Effect of Cefazolin on the corrosion of mild steel in HCl solution

Corrosion Science 52 (2010) 152–160

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Corrosion Science

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Effect of Cefazolin on the corrosion of mild steel in HCl solution

Ashish Kumar Singh, M.A. Quraishi *

Department of Applied Chemistry, Institute of Technology, Banaras Hindu University, Varanasi, Uttar Pradesh 221 005, India

a r t i c l e i n f o

Article history:Received 22 May 2009Accepted 26 August 2009Available online 31 August 2009

Keywords:A. Mild steelB. AFMB. Weight lossC. Acid inhibitionC. Kinetic parameters

0010-938X/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.corsci.2009.08.050

* Corresponding author. Tel.: +91 9307025126; faxE-mail addresses: [email protected], maq

Quraishi).

a b s t r a c t

The adsorption and inhibition effect of Cefazolin on mild steel in 1.0 M HCl at 308–338 K was studied byweight loss, EIS, potentiodynamic polarization and atomic force microscopy techniques. The resultsshowed that inhibition efficiency increased with inhibitor concentration. The adsorption of Cefazolinon mild steel surface obeys the Langmuir adsorption isotherm equation. Both thermodynamic (enthalpyof adsorption DH�ads , entropy of adsorption DS�ads and free energy of adsorption DG�ads) and kinetic param-eters (activation energy DE�a and pre-exponential factor A) were calculated and discussed. Polarizationcurves showed that Cefazolin acted as mixed-type inhibitor controls predominantly cathodic reaction.

� 2009 Elsevier Ltd. All rights reserved.

1. Introduction

The damage by corrosion generates not only high cost forinspection, repairing and replacement, but in addition these consti-tute a public risk, thus the necessity of developing novel sub-stances that behave like corrosion inhibitors especially in acidmedia [1]. There always exists a need for developing new corrosioninhibitors. Acid solutions are widely used in industry such as acidpickling of iron and steel, chemical cleaning and processing, oreproduction and oil well acidification. The use of hydrochloric acidin pickling of metals, acidization of oil wells and in cleaning ofscales is more economical, efficient and trouble-free, comparedto other mineral acids [2].

The corrosion inhibition efficiency of organic compounds is re-lated to their adsorption properties. Studies reported that theadsorption of organic inhibitors mainly depends on some physio-chemical properties of the molecule, related to its functionalgroups, to the possible steric effects and electronic density of donoratoms. Adsorption also depends on the possible interaction of p-orbital of the inhibitor with d-orbital of the surface atoms, whichinduces greater adsorption of the inhibitor molecules on the sur-face of mild steel [3–6].

A large number of organic compounds including heterocycliccompounds [7–12] were studied as corrosion inhibitors for mildsteel [13–15]. Most of them are toxic in nature. This has led tothe development of non-toxic corrosion inhibitors such as Trypt-amine [16], L-ascorbic acid [17], Sulfamethoxazole [18], and Cefa-

ll rights reserved.

: +91 542 [email protected] (M.A.

trexyl [19], Ceftriaxone [20], Cefotaxime [21], sulfa drugs [22],antibacterial drugs [23], antifungal drugs [24], rhodanine azosul-pha drugs [25]. Prabhu et al. [26] reported the inhibitor activityof tramadol on mild steel in HCl and H2SO4.

Cefazolin is a first generation cephalosporin antibiotic [27]. It ismainly used to treat bacterial infections of the skin. It can also beused to treat moderately sever bacterial infections involving thelung, bone, stomach, blood, heart valve [28]. It is clinically effectiveagainst infections caused by Staphylococci and Streptococci ofGram positive bacteria [29]. Cefazolin is the commercial name of(6R,7R)-3-{[(5-methyl-1,3,4-thiadiazol-2-yl)thio]methyl}-8-oxo-7-[(1H-tetrazol-1-ylacetyl)amino]-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylic acid. It is a first generation cephalosporinantibiotic.

In the present work, we have investigated the inhibitive actionof Cefazolin on corrosion of mild steel in 1 M HCl solution at 308 Kusing weight loss, polarization resistance, Tafel polarization andelectrochemical impedance techniques. The effects of temperature,acid concentration, immersion time were also studied. Several iso-therms were tested for their relevance to describe the adsorptionbehaviour of the compound studied.

2. Experimental

2.1. Materials

Tests were performed on mild steel having composition (wt.%)C = 0.17, Mn = 0.46, Si = 0.26, S = 0.017, P = 0.019 and balanceFe were used for weight loss as well as electrochemical studies.The aggressive solution of hydrochloric acid (AR grade) of 1 M

A.K. Singh, M.A. Quraishi / Corrosion Science 52 (2010) 152–160 153

concentration was used for all studies (except in the study of effectof acid concentration).

2.2. Inhibitor

Cefazolin was purchased from medicine shop as a trade nameCefacidal powder injection (m.p. � 463 K) and used without fur-ther purification. Fig. 1 shows the molecular structure of Cefazolin.Cefazolin is a N–S heterocyclic compound containing eight nitro-gen atoms which could be easily protonated in acid solution, anda great deal of p-electrons exists in this molecule.

2.3. Solutions

The aggressive solutions, 1.0 M HCl, were prepared by dilutionof AR grade 37% HCl in distilled water. The stock solution of Cefaz-olin was diluted to a certain conc. of Cefazolin. The inhibitor con-centration in the weight loss and electrochemical study was inthe range of 2.19 � 10�4 M to 10.95 � 10�4 M and 4.76 � 10�4 Mto 8.76 � 10�4 M, respectively.

2.4. Weight loss measurements

The mild steel strips of 2.5 cm � 2.0 cm � 0.025 cm sizes wereabraded with a series of emery paper (grade 600–800–1000–1200) and then washed with distilled water and acetone. Afterweighing accurately, the specimens were immersed in 100 ml of1.0 M HCl with and without addition of different concentrationsof Cefazolin. After 3 h, the strips were taken out washed, driedand weighed accurately. Duplicate experiments were performedin each and the mean value of the weight loss was reported. Theinhibition efficiency ðgW%Þ and surface coverage (h) was deter-mined by using following equation:

gW% ¼Wo �Wi

Wo� 100 ð1Þ

h ¼Wo �Wi

Woð2Þ

where Wi and Wo are the weight loss values in the presence and inthe absence of inhibitor, respectively.

2.5. Electrochemical measurements

The electrochemical studies were carried out using a three-elec-trode cell assembly at room temperature. Mild steel coupons of1 cm � 1 cm (exposed area) with a 7.5 cm long stem (isolated withcommercially available lacquer) were used for electrochemicalmeasurements. A platinum foil of 1 cm2 was used as counter elec-trode and saturated calomel electrode as reference electrode. Theworking electrode was polished with different grades of emery pa-pers, washed with water and degreased with acetone. All electro-chemical measurements were carried out using a Gamry

Fig. 1. Molecular struc

Potentiostat/Galvanostat (Model G-300) with EIS software GamryInstruments Inc., USA. Prior to the electrochemical measurement,a stabilization period of 30 min was allowed, which was provedsufficient to attain a stable value of Ecorr.

For linear polarization resistance measurements, the potentialof the electrode was scanned from �0.02 to +0.02 V vs. corrosionpotential at scan rate of 0.5 mV/s. From the measured polarizationresistance value, the inhibition efficiency was calculated using therelationship;

gRp% ¼R0p � Ro

p

R0p� 100 ð3Þ

where Rop and R0p are the polarization resistance in the absence and

in the presence of inhibitor, respectively.The Tafel polarization curves were obtained by changing the

electrode potential automatically from (+250 mV to �250 mV) atopen circuit potential with a scan rate 0.5 mV s�1 to study the ef-fect of inhibitor on mild steel corrosion. The linear Tafel segmentof cathodic and anodic curves were extrapolated to corrosion po-tential to obtain the corrosion current densities (Icorr).

The inhibition efficiency was evaluated from the calculated Icorr

values using the relationship;

gP% ¼Iocorr � I0corr

Iocorr

� 100 ð4Þ

where Iocorr and I0corr are the corrosion current in the absence and in

the presence of inhibitor, respectively.The impedance studies were carried out using ac signals of

10 mV amplitude for the frequency spectrum from 100 kHz to0.01 Hz. The charge transfer resistance values were obtained fromthe diameter of the semi circles of the Nyquist plots. The inhibitionefficiency of the inhibitor was obtained from the charge transferresistance values using the following equation:

gRct% ¼R0ct � Ro

ct

R0ct

� 100 ð5Þ

where Roct and R0ct are the charge transfer resistance in the absence

and in the presence of inhibitor, respectively.The values of interfacial double layer capacitance (Cdl), were

estimated from the impedance value using bode plot by theformula;

jZj ¼ 12pfCdl

ð6Þ

2.6. Atomic force microscope

The mild steel strips of 1.0 cm � 1.0 cm � 0.025 cm sizes wereprepared as described in section 2.4. After immersion in 1.0 MHCl with and without addition of 8.76 � 10�4 M of Cefazolin at308 K for 3 h, the specimen was cleaned with distilled water, driedand then used for AFM.

ture of Cefazolin.

154 A.K. Singh, M.A. Quraishi / Corrosion Science 52 (2010) 152–160

3. Results and discussion

3.1. Weight loss measurements

3.1.1. Effect of inhibitor concentrationThe corrosion rate values of mild steel with the addition of

Cefazolin in 1.0 M HCl at various temperatures are presented in Ta-ble 1. From Table 1, it can be seen that corrosion rate values in1.0 M HCl solution containing Cefazolin, decreased as the conc. ofinhibitor increased. This result is due to fact that the adsorptionamount and coverage of inhibitor on mild steel surface increaseswith inhibitor concentration.

3.1.2. Effect of temperatureThe values of inhibition efficiencies obtained from weight loss

measurement for the different inhibitor concentrations in 1.0 MHCl are shown in Fig. 2a. From Fig. 2a, it can be seen that inhibitionefficiency decreased with increasing temperature, which indicatesdesorption of inhibitor molecule [30]. However, this decrease ingW% is small at higher inhibitor concentration.

3.1.3. Effect of immersion timeFig. 2b shows the effect of immersion time (3–12 h) at 308 K on

the inhibition efficiency of Cefazolin at different concentrations.Fig. 2b shows that Cefazolin inhibits the corrosion of mildsteel for all immersion times. At lower concentration (2.19–6.57 � 10�4 M), gW% decreased continuously with immersion timebut at higher concentration, gW% was almost constant for differentimmersion times.

3.1.4. Effect of acid concentrationThe effect of acid concentration on corrosion behaviour of mild

steel in the presence of 8.76 � 10�4 M inhibitor concentration wasstudied and the results are shown in Fig. 2c. It is clear that changein acid concentration from 0.5 to 2.5 M results in the inhibitionefficiency varying from 93.76% to 88.9%. This change in theinhibition suggests that the compound is an effective corrosion

Table 1Corrosion rate and Inhibition efficiency (%IE) values for the corrosion of mild steel inaqueous solution of 1 M HCl in the absence and in the presence of differentconcentrations of Cefazolin from weight loss measurements at different temperatures.

Inhibitor concentration(M � 10�4)

Temperature(K)

Corrosion rate(mm/y)

gW%

Blank 308 40.4 –318 58.2 –328 100.9 –338 174.5 –

2.19 308 12.6 68.8318 22.7 60.9328 49.1 51.2338 114.3 34.4

4.38 308 8.5 78.9318 17.5 69.8328 39.4 60.8338 91.1 47.8

6.57 308 5.5 86.3318 12.3 78.8328 30.9 69.3338 72.6 58.4

8.76 308 2.5 93.7318 9.5 83.7328 25.9 74.3338 58.1 66.7

10.95 308 2.4 93.9318 9.3 82.8328 24.9 75.2338 56.5 67.5

inhibitor in acid solution at the studied concentration of the acidsolution.

3.1.5. Adsorption isothermBasic information on the interaction between the inhibitor and

the mild steel surface can be provided using the adsorption iso-therm. For this purpose, the values of surface coverage (h) at differ-ent concentrations of Cefazolin in 1 M HCl acid in the temperaturerange (308–338 K) were calculated to explain the best isotherm todetermine the adsorption process from the experimental dataobtained.

Attempts were made to fit these h values to various isothermincluding Frumkin, Langmuir, Temkin, Freundlich isotherms. Byfar, the experimental data the results were best fitted by Langmuiradsorption isotherm equation [31]:

Ch¼ 1

Kþ C ð7Þ

Fig. 3 shows the relationship between C/h and C at temperatureranges studied. These results show that all the linear correlationcoefficients (R2) are almost equal to unity and all the slopes arevery close to unity, which indicates that the adsorption of Cefazolinfollows Langmuir adsorption isotherm.

3.1.6. Thermodynamic parameters of Cefazolin on mild steel surfaceThermodynamic parameters are important to study the inhibi-

tive mechanism. The thermodynamic functions for dissolution ofmild steel in the absence and in the presence of various concentra-tions of Cefazolin were obtained by applying the Arrhenius equa-tion and the transition state equation [32–35]:

logðrÞ ¼ �E�a2:303RT

þ log A ð8Þ

r ¼ RTNh

expDS�a

R

� �exp �DH�a

RT

� �ð9Þ

where E�a apparent activation energy, A the pre-exponential factor,DH�a the apparent enthalpy of activation, DS�a the apparent entropyof activation, h the Planck’s constant and N the Avogadro number,respectively.

The regression between log(r) and 1/T was calculated and theparameters were calculated and presented Table 2. Arrhenius plotsof log(r) vs. 1/T for the blank and different concentrations of Cefaz-olin are shown in Fig. 4a, and from the Table 2, it can be seen thatapparent activation energy increased with increasing concentra-tion of Cefazolin. The increase in apparent activation energy E�amay be interpreted as physical adsorption [36]. Szauer and Brand[37] explained that the increase in activation energy can be attrib-uted to an appreciable decrease in the adsorption of the inhibitoron the mild steel surface with increase in temperature and a corre-sponding increase in corrosion rates occurs due to the fact thatgreater area of metal is exposed to the acid environment.

According to Eq. (8), corrosion rate (r) is being effected by bothE�a and A. In general, the influence of E�a on the mild steel corrosionwas higher than that of A on the mild steel corrosion. However, ifthe variation in A was drastically higher than that of E�a, the valueof A might be the dominant factor to determine the mild steel cor-rosion. In the present case, E�a and A increased with concentration(the higher E�a and lower A led to lower corrosion rate). As it canbe seen from Table 1, the corrosion rate of steel decreased withincreasing concentration; hence, it was clear that increment of E�awas the decisive factor affecting the corrosion rate of mild steelin 1.0 M HCl.

Fig. 4b shows a plot of log(r/T) vs. 1/T. A straight lines were ob-tained with a slope equal to ð�DH�=2:303RÞ and intercept equal to½logðR=NhÞ þ ðDS�=2:303RÞ�, from which the values of DH�a and DS�a

2 4 6 8 10 1230

40

50

60

70

80

90

100

η W%

Inhibitor concentration (M x 10-4)

2 4 6 8 10 12

4550556065707580859095

54

3

2

1

η W%

Immersion time (h)

(a) (b)

0.5 1.0 1.5 2.0 2.5

90.0

92.5

95.0

97.5η W

%

Acid concentration (M)

(c)

Fig. 2. Variation of inhibition efficiency in 1 M HCl on mild steel of surface area 10 cm2 with (a) different conc. of Cefazolin inhibitor at different temperatures, (b) immersiontime at different conc. and (c) different acid conc. at optimum inhibitor conc. (8.76 � 10�4 M).

2 4 6 8 10 122

4

6

8

10

12

14

16

18

Cin

h/θ(1

0-4 M

)

Cinh

(10-4 M)

308 K (R 2 = 0.9966) 318 K (R 2 = 0.9990)

328 K (R 2 = 0.9988) 338 K (R 2 = 0.9987)

Fig. 3. Langmuir’s adsorption isotherm plots for the adsorption of Cefazolin atdifferent conc. in 1 M HCl on the surface of mild steel.

A.K. Singh, M.A. Quraishi / Corrosion Science 52 (2010) 152–160 155

were calculated and listed in Table 2. Inspection of these data re-veals that the thermodynamic parameters ðDH�a and DS�aÞ of disso-lution reaction of mild steel in 1 M HCl in the presence of Cefazolin

Table 2The values of activation parameters E�a , DH�a and DS�a for mild steel in 1 M HCl in the absen

Inhibitor conc. (M � 10�4) E�a (kJ mol�1) DH�a (kJ mol�1) D

1 M HCl 42.72 39.55 �2.19 63.13 60.47 �4.38 67.71 65.06 �6.57 74.02 71.37 �8.76 89.41 86.76 4410.95 89.24 86.59 43

are higher than in the absence of inhibitor. The positive sign ofenthalpies reflect the endothermic nature of steel dissolution pro-cess meaning that dissolution of steel is difficult [38].

On comparing the values of the entropy of activation ðDS�aÞ gi-ven in Table 2, it is clear that entropy of activation increased pos-itively in the presence of Cefazolin than in the absence ofinhibitor. The increase of DS�a reveals that an increase in disorder-ing takes place on going from reactant to the activated complex[39].

The constant of adsorption, Kads, is related to the standard freeenergy of adsorption, DG�ads from the following equation:

Kads ¼1

55:5exp

�DG�ads

RT

� �ð10Þ

The value 55.5 in the above equation is the concentration of waterin solution in mol l�1 [40]. The standard free energy of adsorptionðDG�adsÞ were calculated and the negative values of DG�ads obtainedindicates the spontaneity of the adsorption process and stabilityof the adsorbed layer on the mild steel surface. Generally valuesof ðDG�adsÞ up to �20 kJ mol�1 are consistent with the electrostaticinteractions between the charged metal (physisorption) while thosearound �40 kJ mol�1 or higher are associated with chemisorptionas a result of sharing or transfer of unshared electron pair or

ce and in the presence of different concentrations of Cefazolin.

S�a (J mol�1 K�1) A (mg cm�2) Linear regression coefficient (R2)

86.75 5.31 � 108 0.988128.82 5.66 � 1011 0.990016.90 2.37 � 1012 0.99700.0103 1.81 � 1013 0.9985.60 3.87 � 1015 0.9918.83 3.53 � 1015 0.9911

2.95 3.00 3.05 3.10 3.15 3.20 3.25

0.5

1.0

1.5

2.0

log

r (m

my

-1)

[(1/T).103]

0K

-1

Blank

2.19 x 10-4 M

4.76 x 10-4 M

6.57 x 10-4 M

8.76 x 10-4 M

10.95 x 10-4 M

2.95 3.00 3.05 3.10 3.15 3.20 3.25

-2.2

-2.0

-1.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

Blank

2.19 x 10-4 M

4.76 x 10-4 M

6.57 x 10-4 M

8.76 x 10-4 M

10.95 x 10-4 M

log

r/T

(mm

y-1

K-1

)

[(1/T).103]

0K

-1

(a) (b)

2.95 3.00 3.05 3.10 3.15 3.20 3.25

-117

-114

-111

-108

-105

-102

-99

-96

-93

Δ G

ads/T

(JK

-1m

ol-1

)

[(1/T).103]

0K

-1

(c)

Fig. 4. (a) Adsorption isotherm plot for log (CR) vs. 1/T; (b) adsorption isotherm plot for log (CR/T) vs. 1/T; (c) adsorption isotherm plot for DG/T vs. 1/T.

156 A.K. Singh, M.A. Quraishi / Corrosion Science 52 (2010) 152–160

p-electrons of organic molecules to the metal surface to form a co-ordinate type of bond (chemisorption) [41,42].

Free energy of adsorption ðDG�adsÞ was calculated using the fol-lowing equations and listed in Table 3.

DG�ads ¼ �RT lnð55:5 KÞ ð11Þ

K ¼ hCð1� hÞ ð12Þ

where h is degree of coverage on the metal surface, C is concentra-tion of inhibitor in mol l�1, R is the molar gas constant and T is tem-perature. The DG�ads value of the inhibitor was found�35.54 kJ mol�1 indicated that it is adsorbed on the metal surfaceby both physical and chemical process [43–45]. The negative valuesof DG�ads indicated the spontaneous adsorption of inhibitor on sur-face of mild steel.

The enthalpy of adsorption was calculated from the Gibbs–Helmholtz equation:

@ðDG�ads=TÞ@T

� �P¼ �DH�ads

T2 ð13Þ

Table 3Thermodynamic parameters for the adsorption of Cefazolin in 1 M HCl on the mildsteel at different temperatures.

Temperature(K)

K(mol�1)

DG�ads

(kJ mol�1)DH�ads

(kJ mol�1)DS�ads

(J mol�1 K�1)

308 19,379 �35.54 �57.04 �69.80318 6594 �33.85 �57.04 �72.90328 3719 �33.35 �57.04 �72.20338 2576 �33.34 �57.04 �70.11

This equation can be rearranged to give the following equation.

DG�ads

T¼ DH�ads

Tþ k ð14Þ

The variation of DG�ads=T with 1/T gives a straight line with a slopethat equals DH�ads (Fig. 4c). It can be seen from the figure thatDG�ads=T decreases with 1/T in a linear fashion. The calculated valuesare depicted in Table 3. The adsorption heat could be approximatelyregarded as the standard adsorption heat ðDH�adsÞ under experimen-tal conditions [33,46]. The negative sign of DH�ads in HCl solutionindicates that the adsorption of inhibitor molecule is an exothermicprocess. Generally, an exothermic adsorption process signifieseither physisorption or chemisorption while endothermic processis attributable unequivocally to chemisorption [47]. Typically, theenthalpy of physisorption process is lower than that 41.86 kJ mol�1

while the enthalpy of chemisorption process approaches100 kJ mol�1 [48]. In the present study, the absolute value of enthal-py is �57.04 kJ mol�1, which is an intermediate case. Then thestandard adsorption entropy ðDS�adsÞwas obtained using the thermo-dynamic basic equation:

DS�ads ¼DH�ads � DG�ads

Tð15Þ

The DS�ads values in the presence of inhibitor are large and neg-ative, meaning a decrease in disordering on going from reactants tothe metal adsorbed species.

3.2. EIS measurements

Electrochemical impedance measurements were performedover the frequency range from 100 kHz to 0.01 Hz at open circuitpotential. The simple equivalent randle circuit for studies is shownin Fig. 5, where RX represents the solution resistance, Rct the chargetransfer resistance and double layer capacitance (Cdl). Nyquist plots

Fig. 5. Electrical equivalent circuit (RX, uncompensated solution resistance; Rct,polarization resistance; Cdl, double layer capacitance).

0 50 100 150 200 250 300

1 = 4.38x10-4 M

2 = 6.57x10-4 M

3 = 8.76x10-4 M

0 5 10 15

0

5

10

15-Z

imag

(Ω c

m2 )

Zreal

(Ω cm2)

250

200

150

100

50

0 321

Blank

-Zim

ag (

Ω c

m2 )

Zreal

(Ω cm2)

Fig. 6. Nyquist plots of mild steel in 1 M HCl with different concentration range ofCefazolin.

0.000 0.001 0.010 0.100 1.000 10.000 100.000-800

-700

-600

-500

-400

-300

-200 1 = Free Acid Solution2 = 4.38x10-4 M3 = 6.57x10-4 M4 = 8.76x10-4 M

43 2 1

Pot

enti

al (

mV

vs

SCE

)

Current density (mA cm-2)

Fig. 7. Tafel Polarization behaviour of mild steel in 1 M hydrochloric acid withdifferent concentration range of Cefazolin.

A.K. Singh, M.A. Quraishi / Corrosion Science 52 (2010) 152–160 157

of mild steel at various concentrations of Cefazolin in 1 M HCl solu-tion are presented in Fig. 6. The impedance spectra for mild steel in1 M HCl are similar in shape with a high frequency (HF) capacitiveloop and low frequency (LF) inductive loop (except at8.76 � 10�4 M). The HF capacitive loop can be attributed to thecharge transfer reaction and time constant of the electric doublelayer. The time constant at high frequencies may be attributed toformation of surface film [49,50]. On the other hand, the inductiveloop has been attributed to a surface or bulk relaxation process orto a dissolution process [51,52]. We have correlated the low fre-quency inductive loop seen in the free acid solution with a surfacedissolution process. The low frequency inductive loop in inhibitedacid solution containing lower concentration of Cefazolin might beattributed to the relaxation process obtained by adsorption of spe-cies as Hads

þ and Cl� on the electrode surface. It might be alsoattributed to the re-dissolution of the passivated surface at low

Table 4Electrochemical parameters for corrosion of mild steel in 1 M HCl in the presence of diffe

Conc. of inhibitor (M � 10�4) Tafel data

Ecorr (mV vs. SCE) Icorr (lA cm�2) ba (mV/dec)

Blank HCl Inh �469 730 734.38 �487 124 546.57 �480 85 628.76 �481 39 67

frequencies. The fact that this semicircle can not be observed afterthe addition of higher concentration of Cefazolin supports ourview. Inhibition efficiencies and other calculated impedanceparameters are given in Table 4. As it can be seen from Table 4,the Rct values increased with increasing the concentration of theinhibitor and the values of Cdl decreased with an increase in theinhibitor concentration. This situation was the result of an increasein the surface coverage by this inhibitor, which led to an increase inthe inhibition efficiency. The thickness of the protective layer, dorg,is related to Cdl by the following equation [53]:

dorg ¼eoer

Cdlð16Þ

where eo is the dielectric constant and er is the relative dielectricconstant. This decrease in the Cdl, which can result from a decreasein local dielectric constant and/or an increase in the thickness of theelectrical double layer, suggested that Cefazolin molecules functionby adsorption at the metal/solution interface. Thus, the change inCdl values was due to the gradual replacement of water moleculesby the adsorption of the organic molecules on the metal surface,decreasing the extent of metal dissolution [54].

3.3. Tafel polarization

Polarization curves for mild steel in 1.0 M HCl at various con-centration of Cefazolin are presented in Fig. 7. The values of corro-sion current densities (Icorr), corrosion potential (Ecorr), the anodicTafel slopes (ba), cathodic Tafel slopes (bc) and inhibition efficiencyðgP%Þ as function of Cefazolin concentration were calculated fromthe curves of Fig. 7 and presented in Table 4. It reveals that the cor-rosion current (Icorr) decreased prominently and inhibition effi-ciency increased with inhibitor concentration. The presence ofCefazolin does not remarkably shift the corrosion potential (Ecorr)and hence, can be said to be a mixed-type inhibitor in 1.0 M HCl.The anodic Tafel slopes changes slightly whereas the change incathodic Tafel slopes are larger which means that Cefazolin

rent concentrations of Cefazolin.

Impedance data Linear polarization

bc (mV/dec) gP% Rct (X cm2) Cdl (lF cm�2) gRct %Rp (X cm2) gRp %

127 – 17.3 1006 – 18.7 –152 83.0 87.3 118 80.1 101.8 81.6152 88.3 126.5 109 86.3 128.9 85.5186 94.7 262.6 81 93.4 256.6 92.7

Fig. 8. Atomic force micrographs of mild steel surface (a) polished mild steel, (b) mild steel in 1 M HCl and (c) inhibited mild steel (1 M HCl + 8.76 � 10�4 inhibitor).

158 A.K. Singh, M.A. Quraishi / Corrosion Science 52 (2010) 152–160

molecules are adsorbed on both sites but under prominent catho-dic control resulting in inhibition of anodic dissolution and catho-dic reduction reaction.

3.4. Linear polarization resistance

The polarization resistance (Rp) values of mild steel in 1 M HClincreases from 18.7 X for the blank to 256.6 X for 8.76 � 10�4 Mconcentration of Cefazolin (Table 4). The increase in the Rp valuesuggests that the inhibition efficiency increases with the increasein the inhibitor concentration.

3.5. Surface characterization: AFM study

The atomic force microscope provides a powerful means ofcharacterizing the microstructure. The three-dimensional AFMimages are shown in Fig. 8a–c. As it can be seen from Fig. 8a themild steel surface before immersion seems smooth compared tothe mild steel surface after immersion in uninhibited 1.0 M HClfor 3 h. The average roughness of polished mild steel (Fig. 8a)and mild steel in 1.0 M HCl without inhibitor (Fig. 8b) was calcu-lated to be 66 and 395 nm, respectively. It is clearly shown inFig. 8b that mild steel sample is getting cracked due to the acid at-tack on mild steel surface. However in the presence of optimumconcentration of inhibitor, the average roughness was reduced to167 nm (Fig. 8c).

4. Mechanism of inhibition

A clarification of mechanism of inhibition requires full knowl-edge of the interaction between the protective compound andthe metal surface. Many of the organic corrosion inhibitors haveat least one polar unit with atoms of nitrogen, sulphur, oxygenand in some cases phosphorous. It has been reported that the inhi-bition efficiency decreases in the order to O < N < S < P. The polarunit is regarded as the reaction centre for the chemisorption pro-

cess. Furthermore, the size, orientation, shape and electric chargeon the molecule determine the degree of adsorption and hencethe effectiveness of inhibitor. On the other hand, iron is wellknown for its co-ordination affinity to heteroatom bearing ligands.

Increase in inhibition efficiencies with the increase of concen-tration of Cefazolin shows that the inhibition action is due toadsorption on the steel surface. Four types of adsorption may takeplace by organic molecules at metal/solution interface namely.

(1) Electrostatic attraction between the charged molecules andcharged metal.

(2) Interaction of unshared electron pairs in the molecule withthe metal.

(3) Interaction of p-electrons with the metal.(4) Combination of (1) and (3) [55].

In HCl solution the following mechanism is proposed for thecorrosion of iron and steel [56].

According to this mechanism anodic dissolution of iron follows:

Feþ Cl�� ðFeCl�Þads

ðFeCl�Þads� ðFeClÞads þ e�

ðFeClÞads ! ðFeClþÞ þ e�

ðFeClþÞ� Fe2þ þ Cl�

The cathodic hydrogen evolution follows:

FeþHþ� ðFeHþÞads

ðFeHþÞads þ e�� ðFeHÞads

ðFeHÞads þHþ þ e� ! FeþH2

In acidic solution, carbonyl group, secondary amine group as well asnitrogen atoms in tetrazole ring (Fig. 9) and adjacent carbonylgroup (O23) can be protonated easily because they all are planarand having greater electron density (N20, N25, N27, N28 and N29).Physical adsorption may take place due to electrostatic interactionbetween protonated molecule and (FeCl�)ads species. Co-ordinate

Fig. 9. Three-dimensional representation of Cefazolin molecule.

A.K. Singh, M.A. Quraishi / Corrosion Science 52 (2010) 152–160 159

covalent bond formation between electron pairs of unprotonated Satom and two N-atoms in thiadiazole ring (S4, N1 and N2) and metalsurface can take place. Further, Cefazolin molecules are chemicallyadsorbed due to interaction of p-orbitals with metal surface follow-ing deprotonisation step of the physically adsorbed protonated mol-ecules. In the presented case, the value of DG�ads is �35.54 kJ mol�1,hence, indicated that adsorption of Cefazolin on the surface of mildsteel involves both physical and chemical process. But, as it can beseen from Table 3, the values of DG�ads decreased with increasingtemperature hence, indicated that adsorption of Cefazolin doesnot favour at higher temperature, indicating that Cefazolin ad-sorbed predominantly physically on the surface of mild steel.

5. Conclusion

(1) Cefazolin acts as a good inhibitor for the corrosion of mildsteel in 1.0 M HCl.

(2) The inhibition efficiency of Cefazolin decreased with tem-perature, which leads to an increase activation energy of cor-rosion process.

(3) The adsorption of Cefazolin obeys Langmuir adsorption iso-therm. The adsorption process is a spontaneous and exother-mic process accompanied by an increase of entropy.

(4) Potentiodynamic polarization curves reveals that Cefazolinis a mixed-type but predominantly cathodic inhibitor.

(5) The results obtained from weight loss, impedance and polar-ization studies are in a good agreement.

Acknowledgement

One of the author A.K.S. is thankful to University Grant Com-mission (UGC), New Delhi, for Senior Research Fellowship.

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