# Inhibiting Effects of Rabeprazole Sulfide on the Corrosion of Mild Steel in Acidic Chloride Solution

Authors: M. K. Pavithra; T. V. Venkatesha; M. K. Punith Kumar
Journal: International Journal of Electrochemistry
 (2013)

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## Abstract

The corrosion inhibition effect of Rabeprazole sulfide (RS) on mild steel in 1 M hydrochloric acid (HCl) was investigated using weight loss, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and chronoamperometric measurements. Protection efficiency of RS increases with the concentration and decreases with the rise in temperature. Adsorption of RS on mild steel surface in 1 M HCl follows Langmuir adsorption isotherm. The kinetic and thermodynamic parameters governing the adsorption process were calculated and discussed. The polarization results suggest that RS performed as an excellent mixed-type inhibitor for mild steel corrosion in 1 M HCl.


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## Body

## 1. Introduction

Mild steel is widely used in many industries due to its excellent mechanical properties and low cost. However, it undergoes corrosion to a greater extent in acidic environment. Acids are employed in several industrial processes such as acid pickling, cleaning, acid descaling, and oil well acidizing. The use of HCl in these processes is more economical, efficient, and trouble-free, compared to other mineral acids [1]. In order to avoid the base metal attack, chemical inhibitors are often used for these processes to control the metal dissolution. The most well-known acid corrosion inhibitors are the heterocyclic compounds containing nitrogen, sulphur, and oxygen atoms [2–5].Generally inhibitors act through the process of surface adsorption and its adsorption depends on the nature and surface charges on the metal, the type of aggressive media, the structure of inhibitor molecules, and its interaction with the metal surface [6]. Previously, large numbers of organic compounds have been investigated as corrosion inhibitors for mild steel in HCl media [7–11]. Even though these compounds show good anticorrosive action, they are toxic in nature. This led investigations to focus on the development of nontoxic corrosion inhibitors like drugs. A few investigations have been reported on the use of drugs such as cefazolin [1], Ampicillin [12], antifungal drugs [13], sulfa drugs [14], and rhodanine azosulpha drugs [15] as corrosion inhibitors. The inhibitory action of tramadol and tacrine on mild steel in 1 M HCl has been reported by Prabhu et al. [16] and Nataraja et al. [17].Rabeprazole sulfide is the commercial name of (2-[[4-(3-Methoxypropoxy)-3-methylpyridine-2-yl]-methylthio]-1H-benzimidazole). It is a metabolite of an antiulcer drug, Rabeprazole. We have investigated the corrosion inhibition effect of this compound on mild steel in 0.5 M H2SO4 media in a previous article [18]. However, according to literature study, there is no report found on corrosion inhibition studies of Rabeprazole sulfide in HCl medium. Hence the present work has been carried out to examine the potential of Rabeprazole sulfide to control corrosion of mild steel in 1 M HCl medium. The aim of this study is to explore the use of Rabeprazole sulfide as an acid corrosion inhibitor for mild steel surface in HCl solution using weight loss, potentiodynamic polarization, EIS, and the chronoamperometric techniques. Also the thermodynamic parameters for the adsorption process and activation parameters for the mild steel dissolution reactions are calculated and discussed.


## 2. Methods and Materials

The experiments were performed with mild steel specimens having the composition 0.04% C, 0.35% Mn, 0.022% P,  and 0.036% S and the remainder being Fe, which were used for weight loss as well as electrochemical studies. The mild steel coupons of dimension 4 cm × 2 cm × 0.1 cm were used for weight loss measurements and those of 1 cm2 area (exposed) with a 5 cm long stem isolated with araldite resin were used for electrochemical experiments. Prior to each experiment, the mild steel samples were abraded with series of emery papers of grade number 220, 660, and 1200 followed by washing in double distilled water and acetone and then dried.The aggressive solutions of 1 M HCl were prepared using AR grade chemicals and double distilled water. The test inhibitor RS was obtained from Ramdev Chemicals India Pvt. Ltd., Mumbai, and its structure is as shown in Figure1. The desired concentrations of inhibitor solutions (0.05, 0.1, 0.5, and 1.0 mM) were prepared by dissolving specified amount of RS in 1 M HCl solution.Figure 1
The chemical structure of RS.The weight loss of precleaned and dried mild steel specimens were determined by weighing the metal samples before and after immersing in 100 cm3 of 1 M HCl in the absence and presence of various concentration of RS at 303, 313, 323, and 333 K. The experiments were performed in triplicate and the mean value is reported. All experiments were carried out in aerated and static conditions.The electrochemical measurements were conducted in a conventional glass cell using CHI 660C electrochemical analyzer (USA make). A mild steel specimen (of 1 cm2 area), a platinum electrode, and a saturated calomel electrode were used as working, auxiliary, and reference electrodes. In case of polarization and electrochemical impedance spectroscopic measurements (EIS) prior to each measurement, a stabilization period of 30 min was allowed to establish a steady state open circuit potential (OCP). Each experiment was carried out in triplicate and the average values of corrosion parameters are reported.The potentiodynamic polarization measurements were carried out over a potential automatically from +200 mV to −200 mV at OCP with a scan rate of 0.5 mV s−1. All the potentials reported were with reference to SCE. The corrosion kinetic parameters such as corrosion potential (Ecorr), corrosion current density (Icorr), and anodic (βa)/cathodic (βc) Tafel slopes were generated from the software installed in the instrument. The EIS measurements were carried at OCP in the frequency range 1 mHz to 100 kHz with 5 mV sine wave as the excitation signal. Impedance data were analyzed using ZSimp-Win 3.21 software. The chronoamperometric experiments were performed by polarizing the working electrode anodically at −0.41 V (SCE) for 600 s.


## 3. Results and Discussion

### 3.1. Weight Loss Measurements

Weight loss method is used for monitoring corrosion rate because of its simple application and reliability. Weight loss of mild steel surface in 1 M HCl was determined at 301–333 K in the absence and presence of different concentration of RS. The obtained corrosion parameters are tabulated in Table1. The corrosion rate (υcorr) of mild steel was determined using the relation
(1)υcorr=ΔmSt,
where Δm is the corrosion weight loss of mild steel (g), S is the surface area of mild steel specimen (cm2), and t is the time of exposure.Table 1
Effect of temperature on the corrosion rate of mild steel in 1 M HCl at different concentrations of RS.
C (mM)
Temperature
303 K
313 K
323 K
333 K
υ
corr   (mg cm−2 h−1)
η
w (%)
υ
corr   (mg cm−2 h−1)
η
w (%)
υ
corr   (mg cm−2 h−1)
η
w (%)
υ
corr   (mg cm−2 h−1)
η
w (%)
Blank
0.85
2.48
8.08
16.95
0.05
0.33
60.62
1.44
42.12
6.11
24.37
14.73
13.08
0.1
0.27
68.35
1.21
51.32
4.87
39.67
13.32
21.41
0.5
0.14
83.47
0.43
82.48
1.53
81.03
3.37
80.12
1.0
0.07
91.42
0.23
90.76
0.82
89.83
1.86
89.02The percentage inhibition efficiencyηw(%) was calculated using the relationship;
(2)ηw(%)=υcorro-υcorrυcorro×100,
where υcorro and υcorr are the corrosion rates of mild steel in the absence and presence of RS, respectively.It can be seen from Table1 that the υcorr decreases and the inhibition efficiency increases with the concentration of the inhibitor. This is due to an increase in the amount of adsorption and coverage of inhibitor on mild steel surface [1]. It is also apparent from the table that the υcorr increases and inhibition efficiency decreases with increasing the temperature. This suggests the physisorption of the inhibitor on metal surface [19]. The increase in υcorr is more pronounced at lower concentration of RS (0.05 and 0.1 mM). However, at higher concentration (0.5 and 1.0 mM) the temperature has very little effect on the υcorr which can be attributed to slight change in the nature of mode of adsorption, where the chemisorption accompanied by physisorption may occur.


#### 3.1.1. Thermodynamic Parameters of the Adsorption Isotherm

Basic information on the interaction between the inhibitor and the mild steel surface can be provided by the adsorption isotherm. Hence in order to know the mode of adsorption of RS on mild steel surface in 1 M HCl at 303–333 K, attempts were made to fit experimental data with several adsorption isotherms like Langmuir, Temkin, Bockris-Swinkels, Freundlich, and Flory-Huggins isotherms. The best fit was obtained with Langmuir isotherm given by the equation [20]
(3)Cθ=1Kads+C,
where C is the molar concentration of inhibitor, Kads is the equilibrium constant of adsorption process, and θ is the degree of surface coverage defined as ηw(%)/100. The plots of C/θ against C for the inhibitor were straight lines and are given in Figure 2. It is found that all the regression coefficients are very close to one, which indicates that the adsorption of RS on the mild steel surface in 1 M HCl follows Langmuir adsorption isotherm.Figure 2
Langmuir adsorption isotherm for adsorption of RS on the mild steel surface in 1 M HCl.TheKads can be calculated from the intercepts of the straight lines on the C/θ axis and it is related to the standard free energy of adsorption (ΔGadso) by the relation [21]
(4)Kads=155.5exp(-ΔGadsoRT).The obtained thermodynamic parameters are given in Table2. The negative values of ΔGadso indicate the spontaneous adsorption of RS on mild steel surface [22]. Also the high values of Kads suggest the strong adsorption ability of RS on mild steel surface.Table 2
Thermodynamic parameters for the adsorption of RS in 1 M HCl on the mild steel at different temperatures.
Temperature (K)
Slope
R
2
K
ads (M−1)
Δ
G
ads   (kJ mol−1)
3
03 K
1.065
0.998
25000
-
35.63
313 K
1.025
0.999
12195
-
34.94
323 K
0.953
0.999
6536
-
34.38
333 K
0.764
0.981
3058
-
33.33It is generally accepted that the values ofΔGadso up to −20 kJ mol−1 are consistent with physisorption and the values around −40 kJ mol−1 or smaller are associated with chemisorption resulting from the sharing or transfer of electrons from organic molecule to the metal surface to form a coordinate bond [23]. However, the calculated ΔGadso values are between −20 kJ mol−1 and −40 kJ mol−1. This signifies that the adsorption of RS on mild steel in 1 M HCl involves comprehensive adsorption where physisorption accompanied by chemisorption will take place.Further the dependence ofΔGadso on temperature can be explained by two cases as follows [24]:(a)
ΔGadso may increase (become less negative) with the increase in temperature, which indicates the occurrence of exothermic process;(b)
ΔGadso may decrease (become more negative) with increasing the temperature, indicating the occurrence of endothermic process.It is clear from Table 2 that with the increase in the temperature, ΔGadso also increases and it specifies the corrosion inhibition of mild steel by RS is an exothermic process. Here the adsorption of RS on metal surface becomes unfavorable with increasing the reaction temperature due to the desorption of inhibitor from the mild steel surface [25]. The enthalpy of adsorption (ΔHadso) and entropy of adsorption can be evaluated by the integrated version of the Van’t Hoff equation expressed as follows [26]:
(5)lnKads=-ΔHadsoRT+ΔSadsoR+ln155.5.The variation oflnKads versus 1/T gives a straight line (Figure 3) with the slope of (-ΔHadso/R) and intercept of (ΔSadso/R+ln1/55.5). The calculated values of ΔHadso and -ΔSadso are −58.76 kJ mol−1 and 76.103 J mol−1 K−1, respectively.Figure 3
The relationship betweenlnKads and 1/T.On the other hand, the enthalpy and entropy for the adsorption of RS on mild steel were also calculated using the thermodynamic equation [27]
(6)ΔGadso=ΔHadso-TΔSadso.A plot ofΔGadso against T was linear (Figure 4) with the slope equal to -ΔSadso and intercept of ΔHadso. The obtained values of ΔHadso and -ΔSadso are −58.87 kJ mol−1 and 76 J mol−1K−1, respectively.Figure 4
The relationshipΔGadso of RS and temperature (T).The negative values ofΔHadso reflect the exothermic behavior of the adsorption of inhibitor on mild steel. Generally, an exothermic adsorption process signifies either physisorption or chemisorption while endothermic process is attributable to chemisorption [28]. In an exothermic process, both physisorption and chemisorption can be distinguished by considering the absolute value of ΔHadso. For physisorption process, ΔHadso is lower, that is 40 kJ mol−1, while that for chemisorption approaches 100 kJ mol−1 [29]. In the present case ΔHadso is 58.87 kJ mol−1, which is an intermediate case clearly specifies the existence of both physical and chemical adsorption. Thus ΔGadso and ΔHadso values complement each other. As expected the values of ΔSadso are negative, because the exothermic adsorption process is associated with the decrease of entropy. Before adsorption, the inhibitor molecules move freely in bulk solution, and as the adsorption progresses, the adsorption of inhibitor molecules onto the mild steel surface becomes more orderly, resulting in a decrease in entropy [29]. Moreover, the values of ΔHadso and -ΔSadso obtained by the two methods are in good agreement.


#### 3.1.2. Effect of Temperature

To assess the effect of temperature on corrosion and corrosion inhibition process, weight loss experiments were carried out for mild steel in 1 M HCl at 303, 313, 323, and 333 K in the absence and presence of different concentrations of RS. Usually corrosion reactions are regarded as Arrhenius processes and theυcorr can be expressed by the relation
(7)lnυcorr=lnA-Ea*RT,
where υcorr is the corrosion rate, Ea* is the apparent activation energy, R is the Universal gas constant (8.314 J K−1mol−1), T is the absolute temperature, and A is the frequency factor. The Arrhenius plot is shown in Figure 5 and the plot of lnυcorr against 1/T gives straight lines with slope -Ea*/R and the intercept of lnA. The obtained values of Ea* and A are presented in Table 3.Table 3
Activation parameters of dissolution reaction of mild steel in 1 M HCl with RS at different concentrations.
C (mM)
A   (g cm−2 h−1)
E
a
* (kJ mol−1)
Δ
H
* (kJ mol−1)
Δ
S
* (J mol−1 K−1)
Blank
4.27
×
10
11
85.22
82.55
-
31.34
0.05
1.338
×
10
15
107.75
105.09
35.92
0.1
2.512
×
10
15
109.83
107.17
40.90
0.5
2.615
×
10
15
112.74
109.99
41.15
1.0
2.695
×
10
15
114.48
111.82
41.49Figure 5
Arrhenius plot for mild steel in 1 M HCl solution with RS at different concentrations.The change in enthalpy(ΔH*) and entropy (ΔS*) of activation was calculated by the transition state equation given below:
(8)lnυcorrT=[lnRNh+ΔS*R]-ΔH*RT,
where h is the Planck’s constant and N is the Avogadro’s number. The plot of ln υcorr/T against 1/T for mild steel corrosion in 1 M HCl without and with RS is shown in Figure 6. Straight lines were obtained with slope of -ΔH*/R and intercept of [lnR/Nh+ΔS*/R) from which the values of ΔH* and ΔS* were calculated and tabulated in Table 3.Figure 6
Transition state plot for mild steel in 1 M HCl solution with RS at different concentrations.It is evident from the table that both theEa* and frequency factor values with increasing concentration of RS and the Ea* in the inhibited solution are higher than those in the uninhibited solution. The higher value of Ea* in presence of RS can be attributed to an increase in the thickness of double layer which increases the Ea* for corrosion process [17]. The Ea* for the corrosion process, both in the absence and presence of inhibitor, is greater than 20 kJ mol−1 and hence the entire process is controlled by surface reaction [30]. These results disclose that the corrosion reaction of mild steel is inhibited by RS.Based on the temperature effects, the relationships between the temperature dependence ofηw(%) of an inhibitor and the Ea* can be classified into three groups [31]:(1)
ηw(%) decreases with the increase in temperature, Ea* (inhibited solution) > Ea* (uninhibited solution);(2)
ηw(%) increases with the increase in temperature, Ea* (inhibited solution) < Ea* (uninhibited solution);(3)
ηw(%) does not change with temperature, Ea* (inhibited solution) = Ea* (uninhibited solution).In the present case,ηw(%) decreases with the increase in temperature and hence Ea* (inhibited solution) > Ea* (uninhibited solution). This suggests the physisorption. But in some cases, the chemical adsorption is accepted as the most probable type of adsorption, although inhibition efficiency decreases with rising the temperature and Ea* is higher than that in the absence of inhibitor [32]. The activation parameters (ΔH* and ΔS*) of mild steel dissolution reaction in 1 M HCl in the presence of RS are higher than those in the absence of inhibitor. The positive values of enthalpy reflect the endothermic nature of mild steel dissolution process. The average difference value of the Ea*-ΔH* is 2.66 kJ mol−1, which is approximately equal to the average value of RT (2.685 kJ mol−1) at 323 K. This infers that the corrosion process is an unimolecular reaction which is characterized by the following equation [25]:
(9)Ea*-ΔH*=RT.The positive values ofΔS* in the presence of inhibitor imply that the rate determining step for the activated complex is dissociation step rather than an association, meaning that the adsorption process is accompanied by an increase in entropy, which is the driving force for the adsorption of inhibitor molecules on the mild steel surface [33].


### 3.2. Electrochemical Measurements

#### 3.2.1. Polarisation Measurements

The influence of RS on the cathodic and anodic potentiodynamic polarization curves of mild steel in 1 M HCl at 303 K is shown in Figure7. The corrosion kinetic parameters such as corrosion potential (Ecorr), corrosion current density (Icorr), and anodic(βa)/cathodic(βc) Tafel slopes are presented in Table 4. The percentage inhibition efficiency ηT(%) was computed from (Icorr) values using the following expression:
(10)ηT(%)=Icorro-IcorrIcorro×100,
where Icorro  and Icorr are the corrosion current densities without and with RS, respectively.Table 4
Polarization parameters of mild steel in 1 M HCl containing different concentrations of RS.
C (mM)
-
E
corr (mV)
β
c  (mV dec−1)
β
a  (mV dec−1)
I
corr  (µA cm−2)
η
T (%)
Blank
494
140.50
77.51
165.6
0.05
490
116.30
56.48
65.48
60.51
0.1
486
149.16
76.38
59.92
63.81
0.5
475
131.06
67.40
32.58
80.33
1.0
462
126.37
63.44
16.54
90.01Figure 7
Polarisation curves for mild steel in 1 M HCl in the presence of various concentrations of RS.As it can be seen from the polarization result, theIcorro decreases and ηT(%) increases with increasing inhibitor concentration. This confirms the corrosion inhibition action of RS. Moreover the cathodic and anodic Tafel slope values changed with the inhibitor concentration, indicating that RS controlled both the cathodic hydrogen evolution and anodic mild steel dissolution reactions. It can be seen that the addition of RS to 1 M HCl shifted the Ecorr values to more positive direction and also the anodic and cathodic branches of polarization curves of pure acid solution towards lower current densities. An inhibitor can be classified as an anodic or cathodic type when the change in Ecorr value is larger than 85 mV [34]. But the largest displacement exhibited by RS was 32 mV versus SCE and hence it acts as a mixed-type inhibitor. So it can be concluded that RS behaves as a mixed-type inhibitor by inhibiting both anodic and cathodic reactions.


#### 3.2.2. Electrochemical Impedance Spectroscopic Measurements

EIS has been widely used in investigating corrosion inhibition process since it provides more information on both the resistive and capacitive behavior at metal/solution interface. The corrosion behavior of mild steel in 1 M HCl with and without RS at 303 K was investigated using this technique and the obtained impedance data represented as Nyquist and Bode plots in Figures8 and 9, respectively. Both these figures endorse that the impedance response of mild steel increases by the addition of RS.Figure 8
Nyquist plot (solid line shows fitted results) of mild steel in 1 M HCl solution without and with different concentrations of RS.Figure 9
Bode plots of mild steel in 1 M HCl solution without and with different concentrations of RS.The impedance spectra (Figure8) exhibit single semicircle which can be attributed to the charge transfer that takes place at electrode/solution interface and this process controls the corrosion of mild steel. The presence of RS does not change the mechanism of mild steel dissolution [35]. Due to frequency dispersion, the Nyquist plots are not perfect semicircle, which is attributed to surface inhomogeneity and roughness [36].The EIS results are simulated using the electrochemical equivalent circuit shown by the inset of Figure8. The equivalent circuit composed of the solution resistance (Rs), the charge transfer resistance (Rct), and the constant phase element (CPE) which describes the interfacial double layer. The impedance of CPE can be represented as follows:
(11)ZCPE=Q-1(jω)-n,
where Q is the CPE constant, ω is the angular frequency, j2=-1 is the imaginary number, and n represents phase shift which gives details about the degree of surface inhomogeneity resulting from surface roughness, inhibitor adsorption, porous layer formation, and so forth [37].Further the double layer capacitance(Cdl) can be calculated by the relation
(12)(Cdl)=(QRct1-n)1/n.The values of impedance parameters derived from Nyquist plots for corrosion of mild steel in 1 M HCl in the absence and presence of RS are tabulated in Table5. The inhibition efficiency ηZ(%) was evaluated from charge transfer resistance (Rct) values using the following equation:
(13)ηZ(%)=Rct-RctoRct×100,
where Rcto and Rct are the charge transfer resistances without and with RS, respectively.Table 5
Electrochemical impedance parameters of mild steel in 1 M HCl containing different concentrations of RS.
C (mM)
R
ct (Ω cm2)
Q (μΩ-1 Sn cm−2)
n
C
dl   (μF cm−2)
η
z (%)
Blank
85
115.20
0.863
55.27
0.05
209.3
92.14
0.870
50.97
59.33
0.1
254.8
89.09
0.864
49.12
66.67
0.5
498.2
73.24
0.856
41.98
82.93
1.0
829.6
31.62
0.877
18.96
89.76It is clear from Table5 that by increasing the concentration of inhibitor, Q and Cdl decrease and Rct increases. The decrease in Cdl and Q values can be attributed to a decrease in local dielectric constant and/or an increase in the thickness of electrical double layer. Meanwhile the increase in Rct indicates the increase in the extent of adsorption of inhibitor molecules and also the adsorbed RS molecules form a protective film on the mild steel surface which becomes a barrier to hinder the mass and charge transfer processes [6]. In Figure 9, only one time constant was observed and the phase angle increases with increasing concentration of RS. This signifies the decrease in surface inhomogeneity [38]. As a result the protection efficiency increases by increasing the concentration of RS. The obtained values of inhibition efficiency are in good agreement with those obtained from weight loss and potentiodynamic polarization measurements.


#### 3.2.3. Chronoamperometric Measurements

The ability of RS to inhibit anodic processes of mild steel was investigated by the chronoamperometric measurements. The experiments were carried out by polarizing anodically the electrode potential at −0.41 V (versus SCE) for 600 s. The current density values obtained during the electrooxidation of mild steel were recorded in 1 M HCl in the absence and presence of different concentrations of RS and the chronoamperometric curves are depicted in Figure10. By the addition of RS, the current density get reduced and the reduction in the current is more pronounced in 1 mM solution compared to uninhibited solution. This confirms that the rate of mild steel dissolution decreases in the presence of RS in 1 M HCl.Figure 10
Chronoamperometric curves of mild steel in 1 M HCl without and with different concentrations of RS.


### 3.3. Mechanism of Inhibition

Based on the experimental results obtained, we could propose a probable mechanism for corrosion inhibition behavior of RS in 1 M HCl. The polarization data suggested the mixed inhibition mechanism of RS.In acidic media, RS might be protonated as follows;(14)RS+nH+⟶[RSHn]n+.The cationic forms ofRS may be adsorbed directly at the cathodic sites and hinder the hydrogen evolution reaction. In acid solutions, mild steel possesses positive charge at the corrosion potential. The chloride ions present in the solution get adsorbed on metal surface by creating an excess negative charge towards solution and it favors the adsorption of protonated inhibitor molecules on metal surface through electrostatic attraction [39, 40]. Therefore the protonated RS molecules get adsorbed on mild steel surface by means of electrostatic interaction between chloride ions and inhibitor cations. Simultaneously, RS may also adsorb at anodic sites of metal surface via chemisorption mechanism by sharing electrons of S, N, and O atoms and also by donor acceptor interactions between π-electrons of methyl pyridine ring, benzimidazole ring, and vacant d orbital of iron [3, 41]. By following the above mechanism, RS shows mixed inhibition behavior by inhibiting cathodic hydrogen evolution and anodic mild steel dissolution reactions.Oguzie et al. reported that sulphur containing substances prefers chemisorption on metal surface in acidic media whereas nitrogen containing compounds tends to favor physisorption [42]. However, RS molecule is composed of both N and S atoms and hence it prefers physisorption accompanied by chemisorption which is also supported by thermodynamic calculations.Even thoughRS shows mixed inhibition behavior via comprehensive adsorption in both H2SO4 and HCl solutions, the performance of inhibitor is not of equal importance in 1 M HCl compared to 0.5 M H2SO4. In the previous work it has been reported that RS shows 98% inhibition efficiency at 1 mM concentration in 0.5 M H2SO4 but in the present work it shows nearly 90% efficiency in 1 M HCl. This may be due to the availability of more sites on the metal surface in sulphuric acid solution because of lesser adsorption of sulfate ions on the mild steel surface [43] but RS is an effective inhibitor in 1 M HCl also. The effectiveness of RS is due to the presence of electron donating S, two O, and three N atoms as well as the π-electrons of methyl pyridine and benzimidazole rings.


## 3.1. Weight Loss Measurements

Weight loss method is used for monitoring corrosion rate because of its simple application and reliability. Weight loss of mild steel surface in 1 M HCl was determined at 301–333 K in the absence and presence of different concentration of RS. The obtained corrosion parameters are tabulated in Table1. The corrosion rate (υcorr) of mild steel was determined using the relation
(1)υcorr=ΔmSt,
where Δm is the corrosion weight loss of mild steel (g), S is the surface area of mild steel specimen (cm2), and t is the time of exposure.Table 1
Effect of temperature on the corrosion rate of mild steel in 1 M HCl at different concentrations of RS.
C (mM)
Temperature
303 K
313 K
323 K
333 K
υ
corr   (mg cm−2 h−1)
η
w (%)
υ
corr   (mg cm−2 h−1)
η
w (%)
υ
corr   (mg cm−2 h−1)
η
w (%)
υ
corr   (mg cm−2 h−1)
η
w (%)
Blank
0.85
2.48
8.08
16.95
0.05
0.33
60.62
1.44
42.12
6.11
24.37
14.73
13.08
0.1
0.27
68.35
1.21
51.32
4.87
39.67
13.32
21.41
0.5
0.14
83.47
0.43
82.48
1.53
81.03
3.37
80.12
1.0
0.07
91.42
0.23
90.76
0.82
89.83
1.86
89.02The percentage inhibition efficiencyηw(%) was calculated using the relationship;
(2)ηw(%)=υcorro-υcorrυcorro×100,
where υcorro and υcorr are the corrosion rates of mild steel in the absence and presence of RS, respectively.It can be seen from Table1 that the υcorr decreases and the inhibition efficiency increases with the concentration of the inhibitor. This is due to an increase in the amount of adsorption and coverage of inhibitor on mild steel surface [1]. It is also apparent from the table that the υcorr increases and inhibition efficiency decreases with increasing the temperature. This suggests the physisorption of the inhibitor on metal surface [19]. The increase in υcorr is more pronounced at lower concentration of RS (0.05 and 0.1 mM). However, at higher concentration (0.5 and 1.0 mM) the temperature has very little effect on the υcorr which can be attributed to slight change in the nature of mode of adsorption, where the chemisorption accompanied by physisorption may occur.


### 3.1.1. Thermodynamic Parameters of the Adsorption Isotherm

Basic information on the interaction between the inhibitor and the mild steel surface can be provided by the adsorption isotherm. Hence in order to know the mode of adsorption of RS on mild steel surface in 1 M HCl at 303–333 K, attempts were made to fit experimental data with several adsorption isotherms like Langmuir, Temkin, Bockris-Swinkels, Freundlich, and Flory-Huggins isotherms. The best fit was obtained with Langmuir isotherm given by the equation [20]
(3)Cθ=1Kads+C,
where C is the molar concentration of inhibitor, Kads is the equilibrium constant of adsorption process, and θ is the degree of surface coverage defined as ηw(%)/100. The plots of C/θ against C for the inhibitor were straight lines and are given in Figure 2. It is found that all the regression coefficients are very close to one, which indicates that the adsorption of RS on the mild steel surface in 1 M HCl follows Langmuir adsorption isotherm.Figure 2
Langmuir adsorption isotherm for adsorption of RS on the mild steel surface in 1 M HCl.TheKads can be calculated from the intercepts of the straight lines on the C/θ axis and it is related to the standard free energy of adsorption (ΔGadso) by the relation [21]
(4)Kads=155.5exp(-ΔGadsoRT).The obtained thermodynamic parameters are given in Table2. The negative values of ΔGadso indicate the spontaneous adsorption of RS on mild steel surface [22]. Also the high values of Kads suggest the strong adsorption ability of RS on mild steel surface.Table 2
Thermodynamic parameters for the adsorption of RS in 1 M HCl on the mild steel at different temperatures.
Temperature (K)
Slope
R
2
K
ads (M−1)
Δ
G
ads   (kJ mol−1)
3
03 K
1.065
0.998
25000
-
35.63
313 K
1.025
0.999
12195
-
34.94
323 K
0.953
0.999
6536
-
34.38
333 K
0.764
0.981
3058
-
33.33It is generally accepted that the values ofΔGadso up to −20 kJ mol−1 are consistent with physisorption and the values around −40 kJ mol−1 or smaller are associated with chemisorption resulting from the sharing or transfer of electrons from organic molecule to the metal surface to form a coordinate bond [23]. However, the calculated ΔGadso values are between −20 kJ mol−1 and −40 kJ mol−1. This signifies that the adsorption of RS on mild steel in 1 M HCl involves comprehensive adsorption where physisorption accompanied by chemisorption will take place.Further the dependence ofΔGadso on temperature can be explained by two cases as follows [24]:(a)
ΔGadso may increase (become less negative) with the increase in temperature, which indicates the occurrence of exothermic process;(b)
ΔGadso may decrease (become more negative) with increasing the temperature, indicating the occurrence of endothermic process.It is clear from Table 2 that with the increase in the temperature, ΔGadso also increases and it specifies the corrosion inhibition of mild steel by RS is an exothermic process. Here the adsorption of RS on metal surface becomes unfavorable with increasing the reaction temperature due to the desorption of inhibitor from the mild steel surface [25]. The enthalpy of adsorption (ΔHadso) and entropy of adsorption can be evaluated by the integrated version of the Van’t Hoff equation expressed as follows [26]:
(5)lnKads=-ΔHadsoRT+ΔSadsoR+ln155.5.The variation oflnKads versus 1/T gives a straight line (Figure 3) with the slope of (-ΔHadso/R) and intercept of (ΔSadso/R+ln1/55.5). The calculated values of ΔHadso and -ΔSadso are −58.76 kJ mol−1 and 76.103 J mol−1 K−1, respectively.Figure 3
The relationship betweenlnKads and 1/T.On the other hand, the enthalpy and entropy for the adsorption of RS on mild steel were also calculated using the thermodynamic equation [27]
(6)ΔGadso=ΔHadso-TΔSadso.A plot ofΔGadso against T was linear (Figure 4) with the slope equal to -ΔSadso and intercept of ΔHadso. The obtained values of ΔHadso and -ΔSadso are −58.87 kJ mol−1 and 76 J mol−1K−1, respectively.Figure 4
The relationshipΔGadso of RS and temperature (T).The negative values ofΔHadso reflect the exothermic behavior of the adsorption of inhibitor on mild steel. Generally, an exothermic adsorption process signifies either physisorption or chemisorption while endothermic process is attributable to chemisorption [28]. In an exothermic process, both physisorption and chemisorption can be distinguished by considering the absolute value of ΔHadso. For physisorption process, ΔHadso is lower, that is 40 kJ mol−1, while that for chemisorption approaches 100 kJ mol−1 [29]. In the present case ΔHadso is 58.87 kJ mol−1, which is an intermediate case clearly specifies the existence of both physical and chemical adsorption. Thus ΔGadso and ΔHadso values complement each other. As expected the values of ΔSadso are negative, because the exothermic adsorption process is associated with the decrease of entropy. Before adsorption, the inhibitor molecules move freely in bulk solution, and as the adsorption progresses, the adsorption of inhibitor molecules onto the mild steel surface becomes more orderly, resulting in a decrease in entropy [29]. Moreover, the values of ΔHadso and -ΔSadso obtained by the two methods are in good agreement.


### 3.1.2. Effect of Temperature

To assess the effect of temperature on corrosion and corrosion inhibition process, weight loss experiments were carried out for mild steel in 1 M HCl at 303, 313, 323, and 333 K in the absence and presence of different concentrations of RS. Usually corrosion reactions are regarded as Arrhenius processes and theυcorr can be expressed by the relation
(7)lnυcorr=lnA-Ea*RT,
where υcorr is the corrosion rate, Ea* is the apparent activation energy, R is the Universal gas constant (8.314 J K−1mol−1), T is the absolute temperature, and A is the frequency factor. The Arrhenius plot is shown in Figure 5 and the plot of lnυcorr against 1/T gives straight lines with slope -Ea*/R and the intercept of lnA. The obtained values of Ea* and A are presented in Table 3.Table 3
Activation parameters of dissolution reaction of mild steel in 1 M HCl with RS at different concentrations.
C (mM)
A   (g cm−2 h−1)
E
a
* (kJ mol−1)
Δ
H
* (kJ mol−1)
Δ
S
* (J mol−1 K−1)
Blank
4.27
×
10
11
85.22
82.55
-
31.34
0.05
1.338
×
10
15
107.75
105.09
35.92
0.1
2.512
×
10
15
109.83
107.17
40.90
0.5
2.615
×
10
15
112.74
109.99
41.15
1.0
2.695
×
10
15
114.48
111.82
41.49Figure 5
Arrhenius plot for mild steel in 1 M HCl solution with RS at different concentrations.The change in enthalpy(ΔH*) and entropy (ΔS*) of activation was calculated by the transition state equation given below:
(8)lnυcorrT=[lnRNh+ΔS*R]-ΔH*RT,
where h is the Planck’s constant and N is the Avogadro’s number. The plot of ln υcorr/T against 1/T for mild steel corrosion in 1 M HCl without and with RS is shown in Figure 6. Straight lines were obtained with slope of -ΔH*/R and intercept of [lnR/Nh+ΔS*/R) from which the values of ΔH* and ΔS* were calculated and tabulated in Table 3.Figure 6
Transition state plot for mild steel in 1 M HCl solution with RS at different concentrations.It is evident from the table that both theEa* and frequency factor values with increasing concentration of RS and the Ea* in the inhibited solution are higher than those in the uninhibited solution. The higher value of Ea* in presence of RS can be attributed to an increase in the thickness of double layer which increases the Ea* for corrosion process [17]. The Ea* for the corrosion process, both in the absence and presence of inhibitor, is greater than 20 kJ mol−1 and hence the entire process is controlled by surface reaction [30]. These results disclose that the corrosion reaction of mild steel is inhibited by RS.Based on the temperature effects, the relationships between the temperature dependence ofηw(%) of an inhibitor and the Ea* can be classified into three groups [31]:(1)
ηw(%) decreases with the increase in temperature, Ea* (inhibited solution) > Ea* (uninhibited solution);(2)
ηw(%) increases with the increase in temperature, Ea* (inhibited solution) < Ea* (uninhibited solution);(3)
ηw(%) does not change with temperature, Ea* (inhibited solution) = Ea* (uninhibited solution).In the present case,ηw(%) decreases with the increase in temperature and hence Ea* (inhibited solution) > Ea* (uninhibited solution). This suggests the physisorption. But in some cases, the chemical adsorption is accepted as the most probable type of adsorption, although inhibition efficiency decreases with rising the temperature and Ea* is higher than that in the absence of inhibitor [32]. The activation parameters (ΔH* and ΔS*) of mild steel dissolution reaction in 1 M HCl in the presence of RS are higher than those in the absence of inhibitor. The positive values of enthalpy reflect the endothermic nature of mild steel dissolution process. The average difference value of the Ea*-ΔH* is 2.66 kJ mol−1, which is approximately equal to the average value of RT (2.685 kJ mol−1) at 323 K. This infers that the corrosion process is an unimolecular reaction which is characterized by the following equation [25]:
(9)Ea*-ΔH*=RT.The positive values ofΔS* in the presence of inhibitor imply that the rate determining step for the activated complex is dissociation step rather than an association, meaning that the adsorption process is accompanied by an increase in entropy, which is the driving force for the adsorption of inhibitor molecules on the mild steel surface [33].


## 3.1.1. Thermodynamic Parameters of the Adsorption Isotherm

Basic information on the interaction between the inhibitor and the mild steel surface can be provided by the adsorption isotherm. Hence in order to know the mode of adsorption of RS on mild steel surface in 1 M HCl at 303–333 K, attempts were made to fit experimental data with several adsorption isotherms like Langmuir, Temkin, Bockris-Swinkels, Freundlich, and Flory-Huggins isotherms. The best fit was obtained with Langmuir isotherm given by the equation [20]
(3)Cθ=1Kads+C,
where C is the molar concentration of inhibitor, Kads is the equilibrium constant of adsorption process, and θ is the degree of surface coverage defined as ηw(%)/100. The plots of C/θ against C for the inhibitor were straight lines and are given in Figure 2. It is found that all the regression coefficients are very close to one, which indicates that the adsorption of RS on the mild steel surface in 1 M HCl follows Langmuir adsorption isotherm.Figure 2
Langmuir adsorption isotherm for adsorption of RS on the mild steel surface in 1 M HCl.TheKads can be calculated from the intercepts of the straight lines on the C/θ axis and it is related to the standard free energy of adsorption (ΔGadso) by the relation [21]
(4)Kads=155.5exp(-ΔGadsoRT).The obtained thermodynamic parameters are given in Table2. The negative values of ΔGadso indicate the spontaneous adsorption of RS on mild steel surface [22]. Also the high values of Kads suggest the strong adsorption ability of RS on mild steel surface.Table 2
Thermodynamic parameters for the adsorption of RS in 1 M HCl on the mild steel at different temperatures.
Temperature (K)
Slope
R
2
K
ads (M−1)
Δ
G
ads   (kJ mol−1)
3
03 K
1.065
0.998
25000
-
35.63
313 K
1.025
0.999
12195
-
34.94
323 K
0.953
0.999
6536
-
34.38
333 K
0.764
0.981
3058
-
33.33It is generally accepted that the values ofΔGadso up to −20 kJ mol−1 are consistent with physisorption and the values around −40 kJ mol−1 or smaller are associated with chemisorption resulting from the sharing or transfer of electrons from organic molecule to the metal surface to form a coordinate bond [23]. However, the calculated ΔGadso values are between −20 kJ mol−1 and −40 kJ mol−1. This signifies that the adsorption of RS on mild steel in 1 M HCl involves comprehensive adsorption where physisorption accompanied by chemisorption will take place.Further the dependence ofΔGadso on temperature can be explained by two cases as follows [24]:(a)
ΔGadso may increase (become less negative) with the increase in temperature, which indicates the occurrence of exothermic process;(b)
ΔGadso may decrease (become more negative) with increasing the temperature, indicating the occurrence of endothermic process.It is clear from Table 2 that with the increase in the temperature, ΔGadso also increases and it specifies the corrosion inhibition of mild steel by RS is an exothermic process. Here the adsorption of RS on metal surface becomes unfavorable with increasing the reaction temperature due to the desorption of inhibitor from the mild steel surface [25]. The enthalpy of adsorption (ΔHadso) and entropy of adsorption can be evaluated by the integrated version of the Van’t Hoff equation expressed as follows [26]:
(5)lnKads=-ΔHadsoRT+ΔSadsoR+ln155.5.The variation oflnKads versus 1/T gives a straight line (Figure 3) with the slope of (-ΔHadso/R) and intercept of (ΔSadso/R+ln1/55.5). The calculated values of ΔHadso and -ΔSadso are −58.76 kJ mol−1 and 76.103 J mol−1 K−1, respectively.Figure 3
The relationship betweenlnKads and 1/T.On the other hand, the enthalpy and entropy for the adsorption of RS on mild steel were also calculated using the thermodynamic equation [27]
(6)ΔGadso=ΔHadso-TΔSadso.A plot ofΔGadso against T was linear (Figure 4) with the slope equal to -ΔSadso and intercept of ΔHadso. The obtained values of ΔHadso and -ΔSadso are −58.87 kJ mol−1 and 76 J mol−1K−1, respectively.Figure 4
The relationshipΔGadso of RS and temperature (T).The negative values ofΔHadso reflect the exothermic behavior of the adsorption of inhibitor on mild steel. Generally, an exothermic adsorption process signifies either physisorption or chemisorption while endothermic process is attributable to chemisorption [28]. In an exothermic process, both physisorption and chemisorption can be distinguished by considering the absolute value of ΔHadso. For physisorption process, ΔHadso is lower, that is 40 kJ mol−1, while that for chemisorption approaches 100 kJ mol−1 [29]. In the present case ΔHadso is 58.87 kJ mol−1, which is an intermediate case clearly specifies the existence of both physical and chemical adsorption. Thus ΔGadso and ΔHadso values complement each other. As expected the values of ΔSadso are negative, because the exothermic adsorption process is associated with the decrease of entropy. Before adsorption, the inhibitor molecules move freely in bulk solution, and as the adsorption progresses, the adsorption of inhibitor molecules onto the mild steel surface becomes more orderly, resulting in a decrease in entropy [29]. Moreover, the values of ΔHadso and -ΔSadso obtained by the two methods are in good agreement.


## 3.1.2. Effect of Temperature

To assess the effect of temperature on corrosion and corrosion inhibition process, weight loss experiments were carried out for mild steel in 1 M HCl at 303, 313, 323, and 333 K in the absence and presence of different concentrations of RS. Usually corrosion reactions are regarded as Arrhenius processes and theυcorr can be expressed by the relation
(7)lnυcorr=lnA-Ea*RT,
where υcorr is the corrosion rate, Ea* is the apparent activation energy, R is the Universal gas constant (8.314 J K−1mol−1), T is the absolute temperature, and A is the frequency factor. The Arrhenius plot is shown in Figure 5 and the plot of lnυcorr against 1/T gives straight lines with slope -Ea*/R and the intercept of lnA. The obtained values of Ea* and A are presented in Table 3.Table 3
Activation parameters of dissolution reaction of mild steel in 1 M HCl with RS at different concentrations.
C (mM)
A   (g cm−2 h−1)
E
a
* (kJ mol−1)
Δ
H
* (kJ mol−1)
Δ
S
* (J mol−1 K−1)
Blank
4.27
×
10
11
85.22
82.55
-
31.34
0.05
1.338
×
10
15
107.75
105.09
35.92
0.1
2.512
×
10
15
109.83
107.17
40.90
0.5
2.615
×
10
15
112.74
109.99
41.15
1.0
2.695
×
10
15
114.48
111.82
41.49Figure 5
Arrhenius plot for mild steel in 1 M HCl solution with RS at different concentrations.The change in enthalpy(ΔH*) and entropy (ΔS*) of activation was calculated by the transition state equation given below:
(8)lnυcorrT=[lnRNh+ΔS*R]-ΔH*RT,
where h is the Planck’s constant and N is the Avogadro’s number. The plot of ln υcorr/T against 1/T for mild steel corrosion in 1 M HCl without and with RS is shown in Figure 6. Straight lines were obtained with slope of -ΔH*/R and intercept of [lnR/Nh+ΔS*/R) from which the values of ΔH* and ΔS* were calculated and tabulated in Table 3.Figure 6
Transition state plot for mild steel in 1 M HCl solution with RS at different concentrations.It is evident from the table that both theEa* and frequency factor values with increasing concentration of RS and the Ea* in the inhibited solution are higher than those in the uninhibited solution. The higher value of Ea* in presence of RS can be attributed to an increase in the thickness of double layer which increases the Ea* for corrosion process [17]. The Ea* for the corrosion process, both in the absence and presence of inhibitor, is greater than 20 kJ mol−1 and hence the entire process is controlled by surface reaction [30]. These results disclose that the corrosion reaction of mild steel is inhibited by RS.Based on the temperature effects, the relationships between the temperature dependence ofηw(%) of an inhibitor and the Ea* can be classified into three groups [31]:(1)
ηw(%) decreases with the increase in temperature, Ea* (inhibited solution) > Ea* (uninhibited solution);(2)
ηw(%) increases with the increase in temperature, Ea* (inhibited solution) < Ea* (uninhibited solution);(3)
ηw(%) does not change with temperature, Ea* (inhibited solution) = Ea* (uninhibited solution).In the present case,ηw(%) decreases with the increase in temperature and hence Ea* (inhibited solution) > Ea* (uninhibited solution). This suggests the physisorption. But in some cases, the chemical adsorption is accepted as the most probable type of adsorption, although inhibition efficiency decreases with rising the temperature and Ea* is higher than that in the absence of inhibitor [32]. The activation parameters (ΔH* and ΔS*) of mild steel dissolution reaction in 1 M HCl in the presence of RS are higher than those in the absence of inhibitor. The positive values of enthalpy reflect the endothermic nature of mild steel dissolution process. The average difference value of the Ea*-ΔH* is 2.66 kJ mol−1, which is approximately equal to the average value of RT (2.685 kJ mol−1) at 323 K. This infers that the corrosion process is an unimolecular reaction which is characterized by the following equation [25]:
(9)Ea*-ΔH*=RT.The positive values ofΔS* in the presence of inhibitor imply that the rate determining step for the activated complex is dissociation step rather than an association, meaning that the adsorption process is accompanied by an increase in entropy, which is the driving force for the adsorption of inhibitor molecules on the mild steel surface [33].


## 3.2. Electrochemical Measurements

### 3.2.1. Polarisation Measurements

The influence of RS on the cathodic and anodic potentiodynamic polarization curves of mild steel in 1 M HCl at 303 K is shown in Figure7. The corrosion kinetic parameters such as corrosion potential (Ecorr), corrosion current density (Icorr), and anodic(βa)/cathodic(βc) Tafel slopes are presented in Table 4. The percentage inhibition efficiency ηT(%) was computed from (Icorr) values using the following expression:
(10)ηT(%)=Icorro-IcorrIcorro×100,
where Icorro  and Icorr are the corrosion current densities without and with RS, respectively.Table 4
Polarization parameters of mild steel in 1 M HCl containing different concentrations of RS.
C (mM)
-
E
corr (mV)
β
c  (mV dec−1)
β
a  (mV dec−1)
I
corr  (µA cm−2)
η
T (%)
Blank
494
140.50
77.51
165.6
0.05
490
116.30
56.48
65.48
60.51
0.1
486
149.16
76.38
59.92
63.81
0.5
475
131.06
67.40
32.58
80.33
1.0
462
126.37
63.44
16.54
90.01Figure 7
Polarisation curves for mild steel in 1 M HCl in the presence of various concentrations of RS.As it can be seen from the polarization result, theIcorro decreases and ηT(%) increases with increasing inhibitor concentration. This confirms the corrosion inhibition action of RS. Moreover the cathodic and anodic Tafel slope values changed with the inhibitor concentration, indicating that RS controlled both the cathodic hydrogen evolution and anodic mild steel dissolution reactions. It can be seen that the addition of RS to 1 M HCl shifted the Ecorr values to more positive direction and also the anodic and cathodic branches of polarization curves of pure acid solution towards lower current densities. An inhibitor can be classified as an anodic or cathodic type when the change in Ecorr value is larger than 85 mV [34]. But the largest displacement exhibited by RS was 32 mV versus SCE and hence it acts as a mixed-type inhibitor. So it can be concluded that RS behaves as a mixed-type inhibitor by inhibiting both anodic and cathodic reactions.


### 3.2.2. Electrochemical Impedance Spectroscopic Measurements

EIS has been widely used in investigating corrosion inhibition process since it provides more information on both the resistive and capacitive behavior at metal/solution interface. The corrosion behavior of mild steel in 1 M HCl with and without RS at 303 K was investigated using this technique and the obtained impedance data represented as Nyquist and Bode plots in Figures8 and 9, respectively. Both these figures endorse that the impedance response of mild steel increases by the addition of RS.Figure 8
Nyquist plot (solid line shows fitted results) of mild steel in 1 M HCl solution without and with different concentrations of RS.Figure 9
Bode plots of mild steel in 1 M HCl solution without and with different concentrations of RS.The impedance spectra (Figure8) exhibit single semicircle which can be attributed to the charge transfer that takes place at electrode/solution interface and this process controls the corrosion of mild steel. The presence of RS does not change the mechanism of mild steel dissolution [35]. Due to frequency dispersion, the Nyquist plots are not perfect semicircle, which is attributed to surface inhomogeneity and roughness [36].The EIS results are simulated using the electrochemical equivalent circuit shown by the inset of Figure8. The equivalent circuit composed of the solution resistance (Rs), the charge transfer resistance (Rct), and the constant phase element (CPE) which describes the interfacial double layer. The impedance of CPE can be represented as follows:
(11)ZCPE=Q-1(jω)-n,
where Q is the CPE constant, ω is the angular frequency, j2=-1 is the imaginary number, and n represents phase shift which gives details about the degree of surface inhomogeneity resulting from surface roughness, inhibitor adsorption, porous layer formation, and so forth [37].Further the double layer capacitance(Cdl) can be calculated by the relation
(12)(Cdl)=(QRct1-n)1/n.The values of impedance parameters derived from Nyquist plots for corrosion of mild steel in 1 M HCl in the absence and presence of RS are tabulated in Table5. The inhibition efficiency ηZ(%) was evaluated from charge transfer resistance (Rct) values using the following equation:
(13)ηZ(%)=Rct-RctoRct×100,
where Rcto and Rct are the charge transfer resistances without and with RS, respectively.Table 5
Electrochemical impedance parameters of mild steel in 1 M HCl containing different concentrations of RS.
C (mM)
R
ct (Ω cm2)
Q (μΩ-1 Sn cm−2)
n
C
dl   (μF cm−2)
η
z (%)
Blank
85
115.20
0.863
55.27
0.05
209.3
92.14
0.870
50.97
59.33
0.1
254.8
89.09
0.864
49.12
66.67
0.5
498.2
73.24
0.856
41.98
82.93
1.0
829.6
31.62
0.877
18.96
89.76It is clear from Table5 that by increasing the concentration of inhibitor, Q and Cdl decrease and Rct increases. The decrease in Cdl and Q values can be attributed to a decrease in local dielectric constant and/or an increase in the thickness of electrical double layer. Meanwhile the increase in Rct indicates the increase in the extent of adsorption of inhibitor molecules and also the adsorbed RS molecules form a protective film on the mild steel surface which becomes a barrier to hinder the mass and charge transfer processes [6]. In Figure 9, only one time constant was observed and the phase angle increases with increasing concentration of RS. This signifies the decrease in surface inhomogeneity [38]. As a result the protection efficiency increases by increasing the concentration of RS. The obtained values of inhibition efficiency are in good agreement with those obtained from weight loss and potentiodynamic polarization measurements.


### 3.2.3. Chronoamperometric Measurements

The ability of RS to inhibit anodic processes of mild steel was investigated by the chronoamperometric measurements. The experiments were carried out by polarizing anodically the electrode potential at −0.41 V (versus SCE) for 600 s. The current density values obtained during the electrooxidation of mild steel were recorded in 1 M HCl in the absence and presence of different concentrations of RS and the chronoamperometric curves are depicted in Figure10. By the addition of RS, the current density get reduced and the reduction in the current is more pronounced in 1 mM solution compared to uninhibited solution. This confirms that the rate of mild steel dissolution decreases in the presence of RS in 1 M HCl.Figure 10
Chronoamperometric curves of mild steel in 1 M HCl without and with different concentrations of RS.


## 3.2.1. Polarisation Measurements

The influence of RS on the cathodic and anodic potentiodynamic polarization curves of mild steel in 1 M HCl at 303 K is shown in Figure7. The corrosion kinetic parameters such as corrosion potential (Ecorr), corrosion current density (Icorr), and anodic(βa)/cathodic(βc) Tafel slopes are presented in Table 4. The percentage inhibition efficiency ηT(%) was computed from (Icorr) values using the following expression:
(10)ηT(%)=Icorro-IcorrIcorro×100,
where Icorro  and Icorr are the corrosion current densities without and with RS, respectively.Table 4
Polarization parameters of mild steel in 1 M HCl containing different concentrations of RS.
C (mM)
-
E
corr (mV)
β
c  (mV dec−1)
β
a  (mV dec−1)
I
corr  (µA cm−2)
η
T (%)
Blank
494
140.50
77.51
165.6
0.05
490
116.30
56.48
65.48
60.51
0.1
486
149.16
76.38
59.92
63.81
0.5
475
131.06
67.40
32.58
80.33
1.0
462
126.37
63.44
16.54
90.01Figure 7
Polarisation curves for mild steel in 1 M HCl in the presence of various concentrations of RS.As it can be seen from the polarization result, theIcorro decreases and ηT(%) increases with increasing inhibitor concentration. This confirms the corrosion inhibition action of RS. Moreover the cathodic and anodic Tafel slope values changed with the inhibitor concentration, indicating that RS controlled both the cathodic hydrogen evolution and anodic mild steel dissolution reactions. It can be seen that the addition of RS to 1 M HCl shifted the Ecorr values to more positive direction and also the anodic and cathodic branches of polarization curves of pure acid solution towards lower current densities. An inhibitor can be classified as an anodic or cathodic type when the change in Ecorr value is larger than 85 mV [34]. But the largest displacement exhibited by RS was 32 mV versus SCE and hence it acts as a mixed-type inhibitor. So it can be concluded that RS behaves as a mixed-type inhibitor by inhibiting both anodic and cathodic reactions.


## 3.2.2. Electrochemical Impedance Spectroscopic Measurements

EIS has been widely used in investigating corrosion inhibition process since it provides more information on both the resistive and capacitive behavior at metal/solution interface. The corrosion behavior of mild steel in 1 M HCl with and without RS at 303 K was investigated using this technique and the obtained impedance data represented as Nyquist and Bode plots in Figures8 and 9, respectively. Both these figures endorse that the impedance response of mild steel increases by the addition of RS.Figure 8
Nyquist plot (solid line shows fitted results) of mild steel in 1 M HCl solution without and with different concentrations of RS.Figure 9
Bode plots of mild steel in 1 M HCl solution without and with different concentrations of RS.The impedance spectra (Figure8) exhibit single semicircle which can be attributed to the charge transfer that takes place at electrode/solution interface and this process controls the corrosion of mild steel. The presence of RS does not change the mechanism of mild steel dissolution [35]. Due to frequency dispersion, the Nyquist plots are not perfect semicircle, which is attributed to surface inhomogeneity and roughness [36].The EIS results are simulated using the electrochemical equivalent circuit shown by the inset of Figure8. The equivalent circuit composed of the solution resistance (Rs), the charge transfer resistance (Rct), and the constant phase element (CPE) which describes the interfacial double layer. The impedance of CPE can be represented as follows:
(11)ZCPE=Q-1(jω)-n,
where Q is the CPE constant, ω is the angular frequency, j2=-1 is the imaginary number, and n represents phase shift which gives details about the degree of surface inhomogeneity resulting from surface roughness, inhibitor adsorption, porous layer formation, and so forth [37].Further the double layer capacitance(Cdl) can be calculated by the relation
(12)(Cdl)=(QRct1-n)1/n.The values of impedance parameters derived from Nyquist plots for corrosion of mild steel in 1 M HCl in the absence and presence of RS are tabulated in Table5. The inhibition efficiency ηZ(%) was evaluated from charge transfer resistance (Rct) values using the following equation:
(13)ηZ(%)=Rct-RctoRct×100,
where Rcto and Rct are the charge transfer resistances without and with RS, respectively.Table 5
Electrochemical impedance parameters of mild steel in 1 M HCl containing different concentrations of RS.
C (mM)
R
ct (Ω cm2)
Q (μΩ-1 Sn cm−2)
n
C
dl   (μF cm−2)
η
z (%)
Blank
85
115.20
0.863
55.27
0.05
209.3
92.14
0.870
50.97
59.33
0.1
254.8
89.09
0.864
49.12
66.67
0.5
498.2
73.24
0.856
41.98
82.93
1.0
829.6
31.62
0.877
18.96
89.76It is clear from Table5 that by increasing the concentration of inhibitor, Q and Cdl decrease and Rct increases. The decrease in Cdl and Q values can be attributed to a decrease in local dielectric constant and/or an increase in the thickness of electrical double layer. Meanwhile the increase in Rct indicates the increase in the extent of adsorption of inhibitor molecules and also the adsorbed RS molecules form a protective film on the mild steel surface which becomes a barrier to hinder the mass and charge transfer processes [6]. In Figure 9, only one time constant was observed and the phase angle increases with increasing concentration of RS. This signifies the decrease in surface inhomogeneity [38]. As a result the protection efficiency increases by increasing the concentration of RS. The obtained values of inhibition efficiency are in good agreement with those obtained from weight loss and potentiodynamic polarization measurements.


## 3.2.3. Chronoamperometric Measurements

The ability of RS to inhibit anodic processes of mild steel was investigated by the chronoamperometric measurements. The experiments were carried out by polarizing anodically the electrode potential at −0.41 V (versus SCE) for 600 s. The current density values obtained during the electrooxidation of mild steel were recorded in 1 M HCl in the absence and presence of different concentrations of RS and the chronoamperometric curves are depicted in Figure10. By the addition of RS, the current density get reduced and the reduction in the current is more pronounced in 1 mM solution compared to uninhibited solution. This confirms that the rate of mild steel dissolution decreases in the presence of RS in 1 M HCl.Figure 10
Chronoamperometric curves of mild steel in 1 M HCl without and with different concentrations of RS.


## 3.3. Mechanism of Inhibition

Based on the experimental results obtained, we could propose a probable mechanism for corrosion inhibition behavior of RS in 1 M HCl. The polarization data suggested the mixed inhibition mechanism of RS.In acidic media, RS might be protonated as follows;(14)RS+nH+⟶[RSHn]n+.The cationic forms ofRS may be adsorbed directly at the cathodic sites and hinder the hydrogen evolution reaction. In acid solutions, mild steel possesses positive charge at the corrosion potential. The chloride ions present in the solution get adsorbed on metal surface by creating an excess negative charge towards solution and it favors the adsorption of protonated inhibitor molecules on metal surface through electrostatic attraction [39, 40]. Therefore the protonated RS molecules get adsorbed on mild steel surface by means of electrostatic interaction between chloride ions and inhibitor cations. Simultaneously, RS may also adsorb at anodic sites of metal surface via chemisorption mechanism by sharing electrons of S, N, and O atoms and also by donor acceptor interactions between π-electrons of methyl pyridine ring, benzimidazole ring, and vacant d orbital of iron [3, 41]. By following the above mechanism, RS shows mixed inhibition behavior by inhibiting cathodic hydrogen evolution and anodic mild steel dissolution reactions.Oguzie et al. reported that sulphur containing substances prefers chemisorption on metal surface in acidic media whereas nitrogen containing compounds tends to favor physisorption [42]. However, RS molecule is composed of both N and S atoms and hence it prefers physisorption accompanied by chemisorption which is also supported by thermodynamic calculations.Even thoughRS shows mixed inhibition behavior via comprehensive adsorption in both H2SO4 and HCl solutions, the performance of inhibitor is not of equal importance in 1 M HCl compared to 0.5 M H2SO4. In the previous work it has been reported that RS shows 98% inhibition efficiency at 1 mM concentration in 0.5 M H2SO4 but in the present work it shows nearly 90% efficiency in 1 M HCl. This may be due to the availability of more sites on the metal surface in sulphuric acid solution because of lesser adsorption of sulfate ions on the mild steel surface [43] but RS is an effective inhibitor in 1 M HCl also. The effectiveness of RS is due to the presence of electron donating S, two O, and three N atoms as well as the π-electrons of methyl pyridine and benzimidazole rings.


## 4. Conclusion

The results reveal that RS is an efficient acid corrosion inhibitor for mild steel in 1 M HCl. It acts as a mixed-type inhibitor by inhibiting both anodic and cathodic reactions. The corrosion rate decreases with inhibitor concentration and increases with the temperature. The adsorption of RS follows the Langmuir adsorption isotherm model and the adsorption is spontaneous and exothermic process. The kinetic and thermodynamic parameters of corrosion and adsorption processes are determined. The results obtained from weight loss measurements are comparable with those obtained from electrochemical measurements.



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