Corrosion inhibition effect of spiropyrimidinethiones on mild steel in 15% HCl solution: insight from electrochemical and quantum studies

Abstract

The effect of synthesized spiropyrimidinethiones, namely, 6′-(4-methoxyphenyl)-1′-phenyl-2′-thioxo-2′,3′-dihydro-1′H-spiro[indoline-3,4′-pyrimidine]-2-one (MPTS) and 6′-(4-methoxyphenyl)-1′-phenyl-2′-thioxo-2′,3′-dihydro-1′H-spiro[indoline-3,4′-pyrimidine]-2-one (CPTS) on the corrosion of mild steel in 15% HCl solution was investigated by using weight loss and electrochemical methods. Both inhibitors act as mixed inhibitors and their adsorption on mild steel obeyed Langmuir's adsorption isotherm. The potential of zero charge (E_PZC) for the mild steel was determined by electrochemical impedance spectroscopy methods. Scanning electron microscopy, energy dispersion X-ray spectroscopy, FTIR, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy were used to characterize the surface morphology of the uninhibited and inhibited mild steel specimens. Density Functional Theory (DFT) was employed for theoretical calculations.

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1. Introduction

Mild steel is extensively used as a construction material in various industries due to its excellent mechanical properties and low cost, but its poor corrosion resistance to acids, restrains its utility. Hydrochloric acid solutions are commonly used for pickling, industrial acid cleaning, acid de-scaling, and in oil well acidifying processes. Acidization of petroleum oil wells for enhancing oil production is commonly brought about by forcing 15% HCl solution through steel tubing into the well to open up near-bore channels in the formation and, hence, to increase the flow of oil. Because of the aggressiveness of the acid solutions, mild steel corrodes severely during these processes, particularly with the use of hydrochloric acid, which results in terrible waste of both resources and money. A corrosion inhibitor is often added to acid solutions for minimizing the effect of corrosion on metal during these processes. The selection of appropriate inhibitors mainly depends on its economic availability, environmental side effects, concentration, temperature, type of acid, the presence of dissolved inorganic and/or organic substances even in minor amounts and, of course, on the type of metallic material supposed to be protected.¹

Heterocyclic organic compounds containing sulfur, phosphorus, oxygen, nitrogen and aromatic rings are reported in literature as most effective corrosion inhibitor for the metals in acidic medium.^2–9 Some spiro compounds¹⁰ have been reported as good corrosion inhibitor for Al in NaCl solution, and exhibit different inhibition performance with the difference in substituent groups and substituent positions on the aromatic rings. The inhibitor, MPTS and CPTS contains indoline and pyrimidine rings as two anchoring sites suitable for bonding with the metal surface. So, it was expected that MPTS and CPTS would work as good corrosion inhibitor for mild steel in 15% hydrochloric acid solution.

Although, the protection mechanism of organic corrosion inhibitors has not been clearly understood,¹¹ it is generally accepted that the organic inhibitors decrease the corrosion rate by adsorbing on the metal/solution interface and blocking the active sites by displacing water molecules and forming a compact barrier film on the metal surface.^12–14 The adsorption process depends upon the nature and surface charge of the metal, the type of aggressive media, the structure of the inhibitor and the nature of its interaction with the metal surface.¹⁵ Research activities in present situations are geared towards developing the cheap, non-toxic and environment friendly corrosion inhibitors. Theoretical calculations using Density Functional Theory (DFT) are being used to explain the mechanism of corrosion inhibition.^16–18

The objective of the present work was to evaluate the corrosion parameters of two selected inhibitors (MPTS and CPTS) in mild steel/inhibitor/15% HCl solution system by means of weight loss measurement, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) techniques. The choice of these compounds is based on molecular structure considerations, i.e., the number of active centers and type of substituent's present in these compounds. Both the compounds have the same number of active centers but they only differ in the substituent type (OCH₃ in MPTS and Cl in CPTS) at the para position of phenyl group and hence if a difference in the inhibition efficiency is observed, it should be predominately attributed to a difference in the electronic effect of the substituent type. Quantum chemical calculations were performed in order to determine the correlation between the corrosion inhibition properties of the investigated spiropyrimidinethiones and their molecular structures.

2. Material and methods

2.1. Synthesis of inhibitors

The studied inhibitors, namely, 6′-(4-methoxyphenyl)-1′-phenyl-2′-thioxo-2′,3′-dihydro-1′H-spiro[indoline-3,4′-pyrimidine]-2-one (MPTS) and 6′-(4-methoxyphenyl)-1′-phenyl-2′-thioxo-2′,3′-dihydro-1′H-spiro[indoline-3,4′-pyrimidine]-2-one (CPTS) were synthesized according to a previously reported experimental procedure¹⁹ as shown in Scheme 1. The purity of the synthesized compounds was checked by thin layer chromatography (TLC). The structure of the MPTS and CPTS was confirmed by elemental analysis, FTIR, and ¹H NMR spectroscopy.

Scheme 1 Synthetic route and structure of MPTS and CPTS.

2.1.1. MPTS. Yield: 58%; mp 187; analytical data calculated for C₂₄H₁₉N₃SO₂; C, 69.73; H, 4.60; N, 10.17; S, 7.75. Found: C, 69.12; H, 4.58%; N, 10.05; S, 7.48.

¹H NMR (300 MHz, DMSO-d6) δ ppm: δ 8.12–8.28 (m, 13H, Ar-H), 6.64 (s, 1H, CH), 9.85 (s, 2H, 2NH), 4.68 (s, 3H, OCH₃).

IR (KBr) λ_max (cm⁻¹): 1510 (C=S), 1660 (C=O), 3240 (NH), 1160 (–OCH₃).

2.1.2. CPTS. Yield: 62%; mp 242; analytical data calculated for C₂₃H₁₆N₃SOCl; C, 66.19; H, 3.84; N, 10.47; S, 7.67. Found: C, 65.96; H, 3.82%; N, 10.36; S, 7.57.

¹H NMR (300 MHz, DMSO-d6) δ ppm: δ 8.24–8.46 (m, 13H, Ar-H), 6.68 (s, 1H, CH), 9.78 (s, 2H, 2NH).

IR (KBr) λ_max (cm⁻¹): 1500 (C=S), 1620 (C=O), 3250 (NH).

2.2. Mild steel specimens

Mild steel specimens of size 6.0 cm × 2.5 cm × 0.1 cm and 1.0 cm × 1.0 cm × 0.1 cm with composition (w%) C, 0.12; Mn, 0.11; Cu, 0.01; Si, 0.02; Sn, 0.01; P, 0.02; Ni, 0.02 and remainder iron were employed for the weight loss and electrochemical studies, respectively. The specimens were abraded using emery papers of different grit sizes up to 1200 grit, polished with Al₂O₃ (1 μm and then 0.3 μm particle size), washed with tap water followed by distilled water, degreased with acetone, dried and stored in desiccator.

2.3. Test solution

The test solution (15% HCl solution) was prepared by dilution of AR grade HCl with double distilled water. The volume of test solution was 250 mL for weight loss study 150 mL for electrochemical study. The concentration range of inhibitors employed was 25 to 150 ppm (mg L⁻¹).

2.4. Weight loss measurements

Weight loss measurements were performed in the absence and presence of different concentrations (25 to 150 ppm) of inhibitors at different temperatures (303–333 K) according to the standard methods.²⁰ The corrosion rate (CR), inhibition efficiency (η%) and surface coverage (θ) were determined by following equations:²¹where, W = weight loss (g), A = area of specimen (cm²) exposed in acidic solution, t = exposure time (h), and D = density of mild steel (g cm⁻³).where, CR₀ and CR_i are corrosion rate in absence and presence of inhibitors.

2.5. Electrochemical methods

The electrochemical studies were conducted in a conventional three-electrode cell consisting of mild steel sample of 1 cm² exposed area as working electrode, a platinum counter electrode and a saturated calomel electrode (SCE) as reference electrode, using CH electrochemical workstation (Model No. CHI 760D, manufactured by CH Instruments, Austin, USA) at 303 K. Potentiodynamic polarization curves were obtained at a scan rate of 1 mV s⁻¹ in the potential range −250 to +250 mV vs. SCE at open circuit potential. The linear Tafel segments of anodic and cathodic curves were extrapolated to obtain corrosion current densities (i_corr). The percentage inhibition efficiency (η%), was calculated using the equationwhere, i⁰_corr and i_corr are the values of corrosion current density in the absence and presence of inhibitor, respectively.

Electrochemical impedance measurements were carried out using AC signals of amplitude 10 mV peak to peak at the open circuit potential in the frequency range of 100 kHz to 10 mHz. The experimental impedance data were fitted to appropriate equivalent circuit using ZSimpWin.3.21 software. The inhibition efficiency (η%) was calculated from charge transfer resistance values obtained from impedance measurements using the following relation

where R_ct(inh) and R_ct are charge transfer resistance in presence and absence of inhibitor, respectively. The values of double layer capacitance (C_dl) were calculated from charge transfer resistance and CPE parameters (Y₀ and n) using the expression²²

C_dl = (Y₀R_ct¹⁻ⁿ)^1/n (6)

where Y₀ is CPE constant and n is CPE exponent. The value of n represents the deviation from the ideal behavior and it lies between 0 and 1.

2.6. FTIR spectrum analysis

The FTIR spectrum of the pure compound and film formed on the surface of N80 steel samples was recorded on Perkin Elmer FTIR, (Spectrum-2000) spectrophotometer.

2.7. Scanning electron microscopic and energy dispersive X-ray spectroscopy analysis

For surface morphological study of the uninhibited and inhibited mild steel samples, SEM and EDX images were recorded using the instrument HITACHI S3400N.

2.8. Atomic force microscopy

The AFM images of polished, uninhibited and inhibited mild steel specimens were carried out using a Nanosurf Easyscan2 instrument.

2.9. XPS analysis

The chemical composition of adsorbed film of inhibitors on mild steel was detected by XPS (VSW Spectrometer) employing Al Kα (1486.6 eV) as the incident radiation source and the binding energy of C 1s (284.6 eV) was used as a reference. XPS analysis was performed without argon sputtering of the specimens.

2.10. Quantum chemical study

Complete geometrical optimization of the investigated inhibitors were performed using Density Functional Theory (DFT) with the Becke's three parameter exchange functional along with the Lee–Yang–Parr nonlocal correlation functional (B3LYP) with 6-31G (d, p) basis set implemented in Gaussian 03 program package.^23,24 Theoretical parameters such as the energy of the highest occupied and lowest unoccupied molecular orbital (E_HOMO and E_LUMO), energy gap (ΔE), dipole moment (μ) absolute electronegativity (χ), global hardness (γ) and softness (σ), and fraction of electrons transferred (ΔN) were determined.

3. Results and discussion

3.1. Weight loss measurements

3.1.1. Effect of inhibitor concentration and temperature. Corrosion parameters for mild steel in 15% HCl solution were determined from weight loss measurement data in the absence and presence of different concentrations (25–250 ppm) of the studied inhibitors at different temperatures (303–333 K) and listed in Table 1. Inspection of the data in Table 1, reveals that inhibition efficiency increases with increasing the concentration of each inhibitor up to 150 ppm concentration and after 150 ppm concentration remains almost constant therefore 150 ppm was taken as optimum concentration and the concentration range was taken as 25 ppm to 150 ppm for other studies. The inhibition efficiency of both inhibitors decreases with an increase in temperature. Such behavior can be interpreted on the basis that the inhibitors exert their action by adsorbing themselves on the mild steel surface and an increase in temperature resulted in desorption of some adsorbed inhibitor molecules leading to a decrease in the inhibition efficiency.²⁵ It is evident from Table 1 that both inhibitors are good inhibitors even at the concentration as low as 25 ppm. The inhibition efficiency of MPTS and CPTS at 150 ppm concentration was found to be 94.8% and 93.4% respectively, while it was 67.4% and 64.7% respectively at 25 ppm concentration at 303 K (Table 1). The inhibition efficiency of MPTS was found to be better than CPTS at all concentrations and temperatures. The inhibitors MPTS and CPTS have nearly same size and number of active centers, the difference between the two inhibitors is that MPTS contains –OCH₃ substituent on phenyl ring whereas CPTS contains –Cl substituent on phenyl ring present in these inhibitors. Thus, the difference in corrosion inhibition efficiency between MPTS and CPTS is due to different nature of these substituent's (–OCH₃ and –Cl). The delocalized π-electron density at phenyl ring in case of MPTS increased due to electron donating nature of methoxy (–OCH₃) substituent whereas the electron density on phenyl ring in case of CPTS decreased due to electron withdrawing nature of chloro (–Cl) substituent. The higher delocalized π-electron density at phenyl ring in case of MPTS as compared to CPTS facilitate greater adsorption of MPTS on mild steel surface than CPTS, leading to higher inhibition efficiency of MPTS than CPTS. The same effect of substituent has been reported in literature by some researchers.²⁶

Table 1 Corrosion parameters of mild steel in 15% HCl solution in the presence and absence of inhibitors at different temperatures, obtained from weight loss measurements

Conc. (ppm)	303 K			313 K			323 K			333 K
	CR (mm per year)	θ	η%	CR (mm per year)	θ	η%	CR (mm per year)	θ	η%	CR (mm per year)	θ	η%
Blank	20.20	—	—	34.55	—	—	55.47	—	—	92.69	—	—
MPTS
25	6.58	0.674	67.4	11.91	0.655	65.5	20.87	0.623	62.3	38.16	0.588	58.8
50	3.97	0.803	80.3	7.34	0.787	78.7	13.12	0.763	76.3	24.96	0.730	73.0
75	2.41	0.880	88.0	4.58	0.867	86.7	8.60	0.844	84.4	17.24	0.814	81.4
100	1.66	0.917	91.7	3.20	0.907	90.7	6.27	0.886	88.6	13.22	0.857	85.7
150	1.05	0.948	94.8	2.29	0.933	93.3	4.71	0.915	91.4	10.32	0.888	88.8
200	0.97	0.952	95.2	2.28	0.934	93.4	4.16	0.915	92.5	10.01	0.892	89.2
250	0.91	0.955	95.5	2.21	0.936	93.6	4.05	0.917	92.7	9.82	0.894	89.4
CPTS
25	7.11	0.647	64.7	12.84	0.628	62.8	22.23	0.599	59.9	40.54	0.562	56.2
50	4.34	0.784	78.4	8.04	0.767	76.7	14.40	0.740	74.0	27.18	0.706	70.6
75	2.75	0.863	86.3	5.17	0.850	85.0	9.67	0.825	82.5	18.99	0.795	79.5
100	1.95	0.903	90.3	3.74	0.891	89.1	7.19	0.870	87.0	14.54	0.843	84.3
150	1.31	0.934	93.4	2.75	0.920	92.0	5.35	0.900	90.0	11.57	0.875	87.5
200		0.946	94.6		0.934	93.4		0.903	90.3		0.877	87.7
250		0.948	94.8		0.936	93.6		0.905	90.5		0.879	87.9

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3.1.2. Effect of exposure period. The variation of the inhibition efficiency for MPTS and CPTS at optimum concentration (150 ppm) with immersion time is shown in Fig. 1. The corrosion parameters such as corrosion rate and corrosion inhibition efficiencies obtained for MPTS and CPTS at optimum concentration (150 ppm) and different exposure periods (6–120 h) are shown in Table 2. It is clear from the Fig. 1 that inhibition efficiency of MPTS and CPTS decreases with increase in exposure time (6–120 h) at 303 K due to the partial desorption of adsorbed inhibitors on increasing the exposure time.²⁷

Fig. 1 Variation of inhibition efficiency with exposure period in presence of 150 ppm concentration of MPTS and CPTS at 303 K.

Table 2 Effect of time on corrosion rate and inhibition efficiency without and with 150 ppm of MPTS and CPTS at 303 K

Exposure period (h)	Blank	MPTS		CPTS
	CR (mm per year)	CR (mm per year)	η (%)	CR (mm per year)	η (%)
6	20.2	1.05	94.8	1.31	93.5
24	109.4	8.75	92.0	9.62	91.2
48	157.9	15.15	90.4	16.57	89.5
72	182.6	20.63	88.7	24.46	86.6
96	202.4	27.12	86.6	31.97	84.2
120	222.6	35.17	84.2	39.40	82.3

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3.1.3. Thermodynamic and activation parameters. The apparent activation energy (E_a) for dissolution of mild steel in 15% HCl was calculated by using the Arrhenius equation:where R is the molar gas constant (8.314 J K⁻¹ mol⁻¹), T is the absolute temperature (K) and A is the Arrhenius pre-exponential factor. Fig. 2(a and b) presents the Arrhenius plot of log CR against 1/T for the corrosion of mild steel in 15% HCl solution in the absence and presence of inhibitors (MPTS and CPTS) at concentrations ranging from 25 to 150 ppm. From Fig. 1(a and b), the activation energy was calculated using the expression E_a = −(slope) × 2.303R. The calculated values of E_a are summarized in Table 3. It is evident from Table 3 that the values of the apparent activation energy for the inhibited solutions were higher than that for the uninhibited solution and continued to increase on increasing the concentration of inhibitor. This explains that the energy barrier of corrosion reaction increases with the concentration of inhibitor.²⁸

Fig. 2 Arrhenius plots of log CR versus 1000/T⁻¹ for mild steel corrosion in 15% HCl solution (a) MPTS (b) CPTS.

Table 3 Activation parameter for mild steel in 15% HCl solution in the absence and presence of inhibitors obtained from weight loss measurements

Inhibitor	Concentration (ppm)	Ea (kJ mol^−1)	ΔH* (kJ mol^−1)	ΔS* (J mol^−1 K^−1)
Blank	—	42.34	39.70	−89.20
MPTS	25	48.96	46.32	−76.75
	50	51.15	48.51	−73.31
	75	54.80	52.16	−66.03
	100	58.15	55.51	−58.18
	150	65.59	60.95	−43.82
CPTS	25	48.42	45.79	−77.90
	50	51.05	48.43	−73.88
	75	53.88	51.24	−67.94
	100	56.04	53.40	−63.73
	150	60.42	57.78	−52.44

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From the thermodynamic and kinetic point of view, the unimolecular reactions is characterized by following equation:

E_a − ΔH* = RT (8)

As observed from Table 3, for both inhibitors E_a > ΔH* by a value which approximately equal to RT. Hence, mild steel sample corrodes in 15% HCl solutions either in absence or presence of different concentrations of the studied inhibitors by a unimolecular reaction.

The values of standard enthalpy of activation (ΔH*) and standard entropy of activation (ΔS*) for the formation of the activation complex in the transition state was calculated by using the transition state equation:

where, h is Planck's constant and N is the Avogadro number, respectively.

A plot of log(CR/T) against 1/T (Fig. 3(a and b)) gave straight lines with a slope of −ΔH*/2.303R and an intercept of [log(R/Nh) + (ΔS*/2.303R)], from which the activation thermodynamic parameters ΔH* and ΔS* were calculated, as listed in Table 3. The negative value of ΔS* for both inhibitors indicates that the formation of the activated complex in the rate determining step represents an association rather than a dissociation step, meaning that a decrease in disorder takes place during the course of the transition from reactants to activated complex.²⁹

Fig. 3 Transition state plots of log CR/T versus 1000/T⁻¹ for mild steel in 15% HCl solution at different concentrations (a) MPTS (b) CPTS.

3.1.4. Adsorption isotherm. The surface coverage (θ) for different concentrations of both studied inhibitors in 15% hydrochloric acid obtained from weight loss method was tested graphically for fitting a suitable adsorption isotherm. Plotting C_inh/θ vs. C_inh yielded a straight line [Fig. 4] with a correlation coefficient (R²) and slope values as given in Table 4 at different temperatures. The correlation coefficient and slope values for both inhibitors as shown in Table 3 are near to unity indicating that the adsorption of these inhibitors obey the Langmuir adsorption isotherm represented by the following equation.where, C_inh is the inhibitor concentration and K_ads is the equilibrium constant for adsorption–desorption process. From the intercept of Fig. 3, the values of K_ads were calculated and represented in Table 4. Large values of K_ads obtained for both studied inhibitors imply more efficient adsorption and hence better corrosion inhibition efficiency. Using the values of K_ads, the values of ΔG⁰_ads was calculated by using the equation:

ΔG⁰_ads = −RT ln(55.5K_ads) (11)

where R is the gas constant and T is the absolute temperature (K). The value of 55.5 is the concentration of water in solution in mol L⁻¹. Calculated values of ΔG⁰_ads are listed in Table 4. Normally, a value of ΔG⁰_ads around −20 kJ mol⁻¹ or less negative is assumed for electrostatic interactions between inhibitor molecules and the charged metal surface (physisorption), and a value around −40 kJ mol⁻¹ or more negative is indicative of charge sharing or transferring from organic species to the metal surface to form a coordinate bond (chemisorption).³⁰ Thus, the negative values of ΔG⁰_ads in the range of −40 kJ mol⁻¹ to −30 kJ mol⁻¹, indicate that the adsorption mechanism of inhibitor on mild is a combination of both physisorption and chemisorptions. The calculated ΔG⁰_ads values for MPTS and CPTS were found in the range of −36.2 to −39.0 and −36.1 to −38.8 kJ mol⁻¹, respectively, at different temperatures (303–333 K). These values are between the threshold values for physical adsorption and chemical adsorption, indicating that the adsorption process of these inhibitors at mild steel surface involve both the physical as well as chemical adsorption. More negative value of ΔG⁰_ads for MPTS indicates a stronger interaction between the inhibitor molecules and the metal surface. It is apparent from the Table 3 that, the negative value of ΔG⁰_ads decreased on increasing the temperatures, suggesting that the adsorption of inhibitor molecules on mild steel surface is not favorable with increasing experimental temperature, indicating that physisorption has the major contribution while chemisorption has the minor contribution in the adsorption process. Similar conclusion was also reported by Ozcan,³¹ who studied the use of cystine as a corrosion inhibitor on mild steel in sulfuric acid.

Fig. 4 Langmuir plots of (C_inh/θ) versus C_inh for (a) MPTS (b) CPTS.

Table 4 Thermodynamic parameters of adsorption for studied inhibitors on mild steel in 15% HCl solution at different temperatures

Inhibitor	Temperature (K)	Kads (M^−1)	ΔG^0ads (kJ mol^−1)	R^2	ΔH^0ads (kJ mol^−1)	ΔS^0ads (J mol^−1 K^−1)
MPTS	303 K	2.4 × 10^4	−39.0	0.999
	313 K	2.7 × 10^4	−38.2	0.996
	323 K	2.9 × 10^4	−37.1	0.997	−67.83	−95
	333 K	3.2 × 10^4	−36.2	0.994
CPTS	303 K	2.2 × 10^4	−38.8	0.993
	313 K	2.5 × 10^4	−38.0	0.996	−66.41	−91
	323 K	2.7 × 10^4	−37.0	0.997
	333 K	3.0 × 10^4	−36.1	0.998

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The enthalpy and entropy changes for the adsorption of inhibitors (MPTS and CPTS) on mild steel surface were determined from the basic thermodynamic equation:³²

ΔG⁰_ads = ΔH⁰_ads − TΔS⁰_ads (12)

where ΔH⁰_ads and ΔS⁰_ads are the standard enthalpy and entropy changes of adsorption process, respectively. The plot of ΔG⁰_ads versus T was linear (Fig. 5) with the slope equal to −ΔS⁰_ads and intercept of ΔH⁰_ads. The calculated values of ΔH⁰_ads and ΔS⁰_ads are listed in Table 4. The ΔH_ads values are negative for both studied inhibitors, suggests that the adsorption of inhibitor's molecules on mild steel surface is an exothermic process. It has been reported in literature that an endothermic adsorption process (ΔH_ads > 0) is due to chemisorption while an exothermic adsorption process (ΔH_ads < 0) may be attributed to physisorption, chemisorption or a mixture of both.³³ In an exothermic process, physisorption is distinguished from chemisorption by considering the absolute value of ΔH_ads, it is lower than 40 kJ mol⁻¹ for the physisorption process whereas for chemisorption process approaches 100 kJ mol⁻¹. In the present study, H_ads values for MPTS and CPTS are −67.83 kJ mol⁻¹ and −66.41 kJ mol⁻¹ respectively, which are larger than the common physical adsorption heat, but smaller than the common chemical adsorption heat, emphasizing that mixed type (both physical and chemical) adsorption take place. The similar results were also reported by other authors.^34–36

Fig. 5 The relationship between ΔG⁰_ads and temperature.

The negative value of ΔS⁰_ads obtained for adsorption of both the inhibitors as shown in Table 4 suggested that before the adsorption of inhibitor's molecules on the mild steel surface, inhibitor molecules might freely move in the bulk solution, but with the progress in the adsorption, inhibitor molecules were orderly adsorbed on the mild steel surface, as a result a decrease in entropy is observed.³⁷ From the thermodynamic principles, since the adsorption is an exothermic process, it must be accompanied by a decrease of entropy.³⁸

3.2. Electrochemical studies

3.2.1. Polarization studies. The potentiodynamic polarization curves of mild steel in 15% HCl solution in the absence and presence of various concentrations of MPTS and CPTS at 303 K are shown in Fig. 6(a and b), respectively. It is clear from the Fig. 6(a and b), that both, the anodic metal dissolution and cathodic hydrogen evolution reactions are inhibited after the addition of inhibitors to the 15% HCl solution. The inhibition of these reactions are more pronounced with the increasing inhibitor concentrations, results increase in inhibition efficiency. The inhibitor molecules are first adsorbed on the mild steel surface and blocking the available reaction sites.³⁹ The surface coverage increases with increase in inhibitor concentrations. The corrosion current densities were calculated by extrapolation of linear parts of anodic and cathodic curves to the point of intersection. The electrochemical corrosion parameters such as corrosion potential (E_corr), anodic Tafel slope (β_a), cathodic Tafel slope (β_c), corrosion current density (i_corr) and percentage inhibition efficiency (η%) obtained from these curves are given in Table 5. It is clear from Table 4 that both the anodic and cathodic current density values are considerably decreased in inhibited solutions when compared with the uninhibited solution. The observed phenomenon may be the result of covering of adsorbed inhibitor molecules on the mild steel surface and decreasing dissolution of mild steel. The minor shift in E_corr value (15 mV) towards negative direction in presence of inhibitor as compared to the E_corr value in absence of inhibitor, indicating that MPTS and CPTS acts as mixed type inhibitor with predominant control of cathodic reaction.⁴⁰

Fig. 6 Potentiodynamic polarization curves for mild steel in 15% HCl solution in the presence and absence of inhibitor (a) MPTS (b) CPTS at 303 K.

Table 5 Electrochemical parameters determined from polarization measurements for mild steel in 15% HCl solution in the presence and absence of different concentrations of inhibitors at 303 K

Conc. (ppm)	Ecorr (mV vs. SCE)	βa (mV per decade)	−βc (mV per decade)	icorr (μA cm^−2)	η%
Blank	−495	93	142	568	—
MPTS
25	−503	73	124	169.4	70.1
50	−498	86	131	107.7	81.0
75	−500	91	148	59.6	89.5
100	−505	62	162	46.5	91.8
150	−505	77	152	29.5	94.8
CPTS
25	−502	94	138	186.3	67.2
50	−504	74	114	113.6	80.0
75	−508	58	157	67.6	88.1
100	−510	78	129	51.1	91.0
150	−500	85	140	39.9	92.9

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3.2.2. EIS studies. Electrochemical impedance measurements were undertaken to provide information on the kinetics of the electrochemical processes at the mild steel/acid interface. Nyquist plots for mild steel corrosion in 15% HCl solution in the absence and presence of different concentrations (25–150 ppm) of the inhibitors MPTS and CPTS are shown in Fig. 7(a and b), respectively. The Nyquist plots for both inhibitors show single semicircles over the frequency range studied, corresponding to one time constant. The capacitive loops are not perfect semicircles, because of non-homogeneity and roughness of the mild steel surface.⁴¹ The impedance spectra were analyzed by fitting the experimental data to the equivalent circuit model shown in Fig. 8, which is a parallel combination of the charge transfer resistance (R_ct) and the constant phase element (CPE), both in series with the solution resistance (R_s). This type of electrochemical equivalent circuit was reported previously to model the iron/acid interface.⁴² The CPE is used in place of a capacitor to compensate deviations from ideal dielectric behavior arising from the inhomogeneous nature of the electrode surfaces. The impedance of the CPE is given by

Z_CPE = Y₀⁻¹(jω)⁻ⁿ (13)

where Y₀ and n stand for the CPE constant and exponent, respectively, j = (−1)^1/2 is an imaginary number, and ω is the angular frequency in rad s⁻¹ (ω = 2πf), where f is the frequency in Hz. The electrochemical parameters such as solution resistance (R_s), charge transfer resistance (R_ct) and CPE constants (Y₀ and n) obtained from fitting the experimental data of Nyquist plots in the equivalent circuit shown in Fig. 8 are presented in Table 6. The data shown in Table 5 reveal that the value of R_ct increases with addition of inhibitors as compared to the blank solution, the increase in R_ct value is attributed to the formation of a protective film at the metal/solution interface. The C_dl value decreases on increasing the concentration of both the inhibitors, indicating the decrease in local dielectric constant and/or to an increase in the thickness of the electrical double layer, suggesting that the inhibitor molecules are adsorbed at the metal/solution interface.⁴³

Fig. 7 Nyquist plot for mild steel in 15% HCl solution containing various concentrations of (a) MPTS (b) CPTS at 303 K.

Fig. 8 Equivalent circuit applied for fitting of the impedance spectra.

Table 6 Electrochemical parameters determined from electrochemical impedance measurements for mild steel in 15% HCl solution in the presence and absence of different concentrations of inhibitors at 303 K

Conc. (ppm)	Rs (Ω cm^2)	Rct (Ω cm^2)	Y0 (μF cm^−2)	n	Cdl (μF cm^2)	η%
Blank	0.93	20	573	0.845	252	—
MPTS
25	0.76	66	237	0.865	123.8	69.7
50	0.54	112	157	0.872	86.7	82.1
75	0.69	161	122	0.878	70.6	87.5
100	0.73	230	91	0.893	57.2	91.3
150	0.82	327	58	0.923	41.6	93.8
CPTS
25	0.82	62	268	0.852	131.5	67.7
50	0.63	98	183	0.861	95.6	79.6
75	0.69	148	145	0.867	80.4	86.5
100	0.58	217	113	0.872	65.5	90.7
150	0.74	296	69	0.912	47.4	93.2

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The Bode phase angle plots (Fig. 9(a and b)) show single maximum (one time constant) at intermediate frequencies, broadening of this maximum in presence of inhibitors accounts for the formation of a protective layer on the electrode surface. Fig. 9(a and b) shows that the impedance value in the presence of both inhibitors is larger than in absence of inhibitors and the value of impedance increases on increasing the concentration of both studied inhibitors. These mean that the corrosion rate is reduced in presence of the inhibitors and continued to decreasing on increasing the concentration of inhibitors.

Fig. 9 Bode (log f vs. log|Z|) and phase angle (log f vs. −phase) plots of impedance spectra for mild steel in 15% HCl solution in the presence and absence of inhibitor (a) MPTS (b) CPTS at 303 K.

Electrochemical results (η%) are in good agreement with the results (η%) obtained by weight loss experiments.

3.2.3. Potential of zero charge (E_PZC). The phenomenon of adsorption is influenced by the chemical structure of the inhibitor, the charge or dipole moment of the inhibitor molecules and the charge of the metal surface. The potential of zero charge (E_PZC) of mild steel in 15% HCl solution in presence of inhibitors was determined by using electrochemical impedance spectroscopy method. The minimum on the double layer capacitance (C_dl) versus applied voltage (E) plots (Fig. 10) is considered as the value of E_PZC of the mild steel. The surface charge of the metal can be defined by the position of the open circuit potential (E_ocp) with respect to the E_PZC. The net surface charge of the mild steel at the open circuit potential was evaluated according to the equation:

E_r = E_ocp − E_PZC (14)

where E_r is the Antropov's “rational” corrosion potential.⁴⁴ The positive and negative values of E_r suggesting the positive and negative net charge on the mild steel surface, respectively, at open circuit potential. Table 7 shows the values of E_ocp, E_PZC and E_r for both inhibitors at 150 ppm concentration. The positive value of E_r for both the inhibitors with respect to E_PZC, as shown in Table 7, suggesting that mild steel surface is positively charged at open circuit potential in presence of 150 ppm of these inhibitors. Since mild steel surface is positively charged therefore anions (Cl⁻ ions) in aqueous hydrochloric acid solution gets adsorbed on the mild steel surface. After the adsorption of the Cl⁻ ions, the mild steel surface becomes negatively charged. Hence, the protonated positively charged form of inhibitor (MPTS⁺ and MPTS⁺) adsorbed on the surface of mild steel via electrostatic interactions with the Cl⁻ ions already adsorbed on mild steel surface. Other sides, the inhibitor molecules may also adsorbed on the mild steel surface via the unshared pair of electrons present on N, O and S atoms.

Fig. 10 The plot of C_dl vs. applied potential for mild steel in 15% HCl solution containing 150 ppm of MPTS and CPTS at 303 K.

Table 7 Values of E_ocp, E_PZC and E_r recorded for mild steel in 15% HCl solution in absence and presence of 150 ppm of MPTS and CPTS at 303 °C

Inhibitor	Eocp (mV vs. SCE)	EPZC (mV vs. SCE)	Er (mV vs. SCE)
MPTS	−508	−520	12
CPTS	−500	−505	05

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3.3. Analysis of FTIR spectra

The FTIR spectra of pure inhibitors and reflectance absorption FTIR spectra of the inhibited mild steel samples were recorded and represented in Fig. 11(a and b). The pure MPTS shows the IR bands around 3240, 1660 and 1510 cm⁻¹ due to presence of –NH, C=O and C=S, respectively, whereas pure CPTS shows the IR band around 3250, 1620 and 1500 cm⁻¹ due to presence of –NH, C=O and C=S group, respectively. The reflectance absorption FTIR spectra of the exposed specimens in MPTS and CPTS show bands around 3180, 1640, 1460 and 3250, 1620, 1500 cm⁻¹ respectively due to presence of –NH, C=O and C=S group, respectively. The shift in peak position of –NH, C=O and C=S in reflectance absorption FTIR spectra of both the inhibitors as compared to their pure compounds spectra, indicating the involvement of these groups in adsorption process of inhibitors with the mild steel surface.

Fig. 11 FTIR spectrum of the pure inhibitors and film formed on the surface of N80 steel specimen.

3.4. Scanning electron microscopy

SEM images of mild steel specimen before and after immersion in 15% HCl solution as well as in presence of 150 ppm of MPTS and CPTS are shown in Fig. 12(a–d). The morphology of the mild steel specimen before immersion (Fig. 12(a)) is very smooth and shows no corrosion while mild steel specimen immersed in 15% HCl solution in the absence of inhibitor (Fig. 12(b)) is very rough and the surface is badly damaged due to metal dissolution. However, the presence of 150 ppm of MPTS and CPTS suppresses the rate of corrosion and surface damage has been diminished considerably (Fig. 12(c and d)) as compared to the blank solution (Fig. 12(b)), suggesting formation of a protective inhibitor film at the mild steel surface.

Fig. 12 SEM images of mild steel in 15% HCl solution after 6 h immersion at 303 K. (a) Before immersion (polished), (b) after immersion without inhibitor (c) after immersion with 150 ppm MPTS (d) after immersion with 150 ppm CPTS.

3.5. Energy dispersive X-ray spectroscopy

Energy dispersive X-ray analysis (EDX) technique was employed in order to get information about the composition of the surface of the mild steel sample in the absence and presence of inhibitors in 15% HCl solution. The results of EDX spectra are shown in Fig. 13(a–d). The EDX spectra of uninhibited mild steel specimen contains the peaks corresponding to the element present in mild steel whereas inhibited mild steel contains the peaks corresponding to all the elements present in the inhibitor molecules, indicating the adsorption of inhibitor molecules at the surface of mild steel. The percentage atomic content of various elements on the surface of mild steel specimen before immersion, after immersion and after inhibition was determined by EDX and summarized in Table 8. The percentage atomic content of Fe for mild steel after immersion in 15% HCl solution is 83.12%, and those for mild steel immersed in presence of 150 ppm of MPTS and CPTS are 72.48% and 76.38%, respectively. The suppressed percentage atomic content of Fe in inhibited specimen as compared with the uninhibited mild steel specimen is due to the inhibitory film formed on the mild steel surface.

Fig. 13 EDX spectra of mild steel specimens (a) before immersion (b) after immersion without inhibitor (c) after immersion with 150 ppm MPTS (d) after immersion with 150 ppm CPTS.

Table 8 Percentage atomic contents of elements obtained from EDX spectra

Inhibitors	Fe	C	S	Mn	Cl	N	O
Mild steel	85.26	12.46	—	0.46	—	—	—
Blank HCl	83.12	15.68	—	0.28	2.29	—	6.36
MPTS	72.48	19.26	1.84	0.16	0.34	5.36	15.24
CPTS	76.38	19.84	1.72	0.12	0.64	5.48	14.12

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3.6. Atomic force microscopy

The three-dimensional AFM images of polished (before immersion), uninhibited and inhibited mild steel specimen are shown in Fig. 14(a–d). The average roughness of polished mild steel specimen (Fig. 14(a)) and mild steel specimen in 15% HCl solution without inhibitor (Fig. 14(b)) were found as 25 and 450 nm. It is clearly shown in Fig. 14(b) that mild steel sample is badly damaged due to the acid attack on surface. However, in presence of optimum concentration (150 ppm) of MPTS and CPTS as shown in Fig. 14(c and d), the average roughness were reduced to 130 and 180 nm, respectively. The lower value of roughness for MPTS than CPTS reveals that MPTS protects the mild steel surface more efficiently than CPTS in 15% HCl solution.

Fig. 14 AFM micrograph of mild steel surface (a) before immersion (b) after immersion without inhibitor (c) after immersion with 150 ppm MPTS (d) after immersion with 150 ppm CPTS at 303 K.

3.7. XPS analysis

XPS analysis was carried out in order to investigate the composition of the adsorbed films on the steel surfaces in the presence of inhibitors. The XPS spectra were obtained for the mild steel surfaces taken out after 24 h immersion in 15% HCl solution containing 150 ppm of MPTS and CPTS separately. The nature of both the XPS spectra were almost similar therefore here we are discussing only the XPS spectra of MPTS protected mild steel surface. The XPS spectra, obtained for MPTS (C 1s, Fe 2p, O 1s, S 2p, N 1s) is shown in Fig. 15(a–e). These spectra show complex forms which have been assigned to the corresponding species through a deconvolution fitting procedure using the XPS Peak-Fit 4.1 software.

Fig. 15 The XPS deconvoluted profiles for C 1s, Fe 2p, O 1s, S 2p and N 1s for mild steel after 24 h immersion period in 15% HCl solution in the presence of 150 ppm of MPTS at 303 K.

The C 1s spectrum of protected mild steel in presence of MPTS are fitted into three peaks (284.6, 287.2 and 288.6 eV) (Fig. 15(a)). The peak with characteristic binding energy of 284.6 eV can be attributed to the C–C and C–H aromatic bonds. The peak at 287.2 eV may be assigned to the carbon atoms bonded to nitrogen. The peak at 288.6 eV may be ascribed to the carbon atom of the C–O and C–S bond.⁴⁵

The Fe 2p spectrum of MPTS adsorbed films (Fig. 15(b)) exhibit two peaks at 711.4 and 715.2 eV for Fe 2p3/2 and Fe 2p1/2, respectively which are attributed to ferric compound such as Fe₂O₃ and FeOOH.⁴⁰ The formation of a stable and insoluble layer of Fe₂O₃ and FeOOH reduces ionic diffusion and thus improves the corrosion resistance of the mild steel in hydrochloric acid solution. The peak, observed at 715.4 eV is characteristic of Fe(II) species.⁴⁵

The deconvolution of the O 1s spectrum of MPTS may be fitted into three main peaks shown in Fig. 15(c). The peak at 530.4 eV can be assigned to O²⁻, bonded with Fe³⁺ in the Fe₂O₃ oxides.⁴⁸ The peak located at 532.4 eV is ascribed to OH⁻, as in FeOOH. The small peak at 533.6 eV may be assigned to the C–O bond and of adsorbed water with mild steel.⁴⁵

The S 2p XPS spectra for MPTS protected mild steel surface displayed three peaks (Fig. 15(d)) at 161.4 eV, 164.4 eV and 169.2 eV assigned as sulfide species, S–C bond and S–Fe bound respectively.⁴⁵

The N 1s XPS spectra for MPTS protected mild steel surface displayed three peaks (Fig. 15(e)). The first peak located at 399.6 is attributed to C–N bond. The second peak at 401.2 eV is assigned to nitrogen atoms of indoline and/or pyrimidine ring coordinated with the steel surface (N–Fe). The last peak at 401.6 eV is attributed to positively charged nitrogen, and could be related to protonated nitrogen atoms in indoline and/or pyrimidine ring.⁴⁵

From these observations it is concluded that the XPS results support the presence of adsorbed MPTS on the mild steel surface.

3.8. Quantum chemical analysis

In order to study the effect of molecular structure on the inhibition efficiency, quantum chemical calculations were performed by using DFT and all the calculations were carried out with the help of complete geometry optimization. Optimized structure, E_HOMO and E_LUMO of MPTS and CPTS are shown in Fig. 16(a and b). For the calculations of quantum chemical parameters the following equations were used:⁴⁶

Fig. 16 The optimized structure (left), HOMO (center) and LUMO (right) distribution for molecules (a) MPTS (b) CPTS [atom legend: white = H; grey = C; blue = N; red = O; yellow = S; green = Cl].

The inverse of the global hardness is designated as the softness, σ as follows:

where, hardness and softness measures the stability and reactivity of a molecule. Soft molecules are considered to be more reactive than hard ones because they can offer electron to acceptors easily. For the simplest transfer of electrons, adsorption could occur at the part of the molecule where σ which is a local property, has the highest value.⁴⁵ Thus, the inhibitor having maximum value of softness adsorbed strongly at the surface of mild steel and shows maximum inhibition efficiency.

The fraction of electrons transferred (ΔN) was calculated by using the equation:⁴⁶

where a theoretical value of χ_Fe ≈ 4.06 eV is taken for iron⁴⁶ and γ_Fe = 0 is taken assuming that I = A for bulk metals.⁴⁷

The quantum chemical parameters such as the energy of the highest occupied molecular orbital (E_HOMO), the energy of the lowest unoccupied molecular orbital (E_LUMO), energy gap ΔE (ΔE = E_LUMO − E_HOMO), ΔE₁ [ΔE₁ = E_{LUMO(inhibitor)} − E_HOMO(Fe)], ΔE₂ [ΔE₂ = E_LUMO(Fe) − E_{HOMO(inhibitor)}], dipole moment (μ), absolute electronegativity (χ), global hardness (γ) and softness (σ), and fraction of electrons transferred (ΔN) were calculated and summarized in Table 9.

Table 9 Quantum chemical parameters for both inhibitors

Inhibitor	EHOMO (eV)	ELUMO (eV)	ΔE (eV)	ΔE1 (eV)	ΔE2 (eV)	μ (D)	γ (eV)	σ (eV^−1)	χ (eV)	ΔN
Fe	−5.075	−1.747		—	—	—	—	—	—	—
MPTS	−4.884	−1.176	3.707	4.899	3.137	6.718	1.854	0.539	3.030	0.278
CPTS	−4.998	−1.146	3.852	3.929	3.251	6.034	1.926	0.519	3.072	0.256

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The reactivity of a chemical species can be defined in terms of frontier orbitals: the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).⁴⁴ According to the frontier molecular orbital (FMO) theory of chemical reactivity, the formation of a transition state is due to interaction between HOMO and LUMO of reacting species. The smaller the orbital energy gap (ΔE) between the participating HOMO and LUMO, the stronger is the interactions between two reacting species.⁴⁸ The results listed in Table 9 show that the interaction between the HOMO of the inhibitor and the LUMO of the Fe atom, as represented by ΔE₂ [ΔE₂ = E_LUMO(Fe) − E_{HOMO(inhibitor)}], is stronger than that between the HOMO of the Fe atom and the LUMO of the inhibitor ΔE₁ [ΔE₁ = E_{LUMO(inhibitor)} − E_HOMO(Fe)]. In principle, the interaction between the HOMO of the inhibitor and the LUMO of the Fe atom, ΔE₂, should be dominated by the interaction between the HOMO of the Fe atom and the LUMO of the inhibitor, ΔE₁, for good inhibitive property.⁴⁹ Adsorption of the inhibitor on the mild steel surface can occur on the basis of donor–acceptor interactions between the lone-pair electron of the heteroatom present in the inhibitors and the vacant d orbital of the mild steel surface Fe atoms. The inhibitors with high values of E_HOMO have a tendency to donate electrons to appropriate acceptors with low-energy, empty molecular orbital.^50,51 The calculated value of E_HOMO for MPTS (−4.884 eV) and CPTS (−4.998 eV) being higher than that for Fe (−5.075 eV) indicates that both inhibitors have a tendency to donate electrons to vacant d orbital of Fe. On the other hand, a lower value of E_LUMO (−1.747 eV) for Fe than that for MPTS (−1.176 eV) and CPTS (−1.146 eV) favors Fe to accept electrons. The lower values of ΔE₂ as compared to ΔE₁ for both the inhibitors suggesting that more donation of electrons occur from inhibitor to metal. The higher value of E_HOMO and lower value of ΔE₂ for MPTS as compared to CPTS, indicating that MPTS is better inhibitor than CPTS.

The higher value of dipole moment of MPTS and CPTS as shown in Table 9, clearly suggest that both inhibitors are polar compounds and can easily, donate electrons to form strong dπ–pπ bond.⁵² According to HSAB theory hard acids prefer to co-ordinate to hard bases and soft acid to soft bases. Fe is considered as soft acid and will co-ordinate strongly to molecule having maximum softness (σ) value. The inhibitor MPTS, which has the highest value of σ (0.278 eV), than CPTS has the higher inhibition efficiency than CPTS (Table 8). Generally, ΔN shows inhibition efficiency resulting from electrons transferred from the inhibitor molecule to the iron atom. According to Lukovits et al.,⁵³ if the value of ΔN is less than 3.6, the inhibition efficiency increases with increasing electron-donating ability of the inhibitor at the metal surface. An improvement in electron-releasing power was shown by replacing one hydrogen atom of phenyl ring by electron-donating substituent (−OCH₃ group) as in case of MPTS, which improved the inhibition efficiency but at the other hand a diminishing effect has been observed by electron-withdrawing substituent (−Cl) as in case of CPTS.

Fig. 16(a and b), reveal that the HOMO location in both inhibitors is mostly distributed in vicinity of the pyrimidine rings and –C=S, C=O groups. The LUMO location in both inhibitors is mostly distributed in vicinity of the pyrimidine, indoline and phenyl rings and –C=S, C=O group. These indicate the reactive sites of the inhibitors for interaction between inhibitor molecules and mild steel surface.

3.9. Mechanism of inhibition

Corrosion inhibition of mild steel in hydrochloric acid solution by MPTS and CPTS can be explained on the basis of molecular adsorption. These compounds inhibit corrosion by controlling both anodic as well as cathodic reactions. In acidic solutions these inhibitors exist as protonated species. In both inhibitors nitrogen atoms present in the molecules can be easily protonated in acidic solution and convert into quaternary compounds. These protonated species adsorbed on the cathodic sites of the mild steel and decrease the evolution of hydrogen. The adsorption on anodic site occurs through π-electrons of aromatic rings and lone pair of electrons of nitrogen, sulphur and oxygen atoms which decrease the anodic dissolution of mild steel. The inhibitors MPTS and CPTS are expected to get adsorbed on surface of mild steel through the lone pairs of electrons on N, S and O atoms present in these inhibitors. The schematic illustration of different modes of adsorption on metal/acid interface is shown in Fig. 17.

Fig. 17 The schematic illustration of different modes of adsorption on metal/acid interface.

4. Conclusions

(1) Both the synthesized spiro compounds are good corrosion inhibitor for the corrosion of mild steel in 15% HCl solutions and inhibition efficiency of MPTS is better than CPTS.

(2) The potentiodynamic polarization studies showed that both the studied inhibitors are mixed type inhibitor.

(3) EIS measurements showed that charge transfer resistance (R_ct) increases and double layer capacitance (C_dl) decreases in presence of inhibitors, suggested the adsorption of the inhibitor molecules on the surface of mild steel.

(4) The results obtained from SEM, AFM and Langmuir adsorption isotherm suggested that the mechanism of corrosion inhibition is occurring mainly through adsorption process.

(5) Quantum chemical results of MPTS and CPTS showed good correlation with the experimental results.

5. Acknowledgments

The authors acknowledge the financial supports of the CSIR New Delhi and Indian School of Mines, Dhanbad, India.

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