Comparative Investigation of Corrosion-Mitigating Behavior of Thiadiazole-Derived Bis-Schiff Bases for Mild Steel in Acid Medium: Experimental, Theoretical, and Surface Study

Abstract

In the present study, comparative analyses of corrosion inhibition property of few thiadiazole-derived bis-Schiff bases for mild steel in 1 M HCl were done. Various electrochemical experiments (electrochemical impedance spectroscopy and potentiodynamic polarization), as well as weight loss experiments, were employed to study the anticorrosion activity of bis-Schiff bases as inhibitors. The highest inhibition efficiency was obtained at an optimum concentration of 125 ppm for all inhibitors. Potentiodynamic polarization studies explain the mixed type but predominantly the cathodic nature of all inhibitors. The Langmuir adsorption isotherm was used to describe the mechanism of adsorption. The change in the value of activation energy on the addition of inhibitors reflects the mixed mode of interaction between the inhibitor and metallic surface. Scanning electron microscopy with energy-dispersive spectroscopy, X-ray diffraction, and X-ray photoelectron spectroscopy analyses confirmed the adsorption of bis-Schiff bases on the metal surface and thereby shielding from corrosion. Besides, the relevance between inhibition efficiency and the molecular structure of an inhibitor was theoretically examined via quantum chemical calculations and molecular dynamics simulations. All the results show consistent agreement with each other.

1. Introduction

Corrosion protection of metallic structures has attracted significant interest due to immense financial and safety misfortunes as a result of corrosion in various industries. Various industries utilize mild steel as an imperative construction material as it is extensively used and has diverse applications, thanks to its low price and exceptional physical and mechanical properties.¹⁻⁵ However, in spite of its compelling use, mild steel employed in industries is easily vulnerable to corrosion, particularly in acidic media. Consequently, owing to low resistance to corrosion, it is necessary to discover certain methods to safeguard mild steel from corrosion. Among various available methods, one of the widely used strategies is employing organic compounds as corrosion inhibitors for mitigating corrosion in acidic solutions.⁶⁻⁹ Several heterocyclic compounds containing heteroatoms (N, S, and O-atoms), π-bonds, and aromatic systems exhibit excellent anticorrosion performance.^10,11 Organic compounds of such a kind are easily adsorbed on the mild steel surface owing to the bonding of lone pairs and/or π-electrons with the metal surface, subsequently diminishing the corrosion.^12,13 Recently, Schiff bases (−N=CH), resulting from the reaction of aldehyde and amines, as proficient corrosion inhibitors have acquired a substantial amount of interest by the researchers.¹⁴⁻¹⁸ Inhibitors having both nitrogen and sulfur are profoundly efficient for corrosion inhibition in contrast with those containing either nitrogen or sulfur.¹⁹ Thiadiazoles are one of such class of compounds containing both nitrogen and sulfur whose Schiff bases have been reported and impacts of such compounds on corrosion inhibition have been examined.^20,21 In addition to this, thiadiazole derivatives also possess antimicrobial properties.^22,23 Noncytotoxic property of thiadiazole derivatives makes them environmental friendly inhibitors.^24,25 In continuation of the research on advancement of thiadiazole compounds as effective corrosion inhibitors in acidic media, the current work discusses the inhibitive performance of four thiadiazole bis-Schiff base derivatives, namely, N,N′-(1,4-phenylenebis(methanylylidene))bis(5-(methylthio)-1,3,4-thiadiazol-2-amine) (PMTTA), 5,5′-((1,4-phenylenebis(methanylylidene))bis(azanylylidene))bis(1,3,4-thiadiazole-2-thiol) (PATT), N,N′-(1,4-phenylenebis(methanylylidene))bis(5-methyl-1,3,4-thiadizol-2-amine) (PMTA), and N,N′-(1,4-phenylenebis(methanylylidene))bis(1,3,4-thiadiazol-2-amine) (PTA) on corrosion of mild steel in 1 M HCl by employing a weight loss technique, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), and modern surface characterization such as X-ray photoelectron spectroscopy (XPS). In addition to these, attenuated total reflectance (ATR) is performed to understand the adsorption of inhibitors by interacting with the mild steel. The effect of change in temperature in different electrolytic solutions of inhibitors is studied and different thermodynamic and kinetic parameters are also determined. All the experimental findings are correlated with the theoretical data using density functional theory (DFT), Fukui indices, and molecular dynamics (MD) simulations to describe the mechanism of effective coordination of the inhibitor with the surface of mild steel.

2. Result and Discussion

2.1. Synthesis

Chemical structures of the synthesized inhibitors were proved by Fourier transform infrared (FT-IR) and ¹H NMR. The ¹H NMR spectra of all the synthesized compounds are given in Figures S1–S4. The ¹H NMR and FT-IR spectral data for all inhibitors are presented as below:

2.1.1. PMTTA

IR (KBr, cm^–1): 1604 cm^–1 (C=N imine), 1446 cm^–1 (C=C stretching), 2924 cm^–1 (C–H stretching), 830 cm^–1 (C–S).

¹H NMR (400 MHz, DMSO-d₆) chemical shift (ppm): δ 8.63 (s, 1H), 7.64 (s, 2H), 3.53 (s, 24H), 2.68 (s, 1H), 2.51 (s, 99H), 1.25 (s, 2H).

2.1.2. PATT

IR (KBr, cm^–1): 1607 cm^–1 (C=N imine), 1560 cm^–1 (C=C stretching), 2928 cm^–1(C–H stretching), 756 cm^–1 (C–S).

¹H NMR (400 MHz, DMSO-d₆) chemical shift (ppm): δ 8.58 (dd, J = 18.0, 14.2 Hz, 1H), 8.01 (s, 1H), 3.47 (s, 22H), 2.60–2.36 (m, 21H).

2.1.3. PMTA

IR (KBr, cm^–1): 1619 cm^–1 (C=N imine), 1590 cm^–1 (C=C stretching), 2922 cm^–1(C–H stretching).

¹H NMR (400 MHz, DMSO-d₆) chemical shift (ppm): δ 10.15 (s, 1H), 8.13 (s, 1H), 7.55 (s, 2H), 3.55 (d, J = 120.6 Hz, 1H), 2.51 (s, J = 20.7 Hz, 50H), 2.46 (s, 1H), 1.25 (s, 1H).

2.1.4. PTA

IR (KBr, cm^–1): 1625 cm^–1 (C=N imine), 1494 cm^–1 (C=C stretching), 2920 cm^–1 (C–H stretching).

¹H NMR (400 MHz, DMSO-d₆) chemical shift (ppm): δ 8.79 (d, J = 12.0 Hz, 1H), 8.39–7.78 (m, 7H), 7.06 (s, 1H), 3.29 (d, J = 16.7 Hz, 62H), 2.49 (dd, J = 16.6, 1.0 Hz, 32H).

2.2. Weight Loss Analysis

2.2.1. Effect of Substituent and Inhibitor’s Concentration

All the four synthesized inhibitors, that is, PATT, PMTA, PMTTA, and PTA are found highly efficient for the corrosion inhibition of mild steel. On varying the concentration of the inhibitor used, change in the rate of corrosion and hence inhibition efficiency was observed as shown in Table 1. The weight loss measurements were done at 308 K. On increasing the concentration of the inhibitor, the corrosion rate declined and thus, inhibition ameliorated which is shown in Figure 1a. This illustrates that on increasing the concentration, inhibitor molecules are basically adsorbed on the metallic surface to the greater extent by providing a wider surface coverage. Consequently, the interaction between metal and acid solution is restricted. The effect of the substituent of the thiadiazole ring on corrosion was analyzed, and it was found to follow the trend as PMTTA > PATT > PMTA > PTA. Thus, the best inhibition performance was shown by PMTTA at 125 ppm, and E_WL % was determined as 92.6%. The superior inhibition efficiency of PMTTA might be due to the introduction of electron-donating functional groups like methylsulfanyl which increase the performance of inhibitor molecules to transfer electrons to the vacant d-orbitals of Fe, thus retarding the corrosion process, as it is well known that the efficiency of inhibitor molecule increases with its electron donating power capacity.²⁶ In any case, an additional increment in the concentration of the inhibitor affords no recognizable modification of inhibition efficiency which could be seen from Table 1 and Figure 1a. This might be due to the saturation of the surface. Hence, 125 ppm is chosen as the optimum concentration.

Table 1. Corrosion Parameters Obtained from Weight Loss Measurements for Mild Steel in 1 M HCl with Various Concentrations of PMTTA, PATT, PMTA, and PTA.

inhibitor	concentration of inhibitor (ppm)	weight loss in triplicate (mg)			mean weight loss (mg)	standard deviation (σ)	EWL % (mean value)	corrosion rate (mm y–1)
1 M HCl		121.4	122.1	122.5	122.0	0.39		90.41
PMTTA	10	28.7	29.1	29.2	29.0	0.18	76.2	21.49
	25	16.7	16.8	17.2	16.9	0.18	86.1	12.52
	50	13.6	14.2	14.2	14.0	0.24	88.5	10.38
	100	9.7	9.8	9.9	9.8	0.07	91.9	7.26
	125	8.8	9.0	9.2	9.0	0.14	92.6	6.67
	150	8.8	8.8	9.1	8.9	0.12	92.7	6.60
	175	8.6	8.7	9.1	8.8	0.18	92.7	6.52
PATT	10	36.1	37.2	37.4	36.9	0.49	69.7	27.35
	25	22.5	22.8	23.7	23.0	0.44	81.1	17.05
	50	15.8	15.9	16.0	15.9	0.07	87.0	11.78
	100	11.6	11.9	11.9	11.8	0.12	90.3	8.74
	125	10.8	10.9	11.3	11.0	0.18	90.9	8.15
	150	10.6	10.8	11.3	10.9	0.25	91.0	8.08
	175	10.6	10.9	10.9	10.8	0.12	91.1	8.00
PMTA	10	58.9	59.0	59.1	59.0	0.07	51.6	43.72
	25	47.5	47.9	48.0	47.8	0.18	60.8	35.42
	50	34.5	34.8	35.4	34.9	0.32	71.3	25.86
	100	22.7	22.9	23.1	22.9	0.14	81.2	16.97
	125	14.8	14.9	15.3	15.0	0.18	87.7	11.12
	150	14.6	14.8	15.3	14.9	0.25	87.7	11.04
	175	14.7	14.7	15.0	14.8	0.12	87.8	10.97
PTA	10	56.5	56.8	57.1	56.8	0.21	53.4	42.09
	25	52.5	53.0	53.2	52.9	0.25	56.6	39.20
	50	35.8	35.9	36.3	36.0	0.18	70.5	26.68
	100	30.8	30.8	31.1	30.9	0.12	74.6	22.90
	125	23.0	24.3	24.7	24.0	0.62	80.3	17.79
	150	23.5	23.9	24.3	23.9	0.28	80.4	17.71
	175	23.5	23.6	24.0	23.7	0.18	80.5	17.56

Figure 1.

(a) Variation of inhibition efficiency with the concentration of various inhibitors, (b) Arrhenius plot of log C_R vs 1/T, (c) plot of log C_R/T vs 1/T, and (d) Langmuir adsorption isotherm plots for all the synthesized inhibitors.

2.3. Calculation of Thermodynamic and Activation Parameters

Arrhenius and transition state equations have been employed to express the dependency of the corrosion rate on temperature. This is illustrated using the following equations

1

2

where E_a represents activation energy, λ is the pre-exponential factor, R is the universal gas constant, ΔH* is enthalpy of activation, and ΔS* is entropy of activation.

A straight line is observed from the plot of log C_R and 1/T having slope (−E_a/2.303R) and intercept log λ, as depicted in Figure 1b. Thus, using slope and intercept of the plot, energy of activation E_a and pre-exponential factor λ were calculated. The estimated activation energies for the all the inhibited solutions was found to be greater in comparison to the solution with bare acid, as summarized in Table 2. Higher the value of activation energy, E_a corresponds to increase in the energy barrier for the corrosion phenomenon.^27,28 As a result, inhibition efficiency is increased in the case of inhibited systems.

Table 2. Activation and Thermodynamic Parameters for Mild Steel Corrosion in 1 M HCl in the Absence and Presence of 125 ppm Concentration of Inhibitors PMTA, PMTTA, PTA, and PATT.

inhibitor	Ea (kJ mol–1)	λ (mg cm–2)	ΔH* (kJ mol–1)	ΔS* (kJ mol–1 K–1)	Kads (M × 104)	–ΔGads° (kJ mol–1)
1 M HCl	32.8	3.74 × 107	30.15	–108.95
PTA	40.3	1.14 × 108	37.64	–99.69	3.33	37.0
PMTA	45.9	7.32 × 108	43.29	–84.22	2.50	36.2
PATT	49.3	1.62 × 109	46.59	–77.65	10.0	39.8
PMTTA	51.3	3.13 × 109	48.66	–72.15	11.1	40.0

In accordance with eq 1, it could be visualized that the corrosion rate is dependent on both E_a and λ. In most of the cases, the effect of E_a is more as compared to λ, or in other words, E_a factor dominates over λ to determine the mild steel corrosion inhibition phenomenon. It is known that higher E_a and lower λ leads to a decreased corrosion rate.⁶ In the current case, values of both E_a and λ are increasing in order PTA < PMTA < PATT < PMTTA. Also, Table 2 depicts the order of corrosion rate as PTA > PMTA > PATT > PMTTA, which implies E_a as a primary decisive factor for the corrosion phenomenon of mild steel in 1 M HCl. An advantageous physical or chemical interaction is indicated by upward or downward change of free energy of activation.^19,29 In the present study, though the value of E_a increased for inhibited solution compared to that of bare acid solution but the change is not so large to consider the interaction as pure electrostatic or physical. Thus, in the present case, a mixed mode of interaction (though it is predominantly physical) is involved between the inhibitor and metal surface.

Similarly, with the help of slope and intercept of the graph log C_R/T and 1/T (presented as Figure 1c), enthalpy and entropy of activation are evaluated and also summarized in Table 2. Positive values of enthalpy of activation (ΔH*) indicated the endothermic behavior, resulting in a slow process of mild steel dissolution.^30,31

On correlating the values of the entropy of activation (ΔS*) depicted in Table 2, it is apparent that values of the entropy of activation increased in the presence of inhibitors than in the absence of inhibitors. The increment in ΔS* suggests the increase in disordering on going from the reactant to the activated complex which means randomness is increased. The adsorption of the organic substrate from the acidic solution on the mild steel is basically a quasi-substitution process among the organic compound in the aqueous phase and water molecules present at the mild steel surface. As a result, the adsorption of the organic inhibitor was accompanied by desorption of water molecules from the mild steel surface. Because of this phenomenon, an increase in entropy of activation was observed which might be due to an increase in solvent entropy.³²

2.4. Adsorption Isotherm

On the basis of adsorption of inhibitors on the metallic surface, a massive amount of information is presented by means of different adsorption isotherms. Among all adsorption isotherms, Langmuir adsorption isotherm was accounted to be the most acceptable to justify the adsorption of all inhibitors on the surface of mild steel on the grounds that the linear regression coefficient is observed very closest to unity.³³ Langmuir adsorption isotherm is well explained by the following equation

3

This relation basically briefs the dependency of the concentration of inhibitor (C_inh) with the surface coverage (θ) of the inhibitor on the surface of metals. The percentage surface coverage is defined as the fraction of the surface covered by an inhibitor molecule and can be calculated as per eq 4.

4

The graph of C_inh/θ versus C_inh (Figure 1d) obtained at 308 K gives straight lines, and hence, adsorption of inhibitors comply Langmuir adsorption isotherm more accurately as compared to other isotherms. K_ads signifies affinity among the adsorbent and adsorbate. Higher values of K_ads inferred greater adsorption and subsequently way better inhibition efficiency.³K_ads can be ascertained using the intercepts of the straight lines, which are summarized in Table 2.

Equation 5 represents the relation between the Gibbs free energy of adsorption and equilibrium constant of adsorption.

5

where R is gas constant, T is absolute temperature, and K_ads represents the equilibrium constant of an adsorption–desorption process.

The negative value of Gibbs free energy of adsorption (ΔG_ads^°) signifies spontaneity of the process; that is the inhibitor molecules are effectively adsorbed on the surface of mild steel.³⁴ It can be seen that the electrostatic interaction and hence physisorption of the inhibitor molecule to the metal surface is characterized by ΔG_ads ≤ −20 kJ mol^–1. However, a highly negative value of free energy of adsorption (>40 kJ mol^–1) signifies a co-ordinate type of bond, that is, chemisorption.⁹ In this case, ΔG_ads^° values are calculated as −37, −36.2, −39.8, and −40 kJ mol^–1 for PTA, PMTA, PATT, and PMTTA, respectively, as given in Table 2. This suggests that the mechanism involves both physisorption and chemisorption, that is, mixed mode of interaction.

2.5. FT-IR Analysis

FT-IR analysis has been performed for the identification and confirmation of different functional groups present in the synthesized compounds. Graphs depicting all the peaks are given in Figure 2. It could be seen from the graphs that peak at 1625, 1607, 1619, and 1604 cm^–1 for PTA, PATT, PMTA, and PMTTA, respectively, confirmed the presence of C=NH (imine group) in the synthesized compounds, thereby confirming the product formation. The FT-IR spectrum was also recorded in the ATR mode and represented in Figure 2. All the characteristic peaks obtained for the inhibitors were found to shift to the lower frequency which confirms the interaction between various functional groups present in the inhibitor and Fe²⁺.

Figure 2.

FT-IR spectra of pure synthesized compounds (a) PMTTA, (b) PATT, (c) PMTA, and (d) PTA and their film deposited on the surface of mild steel.

2.6. Electrochemical Techniques

2.6.1. Potentiodynamic Polarization Studies

Figure 3a–d demonstrates the potentiodynamic polarization plots for all the inhibitors, that is, PMTA, PMTTA, PTA, and PATT in 1 M HCl solution. Extrapolation was done for cathodic and anodic lines and the corresponding intersection results in various polarization parameters, that is, corrosion current density (i_corr), corrosion potential (E_corr), anodic slope (β_a), and cathodic slope (β_c), which are given in Table 3. It can be inferred from the data that i_corr values are suppressed on increasing the concentration of inhibitors. This indicated that the inhibitors are strongly adsorbed on the surface of mild steel, thereby enhancing the corrosion inhibition. Also, the trend for inhibition efficiency follows the same order, that is, PMTTA > PATT > PMTA > PTA as interpreted from the weight loss study. The literature describes that if the variation in the value of E_corr is greater than 85 mV with reference to the value obtained with the blank acid solution, then the inhibitor is said to be a cathodic or anodic inhibitor while if the difference in the E_corr value is less than 85 mV, and it is termed as a mixed type inhibitor.³⁵ In our current work, it could be seen that the E_corr shift for all the described inhibitors with respect to 1 M HCl is less than 85 mV, indicating the mixed type but predominantly cathodic behavior for all described inhibitors. The data of Table 3 reflects that a more pronounced change in the value of β_c than that of β_a, which suggests predominantly the cathodic nature of inhibitors. This mixed type but predominantly cathodic nature of the inhibitors may be due to effective adsorption of the protonated inhibitor molecule on the cathode.

Figure 3.

Potentiodynamic polarization curves of mild steel in 1 M HCl with various concentrations of (a) PMTTA, (b) PATT (c) PMTA, and (d) PTA.

Table 3. Tafel Polarization Parameters for Mild Steel in the Absence and Presence of Different Concentrations of PMTA, PMTTA, PTA, and PATT in 1 M HCl.

inhibitor	concentration (ppm)	–Ecorr (mV vs SCE)	icorr (μA cm–2)	βa (mV dec–1)	βc (mV dec–1)	EPDP % (mean value)	standard deviation (σa)	CR (mm y–1)
1 M HCl		445	3165.4	104.1	174.3		1.52	37.20
PMTTA	10	480	803.3	98.8	195.2	74.6	1.00	9.42
	25	477	502.3	91.3	188.4	84.1	0.84	5.89
	50	476	466.5	79.3	185.2	85.3	1.18	5.47
	100	485	411.0	102.7	188.7	87.0	0.82	4.82
	125	482	302.1	97.8	170.2	90.4	1.25	3.54
PATT	10	490	1497.0	119.8	151.4	52.7	1.70	17.57
	25	478	1113.0	107.4	183.4	64.8	1.61	13.06
	50	482	919.2	85.7	182.5	71.0	0.94	10.78
	100	481	553.6	107.3	272.2	82.5	1.35	6.49
	125	492	373.6	113.1	184.6	88.2	1.00	4.38
PMTA	10	477	1763.7	117.1	158.4	44.3	0.50	20.70
	25	492	1522.7	126.5	160.9	51.9	0.68	17.87
	50	461	1217.2	120.8	198.6	61.5	1.61	14.28
	100	455	687.9	95.2	169.4	78.3	0.70	8.07
	125	481	553.6	107.3	272.2	82.5	0.45	6.49
PTA	10	489	2017.1	120.6	172.6	36.3	0.30	23.67
	25	488	1755.3	122.5	166.9	44.5	0.90	20.60
	50	465	1195.1	119.0	246.6	62.2	1.18	14.02
	100	480	1035.9	117.5	199.0	67.3	1.59	12.15
	125	482	705.3	90.2	196.2	77.7	1.85	8.27

^aCalculated from corrosion current density.

2.6.2. Electrochemical Impedance Spectroscopy

Nyquist plots for mild steel immersed in 1 M HCl solution with and without inhibitors are presented in Figure 4a–d. All the impedance parameters, obtained after fitting the equivalent circuit to the experimental data, are shown as Table 4.

Figure 4.

Nyquist plots of mild steel in 1 M HCl in the absence and presence of different concentrations of (a) PMTTA, (b) PATT (c) PMTA, and (d) PTA and (e) equivalent circuit used to fit the experimental data.

Table 4. Electrochemical Impedance Parameters for Mild Steel in the Absence and Presence of Different Concentrations of PMTA, PMTTA, PTA, and PATT in 1 M HCl.

name of inhibitor	conc. of inhibitor (ppm)	Rs (Ω cm2)	Rct (Ω cm2)	RL (Ω cm2)	L (H)	Q (μF·s(n–1))	n	Cdl (μF cm–2)	EEIS (%)
1 M HCl		0.430	2.40	0.8	10.3	719.2	0.801	157.1
PMTTA	10	0.858	18.16	2.6	7.8	185.3	0.852	68.91	86.8
	25	0.686	25.03	3.3	7.2	150.6	0.863	62.1	90.4
	50	0.623	34.80	3.1	6.5	124.5	0.868	54.4	93.1
	100	0.759	42.60	4.7	5.5	100.8	0.871	44.9	95.0
	125	0.507	47.80	4.9	4.2	90.9	0.874	41.5	94.9
PMTA	10	0.613	6.2	1.6	8.9	302.1	0.831	84.2	61.3
	25	1.421	8.8	2.5	8.0	260.4	0.837	79.7	72.7
	50	0.704	10.0	2.6	7.4	231.6	0.839	72.3	76.0
	100	0.660	15.9	2.7	6.4	200.9	0.840	67.3	84.9
	125	0.996	18.8	2.8	5.7	175.6	0.842	60.1	87.3
PATT	10	0.514	8.70	2.2	8.1	202.1	0.846	63.7	72.4
	25	0.490	11.50	2.3	7.7	176.2	0.855	61.6	79.1
	50	0.760	18.30	2.9	6.2	145.6	0.859	55.1	86.9
	100	0.501	34.80	3.1	5.8	125.1	0.860	51.7	93.1
	125	0.588	46.00	3.8	4.4	104.6	0.863	44.8	94.8
PTA	10	0.608	3.70	0.6	9.6	423.2	0.815	97.7	35.2
	25	0.478	7.3	0.7	8.8	315.3	0.819	82.4	67.1
	50	0.613	8.5	0.7	8.1	279.4	0.822	75.5	71.8
	100	0.662	9.6	0.9	7.0	248.2	0.829	71.4	75.0
	125	0.554	12.2	1.2	5.9	221.8	0.835	68.9	80.3

The impedance behavior of mild steel in HCl solution in the absence and with inhibitors, PATT, PTA, PMTA, and PMTTA, was controlled by the combined effect of resistance, capacitance, and inductance. However, the occurrence of a major portion of the Nyquist plot in positive Y-axis emphasized that the resistance and capacitance of the system controlled its impedance behavior. This is due to the occurrence of the electrical double layer or development of oxide layer at the MS–acid solution interface. The shape of Nyquist plots is depressed semicircles instead of perfect semicircle. The depression is associated with roughness and inhomogeniety of the metal surface.³⁶ The Nyquist plots obtained in this study consists of two loops; one capacitive loop at higher frequency followed by a low-frequency inductive loop. The appearance of inductive loops at lower frequency might be due to more effective adsorption of inhibitors or due to the relaxation process obtained by the adsorption of Cl_ads^– and H_ads⁺ on the surface of the electrode. However, the size of inductive loops increased regularly with increasing concentration of inhibitors which indicates readsorption of the inhibitor over the MS surface.

Considering it, double layer capacitance (C_dl) in the circuit was substituted by a constant phase element (CPE) in the equivalent circuit to obtain more accurate and optimized fit. The equivalent circuit employed for fitting all the experimental results is depicted in Figure 4e.

The impedance (Z_CPE) can be estimated by the following equation

6

where Q represents the magnitude of CPE, j is an imaginary number (j² = −1), ω is angular frequency, and n is the phase exponent which represents the degree of irregularity.

Angular frequency (ω) is attained from eq 7 described below at the frequency with the highest imaginary impedance.

7

Double layer capacitance values are estimated by applying the following relation

8

Value of n is always between 0 and 1 as it justifies the difference from an ideal behavior. As per the Helmholtz model depicted in eq 9, double layer capacitance is inversely linked to the thickness of the electrical double layer which acts as a protective layer.

9

where d is the thickness of the double layer, ε is dielectric constant, ε₀ is vacuum permittivity, and A is an area of the electrode.

So it is apparent from the data in Table 4 that on increasing the concentration of inhibitors, a gradual decrease in the value of C_dl is observed (68.9–41.5 for PMTTA; 84.2 to 60.1 for PMTA; 63.7 to 44.8 for PATT; and 97.7 to 68.9 for PTA), which reflects a lowering in local dielectric constant or/and enlargement in the thickness of the double layer.³⁷ This implies that the addition of the studied inhibitors led to the reduction in C_dl values which might be due to the substitution of water molecules by inhibitor molecules at the electrode surface. As a consequence of which, an increment in surface coverage of mild steel by the inhibitor molecules was observed which increased the efficiency of an inhibitor. Also, the decrease in C_dl leads to a decrease in active area (A), as stated in eq 9 that indeed supports the effectiveness of the inhibitors in providing a surface film that inhibits the anodic dissolution of the metal.

Further, it could be illustrated from the curves that on the addition of the inhibitor, the impedance behavior of mild steel is substantially altered and diameter of the semicircle obtained in the Nyquist plots is enlarged subsequently in the presence of inhibitors. The low solution resistance with all the inhibitor indicated that inhibitor’s solution had no impact on the impedance behavior especially in the high-frequency region.

Bodes-phase angle plots in the absence and with all the four inhibitors are represented in Figure 5a–d. The phase angle curves exhibit single maxima at an intermediate frequency range which signifies one time constant. This could be seen from the graphs that in the case of inhibited solutions, phase angles are somewhat higher as compared to 1 M HCl solution, leading to increased surface smoothness of the mild steel with all inhibitors. More negative values of the phase angle leads to more capacitive behavior which means there is more adsorption of inhibitors on the surface and thus more will be the surface smoothness. Furthermore, broadening of the curves is observed which confirms the accumulation of the molecule of inhibitors on the surface of mild steel. Also, the effect gets more pronounced on increasing the concentration of inhibitors.²⁸ Now, considering all the factors, on comparing all the inhibitors, PMTTA was found to be most efficient with a maximum inhibition efficiency of 94.6% at 125 ppm concentration. The order of E_EIS % was attained as PMTTA > PATT > PMTA > PTA.

Figure 5.

(a–d) Bode-phase angle plots of mild steel in 1 M HCl in absence and presence of different concentrations of all inhibitors.

2.7. Surface Characterization

2.7.1. SEM–EDX

Figure 6a–f presents the SEM images of the surface of mild steel in 1 M HCl solution with and without optimum concentration of the inhibitors. The Figure 6a represents the mild steel surface before immersion in acid solution. The mild steel surface got damaged when exposed to bare acid solution, as shown by Figure 6b. On the contrary, the mild steel surface remained intact appreciably by the presence of inhibitors (Figure 6c–f), indicating suppression in the corrosion phenomenon. This finding followed the other experimental results. The smoother texture of mild steel samples exposed to inhibited acid solutions (Figure 6c–f) compared to that which is exposed to bare acid solution (Figure 6b) indicated that the mild steel surfaces were prevented by the attack of an aggressive solution by the presence of inhibitors.

Figure 6.

SEM images of mild steel for (a) MS surface before immersion in 1 M HCl, (b) after immersion in 1 M HCl, (c) after immersion in 1 M HCl +125 ppm of PMTTA, (d) after immersion in 1 M HCl + 125 ppm of PATT, (e) after immersion in 1 M HCl + 125 ppm of PMTA, and (f) after immersion in 1 M HCl + 125 ppm of PTA.

The formation of the protective film due to adsorption of inhibitors on the surface of mild steel is further confirmed by scanning their energy-dispersive spectroscopy (EDX) spectra. The EDX plots and elemental composition are shown in Figure 7a–f. It could be seen that EDX spectra of mild steel in blank HCl constitutes peak only for iron and carbon. However, new additional peaks are obtained in the EDX spectra in the presence of bis-Schiff bases which are characteristic of nitrogen and sulfur which confirms the presence of these elements on the surface of mild steel. This implies the adsorption of bis-Schiff bases on the surface which inhibits the corrosion. In addition to this, EDX spectra demonstrate that the percentage composition of iron subsequently decreases on the addition of bis-Schiff bases which may be due to surface coverage (Table S1).The deposition of different elements over the MS surface can be seen as mapping images provided in Figures S5–S10.

Figure 7.

EDX spectra of mild steel for the (a) MS surface before immersion in 1 M HCl, (b) after immersion in 1 M HCl, (c) after immersion in 1 M HCl + 125 ppm of PMTTA, (d) after immersion in 1 M HCl + 125 ppm of PATT, (e) after immersion in 1 M HCl + 125 ppm of PMTA, and (f) after immersion in 1 M HCl + 125 ppm of PTA.

2.7.2. AFM

The topography of the mild steel samples was analyzed with the help of an atomic force microscope. Figure 8a–f demonstrates 2-D and 3-D AFM images of mild steel after immersing in different HCl solutions for 3 h. Also, Figure 9a–f represents a roughness profile diagram of mild steel after immersing in different HCl solutions for 3 h.

Figure 8.

Atomic force micrographs of mild steel (a) before immersion in 1 M HCl, (b) after immersion in 1 M HCl in the absence of an inhibitor, (c) after immersion in 1 M HCl + 125 ppm PMTTA, (d) after immersion in 1 M HCl + 125 ppm PATT, (e) after immersion in 1 M HCl + 125 ppm PMTA, and (f) after immersion in 1 M HCl + 125 ppm PTA.

Figure 9.

Surface roughness profile diagram of mild steel (a) before immersion in 1 M HCl solution, (b) after immersion in 1 M HCl, (c) after immersion in 1 M HCl + 125 ppm PMTTA, (d) after immersion in 1 M HCl + 125 ppm PATT, (e) after immersion in 1 M HCl + 125 ppm PMTA, and (f) after immersion in 1 M HCl + 125 ppm PTA.

It is apparent from Figure 8b that the average roughness of abraded mild steel before immersion in acid solution was found to be 22.2 nm while the average roughness of the mild steel sample immersed in 1 M HCl solution reached the maximum value, that is 1335 nm (Figure 8b). This infers that mild steel corroded by aggressive HCl solution and developed some cracks. However, the values of average roughness for inhibited solutions were somewhat reduced in comparison to the uninhibited solution and were found to follow the order PMTTA < PATT < PMTA < PTA, and the values obtained were 22.9, 39.3, 69.1, and 291 nm respectively. This may be due to the adsorption of the inhibitors on the surface of mild steel which helps in protection from getting corroded. The effect of an inhibitor on the texture of the surface of mild steel can be seen from the roughness profile diagram presented in Figure 9.

2.7.3. XRD Analysis

XRD is known to be an excellent technique for material characterization, identification of crystalline phase, and quantitative phase analysis. Thus, the XRD method could be utilized for the characterization of corrosion products obtained after immersion of mild steel in aggressive HCl solution. The supremacy of the inhibitor, PMTTA leads us to select it to study its effect on the XRD pattern. Figure 10 depicts the comparison of XRD patterns of corrosion products formed on the mild steel in 1 M HCl in the absence and presence of the inhibitor, PMTTA. The XRD pattern for mild steel immersed in 1 M HCl shows three peaks at 2θ = 44.88, 82.38, 65°. When the XRD pattern of mild steel immersed in inhibited acid solution (1 M HCl + 125 ppm PMTTA) was scanned, the same three peaks were observed at 2θ = 44.66, 64.27, and 82.34°. Peaks at 2θ = 44.66, 44.88, 82.34, and 82.38° are assigned metallic iron (Fe). The peak at 2θ = 64.27 and 65° is characteristic for oxides of iron such as iron oxy-hydroxide (FeOOH).³⁸ Thus, it can be observed that the peak due to FeOOH is present in the presence of an inhibitor but its intensity is reduced. Also, the increase in the intensity of Fe peaks is observed. This clearly indicates the protection of the metal surface from corrosion by forming a protective layer of the inhibitor which resists the formation of any corrosion products such as oxides of oxy-hydroxides.

Figure 10.

XRD patterns of corrosion products formed on the mild steel in 1 M HCl in the absence and presence of the inhibitor, PMTTA.

2.7.4. X-Ray Photoelectron Spectroscopy

The composition of the film developed by adsorption of different substances on the mild steel surface was detected by XPS. The XPS spectra of mild steel were scanned in the absence and presence of the inhibitor, PMTTA, which is predicted to be a highly efficient compound among all. The XPS spectrum of mild steel immersed in bare HCl solution was also obtained in order to compare and confirm the adsorption of the inhibitor on the surface of mild steel. The XPS spectra in the absence and presence of PMTTA are presented in Figure 11a–i. The deconvolution procedure is used for fitting various peaks in order to assign peak for respective elements. This could be done by making use of XPS Peak Fit 4.1 software.

Figure 11.

XPS spectra for mild steel immersed in 1 M HCl solution without an inhibitor: (a) C 1s; (c) Fe 2p; and (e) O 1s and XPS spectra for mild steel immersed in 1 M HCl solution with the inhibitor, PMTTA: (b) C 1s; (c) Fe 2p; (f) O 1s; (g) N 1s; and (h) S 2p and (i) a survey scan for both inhibited as well as the uninhibited solution.

The C 1s spectrum of the uninhibited system shows three peaks at 248.9, 285.1, and 288.4 eV, whereas four peaks, that is, 248.8, 285.2, 286.2, and 288.2 eV are ascribed for the inhibited system as given in Figure 11a,b. Peaks at 248.9, 285.1, 284.8, and 285.2 eV can be due to C–H, C–C, and C=C and also might be due to the presence of contaminant hydrocarbons on the surface for uninhibited and inhibited systems, respectively.^39,40 The peak at 286.2 eV in the PMTTA-inhibited system is due to the presence of C=N and C–N bond in imine and thiadiazole species or the C–S bond which confirms the adsorption of the inhibitor on the mild steel surface.^39,41 The peak at 288.4 and 288.2 eV for uninhibited and inhibited solution, respectively,⁴² might be due to carbon contamination on the surface due to O–C=O.

The XPS spectra of Fe 2p can be deconvoluted into two peaks corresponding to Fe 2p_3/2 and Fe 2p_1/2. Fe 2p_3/2 shows four peaks, that is, 710.8, 713, 715, and 719.8 eV for blank 1 M HCl, while three peaks for the inhibited system, that is, PMTTA which are given as 710.8, 713, 715.4 eV. These are presented in Figure 11c–d. The peak at 710.8 eV is ascribed as a ferric compound Fe³⁺ of Fe₂O₃ and/or FeO(OH).^39,43 However, Fe₂O₃ and FeO(OH) were differentiated from the O 1s XPS spectrum. The peak at 713 eV is ascribed to FeCl₃ accumulated on the metal surface.⁴¹ The peak at 715.4 eV in inhibited solution is characteristic of Fe(II) species.⁴⁴ This means there is no transformation of Fe(II) to Fe(III) in the presence of PMMTA, or in other words, there is the existence of stable ferrous compounds on the PMTTA-treated surface. This effectively supports the anti-corrosion activity of PMTTA as an inhibitor as the inhibitor molecules are coordinated with ferrous species to form a stable and insoluble PMMTA–Fe(II) complex which retards the corrosion process of mild steel.⁴⁵ The peaks appeared at 715 and 719.8 eV for 1 M HCl are attributed to the satellites of Fe(II) and Fe(III), respectively.⁴⁶ Similarly, Fe 2p_1/2 can be deconvoluted into two peaks, that is, 724.4 and 725.9 eV for an uninhibited solution while one peak is obtained at 724.4 eV for an inhibited solution. These peaks are also represented to Fe³⁺ species, that is, Fe₂O₃ and FeO(OH).⁴⁷ In addition, the intensity of Fe peaks is somewhat lower in the case of an inhibited system as compared to the uninhibited system.

The XPS spectrum of O 1s consists of three peaks for blank 1 M HCl while two peaks in the case of an inhibited system, as presented in Figure 11e,f. The peaks located at 530 and 530.1 eV can be attributed as the peak of O^2– of ferric oxides Fe₂O₃ or/and Fe₃O₄, which means the oxygen atom that is coordinated to the Fe atom in oxides, respectively, for uninhibited and inhibited solutions.^43,47,48 Other peaks located at 531.6 and 531.5 eV are interpreted as OH^– of hydrous iron oxides FeO(OH) which usually appears at 531.7 ± 0.2 eV.⁴⁹ The latter peak at 529.9 eV in uninhibited, that is, blank 1 M HCl is ascribed to oxygen atoms of absorbed water molecules. However, this peak is absent in the case of the inhibited solution. This means that the absorbed water molecules present on the surface are being replaced by the inhibitor molecules.⁴⁸

The high-resolution N 1s XPS spectrum can be deconvoluted into two peaks at 399.3 and 400.1 eV, as displayed in Figure 11g. The first peak at around 399.3 eV is representative of the unprotonated C–N and −C=N bond which is present itself in the inhibitor.^39,50 On the other hand, the second peak at 400.1 eV is basically ascribed to the N–Fe bond which arises due to coordination of nitrogen in the inhibitor complex with Fe on the steel surface.³⁹

The XPS spectrum for S 2p can be deconvoluted into three peaks located at 161.8, 163.7, and 168.5 eV, as presented in Figure 11h. The first peak centered on 161.8 eV might be due to disulfide species, that is, FeS₂. The peak centered on 163.7 eV is attributed to neutral sulfur (−S) in the inhibitor. The third peak at 168.5 eV is ascribed to sulfur coordinated to Fe to form the S–Fe complex and/or it can also be due to sulfur atoms which are in a more positive environment.⁴¹

In addition to this, a survey scan for both inhibited as well as the uninhibited solution is presented in Figure 11i, which represents all the elements presents on the surface of mild steel. The data drawn from XPS analysis for blank 1 M HCl and PMTTA steel surface gives direct evidence of adsorption of the PMTTA inhibitor on the mild steel surface.

2.8. DFT Calculations

2.8.1. Global Reactivity Descriptors

Quantum chemical calculations offer unprecedented and precise information into the geometric and electronic structure of newly synthesized corrosion inhibitors.⁵⁵ Herein, DFT calculations using the hybrid functional B3LYP, generalized gradient approximated (GGA), and meta-GGA functionals are conducted to investigate global and local reactivity descriptors of four corrosion inhibitors. Optimized geometry and Frontier molecular orbital’s density distribution obtained using meta-GGA are depicted in Figure 12. Some of the quantum chemical parameters are shown in Table 5.

Figure 12.

Optimized molecular structures and Frontier molecule orbital density distributions of investigated compounds obtained using m-GGA functional.

Table 5. Computed Quantum Chemical Parameters for the Four Inhibitor Molecules Using B3LYB, GGA, and mGGA Functionals.

methods	inhibitors	EHOMO (eV)	ELUMO (eV)	ΔEgap (eV)	ΔN110
GGA	PMTTA	–5.307	–3.566	1.741	0.22
	PTA	–5.854	–3.665	2.189	0.027
	PATT	–5.482	–3.647	1.835	0.139
	PMTA	–5.698	–3.47	2.228	0.105
mGGA	PMTTA	–5.723	–3.691	2.032	0.05
	PTA	–6.787	–3.259	3.528	–0.057
	PATT	–5.855	–3.782	2.073	0.0007
	PMTA	–6.13	–3.513	2.617	–0.0005
B3LYB	PMTTA	–6.381	–3.125	3.256	0.02
	PTA	–6.786	–3.261	3.525	–0.057
	PATT	–6.411	–3.263	3.148	–0.005
	PMTA	–6.806	–3.053	3.753	–0.029

The assumption is that a corrosion inhibitor with a relatively flat (or planar) orientation can cover a large surface area when binding to a metal surface.⁵⁶ On the other hand, functional groups are very recognized by their ability to increase the interactive force of an inhibitor molecule.⁵⁷ From the results, we can get valuable conclusions for the geometry of each inhibitor molecule. We can see that the molecular structure of PTA and PATT compounds seem to be more planar than others, that is, PMTA and PMTTA. The introduction of methyl and sulfanyl groups at both thiadiazole nuclei alters the planarity of PMTA and PMTTA while an unobvious effect was observed in the case of the thiol group. In the case of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) distribution, we can observe that the isodensity in HOMO and LUMO of all compounds is equally distributed throughout the entire molecular structure of inhibitors. Particularly, in the case of PMTTA, the LUMO orbital is more concentrated on the phenyl and its linked (sulfanyl) thiadiazole while the HOMO orbital is mainly dispersed on the other (sulfanyl) thiadiazole. The distribution of HOMO and LUMO orbitals in all compounds is very similar to each other. This could lead to further increased adsorption of inhibitors on the metal surface through dual interactions, that is, interactions between the unshared electron pair of heteroatoms and vacant d-orbitals of iron atoms. Unpaired electrons are attracted to the metal surface, which makes them more reactive.

The calculated quantum chemical parameters, that is, the highest occupied molecular orbital energy (E_HOMO), lowest unoccupied molecular orbital energy (E_LUMO), the energy gap (ΔE_gap = E_LUMO – E_HOMO), and the number of transferred electrons (ΔN) are given in Table 5. The HOMO energy corresponds to the ionization potential and thus represents the propensity of an inhibitor molecule to share an electron.⁵⁸ Higher HOMO energy potentially leads to a higher electron donating tendency, whereas the LUMO energy corresponds to the electron affinity and it is a measure of the electron transport level.⁵⁹ Lower LUMO energy means a higher electron-accepting tendency of an inhibitor molecule. The donor–acceptor interactions can also be measured by the number of transferred electrons from the corrosion inhibitors to the metal surface. A positive ΔN value means the ability of an inhibitor molecule to share its electrons and vice-versa for a negative value, and it usually follows the HOMO energy order.⁶⁰ According to published studies,^59,61 the energy gap (ΔE) is the primary key factor predicting chemical reactivity and stability of inhibitor molecules. As the value of ΔE is decreased, an inhibitor molecule becomes more reactive. The data in Table 5 suggests that the effect of LUMO energy to predict the inhibition efficiency was inaccurate in all conducted methods. In contrast, except in B3LYB results, the trend of E_HOMO is PMTTA > PATT > PMTA > PTA, which agrees with the inhibition efficiency order. The same trend can be observed from ΔN values. These outcomes showed that the ability of inhibitor molecules to donate electrons was improved by introducing electron-donating functional groups like methyl and sulfanyl, which explains the superior inhibition efficiency of PMTTA. From Table 5, we also noticed that the trend for the ΔE values obtained from GGA and B3LYB results failed to agree with that from experimental results while the results from mGGA show a good agreement. Unsurprisingly, these outcomes show that the reactivity of inhibitor molecules increases as the electron-donating power increases. However, despite their importance, these data are, in some cases, insufficient to provide a full understanding of factors controlling inhibitor’s effectiveness and fail to provide a precise correlation. Next, we investigated the local reactivity of our compounds using Fukui functions and got some useful insights.

2.8.2. Local Reactivity: Fukui Functions

Besides global reactivity descriptors, which might be helpful for the prediction of reactive parts in inhibitor molecules, actually, Fukui functions and dual descriptor indices provide a useful tool for the prediction of local reactive sites and are used as a key analysis tool in identifying potential atoms responsible for the inhibitor performance.⁶² An atomic site with more positive charges (f_k⁺) is favorable for a nucleophile to attack, while an atomic site having more surface negative charges (f_k^–) is favorable for an electrophile to attack.⁶² The various condensed Fukui functions along with the dual descriptor of our compounds are calculated by applying Hirshfeld population analysis, which is depicted in Table S2. To well understand the key factors governing the inhibitor performance, it is very important to determine the effect of the functional groups on the reactivity of heteroatoms, which are definitely the main reason responsible on inhibitor effectiveness. The data in Table S2 indicate that all reactive sites in inhibitor PTA have almost an equal strength in terms of electrophilic and nucleophilic power. The addition of functional groups to this compound affects the reactivity of its reactive sites, and a tendency toward the nucleophilic or electrophilic character is clearly observed in three-substituted compounds. This leads to an obvious effect on the interactive force of each compound and thus its corrosion inhibition performance. The existing data from the literature show that organic compounds containing heteroatoms are considered as promising corrosion inhibitors; thus heteroatoms have an extraordinary potential to improve the inhibition efficiency of those compounds.⁵⁷ A deeper analysis of results in Table S2 shows that PMTTA and PMTA have widespread distribution and substantial values of f_k^– sites, which reflect the higher ability to donate electrons. This could explain the superior inhibition performance and the strong interaction of PMTTA and PMTA with the metal surface compared to other compounds. If we now consider reactive site distribution in these compounds, that is, PMTTA and PMTA, we can see that the major difference between them comes from the presence of S₁₇ and S₂₂ in PMTTA, which do not exist in PMTA. This leads us to conclude that the electron-donating sulfanyl groups attached to thiadiazole rings have a greater effect on inhibition efficiency than methyl groups. Thus, a better inhibition performance was obtained for this compound (PMTTA).

2.9. MD Simulations

Although powerful in investigating electronic properties of an inhibitor molecule, quantum chemical parameters are not enough to predict the trend of the inhibition efficiency of inhibitors with high accuracy. Therefore, it is imperative to search for a robust method for the modeling of inhibitor–metal interactions in an environment that can mimic the real experimental condition.^63,64 The results from MD simulations provide a basis for judging an inhibition performance of an inhibitor molecule and whether it can effectively protect a metal surface.⁶⁵ Herein, the interaction of four inhibitor molecules and the Fe(110) surface was analyzed and their adsorption configuration on the metal surface was obtained, as shown in Figure 13. By carefully inspecting the results summarized in Figure 13, almost all inhibitor molecules showed a strong binding affinity toward the iron surface and displayed a nearly flat or parallel disposition to the metal surface. Further, the outcomes reveal that the four inhibitor molecules exhibited a similar adsorption profile, and only small deviations from complete parallel adsorption can be observed. In such a situation, the direct close contact of the inhibitor molecule with the surface of mild steel would facilitate its adsorption and help in the formation of a compact adsorption film. Furthermore, the widely dispersed potential reactive sites of inhibitor’s molecule could provide a large effective coverage area and strong interactions. As we have seen in the HOMO and LUMO orbitals distribution and Fukui function indices, a large number of potential nucleophilic and electrophilic sites could have a very strong impact on inhibitor–metal interactions. On the other hand, the interaction and binding energy (BE) are a significant criterion to estimate extent of the adsorption with inhibitor molecules. Higher interaction energy basically indicates better adsorption of an inhibitor molecule with an iron surface.⁶⁶ Interestingly, all inhibitor molecules have strong interaction energy and its increasing trend follows the experimental results, as shown in Table 6. However, the data in Table 6 clearly show that the PMTTA compound gives the maximum BE (defined as the inverse value of the interaction energy) during the whole simulation process. Thus, PMTTA is accepted as the most efficient inhibitor compared to other inhibitors, and the result is consistent with DFT/mGGA results.

Figure 13.

Side view and top view of equilibrium adsorption configurations of different inhibitors, PMTTA, PATT, PMTA, and PTA on the Fe (1 1 0) surface obtained by MD simulations.

Table 6. MD Parameters for the Four Compounds Adsorbed on the Fe(110) Surface in the Presence of Solvent Species.

simulation models	Einteraction (kJ mol–1)
PMTTA	–758.79
PTA	–497.87
PATT	–691.69
PMTA	–633.13

As previously mentioned, the presence of an electron-rich moiety along with π-electron should affect the electronic structure of an inhibitor molecule and help it to easily expel the water molecules from the metal surface.⁶⁷ It is apparent from the results achieved from MD simulations were in good consistency with the results obtained from meta-GGA calculations.

3. Conclusions

The present research work can be summarized by the following conclusions:

• 1.All the synthesized thiadiazole bis-Schiff base derivatives, that is, PTA, PMTA, PATT, and PMTTA were proven to be effective corrosion inhibitors for mild steel in 1 M HCl medium. The inhibition efficiency was found to abide the order PMTTA > PATT > PMTA > PTA.
• 2.Potentiodynamic polarization data revealed the mixed type nature of all the inhibitors. EIS experiments showed that the values of R_ct increased and double layer capacitance declined in the presence of bis-Schiff bases as a result of adsorption of these on the metal surface.
• 3.Surface characterization such as SEM–EDX, AFM, and XPS reflected the protective barrier of inhibitors on the steel surface mitigating corrosion.
• 4.All the experimental findings were in fine consistency with the theoretical studies viz. DFT, Fukui indices, and MD simulations which effectively explained the relation of the inhibition efficiency with molecular structures of inhibitors.

4. Experimental Section

4.1. Material and Sample Preparation

The composition of mild steel employed for performing corrosion experiments is depicted in Table 7. HCl (37%, Merck India) was used to prepare 1 M HCl solution from double distilled water. Stock solution of all the inhibitors was prepared in 1 M HCl and ethanol (used in 9:1 ratio) and then was diluted to prepare various concentrations of inhibitors. All the weight loss and electrochemical analysis were carried out in 1 M HCl solution having a distinctive amount, viz. 10, 25, 50, 100, and 125 ppm, of an inhibitor. Prior to each experiment, the surface of mild steel was polished with emery papers of 200, 400, 600, 1000, 1200, and 1500 grades and was cleaned altogether with distilled water and acetone.

Table 7. Chemical Composition of Mild Steel.

elements	C	Si	P	Mn	S	Ni	Cr	Fe
% wt	0.05	0.009	0.012	0.20	<0.01	0.0025	0.001	remainder

4.2. Synthesis of Inhibitors

All the thiadiazole-derived bis-Schiff bases were synthesized by refluxing terephthaldehyde (0.01 mol) with different derivatives of 2-amino-1,3,4-thiadiazole (0.02 mol), that is, 1:2 molar ratio in ethanol (40 mL) using hydrochloric acid as a catalyst for 8–10 h, as per scheme shown in Figure 14. The solid precipitate thus obtained was filtered and dried. The synthesized compounds were re-crystallized using ethanol in order to obtain pure compounds.

Figure 14.

Scheme of synthesis of all the four inhibitors.

4.3. FT-IR Analysis

FT-IR analysis was performed using a Thermo Fisher FT-IR spectrometer for confirmation and identification of different functional groups present in the synthesized compounds. In addition, the ATR mode was also recorded in order to identify and understand the interaction between inhibitor molecules and the metallic surface.

4.4. Weight Loss Analysis

Weight loss estimations have been carried out using mild steel strips of size 2.5 × 2.0 × 0.025 cm³. Weight loss measurements were performed in 1 M HCl as per ASTM G1⁶⁸ with varied concentration of inhibitors specified above at 308 K. Weight loss analysis was performed at 308 K as it was chosen parallel to the room temperature. The strips were suspended into different solutions in the hanging position in a conical flask using rubber corks and by ensuring complete immersion of strips in the solution. After completion of experiments, mild steel strips were weighed accurately. A digital electronic weighing balance has been used for accurate weighing measurements of mild steel specimens. All the experiments have been performed by maintaining constant temperatures with the help of a digital thermostat (Macwin India Ltd.). Weight loss studies were likewise performed at various temperatures, that is, 318, 328, and 338 K to analyze the effect of temperature on the corrosion phenomenon. The loss of weight of mild steel strips was computed which is basically the difference of weight before and after immersion in the solution. All the experiments were performed in triplicate sets to get precise results, and standard deviations were estimated as given in Table 1. Inhibition efficiency (E_WL %) and corrosion rate (C_R; mm y^–1) can be calculated from weight loss measurement as per eqs 10 and 11 as²⁹

10

11

where w₀ represents weight loss of mild steel strip in 1 M HCl, w_i is weight loss of mild steel bearing various inhibitors, w signifies weight loss (mg) of mild steel, A is area of mild steel strip utilized (cm²), t is time (h), and D represents the density of mild steel (g cm^–3).

4.5. Electrochemical Techniques

Electrochemical experiments were performed using SP-240 Bio-Logic Instrument with EC lab software for analyzing the data. A three electrode cell setup was prepared for electrochemical measurements maintained at 308 K. A platinum electrode was used as a counter electrode, a saturated calomel electrode (SCE) as a reference electrode, and mild steel as the working electrode. Rectangular mild steel strips of size 1.0 × 1.0 × 7.5 cm³ with only 1 cm² area exposed to the electrolyte and the remaining being covered by using commercially available lacquer were used to carry out all measurements.

The electrochemical setup was allowed to stand for 0.5 h to obtain the steady open circuit potential (OCP) prior to each and every electrochemical measurements. Potentiodynamic polarization data were recorded in the potential range of ±250 mV with respect to OCP with a scan rate 1 mV s^–1. Cathodic and anodic curves obtained were extrapolated using Tafel fit to obtain corrosion potential (E_corr) and corrosion current density (i_corr) directly using the software.

Using the values of corrosion current densities, the inhibition efficiency from potentiodynamic polarization (E_PDP %) could be determined using the following equation

12

where i_corr⁰ is corrosion current density for 1 M HCl, that is, in the absence of an inhibitor while i_corr is corrosion current density in the presence of an inhibitor.

Using eq 13, the value of the corrosion rate (C_R) can also be calculated using corrosion current density as

13

where i_corr represents corrosion current density in mA cm^–2 and equiv wt and d are the equivalent weight and density of mild steel, respectively.

EIS studies were done in the frequency range of 100 kHz to 0.01 Hz with a sinus amplitude of 20 mV using an AC signal at OCP. EIS parameters were analyzed using Z Fit in EC Lab software. Further, the inhibition efficiency from EIS (E_EIS %) can be evaluated using charge transfer resistance as per the following equation

14

where R_ctⁱ and R_ct represent charge transfer resistance in the presence and absence of an inhibitor in the electrolyte solution, respectively.

4.6. Surface Characterization

4.6.1. SEM–EDX

The surface morphology of mild steel strips was analyzed by a scanning electron microscope (Hitachi TM3000) instrument. Also, the elemental detection was carried out by recording EDX spectra with Oxford SwiftED 3000. The analysis was performed by using 1 × 1 × 0.025 cm³ mild steel strips which were immersed in 1 M HCl in the absence and presence of 125 ppm concentration of all the inhibitors. After 3 h, strips were cleaned with acetone, then dried, and subsequently used for analysis. SEM examination was done at an accelerating voltage of 15 kV, and images were obtained at 2500k× magnification. EDX data was also obtained at 2500k× magnification at a voltage ≥ 15 kV.

4.6.2. Atomic Force Microscopy

For AFM characterization, mild steel strips of sizes 2 × 2.5 × 0.025 cm³ were immersed in 1 M HCl in the absence and presence of 125 ppm concentration of different inhibitors for 3 h. The strips were thereafter cleaned and finally subjected for AFM characterization. The atomic force microscope of Model-Bruker, Dimension ICON with ScanAsyst was utilized to scan the surface of mild steel in the tapping mode. The images were scanned in the chosen area of 10 × 10 μm². Nanoscope Analysis software 8.2 was used for analyzing the atomic force micrographs of MS samples.

4.6.3. XRD Analysis

For XRD characterization, mild steel strips were immersed in 1 M HCl solution in the absence and presence of 125 ppm PMTTA for 3 h. The surface film formed on mild steel for both uninhibited and inhibited acid solution was analyzed using a X-ray diffractometer (model: Rigaku Ultima IV, Ri). The diffraction patterns obtained were recorded in the 2θ range of 30–90° with a scan rate 5°/min.

4.6.4. X-ray Photoelectron Spectroscopy

X-ray photoemission spectroscopic (XPS) measurements were achieved in a multiprobe surface analysis system (Scienta Omicron, Germany) at a base pressure of 5 × 10^–11 Torr. XPS spectra were recorded via nonmonochromatic Mg Kα (1253.6 eV) radiation sources. The high-resolution XPS spectra were recorded at a pass energy of 20 eV for CLs and 5 eV for the valence band spectra. The BE of the photoemission line and spectrometer work function calibration was carried out referring Au 4f_7/2 emission line and Au Fermi level.

4.7. Theoretical Studies

4.7.1. Quantum Chemical Calculations

To theoretically understand the origin of the inhibitor’s effectiveness and to identify the most influential parameters that may govern the inhibition efficiency, we carried out multiple quanta chemical calculations. Quantum chemical parameters and Fukui functions were determined from DMol³ (quantum mechanical code under DFT approximation of a Materials Studio Software).^69,70 DFT calculations at generalized gradient first-principles approximation, GGA, using the Perdew, Burke, and Ernzerhof formalism recognized as PBE, the meta-generalized gradient approximation (meta-GGA) exchange–correlation density functional and the Becke’s three-parameter hybrid functional using the LYP correlation functional (B3LYP) were done to investigate the electronic properties of tested compounds.⁷¹⁻⁷⁴ The COSMO implicit solvent model was utilized for performing all the calculations.⁷⁵ The ionization energy and the electronic affinity were determined using the energies of the HOMO and LUMO orbitals and from which, the electronegativity and the global hardness of the extract molecules were calculated⁷⁶

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The fraction of transferred electrons (ΔN) was calculated using the following equation⁷⁷

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The work function (ϕ) of Fe(110) was mainly accepted as 4.82 eV, whereas the absolute hardness η_Fe of iron was determined at 0 as I = A for bulk metals.^78,79

The condensed Fukui functions and the dual descriptor were predicted on the basis of Hirschfield population analysis and the finite difference approximation as follows⁸⁰

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In the abovementioned equations, q_k represents the electronic population of an atomic site within a molecule in its neutral (N), anionic (N + 1), or cationic (N – 1) state.

4.7.2. MD Simulations

MD simulations were carried to generate insights on interactions between inhibitor molecules and the iron surface using Materials Studio package.⁶⁹ The Fe(110) plane was used as the metallic substrate as a result of its higher stabilization energy and its extremely packed structure.⁸¹ Two layers were constructed: a solvent layer with water molecules (491), chlorine, and hydronium ions (9) along with an inhibitor molecule and the iron substrate layer. Both layers were collected in one simulation box (24.82 × 24.82 × 35.69 ų) and optimized by steepest descent and conjugated gradient algorithms.⁸² The COMPASS force field⁸³ and the NVT canonical ensemble were used for all simulations. The simulations were completed by time step of 1 fs and simulation time of 2000 ps at 303 K, which is controlled using the Andersen algorithm.⁸⁴ The interaction and the binding energies (E_Binding = −E_interaction) were determined when the system attains the equilibrium state by using the following equation⁸⁵

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In the above equation, E_{surface+solution} signifies the total energy of the Fe(110) and solution without an inhibitor molecule, E_inhibitor denotes the total energy of an inhibitor molecule alone, and E_total is the total energy of the full system.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.9b04274.

• ¹H NMR spectra of synthesized compounds, mapping of different elements over the mild steel surface before and after immersion in different electrolytic solutions, percentage composition of the mild steel samples immersed in different aggressive solutions, and Fukui function and dual descriptor indices of studied compounds calculated at DFT/m-GGA (PDF)

The authors declare no competing financial interest.