concentration of NaCl, the electrochemical corrosion tendency of the steel specimen changed depending on the concentration of Na 2 SO 4 (2) In the Ca(OH) 2 solution mixed with NaCl and Na 2 SO 4 , the lithium nitrite inhibitor had a corrosion prevention efficiency of 69.36-75.93%, diethanolamine 35.69-39.91%, methyl diethanolamine 66.07-69.09%, and 4-aminobutyric acid 67.87-77.80% (3) In the Ca(OH) 2 solution mixed with NaCl, as the Na 2 SO 4 concentration increased, the corrosion prevention efficiency decreased (4) EIS studies confirmed the corrosion resistance of the inhibitors with different concentrations of NaCl and Na 2 SO 4 (5) e results of potentiodynamic tests with the inhibitors showed the passivation behavior of reinforcing steel in the Ca(OH) 2 solution in which chloride and sulfate ions were present

Data Availability
e data used to support the findings of this study are included within the article.

Conflicts of Interest
e authors declare that there are no conflicts of interest.