Inhibition of Mild Steel Corrosion in Hydrochloric and Sulfuric Acid

Article pubs.acs.org/IECR

Inhibition of Mild Steel Corrosion in Hydrochloric and Sulfuric Acid Media Using a Thiosemicarbazone Derivative Punita Mourya, Sitashree Banerjee, Rashmi Bala Rastogi, and Madan Mohan Singh* Department of Applied Chemistry, Indian Institute of Technology, Banaras Hindu University, Varanasi 221005, India

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S Supporting Information *

ABSTRACT: The inhibitive effect of 4-(N,N-dimethylamino)benzaldehyde thiosemicarbazone (DMABT) on the corrosion of mild steel (MS) in 1 N HCl and 1 N H2SO4 solutions was investigated by weight loss, potentiodynamic polarization, and electrochemical impedance spectroscopic measurements. It is inferred on the basis of the obtained results that DMABT is a mixed-type inhibitor, predominantly retarding cathodic reaction in both acidic media through adsorption on a MS surface. Adsorption obeys Langmuir’s adsorption isotherm in both acidic media. The observations regarding energy-dispersive X-ray, scanning electron microscopy, and atomic force microscopy confirm the existence of a protective film of the inhibitor on a MS surface. The molecular adsorption of DMABT was ascertained by density functional theory data.

1. INTRODUCTION Acid solutions, especially hydrochloric and sulfuric acids, are widely used in various industrial processes, such as pickling of iron, chemical cleaning, descaling of boilers, and oil well acidification in petroleum exploration. The use of inhibitors is one of the most practical methods for corrosion protection of metallic objects in acidic media,1 as well as for reduction of acid consumption occurring during the course of corrosion.2 Corrosion inhibitors are the substances that minimize or completely prevent corrosion when added at low concentrations in an aggressive environment. The known inhibitors in acidic media are mostly organic compounds containing N, O, and S atoms and/or delocalized π electrons. The effectiveness of the organic inhibitors depends on their adsorption rates and covering capabilities on metal surfaces. It has been realized from many sources3,4 that adsorption depends on the molecular structure, surface charge of the metal, and type of electrolytes. In an aqueous solution, inhibitors are adsorbed by replacing water molecules already adsorbed on the surface. For this, electrostatic interaction between an inhibitor molecule and a metal should be more dominant over that between metal and water molecules. In an inhibitor, the electron densities of different functional groups and their polarizability and electronegativity are the main factors for such interactions. In the case of organic inhibitors containing N, O, and S, the inhibition efficiency5,6 generally increases in the order O < N < S because of their increasing tendency to form a coordination bond. The inhibitors having more than one heteroatom show better corrosion inhibition in comparison to those possessing just one of them. Among such inhibitors, thiosemicarbazide derivatives, particularly Schiff bases, have been reported as efficient corrosion inhibitors for different metal−environment systems like mild steel (MS),7−9 aluminum,10,11 copper,12,13 and zinc14 in different acidic media. Schiff bases occupy a special place in the field of corrosion inhibition owing to their ecofriendly nature and the ease of their synthesis from relatively less expensive starting materials.15,16 These compounds usually form very thin and persistent adsorbed films on the metal surface, © 2013 American Chemical Society

causing a decrease in the corrosion rate by retarding anodic, cathodic, or both reactions.17 In 2011, Poornima et al.18 studied the inhibitive property of 4(N,N-diethylamino)benzaldehyde thiosemicarbazone (DEABT) on the corrosion of 18 Ni 250 grade maraging steel in a phosphoric acid solution and reported its maximum inhibition efficiency as 95% at 1.2 × 10−3 M. Pinto et al.19 investigated the inhibitive action of 4-(N,N-dimethylamino)benzaldehyde thiosemicarbazone (DMABT) on the corrosion of a 6061 Al−15 vol % SiC(p) composite and its base alloy in mixtures of hydrochloric and sulfuric acids at their different concentrations and at different experimental temperatures. Their results exhibited 90 and 74.4% inhibition efficiency for the composite and base alloy, respectively. In view of this, it is expected that DMABT may prove to be an effective inhibitor for MS corrosion in HCl and H2SO4 separately. Accordingly, a comparative study of the inhibitive characteristics of DMABT on the corrosion of MS in 1 N HCl and 1 N H2SO4 was taken up during the present investigation. Weight loss measurements, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) have been employed to determine the inhibition efficiency in order to accomplish the possible mechanism of inhibition. The corroded and inhibited metal surfaces have been characterized by energy-dispersive Xray (EDX), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The structural correlation of the inhibition efficiency of DMABT has been further ascertained through molecular modeling using density functional theory (DFT).

2. EXPERIMENTAL SECTION 2.1. Coupon Preparation. The composition (wt %) of mild steel (MS) used for all of the experiments was as follows: C, Received: Revised: Accepted: Published: 12733

April 19, 2013 August 3, 2013 August 12, 2013 August 12, 2013 dx.doi.org/10.1021/ie4012497 | Ind. Eng. Chem. Res. 2013, 52, 12733−12747

Industrial & Engineering Chemistry Research

Article

thus obtained, the inhibition efficiency (η) was calculated as follows:

0.253; Si, 0.12; P, 0.013; S, 0.024; Cr, 0.012; Mn, 0.03; Fe, balance. Coupons cut into 3 × 4 × 0.05 cm3 dimensions were used for weight loss measurements, whereas specimens of size 3 × 1 × 0.05 cm3 were used as working electrodes for polarization and EIS measurements. Prior to the experiments, the specimens were mechanically abraded with 320, 400, 600, 800, 1000, 1500, and 2000 grade emery papers. They were then degreased with acetone, washed with double-distilled water, and dried in air before immersion in a corrosive medium. 2.2. Electrolytic Solutions. The corrosive solutions, 1 N each of H2SO4 and HCl, were prepared by dilution of analyticalgrade H2SO4 and HCl of predetermined normality with tripledistilled water, respectively. The concentration range of DMABT was 45−450 μM, and the volume of the electrolyte used was 150 mL in each experiment. 2.3. Inhibitor. The inhibitor 4-(N,N-dimethylamino)benzaldehyde thiosemicarbazone (DMABT) was synthesized as per the reported procedure20,21 and recrystallized. A mixture containing equimolar ethanolic solutions of 4-(N,Ndimethylamino)benzaldehyde and -thiosemicarbazide was taken in a round-bottomed flask. The reaction mixture was refluxed on a hot water bath for about 3 h, and the pale-yellow product obtained was separated by filtration and dried. The product was recrystallized from ethanol and characterized by melting point (214 °C). IR (ν, cm−1): 3307 (w, NH2), 3101 (w, NH), 1600 (s, CN), 1155 (s, CS), 837 (w, NH−CS). 1H NMR (300 MHz, DMSO-d6): δ 11.17 (1H, −CHN−), 7.99 (br s, 1H, −NH−), 7.93 (s, 1H, −NH−), 7.75 (br s, −NH−), 7.75 (d, 2H, JH2,H3 = 8.6 Hz, 2 × H-2), 6.68 (d, 2H, JH3,H2 = 8.6 Hz, 2 × H-3), 2.95 (s, 6H, 2 × −CH3). The structure of the molecule is shown in Figure 1.

η%=

η%=

R p − R p° Rp

× 100 (3)

where Rp° and Rp are the polarization resistance values without and with the addition of inhibitor. 2.6.2. Potentiodynamic Polarization Measurement. In the Tafel polarization study, polarization curves were recorded from −250 to +250 mV with respect to the corrosion potential with a scan rate of 1.0 mV s−1. The linear Tafel segments of the anodic and cathodic curves were extrapolated until the point of intersection to obtain the corrosion potential (Ecorr) and corrosion current density (icorr). The inhibition efficiency has been calculated by using the respective icorr values in place of the corrosion rates (CR) in eq 2. 2.6.3. EIS. The impedance measurements were carried out using alternating-current signals of 5 mV amplitude for the frequency spectrum from 100 kHz to 0.01 Hz. The Nyquist representations of the impedance data were analyzed with Zsimpwin software. The electrode was kept for 0.5 h in the test solution before starting the impedance measurements. The charge-transfer resistance (Rct) was obtained from the diameter of the semicircle of the Nyquist plot. The inhibition efficiency of the inhibitor has been calculated from Rct values in the presence of an inhibitor. 2.7. Surface Characterization. 2.7.1. SEM/EDX. The morphologies of the uninhibited and inhibited MS surfaces in normal HCl and H2SO4 were analyzed by SEM. The EDX spectra of abraded, corroded, and inhibited MS samples showed the characteristic peaks of the elements constituting MS samples and changes therein. An electron microscope (FEI Quanta 200F) was used for SEM and EDX studies. 2.7.2. AFM. Observation and characterization of the uninhibited and inhibited MS surfaces were done by AFM.

2.4. Weight Loss Measurement. Finely polished and dried MS specimens of dimensions 3 × 4 × 0.05 cm3 were weighed on a digital balance with a sensitivity of 0.001 g and immersed for 24 h in each 1 N acidic solution (HCl and H2SO4) in the absence and presence of 45, 90, 180, 270, 360, and 450 μM of the inhibitor at 298 K. The experiments were repeated for two selective concentrations of 180 and 360 μM in the temperature range 298−338 K. The corroded/inhibited specimens were washed thoroughly by liquid soap, rinsed several times with distilled water, cleaned, dried using acetone, and reweighed. The weight loss was calculated as the difference in the weight of the specimen before and after immersion in corrosion media. In all of the above measurements, three close values were considered, and their average values are reported. The corrosion rate (CR in mg cm−2 h−1) was calculated from the following equation: ΔW st

(2)

where CR° and CR are the corrosion rates of MS specimens in the absence and presence of inhibitor, respectively. 2.5. Polarization Cell Assembly. Electrochemical measurements were carried out in a conventional three-electrode cell assembly. This assembly consisted of a flat-bottomed pyrex glass flask with three openings: for the working, reference, and counter electrodes. A rectangular working electrode of MS with an exposed surface area of 1 cm2 was attached to a copper rod fixed with the help of a screw and the rest covered with lacquer. A platinized platinum electrode and a silver−silver chloride electrode with a KCl salt bridge were used as the counter and reference electrodes, respectively. 2.6. Electrochemical Measurements. The working electrode was immersed in the test solution, and the constant steady-state (open-circuit) potential was recorded when it became virtually a constant. After the steady state was reached, anodic and cathodic polarization studies were conducted using an electrochemical analyzer (model CHI604A). 2.6.1. Polarization Resistance Measurement. Polarization resistance (Rp) measurements were performed in a potential range ±10 mV with respect to the open-circuit potential. Rp values were obtained from the resulting current versus potential plot. From the measured polarization resistance values, η % has been calculated using the relation

Figure 1. Chemical molecular structure of DMABT.

CR =

CR° − CR × 100 CR°

(1)

where ΔW is the average weight loss, s is the total area of the specimen, and t is the immersion time. From the corrosion rate 12734

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Table 1. Variation of the Inhibition Efficiency with Different Concentrations of DMABT Obtained from Weight Loss Experiments at 298 K Temperature in 1 N HCl and 1 N H2SO4 Solutions 1 N HCl Cinh (M × 10−6)

wt loss (mg cm−2)

CR (mg cm−2 h−1)

0 45 90 180 270 360 450

20.35 3.530 1.992 1.560 1.464 1.248 0.888

0.848 0.147 0.083 0.065 0.061 0.052 0.037

1 N H2SO4 η (%)

wt loss (mg cm−2)

CR (mg cm−2 h−1)

η (%)

82.5 90.1 92.2 92.8 94.0 95.7

76.65 25.85 9.240 8.760 3.216 2.280 1.512

3.194 1.077 0.385 0.365 0.134 0.095 0.063

66.2 88.0 88.6 95.7 97.0 97.8

Figure 2. Variation of the inhibition efficiency (η) and corrosion rate (CR) with 180 and 360 μM DMABT obtained from weight loss experiments at different temperatures in 1 N HCl and 1 N H2SO4 solutions.

H2SO4 solutions, respectively. The lower CR in HCl compared to H2SO4 at the same concentration was reported earlier.3 In both media, CR decreases noticeably with an increase in the DMABT concentration; i.e., the corrosion inhibition efficiency enhances with the inhibitor concentration. From Table 1, it could be seen that, at any given inhibitor concentration, the corrosion rate in 1 N H2SO4 is comparatively higher than that in 1 N HCl. The CR values decreased from 0.147 to 0.037 mg cm−2 h−1 and from 1.077 to 0.063 mg cm−2 h−1 as the concentration of the inhibitor was increased from 45 and 450 μM in HCl and H2SO4 solutions, respectively. This behavior is due to fact that the extent of adsorption and surface coverage due to the inhibitor on the MS surface increases with the inhibitor concentration.23 The values of η obtained at 450 μM in 1 N HCl and 1 N H2SO4 are 95.7 and 97.8%, respectively, which indicates that DMABT is a very good corrosion inhibitor for MS in both acidic media. The data tabulated in Table 1 indicate that η values at lower concentrations of the inhibitor are significantly higher in HCl compared to those in H2SO4. However, at higher inhibitor concentrations, η values are almost identical in both acids though marginally higher in H2SO4. It was established earlier that the adsorbability of SO42− on the steel surface is essentially less than that of Cl−.24 The surface charge of the metal is due to the electrical field that emerges at the interface by immersion in the electrolyte. This surface charge can be determined by comparing the potential of zero charge (PZC)

Observation was performed by contact-mode AFM (model BT02218, Nanosurf easyscan 2 Basic AFM, Switzerland) supported by a Si3N4 cantilever (Nanosensor, CONTR type) having a spring constant of 0.1 N m−1 and tip radii of more than 10 nm. The measurements were done over a square area with an image size of 50.0 μm. The resolution was 256 points line−1; the imaging rate was 1 s line−1. The images were obtained in “scan forward” mode, and scanning was carried out from bottom to top in a static regime. 2.8. DFT Study. Quantum-chemical calculations were performed using the DFT method, and structural parameters were geometrically optimized using functional hybrid B3LYP with electron basis set 6-31G(d,p) for all atoms. All of the calculations were performed with Gaussian 03, revision E.01.22 The quantum-chemical parameters obtained were EHOMO, ELUMO, and EHOMO−LUMO (ΔE) with a Mulliken charge on heteroatoms (N and S).

3. RESULTS AND DISCUSSION 3.1. Weight Loss Measurements. 3.1.1. Effect of the Inhibitor Concentration on the Corrosion Rate (CR) and Inhibition Efficiency (η). Table 1 lists the corrosion rates and inhibition efficiencies obtained by weight loss measurements in 1 N solutions of HCl and H2SO4 in the absence and presence of different concentration of DMABT at 298 K. The corrosion rates were found to be 0.848 and 3.194 mg cm−2 h−1 in HCl and 12735



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Figure 3. Arrhenius plots for MS corrosion rates (CR) in acidic media in the absence and presence of 180 and 360 μM DMABT: (a) 1 N HCl; (b) 1 N H2SO4.

Figure 4. Transition-state plots for MS corrosion rates (CR) in acidic media in the absence and presence of 180 and 360 μM DMABT: (a) 1 N HCl; (b) 1 N H2SO4.

and the open-circuit potential of the metal in the corresponding medium.24,25 Because PZC corresponds to the potential at which the surface of electrode is charge-free, at the corrosion potential the metal surface can be positively or negatively charged with respect to PZC. A negative surface charge favors the adsorption of cations, while an anion adsorption is favored by a positively charged surface. The surface charge on iron immersed in a H2SO4 solution is less negative with respect to PZC compared to that in HCl. Hence, a poor adsorption of organic cations can occur in the H2SO4 medium.24−26 On the other hand, the ability of Cl− ions in a HCl solution to be strongly adsorbed on the metal surface facilitates the physical adsorption of cations.27,28 Accordingly, a comparison of the inhibiting characteristics of

DMABT in 1 N HCl and 1 N H2SO4 solutions could give insight into the mode of adsorption of the inhibiting species.23−26 In the present case, both solutions have the same anion equivalent concentrations; if corrosion inhibition is due exclusively to protonated species that are adsorbed physically, the inhibitor should be less effective in H2SO4 solution than HCl. On the other hand, if they are adsorbed as molecules, the inhibiting effect in both acid solutions would be comparable.24,25 At lower concentration, the obtained results show that DMABT is adsorbed physically in both acidic media through Coulombic attraction between the positively charged protonated inhibitor molecule and the negatively charged surface. This is based on the

Table 2. Thermodynamic Activation Parameters of MS in 1 N HCl and 1 N H2SO4 Solutions Obtained from the Weight Loss Method thermodynamic parameters 1 N HCl

1 N H2SO4

Cinh (M × 10−6)

Ea (kJ mol−1)

ΔHa (kJ mol−1)

−ΔSa (J mol−1 K−1)

Ea (kJ mol−1)

ΔHa (kJ mol−1)

−ΔSa (J mol−1 K−1)

0 180 360

47.3 68.1 63.1

47.2 68.0 63.0

87.7 40.0 58.6

25.5 67.7 101.2

19.7 54.2 87.5

168.2 66.01 −32.5

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Figure 5. Langmuir adsorption isotherm plot of MS in 1 N HCl and 1 N H2SO4 containing different concentrations of DMABT at 298 K.

Table 3. Thermodynamic Parameters for Adsorption of DMABT on the MS Surface in 1 N HCl and 1 N H2SO4 Acidic Solutions at Different Temperatures DMABT in HCl

DMABT in H2SO4

temperature (K)

Kads (M)

ΔGads (kJ mol−1)

Kads (M)

ΔGads (kJ mol−1)

298 308 318 328 338

167 × 103 115.2 × 103 100.5 × 103 71.7 × 103 4.89 × 103

−39.74 −40.12 −41.07 −41.44 −41.63

54.5 × 103 14.7 × 103 9.12 × 103 4.49 × 103 1.79 × 103

−36.97 −34.85 −34.72 −33.89 −32.33

Figure 6. Anodic and cathodic polarization curves for MS in acidic media in the absence and presence of various concentrations of DMABT at 298 K: (a) 1 N HCl; (b) 1 N H2SO4.

lower inhibition efficiency of DMABT in a H2SO4 solution. However, because the inhibition efficiencies of DMABT at higher concentrations in both acidic solutions are almost identical and independent of the type of acid anion (Table 1), corrosion inhibition is due to adsorption of the inhibitor in its molecular form rather than the physical adsorption of its cationic form. 3.1.2. Effect of the Temperature on the Corrosion Rate (CR) and Inhibition Efficiency (η). The effect of the temperature on CR and η was evaluated by the weight loss method in the temperature range 298−338 K, in the absence and presence of the inhibitor in both test solutions. Figure 2 shows variation of CR and η with the temperature for 180 and 360 μM concentrations of DMABT in both electrolytes. It is apparent from the figure that η decreases with an increase in the whole range of temperature. In the case of 1 N HCl containing 180 μM inhibitor, CR increases from 0.065 to 1.301 mg cm−2 h−1 as the temperature is increased from 298 to 338 K. However, at its 360 μM concentration, CR increases from 0.052 to 0.800 mg cm−2 h−1. The corresponding increase in CR with the temperature was found to be from 0.363 to 9.090 mg cm−2 h−1 at 180 μM inhibitor and from 0.095 to 6.103 mg cm−2 h−1 at 360 μM inhibitor concentration in 1 N H2SO4. In acidic solutions, metal dissolution is generally governed by the evolution of hydrogen gas. A rise in the temperature usually increases the rate of the hydrogen evolution reaction on the cathode, which results in a higher metal dissolution rate. The higher corrosion rate observed at elevated temperature can be attributed to an appreciable increase in desorption of the

inhibitor on the MS surface with a rise in the temperature. Because of more desorption of inhibitor molecules at higher temperatures, the greater surface area of MS comes into contact with the acid environment, resulting in increased corrosion rates with increasing temperature.29 The values of the apparent activation energy (Ea) were calculated using the Arrhenius equation log CR =

−Ea +λ 2.303RT

(4)

where Ea is the apparent activation energy, R is the universal gas constant, and λ is the Arrhenius preexponential factor. A plot of log CR versus 1/T obtained by weight loss measurement in the absence and presence of 180 and 360 μM inhibitor gave straight lines in both test solutions (Figure 3a,b). The Ea values from the slope of these lines are listed in Table 2. The observed higher values of Ea in the presence of DMABT confirm that this molecule acts as an inhibitor. Its inhibition activity can be explained in terms of its physical adsorption on the metal surface.30−32 The values of the enthalpy of activation (ΔHa) and entropy of activation (ΔSa) were calculated using the following equation:33 CR = 12737

⎧ ΔHa ⎫ ⎧ ΔS ⎫ RT ⎬ exp⎨ a ⎬ exp⎨− ⎩ RT ⎭ ⎩ R ⎭ Nh

(5)



Article

Table 4. Electrochemical Parameters Obtained from the Polarization Curves of DMABT in 1 N HCl and 1 N H2SO4 Solutions at 298 K 1 N HCl

1 N H2SO4

Tafel

LPR

Tafel

LPR

Cinh (M × 10−6)

−Ecorr (mV)

βa (mV dec−1)

−βc (mV dec−1)

icorr(μA cm−2)

Rp(Ω cm2)

−Ecorr (mV)

βa (mV dec−1)

−βc (mV dec−1)

icorr(μA cm−2)

Rp(Ω cm2)

0 45 90 180 270 360 450

446 480 475 491 484 469 462

151 102 109 105 117 118 154

117 158 150 138 131 135 166

917 140 93 76 59 53 35

31 192 296 341 460 516 986

458 499 488 478 465 472 471

130 122 121 70 85 109 92

157 141 154 130 137 130 164

2049 560 238 163 84 82 69

15 51 119 124 278 306 372

plot of log CR/T versus 1/T yielded straight lines (Figure 4a,b) with a slope of −ΔHa/2.303R and an intercept of log(R/Nh) + ΔSa/2.303R from which values of ΔHa and ΔSa were calculated and are listed in Table 2. The values of Ea and ΔHa are close to each other, as expected from the concept of transition-state theory, and follow the same pattern of variation with different concentrations of the inhibitor. The positive sign of ΔHa has been attributed to the endothermic nature of the MS dissolution process.34 From Table 2, it was observed that the values of ΔSa were higher for inhibited solutions than uninhibited solutions. 3.1.3. Adsorption Isotherm and Standard Adsorption Free Energy. The interaction between the inhibitor and MS surface can be very well understood in terms of the adsorption isotherm. Attempts were made to fit experimental data to various isotherms including Frumkin, Langmuir, Temkin, and Freundlich. The results were best fitted by the Langmuir adsorption isotherm equation35 C inh 1 = + C inh θ K ads

(6)

where Kads is the equilibrium constant of the inhibitor adsorption process, Cinh is the inhibitor concentration, and θ (η/100) is the fraction of the metal surface covered with the inhibitor as a result of adsorption. The surface coverage (θ) of different concentrations of inhibitor in the two acids was evaluated from weight loss measurements. For both media, straight lines were observed upon plotting Cinh/θ against Cinh (Figure 5) with a correlation coefficient higher than 0.99 and suggest that the Langmuir adsorption isotherm fits very well with the experimental data. The slope value being very near to unity (1.03 for 1 N HCl and 0.974 for 1 N H2SO4) further confirms the validity of the Langmuir adsorption isotherm. On the basis of these observations, it may be argued that a monolayer of DMABT is formed on the surface and thereby inhibits corrosion. The Kads values were calculated from the intercept on the Cinh/θ axis. This value is related to the standard free energy of adsorption (ΔGads) by the equation36,37

Figure 7. Nyquist plots of the corrosion of MS in acidic media without and with different concentrations of DMABT at 298 K: (a) 1 N HCl; (b) 1 N H2SO4.

ΔGads = −RT ln(55.5K ads)

(7)

where all of the terms have the usual meaning. The constant value of 55.5 is the concentration of water in solution in mol L−1. The values of the free energy of adsorption thus obtained are −39.74 and −36.97 kJ mol−1 in HCl and H2SO4, respectively. The negative sign of ΔGads indicates that the inhibitors are spontaneously adsorbed on the metal surface.38 Generally, the magnitude of ΔGads around −20 kJ mol−1 or less negative is assumed to be due to electrostatic interaction between the

Figure 8. Equivalent circuit for MS in both electrolytes.

where h is Plank’s constant, N is Avogadro’s number, ΔSa is the entropy of activation, and ΔHa is the enthalpy of activation. The 12738



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Table 5. EIS Parameters for the Corrosion of MS in 1 N HCl and 1 N H2SO4 Solutions Containing DMABT at 298 K 1 N HCl

1 N H2SO4 CPE

Cinh (M × 10−6)

Rs (Ω cm2)

Rct (Ω cm2)

Cdl(μF cm−2)

Y0(μ Ω s cm−2)

0 45 90 180 270 360 450

7.47 8.04 8.34 8.45 8.40 8.60 8.76

28 220 340 357 581 632 812

99.2 89.5 46.7 45.2 39.3 33.9 20.1

234 99 68 60 56 46 21

CPE n

χ2

Rs (Ω cm2)

Rct (Ω cm2)

Cdl(μF cm−2)

Y0(μ Ω sn cm−2)

n

χ2

0.85 0.89 0.90 0.90 0.91 0.91 0.89

0.00041 0.0050 0.0048 0.0054 0.0029 0.0033 0.0016

4.27 4.44 4.56 4.40 5.08 5.77 5.73

13 53 67 201 373 412 730

304.5 267.8 63.1 39.3 37.1 29.6 26.9

328 76 69 60 48 36 32

0.95 0.88 0.93 0.95 0.95 0.97 0.94

0.00099 0.0013 0.0037 0.0028 0.0048 0.0014 0.0026

n

Table 6. Variation of η for MS in Acidic Media with Different Concentrations of DMABT by Weight Loss and Electrochemical Methods in 1 N HCl and 1 N H2SO4 inhibition efficiency η (%) 1 N H2SO4

1 N HCl Cinh (M × 10−6)

wt loss

Tafel extrapolation

LPR

EIS

wt loss

Tafel extrapolation

LPR

EIS

45 90 180 270 360 450

82.5 90.1 92.2 92.8 94.0 95.7

84.7 89.9 91.7 93.6 94.2 96.2

83.7 89.4 90.8 93.2 94.0 96.8

87.3 92.0 92.1 95.2 95.5 96.5

66.2 88.0 88.6 95.7 97.0 97.8

72.6 88.4 92.0 95.9 96.0 96.6

70.6 87.4 88.0 94.6 95.1 96.0

75.5 80.6 93.5 96.5 96.8 98.2

chemisorption. The obtained values of ΔSads are +52.87 and −100.4 J mol−1 K−1 in HCl and H2SO4, respectively. The positive value of ΔSads (52.87 J mol−1 K−1) in HCl arises from the substitution process, which can be attributed to an increase in the solvent entropy and a more positive water desorption entropy. The increase of disorder is also interpreted in terms of more water molecules being desorbed from the metal surface by one inhibitor molecule.43 However, the negative values of ΔSads in H2SO4 imply that inhibitor molecules were freely moving in the bulk of solution before the adsorption process, and as a result of adsorption, the inhibitor molecules were orderly adsorbed on the metal surface.44 3.2. Potentiodynamic Polarization Measurements. Potentiodynamic polarization curves for MS in 1 N HCl and 1 N H2SO4 solutions containing some selective concentrations of DMABT at 298 K are shown in parts a and b of Figure 6, respectively. In both acidic solutions, the addition of DMABT shifts both the anodic and cathodic curves to lower current densities. It is further observed that the magnitude of the shift in the cathodic direction is much larger than that in the anodic direction. In other words, the cathodic reaction of the MS electrode is more drastically inhibited by DMABT compared to the anodic reaction. This may be due to adsorption of a protonated inhibitor on the negatively charged surface and through coordination bonding between the DMABT molecule and MS on the positively charged surface with respect to PZC. The electrochemical corrosion parameters icorr, Ecorr, and cathodic and anodic Tafel slopes (βc and βa) are presented in Table 4. The values of η calculated with the help of icorr are listed in Table 6. It is apparent from Figure 6a,b that the shapes of the cathodic curves in both media are similar but those of the anodic curves are different. It is noted that the nature of the anodic polarization curve observed in HCl remains unaltered even upon the addition of different amounts of inhibitor. This indicates that the mechanisms of anodic dissolution in the absence and presence

inhibitor and the charged metal surface (i.e., physisorption). The values of ΔGads around −40 kJ mol−1 or more negative indicates that a charge sharing or transferring from organic species to the metal surface occurs to form a coordinate type of bond (i.e., chemisorption).26,39 The calculated ΔGads values for DMABT at 298 K temperature in both solutions being just less than −40 kJ mol−1 indicate that besides electrostatic interaction there is predominant coordinate bonding between the inhibitor and charged metal surface (Table 3). To accumulate Supporting Information (SI) about the mechanism of corrosion inhibition, other thermodynamic parameters such as enthalpy (ΔHads) and entropy (ΔSads) were calculated by using the following equation:40 ΔGads = ΔHads − T ΔSads

ln K ads =

(8)

−ΔH ads ΔS ads + − ln(55.5) RT R

(9) −1

Figure S1 (SI) represents the plots of ln Kads versus T for adsorption of DMABT in hydrochloric and sulfuric acid, respectively. The lines obtained represent a slope of −ΔHads/R and intercept of (ΔSads/R) − ln(55.5). It is indisputable that an endothermic adsorption process (ΔHads > 0) refers to chemisorption, whereas an exothermic adsorption process (ΔHads < 0) may involve either physisorption or chemisorption or both of the processes occurring together.41 In an exothermic process, the distinction between physisorption and chemisorption is based upon the absolute value of ΔHads. For a physisorption process, the enthalpy of adsorption is considered to be lower than 40 kJ mol−1, while for chemisorption, it approaches 100 kJ mol−1.42 The calculated values of ΔHads in HCl and H2SO4 are −24.03 and −66.51 kJ mol−1, respectively. On the basis of the results of this study, it is apparent that the mechanism of adsorption between the inhibitor and metal surface in the case of HCl is essentially physisorption, while in the case of H2SO4, it is a combination of physisorption and 12739



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Figure 9. SEM micrographs of the MS surface: (a) abraded MS; (b) blank in 1 N HCl; (c) with DMABT in HCl; (d) blank in 1 N H2SO4; (e) with DMABT in H2SO4.

inhibitor is significantly different from that in the blank solution;

of the inhibitor are not much different. The decrease in the current density values at different inhibitor concentrations is due to the adsorption of InhH+ onto the adsorbed FeCl− present at the MS electrolyte interface, as indicated by eq 24. However, the nature of the anodic polarization curve in H2SO4 containing

a distinct Tafel region is observed in the case of inhibited solutions. Obviously, the mechanisms of anodic dissolution in the blank and inhibited solutions are not the same. This is clearly 12740



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Table 7. Atomic Percentage of Elements Obtained from EDX Spectra of MS Surfaces in 1 N HCl and 1 N H2SO4 Solutions without and with DMABT at 298 K inhibitor abraded MS blank

1 N HCl 1 N H2SO4 1 N HCl 1 N H2SO4

DMABT

Fe

C

Si

S

Mn

P

N

95.4 90.0 90.2 74.1 64.2

18.6 22.5 25.3 26.2 29.3

2.32 0.55 0.62 0.59 0.75

0.37 0.44 0.34 0.44 0.47

0.27 0.27 0.38 0.34 0.32

0.21 0.35 0.50 0.29 0.41

1.1 1.3 2.7 2.6 4.6

reduction mechanism is not affected by the presence of the inhibitors.50 On the basis of the facts that the mechanism of hydrogen evolution remains unaffected even in the presence of the inhibitor and there is retardation in the rate of the cathodic reaction by the inhibitor, possible pathways49 of the cathodic reaction can be summarized as follows:

demonstrated by the two mechanisms mentioned in the subsequent paragraphs. Iron electrodissolution in an acidic sulfate solution depends primarily on the adsorbed intermediate FeOHads according to the mechanism45 reproduced below: Fe + H 2O ↔ Fe ·H 2Oads

(10)

FeH 2Oads ↔ FeOHads + H+ + e−

(11)

FeOHads → FeOH+ + e−

(12)

FeOH+ + H+ ↔ Fe 2 + + 2e−

(13)

(14)

(FeCl−)ads ↔ (FeCl)ads + e−

(15)

(FeCl)ads → FeCl + e−

(16)

FeCl+ + e− ↔ Fe 2 + + Cl−

(17)

The difference in the anodic curves may be attributed to the different mechanisms of anodic dissolution in HCl and H2SO4 solutions. In a H2SO4 solution, the anodic dissolution of iron depends primarily on the adsorbed intermediate (FeOH)ads,46 while it depends on (FeCl)ads in a HCl solution.47 Upon the addition of inhibitor to acid solutions, different additional steps are involved depending on the nature of the electrolyte in the mechanism of anodic dissolution of MS. In sulfuric acid, these additional steps in an anodic oxidation mechanism as described in the literature48,49 are as follows: Fe· H 2Oads + Inh ↔ FeOHads− + H+ + Inh

(18)

Fe· H 2Oads + Inh ↔ Fe· Inh ads + H 2O

(19)

FeOHads− → FeOHads + e−

(rate‐determining step) (20)

+

Fe· Inhads ↔ Fe· Inh ads + e

−

+

(21) +

FeOHads + Fe ·Inh ads ↔ FeOH + Fe ·Inh ads

(22)

FeOH+ + H+ ↔ Fe 2 + + H 2O

(23)

whereas in the case of hydrochloric acid in the presence of an inhibitor follows the additional chemical equations: (FeCl−)ads + InhH+ ↔ (FeCl−InhH+)ads −

+

+

(24) −

(FeCl )ads + InhH ↔ (Fe ·InhH )ads + Cl

(26)

Fe + (InhH+) + e− ↔ (Fe ·InhH)ads

(27)

(FeH)ads + (FeH)ads → Fe + H 2

(28)

It is evident from the first two chemical equations that there is a competition between H+ and InhH+ (protonated inhibitor) for the same active site to get adsorbed on the MS surface. Further, from the activation-controlled nature of the cathodic process as mentioned above, the first step is very likely to be the ratecontrolling step. The higher values of βc compared to those of βa at each concentration of the inhibitor further confirm that the inhibitor is predominantly cathodic.51 The anodic Tafel curves in both acidic solutions containing inhibitor shifted to the direction of lower current density, which suggests that DMABT could also suppress the anodic reaction. Generally, if the displacement in Ecorr is >85 mV with respect to Ecorr in an uninhibited solution, the inhibitor can be defined as a cathodic or anodic type.52 From the results shown in Table 4, no systematic variation in Ecorr is seen with a change in the concentration of DMABT and the maximum displacement is observed as 45 mV, which indicates that DMABT is a mixed type of inhibitor. From Table 4, it is apparent that icorr decreases considerably in the presence of DMABT at each experimental concentration. Correspondingly, η increases with the inhibitor concentration. The value of η of the inhibitor in 1 N HCl, which was observed to be 84.7% for 45 μM, increases steadily with the concentration and approaches its maximum value of 96.2% at 450 μM. Similarly, the η values increase from 72.6% to 96.6% on increasing inhibitor concentration in the same range as that listed in Table 6. It was observed from linear polarization studies that the polarization resistance (Rp) increases from 31.0 to 986.0 Ω cm2 and from 15.0 to 372.0 Ω cm2 from the blank to the electrolyte containing 450 μM DMABT in 1 N HCl and 1 N H2SO4 solutions, respectively (Table 4). The increase in the polarization resistance in the presence of the inhibitor suggests that a nonconducting physical barrier of DMABT is formed on the MS surface, giving the highest inhibiting efficiencies of 96.8% and 96.0% at 450 μM in 1 N HCl and 1 N H2SO4, respectively, for which the polarization resistance is also the highest. 3.3. EIS. EIS measurements were performed to determine the impedance parameter of the MS−hydrochloric acid and MS− sulfuric acid interfaces in the absence and presence of 270, 360,

However, in the case of HCl, the process of iron dissolution follows different pathways: Fe + Cl− ↔ (FeCl−)ads

Fe + H+ + e− ↔ (FeH)ads

(25)

The nature of cathodic Tafel curves in Figure 6a,b suggests that the hydrogen evolution is activation-controlled and the 12741



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Figure 10. AFM micrographs of the MS surface: (a) abraded MS; (b) blank in 1 N HCl; (c) with DMABT in HCl; (d) blank in 1 N H2SO4; (e) with DMABT in H2SO4.

and 450 μM concentrations of DMABT. Parts a and b of Figure 7 show the Nyquist plots at 298 K for MS in 1 N HCl and 1 N H2SO4, respectively. The existence of a single semicircle with its center below the x axis in both plots indicates the presence of a single charge-transfer process during metal dissolution. The

impedance spectra consist of a large capacitive loop at high frequencies followed by a small inductive loop at low-frequency values. The high-frequency capacitive loop is usually related to charge transfer of the corrosion process and double-layer behavior. On the other hand, the low-frequency inductive loop 12742



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Table 8. Area and Line Roughness Obtained from AFM of MS Surfaces in 1 N HCl and 1 N H2SO4 Solutions without and with DMABT at 298 K 1 N HCl

Table 9. Energy Order of the Frontier Orbital (eV)

1 N H2SO4

AFM data

abraded MS

blank

DMABT

blank

DMABT

area roughness (nm) line roughness (nm)

42.23 53.15

626.9 586.4

124.6 157.3

660.0 518.9

131.9 67.9

Fe5 DMABT

EHOMO

ELUMO

ΔE1 = ELUMO(DMABT) − EHOMO(Fe)

−5.075 −0.1974

−1.747 −0.6068

5.014

ΔE2 = ELUMO(Fe) − EHOMO(DMABT) −1.550

describe the frequency dependence of nonideal capacitive behavior. The impedance of the CPE is mathematically expressed57 as

may be attributed to the relaxation process of the adsorbed intermediates controlling the anodic process.53 The shape of the curve remains unchanged in the two electrolytes in both the absence and presence of an inhibitor. This indicates that the mechanism of corrosion is not affected by the addition of an inhibitor.54 These capacitive loops in all cases are not perfect semicircles, which can be attributed to the frequency dispersion effect as a result of the roughness and inhomogenity of the electrode surface.55 Furthermore, the diameter of the capacitive loop in the presence of an inhibitor is bigger than that in its absence, and its magnitude is a function of the inhibitor concentration. This indicates that the impedance of inhibited substrate becomes larger with an increase in the DMABT concentration. The corresponding Bode plots are illustrated in Figure S2 (SI) for MS in different media at 298 K in the absence and presence of an inhibitor. It is apparent from these curves that the addition of an inhibitor causes an increase in the interfacial impedance, which further increases upon increasing concentration of the inhibitor. The single narrow peak in the phase-angle plots [Figure S3 (SI)] again indicate a single time constant for the corrosion process at the metal−solution interface in both cases. The increase in the peak heights indicates a more capacitive response of the interface due to the presence of inhibitor molecules at the interface. All of the impedance diagrams were analyzed in terms of the equivalent circuit (Figure 8), which is a parallel combination of Rct and the constant phase element (CPE) of the double layer, both in series with Rs.30,56 The CPE is generally introduced to

ZCPE = Y0−1(iω)−n

(29)

where Y0 is a proportionality factor and n has the meaning of phase shift. The value of n represents the deviation from ideal behavior,58 and it lies between 0 and 1. The value of the doublelayer capacitance (Cdl) can be calculated from CPE parameter values Y0 and n using the expression59 Cdl =

Y0ωn − 1 sin[n(π /2)]

(30)

The values of Rs, Rct, Cdl, CPE, and goodness of fit (χ ) were obtained from the above-mentioned equivalent circuit and are presented in Table 5. The quality of the fit to the equivalent circuit was judge by the χ2 value.60 The obtained χ2 values (0.00043−0.0013) in Table 5 indicate a good fitting to the proposed circuit. The value of Rct increases while the doublelayer capacitance decreases with the concentration of DMABT in both acid solutions. The largest effect was observed at 450 μM DMABT, which gives Rct values of 812.0 Ω cm2 in 1 N HCl and 730.0 Ω cm2 in 1 N H2SO4 and Cdl values of 20.1 μF cm−2 in 1 N HCl and 26.9 μF cm−2 in 1 N H2SO4. The increase in the Rct values is attributed to the formation of an insulating protective film at the metal−solution interface. The decrease in the Cdl values can be attributed to a decrease in the local dielectric constant and/or to an increase in the thickness of the electrical double layer, suggesting that the inhibitor molecules are adsorbed at the metal−solution interface.56 The value of η increases with the concentration of DMABT and lies at 96.5% in 2

Figure 11. Optimized structure of DMABT calculated with B3LYP/6-31G(d) model chemistry. 12743



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Figure 12. FMO diagrams (a) HOMO and (b) LUMO of DMABT by the B3LYP/6-31G(d) model chemistry.

morphologies (3D) of the abraded MS sample and in the absence and presence of DMABT in 1 N HCl and 1 N H2SO4 are shown. The area and line roughness are listed in Table 8. It is clearly seen from the figure that the steel sample shows a rough surface due to acid corrosion. However, the presence of 450 μM DMABT retarded corrosion and the surface of the inhibited MS specimen gets smoothened, as shown in Figure 10c in HCl with DMABT and Figure 10e in H2SO4 with DMABT. The decrease in roughness was probably due to the formation of adsorbed protective film of DMABT on the MS surface. 3.5. DFT Study: Interaction Pattern between the Inhibitor (DMABT) and Metal Surface. The optimized structure of DMABT in its ground state is shown in Figure 11. The reactivity of a chemical species is very well-defined in terms of frontier orbitals:61 the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). According to the frontier molecular orbital (FMO) theory of chemical reactivity, the formation of a transition state is due to interaction between HOMO and LUMO of reacting species. The smaller the orbital energy gap (ΔE) between the participating HOMO and LUMO, the stronger the interactions between two reacting species.62 The results listed in Table 9 show that the interaction between the HOMO of the inhibitor and the LUMO of the metal atom, as represented by ΔE2, is stronger than that between the HOMO of the metal atom and the LUMO of the inhibitor (ΔE1). In principle, the interaction between the HOMO of the inhibitor and the LUMO of the Fe atom, ΔE2, should be dominated by the interaction between the HOMO of the Fe atom and the LUMO of the inhibitor, ΔE1, for good inhibitive property.63 Adsorption of the inhibitor on the metal surface can occur on the basis of donor−acceptor interactions between the lone-pair electron of the heteroatoms present in the thiosemicarbazone compound and the vacant d orbitals of the metal surface atoms.64 Substances with high values of EHOMO have a tendency to donate electrons to appropriate acceptors with low-energy, empty molecular orbitals.64,65 The calculated value of EHOMO (−0.1973

1 N HCl and 98.2% in 1 N H2SO4. These results again confirm that DMABT exhibits a good inhibitive performance for MS in both solutions. It is heartening to note that the inhibition efficiencies obtained from the employed techniques, weight loss measurement, electrochemical polarization, and linear polarization are reasonably in good agreement, as shown in Table 6 for HCl and H2SO4. Thus, DMABT is a very good inhibitor in both corrodants, and the inhibition efficiency in 1 N HCl is higher than that in 1 N H2SO4 at lower concentration, while at higher concentrations, inhibition efficiencies are almost the same in both solutions. 3.4. Surface Analysis. 3.4.1. SEM/EDX. SEM micrographs (Figure 9a−e) and atomic percentage (atom %) of elements obtained from EDX analysis of the MS surface (Table 7) in 1 N HCl and 1 N H2SO4 solutions exhibit the changes that occurred during the corrosion process in the absence and presence of an inhibitor. The MS surface in 1 N H2SO4 (Figure 9d) was more damaged in comparison to the 1 N HCl (Figure 9b) solution in the absence of an inhibitor. However, in the presence of 450 μM DMABT, the surface was remarkably improved and less damage occurred in comparison to their surfaces in the absence of an inhibitor. This improvement in the surface morphology is due to the formation of a good protective film of DMABT on the MS surface, which is responsible for inhibition. The obtained atomic percentage values of the elements from EDX spectra of an inhibited MS surface show more intensity of nitrogen and sulfur because of the nitrogen and sulfur of DMABT. The values of atomic percentage due to iron in inhibited MS are comparatively smaller than those of the abraded and corroded MS samples. The reduction in the intensity of the atomic percentage might be due to an overlying inhibitor film. This indicated that the MS surface was covered with a protective film of inhibitor molecules. 3.4.2. AFM. The surface morphology of the MS specimens was further studied by AFM before and after corrosion in the absence and presence of 450 μM DMABT. In Figure 10a−e, the surface 12744



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eV) for DMABT being higher than that for Fe (−5.075 eV)63 indicates that DMABT has a tendency to donate electrons to vacant d orbitals of Fe. On the other hand, a lower value of ELUMO (−1.747 eV)63 for Fe than that for DMABT (−0.0606 eV) favors Fe to accept electrons. The HOMO and LUMO populations of the studied DMABT are shown in Figure 12a,b. It could be easily seen that DMABT is fully planar, which may result in its significant interaction with the metal surface. This observation is in good agreement with the findings of the present work.

(5) Li, X.; Deng, S.; Fu, H. Triazolyl blue tetrazolium bromide as a novel corrosion inhibitor for steel in HCl and H2SO4 solutions. Corros. Sci. 2011, 53, 302−309. (6) Bentiss, F.; Traisnel, M.; Gengembre, L.; Lagrenée, M. A new triazole derivative as inhibitor of the acid corrosion of mild steel: electrochemical studies, weight loss determination, SEM and XPS. Appl. Surf. Sci. 1999, 152, 237−249. (7) Jacob, K. S.; Parameswaran, G. Corrosion inhibition of mild steel in hydrochloric acid solution by Schiff base furoin thiosemicarbazone. Corros. Sci. 2010, 52, 224−228. (8) Obot, I. B.; Obi-Ebbedi, N. O. Adsorption properties and inhibition of mild steel corrosion in sulfuric acid solution by ketoconazole: Experimental and theoretical investigation. Corros. Sci. 2010, 52, 198− 204. (9) Kumar, S. L. A.; Gopiraman, M.; Kumar, M. S.; Sreekanth, A. 2Acetylpyridine-N(4)-Morpholine Thiosemicarbazone (HAcpMTSc) as a Corrosion Inhibitor on Mild Steel in HCl. Ind. Eng. Chem. Res. 2011, 50 (13), 7824−7832. (10) Singh, D. D. N.; Singh, M. M.; Chaudhary, R. S.; Agrawal, C. V. Inhibitive effects of isatin, thiosemicarbazide and isatin-3-(3-thiosemicarbazone) on the corrosion of aluminium alloys in nitric acid. J. Appl. Electrochem. 1980, 10, 587−592. (11) Aytac, A.; Ozmen, U.; Kabasakaloglu, M. Investigation of some Schiff bases as acidic corrosion of alloy AA3102. Mater. Chem. Phys. 2005, 89, 176−181. (12) Li, S. L.; Ma, H. Y.; Lei, S. B.; Yu, R.; Chen, S. H.; Liu, D. X. Inhibition of copper corrosion with Schiff base derived from 3methoxysalicylaldehyde and O-phenyldiamine in chloride media. Corrosion 1998, 54, 947. (13) Li, S.; Chen, S.; Lei, S.; Ma, H.; Yu, R.; Liu, D. Investigation on some Schiff bases as HCl corrosion inhibitors for copper. Corros. Sci. 1999, 41, 1273−1287. (14) Fouda, A. S.; Madkour, L. H.; El-Shafei, A. A.; Abd Elmaksoud, S. A. Corrosion inhibitor for Zinc in 2 M HCl solution. Bull. Korean Chem. Soc. 1995, 16, 454−458. (15) Lashgari, M.; Arshadi, M. R.; Miandari, S. The enhancing power of iodide on corrosion prevention of mild steel in the presence of a synthetic soluble Schiff base: electrochemical and surface analyses. Electrochim. Acta 2010, 55, 6058−6063. (16) Küstü, C.; Emregül, K. C.; Atakol, O. Schiff bases of increasing complexity as mild steel corrosion inhibitors in 2 M HCl. Corros. Sci. 2007, 49, 2800−2814. (17) Asan, A.; Kabasakaloglu, M.; Isıklan, M.; Kılıç, Z. Corrosion inhibition of brass in presence of terdentate ligands in chloride solution. Corros. Sci. 2005, 47, 1534−1544. (18) Poornima, T.; Nayak, J.; Shetty, A. N. Effect of 4-(N,Ndiethylamino)benzaldehyde thiosemicarbazone on the corrosion of aged 18 Ni 250 grade maraging steel in phosphoric acid solution. Corros. Sci. 2011, 53, 3688−3696. (19) Pinto, G. M.; Nayak, J.; Shetty, A. N. Corrosion inhibition of 6061 Al-15 vol. pct. SiC(p) composite and its base alloy in a mixture of sulphuric acid and hydrochloric acid by 4-(N,N-dimethylamino)benzaldehyde thiosemicarbazone (DMABT). Mater. Chem. Phys. 2011, 125, 628−640. (20) Shah, P. T.; Daniels, T. C. Thiosemicarbazide as a reagent for the identification of aldehydes, ketones, and quinines. Recl. Trav. Chim. PaysBas 1950, 69, 1545. (21) Li, X. H.; Mu, G. N. Tween-40 as corrosion inhibitor for cold rolled steel in sulfuric acid: weight loss study, electrochemical characterization and AFM. Appl. Surf. Sci. 2005, 252, 1254−1265. (22) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratman, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;

4. CONCLUSIONS (1) DMABT acts as a very good inhibitor for the corrosion of MS in both 1 N HCl and 1 N H2SO4 solutions. The inhibition efficiency increases with the inhibitor concentration, and the maximum η values are 95.7% (1 N HCl) and 97.8% (1 N H2SO4) at 450 μM. (2) Adsorption of DMABT is a spontaneous process and obeys the Langmuir adsorption isotherm irrespective of the nature of the electrolyte. The free energy of adsorption indicates that adsorption of DMABT involves both physical and chemical adsorption. (3) DMABT acts as a mixed-type inhibitor, although it retards cathodic reaction to a greater extent. (4) DFT studies and surface characterization support the experimental data and favor the adsorption mechanism of the present inhibitor.

■

ASSOCIATED CONTENT

* Supporting Information S

Details of DMABT adsorption in both acidic media: plots of ln Kads versus T−1 (Figure S1) and representations of Z Bode spectra (Figure S2) and phase-angle Bode spectra (Figure S3) of corrosion of MS in acidic media without and with different concentrations of DMABT. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./fax: +91 542 6702859. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are highly thankful to Prof. G. V. S. Shastri, Department of Metallurgical Engineering, Indian Institute of Technology (BHU), Varanasi, India, for providing SEM/EDX facilities. They are also thankful to the Head, Chemistry Department, Faculty of Science, Banaras Hindu University, Varanasi, India, for carrying out theoretical calculations.

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REFERENCES

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Inhibition of Mild Steel Corrosion in Hydrochloric and Sulfuric Acid

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