2‑Amino-3,5-dicarbonitrile-6-thio-pyridines: New and EffectiveCorrosion Inhibitors for Mild Steel in 1 M HClSudheer 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-3,5-dicarbonitrile-4-(4-nitrophenyl)-6-(phenylthio) pyridine (ADTP III) was investigated by electrochemical impedance spectros-copy, potentiodynamic polarization, and weight loss techniques. The results of potentiodynamic polarization studies show thatthe three compounds under investigation show mixed-type inhibition behavior. Among them, ADTP I shows the highestinhibition efficiency of 97.6% at 1.22 mmol L−1. The effect of temperature and related activation parameters were worked out. Toinspect 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 ofcorrosion inhibitors.1−3 Nitrogen-containing compounds func-tion more effectively as corrosion inhibitors in hydrochloricacid4,5 whereas sulfur-containing inhibitors are preferred forH2SO4.
6,7 Heterocyclic compounds containing both nitrogenand sulfur are of particular importance as they often provideexcellent inhibition.8−11 The aim of the present work is tosynthesize heterocyclic compounds containing both sulfur andnitrogen atoms in the same ring. A perusal of the literaturereveals that some pyridine compounds12−17 have beeninvestigated for their corrosion inhibition behavior in hydro-chloric 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 pyridinederivatives: 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-3,5-dicarbonitrile-4-(4-nitrophenyl)-6-(phenylthio) pyridine(ADTP III) and marked as ADTPS. The inhibition analysesof above pyridine derivatives were carried out following weightloss, electrochemical impedance spectroscopy (EIS), andpotentiodynamic polarization measurement.23 The SEM andEDX techniques were also used to examine surface morphologyof mild steel with and without inhibitors in 1 M HCl.
2. MATERIALS AND METHOD
2.1. Electrodes and Solutions. The mild steel used forexperimental whose chemical composition by wt % is asfollows: 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 mildsteel coupons, for weight loss and electrochemical study, werecut 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 impuritiesfrom the surface, coupons were abraded with SiC papers (800−1500 grade) and washed with distilled water, followed by
degreasing with acetone. The test solution 1 M HCl wasprepared from 37% hydrochloric acid (Merck) analytical gradechemical and bidistilled water. The inhibitors concentrationswere taken in millimoles per liter for all investigation.
2.2. Inhibitors. The inhibitors were prepared as reportedearlier,24 and the scheme is shown in Figure 1. The completion
of reaction was monitored by TLC (ethyl acetate/n-hexane,1:7) and product purification was performed by ethanolrecrystallization. 1H NMR (300 MHz) spectra of selectedcompounds was determined via JEOL AL 300 FT-NMR inDMSO with TMS as internal standard. The chemicalstructures, names, spectral data, and their melting points areprovided in Table 1.
2.3. Weight Loss Measurement. The weight lossexperiments were carried out as descried previously.19 Themild steel was immersed in 1 M HCl in absence and presenceof various concentrations (0.31−1.22 mmol L−1) of inhibitorsfor 3 h duration at constant temperature 308 K. The effect oftemperature on inhibition of all the three inhibitors was studiedat different temperature (308−328) using lower and optimumconcentration of inhibitors. The maximum standard deviationwas observed ±2.5% in the weight loss study.
Received: May 23, 2013Revised: December 30, 2013Accepted: January 24, 2014Published: January 24, 2014
2.4. Electrochemical Measurements. Electrochemicalexperiments were conducted using a Gamry Potentiostat/Galvanostat with an ESA400 Gamry framework system. Aconventional cell setup had three electrode mild steel asworking electrode, platinum foil as auxiliary electrode, and asaturated calomel electrode (SCE) as reference electrode wasutilized for above measurement. The open circuit potential(OCP) was obtained by immersing the working electrode inthe test solution 1 M HCl for 30 min. Electrochemicalimpedance spectroscopy (EIS) measurements were carried outat corrosion potentials (OCP) across the frequency range 100kHz−10 mHz, with a 10 mV amplitude of waveform. Forpotentiodynamic polarization measurements, potential wasscanned in the range −250 to +250 mV at a scan rate 1 mVs−1. The electrode was allowed to corrode freely prior to EISand polarization measurements. During this time the OCP wasrecorded for 200s to obtain a steady-state value representingthe corrosion potential (Ecorr) of the working electrode. All datafor electrochemical measurements were analyzed using GamryEChem Analyst 6.03 software package.2.5. Surface Analysis. Surface morphological studies of the
mild steel electrode were studied through scanning electronmicroscopy using a SEM model FEI Quanta 200F microscopeat 5000× magnification. The following two case were selectedfor SEM study (a) mild steel coupon in 1 M HCl solution withno inhibitor and (b) with optimum concentration of (ADTP I),after 3 h immersion.
3. RESULTS AND DISCUSSION
3.1. Weight Loss Measurements. Effect of InhibitorConcentration. In order to study the effect of inhibitor oncorrosion rate of mild steel weight loss method was performed,and results are compiled in Table 2. The corrosion ratesdecreased and inhibition efficiency increased as a result ofincreasing in the concentration of all inhibitors. The maximuminhibition efficiency was achieved at a concentration of 1.22mmol L−1 and after adding more inhibitor did not create anysignificant change in the efficiency. Inhibition efficiency wascalculated from corrosion rates by the following relationship:
η =−
×C C
C100%
ro
ri
ro
(1)
where Cro and Cr
i represent the corrosion rates values withoutand with inhibitors, respectively. The corrosion rate (mg cm−2
h−1) is Cr = Δw/At where Δw is the weight loss of mild steelcoupons (mg), A is the area of coupon (cm2), t is the exposuretime (h).25 Among the investigated inhibitors ADTP I showedhighest efficiency 97.6% and order of inhibition is as follows:ADTP I > ADTP II > ADTP III. The difference in corrosioninhibition efficiency for the ADTPs is attributive to thedifference in their structure and molecular weight. The betterperformance of ADTP I may due to the +I effect of donatinggroup (−OCH3).
3.2. Thermodynamic Consideration. 3.2.1. Applicationof Adsorption Isotherm. The inhibition through adsorption byorganic inhibitors is considered as a quasi-substitution reaction
Table 1. Chemical Structures and Names of the Inhibitors under Investigation
Table 2. Weight Loss Observations for Mild Steel in 1 MHCl and with Different Concentrations of ADTP (I−III) at308 K
between organic molecules and water molecules at a corrodinginterface as26
+
→ +
x
x
organic (sol) H O (ads)
organic (ads) H O (sol)2
2 (2)
On account of quasi-substitution, adsorption is now consideredin thermodynamic terms by testing the suitable adsorptionisotherm. The best approach to understand the adsorptionbehavior of inhibitors on the metal surface is to plot the surfacecoverage values (θ) against logarithm of molar concentration ofinhibitor (log Cinh). In the present study a straight line wasobtained. From which it can be observed that the adsorption ofinvestigated inhibitors obeys Langmuir’s isotherm and followsthe following equation:27,28
θ= +
CK
C1inh
adsinh
(3)
Kads is the equilibrium constant. The correlation coefficient(R2) is concerned to determine the best fits. From Figure 2 thelinearity with R2 varying from 0.9593 to 0.9794 for theLangmuir isotherm provided the most satisfactory fit for theexperimental data of ADTPs. The values of Kads calculated fromthe isotherm fit in Figure 2 are given in Table S1 in theSupporting Information. The high values of Kads representgreater efficient of adsorption and thus better inhibitionefficiency. Kads is related to the free energy of adsorption(ΔGads
o ) by the following equation:
Δ = −G RT Kln(55.5 )adso
ads (4)
Figure 2. Langmuir adsorption isotherm plots for ADTP derivatives in1 M HCl solution.
Figure 3. Arrhenius plots of log Cr vs 1/T at 1.22 mmol L−1 for ADTPderivatives in 1 M HCl solution.
Table 3. Summary of Activation Values of Mild Steel in 1 MHCl without and Containing Lower and HigherConcentrations of ADTPs
which can characterize the interaction of adsorption moleculesand metal surface. The negative ΔGads
o values ensure aspontaneous adsorption and lead to a stable adsorbed layeron the mild steel surface.29 A −20 kJ mol−1 or lower value ofΔGads
o shows electrostatic interaction between charged organicmolecules present in bulk solution and charged metal surface.On the other hand, a ΔGads
o value near or greater than −40 kJmol−1 includes charge sharing or transferring between the
organic molecules and metal surface.30−32 In this study thecalculated values of ΔGads
o for ADTPs ranged from −28.61 to−38.86 kJ mol−1. These findings show that the adsorption of
Figure 6. Nyquist plots for mild steel in 1 M HCl containing differentconcentrations of (a) ADTP I, (b) ADTP II, and (c) ADTP III at 308K.
Figure 7. Phase impedance plots for mild steel in 1 M HCl in absenceand presence of different concentrations of (a) ADTP I, (b) ADTP II,and (c) ADTP III.
Figure 8. Equivalent circuit model used to fit the impedance spectra.
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ADTPs, on the surface of mild steel, involves complexinteractions 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 optimumconcentration, was studied by weight loss method and resultsare given in Table S2 (Supporting Information). It can beobserved that ADTPs had good inhibition efficiencies againstcorrosion of mild steel, but inhibition efficiencies decreasedwith increasing temperature as desorption take place at highertemperature.21 The Arrhenius equation given below was used,to derive thermodynamic activation parameters in the absenceand presence of inhibitors.
λ= − +CE
RTlog
2.303logr
a
(5)
where Ea and λ represent the apparent activation energy andthe frequency factor, along with R and T is the molar gasconstant (8.314 J K−1 mol−1) and the absolute temperature,respectively. The resultant Arrhenius plots gives the straightline, Figure S1 (Supporting Information) and Figure 3, theslopes of which give the apparent activation energy (Ea) givenin Table 3. It is obvious that Ea values for systems containinglower and higher concentrations of inhibitors (ranged from51.96 to 87.64 kJ mol−1) are higher in comparison to inhibitor-free solution (31.32 kJ mol−1). Usually, an increase in theactivation energy indicates physical adsorption, while un-changed or decreased energy is correlated with the occurrenceof chemisorptions on the metal surface.33,34 The data presentedin Table 3 provide significant support to the nature of theinteraction between adosrbate and metal, i.e., the electrostaticinteraction of all ADTPs. However, the adsorption of organicinhibitor can be governed by two considerations: (i) thecompetitive adsorption of absorbable species present in thesolution35 and (ii) molecular interactions within adsorbedlayers.36 Thus the adsorption of organic inhibitor molecules onthe metal surface is complex in nature, and it is not possible toconsider 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 stateequation:
= − Δ * Δ *⎜ ⎟ ⎜ ⎟⎛⎝
⎞⎠
⎛⎝
⎞⎠C
RTNh
HRT
SR
exp expr (6)
where h is the Planck constant, N is the Avogadro number, R isthe universal gas constant, ΔH* is the enthalpy of activation,and ΔS* is the entropy of activation respectively. For thispurpose, relationship of log Cr/T and 1/T are illustrated inFigure S2 (Supporting Information) and Figure 4. The slopeand intercept of the straight line are (−ΔH*/2.303R) and(log(R/Nh) + (ΔS*/2.303R)), respectively. These are used tocompute the values of ΔH* and ΔS*, respectively, andcompiled in Table 3. Considering these data of activationfunction ΔH*, it is useful to emphasize that the more energybarrier is required for the dissolution of mild steel in thepresence of the inhibitors.37 One can arrive at a similarconclusion which is examined with activation energy. There isalso an agreement between the values of ΔH* and Ea as theychange in the same manner which is qualified by followingequation ΔH* = Ea − RT. Consequently, the higher value ofΔS* in the presence of inhibitors suggests that in the formationof the activated complex, the rate determining step isdissociation rather than association.20,38 It can conclude thatquasi-equilibrium exists between the water molecules andADTPs 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 CircuitPotential vs Time. The variation in OCP of mild steel vsreference electrode in 1 M HCl without and containingoptimum concentrations of ADTP I, ADTP II, ADTP III at 308K is graphically represented in Figure 5. Prior to steady-statecondition (30 min of immersion) the OCP values, in blanksolution, are more positive than that of Eocp at t = 0 as a resultof air oxide film dissolution at the electrode surface.19 FromFigure 5, a nobler shift in Eocp value without altering thecommon characteristics of the E−t plots on the addition ofADTP I, ADTP II, and ADTP III in 1 M HCl solutionindicating their role in catalysis of oxide film dissolution. Thenoble shift in the OCP is attributable to the generation of aprotective layer of inhibitor on the electrode surface.20
3.3.2. Electrochemical Impedance Spectroscopy Analysis.Nyquist and Bode plots of different concentrations of ADTPsare represented in Figures 6 and 7, respectively. The Nyquistplots exhibit single depressed semicircles across the frequencyrange studied, parallel to one time constant in the Bode plots,which denotes that the dissolution process is controled bycharge-transfer reaction.39 The depression of the Nyquist
Table 4. Electrochemical Impedance Parameters in 1 M HCl at Various Concentrations of ADTPs (I−III)
inhibitors (mmol L−1) Rs (Ω cm2) Rct (Ω cm2) Y (Ω−1 sn cm−2) n Cdl (μF cm−2) inhibition efficiency η%
semicircle for solid electrodes is often termed as frequencydispersion which can be ascribed to different physicalphenomena such as surface roughness, active sites, andnonhomogeneity of the solids.40 The EIS spectra wereexamined via fitting to the equivalent circuit model arrangedin such a way Rs in series to the parallel of CPE and Rct shownin Figure 8. Where, Rs is the solution resistance and assigned atthe high frequency intercept and Rct is the charge transferresistance at low frequency intercept, with the real axis in theNyquist plots. The CPE, constant phase element, which is usedin place of pure capacitor for the deviations of ideal dielectricbehaviors related to electrode surface inhomogeniety.41 The
impedance, ZCPE, for the rough solid electrode is described bythe expression:18,42
ω= − −Z Y i( ) nCPE 0
1(7)
where Yo is a proportionality factor and ω is angular frequencywhile n is a CPE exponent that can be used as a gauge of thesurface roughness/heterogeneity or to continuously distributedtime constants for the charge-transfer reactions. The constantphase elements (CPE), with their n values changed from 0.79to 0.89, identify the mechanism of mild steel dissolution in theabsence and presence of ADTPs. The CPE only describes anideal capacitor when n = 1. The electrochemical parameters arelisted in Table 4 and illustrate that an increase in the magnitudeof Rct with corresponding increase in the concentrations ofADTPs and reaches a maximum values 435 Ω in the case ofADTP I. Additionally, the value of proportionality factor Yo ofCPE changes uniformly with the concentration of inhibitors.Variation in the values of Rct and Yo can be correlated with thegradual displacement of water molecules with those of theinhibitor on the electrode surface leading to a decrease in theactive sites and slowing down the corrosion process. Anincrease in the concentrations of ADTPs resulted increases inthe magnitude of the phases in the Bode plots. On the otherhand there is a decrease in Cdl values along with increasing inthe concentration of inhibitors. Such findings also support thedisplacement of water molecules by means of the adsorption ofinhibitors on the mild steel surface.43 The decrease in Cdl valuesperhaps ascribed to decrement in the local dielectric constantand/or increment in the thickness of the electrical double layer.The inhibition efficiency was under taken by putting the valuesof the charge transfer resistance in the absence Rct
0 and presenceof inhibitor Rct
i as follows:
η =−
×R R
R% 100ct
ict0
cti
(8)
The order of inhibition efficiency is ADTP I > ADTP II >ADTP III at optimum concentration and maximum inhibitionefficiency is 97.8% for ADTP I.
Figure 9. Potentiodynamic polarization curves for mild steel in 1 MHCl containing different concentrations of (a) ADTP I, (b) ADTP II,and (c) ADTP III at 308 K.
Table 5. Parameters Obtained by Tafel PolarizationMeasurements in the Absence and Presence of DifferentConcentrations of ADTP (I−III)
Bode-phase formats (log f vs phase) were chosen as a modelsystem to explain the various phenomena taking place at theinterfaces by admitting the frequency range. For the idealcapacitance the phase angle (α) and slope (S) values are −90°and −1, respectively.44 In the present investigation at the regionof intermediate frequency, a linearity between log|Z| vs log fwith a slope close to −0.83 and a phase angle of −73° isobserved and listed in Table S3 (Supporting Information). Thehigher slope and phase angle values for solutions with inhibitorsthan those without inhibitors suggest the growth of a protectivefilm of inhibitors on the electrode surface.3.3.3. Potentiodynamic Polarization Measurements. Polar-
ization measurements furnish the information about thekinetics of corrosion reactions.45,46 The potentiodynamicpolarization curves for mild steel in 1 M HCl in the absenceand presence of the ADTPs are illustrated in Figure 9,respectively, and electrochemical parameters are summarized inTable 5. It is clear from the outcomes that the anodic andcathodic reactions are affected by addition of ADTPs. Table 5points out that ADTPs markedly affect the cathodic reaction asthe change in βc values is more; whereas, βa values change onlya little. The addition of ADTPs leads to find noticeable shifts in
potential toward negative direction in comparison to thepotential of inhibitor-free solution. Such findings in potentialsupport a mixed-type inhibitor behavior of ADTPs in 1 MHCl.47 Inhibition efficiency was calculated by equating thecorrosion current densities values, in the absence (Icorr
0 ) andpresence of inhibitor (Icorr
i ), as follows:
η =−
×I I
I% 100corr
0corri
corr0
(9)
The inhibition efficiency increases with increasing concen-tration of ADTPs and follows the order ADTP I > ADTP II >ADTP III with the highest inhibition efficiency being 96.6% forADTP I. The same trend of inhibition has been againhighlighted by this method, as with the gravimetric andimpedance data, and in good agreement with them.
3.4. Scanning Electron Microscopy Analysis. Scanningelectron microscopy (SEM) pictures and EDX pattern of mildsteel surface without and containing optimum concentration ofADTP I after immersion of 3 h in 1 M HCl are shown in Figure10 and 11, respectively. The morphology of mild steel surfacein Figure 10a shows pits and cracks indicating that the steelsurface was rigorously scratched in the absence of ADTP. Onthe other hand Figure 10b appears to be less scratched in thepresence of ADTP I. Figure 11 and Table S4 (SupportingInformation) represent the EDX spectra and percentage ofatomic content in mild steel samples, respectively. Figure 11ashows the characteristics peaks of the elements constituting themild steel in absence of ADTP. The EDX patterns in thepresence of ADTP I, Figure 11b, show an additional peak dueto presence of N and S. The presence of N and S peaks in theEDX patterns of inhibitors on the surface indicated thatinhibitor is adsorbed on the mild steel surface, preventing itfrom being corroded.
4. CONCLUSIONThis study has revealed that ADTPs are good corrosioninhibitors for mild steel in 1 M HCl. The order of inhibitionefficiency is ADTP I > ADTP II > ADTP III at optimum
Figure 10. SEM micrographs of mild steel surfaces: (a) uninhibited 1M HCl and (b) in the presence of ADTP I.
Figure 11. EDX spectra of mild steel specimens: (a) uninhibited 1 MHCl and (b) in the presence of ADTP I.
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concentration. Polarization measurements show that they aremixed-type inhibitors. However, the cathodic inhibiting effect ismore prominent. Impedance data and SEM results specify thatdissolution of mild steel was prevented by the adsorption ofADTPs on its surface. The adsorption of ADTPs on mild steelfollowed the Langmuir adsorption.
■ ASSOCIATED CONTENT
*S Supporting InformationLangmuir adsorption isotherm parameters at 308 K (Table S1),Parameters form weight loss method of ADTPs at differenttemperature (Table S2), Maximum phase angles (α) and slopes(S) of Bode plots in the intermediate frequency region (TableS3), and Elements found from EDX spectra (Table S4).Arrhenius plots of log Cr vs 1/T and log Cr/T vs 1/T at lower0.31 mmol L−1 for ADTP derivatives (Figure S1 and FigureS2). This material is available free of charge via the Internet athttp://pubs.acs.org.
NotesThe authors declare no competing financial interest.
■ ACKNOWLEDGMENTS
Sudheer is grateful to the University Grant Commission(UGC), New Delhi, India, for financial assistance underRFSMS scheme.
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