New and Effective Corrosion Inhibitors for Mild Steel in 1 M HCl

Article pubs.acs.org/IECR

2‑Amino-3,5-dicarbonitrile-6-thio-pyridines: New and Effective Corrosion Inhibitors for Mild Steel in 1 M HCl Sudheer and Mumtaz Ahmad Quraishi* Department of Chemistry, Indian Institute of Technology (Banaras Hindu University), Varanasi -221 005, India S Supporting Information *

ABSTRACT: The corrosion protection efficiency of three pyridines namely 2-amino-3,5-dicarbonitrile-4-(4-methoxyphenyl)-6(phenylthio)pyridine (ADTP I), 2-amino-3,5-dicarbonitrile-4phenyl-6-(phenylthio) pyridine (ADTP II), and 2-amino-3,5dicarbonitrile-4-(4-nitrophenyl)-6-(phenylthio) pyridine (ADTP III) was investigated by electrochemical impedance spectroscopy, potentiodynamic polarization, and weight loss techniques. The results of potentiodynamic polarization studies show that the three compounds under investigation show mixed-type inhibition behavior. Among them, ADTP I shows the highest inhibition efficiency of 97.6% at 1.22 mmol L−1. The effect of temperature and related activation parameters were worked out. To inspect the surface morphology and composition of inhibitor film on the mild steel surface, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) technique were used.

1. INTRODUCTION Heterocyclic compounds exemplify an attractive class of corrosion inhibitors.1−3 Nitrogen-containing compounds function more effectively as corrosion inhibitors in hydrochloric acid4,5 whereas sulfur-containing inhibitors are preferred for H2SO4.6,7 Heterocyclic compounds containing both nitrogen and sulfur are of particular importance as they often provide excellent inhibition.8−11 The aim of the present work is to synthesize heterocyclic compounds containing both sulfur and nitrogen atoms in the same ring. A perusal of the literature reveals that some pyridine compounds12−17 have been investigated for their corrosion inhibition behavior in hydrochloric acid media. In continuance of our research, development of improved version of heterocyclic compounds as corrosion inhibitors,18−22 we report in the present work synthesis of three pyridine derivatives: 2-amino-3,5-dicarbonitrile-4-(4-methoxyphenyl)-6(phenylthio)pyridine (ADTP I), 2-amino-3,5-dicarbonitrile4phenyl-6-(phenylthio) pyridine (ADTP II), and 2-amino-3,5dicarbonitrile-4-(4-nitrophenyl)-6-(phenylthio) pyridine (ADTP III) and marked as ADTPS. The inhibition analyses of above pyridine derivatives were carried out following weight loss, electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization measurement.23 The SEM and EDX techniques were also used to examine surface morphology of mild steel with and without inhibitors in 1 M HCl.

degreasing with acetone. The test solution 1 M HCl was prepared from 37% hydrochloric acid (Merck) analytical grade chemical and bidistilled water. The inhibitors concentrations were taken in millimoles per liter for all investigation. 2.2. Inhibitors. The inhibitors were prepared as reported earlier,24 and the scheme is shown in Figure 1. The completion

Figure 1. Synthetic route of ADTP derivatives.

of reaction was monitored by TLC (ethyl acetate/n-hexane, 1:7) and product purification was performed by ethanol recrystallization. 1H NMR (300 MHz) spectra of selected compounds was determined via JEOL AL 300 FT-NMR in DMSO with TMS as internal standard. The chemical structures, names, spectral data, and their melting points are provided in Table 1. 2.3. Weight Loss Measurement. The weight loss experiments were carried out as descried previously.19 The mild steel was immersed in 1 M HCl in absence and presence of various concentrations (0.31−1.22 mmol L−1) of inhibitors for 3 h duration at constant temperature 308 K. The effect of temperature on inhibition of all the three inhibitors was studied at different temperature (308−328) using lower and optimum concentration of inhibitors. The maximum standard deviation was observed ±2.5% in the weight loss study.

2. MATERIALS AND METHOD 2.1. Electrodes and Solutions. The mild steel used for experimental whose chemical composition by wt % is as follows: C = 0.1, Mn = 0.46, Si = 0.026, Cr = 0.050, P = 0.012, Cu = 0.135, Al = 0.023, Ni = 0.05, and balance Fe. The mild steel coupons, for weight loss and electrochemical study, were cut into size 2.5 cm × 2.0 cm × 0.025 cm and 8 cm × 1 cm × 0.025 cm, respectively. Before testing, to remove the impurities from the surface, coupons were abraded with SiC papers (800− 1500 grade) and washed with distilled water, followed by © 2014 American Chemical Society

Received: Revised: Accepted: Published: 2851

May 23, 2013 December 30, 2013 January 24, 2014 January 24, 2014 dx.doi.org/10.1021/ie401633y | Ind. Eng. Chem. Res. 2014, 53, 2851−2859

Industrial & Engineering Chemistry Research

Article

Table 1. Chemical Structures and Names of the Inhibitors under Investigation

2.4. Electrochemical Measurements. Electrochemical experiments were conducted using a Gamry Potentiostat/ Galvanostat with an ESA400 Gamry framework system. A conventional cell setup had three electrode mild steel as working electrode, platinum foil as auxiliary electrode, and a saturated calomel electrode (SCE) as reference electrode was utilized for above measurement. The open circuit potential (OCP) was obtained by immersing the working electrode in the test solution 1 M HCl for 30 min. Electrochemical impedance spectroscopy (EIS) measurements were carried out at corrosion potentials (OCP) across the frequency range 100 kHz−10 mHz, with a 10 mV amplitude of waveform. For potentiodynamic polarization measurements, potential was scanned in the range −250 to +250 mV at a scan rate 1 mV s−1. The electrode was allowed to corrode freely prior to EIS and polarization measurements. During this time the OCP was recorded for 200s to obtain a steady-state value representing the corrosion potential (Ecorr) of the working electrode. All data for electrochemical measurements were analyzed using Gamry EChem Analyst 6.03 software package. 2.5. Surface Analysis. Surface morphological studies of the mild steel electrode were studied through scanning electron microscopy using a SEM model FEI Quanta 200F microscope at 5000× magnification. The following two case were selected for SEM study (a) mild steel coupon in 1 M HCl solution with no inhibitor and (b) with optimum concentration of (ADTP I), after 3 h immersion.

Table 2. Weight Loss Observations for Mild Steel in 1 M HCl and with Different Concentrations of ADTP (I−III) at 308 K inhibitors blank ADTP I

ADTP II

ADTP III

η% =

concentration (mmol L−1)

corrosion rate (mg cm−2 h−1)

surface coverage (θ)

inhibition efficiency (η%)

0.31 0.61 0.91 1.22 1.52 0.31 0.61 0.91 1.22 1.52 0.31 0.61 0.91 1.22 1.52

7.00 0.83 0.46 0.30 0.17 0.17 0.73 0.53 0.37 0.27 0.27 0.90 0.60 0.47 0.37 0.37

0.88 0.93 0.96 0.98 0.98 0.89 0.92 0.95 0.96 0.96 0.87 0.91 0.93 0.95 0.95

88.1 93.3 95.7 97.6 97.6 89.5 92.4 94.7 96.1 96.1 87.1 91.4 93.3 94.7 94.7

Cro − Cri × 100 Cro

(1)

where Cor and Cir represent the corrosion rates values without and with inhibitors, respectively. The corrosion rate (mg cm−2 h−1) is Cr = Δw/At where Δw is the weight loss of mild steel coupons (mg), A is the area of coupon (cm2), t is the exposure time (h).25 Among the investigated inhibitors ADTP I showed highest efficiency 97.6% and order of inhibition is as follows: ADTP I > ADTP II > ADTP III. The difference in corrosion inhibition efficiency for the ADTPs is attributive to the difference in their structure and molecular weight. The better performance of ADTP I may due to the +I effect of donating group (−OCH3). 3.2. Thermodynamic Consideration. 3.2.1. Application of Adsorption Isotherm. The inhibition through adsorption by organic inhibitors is considered as a quasi-substitution reaction

3. RESULTS AND DISCUSSION 3.1. Weight Loss Measurements. Effect of Inhibitor Concentration. In order to study the effect of inhibitor on corrosion rate of mild steel weight loss method was performed, and results are compiled in Table 2. The corrosion rates decreased and inhibition efficiency increased as a result of increasing in the concentration of all inhibitors. The maximum inhibition efficiency was achieved at a concentration of 1.22 mmol L−1 and after adding more inhibitor did not create any significant change in the efficiency. Inhibition efficiency was calculated from corrosion rates by the following relationship: 2852

dx.doi.org/10.1021/ie401633y | Ind. Eng. Chem. Res. 2014, 53, 2851−2859


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Figure 4. Arrhenius plots of log Cr/T vs 1/T 1.22 mmol L−1 for ADTP derivatives in 1 M HCl solution.

Figure 2. Langmuir adsorption isotherm plots for ADTP derivatives in 1 M HCl solution.

Figure 5. Chrono-potentiometric (zero current) curves for mild steel in 1 M HCl without and with optimum concentrations of different ADTPs at 308 K. Figure 3. Arrhenius plots of log Cr vs 1/T at 1.22 mmol L−1 for ADTP derivatives in 1 M HCl solution.

On account of quasi-substitution, adsorption is now considered in thermodynamic terms by testing the suitable adsorption isotherm. The best approach to understand the adsorption behavior of inhibitors on the metal surface is to plot the surface coverage values (θ) against logarithm of molar concentration of inhibitor (log Cinh). In the present study a straight line was obtained. From which it can be observed that the adsorption of investigated inhibitors obeys Langmuir’s isotherm and follows the following equation:27,28

Table 3. Summary of Activation Values of Mild Steel in 1 M HCl without and Containing Lower and Higher Concentrations of ADTPs inhibitors (mmol L−1) blank 0.31 1.22 0.31 1.22 0.31 1.22

Ea (kJ mol−1)

ΔH* (kJ mol−1)

31.32 ADTP 1 58.15 87.64 ADTP II 89.22 83.22 ADTP III 51.96 78.51

ΔS* (J mol−1)

29.75

−132.80

55.51 84.99

−67.31 15.64

56.58 80.57

−63.69 5.36

49.31 75.86

−83.54 −7.65

C inh 1 = + C inh θ K ads

Kads is the equilibrium constant. The correlation coefficient (R2) is concerned to determine the best fits. From Figure 2 the linearity with R2 varying from 0.9593 to 0.9794 for the Langmuir isotherm provided the most satisfactory fit for the experimental data of ADTPs. The values of Kads calculated from the isotherm fit in Figure 2 are given in Table S1 in the Supporting Information. The high values of Kads represent greater efficient of adsorption and thus better inhibition efficiency. Kads is related to the free energy of adsorption (ΔGoads) by the following equation:

between organic molecules and water molecules at a corroding interface as26 organic (sol) + x H 2O (ads) → organic (ads) + x H 2O (sol)

(3)

o ΔGads = −RT ln(55.5K ads)

(2) 2853

(4)



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Figure 7. Phase impedance plots for mild steel in 1 M HCl in absence and presence of different concentrations of (a) ADTP I, (b) ADTP II, and (c) ADTP III.

Figure 6. Nyquist plots for mild steel in 1 M HCl containing different concentrations of (a) ADTP I, (b) ADTP II, and (c) ADTP III at 308 K.

which can characterize the interaction of adsorption molecules o values ensure a and metal surface. The negative ΔGads spontaneous adsorption and lead to a stable adsorbed layer on the mild steel surface.29 A −20 kJ mol−1 or lower value of ΔGoads shows electrostatic interaction between charged organic molecules present in bulk solution and charged metal surface. On the other hand, a ΔGoads value near or greater than −40 kJ mol−1 includes charge sharing or transferring between the

Figure 8. Equivalent circuit model used to fit the impedance spectra.

organic molecules and metal surface.30−32 In this study the calculated values of ΔGoads for ADTPs ranged from −28.61 to −38.86 kJ mol−1. These findings show that the adsorption of 2854



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Table 4. Electrochemical Impedance Parameters in 1 M HCl at Various Concentrations of ADTPs (I−III) inhibitors (mmol L−1)

Rs (Ω cm2)

1 M HCl

1.12

0.31 0.61 0.91 1.22

1.368 0.891 2.348 1.195

160.9 217.5 287.8 435.4

0.31 0.61 0.91 1.22

0.902 2.434 2.205 2.402

163.4 205.6 258.9 352.7

0.31 0.61 0.91 1.22

1.027 2.065 1.604 1.610

130.1 176.2 244.8 297.5

Rct (Ω cm2) 9.55

Y (Ω−1 sn cm−2)

n

Cdl (μF cm−2)

482.0 ADTP I 143.0 132.5 55.94 51.59 ADTP II 162.4 172.1 83.19 79.33 ADTP III 179.2 90.63 98.46 72.60

0.789

124.8

Ea + log λ 2.303RT

(5)

where Ea and λ represent the apparent activation energy and the frequency factor, along with R and T is the molar gas constant (8.314 J K−1 mol−1) and the absolute temperature, respectively. The resultant Arrhenius plots gives the straight line, Figure S1 (Supporting Information) and Figure 3, the slopes of which give the apparent activation energy (Ea) given in Table 3. It is obvious that Ea values for systems containing lower and higher concentrations of inhibitors (ranged from 51.96 to 87.64 kJ mol−1) are higher in comparison to inhibitorfree solution (31.32 kJ mol−1). Usually, an increase in the activation energy indicates physical adsorption, while unchanged or decreased energy is correlated with the occurrence of chemisorptions on the metal surface.33,34 The data presented in Table 3 provide significant support to the nature of the interaction between adosrbate and metal, i.e., the electrostatic interaction of all ADTPs. However, the adsorption of organic inhibitor can be governed by two considerations: (i) the competitive adsorption of absorbable species present in the solution35 and (ii) molecular interactions within adsorbed layers.36 Thus the adsorption of organic inhibitor molecules on the metal surface is complex in nature, and it is not possible to consider the same solely as a physical or chemical adsorption. In an attempt to elucidate the enthalpy of activation ΔH*, and entropy of activation ΔS* for the following transition state equation: Cr =

RT ⎛⎜ ΔH * ⎞⎟ ⎛⎜ ΔS* ⎞⎟ exp − exp Nh ⎝ RT ⎠ ⎝ R ⎠

0.890 0.825 0.836 0.868

89.70 62.40 24.90 29.00

94.1 95.6 96.6 97.8

0.835 0.838 0.847 0.847

79.30 90.20 41.60 41.60

94.2 95.3 96.3 97.2

0.850 0.854 0.840 0.885

92.30 44.70 48.40 44.10

92.6 94.5 96.1 96.7

where h is the Planck constant, N is the Avogadro number, R is the universal gas constant, ΔH* is the enthalpy of activation, and ΔS* is the entropy of activation respectively. For this purpose, relationship of log Cr/T and 1/T are illustrated in Figure S2 (Supporting Information) and Figure 4. The slope and intercept of the straight line are (−ΔH*/2.303R) and (log(R/Nh) + (ΔS*/2.303R)), respectively. These are used to compute the values of ΔH* and ΔS*, respectively, and compiled in Table 3. Considering these data of activation function ΔH*, it is useful to emphasize that the more energy barrier is required for the dissolution of mild steel in the presence of the inhibitors.37 One can arrive at a similar conclusion which is examined with activation energy. There is also an agreement between the values of ΔH* and Ea as they change in the same manner which is qualified by following equation ΔH* = Ea − RT. Consequently, the higher value of ΔS* in the presence of inhibitors suggests that in the formation of the activated complex, the rate determining step is dissociation rather than association.20,38 It can conclude that quasi-equilibrium exists between the water molecules and ADTPs on the metal electrode surface. Increment in the ΔS* drives the adsorption of inhibitors on the metal surface. 3.3. Electrochemical Measurements. 3.3.1. Open Circuit Potential vs Time. The variation in OCP of mild steel vs reference electrode in 1 M HCl without and containing optimum concentrations of ADTP I, ADTP II, ADTP III at 308 K is graphically represented in Figure 5. Prior to steady-state condition (30 min of immersion) the OCP values, in blank solution, are more positive than that of Eocp at t = 0 as a result of air oxide film dissolution at the electrode surface.19 From Figure 5, a nobler shift in Eocp value without altering the common characteristics of the E−t plots on the addition of ADTP I, ADTP II, and ADTP III in 1 M HCl solution indicating their role in catalysis of oxide film dissolution. The noble shift in the OCP is attributable to the generation of a protective layer of inhibitor on the electrode surface.20 3.3.2. Electrochemical Impedance Spectroscopy Analysis. Nyquist and Bode plots of different concentrations of ADTPs are represented in Figures 6 and 7, respectively. The Nyquist plots exhibit single depressed semicircles across the frequency range studied, parallel to one time constant in the Bode plots, which denotes that the dissolution process is controled by charge-transfer reaction.39 The depression of the Nyquist

ADTPs, on the surface of mild steel, involves complex interactions as result of both physisorption and chemisorption. 3.2.2. Effect of Temperature. The effect of temperature on the performance of inhibitors, at lower and optimum concentration, was studied by weight loss method and results are given in Table S2 (Supporting Information). It can be observed that ADTPs had good inhibition efficiencies against corrosion of mild steel, but inhibition efficiencies decreased with increasing temperature as desorption take place at higher temperature.21 The Arrhenius equation given below was used, to derive thermodynamic activation parameters in the absence and presence of inhibitors. log Cr = −

inhibition efficiency η%

(6) 2855



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Table 5. Parameters Obtained by Tafel Polarization Measurements in the Absence and Presence of Different Concentrations of ADTP (I−III) inhibitors (mmol L−1)

Ecorr (mV vs SCE)

βa (mV dec−1)

blank

−445.8

62.70

0.31 0.61 0.91 1.22

−491.0 −523.0 −514.0 −489.0

0.31 0.61 0.91 1.22

−475.0 −502.0 −491.0 −523.0

0.31 0.61 0.91 1.22

−483.0 −487.0 −507.0 −499.0

βc Icorr (mV dec−1) (μA cm−2)

116.7 ADTP I 77.10 144.8 124.7 149.1 139.9 212.2 85.40 193.0 ADTP II 72.20 109.5 107.9 124.4 89.60 236.4 96.30 109.5 ADTP III 51.80 128.4 76.80 163.2 92.80 127.4 110.9 184.6

inhibition efficiency (η%)

1320 124.0 106.0 70.70 44.80

90.6 91.9 94.6 96.6

182.0 119.0 81.30 59.30

86.2 90.9 93.8 95.5

395.0 159.0 118.0 62.60

70.1 87.9 91.0 95.2

impedance, ZCPE, for the rough solid electrode is described by the expression:18,42

ZCPE = Y0−1(iω)−n

(7)

where Yo is a proportionality factor and ω is angular frequency while n is a CPE exponent that can be used as a gauge of the surface roughness/heterogeneity or to continuously distributed time constants for the charge-transfer reactions. The constant phase elements (CPE), with their n values changed from 0.79 to 0.89, identify the mechanism of mild steel dissolution in the absence and presence of ADTPs. The CPE only describes an ideal capacitor when n = 1. The electrochemical parameters are listed in Table 4 and illustrate that an increase in the magnitude of Rct with corresponding increase in the concentrations of ADTPs and reaches a maximum values 435 Ω in the case of ADTP I. Additionally, the value of proportionality factor Yo of CPE changes uniformly with the concentration of inhibitors. Variation in the values of Rct and Yo can be correlated with the gradual displacement of water molecules with those of the inhibitor on the electrode surface leading to a decrease in the active sites and slowing down the corrosion process. An increase in the concentrations of ADTPs resulted increases in the magnitude of the phases in the Bode plots. On the other hand there is a decrease in Cdl values along with increasing in the concentration of inhibitors. Such findings also support the displacement of water molecules by means of the adsorption of inhibitors on the mild steel surface.43 The decrease in Cdl values perhaps ascribed to decrement in the local dielectric constant and/or increment in the thickness of the electrical double layer. The inhibition efficiency was under taken by putting the values of the charge transfer resistance in the absence R0ct and presence of inhibitor Rict as follows:

Figure 9. Potentiodynamic polarization curves for mild steel in 1 M HCl containing different concentrations of (a) ADTP I, (b) ADTP II, and (c) ADTP III at 308 K.

semicircle for solid electrodes is often termed as frequency dispersion which can be ascribed to different physical phenomena such as surface roughness, active sites, and nonhomogeneity of the solids.40 The EIS spectra were examined via fitting to the equivalent circuit model arranged in such a way Rs in series to the parallel of CPE and Rct shown in Figure 8. Where, Rs is the solution resistance and assigned at the high frequency intercept and Rct is the charge transfer resistance at low frequency intercept, with the real axis in the Nyquist plots. The CPE, constant phase element, which is used in place of pure capacitor for the deviations of ideal dielectric behaviors related to electrode surface inhomogeniety.41 The

η% =

R cti − R ct0 R cti

× 100 (8)

The order of inhibition efficiency is ADTP I > ADTP II > ADTP III at optimum concentration and maximum inhibition efficiency is 97.8% for ADTP I. 2856



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Figure 11. EDX spectra of mild steel specimens: (a) uninhibited 1 M HCl and (b) in the presence of ADTP I.

potential toward negative direction in comparison to the potential of inhibitor-free solution. Such findings in potential support a mixed-type inhibitor behavior of ADTPs in 1 M HCl.47 Inhibition efficiency was calculated by equating the corrosion current densities values, in the absence (I0corr) and presence of inhibitor (Iicorr), as follows: η% =

0 i Icorr − Icorr 0 Icorr

× 100 (9)

The inhibition efficiency increases with increasing concentration of ADTPs and follows the order ADTP I > ADTP II > ADTP III with the highest inhibition efficiency being 96.6% for ADTP I. The same trend of inhibition has been again highlighted by this method, as with the gravimetric and impedance data, and in good agreement with them. 3.4. Scanning Electron Microscopy Analysis. Scanning electron microscopy (SEM) pictures and EDX pattern of mild steel surface without and containing optimum concentration of ADTP I after immersion of 3 h in 1 M HCl are shown in Figure 10 and 11, respectively. The morphology of mild steel surface in Figure 10a shows pits and cracks indicating that the steel surface was rigorously scratched in the absence of ADTP. On the other hand Figure 10b appears to be less scratched in the presence of ADTP I. Figure 11 and Table S4 (Supporting Information) represent the EDX spectra and percentage of atomic content in mild steel samples, respectively. Figure 11a shows the characteristics peaks of the elements constituting the mild steel in absence of ADTP. The EDX patterns in the presence of ADTP I, Figure 11b, show an additional peak due to presence of N and S. The presence of N and S peaks in the EDX patterns of inhibitors on the surface indicated that inhibitor is adsorbed on the mild steel surface, preventing it from being corroded.

Figure 10. SEM micrographs of mild steel surfaces: (a) uninhibited 1 M HCl and (b) in the presence of ADTP I.

Bode-phase formats (log f vs phase) were chosen as a model system to explain the various phenomena taking place at the interfaces by admitting the frequency range. For the ideal capacitance the phase angle (α) and slope (S) values are −90° and −1, respectively.44 In the present investigation at the region of intermediate frequency, a linearity between log|Z| vs log f with a slope close to −0.83 and a phase angle of −73° is observed and listed in Table S3 (Supporting Information). The higher slope and phase angle values for solutions with inhibitors than those without inhibitors suggest the growth of a protective film of inhibitors on the electrode surface. 3.3.3. Potentiodynamic Polarization Measurements. Polarization measurements furnish the information about the kinetics of corrosion reactions.45,46 The potentiodynamic polarization curves for mild steel in 1 M HCl in the absence and presence of the ADTPs are illustrated in Figure 9, respectively, and electrochemical parameters are summarized in Table 5. It is clear from the outcomes that the anodic and cathodic reactions are affected by addition of ADTPs. Table 5 points out that ADTPs markedly affect the cathodic reaction as the change in βc values is more; whereas, βa values change only a little. The addition of ADTPs leads to find noticeable shifts in

4. CONCLUSION This study has revealed that ADTPs are good corrosion inhibitors for mild steel in 1 M HCl. The order of inhibition efficiency is ADTP I > ADTP II > ADTP III at optimum 2857



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concentration. Polarization measurements show that they are mixed-type inhibitors. However, the cathodic inhibiting effect is more prominent. Impedance data and SEM results specify that dissolution of mild steel was prevented by the adsorption of ADTPs on its surface. The adsorption of ADTPs on mild steel followed the Langmuir adsorption.

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ASSOCIATED CONTENT

S Supporting Information *

Langmuir adsorption isotherm parameters at 308 K (Table S1), Parameters form weight loss method of ADTPs at different temperature (Table S2), Maximum phase angles (α) and slopes (S) of Bode plots in the intermediate frequency region (Table S3), and Elements found from EDX spectra (Table S4). Arrhenius plots of log Cr vs 1/T and log Cr/T vs 1/T at lower 0.31 mmol L−1 for ADTP derivatives (Figure S1 and Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail addresses: [email protected]; maquraishi@ rediffmail.com. Tel.: +91-9307025126. Fax: +91-542-2368428. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Sudheer is grateful to the University Grant Commission (UGC), New Delhi, India, for financial assistance under RFSMS scheme.

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REFERENCES

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New and Effective Corrosion Inhibitors for Mild Steel in 1 M HCl

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