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. The corrosion resistance of inhibitors was evaluated by corrosion potential, electrochemical impedance spectroscopy, and potentiodynamic techniques. The 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%).
The corrosion of steel reinforcements within concrete occurs as the passive films, formed on the steel surfaces in the alkaline environment of concrete, are destroyed. The 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. The 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 [
Inorganic corrosion inhibitors are mostly nitrites, which have been used since the 1940s. Nitrite-based corrosion inhibitors are environmentally unfriendly with hazardous biological effects [
Before corrosion begins or at relatively low corrosion levels, amino alcohol inhibitors are more effective [
Studies about corrosion of reinforcing steel-simulating concrete environments using saturated Ca(OH)2 solutions have been reported elsewhere [
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 Na2SO4 in saturated Ca(OH)2 that contained NaCl, which simulated the concrete environment, were investigated. The corrosion resistance of inhibitors was evaluated by various electrochemical tests, such as potential times, electrochemical impedance spectroscopy, and potentiodynamic techniques.
Commercially available inorganic nitrite corrosion inhibitor and amino acid corrosion inhibitors were used, and their physical properties are shown in Table
Physical properties of corrosion inhibitors.
Corrosion inhibitor | Value | |||
---|---|---|---|---|
Specific gravity | pH | Viscosity (cP) | Solid content (wt.%) | |
Lithium nitrite (LiNO2) | 1.052 | 12.84 | 8.0 | 75.0 |
Diethanolamine (HN(CH2CH2OH)2) | 1.097 | 12.21 | 10.0 | 90.0 |
Methyl diethanolamine (C₅H₁₃NO₂) | 1.038 | 11.26 | 11.0 | 90.0 |
4-Aminobutyric acid (H2N(CH2)3COOH) | 1.110 | 7.99 | 9.0 | 80.0 |
Steel specimens of 16 mm in diameter were cut and placed in acid/alkali-resistant thermosetting resin. The mounted samples were polished with 180–1,200
Chemical composition of steel specimens (wt.%).
C | Si | Mn | P | S | Ni | Cr | Mo | Cu | Sn | Fe |
---|---|---|---|---|---|---|---|---|---|---|
0.24 | 0.26 | 0.95 | 0.016 | 0.008 | 0.03 | 0.04 | 0.01 | 0.02 | 0.0005 | Balance |
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
Representation of studied solutions.
Inhibitors | Concentration of NaCl (g/L) | Concentration of NaSO4 (g/L) | Concentration of inhibitor (g/L) | Solution names |
---|---|---|---|---|
None | 0.98 | 0.89 | 0.00 | C0.98-S0.89 |
1.77 | C0.98-S1.77 | |||
Lithium nitrite (LiNO2) | 0.89 | 2.04 | C0.98-S0.89-L2.04 | |
1.77 | C0.98-S1.77-L2.04 | |||
Diethanolamine (HN(CH2CH2OH)2) | 0.89 | 0.51 | C0.98-S0.89-D0.51 | |
1.77 | C0.98-S1.77-D0.51 | |||
Methyl diethanolamine (C₅H₁₃NO₂) | 0.89 | 0.50 | C0.98-S0.89-M0.50 | |
1.77 | C0.98-S1.77-M0.50 | |||
4-Aminobutyric acid (H2N(CH2)3COOH) | 0.89 | 0.38 | C0.98-S0.89-A0.38 | |
1.77 | C0.98-S1.77-A0.38 |
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). The areas of the working electrodes were 0.78 cm2, held constant for all specimens. The 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. The DC polarization was performed from −0.3 V to +0.3 V vs. the open-circuit potential with a scanning rate of 1 mV/s. The 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.
Cement consists of calcium silicate phases termed C2S and C3S and calcium aluminate phases termed C3A and C4AF in cement chemistry notation. Among these compounds, C2S (belite) and C3S (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 [
When sulfates penetrate the concrete, calcium hydroxide reacts with them to produce dihydrate gypsum. Through this process, an inflation 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 film of reinforcing steel in the acidifying environment.
Figure
Results of pH measurement.
Corrosion potential was measured for the steel specimens in saturated Ca(OH)2 solutions containing different concentrations of NaCl, Na2SO4, and corrosion inhibitors. The changes in the corrosion potential are shown in Figure
Corrosion potential-time plots of steel specimens exposed in saturated Ca(OH)2 solution with 0.98 g/L NaCl, different concentrations of Na2SO4, and inhibitors.
In the saturated Ca(OH)2 solutions containing 0.98 g/L NaCl and 0.89, 1.77 g/L Na2SO4, EIS was conducted for steel specimens with various corrosion inhibitors. The size of the semicircular loop in the EIS is decreased as the concentration of sulfate ions increases (Figure
Nyquist plots of steel specimens exposed in saturated Ca(OH)2 solution with 0.98 g/L NaCl and different concentrations of Na2SO4.
Figure
Impedance-frequency bode plots of steel specimens exposed in saturated Ca(OH)2 solution with 0.98 g/L NaCl and different concentrations of Na2SO4.
For all reactions, it can be written as follows:
Fe(OH)2 is unstable; by reacting with chloride ions in the solution, acidic ferrous chloride is formed:
FeCl2 (acidic ferrous chloride) and OH− at the anode and cathode parts induce the anodic dissolution of the reinforcing steel [
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. The pH can be decreased by the carbonation of concrete via acidic substances present in solution. This reduction in pH causes destruction of the passive film, and in the presence of anions such as Cl−, the steel rebars produce corrosion products by the same reaction as in equations (
Nyquist plots and impedance-frequency bode plots of the steel specimens in the solution with various inhibitors are shown in Figures
Nyquist plots of steel specimens exposed in saturated Ca(OH)2 solution with 0.98 g/L NaCl, 0.89 g/L Na2SO4, and inhibitors.
Impedance-frequency bode plots of steel specimens exposed in saturated Ca(OH)2 solution with 0.98 g/L NaCl, 0.89 g/L Na2SO4, and inhibitors.
Phase-frequency bode plots of steel specimens exposed in saturated Ca(OH)2 solution with 0.98 g/L NaCl, 0.89 g/L Na2SO4, and inhibitors.
In the case of 0.98 g/L NaCl and 1.77 g/L NaSO4 mixed in the saturated Ca(OH)2 solutions, the inhibitor effects are confirmed with 0.6 mol lithium nitrite, diethanolamine, methyl diethanolamine, and 4-aminobutyric acid, as shown in Figures
Nyquist plots of steel specimens exposed in saturated Ca(OH)2 solution with 0.98 g/L NaCl, 1.77 g/L Na2SO4, and inhibitors.
Impedance-frequency bode plots of steel specimens exposed in saturated Ca(OH)2 solution with 0.98 g/L NaCl, 1.77 g/L Na2SO4, and inhibitors.
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
For 0.98 g/L NaCl and 0.89 or 1.77 g/L Na2SO4 mixed in the saturated Ca(OH)2 solution, polarization resistance of the steel specimen was measured following the addition of the corrosion inhibitors. The polarization resistance decreases as the conductivity of the solution is increased. An increase in the conductivity of the solution, i.e., distribution of anions such as chloride ions that destroy the passive film, induces localized corrosion of the steel. Table
Polarization resistance of steel specimens and Efficiency of inhibitors exposed in saturated Ca(OH)2 solution 0.98 g/L NaCl and different concentrations of Na2SO4.
Inhibitors | Concentration of NaCl (g/L) | Concentration of NaSO4 (g/L) | Concentration of inhibitor (g/L) | Polarization resistance, |
Efficiency, |
---|---|---|---|---|---|
None | 0.98 | 0.89 | 0.00 | 4356 | |
1.77 | 3227 | ||||
Lithium nitrite (LiNO2) | 0.89 | 2.04 | 14219 | 69.36 | |
1.77 | 13407 | 75.93 | |||
Diethanolamine (HN(CH2CH2OH)2) | 0.89 | 0.51 | 7249 | 39.91 | |
1.77 | 5018 | 35.69 | |||
Methyl diethanolamine (C₅H₁₃NO₂) | 0.89 | 0.50 | 14093 | 69.09 | |
1.77 | 9512 | 66.07 | |||
4-Aminobutyric acid (H2N(CH2)3COOH) | 0.89 | 0.38 | 19623 | 77.80 | |
1.77 | 10044 | 67.87 |
The efficiency of each corrosion inhibitor can be calculated with the following equation [
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 Na2SO4. Figure
Phase-frequency bode plots of steel specimens exposed in saturated Ca(OH)2 solution with 0.98 g/L NaCl, 1.77 g/L Na2SO4, and inhibitors.
Figure
Potentiodynamic plots of steel specimens exposed in saturated Ca(OH)2 solution with 0.98 g/L NaCl and different concentrations of Na2SO4. (a) C0.98-S0; (b) C0.98-S0.89; (c) C0.98-S1.77.
Potentiodynamic plots of steel specimens exposed in saturated Ca(OH)2 solution with 0.98 g/L NaCl, 0.89 g/L Na2SO4, and inhibitors. (a) C0.98-S0.89-L2.04; (b) C0.98-S0.89-D0.51; (c) C0.98-S0.89-M0.50; (d) C0.98-S0.89-A0.38.
Figure
Potentiodynamic plots of steel specimens exposed in saturated Ca(OH)2 solution with 0.98 g/L NaCl, 1.77 g/L Na2SO4, and inhibitors, (a) C0.98-S1.77-L2.04; (b) C0.98-S1.77-D0.51; (c) C0.98-S1.77-M0.50; (d) C0.98-S1.77-A0.38.
The principle of corrosion inhibiting is that the functional groups of the inhibitor react with iron ions, thereby forming a protective film. For inorganic lithium nitrite, the nitrite reacts with the iron ions and forms a passive film. For organic diethanolamine and methyl diethanolamine, containing hydroxyl and amine groups, they act as nucleophiles and react with the metal. Afterwards, the diethanolamine and methyl diethanolamine inhibitors form very thin and protective oxide layers on the surfaces of reinforcing steels. Furthermore, 4-aminobutyric acid contains carboxyl and amine groups; like the other organic inhibitors, it acts as a nucleophile, thereby donating to the vacant
This study evaluated the performances of inorganic nitrite and amino acid corrosion inhibitors and their effects on the pH with various concentrations of Na2SO4 in saturated Ca(OH)2 solutions that contained NaCl, which simulated the concrete environment. The results are as follows: 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 Na2SO4 In the Ca(OH)2 solution mixed with NaCl and Na2SO4, 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% In the Ca(OH)2 solution mixed with NaCl, as the Na2SO4 concentration increased, the corrosion prevention efficiency decreased EIS studies confirmed the corrosion resistance of the inhibitors with different concentrations of NaCl and Na2SO4 The 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
The data used to support the findings of this study are included within the article.
The authors declare that there are no conflicts of interest.
This research was supported by the research grant 18CTAP-C130223-02 through the Korea Agency for Infrastructure Technology Advancement funded by the Ministry of Land, Infrastructure and Transport of Korean Government.