Prevention of Reinforcement Corrosion in Concrete by Sodium Lauryl Sulphate : Electrochemical and Gravimetric Investigations

Prolonged corrosion inhibition response of sodium lauryl sulphate (SLS) on steel reinforcement in contaminated concrete was investigated by gravimetric method and electrochemical techniques such as potentiodynamic polarization and electrochemical impedance spectroscopy. Using half cell potential measurements probability of steel reinforcement corrosion was monitored for a period of 480 days. FT-IR spectroscopic analysis of the corroded products deposited on the steel reinforcement revealed the mechanism of corrosion inhibition. Modification in the surface morphology of steel specimens in the concrete was examined by optical microscopy. During the period of investigation (480 days), SLS showed appreciable corrosion inhibition efficiency on the steel reinforcement in concrete.


Introduction
Reinforced concrete structures are developed to ensure the strength and existence throughout their lifetime.But the deterioration and collapse of reinforced concrete structure are a major issue in construction field [1][2][3].CO 2 from the air and chloride ions from the sea atmosphere enter into the concrete and make the concrete interstitial solution more aggressive than before [4][5][6][7][8].Depassivation of the steel reinforcement occurs locally in the presence of chloride ions leading to pitting corrosion [9][10][11][12].Passivation of steel reinforcement takes place due to the alkaline compounds present in the concrete.The cost of replacing deteriorated concrete structures may affect the GDP (Gross Domestic Product) of a country.To find out an effective and economic method to combat the steel reinforcement corrosion in concrete has been attracted by the researchers and engineers for the last two decades.
Addition of the corrosion inhibitors to the concrete mixture is the most practical way to minimize steel reinforcement corrosion in contaminated concrete [13][14][15][16].In ideal situations, corrosion inhibitors will not affect the properties of the concrete.Some of the previous researchers have been established that inhibitor molecules generally affect the kinetics of electrochemical process of corrosion of steel [17][18][19].The mechanism of corrosion inhibition by the added compounds may include lowering of the rate of diffusion of chloride ion, enhancing the threshold value of chloride ion, and lowering the rate of anodic and cathodic process of corrosion [20][21][22].Corrosion inhibitors can be added to the fresh concrete as an admixture during concrete casting step or can be applied on the concrete surface.
Commercially available corrosion inhibitors may contain either organic or inorganic compounds like sodium nitrite, sodium monofluorophosphate, aminoethanols, and so forth [23][24][25][26][27]. Nowadays, use of nitrites is not recommended due to its toxicity.The efficacies of organic inhibitors are sometimes questionable at elevated chloride concentrations.Researchers, scientists, and engineers are always in search of potent, economic, and environmental-friendly admixtures to combat steel reinforcement corrosion in concrete.Some recently invented green inhibitors like silver nanoparticle doped palm oil leaf extracts [28] and naturally obtained Welan gum and Neem gum [29] have predominant concrete corrosion inhibition efficiency.
Present investigation was conducted to check the corrosion preventing capacity of SLS on steel reinforcement in contaminated concrete [30,31].Preliminary examinations were done by measuring the electrode potentials of steel reinforcements (against SCE) dipped in concrete pore solution contaminated with 3.5% NaCl containing various concentrations of SLS.After this examination, we decided to investigate the corrosion response of steel reinforcement in concrete specimen containing 2.5% of SLS by weight of cement.

Experimental
2.1.Materials.Sodium lauryl sulphate and sodium chloride (>99.9 %) were procured from Merck Millipore.Portland cement (Indiana Cements, Grade 53) used for making concrete specimens was purchased from the market.3.5% NaCl was added to the reservoir of the specimens and kept for 2 days.This will ensure the complete diffusion of NaCl throughout the specimen.This process was repeated three times during 30-60 days.

Electrochemical Investigations
(i) Potentiodynamic Polarization Studies. Figure 1 shows the schematic diagram of the test specimen and the experimental setup for electrochemical studies.Three-electrode cell assembly consisting of steel reinforcement present in concrete as working electrode, concrete specimen covered with a stainless steel mesh as counter electrode, and saturated calomel electrode (SCE) as reference electrode were used for the electrochemical studies [32].For good electrical contact, a wet sponge was used on the concrete surface between SCE and concrete surface.
After curing and treatment of test specimens with NaCl, polarization studies were performed three times during a period of 480 days in 160 days' interval.Steel reinforcement in the concrete was scanned for a potential range of +250 to -250 mV having a scan rate of 1mV/s.Tafel curves were analyzed to get the slopes, which are extrapolated to get the corrosion current densities.The corrosion inhibition efficiency of SLS was determined using the following equation: where I corr and I  corr are corrosion current densities of steel reinforcement in test specimens in the absence and presence of SLS, respectively.Experiments were repeated with the duplicate concrete block and the average values were taken.
(ii) Electrochemical Impedance Spectroscopic (EIS) Studies.EIS experiments were carried out at constant potential in the frequency range of 1 KHz to 100 mHz with excitation signal of 10mV amplitude.Analysis of the Nyquist curves using the equivalent circuit gave the charge transfer resistance (R ct ).Using R ct values corrosion inhibition efficacy of SLS was determined using the following equation: where R  ct and R ct are the charge transfer resistance of working electrode with and without SLS, respectively.Polarization and EIS analyses were done using Ivium CompactStat electrochemical work station (IviumSoft).
(iii) Half Cell Potential Measurements.For understanding the corrosion response of steel reinforcement in concrete, half cell potential measurements were carried out using saturated calomel electrode (SCE) and high impedance voltmeter (HP E2378A).In this electrochemical cell, steel reinforcement and SCE were acted as anode and cathode, respectively.The potential of the steel rebar was obtained by deducting the cell potential from the standard electrode potential of saturated calomel electrode.Sixteen readings were taken with a regular interval of 1 reading per month during 480 days.

Gravimetric Method.
Before casting the concrete blocks, each steel rebar was pickled with 2M HCl for 10 minutes, washed with water, degreased with acetone, dried, and weighed.The surface areas of all reinforcements were the same (10.5cm 2 ).After 480 days, concrete blocks were broken and the steel reinforcements were taken outside, pickled using 2M HCl for 15 minutes to remove the adhered rust, washed with water, dried, and weighed.From the weight loss of the steel specimens, approximate corrosion inhibition efficiency of SLS was calculated by the following equation: where w 0 and w are the weight loss of steel rebars of SAMPLE 1 and SAMPLE 2.
2.6.Spectroscopic Studies.Fourier Transform Infrared (FT-IR) spectroscopic study of corroded product on the steel reinforcement was conducted using KBr pellet method.The spectrum was recorded in the range of 400-4000cm −1 using Shimadzu IR Affinity-1 model FT-IR spectrophotometer.

Microscopic Surface Analysis.
After 480 days of electrochemical investigation, the steel specimens were taken outside from the concrete and analyzed using a highresolution optical microscope (Leica Stereo Microscope, No. S8ACO).This study mainly aims to understand the surface modifications that occurred for steel specimen during the period of investigation.

Potentiodynamic Polarization Studies.
The inhibition efficacy of SLS and site of corrosion inhibition on steel reinforcement in the contaminated concrete was monitored using potentiodynamic polarization studies.Analysis of polarization curve gave Tafel slopes, current densities, and percentage of inhibition efficiency which are given in Table 1.From the data, it is understandable that the steel reinforcement in the concrete containing SLS was less corroded than reinforcement in the SAMPLE 1.During 480 days, three sets of measurements were taken in 160 days' interval.It was noticed that SLS exhibited 89% corrosion inhibition efficiency on 160 th day of study.On 320 th and 480 th days of investigation, inhibition efficiency changed to 71% and 80%, respectively.
From these results, it can be assured that SLS can act as an efficient admixture, which resists the steel reinforcement corrosion in the contaminated concrete structures for a long time.SLS mainly acted on cathodic sites of corrosion up to 2/3 rd period of investigation, since the cathodic slopes of the Tafel lines altered appreciably (Figure 2).But the corrosion inhibition response of SLS was changed into mixed type (affecting both anodic and cathodic sites) towards the end of study.
The gradation in the inhibition efficiency of SLS can be explained with the help of following hypothesis.The entire period of investigation can be divided into three stages: first stage (0-160 days), second stage (161-320 days), and third stage (321-480 days).In the first stage, SLS molecules adjacent to the steel rebar make coordinate bonds with ferrous ions and prevent chloride ions and water molecules diffusing towards the steel surface considerably.Thus, during this period, SLS protects well the corrosion of reinforcement.Towards the end of second stage (320 days), it was observed that the rate of corrosion slightly increased.Since the rate of diffusion of chloride ion is greater than that of SLS, higher percentage of chloride ions can be expected near the steel surface than that of first stage.The porous nature of the hydrated ferric oxide helps to enter water molecules, chloride, and oxygen more towards the steel surface.Thus SLS showed less corrosion  protection efficacy at this stage.In the third stage (480 days), the rate of corrosion was seemed to decrease again (but not as that much of first stage).During the end of this stage, it can be believed that more and more SLS molecules migrated towards the steel surface from the bulk and make more coordinate bonds with ferrous ions and protect steel surface.This led to higher corrosion protection efficiency of SLS than in second stage.

EIS Measurements.
The corrosion response of steel reinforcement in concrete in the presence and absence of SLS was monitored using electrochemical impedance spectroscopy and the corresponding Nyquist plots are represented in Figure 3.The corrosion behaviour of steel reinforcement was significantly differed in the presence and absence of inhibitor.The equivalent circuit fitted for EIS measurements (Figure 4) consists of solution resistance (R s ) which is connected in series to double layer capacitance (C 1 ) and this combination is connected in parallel to a series combination of Warburg resistance (W) and charge transfer resistance (R ct ) [33,34].
The EIS parameters of the test specimens are given in Table 2.
Impedance data show that addition of SLS leads to the decrease in double-layer capacitance and increase of charge transfer resistance.This is due to the adsorption of SLS molecules on the steel rebar.SLS exhibited 87.2% corrosion inhibition efficiency on 160 th day of investigation.Inhibition efficiencies changed to 78 and 86%, respectively, on the 320 th and 480 th days of study.

Half Cell Potential Measurements.
The probability of corrosion of steel reinforcement in concrete was visualized with the help of half cell potential measurements.It was established by the previous researchers that tendency of corrosion increases as the potential of the steel reinforcement changes to more anodic side.Half cell potentials (Vs SCE) of steel reinforcements in concrete blocks during 480 days were measured at a time interval of 30 days and are given in Table 3.The half cell potential values are compared with the help of a line graph and are represented in Figure 5. Data show that the steel reinforcement in concrete block (SAMPLE 1) exhibited high anodic potential values, which is a clear reflection of high corrosion rate in the contaminated concrete.Half cell potentials of steel specimen in SAMPLE 2 were less negative (more cathodic) than SAMPLE 1 during the period of investigation.These results show that SLS is acting as a good corrosion inhibitor on steel reinforcement through the period of investigation.

Gravimetric Analysis.
Gravimetric analysis of steel reinforcement in SAMPLE 1 and SAMPLE 2 was done for examining the percentage of weight loss and the approximate corrosion inhibition efficacy of SLS.After the period of investigation, steel specimens were taken outside from the concrete block, pickled with HCl, dried, and weighed.Steel reinforcements of SAMPLE 1 and SAMPLE 2 displayed weight losses of 24% and 3.6%, respectively.Thus the intervention of SLS on the steel rebar in concrete contaminated with NaCl was well established.Approximate value of corrosion inhibition efficiency of SLS determined by gravimetric studies was 86%.

FT-IR Analysis.
IR spectral studies of the corroded product gave an idea about the nature of interaction of SLS on steel reinforcement.Surface deposits were removed and subjected to spectral analysis.Various compounds such as ferrous and ferric hydroxides, hydrated ferric oxide, and ferrous chloride may be deposited on the steel reinforcement.A broad signal observed at 3375 cm −1 in the spectrum of corroded product of SAMPLE 1 was due to -OH stretching frequency of ferrous hydroxide or hydrated ferric oxide (or both) [35].In the IR spectrum of corroded product of SAMPLE 2, it was noticed International Journal of Corrosion  that the peak due to -OH frequency shifted to 3349 cm −1 .This may be due to the interaction of SLS with the ferrous or ferric hydroxide.The IR spectrum of sodium lauryl sulphate contains peaks at 2922 and 2821 cm −1 which can assigned to the stretching frequencies of various C-H.This peak was visible at 2895 cm −1 in the spectrum of surface product of SAMPLE 2 steel specimen.In the IR spectrum of SLS, two IR regions corresponding to symmetric and asymmetric stretching frequency of sulphate group appeared at 826-1240cm −1 and 1240-1466cm −1 , respectively.These peaks were shifted to 789-1104cm −1 and 1104-1420cm −1 in the spectrum of corroded product of SAMPLE 2. This is clear indication of the interaction of SLS through the sulphate end.A weak peak corresponding to the C-C stretching frequency of carbon skeleton displayed at 964cm −1 in the IR spectrum of SLS was shifted to 955cm −1 .Two intense peaks at 2821cm −1 and 2922cm −1 corresponding to symmetric and asymmetric stretching frequencies of CH 2 groups of SLS also shifted to 2802cm −1 and 2895cm −1 in the FT-IR spectrum of surface deposits of steel in SAMLE 2. Figure 6 represents the spectrum of SLS and the IR spectrum of oxide film deposited on the steel reinforcement in SAMPLE 1and SAMPLE 2. steel rebar occurred due to the continuous contact of the contaminated concrete pore solution.It was noticed that the surface of steel reinforcement in SAMPLE 2 contains little or no oxide.Thus the ability of SLS to prevent the steel reinforcement corrosion in contaminated concrete was once again proven by this analysis.

Mechanism of Inhibition.
Sodium lauryl sulphate is a surfactant molecule and can perform well on the steel surface to decrease the rate of anodic and cathodic processes of corrosion [36,37].It can be assumed that SLS molecules present in concrete pore solution slowly migrate towards the steel surface and make coordinate bonds with the ferrous ions on the surface through the polar end.Since SLS has a long hydrophobic chain pointing away from the steel surface, it can repel water molecules and chloride ions diffusing towards the surface.This process is really beneficial to control the electrochemical process of corrosion.In other words, SLS efficiently resists the conversion of ferrous ions into hydrated ferric oxide (rust).The overall mechanism of the inhibition and the structure of SLS are represented in Figure 8.

Conclusion
Investigations revealed that sodium lauryl sulphate has been a potent corrosion inhibitor for the steel reinforcement in contaminated concrete for a long time.Electrochemical studies proved that the corrosion inhibition efficacy of SLS did not alter appreciably.SLS generally affect the cathodic and anodic process of corrosion.According to gravimetric studies, SLS displayed 86% of corrosion inhibition efficiency on steel reinforcement in contaminated concrete.These results are in good agreement with the electrochemical investigation results.FT-IR and microscopic surface analysis proved the interaction of SLS on steel reinforcement.

Figure 2 :
Figure 2: Tafel curves of steel reinforcement in concrete (a) in the absence and (b) presence of SLS for 160, 320, and 480 days.

Figure 3 :Figure 4 :
Figure 3: Nyquist plots and Bode plots of steel reinforcement in concrete (a) in the absence and (b) presence of SLS for 160, 320, and 480 days.

Figure 5 :Figure 6 :
Figure 5: Variation of half cell potentials of steel reinforcement in contaminated concrete in the absence (SAMPLE 1) and presence (SAMPLE 2) of SLS during 480 days.

Figure 8 :
Figure 8: (a) Structure of sodium lauryl sulphate; (b) SAMPLE 1: water molecules and chloride ions are attached on steel surface; (c) SAMPLE 2: coordinated SLS prevent the migration of water molecules and chloride ions towards steel surface.

Table 1 :
Tafel data of steel reinforcement in concrete in the presence and absence of SLS for different periods.

Table 2 :
EIS parameters of steel reinforcement in concrete in the presence and absence of inhibitor for different periods.

Table 3 :
Half cell potential (mV) of steel reinforcement in concrete in the presence and absence of SLS for 480 days.