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Original article
10 (
1_suppl
); S1364-S1372
doi:
10.1016/j.arabjc.2013.04.006

Synergistic effect of sodium dodecyl sulfate and cetyltrimethyl ammonium bromide on the corrosion inhibition behavior of l-methionine on mild steel in acidic medium

, ,
 Corrosion Research Laboratory, Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh 202 002, India

⁎Corresponding author. Tel.: +91 9411491161; fax: +91 571 2701895. drmmobin@hotmail.com (M. Mobin)

Received: , Accepted: ,
© The Author(s) 2013
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
1 Present address: Department of Chemistry, K.S.U., Riyadh, Saudi Arabia.

Abstract

The corrosion inhibition behavior of amino acid l-methionine (LMT) separately and in combination with very low concentration of surfactants sodium dodecyl sulfate (SDS) and cetyltrimethyl ammonium bromide (CTAB) on mild steel in 0.1 M H2SO4 solution was studied, using weight loss and potentiodynamic polarization measurement techniques. The studies were carried out in the temperature range of 30–60 °C. The surface morphology of the corroded steel samples was studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM).The results show that LMT is an effective inhibitor for mild steel corrosion in 0.1 M H2SO4 which is synergistically improved in the presence of SDS and CTAB. The mixed LMT and CTAB is more effective as an inhibitor than mixture of LMT and SDS. The SEM and AFM photographs show a clearly different surface morphology in the presence of additives. LMT alone and in combination with surfactants obeys Langmuir adsorption isotherm from the fit of the experimental data of all concentrations and temperatures studied. Phenomenon of physical adsorption is proposed from the trend of the IE with temperature and also the values of activation energy (Ea), standard enthalpy of adsorption (ΔHads), and standard free energy of adsorption (ΔGads) obtained. The results obtained by potentiodynamic polarization measurements are consistent with the results of the weight loss measurements. LMT acts as a mixed type inhibitor.

Keywords

l-Methionine
Corrosion inhibition
Mild steel
Synergistic effect
Surfactant
Adsorption isotherm
1

1 Introduction

The corrosion of mild steel is a subject of fundamental, academic and industrial concern and has received a considerable attention during the last few decades (Uhlig and Revic, 1985). The use of inhibitors is one of the most effective practical and economic methods to protect metallic surfaces against corrosion in aggressive acidic media (Behpour et al., 2009; Demadis et al., 2007; El-Meligi et al., 2000). Mineral acids are widely used in various industries for pickling of steel at elevated temperatures up to 60 °C. This technique besides being used to remove corrosion scales from the steel surface without causing acid attack of the bulk metal is also effectively applied in cleaning of industrial equipment and acidization of oil well (Clubley, 1990). Most of the efficient pickling inhibitors are organic compounds containing hetero atoms such as sulfur, nitrogen, oxygen, phosphorus, and multiple bonds or aromatic rings in their structures (Ostovari et al., 2009; Kertit and Hammouti, 1996).The number of lone pair of electrons and loosely bound π- electrons in these functional groups are the key structural features that determine the inhibitive action of these compounds (Shukla et al., 2009; Shukla and Quraishi, 2009). These compounds prevent corrosion by blocking the active corrosion sites either by getting adsorbed, or by forming a protective layer or an insoluble complex on the metal surface. However, most of the organic compounds used as corrosion inhibitors are toxic and hazardous to both human beings and the environment and need to be replaced by nontoxic, environment friendly compounds. As a result, the current research trends are toward the development of nontoxic, economical and more environmentally safe green chemicals as corrosion inhibitors (Abiola and James, 2010; Singh and Quraishi, 2010a,b; de Souza and Spinelli, 2009; Morad, 2008; Umoren et al., 2009).

It has been shown by a number of investigators that some amino acids can act as corrosion inhibitors, which has generated an increasing interest in these compounds (Fu et al., 2010; Saifi et al., 2010; Toufari et al., 2008; Oguzie et al., 2007a,b; Silva et al., 2006; Ashassi-Sorkhabi et al., 2005, 2004; Zerfaoui et al., 2004; Zhang et al., 2008, 2005; Diab et al., 2005). Amino acids are attractive as corrosion inhibitors because they are relatively easy to produce with high purity at low cost and are nontoxic, biodegradable and completely soluble in aqueous media. The sulfur containing amino acid l-methionine has been found to be an efficient corrosion inhibitor (Khaled, 2009; Ozcan et al., 2008; Abd-El-Nabey et al., 1985; Abiola, 2005, 2004; Morad, 2005). Nineteen different naturally occurring amino acids including Methionine were studied as corrosion inhibitors for mild steel in H2SO4 by measuring Tafel polarization curves (Rahim et al., 1997). The best corrosion inhibition was obtained with the S-containing amino acids. The inhibition effects of two amino acids Methionine and Tyrosine on the corrosion of iron in 0.1 M HCl solution were studied (Zor et al., 2009). The inhibition efficiency of the compounds was found to increase with increase in their concentration. Methionine was found to be more effective than Tyrosine. The adsorption of the inhibitors accords with the Langmuir adsorption isotherm.

Synergism is one of the most important effects in the inhibition process and serves as the basis for most of the modern corrosion inhibiting formulations. The addition of halide ions to organic compounds has shown synergistic effect and resulted in improved inhibition efficiency of many organic compounds (Ebenso, 2003).The IE of methionine was also found to be synergistically increased in the presence of KI (Oguzie et al., 2007b). However, there remain relatively few works directed toward the synergistic effect between the different organic compounds and surfactants (Rafiquee et al., 2008; Mobin et al., 2011). In a very recent paper the inhibition behavior of methionine combined with cetrimonium bromide (CTAB) and cetylpyridinium bromide (CPB) for Cu corrosion in 0.5 M HCl solution has been reported. It has been shown that combination of methionine with CTAB or CPB provides strong synergistic inhibition effect (Zhang et al., 2011).

The corrosion inhibition by surfactant molecules is related to the surfactant's ability to aggregate at interfaces and in solution. The effectiveness of surfactant inhibitor can be studied on the basis of their micellar properties in a particular medium. The adsorbed molecules form monolayer or bilayer hemimicelles or admicelles, depending upon the surfactant concentration and prevent the acid to attack the surface, and thus reduce the corrosion attack (Migahed and Al-Sabagh, 2009; Free, 2002; Saleh and Atha, 2006). The surfactant can be used either alone or in mixture with other compounds to improve their performance as inhibitors. Amino acids are likely to interact with the surfactants to form complex structure and help to adhere to the surface and offer greater resistance to corrosion.

The present work was undertaken to investigate the corrosion inhibition behavior of LMT separately and in combination with very low concentration of the surfactants SDS and CTAB on mild steel in 0.1 M H2SO4 solution. The techniques used are weight loss measurements, potentiodynamic polarization measurements, SEM, and AFM.

2

2 Experimental

2.1

2.1 Materials

The composition of mild steel used for corrosion inhibition studies was (wt%): 0.20 C, 0.53 Mn, 0.036 Si, 0.11 S, 0.098 P, and balance being Fe. The specimens of size 2.5 × 2.0 × 0.03 cm were press cut from the mild steel sheet, were machined and abraded with a series of emery papers. This was followed by rinsing in acetone and double distilled water and finally dried in air. Before any experiment, the substrates were treated as described and freshly used with no further storage. The inhibitor LMT [(S)-2-amino-4-(methylmercapto) butyric acid, molecular mass 149.21 g mole−1], SDS, and CTAB (CDH, India) were used as received. A stock solution of 1000 ppm of LMT was prepared in 0.1 M H2SO4 (AR grade) and the desired concentration was obtained by appropriate dilution. The concentration of LMT used for the study ranges from 10 to 500 ppm. All solutions were made using double distilled water. The study was carried out at 30–60 °C maintaining the temperatures using a thermo stated water bath. The molecular structure of the LMT is given in Fig. 1.

Molecular structure of l-methionine (LMT).
Figure 1
Molecular structure of l-methionine (LMT).

2.2

2.2 Weight loss measurements

The freshly prepared mild steel specimens were suspended in 250 ml beakers containing 200 ml of test solution maintained at 30–60 °C in a thermo stated water bath with the aid of glass rods and hooks. The weight loss taken was the difference between the weight at a given time and the original weight of the specimens. The measurements were carried out for the uninhibited solution and the solution containing LMT and LMT–surfactant mixtures. Weight loss experiments were performed for the duration of 6 h, as per ASTM designation G1–90. The specimens were immersed in triplicate and the average corrosion rate was calculated. These uncertainties or RSD for three replicate measurements were less than 5%. The corrosion rates were determined using the equation:

(1)
Corrosion rate (mpy) = 534 W ρ At where W, is weight loss in mg, ρ, is the density of specimen in g/cm3, A, is the area of specimen in sq. and t, is exposure time in h.

The IE of LMT, surfactants and LMT–surfactant mixture was calculated by using the following equation:

(2)
% IE = CR 0 - CR i CR 0 × 100 where CR0 = Corrosion rate of mild steel in the absence of inhibitor; CRi = Corrosion rate of mild steel in the presence of inhibitor.

2.3

2.3 Potentiodynamic polarization measurements

The potentiodynamic polarization measurements were carried out using three electrode assembly, potentiostat/Galvanostat, model: PGSTAT30 controlled by a PC through the general purpose electrochemical system (GPES) software provided by AUTOLAB. The experiments were carried out using Ag/AgCl electrode (saturated KCl) as the reference electrode, Platinum wire as the counter electrode and mild steel specimens as the working electrode. The experiments were performed using a scan rate of 0.5 mV/s commencing at a potential above 250 mV more active than the stable open circuit potential. All the measurements were carried out at room temperature (30 ± 1 °C). Before starting the measurements, the specimen was left in the solution for 30 min to attain a steady state which was indicated by a constant potential. The IE was calculated from the measured Icorr values using the relationship:

(3)
IE ( % ) = 1 - i corr i corr o × 100 where icorr = inhibited current density and icorr = uninhibited current density.

2.4

2.4 SEM and AFM studies

The surface morphology of the corroded steel sample surface in the presence and absence of the inhibitors was studied using SEM (model: 430 LEO electron microscopy Ltd. Cambridge, England) and AFM (model: Innova SPM, Veeco). To study the surface morphology of mild steel, polished specimens prior to initiation of any corrosion reaction, were examined in optical microscope to find out any surface defect, such as pit or noticeable irregularities like cracks etc. Only those specimens, which had a smooth pit-free surface, were subjected to immersion. The specimens were immersed for 6 h at 30 °C. After completion of the tests specimens were thoroughly washed with double distilled water and dried and then subjected to SEM and AFM examination.

3

3 Results and discussion

3.1

3.1 Weight loss measurements

The corrosion behavior of mild steel in 0.1 M H2SO4 in the absence and presence of different concentrations of LMT alone and in combination with surfactants SDS and CTAB, was studied in the temperature range of 30–60 °C using a weight loss technique and data obtained after 6 h of immersion are shown in Table 1. The corrosion rate of mild steel is reduced in the presence of LMT as compared to free acid solution and depends upon inhibitor concentration and temperature. The IE increases with increasing LMT concentrations showing a maximum IE of 75.79% at 200 ppm. The increased IE with increasing inhibitor concentrations indicates that more LMT molecules are adsorbed on the steel surface at higher concentrations, leading to greater surface coverage and hence the formation of a protective film (Rao and Singhal, 2009). A relatively low IE at lower concentrations of LMT could be attributed to the modest surface coverage owing to its small molecular area and solubility of adsorbed intermediate formed on the surface. A further increase in LMT concentrations causes a slight lowering in IE. This phenomenon is attributed to the dissolution of adsorbed inhibitor film (Zhang et al., 2005). Also a decrease in IE is observed with increase in temperature at all the concentrations studied. This suggests physical adsorption as the weak Vander Waal's forces responsible for such type of interaction tends to disappear at elevated temperatures.

Table 1 Calculated values of corrosion rate (mpy) and inhibition efficiency (%IE) for mild steel in 0.1 M H2SO4 in the absence and presence of various concentrations of LMT with surfactants SDS/CTAB at 30–60 °C from weight loss measurement.
Inhibitor conc. (ppm) Surfactant conc. (ppm) Corrosion rate (mpy) Inhibition efficiency (%IE)
30 °C 40 °C 50 °C 60 °C 30 °C 40 °C 50 °C 60 °C
Blank – 786.50 1486.55 2561.71 3960.41 – – – –
10 – 367.45 797.66 1603.69 3156.47 53.28 46.34 37.39 20.29
25 – 351.42 716.78 1419.61 2790.41 55.32 51.78 44.58 29.54
50 – 310.98 689.58 1319.21 2426.45 60.46 53.61 58.50 38.73
100 – 234.28 642.87 1250.87 2208.90 70.21 56.75 51.17 44.22
200 – 190.35 541.07 1051.46 2171.25 75.79 63.61 62.11 45.49
300 – 193.84 464.37 962.21 2027.62 75.35 68.76 62.44 48.80
500 – 197.32 477.62 970.58 2038.08 74.02 67.87 62.11 48.54
– 1 SDS 555.71 1250.87 2379.03 3748.44 29.34 15.85 7.13 5.35
– 5 SDS 541.07 1207.64 2345.57 3687.08 31.21 18.76 8.44 6.90
10 5 ” 260.77 543.86 1434.95 2984.25 66.84 63.41 43.98 21.41
25 5 ” 220.33 524.34 1358.25 2571.48 71.98 64.73 46.98 35.07
50 5 ” 174.31 504.81 1305.26 2348.35 77.84 66.04 49.05 40.70
100 5 ” 147.12 451.82 1221.59 2151.73 81.29 69.61 52.31 45.67
200 5 ” 136.66 414.17 875.75 2070.85 82.62 72.14 65.81 47.71
300 5 ” 118.53 380.70 872.96 1981.59 84.62 74.39 65.92 49.96
500 5 ” 137.36 384.88 881.33 1983.69 82.09 74.11 65.59 49.81
– 1 CTAB 302.61 868.78 2250.74 3618.75 61.52 41.56 12.14 8.63
– 5 CTAB 185.68 592.78 1575.14 3270.81 76.51 62.63 41.94 21.31
10 1 CTAB 112.11 547.94 1348.12 3321.26 73.58 68.67 44.09 26.48
25 1 ” 103.70 540.93 1342.52 3231.57 76.33 69.42 49.75 41.18
50 1 ” 101.59 400.79 947.33 2776.12 78.19 71.81 53.13 44.38
100 1 ” 100.89 368.56 884.27 2481.83 82.62 72.79 56.12 45.69
200 1 ” 95.49 366.46 880.25 2324.88 84.22 75.61 66.74 48.49
300 1 ” 92.49 351.74 870.25 2216.27 84.66 76.36 67.53 52.31
500 1 ” 114.91 372.89 992.17 2220.48 83.69 75.66 65.16 53.59

The effectiveness of the adsorption of LMT in acidic solution can be attributed to the presence of protonated amine group and S-atom in the molecule. The presence of S-CH3 though decreases the stability of the positive charge, the inhibitor could interact with the corroding steel surface via the protonated amino functional group which can be adsorbed at the cathodic sites and hinder the hydrogen evolution reaction, or via the S atom in aliphatic chain, which may be adsorbed at anodic sites and retard the iron dissolution. This suggests a mixed inhibition mechanism where both the anodic and cathodic partial reactions are influenced by the LMT (Ashassi-Sorkhabi et al., 2005; Oguzie et al., 2007a,b). Considering the adsorption of LMT molecules on the steel surface, it may be deduced that the LMT molecules self aggregate at the steel surface arranging into an array of well-ordered stripes of uniform width and separation, depending upon its concentration. The molecules have a long range interaction among them and thus able to cover the surface effectively and results in lowering of the corrosion rate. Since the self-assembly of LMT molecules does not lead to chemical bonding among them, the optimization of the adsorption geometry is only governed by the non-bonding forces e.g. Vander Waal's repulsive forces at close range, Lennard–Jones long-range attractive interactions, electrostatic coulomb forces and hydrogen bonding forces. The LMT molecules lie flat on the steel surface. The energetically most favorable situations are attained when the amino and carboxyl groups as well as the sulfur in the side chain are close to the steel surface. One of the inertial axes of the molecule is nearly parallel with the surface. After the adsorption of the first molecule, the next molecule of LMT is preferably arranged in dimer rows with the carboxyl and amino groups facing each other. The bonding of the molecules is accomplished through hydrogen bonds between the amino group and the carboxyl group. These bonds enable the formation of stable dimer rows. Thus it is inferred that the whole of the steel surface is covered by dimer LMT where the two molecules are facing each other. This configuration for LMT molecules in anti-parallel arrangement is responsible for the high values of IE.

To observe the effect of SDS and CTAB on the corrosion inhibition behavior of LMT, the corrosion of mild steel in 0.1 M H2SO4 in the absence and presence of different concentrations of LMT, in combination with 5 ppm of SDS and 1 ppm of CTAB was separately studied in the temperature range of 30–60 °C. The results are shown in Table 1. The SDS and CTAB also exhibit corrosion inhibition of mild steel in 0.1 M H2SO4. The corrosion rates of mild steel in the presence of LMT in combination with surfactants are further reduced in comparison to LMT alone. It is observed that the mixture of LMT and SDS or LMT and CTAB, increases the IE more than either LMT or surfactants alone indicating a synergistic effect between LMT and surfactants. The mixed LMT and CTAB is more effective as an inhibitor for steel corrosion than that the mixture of LMT and SDS.

Considering the adsorption of surfactant molecules at the surfaces, they also have tendency to get adsorbed either as individual molecules or as aggregates of various types, depending upon their nature and concentration. Adsorption of ionic surfactants on a like-charged surface is less understood, but can occur via hydrogen bonding or attractive dispersion forces as is the case for nonionic surfactants. At low surfactant concentrations, the adsorption behavior can usually be described by the simple electrical double-layer model. Here ionic surfactant monomers adsorb as individual ions without mutual interaction. At higher concentrations tail–tail interactions may begin to cause association of the adsorbed surfactants into aggregates, with the headgroups facing the surface. Bilayer at the surface is formed, with surfactant monomer headgroups in the first layer facing the surface while those of the second layer face the bulk solution. The surfactant molecule adsorption to the steel surface decreases the availability of electrons for undergoing corrosion reaction and thereby decreases the corrosion rate. The micelles like aggregates are formed spontaneously at concentrations well below the bulk cmc and that a complete bilayer is formed at the maximum adsorption of ionic surfactants adsorbing onto the surfaces of opposite charge.

Considering the effect of surfactants on the corrosion inhibition behavior of LMT, in the presence of surfactant molecules, the headgroups of CTAB and protonated SDS molecules are adsorbed to the steel surface through electrostatic coulomb forces and hydrogen bonding. The surfactant molecules help and direct LMT molecules to adsorb to the steel surface through hydrophobic hydrocarbon chain-side chain (sulfur containing) of LMT. The possible interactions between SDS and LMT are –NH3+–O3S−, –COO–H–O3S− and tail of SDS-side chain of LMT. The possible interactions between CTAB and LMT are –COO-headgroup of CTAB, –NH2-headgroup and tail of CTAB-side chain of LMT. The effect of surfactants on the IE of LMT in 0.1 M H2SO4 appears to be synergistic in nature.

3.2

3.2 Synergism considerations

The interaction of LMT and surfactants SDS/CTAB can be described by a synergism parameter S1 (Aramaki and Hackerman, 1969) which is defined as:

(4)
S 1 = 1 - I 1 + 2 1 - I 1 + 2 ′ where, I1+2 = (I1 + I2); I1 is IE of LMT; I2 is IE of surfactants, SDS/CTAB and I′1+2 being IE of LMT in combination with surfactants. S1 approaches 1 when no interaction between the inhibitor molecules exists while S1 > 1 indicates a synergistic effect. In the case of S1 < 1, antagonistic behavior prevails which may be attributed to competitive adsorption. The values of synergism parameter for the various concentrations of LMT studied were calculated from the gravimetric data at 30–60 °C and the results are given in Table 2. The values are all greater than unity. This is an indication that the enhanced IE resulting from the addition of surfactants to LMT is synergistic in nature and proved that the addition of a very small concentration of surfactants can significantly improve the adsorption of LMT on the mild steel surface.
Table 2 Calculated values of synergism parameter (S1) for mild steel in 0.1 M H2SO4 in the absence and presence of LMT and surfactants SDS/CTAB at 30–60 °C from weight loss measurement.
Inhibitor conc (ppm) Surfactant conc (ppm) Synergism parameter (S1)
30 °C 40 °C 50 °C 60 °C
10 5 SDS 1.27 1.03 1.04 1.14
25 5″ 1.20 1.09 1.13 1.04
50 5″ 1.18 1.09 1.16 1.12
100 5″ 1.25 1.09 1.14 1.12
200 5″ 1.29 1.14 1.02 1.10
300 5″ 1.26 1.18 1.08 1.12
500 5″ 1.29 1.17 1.08 1.11
10 1 CTAB 1.57 1.21 1.13 1.09
25 1″ 1.54 1.36 1.14 0.92
50 1″ 1.57 1.33 1.14 1.07
100 1″ 1.60 1.35 1.11 1.06
200 1″ 1.64 1.39 1.07 1.08
300 1″ 1.62 1.45 1.10 1.09
500 1″ 1.62 1.45 1.10 1.05

3.3

3.3 Adsorption isotherms

Adsorption isotherms are very important in determining the mechanism of organo-electrochemical reaction. The inhibition of mild steel corrosion in the presence of various organic compounds has been attributed to their adsorption on the steel surface and is generally confirmed from the fit of the experimental data to various adsorption isotherms. The degree of surface coverage (θ) for various concentrations of LMT and LMT in combination with surfactants has been used to explain the best isotherm to determine the adsorption process. The data were tested graphically by fitting to various isotherms and the best result was obtained for Langmuir adsorption isotherm. The plots of C/θ against C are drawn which are the characteristics of Langmuir adsorption isotherm and given by equation:

(5)
C θ = 1 K + C where θ is the degree of surface coverage, C is the inhibitor concentration; K is the equilibrium constant of adsorption.

The plots of C/θ against C at 30–60 °C gave a straight line for mild steel in 0.1 M H2SO4 in the presence of LMT alone and in combination with surfactants. A linear correlation of slope close to unity suggest that adsorption of LMT alone and in combination with surfactants on mild steel interface obeys Langmuir adsorption isotherm at all the temperatures studied. The typical Langmuir adsorption isotherm for LMT in combination with 1 ppm of CTAB, adsorbed on the mild steel surface in 0.1 M H2SO4 at different temperatures is shown in Fig. 2.

Langmuir adsorption isotherm for LMT + 1 ppm CTAB adsorbed on the mild steel surface in 0.1 M H2SO4 at different temperatures.
Figure 2
Langmuir adsorption isotherm for LMT + 1 ppm CTAB adsorbed on the mild steel surface in 0.1 M H2SO4 at different temperatures.

3.4

3.4 Effect of temperature

The corrosion of mild steel was studied in the temperature range of 30–60 °C in the absence and presence of LMT, surfactants and LMT in combination with the surfactants. The dependence of logarithm of corrosion rate (log CR) on the reciprocal of absolute temperature (1/T) for 0.1 M H2SO4 for blank and LMT alone and in combination with SDS and CTAB is presented in Fig. 3. Linear plots were obtained which indicates that it follows Arrhenius equation (Bentiss et al., 2001):

(6)
log CR = l og A - E a 2.303 RT where, ‘CR’ is the corrosion rate, A is the Arrhenius constant, Ea is the apparent activation energy R is the molar gas constant and T is absolute temperature.
Adsorption isotherm plot for log CR versus 1/T in the absence and presence of LMT, SDS, CTAB and LMT in combination with SDS/CTAB.
Figure 3
Adsorption isotherm plot for log CR versus 1/T in the absence and presence of LMT, SDS, CTAB and LMT in combination with SDS/CTAB.

The activation energy (Ea) values obtained from the slope of the linear plots are shown in Table 3. The values are higher in the presence of the additives compared to the blank. This is suggestive of physical adsorption (Umoren et al., 2009). The energy barrier of corrosion process increases with the addition of surfactants indicating that the physisorption creates an adsorption film to retard the charge and mass transfer process. Standard enthalpy of adsorption, ΔH and standard entropy of adsorption, ΔS for the corrosion of mild steel in 0.1 M H2SO4 in the presence of LMT alone and in combination with surfactants were obtained by alternative formulation of Arrhenius equation also called transition state plot which is given by the equation:

(7)
CR = RT Nh exp Δ S R exp - Δ H RT where h is the Planck's constant, N is the Avogadro's number, R is the molar gas constant and T is the absolute temperature. Fig. 4 shows the plot of log (CR/T) versus 1/T for blank, LMT alone and in combination with surfactants. From the slope - Δ H 2.303 R and intercept log R Nh + Δ S 2.303 R of the linear plots ΔH and ΔS, respectively were obtained. The calculated values are shown in Table 3. The values of ΔH increase in the presence of additives compared to the free acid solution, this further indicates physical adsorption. In all cases, values of ΔS are positive which indicates a decrease in the system order in the presence of additives (Refaey et al., 2004). The values of standard free energy of adsorption (ΔGads) listed in Table 3, were calculated using the following equation (Cases and Villieras, 1992):
(8)
Δ G ads = - RT ln ( 55.5 K ) where K is equilibrium constant and is given by:
(9)
K = θ C ( 1 - θ ) where, θ is the degree of surface coverage, C the concentration of inhibitors in mol dm−3, R is gas constant and T is the solution temperature. The calculated values of ΔGads from 30 °C to 60 °C for the various systems studied are presented in Table 3. The negative values of ΔGads indicate the stability of the adsorbed inhibitor on the mild steel surface and the spontaneity of the process. An increase in ΔGads (becomes less negative) with increase in temperature suggests the occurrence of exothermic. The values of ΔGads obtained in this study are between −15.07 and −28.70 kJ/mol. This is consistent with electrostatic interaction between the charged organic molecules and the charged metal surface which is indicative of physical adsorption.
Table 3 Calculated values of kinetic/thermodynamic parameters for mild steel in 0.1 M H2SO4 in the absence and presence of LMT and LMT–surfactant SDS/CTAB mixtures from weight loss measurement.
Additives Conc. (ppm) Ea (kJ mol−1) ΔH (kJ mol−1) ΔS (kJ mol−1K−1) ΔGads(kJ mol−1)
30 °C 40 °C 50 °C 60 °C
Blank – 45.21 47.52 5.132 – – – –
LMT 200 68.06 68.85 64.17 −17.07 −16.08 −16.43 −15.07
SDS 5 53.66 55.99 30.26 −21.47 −20.43 −18.61 −18.58
CTAB 1 69.37 72.45 80.05 −28.70 −27.54 −24.02 −23.69
LMT + SDS 200 + 5 76.00 76.68 87.13 −18.08 −17.10 −16.88 −15.31
LMT + CTAB 200 + 1 78.29 80.59 99.93 −18.36 −17.57 −16.96 −15.41
Adsorption isotherm plot for log CR/T versus 1/T in the absence and presence of LMT, SDS, CTAB and LMT in combination with SDS/CTAB.
Figure 4
Adsorption isotherm plot for log CR/T versus 1/T in the absence and presence of LMT, SDS, CTAB and LMT in combination with SDS/CTAB.

3.5

3.5 Potentiodynamic polarization measurements

Potentiodynamic polarization curves for the corrosion of mild steel in 0.1 M H2SO4 in the absence and presence of LMT, SDS, CTAB and LMT in combination with SDS and CTAB are shown in Figs. 5 and 6. The values of electrochemical parameters as deduced from these curves e.g., corrosion potential (Ecorr), corrosion current density (icorr), and% IE are shown in Table 4. The IE was calculated using the equation:

(10)
% IE = i corr o - i corr i corr o × 100 where icorr and icorr are the corrosion current density in the absence and presence of inhibitors, respectively. The value of icorr decreases in the presence LMT which is further decreased in the presence of LMT–surfactant mixture. The values of Ecorr in the presence of LMT and LMT–surfactant mixture shift to more positive values compared to the blank. The positive shift in Ecorr is more pronounced in the presence of LMT–surfactant mixture, suggesting the dominant role of anodic suppression in the process (Hu et al., 2010). The displacement in Ecorr is less than 85 mv/SCE suggesting that compounds act as mixed type inhibitors. The results as obtained by electrochemical studies are consistent with the results of the weight loss measurements.
Potentiodynamic curves for mild steel in 0.1 M H2SO4 in the absence and presence of various concentrations of LMT (a) Blank, (b) LMT 100 ppm, (d) LMT 200 ppm and (e) LMT 500 ppm.
Figure 5
Potentiodynamic curves for mild steel in 0.1 M H2SO4 in the absence and presence of various concentrations of LMT (a) Blank, (b) LMT 100 ppm, (d) LMT 200 ppm and (e) LMT 500 ppm.
Potentiodynamic curves for mild steel in 0.1 M H2SO4 in the absence and presence of various concentrations of additives (a) Blank, (b) 1 ppm CTAB, (c) 5 ppm SDS, (d) LMT 200 ppm, (e) LMT 200 ppm + CTAB 1 ppm, (f) LMT 200 ppm + SDS 5 ppm.
Figure 6
Potentiodynamic curves for mild steel in 0.1 M H2SO4 in the absence and presence of various concentrations of additives (a) Blank, (b) 1 ppm CTAB, (c) 5 ppm SDS, (d) LMT 200 ppm, (e) LMT 200 ppm + CTAB 1 ppm, (f) LMT 200 ppm + SDS 5 ppm.
Table 4 Potentiodynamic polarization parameters for corrosion of mild steel in 0.1 M H2SO4 in the absence and presence of various concentrations of LMT and LMT with surfactant SDS/CTAB mixtures at 30 °C.
Additives Conc. (ppm) Ecorr (mv) Icorr (A/cm2) (%IE)
Blank Blank −507 3.398 × 10−4 –
SDS 5 −490 2.455 × 10−4 27.75
CTAB 1 −489 9.770 × 10−5 71.25
LMT 100 −478 1.106 × 10−4 67.45
LMT 200 −499 7.891 × 10−5 76.77
LMT 500 −474 1.080 × 10−4 68.13
LMT + SDS 200 + 5 −467 6.766 × 10−5 80.09
LMT + CTAB 200 + 1 −498 4.484 × 10−5 86.81

3.6

3.6 Surface morphological studies

Surface photographs of the mild steel specimens were obtained by means of SEM and AFM so as to determine if the corrosion inhibition is due to the formation of a protective film by adsorption of inhibitors. Considering the results of the SEM studies on steel prior to its immersion in the solutions, except the presence of polishing scratches the surface shows the absence of noticeable defects such as pits and cracks. In the presence of uninhibited 0.1 M H2SO4 solutions a damaged and heterogeneous surface is observed. The surface heterogeneity is considerably decreased in the presence of inhibitor LMT which is further reduced in the presence of LMT- surfactant additive. The typical SEM photomicrograph of the surface of mild steel after immersion in LMT inhibited 0.1 M H2SO4 solution is shown in Fig. 7.

SEM photomicrograph of the surface of mild steel after immersion in LMT inhibited 0.1 M H2SO4 solution for 6 h at 30 °C.
Figure 7
SEM photomicrograph of the surface of mild steel after immersion in LMT inhibited 0.1 M H2SO4 solution for 6 h at 30 °C.

The SEM results are further proved by AFM results of steel specimens taken in uninhibited and inhibited acid solutions at room temperature in the range of 0–2 μm and 0–3 μm, respectively. In uninhibited acid solution the AFM photograph clearly shows a rough surface (maximum surface roughness ∼0.29 μm) due to rapid corrosion of steel specimen. In the presence of LMT the steel surface is less corroded and a different surface morphology having comparatively smoother surface (maximum surface roughness ∼0.125 μm) is observed. A smoother layer with a clearly different morphology is as a result of the formation of a protective layer by the adsorbed inhibitor. The inhibitor layer is not very compact and as such does not provide absolute coverage, with some metal sites still exposed to acid attack. In the presence of surfactant SDS the adsorbed LMT layer more homogeneously covered the steel surface and further reduced the surface roughness (maximum surface roughness ∼0.05 μm). The typical AFM photograph of the surface of mild steel after immersion in LMT + SDS inhibited 0.1 M H2SO4 solution is shown in Fig. 8.

AFM photograph of the surface of mild steel after immersion in LMT + 5 ppm SDS inhibited 0.1 M H2SO4 solution for 6 h at 30 °C.
Figure 8
AFM photograph of the surface of mild steel after immersion in LMT + 5 ppm SDS inhibited 0.1 M H2SO4 solution for 6 h at 30 °C.

4

4 Conclusions

  1. l-Methionine showed good performance as corrosion inhibitor for mild steel in 0.1 M H2SO4 which is further improved in the presence of surfactants SDS and CTAB. The effect of surfactants on corrosion inhibition behavior of l-methionine appears to be synergistic in nature.

  2. The data obtained from weight loss measurements suggest corrosion inhibition by adsorption mechanism and fit well the Langmuir adsorption isotherm at all the concentrations and temperatures studied.

  3. The negative free energy of adsorption ΔGads indicates stability of the adsorbed inhibitor on the mild steel surface and confirms the spontaneity of the process and its physical nature.

  4. l-Methionine acts a mixed inhibitor.

  5. SEM and AFM studies further confirm the inhibitive character of the additives.

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