Inhibition behaviour of mild steel by three new benzaldehyde thiosemicarbazone derivatives in 0.5 M H2SO4: experimental and computational study.

Three new benzaldehyde thiosemicarbazone derivatives namely benzaldehyde thiosemicarbazone (BST), 4-carboxyl benzaldehyde thiosemicarbazone (PBST) and 2-carboxyl benzaldehyde thiosemicarbazone (OCT) were synthesized and their inhibition effects on mild steel corrosion in 0.5 M H2SO4 solution were studied systematically using gravimetric and electrochemical measurements. Weight loss results revealed that PBST exhibited the highest inhibition efficiency of 96.6% among the investigated compounds when the concentration was 300 µM. The analysis of polarization curves indicated that the three benzaldehyde thiosemicarbazone derivatives acted as mixed type inhibitors and PBST and OCT predominantly anodic. The adsorption process of all these benzaldehyde thiosemicarbazone derivatives on Q235 steel surface in 0.5 M H2SO4 solution conformed to Langmuir adsorption isotherm. Scanning electron microscopy was conducted to show the presence of benzaldehyde thiosemicarbazone derivatives on Q235 mild steel surface. The results of theoretical calculations were in good agreement with that of experimental measurements.

Introduction

Mild steel has been applied as a popular construction material in petroleum, food, chemical and engineering industries [1-3]. However, mild steel is easily corroded in acidic solutions when they are serving in industrial washing, acid de-scaling and oil well acidization [4-6], which may cause significant economic losses and security risks. The use of inhibitors to prevent or minimize the considerable damage of mild steel in acid environment has been found to be one of the most economical and efficient methods [7-10]. It is generally believed that organic compounds containing heteroatoms such as nitrogen, oxygen and sulfur, or their molecular structure containing heterocyclic rings or polar functional groups serve as excellent organic inhibitors in acidic media [10]. The reason is that these compounds can form a strong chemical bond with the metal on the solid/liquid surface through charge transfer [11-13] and block the active site on the mild steel surface, thereby resisting the corrosion of mild steel in corrosive environment [14-17].

Gravimetric measurements [18,19], potentiodynamic polarization curves [20-22] and electrochemical impedance spectroscopy (EIS) [23-26] are classical methods to evaluate the inhibition behaviour. Theoretical calculation is a powerful technique to establish the relationship between the inhibition behaviour and molecular structure [27-29], which is helpful for designing a more effective inhibitor molecule. Moreover, some useful parameters, including the energy of the highest occupied molecular orbital, the energy of the lowest unoccupied molecular orbital, the energy gap and dipole moment can supply important information about the inhibition mechanism.

The objective of the present work is to investigate the inhibition behaviour of Q235 mild steel in 0.5 M H2SO4 solution containing three new synthesized benzaldehyde thiosemicarbazone derivatives namely benzaldehyde thiosemicarbazone (BST), 4-carboxyl benzaldehyde thiosemicarbazone (PBST) and 2-carboxyl benzaldehyde thiosemicarbazone (OCT) (table 1). The reason of choosing these compounds is that, firstly, these compounds contain various adsorption centres including oxygen, nitrogen and sulfur heteroatoms and –NH2, –OH functional groups. Secondly, benzaldehyde thiosemicarbazone exhibited high inhibition efficiency for iron-base metallic glassy alloy in 0.5 M H2SO4 solution at 30°C, as previous reported [30]. Thirdly, these inhibitors can be easily synthesized with high yield. The study was carried out using weight loss measurement, potentiodynamic polarization curves, EIS and scanning electron microscopy (SEM). The correlation between the inhibition efficiencies of different substitution on the thiosemicarbazone compounds is discussed. Moreover, theoretical calculations were conducted to evaluate the inhibition mechanism.

Table 1.

Physical and chemical properties of the synthesized compounds.

no.	molecular structure	abbreviations	structure characterizations
1		BST	C8H9N3S (mol. wt. 179)M.P. 164–165°CIR spectrum (KBr, cm−1)3399, 3144, 1599, 1283, 1098
2		PBST	C9H9O2N3S (mol. wt. 223)M.P. 197–199°CIR spectrum (KBr, cm−1)3482, 3148, 1613, 1277, 1092
3		OCT	C9H9O2N3S (mol. wt. 223)M.P. 203–205°CIR spectrum (KBr, cm−1)3173, 1606, 1270, 1105

Methods

Materials

The studied benzaldehyde thiosemicarbazone derivatives were synthesized according to the literature [30,31] and the chemical reaction equation is shown in figure 1. Table 1 depicts the physical and chemical properties of the synthesized compounds.

Figure 1.

Chemical reaction equation of the studied compounds.

Weight loss measurements

Weight loss experiments were performed in 500 ml 0.5 M H2SO4 solution containing different concentrations of benzaldehyde thiosemicarbazone derivatives. The testing time is 8 h at 298 K. Square specimens of Q235 carbon steel having dimensions 50 × 25 × 5 mm were used for the gravimetric tests. The chemical composition of Q235 carbon steel is shown in table 2. The Q235 specimens were accurately weighed after degreasing with acetone and drying in N2. After 8 h corroding time, the Q235 samples were moved out and the surface was scrubbed with a bristle brush, and then weighed again. For each case, at least triplicate experiments were conducted and the average results are reported.

Table 2.

Chemical composition (mass fraction, wt%) of Q235 mild steel samples.

C	Mn	Si	S	P	Fe
0.16	0.53	0.30	<0.055	<0.045	Bal.

The corrosion rate for Q235 mild steel was derived from the following expression [32]:where W is the mass loss of Q235 mild steel without and with addition of inhibitors in milligrams, A equals 32.5 cm2 in our experimental condition, t is the testing time of 8 h, ρ is the Q235 mild steel density of 7.86 × 103.

Thus, the inhibition efficiency (IE%) and surface coverage (θ) can be obtained from the corrosion rate using the following equation [32]:andwhere CRinhi and CRfree are the obtained corrosion rates of Q235 mild steel with and without benzaldehyde thiosemicarbazone derivatives in 0.5 M H2SO4 solution, respectively.

Potentiodynamic polarization studies

The electrochemical measurements were carried out in a cylindrical glass cell of 250 ml with traditional three electrodes. A saturated calomel electrode (SCE) and a large platinum foil were employed as reference and counter electrode, respectively. Cylindrical Q235 mild steel sealed with Teflon was used as working electrode and the exposed area was 0.50 cm2. The exposed surface was abraded with fine sand paper and then polished to mirror using 2.5 µm diamond paste, cleaned with double-distilled water and finally immersed into the electrochemical glass cell containing 0.5 M H2SO4 solution without and with different concentrations of benzaldehyde thiosemicarbazone derivatives for at least 1 h. When the open circuit potential (Eocp) reached a steady state, the potentiodynamic polarization study was performed using CHI660A electrochemical workstation at a scan rate of 1 mV s−1. The scanning potential ranges from Eocp − 250 mV to Eocp + 250 mV. Tafel extrapolation method was employed to obtain some useful parameters, including corrosion potential (Ecorr), corrosion current density (jcorr), anodic and cathodic Tafel slopes. The inhibition efficiency η (%) is then derived from the corrosion current density as follows:where and are the obtained corrosion current densities with and without BST, PBST and OCT inhibitors, respectively.

Electrochemical impedance experiments

EIS was conducted using PARSTAT 2273 measurement unit in the frequency range of 10 mHz–100 kHz at Eocp and the scanning always initiated from high frequency to low frequency. The voltage amplitude was 5 mV. Each concentration was repeatedly tested three times or more and the average results were calculated. The EIS experimental data was analysed using Z-View software. The inhibition efficiency ηEIS (%) can be obtained from the charge transfer resistance as follows:where and Rct are the charge transfer resistance for Q235 mild steel in uninhibited and inhibited solution, respectively.

Scanning electron microscopy measurements

The Q235 mild steel surface after corroded in 0.5 M H2SO4 solution at 298 K without and with inhibitors was observed with SEM model Hitachi SU80 instrument at an accelerating voltage of 5 kV at 2000× magnification. EDX detector model coupled with SEM was used to evaluate the surface composition of Q235 mild steel.

Theoretical calculations

As described in previous literature [33,34], the geometric optimizations of the synthesized derivatives and quantum chemical calculations were performed using the functional hydride B3LYP density functional theory (DFT) formalism. During the calculations, the electron basis set 6-31G (d, p) in the standard Gaussian-03 software package was employed. As a result, some useful quantum chemical parameters, such as energy of the lowest unoccupied molecular orbital (ELUMO), the energy of the highest occupied molecular orbital (EHOMO), the energy gap (ΔE) between LUMO and HOMO, the ionization potential (I), the electron affinity (A), dipole moment, the global hardness (η) and the global softness (σ) were calculated.

Results and discussion

Weight loss tests

The inhibitive effect of the synthesized benzaldehyde thiosemicarbazone derivatives (BST, PBST and OCT) for Q235 mild steel in 0.5 M H2SO4 solution at 298 K was initially investigated with weight loss measurements. The calculated values of corrosion rate, inhibition efficiency and surface coverage are summarized in table 3. Obviously, the corrosion rate decreased considerably with addition of these compounds compared to the blank, which may be related to the strong adsorption of these compounds onto Q235 mild steel surface and forming a protective physical barrier to resist the acid attack [35]. It is clear that with increasing the inhibitor concentration, the inhibition efficiency also increased and the highest value was found to be 85.7%, 96.6% and 93.4% for BST, PBST and OCT compounds, respectively at 300 µM, suggesting that these inhibitors effectively inhibited the Q235 mild steel corrosion in acidic medium. It can also be deduced from table 3 that at the same concentration, the inhibition efficiency follow the order: PBST > OCT > BST, indicating that PBST exhibits the best inhibitive performance compared to other two benzaldehyde thiosemicarbazone derivatives. This result may be correlated with its molecular structure of –COOH functional group at the ρ-substitution (table 1).

Table 3.

The weight loss parameters for Q235 steel in 0.5 M H2SO4 containing different concentrations of BST, PBST and OCT inhibitor at 298 K.

inhibitor	Cinh (μM)	CR (mg cm−2 h−1)	η (%)	θ
blank	0	6.09	—
BST	50	2.75	54.8	0.548
	100	1.71	71.9	0.719
	200	1.12	81.6	0.816
	300	0.87	85.7	0.857
PBST	50	2.33	61.7	0.617
	100	1.24	79.6	0.796
	200	0.66	89.2	0.892
	300	0.21	96.6	0.966
OCT	50	2.11	65.4	0.654
	100	1.37	77.5	0.775
	200	0.76	87.5	0.875
	300	0.40	93.4	0.934

Open circuit potential curves

The open circuit potential for Q235 mild steel in 0.5 M H2SO4 solution without and with 300 µM BST, PBST and OCT inhibitor is depicted in figure 2. It can be seen that the OCP reached a steady state after 1 h immersion time. Apparently, the OCP value moved in the negative direction with addition of 300 µM BST, PBST and OCT inhibitor compared to the blank. This shift may be correlated to the adsorption of these compounds on mild steel surface.

Figure 2.

Open circuit potential for Q235 mild steel in 0.5 M H2SO4 solution without and with 300 µM BST, PBST and OCT inhibitor.

Potentiodynamic polarization curves

Figure 3 shows the polarization curves for Q235 mild steel in 0.5 M H2SO4 solution containing different concentrations of BST, PBST and OCT inhibitor. Apparently, the current densities of both the anodic and cathodic branches decreased with addition of benzaldehyde thiosemicarbazone derivatives, indicating that both the anodic and cathodic reaction rates were resisted which was generally due to the adsorption of these inhibitors at the active sites on the surface. It is noticeable that the shape of polarization curves without inhibitors is similar to that with addition of these three inhibitors. This phenomenon demonstrated that the addition of these inhibitors did not change the corrosion mechanism of Q235 mild steel dissolution in 0.5 M H2SO4 solution [36] and the inhibitive effect of these inhibitors is originated from the coverage of inhibitor molecules at the active sites to restrain their exposure to the acidic environment. In addition, it can be seen that there was no obvious trend observed in the Ecorr values for BST inhibitor compared to the blank, which moves to the positive direction with less than 85 mV in the presence of PBST and OCT inhibitors, suggesting that these inhibitors were of mixed type and PBST and OCT predominantly anodic [37-39]. The values of Ecorr, jcorr, cathodic and anodic Tafel slopes, and η (%) are listed in table 4. Obviously, jcorr values decreased remarkably with addition of these inhibitors compared to the uninhibited. With increasing inhibitors concentration from 0 to 300 µM, the values of jcorr decreased from 375.8 µA cm−2 to 50.2, 34.8 and 36.1 µA cm−2 for BST, PBST and OCT inhibitors, respectively. Therefore, η exhibited a maximum value of 87.1%, 90.7% and 90.4% for BST, PBST and OCT inhibitor, respectively. Observation of table 4 shows that the inhibition efficiency obeys the order: PBST > OCT > BST, which is in good accordance with the gravimetric tests.

Figure 3.

Polarization curves for Q235 mild steel in 0.5 M H2SO4 solution containing different concentrations of (a) BST, (b) PBST and (c) OCT inhibitor.

Table 4.

Polarization parameters for Q235 mild steel corroded in 0.5 M H2SO4 containing different concentrations of BST, PBST and OCT inhibitor at 298 K.

inhibitor	Cinh(μM)	Ecorr(mV)	βa(mV dec−1)	−βc(mV dec−1)	jcorr(μA cm−2)	ηP (%)
blank	0	−479.1	113.9	102.4	375.8
BST	50	−477.2	119.5	105.0	218.5	41.9
	100	−479.9	121.5	102.6	122.9	67.3
	200	−478.8	116.7	97.7	54.1	85.6
	300	−475.6	109.0	100.2	50.2	87.1
PBST	50	−457.1	102.9	110.1	167.3	55.5
	100	−448.6	105.2	108.3	67.2	82.1
	200	−458.8	98.6	110.4	51.9	86.2
	300	−460.4	100.4	108.2	34.8	90.7
OCT	50	−456.2	98.4	126.8	198.6	47.1
	100	−450.6	102.1	124.3	90.6	75.9
	200	−455.8	98.3	112.9	53.0	85.8
	300	−459.3	100.9	121.5	36.1	90.4

Electrochemical impedance spectroscopy measurements

The representative Nyquist plots for Q235 mild steel dissolution in 0.5 M H2SO4 solution at 298 K in the absence and presence of different concentrations of BST, PBST and OCT inhibitors are shown in figure 4. It is apparent that the Nyquist plots were considerably influenced after the addition of these inhibitors into 0.5 M H2SO4 solution, which diameter was greater than that in the blank solution, suggesting that the Q235 mild steel dissolution process was remarkably restrained by these inhibitors. As observed, for the uninhibited case, the impedance spectrum contains only one depressed capacitive loop, while after the addition of BST, PBST and OCT inhibitors, the Nyquist plots show two capacitive loops which may correspond to the double electric layer and film capacitance, respectively. Obviously, the diameter of the capacitive loops became larger with the increase of inhibitor concentration. Figure 3 also shows that the centres of the impedance loops are below the real axis. This finding indicates a non-ideal electrochemical behaviour at the metal/solution interface [40,41], which generally resulted from the surface roughness and heterogeneities [42,43]. Therefore, a constant phase element CPE (Q) was used to replace capacity [44] and the thus improved electrochemical equivalent circuit (EEC) is depicted in figure 4, which was used to analyse the EIS data. The admittance of a CPE can be calculated using the following expression [45]:where Y0 is the magnitude, j equals −1, ω is the angular frequency, and n is the phase shift, representing the surface inhomogeneity [46] (figure 5).

Figure 4.

Nyquist plots for Q235 mild steel corrosion in 0.5 M H2SO4 solution at 298 K containing different concentrations of (a) BST, (b) PBST and (c) OCT.

Figure 5.

EEC model used to simulate the EIS data. (Rs: solution resistance, Qdl: double layer capacitance; Rct: charge transfer resistance; Qf: film capacitance; Rf: film resistance).

The fitted results are summarized in table 5. It can be seen that the values of Rct increased upon rising of inhibitors concentration, suggesting a higher corrosion resistance by the adsorption of these inhibitors onto Q235 mild steel surface. The Rct values reached 408.9, 485.3 and 460.8 Ω cm2 for BST, PBST and OCT inhibitors respectively, when their concentration was 300 µM. Accordingly, the inhibition efficiency exhibited a maximum value of 91.8%, 93.2% and 92.8% for BST, PBST and OCT, respectively. Oppositely, the Qdl values decreased with the increase of inhibitors concentration. The reason may be that the synthesized benzaldehyde thiosemicarbazone derivatives adsorbed on the metal surface and formed a stronger chemical bond with steel than water molecules, thus the previously absorbed water molecules were replaced [47,48]. Additionally, the values of n were all near to 1 in the absence and presence of these inhibitors (table 5), suggesting the homogeneous nature of the surface. Moreover, it is worth noting that the inhibition efficiencies of PBST and OCT inhibitor are higher than that of BST at the same concentration, which may be correlated to the presence of –COOH functional group in the molecular structure.

Table 5.

EIS parameters for Q235 mild steel in 0.5 M H2SO4 containing different concentrations of BST, PBST and OCT inhibitor.

	Cinh(μM)	Rs(Ω cm2)	Qf(Ω−1 sn cm−2)	Rf(Ω cm2)	Rct(Ω cm2)	Qdl (Ω−1 sn cm−2)	ndl	ηEIS(%)
blank	0	0.88 ± 0.02			33.4 ± 1.1	175 ± 6.1	0.968	—
BST	50	1.05 ± 0.03	242 ± 10	2.16 ± 0.08	78.4 ± 1.8	84.6 ± 4.3	1	57.4
	100	0.99 ± 0.03	206 ± 8	2.44 ± 0.04	126.9 ± 3.2	72.2 ± 2.0	1	73.7
	200	1.02 ± 0.02	169 ± 4	12.1 ± 0.16	306.4 ± 8.3	56.6 ± 2.2	0.945	89.1
	300	1.02 ± 0.03	132 ± 5	16.8 ± 0.12	408.9 ± 7.6	51.4 ± 3.1	0.922	91.8
PBST	50	1.08 ± 0.02	213 ± 7	9.24 ± 0.14	175.6 ± 5.2	73.6 ± 3.5	0.948	80.9
	100	0.93 ± 0.02	182 ± 5	11.2 ± 0.16	252.3 ± 4.9	71.4 ± 2.4	0.916	86.7
	200	0.96 ± 0.03	130 ± 3	13.6 ± 0.12	371.6 ± 8.2	46.4 ± 1.3	0.953	91.0
	300	0.89 ± 0.02	114 ± 2	17.4 ± 0.23	485.3 ± 7.1	32.2 ± 0.9	0.942	93.2
OCT	50	0.84 ± 0.03	225 ± 6	3.58 ± 0.11	113.4 ± 2.6	84.6 ± 1.7	0.974	70.5
	100	1.03 ± 0.02	196 ± 4	6.26 ± 0.15	242.2 ± 5.6	58.2 ± 1.3	0.906	86.2
	200	1.06 ± 0.02	143 ± 3	11.4 ± 0.12	356.5 ± 8.2	41.2 ± 1.2	0.930	90.6
	300	0.97 ± 0.03	128 ± 4	15.7 ± 0.14	460.8 ± 11	24.8 ± 0.8	0.961	92.8

Surface investigation

SEM images of Q235 mild steel samples after being corroded in 0.5 M H2SO4 solution without and with addition of 300 µM BST, PBST and OCT inhibitors are shown in figure 6. It is observed that the Q235 mild steel surface was strongly damaged without inhibitors (figure 6a), while the surface was smooth and compact when 300 µM inhibitors was added (figure 6b–d), indicating that benzaldehyde thiosemicarbazone derivatives formed a protective physical barrier on the mild steel surface and retarded the aggressive acid attack. The presence of these inhibitors was further confirmed by EDX spectra, as shown in figure 7. It is worth noting that no characteristic peaks for nitrogen (N) and sulfur (S) can be found in the uninhibited solution (figure 7a), whereas both of them appeared on Q235 mild steel surface in 0.5 M H2SO4 solution containing 300 µM inhibitors (figure 7b–d), which indicated the presence of these inhibitor molecules to form a protective film on the Q235 mild steel surface. Moreover, the percentage atomic contents of elements obtained from EDX measurements for Q235 mild steel samples in the absence and presence of 300 μM BST, PBST and OCT is given in table 6. It also can be seen that the percentage atomic content of Fe reduced sharply with addition of 300 µM BST, PBST and OCT inhibitors compared to the blank, which was due to the surface coverage of these inhibitor molecules on Q235 mild steel surface.

Figure 6.

Surface morphology of Q235 mild steel after being corroded in 0.5 M H2SO4 solution at 298 K in the absence (a) and presence of 300 µM (b) BST, (c) PBST and (d) OCT.

Figure 7.

EDX spectra of Q235 mild steel surface in 0.5 M H2SO4 at 298 K in the absence (a) and presence of 300 µM (b) BST, (c) PBST and (d) OCT.

Table 6.

EDX spectra results for mild steel samples in the absence and presence of 300 µM BST, PBST and OCT.

inhibitor	Fe	C	N	S
blank	70.78	29.22	—	—
BST	67.84	27.53	2.82	1.81
PBST	62.54	26.52	6.64	4.30
OCT	64.65	26.65	5.07	3.63

Adsorption isotherm

To further explore the adsorption mechanism of BST, PBST and OCT inhibitors onto Q235 mild steel in 0.5 M H2SO4 solution, different adsorption isotherms, including Langmuir, Flory-Huggins, Temkin, Freundlich and Frumkin isotherm models were employed. In the present study, a linear relationship between c/θ values and inhibitors concentration c was established, as shown in figure 8, which indicated that the adsorption of these inhibitors on Q235 mild steel surface in 0.5 M H2SO4 solution conformed to Langmuir adsorption isotherm with the following expression [49,50]:where Kads is the equilibrium constant of inhibitors adsorption onto Q235 mild steel surface, which values can be obtained from the intercept of figure 8. According to the relationship between the standard free energy of adsorption and Kads, can be calculated from the value of Kads using the following equation [51]:where R is the molar gas constant and T is the absolute temperature. The calculated equilibrium constant Kads and standard free energy of adsorption are summarized in table 7.

Figure 8.

Langmuir isotherm for adsorption of (a) BST, (b) PBST and (c) OCT molecules onto Q235 mild steel in 0.5 M H2SO4 solution.

Table 7.

The values of Kads and for Q235 mild steel in the presence of BST, PBST and OCT inhibitors in 0.5 M H2SO4 solution.

inhibitor	Kads(×104/M)	ΔGads0 (kJ mol−1)	slope	R2
BST	2.62	−35.1	1.04	0.999
PBST	3.26	−35.7	0.98	0.999
OCT	2.97	−35.4	1.00	0.992

In our present measurements, the values of are found to be −35.1, −35.7 and −35.4 kJ mol−1 for BST, PBST and OCT inhibitors, respectively. It is reported that the adsorption of organic inhibitor molecules onto metal surface follows physical adsorption through electrostatic interaction when the value of was positive to −20 kJ mol−1, which conforms to chemisorptions involving charge sharing or charge transfer between the metal surface and inhibitor molecules when value was negative to −40 kJ mol−1 [52-54]. Therefore, it is reasonable to deduce that the adsorption process of BST, PBST and OCT inhibitors onto Q235 mild steel surface is a combination of both chemisorptions and physisorption, which are predominant chemisorptions.

Effect of temperature

To further obtain the thermodynamic and activation parameters, weight loss experiments were performed at different temperatures ranging from 25°C to 55°C. The calculated corrosion rate and inhibition efficiency under different temperatures are listed in table 8. It is apparent that the corrosion rate increases with raising temperature for all these inhibitors. Meanwhile, the inhibition efficiency increases with temperature as well, which corresponds to the chemisorptions mechanism of inhibitor molecule onto metal surface. This phenomenon has been explained by the specific interaction between inhibitor molecule and mild steel [55-58]. The apparent activation energy (Ea) is calculated by Arrhenius equation [59],where Ea is the apparent activation energy and A is the Arrhenius pre-exponential factor.

Table 8.

Weight loss parameters for Q235 mild steel corroded in 0.5 M H2SO4 solution with addition of 200 µM BST, PBST and OCT inhibitors at different temperatures.

inhibitor	temperature(°C)	blankCR (mg cm−2 h−1)	200 µMCR (mg cm−2 h−1)	η (%)
BST	25	6.09	1.12	81.6
	35	6.79	1.24	81.7
	45	7.83	1.39	82.2
	55	9.16	1.59	82.6
PBST	25	6.09	0.66	89.2
	35	6.79	0.70	89.7
	45	7.83	0.75	90.4
	55	9.16	0.80	91.3
OCT	25	6.09	0.76	87.5
	35	6.79	0.83	87.8
	45	7.83	0.92	88.3
	55	9.16	1.01	90.0

The Ea values were obtained from the slope of Arrhenius plot as shown in figure 9 and the results are shown in table 9. It is obvious that Ea values for the Q235 mild steel dissolution in 0.5 M H2SO4 solution containing 200 µM inhibitors were smaller than that in the uninhibited. It was previously reported that the adsorption of organic inhibitor molecules follows chemisorptions mechanism when Ea value was unchanged or lower compared to the blank [37,60], which was explained by some of the energy being consumed in the chemical reaction. This finding furthermore supports the conclusion that was inferred from the Langmuir isotherm that the adsorption behaviour of the synthesized compounds on mild steel surface conforms to chemisorptions.

Figure 9.

Log CR versus 1/T for Q235 mild steel in 0.5 M H2SO4 solution.

Table 9.

The apparent activation energy of mild steel corroded in 0.5 M H2SO4 without and with addition of 200 µM BST, PBST and OCT inhibitors.

inhibitor	Ea (KJ mol−1)
blank	11.1
BST	9.45
PBST	5.25
OCT	7.76

Effect of immersion time

The impact of immersion time on the inhibition efficiency for 300 µM BST, PBST and OCT inhibitors onto Q235 mild steel in 0.5 M H2SO4 solution at 298 K is shown in figure 10. It can be seen that during the initial 8 h, the inhibition efficiency increased with immersion time for PBST and OCT inhibitor, whereas 12 h for BST inhibitor, which may be correlated to the film growth and rearrangement of the BST, PBST and OCT inhibitor molecules on Q235 surface. After that, the inhibition efficiency decreased with prolonging immersion time, which may be linked to desorption or dissolution of adsorbed inhibitor molecules [61]. It is noticeable that during the whole testing immersion time, the inhibition efficiency of PBST and OCT inhibitors is higher than that of BST inhibitor. Moreover, the inhibition efficiencies of all these inhibitors were still over 90% after 96 h immersion time, suggesting that these synthesized inhibitors were all long-term effective inhibitors for Q235 mild steel in 0.5 M H2SO4 solution.

Figure 10.

Effect of immersion time on the inhibition efficiency for 300 µM BST, PBST and OCT inhibitors onto Q235 mild steel in 0.5 M H2SO4 at 298 K.

Quantum chemical calculations

To explore the correlation between the inhibition behaviour and molecular structures, the quantum chemical calculation were performed, and the optimized geometry structures and the frontier molecule orbital density distributions of these inhibitors as well as their protonated forms are presented in figures 11 and 12. The useful parameters, such as EHOMO, ELUMO, energy gap ΔE and dipole moment were determined and applied to explore the correlation between the inhibitor molecular structure and mild steel. According to the frontier molecular orbital theory, EHOMO is related to the ability of a molecule to donate electrons to appropriate electron acceptors, thus, a molecule exhibits stronger tendency to donate electrons to the steel vacancy d-orbital in the present study when the calculated EHOMO value is higher. Whereas, ELUMO corresponds to the electron accepting ability of the molecule, and a molecule has higher capability of accepting electrons when the calculated ELUMO value is lower [27,62,63]. Furthermore, the energy gap ΔE is an important parameter to evaluate the inhibitive effect of the inhibitor molecules. It was previously inferred [64,65] that an inhibitor possesses higher inhibition efficiency when its ΔE value is smaller, because lower energy is needed to remove an electron from the last occupied orbital.

Figure 11.

Optimized structure of BST, PBST and OCT and the protonated forms.

Figure 12.

Frontier molecule orbital density distributions of these inhibitors and the protonated forms.

According to Lukovits theorem [61], the value of ionization potential (I) and the electron affinity (A) can be derived from ELUMO and ELUMO by the following equations:and

Additionally, the absolute electronegativity (χ), the global hardness (ρ) and softness (σ) of the inhibitor molecule are defined as follows:and

All of these calculated quantum chemical parameters are summarized in table 10. It is apparent that the neutral form of PBST inhibitor as well as its protonated form PBST-H+ has a minimum value of ELUMO and lowest value of ΔE which is well in agreement with its highest inhibition efficiency. Moreover, PBST inhibitor shows the lowest value of dipole moment (μ), which will favour accumulation of the inhibitor [66]. However, there is still a controversy about the correlation between the dipole moment and inhibition efficiency that many researchers suggested that the inhibition efficiency increased with the increase of dipole moment [67,68], while others stated that inhibitor molecule with a lower value of dipole moment revealed higher inhibition efficiency [66]. Generally, the value of electronegativity χ represents the chemical potential and a higher value indicates better inhibition performance. In addition, an inhibitor always shows a higher inhibition efficiency when the value of global hardness is smaller according to the hard-soft acid base (HSAB) principle [69]. Inspection of table 10 also demonstrates that PBST has the highest electronegativity and lowest global hardness, resulting in the maximum inhibition efficiency compared to the other two inhibitors, which is consistent with the result of weight loss and electrochemical measurements.

Table 10.

Theoretical parameters of BST, PBST and OCT inhibitor and the protonated forms.

parameters	BST	PBST	OCT	BST-H+	PBST-H+	OCT-H+
EHOMO (eV)	−5.9772	−6.1944	−6.0986	−10.1295	−10.3085	−10.3322
ELUMO (eV)	−2.1490	−2.7433	−2.5512	−6.2983	−6.5176	−6.4123
ΔE (eV)	3.8282	3.4511	3.5474	3.8312	3.7909	3.9199
μ (D)	5.4560	3.1922	4.3649	4.3037	10.8639	9.4524
I (eV)	5.9772	6.1944	6.0986	10.1295	10.3085	10.3322
A (eV)	2.1490	2.7433	2.5512	6.2983	6.5176	6.4123
χ (eV)	4.0631	4.4689	4.3249	8.2139	8.4131	8.3722
ρ (eV)	1.9141	1.7256	1.7737	1.9156	1.8954	1.9599
σ [(eV)−1]	0.5224	0.5795	0.5638	0.5220	0.5276	0.5102

Conclusion

Gravimetric measurements, polarization curves, electrochemical impedance spectroscopy and scanning electron microscopy (SEM) were used to study the inhibition behaviour of three new benzaldehyde thiosemicarbazone derivatives for mild steel in 0.5 M H2SO4 solution. Results revealed that all these compounds are good inhibitors for Q235 steel in 0.5 M H2SO4 solution and PBST inhibitor showed the maximum inhibition efficiency of 96.6% at 300 µM. The inhibition efficiency increases with increasing inhibitors concentration and temperature. The results of polarization curves indicated that these three compounds behaved as mixed type and PBST and OCT predominantly anodic. The adsorption of these inhibitors on Q235 steel surface was according to Langmuir adsorption isotherm. The results of theoretical calculation and SEM studies were found to be in good agreement with that of weight loss and electrochemical measurements.