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Adsorption and inhibitive properties of Tryptophan on low alloy steel corrosion in acidic media
⁎Corresponding author at: Central Chemical Laboratories, Egyptian Electricity Holding Company, Sabtia, Cairo, Egypt. Tel.: +20 12 22678654. hesham_tm@yahoo.com (Hesham T.M. Abdel-Fatah)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
The inhibition efficiency of Tryptophan (Trp) has been studied for the corrosion of low alloy steel ASTM A213 grade T22 in sulfamic (HSO3NH2) and hydrochloric (HCl) acid solutions.
Corrosion inhibition was studied using electrochemical methods (electrochemical impedance spectroscopy; EIS and the new technique electrochemical frequency modulation; EFM) and weight loss measurements. The influence of inhibitor concentration, solution temperature, and immersion time on the corrosion resistance of low alloy steel (LAS) has been investigated. Trp proved to be a very good inhibitor for low alloy steel acid corrosion. EFM measurements showed that Trp is a mixed type inhibitor. Trp behaved better in 0.6 M HCl than in 0.6 M HSO3NH2. Moreover, it was found that the inhibition efficiency increased with increasing inhibitor concentration, while a decrease was detected with the rise of temperature and immersion time. The associated activation energy (Ea) has been determined. The values of Ea indicate that the type of adsorption of Trp on the steel surface in both acids belongs to physical adsorption. The adsorption process was tested using Temkin adsorption isotherm.
Keywords
Corrosion inhibition
Acid cleaning
Amino acids
EIS
EFM
1 Introduction
Low alloy steels are widely employed in the power and petrochemical industries for boilers, piping, and chemical reaction vessels (Dobrzanski, 2004; Ghanem et al., 1996).
The fouling deposits of boiler and heat exchange tubes is a major problem in the operation of steam generating units. Therefore, the internal surfaces of boiler tubes in contact with water and steam must be kept clean and free of deposits (Majnouni and Jaffer, 2003). Industrial acid cleaning is applied chiefly to remove scale and undesired deposits from steam generating equipment (Natarajan and Sivan, 2003). Several acid solutions will effectively remove waterside deposits. Hydrochloric, sulfamic, sulfuric, and citric acids are employed for such purpose (Sathiyanarayanan et al., 2006).
Hydrochloric acid is one of the most important pickling acids, which is widely used in steel and ferrous alloy industry for acid cleaning, acid descaling, oil well oxidizing and other petrochemical processes.
Sulfamic acid is widely used in many diversified industrial acid cleaning applications. In addition to its strength as an effective solvent for iron oxides and a variety of water-formed scales it has also many other advantages such as that it is suitable for use with alloy steels and austenitic stainless steels (Morad, 2008; Majnouni and Jaffer, 2003; McCoy, 1984).
The usual concentration of sulfamic acid and hydrochloric acid is around 4–10% by weight and at a temperature range around 60–80 °C (Majnouni and Jaffer, 2003; McCoy, 1984).
The main problem in using low alloy steel in acidic solution is its degradation due to corrosion. Therefore, the prevention of corrosion of low alloy steel in acidic solutions is of practical importance. Among the several methods of corrosion control, the use of chemical inhibitors is often considered as the most effective and practical method of corrosion prevention.
Amino acids are from a class of organic compounds that are completely soluble in aqueous media, relatively cheap, easy to produce with high purity; non-toxic and considered as environmentally friendly compounds. These properties enhance their use as corrosion inhibitors for iron, steel and stainless steel (Oguzie et al., 2007; Ashassi-Sorkhabi et al., 2005; Kalota and Silverman, 1994; Madkour and Ghoneim, 1997; Morad et al., 2002).
Tryptophan is a unique amino acid that is an essential component of the human diet. The principal role of Tryptophan in the human body is as a constituent of protein synthesis.
Tryptophan is a routine constituent of most protein-based foods or dietary proteins. It is particularly plentiful in turkey, chocolate, dried dates, milk, yogurt, red meat, eggs, fish, poultry, sesame, sunflower seeds, bananas, and peanuts (Richard et al., 2009; Sainio et al., 1996).
The purpose of this article is to study the inhibition properties of Tryptophan (Trp) on the corrosion behavior of low alloy steel (LAS) ASTM A213 grade T22 in sulfamic (HSO3NH2) and hydrochloric (HCl) acid solutions. Corrosion inhibition was investigated using weight loss, electrochemical impedance spectroscopy (EIS) and electrochemical frequency modulation (EFM) methods.
2 Experimental
The experiments were performed on low alloy steel (ASTM A213 grade T22) strips. Table 1 presents the chemical composition (wt.%) of the used alloy.
| Alloy | Si | Mn | Cr | Fe | Mo |
|---|---|---|---|---|---|
| ASTM A213 grade T22 | 0.35 | 0.64 | 2.30 | 95.85 | 0.86 |
Suitable amounts of Tryptophan (Trp) were dissolved in 0.6 M HSO3NH2 and HCl acid solutions to obtain the desired concentration (0.005 M–0.04 M). The chemical structure of Trp is shown in Fig. 1.
All solutions were freshly prepared from analytical grade chemical reagents using ultrapure water. The experiments were conducted in stagnant solutions at different temperatures 25, 40, 50 and 60 ± 1 °C.
2.1 Weight loss measurements
The most common method for estimating a corrosion rate from mass loss is to weigh the corroding sample before and after exposure and divide by the total exposed area and the total exposure time.
Weight loss measurements were performed with the dried rectangular strips of size 1.5 cm × 1 cm × 0.2 cm with total exposed area of 4 cm2. The strips were immersed in the acid solutions in the absence and presence of various concentrations of Trp for different time intervals, up to 5 days in steps of 1 day at room temperature.
The corrosion rate in units of millimeters per year (mm/year) can be the following equation (Fontana and Mars, 1987):
The inhibition efficiency (IE%) of Trp was calculated under different experimental conditions by using the following equation:
2.2 Electrochemical measurements
Gamry PCI300/4 Potentiostat and Echem Analyst 5.21 software were used for electrochemical data acquisition and analysis. The conventional three electrode system consisting of saturated calomel electrode (SCE) as reference electrode, platinum foil (1 cm2) as counter electrode and T22 steel strips having exposed area of 1 cm2 as working electrode was used. The working electrodes were mechanically abraded with different grades (240, 400, 600 and 1200) of abrasive papers, degreased with acetone in an ultrasonic bath, then washed with ultrapure water and finally dried before use.
The electrochemical impedance spectroscopy (EIS) measurements were carried out using AC signals of 5 mV amplitude and sweeping the frequency from 20 kHz to 0.3 Hz. The impedance data were analyzed with EIS300 software. The charge transfer resistance obtained by fitting the semicircles of the Nyquist representations has been used to calculate inhibition efficiencies of Trp using the relation;
The electrochemical frequency modulation (EFM) is a new electrochemical technique in which two sinusoidal potential signals are summed and applied to a corrosion sample through a potentiostat. The resulting current is measured and the time-domain data are converted to the frequency domain. This frequency domain is used to measure the signal at the applied fundamental frequencies, at harmonics of the fundamental frequencies, and at intermodulation frequencies. By the appropriate mathematical manipulation, the large peaks are used to directly determine the values of corrosion current density (Icorr), corrosion rate, Tafel constants (βc and βa) and the causality factors (CF2 & CF3). This is in accordance with the theory of EFM technique, which was reported before (Bosch et al., 2001).
EFM technique has many advantageous features such as (Bosch et al., 2001; Abd El Rehim et al., 2006; Amin et al., 2009):
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The test requires only 2–10 min.
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It is considered as a non-destructive technique.
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It has an internal self-check in the form of two “Causality Factors”. These two factors should have the values 2.0 and 3.0 if all of the conditions of EFM theory have been met.
The EFM measurements are performed by applying a potential perturbation signal with amplitude of 10 mV with two sine waves of 2 and 5 Hz and the inhibition efficiency was evaluated from the measured Icorr values using the following relationship:
3 Results and discussion
3.1 Electrochemical frequency modulation studies
The EFM intermodulation spectra of LAS in the presence of 0.02 M of Trp in 0.6 M HSO3NH2 and 0.6 M HCl at 25 °C as an example are shown in Fig. 2.
The EFM results; corrosion current density (Icorr), Tafel constants (βc and βa) and the causality factors (CF2 and CF3) are given in Table 2.
| Acid | Trp conc. (M) | βa (mV dec−1) | βc (mV dec−1) | CF2 | CF3 | Icorr (μA cm−2) | IE% |
|---|---|---|---|---|---|---|---|
| HSO3NH2 | 0.00 | 87.5 | 178.7 | 1.99 | 2.87 | 429.7 | 0.00 |
| 0.005 | 88.9 | 174.1 | 2.00 | 2.74 | 281.6 | 34.47 | |
| 0.01 | 89.6 | 181.2 | 1.79 | 2.99 | 172.8 | 59.79 | |
| 0.02 | 87.5 | 179.9 | 1.98 | 2.76 | 87.6 | 79.61 | |
| 0.04 | 86.7 | 180.7 | 2.00 | 2.85 | 36.2 | 91.58 | |
| HCl | 0.00 | 82.7 | 172.8 | 1.87 | 2.73 | 191.3 | 0.00 |
| 0.005 | 80.2 | 171.1 | 1.69 | 2.94 | 118.3 | 38.16 | |
| 0.01 | 81.5 | 170.6 | 2.00 | 3.00 | 69.81 | 63.51 | |
| 0.02 | 83.2 | 178.4 | 1.73 | 2.87 | 35.28 | 81.56 | |
| 0.04 | 76.8 | 171.5 | 1.98 | 2.65 | 10.59 | 94.46 | |
The values of corrosion current density (Icorr) decreased in the presence of Trp which suggest that the rate of electrochemical reaction was reduced due to the formation of a barrier layer over the LAS surface by the inhibitor. The values of βc and βa did not show any appreciable change indicating that the studied inhibitor in both acids is a mixed type inhibitor (Abd El-Maksoud et al., 2005).
From Table 2, it can also be shown that the values of causality factors (CF2 and CF3) were approximately equal to the theoretical values 2 and 3 indicating that the measured data are reliable (Bosch et al., 2001).
The inhibition efficiency (IE%) of Trp was calculated from the corrosion current density (Eq. (4)) and provided in Table 2. The inhibition efficiency increased with inhibitor concentrations in both acids. Moreover, the inhibition efficiency of Trp in 0.6 M HCl is found to be more than that for HSO3NH2.
Weight loss and EIS studies were conducted in order to ensure validity of the results obtained from the EFM technique as an effective corrosion monitoring technique.
3.2 Weight loss studies
The rate of corrosion was calculated from the weight loss, the time of exposure and the original exposed surface area of LAS samples using Eq. (1).
Fig. 3a illustrates the influence of the presence and increasing concentrations of Trp on the corrosion rate (mm/year) of LAS in 0.6 M HSO3NH2 and 0.6 M HCl acid solutions.
It is obvious that the corrosion rate decreases with increasing inhibitor concentration which suggests the retardation of LAS corrosion in the presence of Trp with respect to the plain acid (0.6 M HSO3NH2 and 0.6 M HCl).
It is also observed from Fig. 3a that at a given inhibitor concentration, the corrosion rate of LAS enhances in case of sulfamic acid more than that in hydrochloric acid.
The inhibition efficiency (IE%) of Trp for each concentrations was calculated using the Eq. (2). The variation in inhibition efficiency of Trp with its concentrations is shown in Fig. 3b. It seems from this figure that the inhibition efficiency (IE%) of Trp increases with increasing their concentration as a result of increasing the surface coverage by inhibitor species. It is also found that Trp is more efficient in 0.6 M HCl than in 0.6 M HSO3NH2.
3.3 Electrochemical impedance studies
Nyquist plots of LAS in uninhibited and inhibited HCl acid solutions containing various concentrations of Trp at 50 °C (as an example) are shown in Fig. 4. It is observed that the plots are not perfect semicircles. This is due to the frequency dispersion that occurred during the formation of a double layer at the metal/solution interface (Paskossy, 1994; Growcock and Jasinski, 1989).
The electrical equivalent circuit models shown in Fig. 5 were used to analyze the obtained impedance data. In the absence of inhibitor molecules, the standard Randles circuit model was used as shown in Fig. 5a. The values of polarization resistance (Rp) consist of only charge transfer resistance (Rct). While, in the presence of inhibitor (Fig. 5b), the polarization resistance, which corresponds to the diameter of Nyquist plot, consist of Rct, Rd (diffuse layer resistance), and Rf (film resistance) i.e. Rp = Rct + Rd + Rf + Ra (Lorenz and Mansfeld, 1986; Solmaz et al., 2008; Sam et al., 2010).
The values of polarization resistance (Rp) and the double layer capacitance (Cdl) obtained from impedance data at various concentrations of Trp and at different temperatures are presented in Table 3.
| Temp (°C) | Trp Conc. (M) | 0.6 M HSO3NH2 | 0.6 M HCl | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Cdl (μF.cm−2) | Rct (ohm.cm2) | IE% | θ | Cdl (μF.cm−2) | Rct (ohm.cm2) | IE% | θ | ||
| 25 | 0.00 | 278.2 | 58.83 | 0.00 | 0.000 | 101.2 | 124.6 | 0.00 | 0.000 |
| 0.005 | 195.2 | 91.78 | 35.90 | 0.359 | 66.22 | 200.3 | 37.79 | 0.378 | |
| 0.01 | 121.6 | 151.4 | 61.15 | 0.611 | 34.26 | 351.21 | 64.52 | 0.645 | |
| 0.02 | 72.81 | 282.4 | 79.17 | 0.792 | 22.98 | 709.7 | 82.44 | 0.824 | |
| 0.04 | 31.56 | 745.5 | 92.11 | 0.921 | 9.385 | 2019 | 93.83 | 0.938 | |
| 40 | 0.00 | 673.8 | 34.93 | 0.00 | 0.000 | 199.5 | 62.51 | 0.00 | 0.000 |
| 0.005 | 499.6 | 48.98 | 28.69 | 0.287 | 128.3 | 92.38 | 32.33 | 0.323 | |
| 0.01 | 318.4 | 73.36 | 52.39 | 0.524 | 90.07 | 140.2 | 55.42 | 0.554 | |
| 0.02 | 201.6 | 130.2 | 73.17 | 0.732 | 48.96 | 266.9 | 76.58 | 0.766 | |
| 0.04 | 92.87 | 226.4 | 84.57 | 0.846 | 29.81 | 500.8 | 87.52 | 0.875 | |
| 50 | 0.00 | 867.9 | 24.17 | 0.00 | 0.000 | 468 | 40.29 | 0.00 | 0.000 |
| 0.005 | 632.7 | 32.18 | 24.89 | 0.249 | 345.8 | 53.81 | 25.13 | 0.251 | |
| 0.01 | 500.2 | 41.36 | 41.56 | 0.416 | 274.8 | 71.42 | 43.59 | 0.436 | |
| 0.02 | 286.9 | 65.15 | 62.90 | 0.629 | 157.6 | 115.7 | 65.18 | 0.652 | |
| 0.04 | 186.7 | 123.4 | 80.41 | 0.804 | 91.84 | 227.4 | 82.28 | 0.823 | |
| 60 | 0.00 | 1218 | 15.47 | 0.00 | 0.000 | 854.2 | 24.28 | 0.00 | 0.000 |
| 0.005 | 978.3 | 18.91 | 18.19 | 0.182 | 702.7 | 30.37 | 20.05 | 0.201 | |
| 0.01 | 752.4 | 23.82 | 35.05 | 0.351 | 540.5 | 39.84 | 39.06 | 0.391 | |
| 0.02 | 654.7 | 31.46 | 50.83 | 0.508 | 378.6 | 54.43 | 55.39 | 0.554 | |
| 0.04 | 315.5 | 58.47 | 73.54 | 0.735 | 210.7 | 107.1 | 77.33 | 0.773 | |
The value of Rp increases, while Cdl decreases as the concentration of Trp increased from 0.005 M to 0.04 M. This is attributed to the increase in the surface coverage by the inhibitor molecules leading to an increase in the thickness of the electrical double layer which is responsible for the decrease in Cdl values. This suggests that Trp acts by adsorption at the metal/solution interface by the gradual replacement of water molecules and the resulting adsorption film isolate the metal surface from the corrosive medium and decrease metal dissolution (Mansfield, 1987; McCafferty and Hackerman, 1972).
The calculated values of the inhibition efficiency (from Eq. (3)) for different concentrations of Trp at different temperatures are also presented in Table 3. It is found that the values of IE% increase with the increase of the inhibitor concentration. Additionally, the values of IE% decrease with the increase of solution temperature. This is due to the instability of the adsorbed film. But even for the higher temperatures (60 °C) the inhibition efficiency did not fall below 74% and 77% for sulfamic and hydrochloric acids respectively.
Moreover, this compound reduces the corrosion of LAS more effectively in hydrochloric acid than in sulfamic acid solutions.
3.4 Apparent activation energy
In order to deduce the possible inhibition mechanism of Trp in 0.6 M hydrochloric and sulfamic acid solutions, a further study of the effect of temperature on the inhibition efficiency was carried out using the obtained data from EFM and EIS techniques. The temperature dependence of LAS dissolution in both acids with the presence of Trp was employed using the following Arrhenius equation:
A plot of logarithm of the corrosion rate of LAS versus 1/T gives straight lines; and its slope is −Ea/2.303R (curves are not attached).
The type of inhibitor adsorption, with respect to the activation energy (Ea) can be classified into the following two cases (Popova et al., 2003; Mora-Mendoza and Turgoose, 2001; Jovancicevic et al., 1999; Hirozawa, 1995):
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Case (1): an increase in inhibition efficiency with the rise in temperature and increase in corrosion activation energy was detected in the absence of the inhibitor, rather than in its presence, which suggests a chemisorption mechanism.
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Case (2): contrast to the case (1), where a decrease in inhibition efficiency with the rise in temperature, and increase in corrosion activation energy in the presence of the inhibitor was deduced compared to its absence. This is frequently interpreted as being suggestive of formation of an adsorption film of physical nature (electrostatic).
The calculated values of Ea extracted from EFM and EIS techniques, are given in Table 4. The analysis of Ea values shows that an increase in corrosion activation energy in the presence of Trp compared to its absence with a decrease in inhibition efficiency with the rise in temperature indicates that the type of adsorption of Trp on the steel surface in sulfamic acid solutions belongs to physical adsorption.
| Trp Conc. (M) | Ea (kJ mol−1) | |||
|---|---|---|---|---|
| 0.6 M HSO3NH2 | 0.6 M HCl | |||
| EFM | EIS | EFM | EIS | |
| 0.00 | 29.50 | 25.79 | 35.07 | 29.34 |
| 0.005 | 36.13 | 33.87 | 44.88 | 43.58 |
| 0.01 | 38.57 | 37.08 | 49.70 | 53.47 |
| 0.02 | 47.76 | 42.81 | 59.23 | 58.21 |
| 0.04 | 56.26 | 52.76 | 66.43 | 62.87 |
3.5 Adsorption Isotherm
Adsorption of organic compounds can be described by two main types of interaction: physical adsorption and chemisorption processes, which are influenced by the nature and charge of the metal, the chemical structure of the inhibitor and the type of electrolyte.
The adsorption isotherm can give valuable information about the interactions of inhibitor with the metal. The degree of surface coverage (θ = IE%/100) obtained from impedance measurements for various concentrations of Trp at different temperatures has been tested graphically to fit a suitable isotherm. The plots of θ versus logarithm of inhibitor concentration (Cinh) yield a straight line, proving that the adsorption of Trp on the LAS surface obeys the Temkin adsorption isotherm. Fig. 6 is an example of Temkin adsorption isotherm plots for T22 low alloy steel in 0.6 M hydrochloric acid with various concentrations of Tryptophan at different temperatures using the obtained data from the EIS measurements.
The values of equilibrium constant (Kads) are listed in Table 5. These values were calculated from the intercepts and slopes of the straight lines of Temkin isotherm curves. Inspection of this table reveals that the values of Kads were relatively small confirming the suggestion that Trp is physically adsorbed on the metal surface (Keera and Deyab, 2005). Additionally, values of Kads decreased with an increase in the temperature. This result confirmed the suggestion that the strength of the adsorption decreased with temperature and the inhibitor species are easily removable by the solvent molecules from the steel surface (Keera and Deyab, 2005; Abd El Rehim et al., 2002; El Azhar et al., 2002).
| Acid | Equilibrium constant of adsorption, Kads | |||
|---|---|---|---|---|
| 25 °C | 40 °C | 50 °C | 60 °C | |
| HSO3NH2 | 853.10 | 635.72 | 490.01 | 385.04 |
| HCl | 948.18 | 730.93 | 491.62 | 415.05 |
3.6 Examination of surface morphology
The formation of a protective film of the species of inhibitor on the surface of low alloy steel (T22) was further confirmed by Optical Microscope examinations. Fig. 7a and d represent the micrographs of low alloy steel samples after 5 days of immersion in 0.6 M hydrochloric and sulfamic acid solutions in the absence and presence of 0.04 M Trp.
The surfaces of low alloy steel that has been exposed to both acids are demonstrated in Figs. 7a and b. Uniform corrosion can be observed and the surface is highly corroded with areas of localized corrosion. However, Fig. 7c and d clearly show that the corroded area is dramatically diminished in the presence of the inhibitor in both acids, which indicates that the rate of corrosion is highly suppressed by covering most of the alloy surface by inhibitor species. This observation indicates that the formation of adsorption layer by Trp species protects the specimen surface in the presence of inhibited hydrochloric and sulfamic acid solutions. It is clearly evident in these pictures (7c and 7d) that the adsorption layer of Trp is more dense and cohesive in hydrochloric acid more than in sulfamic acid. This corresponds with the obtained results of chemical and electrochemical methods.
3.7 Mechanism of Inhibition
All the obtained results support that Trp actually inhibits the corrosion of LAS in both sulfamic and hydrochloric acid solutions, to an appreciable extent.
The inhibitive effect of Trp may be attributed to the accumulation of the inhibitor molecules onto the metal surface, which reduces the direct contact of the metal with the corrosive environment.
It is known that, in aqueous acid solution, the amino acids exist either as neutral molecules or in the form of cations (protonated) (Amin et al., 2009; Bockris and Yang, 1991; Abdallah and Megahed, 1995).
It has been reported previously that, in aqueous acid solutions, the surface of steel is positively charged (Solmaz et al., 2008; Lagrenée et al., 2002; Wahdan et al., 2002).
Therefore, the amino acid may be adsorbed on the positively charged surface in the form of neutral molecules (Bentiss et al., 1999).
Furthermore, adsorption can occur via Cl− and SO3NH2− anions at the positively charged steel surface. Then, the adsorbed Cl− and SO3NH2− anions make a negatively charged layer and consequently increase in the adsorption capability of the protonated amino acids (Wahdan et al., 2002; Larabi et al., 2004).
A noteworthy point of the present investigation is that the studied inhibitor gave a better performance in 0.6 M HCl than in 0.6 M HSO3NH2. The better inhibition efficiencies (IE%) observed in the case of 0.6 M HCl may be attributed to the difference in the extent of adsorption of chloride and sulfamate ions.
Some studies suggest that the chloride ions have a stronger tendency to adsorb on the metal surface (Quraishi and Sardar, 2002; Muralidharn and Iyer, 1995).
The positive synergistic effect of halide ions has been investigated. According to Lorenz (1970), when an inhibited solution contains adsorbable anions, such as halide ions, these adsorb on the metal surface by creating oriented dipoles and consequently increase the adsorption of the organic cations on the dipoles. This model could be used to explain the higher inhibition efficiency of organic inhibitors in HCl solutions.
Figs. 8a and b illustrate the proposed mechanism for inhibition of Trp in both HSO3NH2 and HCl acid solutions.
4 Conclusions
In this study, the inhibition property of Tryptophan (Trp) was tested by using chemical and electrochemical measurements. Tryptophan has been found to perform well in both sulfamic and hydrochloric acid solutions, but a better performance is noticed in the case of HCl. The adsorption of the inhibitor molecules onto low alloy steel surface obeys the Temkin isotherm. The protection efficiency increased with increasing inhibitor concentration, but decreased slightly with the rise of temperature. The produced results from weight loss and EIS measurements were comparable with those obtained from the EFM method.
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