Corrosion Science 62 (2012) 163–175

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Corrosion Science

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Inhibition of the corrosion of steel in HCl, H2SO4 solutions by bamboo leaf extract

Xianghong Li a,⇑, Shuduan Deng b, Hui Fu aa Faculty of Science, Southwest Forestry University, Kunming 650224, PR Chinab Faculty of Materials Engineering, Southwest Forestry University, Kunming 650224, PR China

a r t i c l e i n f o

Article history:Received 31 December 2011Accepted 17 May 2012Available online 26 May 2012

Keywords:A. SteelB. AFMB. EISB. PolarizationB. Weight lossC. Acid inhibition

0010-938X/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.corsci.2012.05.008

⇑ Corresponding author. Tel.: +86 871 3863375; faxE-mail address: xianghong-li@163.com (X. Li).

a b s t r a c t

The inhibition effect of the bamboo of Dendrocalmus sinicus Chia et J.L. Sun leaf extract (DSCLE) on the cor-rosion of cold rolled steel (CRS) in 1.0–5.0 M HCl, 0.5–5.0 M H2SO4 solutions was studied by weight loss,potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS) and atomic forcemicroscope (AFM) methods. The results show that DSCLE is a good inhibitor in 1.0 M HCl and 0.5 MH2SO4, and inhibition efficiency follows the order: HCl > H2SO4. The adsorption of DSCLE on CRS surfaceobeys Langmuir adsorption isotherm, and acts as a mixed-type inhibitor in both acids.

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1. Introduction

The use of inhibitors is one of the most practical methods forprotecting metals against corrosion, especially in acidic media[1]. Acid solutions are widely used in industry; some of the impor-tant fields of application are acid pickling of steel, chemical clean-ing and processing, ore production and oil well acidizing. Asordinary acids, HCl and H2SO4 are usually used as industrial acidcleaning and pickling acids. Because of the general aggression ofacid solutions, inhibitors are commonly used to retard the corro-sive attack on metallic materials. During past decades, some com-mercial inhibitors have been synthesized and used successfully toinhibit corrosion of steel in acidic media. However, the major prob-lem associated with most of these inhibitors is that they are noteco-friendly but toxic and expensive. Therefore, the study of newnon-toxic or low-toxic corrosion inhibitors is essential to overcomethis problem. The research in the field of eco-friendly corrosioninhibitors has been addressed toward the goal of using cheap,effective compounds at low or ‘‘zero’’ environmental impact.

Plant extracts are low-cost and biodegradable, and so the studyof plant extracts as corrosion inhibitors is an important scientific re-search field due to both economic and environmental benefits. Asearly as in 1930, plant extracts (dried stems, leaves and seeds) ofChelidonium majus and other plants were used as corrosion inhibi-tors for steel in H2SO4 pickling baths [2]. In 1972, El Hosary et al. [3]studied the extract of Hibiscus subdariffa (Karkode) as the corrosioninhibitor for Al and Zn in HCl and NaOH solutions. In 1980s, Saleh

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et al. [4,5] reported the inhibition effect of aqueous extracts of someplant leaves (Opuntia, Aleo eru) and fruit peels (orange, mango) onthe corrosion of steel, aluminum, zinc and copper in acids and alu-minum in NaOH solution. In 1990s, Azadirachta [6] and Vernoniaamygdalina (bitter leaf) [7] leaves extracts were reported as goodcorrosion inhibitors for steel in HCl and H2SO4 solutions. Enteringinto the 21st century, along with people’s increasing awareness ofprotecting the environment, a large number of scientific manu-scripts about plant leaves extracts as effective corrosion inhibitorsof iron or steel in acidic media (HCl, H2SO4, HNO3, H3PO4, etc.) havebeen published; the studied plants such as henna [8–11], Nypa frut-icans Wurmb [12,13], Azadirachta indica [14], Acalypha indica [15],Zenthoxylum alatum [16,17], Damsissa [18], Mentha pulegium [19],olive [20], Phyllanthus amarus [21,22], Occimum viridis [23,24], Car-ica papaya [25], Murraya koenigii [26], Ocimum gratissimum [27], lu-pine [28], Palicourea guianensis [29], Ananas comosus [30],Lasianthera africana [31], Strychnos nux-vomica [32,33], Rauvolfiaserpentine [34], Justicia gendarussa [35], Oxandra asbeckii [36], Ferulaassa-foetida [37], coffee [38], fruit peel [39], Halfabar [40], KopsiaSingapurenis [41], Jasminum nudiflorum [42] and ginkgo [43].Through these studies, it is agreed that the inhibition performanceof plant extract is normally ascribed to the presence in their compo-sition of complex organic species such as tannins, alkaloids andnitrogen bases, carbohydrates, amino acids and proteins as wellas hydrolysis products. These organic compounds contain polarfunctions with N, S, O atoms as well as conjugated double bondsor aromatic rings in their molecular structures, which are the majoradsorption centers. However, confronting with the vast varieties ofplant, the data regarding the use of plant leaves extract as the cor-

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164 X. Li et al. / Corrosion Science 62 (2012) 163–175

rosion inhibitor are still poor. In the present work, bamboo leaf ex-tract is chosen to be the corrosion inhibitor.

Bamboo is a group of perennial evergreens in the Bambusoideaesubfamily, and features a short growth cycle, easy renewal, fastgrowth and strong regeneration capacity. There are about 1200species and 70 genera of bamboo in the world. China is one ofthe richest countries in the world in terms of 500 bamboo species.Bamboo leaf extract (BLE) is virtually nonpoisonous [44,45], andrich in flavonoids, amino acids, and active sugar [46]. In our labo-ratory, much work has been conducted to study the inhibition byBLE on the corrosion of metal in different media. Recently, Dendro-calamus brandisii leaves extract (DBLE) has been reported as a goodinhibitor for steel in 1.0 M HCl solution [47]. However, in the ear-lier work [47], the effects of acid concentration and immersiontime on inhibitive performance were still uncertain. In addition,the corrosion inhibition of BLE in H2SO4 media is little studied.For these reasons, the present work is to investigate the corrosioninhibition of another bamboo, namely Dendrocalmus sinicus Chia etJ.L. Sun leaves extract (DSCLE) in HCl and H2SO4 solutions usingweight loss, polarization curves, impedance spectroscopy (EIS)and atomic force microscope (AFM) methods. Effects of inhibitorconcentration, temperature, immersion time and acid concentra-tion on the corrosion inhibition were fully investigated and dis-cussed. Furthermore, the comparison of inhibition performancein HCl and that in H2SO4 is discussed in detail. It is expected toaccumulate useful information about the inhibitive performanceof bamboo leaf extract for steel in HCl and H2SO4 media.

2. Experimental

2.1. Materials

The experiments were performed with cold rolled steel (CRS)with the following composition (wt%): C 0.07%, Mn 0.3%, P0.022%, S 0.010%, Si 0.01%, Al 0.030% and Fe balance. The aggressivesolutions of 1.0–7.0 M HCl and 0.5–7.0 M H2SO4 were prepared bydilution of AR grade 37% HCl and 98% H2SO4 with distilled water,respectively.

Fresh Dendrocalmus sinicus Chia et J.L. Sun leaves were picked incampus of Southwest Forestry University, and then cleaned withtap water to eliminate ash of mud, dried for 2 days in an oven at60 �C, and ground to powder. Fifteen g sample of the powder wasrefluxed in 450 ml 80% (percent by volume) C2H5OH at 75 �C for2 h. The refluxed solution was filtered, and the filter liquor wasevaporated to 100 ml of dark brown residue, and then degreasedwith petroleum ether (boiling bracket: 60–90 �C) using separatingfunnel. Thereafter, the solution was evaporated to about 50 mldark brown residues using rotary evaporator, dried in vacuum dry-ing oven at 60 �C until complete dryness (about 2 days). Then thedark brown solid residue (about 1.6 g) of DSCLE was obtainedand preserved in a desicator.

The stock solution (1000 mg l�1) of DSCLE with dark yellow col-or was prepared from distilled water, and then used to prepare thedesired concentration by dilution with distilled water. The concen-tration range of DSCLE used was 5–200 mg l�1.

2.2. Characterization of DSCLE

The solid plant extract of DSCLE was characterized by Fouriertransform infrared (FTIR) spectroscopy. FTIR spectra was recordedin an AVATAR-FTIR-360 spectrophotometer (Thermo Nicolet Com-pany, USA), which extended from 4000 to 400 cm�1, using the KBrdisk technique.

On the other hand, the solutions of DSCLE were analyzed by UVspectral measurements using UV-2401PC spectrophotometer (Shi-

madzu Company, Japan). The absorption spectra of these solutionswere determined with distilled water as reference.

2.3. Weight loss measurements

The cold rolled steel (CRS) sheets of 2.5 � 2.0 � 0.06 cm wereabraded with a series of emery paper (grade 320-500-800) andthen washed thoroughly with distilled water and acetone. Afterweighing accurately, the specimens were immersed in beakerswhich contained 250 ml acid solutions with different concentra-tions of DSCLE at a certain temperature remained by a water ther-mostat. All the aggressive acid solutions were open to air. After 6 hthe specimens were taken out, washed, dried, and weighed accu-rately. Then the tests were repeated at different temperature andimmersion time. In order to get good reproducibility, experimentswere carried out in duplicate. The average weight loss of two par-allel CRS sheets was obtained. The corrosion rate (v) was calculatedfrom the following equation:

v ¼WSt

ð1Þ

where W is the average weight loss of two parallel CRS sheets (g), Sthe total area of one CRS specimen (m2), and t is the immersiontime. With the calculated corrosion rate, the inhibition efficiency(gw) was calculated as follows:

gw ¼v0 � v

v0� 100% ð2Þ

where v0 and v are the values of corrosion rate without and withinhibitor, respectively.

2.4. Electrochemical measurements

Electrochemical experiments were carried out in the conven-tional three-electrode cell with a platinum counter electrode (CE)and a saturated calomel electrode (SCE) coupled to a fine Luggincapillary as the reference electrode. To minimise the ohmic contri-bution, the Luggin capillary was kept close to the working electrode(WE) which was in the form of a square CRS embedded in PVCholder using epoxy resin so that the flat surface was the only sur-face in the electrode. The working surface area was 1.0 � 1.0 cm,and prepared as described above (Section 2.3). Before measurementthe electrode was immersed in test solution at open circuit poten-tial (OCP) for 2 h to ensure OCP to reach steady state. All electro-chemical measurements were carried out using PARSTAT 2273advanced electrochemical system (Princeton Applied Research).Each experiment was repeated at least three times to check thereproducibility.

The potential of potentiodynamic polarisation curves was in-creased at 0.5 mV s�1 and started from a potential from �250 to+250 mV vs. OCP. Inhibition efficiency (gp%) is defined as:

gp ¼icorr � icorrðinhÞ

icorr� 100% ð3Þ

where icorr and icorr(inh) represent corrosion current density valueswithout and with inhibitor, respectively.

Electrochemical impedance spectroscopy (EIS) was carried outat OCP in the frequency range of 0.01 Hz–100 kHz using a 10 mVrms voltage excitation. Inhibition efficiency (gR%) is estimatedusing the following relation:

gR ¼RtðinhÞ � Rtð0Þ

RtðinhÞ� 100% ð4Þ

where Rt(0) and Rt(inh) are charge transfer resistance in the absenceand presence of the inhibitor, respectively.

X. Li et al. / Corrosion Science 62 (2012) 163–175 165

2.5. Atomic force microscope (AFM)

The CRS specimens of size 1.5 � 1.0 � 0.06 cm were prepared asdescribed above (Section 2.3). After immersion in 1.0 M HCl and0.5 M H2SO4 solutions without and with addition of 200 mg l�1

DSCLE at 20 �C for 6 h, the specimen was cleaned with distilledwater, dried with a cold air blaster, and then used for a Japaninstrument model SPA-400 SPM Unit atomic force microscope(AFM) examinations. The AFM images were measured in tappingmode using Si3N4 tips.

3. Results and discussion

3.1. FTIR and UV of DSCLE

Fourier transform infrared (FTIR) spectroscopy of DSCLE is shownin Fig. 1. The strong absorption band at 3414 cm�1 is attributed to N–H or O–H stretching vibration, and that at 2932 cm�1 is related to C–H stretching vibration. The strong band at 1621 cm�1 is assigned toC@C and C@O stretching vibration. Owing to the conjugation effectof flavonoids of DSCLE, the C@O peak shifts from about 1700 cm�1

to lower wavenumber (approximately 1620 cm�1), C@C and C@Ostretching vibration bands are superposition [48]. The C–H bending

4000 3500 3000 2500 2000 1500 1000 5000

20

40

60

80

100

493

615

893

938

10681

113

1264

1338

1402

1501

1621

2932

3414

Tran

smitt

ance

(%)

Wavenumbers (cm-1)

Fig. 1. FTIR spectra of DSCLE.

200 300 4000.0

0.5

1.0

1.5

2.0

2.5

3.0

2000.0

0.5

1.0

1.5

2.0

2.5

3.0

Abs

orba

nce

a b

Wavel

Fig. 2. UV spectra of DSCLE (vs. distilled water). (a) 100 mg l�1 DSCLE; (b)

bands in –CH2 and –CH3 are found to be at 1402 and 1338 cm�1,respectively. The absorption bands at 1501 and 1264 cm�1 could beassigned to the framework vibration of aromatic ring. Besides these,there are absorption bands at 1113 and 1068 cm�1, which can be as-cribed to the C–N or C–O stretching vibration. The absorption bandsbelow 1000 cm�1 correspond to aliphatic and aromatic C–H group.These results indicate that DSCLE contains O and N atoms in func-tional groups (O–H, N–H, C@C, C@O, C@N, CAN, CAO) and aromaticring.

Fig. 2 shows the UV absorption spectra of 100 mg l�1 DSCLE,100 mg l�1 DSCLE in 1.0 M HCl and 100 mg l�1 DSCLE in 0.5 MH2SO4. In all cases, UV exhibits almost the same absorption curve,which indicates that chemical structures of the components inDSCLE are not broken down in either HCl or H2SO4 medium. Gen-erally, UV of flavonoids has two characteristic absorption peaks at300–400 nm (band I) and 240–280 nm (band II) [49]. From Fig. 2,the absorption spectra appear three peaks at 197, 271 and333 nm, respectively. The weak band at 333 nm (band I) is attrib-uted to the electron transition of cinnamic acyl with B ring of flavo-noids. The absorption band at 271 nm (band II) corresponds to theelectron transition of benzoyl A ring of flavonoids. Noticeably, thestrong peak at 197 nm is caused by the electron transition ofn ? r⁄ in N and O atoms. The result implies that the main consti-tution compounds in DSCLE are flavonoids and other compoundscontaining N or O atoms.

3.2. Weight loss measurements

3.2.1. Effect of DSCLE on corrosion rateFigs. 3 and 4 show the logarithmic corrosion rate values (log v) of

CRS without and with different concentrations of DSCLE at 20–50 �Cin 1.0 M HCl and 0.5 M H2SO4 solutions, respectively. Either in 1.0 MHCl or 0.5 M H2SO4, along with the increase of DSCLE concentration,the values of log v decreases gradually, i.e. the corrosion of steel isretarded by DSCLE, or the inhibition enhances with the inhibitorconcentration. This behaviour is due to the fact that the adsorptioncoverage of inhibitor on steel surface increases with the inhibitorconcentration. But when the inhibitor concentration is up to100 mg l�1, the corrosion rates reach certain values either in 1.0 MHCl (0.50, 1.04, 2.06 and 5.24 g m�2 h�1 for 20, 30, 40 and 50 �C,respectively) or 0.5 M H2SO4 (1.87, 3.06, 5.29 and 8.34 g m�2 h�1

for 20, 30, 40 and 50 �C, respectively), and then do not vary remark-ably. At same temperature and inhibitor concentration, the corro-

300 400 200 300 4000.0

0.5

1.0

1.5

2.0

2.5

3.0

ength (nm)

c

1.0 M HCl + 100 mg l�1 DSCLE; (c) 0.5 M H2SO4 + 100 mg l�1 DSCLE.

0 20 40 60 80 100 120 140 160 180 200 220-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

log

v (g

m-2 h

-1)

c (mg l-1)

20 oC 30 oC 40 oC 50 oC

Fig. 3. Relationship between logarithmic corrosion rate (log v) and concentration ofDSCLE (c) in 1.0 M HCl (weight loss method, immersion time is 6 h).

0 20 40 60 80 100 120 140 160 180 200 220

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

log

v (g

m-2 h

-1)

c (mg l-1)

20 oC 30 oC 40 oC 50 oC

Fig. 4. Relationship between logarithmic corrosion rate (log v) and concentration ofDSCLE (c) in 0.5 M H2SO4 (weight loss method, immersion time is 6 h).

0 20 40 60 80 100 120 140 160 180 200 22030

40

50

60

70

80

90

100

η w (%

)

c (mg l-1)

20 oC 30 oC 40 oC 50 oC

Fig. 5. Relationship between inhibition efficiency (gw) and concentration of DSCLE(c) in 1.0 M HCl (weight loss method, immersion time is 6 h).

0 20 40 60 80 100 120 140 160 180 200 22030

40

50

60

70

80

90

η w (%

)

c (mg l-1)

20 oC 30 oC 40 oC 50 oC

Fig. 6. Relationship between inhibition efficiency (gw) and concentration of DSCLE(c) in 0.5 M H2SO4 (weight loss method, immersion time is 6 h).

166 X. Li et al. / Corrosion Science 62 (2012) 163–175

sion rate in 0.5 M H2SO4 is comparatively higher than that in 1.0 MHCl solution.

Also, the corrosion rate increases with temperature both inuninhibited and inhibited solutions, especially goes up more rap-idly in the absence of inhibitor. These results confirm that DSCLEacts as an effective inhibitor in the range of temperature studied.

3.2.2. Effect of DSCLE on inhibition efficiencyThe values of gw for different DSCLE concentrations at 20–50 �C

in 1.0 M HCl and 0.5 M H2SO4 solutions are presented in Figs. 5 and6, respectively. In each acid medium, gw increases with the inhib-itor concentration. Noticeably, when the concentration of DSCLE isless than 100 mg l�1, gw increases sharply with an increase in con-centration, but a further raise inhibitor concentration causes noappreciable change in inhibitive performance. At the inhibitor con-centration of 200 mg l�1, the maximum gw in 1.0 M HCl is 89.0% at20 �C; 90.2% at 30 �C; 91.2% at 40 �C; and 92.1% at 50 �C. In 0.5 MH2SO4, gw of 200 mg l�1 DSCLE is 79.4% at 20 �C; 80.9% at 30 �C;82.4% at 40 �C; and 86.5% at 50 �C. The results indicate DSCLE is agood inhibitor for steel in both 1.0 M HCl and 0.5 M H2SO4 solu-tions. According to our recent work of the bamboo leaves extractof DBLE [47], through comparison, the difference of inhibitive ac-tion between two bamboo species is within 5%, which implies that

the series of bamboo leaf extract could be seemed as the good po-tential corrosion inhibitors for steel in acid solution.

At certain inhibitor concentration and temperature, gw followsthe order: 1.0 M HCl > 0.5 M H2SO4. Fig. 5 shows that gw decreaseswith the experimental temperature within inhibitor concentra-tions from 10 to 80 mg l�1, but then gw increases slightly with anincrease of temperature. Similar results were reported for anotherbamboo leaf extract of DBLE under same conditions [47]. For theacid media of 0.5 M H2SO4, Fig. 6 illustrates that enhancing tem-perature results in reducing inhibitive ability at 10–40 mg l�1,but in turn improves the inhibition performance at 70–200 mg l�1.

3.2.3. Adsorption isothermOrganic compounds show inhibitive effect via adsorption on

metal surface, and some adsorption isotherms have been widelyused to study the mechanism of corrosion inhibition. Attemptswere made to fit the experimental data to various isothermsincluding Frumkin, Langmuir, Temkin, Freundlich, Bockris-Swin-kels and Flory-Huggins. In this study, Langmuir adsorption iso-therm is applied to study the adsorption of inhibitor on steelsurface, and it has the following equation [50]:

ch¼ 1

Kþ c ð5Þ

Table 1Parameters of the linear regression between c/h and c in 1.0 M HCl.

Acid solution Temperature (�C) r Slope K (l mg�1)

1.0 M HCl 20 0.9999 0.97 0.239530 0.9999 0.97 0.168440 0.9999 0.95 0.109950 0.9996 0.89 0.05246

0.5 M H2SO4 20 0.9996 0.98 0.182430 0.9997 0.97 0.136340 0.9999 0.93 0.0842650 0.9990 0.89 0.04529

-2.5

-2.0

-1.5

-1.0

1.0 M HCl

ln K

(l m

g-1 )

X. Li et al. / Corrosion Science 62 (2012) 163–175 167

where c is the concentration of inhibitor, K the adsorptive equilib-rium constant, and h is the surface coverage, and calculated bythe Sekine and Hirakawa’s method [51]:

h ¼ v0 � vv0 � vmð6Þ

where vm is the smallest corrosion rate.The linear regression parameters between c/h and c are listed in

Table 1, and the straight lines of c/h versus c in both acids at 20 �C isshown in Fig. 7. It is evident that all linear correlation coefficients(r) are almost equal to 1, and the slope values are also close to 1,which indicates the adsorption of DSCLE on steel surface obeysLangmuir adsorption isotherm. As shown in Table 1, the adsorptiveequilibrium constant (K) decreases with the temperature in eitherHCl or H2SO4 solution, which could be ascribed to that it is easy forinhibitor to adsorb on the steel surface at relatively lower temper-ature. But when the temperature is gone up, the adsorbed inhibitortends to desorb from the steel surface. At certain temperature, K(1.0 M HCl) > K (0.5 M H2SO4), which means that DSCLE exhibitsa stronger tendency to adsorb on steel surface in 1.0 M HCl com-pared with that in 0.5 M H2SO4. Generally, large value of K is boundup with better inhibition efficiency of a given inhibitor. This is ingood agreement with the values of gw obtained from Figs. 5 and 6.

3.2.4. Thermodynamic parametersThermodynamic parameters are important to further under-

stand the adsorption process of inhibitor on steel/solution inter-face. The standard adsorption enthalpy (DH0) could be calculatedon the basis of Van’t Hoff equation:

d ln KdT

¼ DH0

RT2ð7Þ

0 20 40 60 80 100 120 140 160 180 200 2200

20

40

60

80

100

120

140

160

180

200

220

1.0 M HCl 0.5 M H

2SO

4

c/θ (

mg

l-1)

c (mg l-1)

Fig. 7. Langmuir isotherm adsorption mode of DSCLE on the CRS surface in 1.0 MHCl and 0.5 M H2SO4 at 20 �C from weight loss measurement.

where R is the gas constant (8.314 J K�1 mol�1), T the absolute tem-perature (K), and K is the adsorptive equilibrium constant (M�1). Eq.(7) can also be changed as follows [50]:

ln K ¼ �DH0

RTþ D ð8Þ

where D is integration constant. Fig. 8 represents the straight linesof ln K versus 1/T with good linear relationship (the linear correla-tion coefficients are 0.9684 and 0.9740 in HCl and H2SO4, respec-tively). DH0 is calculated from the slope (�DH0/R), and listed inTable 2.

The adsorptive equilibrium constant (K) is related to the stan-dard adsorption free energy (DG0) obtained according to [52]:

K ¼ 1csolvent

expð�DG0

RTÞ ð9Þ

where csolvent is the concentration of water in solution. It shouldbe noted that the unit of csolvent lies in that of K. As can be seen fromTable 1, the unit of K is l mg-1, which results in that the unitof csolvent is mg l-1 with the value of approximate 1.0 � 106.

With the obtained both parameters of DG0 and DH0, the stan-dard adsorption entropy (DS0) can be calculated using the follow-ing thermodynamic basic equation:

DS0 ¼ DH0 � DG0

Tð10Þ

All the standard thermodynamic parameters are listed in Table2. In any case, the negative sign of DH0 suggests that the adsorptionof inhibitor is an exothermic process, which means that inhibitionefficiency decreases with rise in the temperature. The behaviorcan be interpreted on the basis that increasing temperature leads

0.0030 0.0031 0.0032 0.0033 0.0034 0.0035-3.5

-3.0 0.5 M H2SO4

1/T (K-1)

Fig. 8. Straight lines of ln K versus 1/T.

Table 2Standard thermodynamic parameters of the adsorption of DSCLE on CRS surface inacid solution.

Acid solution(�C)

Temperature DG0

(kJ mol�1)DH0

(kJ mol�1)DS0

(J mol�1 K�1)

1.0 M HCl 20 �30.19 �39.02 �30.1230 �30.33 �39.02 �28.6740 �30.22 �39.02 �28.1050 �29.20 �39.02 �30.39

0.5 M H2SO4 20 �29.52 �36.52 �23.8830 �29.80 �36.52 �22.1740 �29.53 �36.52 �22.3250 �28.80 �36.52 �23.89

Table 3Parameters of the regression between ln v and 1/T.

Acidsolution

c(mg l�1)

Ea(kJ mol�1)

A(g m�2 h�1)

Linear regressioncoefficient (r)

1.0 M HCl 0 57.36 6.74 � 1010 0.998310 71.68 9.03 � 1012 0.995320 73.37 1.31 � 1013 0.993130 75.60 2.31 � 1013 0.989340 74.15 1.06 � 1013 0.982950 71.36 3.03 � 1012 0.986860 69.81 1.46 � 1012 0.989770 68.57 8.11 � 1011 0.991280 65.68 2.49 � 1011 0.993490 63.57 1.00 � 1011 0.9938

100 60.74 3.14 � 1010 0.9928120 57.84 9.56 � 109 0.9946140 53.15 1.38 � 109 0.9965160 54.31 2.07 � 109 0.9954180 53.15 4.11 � 108 0.9963200 48.93 2.33 � 108 0.9977

0.5 MH2SO4

0 49.62 5.07 � 109 0.996210 60.74 2.12 � 1011 0.997120 62.77 3.70 � 1011 0.991930 60.57 1.35 � 1011 0.990840 61.11 1.50 � 1011 0.992150 58.09 4.13 � 1010 0.989260 55.05 1.17 � 1010 0.990370 53.69 6.57 � 109 0.987580 51.69 2.87 � 109 0.992090 48.73 8.45 � 108 0.9951

100 45.14 1.98 � 108 0.9962120 41.76 4.88 � 107 0.9970140 41.14 3.57 � 107 0.9971160 41.23 3.57 � 107 0.9961180 40.64 2.71 � 107 0.9959200 39.25 2.54 � 107 0.9939

168 X. Li et al. / Corrosion Science 62 (2012) 163–175

to the desorption amount of some adsorbed inhibitor moleculesfrom the steel surface rises as well. Generally, values of DG0 up to�20 kJ mol�1 are consistent with the electrostatic interaction be-tween the charged molecules and the unlike charged metal (phys-ical adsorption) while those more negative than about�40 kJ mol�1 involve sharing or transfer of electrons from theinhibitor molecules to the metal surface to form a co-ordinate bond(chemisorption) [53]. Table 2 shows the values of DG0 in both acidsare with the range from �20 to �40 kJ mol�1, probably means thatboth physical adsorption and chemical adsorption (mixed adsorp-tion) would take place. What’s more, the values of DG0 in 1.0 MHCl is lower than that in 0.5 M H2SO4, demonstrating that it exhib-its a stronger tendency for DSCLE adsorb on steel surface in HClsolution. As for DS0 in Table 2, the sign is negative; means thatthe process of adsorption is accompanied by a decrease in entropy.It might be explained as follows: before the adsorption of inhibitoronto the steel surface, the chaotic degree was high, but when inhib-itor molecules were orderly adsorbed onto the steel surface, as a re-sult, a decrease in entropy.

3.2.5. Apparent activation energy (Ea) and pre-exponential factor (A)On the basis of Arrhenius equation, the natural logarithm of the

corrosion rate (ln v) is a linear function with 1/T for the acid corro-sion of steel [47,50]:

ln v ¼ �EaRTþ ln A ð11Þ

where Ea and A represent apparent activation energy and pre-expo-nential factor, respectively.

Arrhenius plots of ln v vs. 1/T for the blank acid solutions andcontaining 200 mg l�1 DSCLE are shown in Fig. 9, and the linearregression parameters for all inhibitor concentrations are summa-rized in Table 3. Through Fig. 9 and Table 3, all linear regressioncoefficients (r) are very close to 1, which suggests that the corrosionof steel in HCl and H2SO4 solutions without and with inhibitor fol-lows the Arrhenius equation. Fig. 10 shows the relationship be-tween Ea and inhibitor concentration in both media. Clearly, bothcurves appear similar changing rule, and Ea in H2SO4 is higher thanthat in HCl at every inhibitor concentration. At relative lower con-centrations (0–30 mg l�1 in 1.0 M HCl; 0–20 mg l�1 in 0.5 MH2SO4), Ea firstly increases with the concentration of inhibitor andreaches maximum values, then decreases sharply with increasingconcentration of inhibitor to about 140 mg l�1, lastly remains al-most constant for the inhibitor concentration rises from 140 to

0.0030 0.0031 0.0032 0.0033 0.0034 0.0035-1

0

1

2

3

4

5

ln v

(g m

-2 h

-1)

1/T (K-1)

Fig. 9. Arrhenius plots related to the corrosion rate of CRS in 1.0 M HCl and 0.5 MH2SO4. –j– 1.0 M HCl; –d– 0.5 M H2SO4; –h– 1.0 M HCl + 200 mg l�1 DSCLE; –s–0.5 M H2SO4 + 200 mg l-1 DSCLE.

200 mg l�1. That is to say, in the present system there is a ‘‘peak-like’’ value for Ea. The similar result was also reported for the bam-boo leaves extract of DBLE in HCl [47]. It is noted that when theDSCLE concentration is 140–200 mg l�1 in 1.0 M HCl as well as90–200 mg l�1 in 0.5 M H2SO4, the apparent activation energy valueis lower than that of blank. The increase in the apparent activationenergy Ea at low inhibitor concentrations may be interpreted asphysical adsorption that occurs in the first stage [37,54–57].Whereas a drop in Ea with respect to the blank solution, observedat the higher inhibitor concentration suggests chemisorption[28,53,58–61]. This result may be considered in line with the sug-gestion that the adsorption lies in the inhibitor concentration, and

0 20 40 60 80 100 120 140 160 180 200 22035

40

45

50

55

60

65

70

75

80

E a (k

J m

ol-1)

c (mg l-1)

1.0 M HCl 0.5 M H

2SO

4

Fig. 10. Relationship between apparent activation energy (Ea) and concentration ofDSCLE (c) in 1.0 M HCl and 0.5 M H2SO4.

Table 4Parameters of the linear regression between ln v and C for the corrosion of steel in HCland H2SO4 containing DSCLE at 20 �C (calculated from the data in Fig. 6 using Eq. (7)).

Acid solution c (mg l�1) r k (g m�2 h�1) B (M�1)

HCl 0 0.9995 2.80 0.42200 0.9941 0.17 0.85

H2SO4 0 0.9964 6.28 0.51200 0.9895 0.82 0.89

0 12 24 36 48 60 72 84 96 108 120 132 144 156 168 18075

80

85

90

95

100

η w (%

)

immersion time t (h)

1.0 M HCl 0.5 M H

2SO

4

X. Li et al. / Corrosion Science 62 (2012) 163–175 169

manifests mainly physisorption at lower concentration whilechemisorption at higher concentration for a given inhibitor [62].

From Table 4, it is also observed that the variance of A is similarto that of Ea. Influenced by the altogether effect of the magnitudesof Ea and A, the corrosion rate decreases with the inhibitorconcentration.

Fig. 12. Effect of immersion time (t) on inhibition efficiency (gw) in 1.0 M HCl and0.5 M H2SO4 at 20 �C (weight loss method).

3.2.6. Effect of immersion time on corrosion inhibition

The immersion time is another important parameter in assess-ing the stability of inhibitive behaviour, so it is necessary to eval-uate the inhibition efficiency for a long immersion time. In thepresent study, effect of immersion time (6–168 h) on corrosioninhibition of 200 mg l�1 DSCLE in 1.0 M HCl and 0.5 M H2SO4 at20 �C was investigated using weight loss method. Fig. 11 showsthe corrosion rates obtained in the absence and presence ofDSCLE act as a function of time. Without inhibitor, the corrosionrates increases with the immersion time from 6 to 60 h, then re-mains at about 6.5 and 9.9 g m�2 h�1 in 1.0 M HCl and 0.5 MH2SO4, respectively. That is to say, it takes about 60 h for the cor-rosion rates in both acids reach certain values, and it is worth-while to study the effect of immersion time on the corrosioninhibition. Also through Fig. 11, it is found that the corrosionrates in the presence of DSCLE remain almost constant with theimmersion time studied, which may be ascribed to the adsorptionfilm formed on the steel surface. Dependence inhibition efficiency(gw) on the concentration of DSCLE is shown in Fig. 12. It can beseen that the changed rule of gw in both acids is similar. At first,gw increases with immersion time from 6 to 36 h, while remainsconstant in 6–160 h (95% in 1.0 M HCl, 86% in 0.5 M H2SO4). Thereasons could be attributed to the adsorptive film of inhibitor thatrests upon the immersion time [50]. The adsorptive film reachesmore compact and uniform along with prolonging immersiontime (6–36 h), and then the adsorptive film becomes saturatedstate within 36–160 h.

0 12 24 36 48 60 72 84 96 108 120 132 144 156 168 1800

1

2

3

4

5

6

7

8

9

10

v (g

m-2 h

-1)

immersion time t (h)

0.5 M H2SO

4

1.0 M HCl 0.5 M H

2SO

4 + 200 mg l-1 DSCLE

1.0 M HCl + 200 mg l-1 DSCLE

Fig. 11. Effect of immersion time (t) on corrosion rate (v) of CRS in 1.0 M HCl and0.5 M H2SO4 at 20 �C (weight loss method).

3.2.7. Effect of acid concentration on corrosion inhibitionIn order to study the effect of acid concentration on the corro-

sion of steel in the presence of 200 mg l�1 DSCLE, dependence ofinhibition efficiency (gw) on the concentration of acid (1.0–5.0 MHCl, 0.5–5.0 M H2SO4) at 20 �C is shown in Fig. 13. For either HClor H2SO4 media, increasing acid concentration resulted in decreas-ing gw gradually, and especially for H2SO4, gw decreases almost lin-early with the concentration of H2SO4. The minimum gw values are55.6% and 26.3% in 5.0 M HCl and 5.0 M H2SO4 solutions, respec-tively. At same acid concentration solution, inhibition performancefollows the order: HCl > H2SO4.

Assuming the corrosion rate (v) against the molar concentrationof acid (C) obeys the kinetic expression proposed by Mathur andVasudevan [63]:

ln v ¼ ln kþ BC ð12Þ

where k is the rate constant, and B is the reaction constant. Thestraight lines of ln v versus C are shown in Fig. 14, and the corre-sponding kinetic parameters are listed in Table 4.

As shown in Eq. (12), k can be deemed as a commencing rate atzero acid concentration, so k means the corrosion ability of acid formetal [63]. Inspection of Table 4 reveals that in the presence ofDSCLE, there is a drop of k to more extent, which indicates thatthe steel corrosion is retarded by the inhibitor of DSCLE. Further-

0 1 2 3 4 5 620

30

40

50

60

70

80

90

100

η w (%

)

concentration of acid C (M)

1.0 M HCl 0.5 M H

2SO

4

Fig. 13. The relationship between inhibition efficiency (gw) of 200 mg l-1 DSCLE andacid concentration (C) at 20 �C (weight loss method).

0 1 2 3 4 5 6-2

-1

0

1

2

3

4

5

H2SO

4

HCl H

2SO

4 + 200 ppm DSCLE

HCl + 200 ppm DSCLE

ln v

(g m

-2 h

-1)

concentration of acid C (M)

Fig. 14. The straight lines of ln v versus C at 20 �C.

-6 -5 -4 -3 -2 -1-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

E (

V v

s. S

CE

)

log i (A cm-2)

blank

10 mg l-1

50 mg l-1

100 mg l-1

200 mg l-1

Fig. 16. Potentiodynamic polarization curves for CRS in 0.5 M H2SO4 without andwith different concentrations of DSCLE at 20 �C (immersion time is 2 h).

170 X. Li et al. / Corrosion Science 62 (2012) 163–175

more, the values of k in 1.0–5.0 M HCl uninhibited and inhibitedsolutions are respective lower than those in 0.5–5.0 M H2SO4 unin-hibited and inhibited solutions, which confirms that the corrosionof CRS in H2SO4 solution is stronger than that in HCl, and the inhi-bition performance in 1.0–5.0 M HCl is more superior to that in0.5–5.0 M H2SO4. On the basis of Eq. (12), B is the slope of the lineln v-C, thus B implies the changed extent of v with the acid concen-tration [63]. It is observed that the values of B in the presence ofinhibitor increase compared with those of blank HCl and H2SO4solutions, which suggests that the changed extent of corrosion ratewith acid concentration in inhibited acid is larger than that inuninhibited acid. In addition, the value of B in H2SO4 solution ishigher than that in corresponding HCl solution, which indicatesthat the changed degree of corrosion rate with acid concentrationin H2SO4 is greater than that in HCl.

3.3. Potentiodynamic polarization curves

Figs. 15 and 16 show the potentiodynamic polarization curvesfor CRS in 1.0 M HCl and 0.5 M H2SO4 solutions without and withdifferent concentrations of DSCLE at 20 �C (immersion time is2 h), respectively. In 1.0 M HCl solution, the presence of DSCLEcauses a remarkable decrease in the corrosion rate i.e., shifts the

-6 -5 -4 -3 -2 -1-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

E (

V v

s. S

CE

)

log i (A cm-2)

blank

10 mg l-1

50 mg l-1

100 mg l-1

200 mg l-1

Fig. 15. Potentiodynamic polarization curves for CRS in 1.0 M HCl without and withdifferent concentrations of DSCLE at 20 �C (immersion time is 2 h).

both anodic and cathodic curves to lower current densities. Inother words, DSCLE acts as a mixed-type inhibitor in HCl solution.As for 0.5 M H2SO4 solution, DSCLE also inhibits both anodic andcathodic reactions, while prominently inhibits the cathodic reac-tion compared with the anodic reaction, especially for 10 and50 mg l�1. The potentiodynamic polarization parameters includingcorrosion current densities (icorr), corrosion potential (Ecorr), catho-dic Tafel slope (bc), anodic Tafel slope (ba), and inhibition efficiency(gP) are presented in Table 5.

The corrosion current density (icorr) of uninhibited 0.5 M H2SO4is higher than that of uninhibited 1.0 M HCl, and that the reason forthis is not yet known. The polarization curves show that the highercurrent in H2SO4 is observed in the anodic branch, and not in thecathodic branch, so the effect is in the kinetics of anodic dissolu-tion. The acid anions (Cl�, SO42�) are firstly adsorbed on the steelsurface to form the surface complexes in the anodic process, andthe complexes desorbed from the surface [64].

Table 5 shows that icorr decreases considerably in the presenceof DSCLE in both acids, and decreases with the increase of inhibitorconcentration. In turn, inhibitive ability improves with the inhibi-tor concentration. gp of 200 mg l�1 DSCLE reaches up to a maxi-mum of 90.3% in 1.0 M HCl; and 79.2% in 0.5 M H2SO4. In bothacids of HCl and H2SO4 solutions, Ecorr almost does not alter inthe presence of DSCLE, which indicates DSCLE acts as a mixed-typeinhibitor. According to Cao [65], the inhibition category belongs togeometric blocking effect. That is, the inhibition action manifests

Table 5Potentiodynamic polarization parameters for the corrosion of CRS in 1.0 M HCl and0.5 M H2SO4 solutions containing different concentrations of DSCLE at 20 �C.

Acid solutionc (mg l�1)

Ecorr(mV vs.SCE)

icorr(lA cm�2)

�bc(mV dec�1)

ba(mV dec�1)

gp(%)

1.0 M HCl 0 �453 152.5 123 66 –10 �462 59.0 129 70 61.350 �469 22.7 114 61 85.1

100 �464 16.1 115 64 89.4

0.5 MH2SO4

200 �461 14.8 119 72 90.30 �481 481.4 140 41 –

10 �484 237.8 133 30 50.650 �485 138.9 120 31 71.1

100 �495 126.0 125 52 73.8200 �472 100.1 122 48 79.2

X. Li et al. / Corrosion Science 62 (2012) 163–175 171

through the reduction of the reaction area on the surface of thecorroding metal [65]. The slight vary of both bc and ba suggests thatthe corrosion mechanism of steel does not change in the presenceof DSCLE. Table 5 again reveals that at same inhibitor concentra-tion, icorr (1.0 M HCl) < icorr (0.5 M H2SO4); whereas gp (1.0 MHCl) < gp (0.5 M H2SO4).

3.4. Electrochemical impedance spectroscopy (EIS)

Figs. 17 and 18 show the Nyquist diagrams for CRS in 1.0 M HCland 0.5 M H2SO4 at 20 �C (immersion time is 2 h), respectively.Obviously, all impedance spectra have typical characteristics thathave been widely reported for steel in strong acid media. As shownin Fig. 17, in uninhibited and inhibited 1.0 M HCl solutions, theimpedance spectra exhibit one single capacitive loop, which indi-cates that the corrosion of steel is mainly controlled by the chargetransfer process [66]. On the other hand, in 0.5 M H2SO4 solution,the impedance spectra shown in Fig. 18 consist of two loops, onelarge capacitive loop at high frequencies (HF), and one small induc-

0 50 100 150 200 250 300 350 400 450 500 550

0

50

100

150

200

250

300

350

400

450

500

550

-Zi (

Ωcm

2 )

Zr (Ω cm2)

10 mg l-1

50 mg l-1

100 mg l-1

200 mg l-1

0 10 20 30 40 50 60 70

010203040506070

blank

-Zr (

Ω c

m2 )

Zr (Ω cm2)

Fig. 17. Nyquist plots of the corrosion of CRS in 1.0 M HCl without and withdifferent concentrations of DSCLE at 20 �C (immersion time is 2 h).

0 40 80 120 160 200 240

0

40

80

120

160

200

240

0 10 20 30 40 50

0

10

20

30

40

50

-Zr (

Ω c

m2 )

Zr (Ω cm2)

blank

-Zi (

Ωcm

2 )

Zr (Ω cm2)

10 mg l-1

50 mg l-1

100 mg l-1

200 mg l-1

Fig. 18. Nyquist plots of the corrosion of CRS in 0.5 M H2SO4 without and withdifferent concentrations of DSCLE at 20 �C (immersion time is 2 h).

tive loop at low frequencies (LF). The capacitive loop at HF is gen-erally related to the charge transfer of the corrosion process anddouble layer behavior. In contrast, the inductive loop at LF maybe attributed to the relaxation process obtained by adsorption spe-cies like FeSO4 [67] or inhibitor species [68] on the electrode sur-face. In both acids, with respect to blank solution, the shape ismaintained throughout all tested concentrations, indicating thatthere is almost no change in the corrosion mechanism occursregardless of the inhibitor addition [69].

It is noted that these capacitive loops in both acids are not per-fect semicircles which can be attributed to the frequency disper-sion effect as a result of the roughness and inhomogeneous ofelectrode surface [70]. Furthermore, the diameter of the capacitiveloop in the presence of inhibitor is larger than that in blank solu-tion, and enlarges with the inhibitor concentration. This indicatesthat the impedance of inhibited substrate increases with the inhib-itor concentration, and leads to good inhibitive performance.

The EIS results of these capacitive loops are fitted using theequivalent circuit shown in Fig. 19 to pure electric models thatcould verify or rule out mechanistic models and enable the calcu-lation of numerical values corresponding to the physical and/orchemical properties of the electrochemical system under investiga-tion [61]. The circuit employed allows the identification of bothsolution resistance (Rs) and charge transfer resistance (Rt). It isworth mentioning that the double layer capacitance (Cdl) value isaffected by imperfections of the surface, and that this effect is sim-ulated via a constant phase element (CPE) [71]. The CPE containsthe component Qdl and the coefficient a that quantifies differentphysical phenomena like surface inhomogeneousness resultingfrom surface roughness, inhibitor adsorption, porous layer forma-tion, etc. The double layer capacitance (Cdl) is accounted usingthe following relation [72]:

Cdl ¼ Q dl � ð2pfmaxÞa�1 ð13Þ

where fmax represents the frequency at which imaginary valuereaches a maximum on the Nyquist plot. The electrochemicalparameters of Rt, Cdl and gR are calculated by ZSimpWin softwareand listed in Table 6.

Clearly, Rt increases significantly whereas Cdl reduces with theconcentration of DSCLE in both acids. The greatest inhibitive effect

CPE

Rt

Rs

Fig. 19. Equivalent circuit used to fit the capacitive loop.

Table 6EIS parameters for the corrosion of CRS in 1.0 M HCl and 0.5 M H2SO4 solutionscontaining DSCLE at 20 �C.

Acid solution c (mg l�1) Rt (X cm2) Cdl (lF cm�2) gR (%)

1.0 M HCl 0 64.8 95.3 –10 149.8 74.5 56.750 369.3 65.3 82.4

100 484.2 57.2 86.6200 513.6 39.3 87.4

0.5 M H2SO4 0 46.3 103.5 –10 96.55 86.8 52.050 149.2 72.6 69.0

100 188.3 67.1 75.4200 218.6 66.7 78.8

172 X. Li et al. / Corrosion Science 62 (2012) 163–175

is observed at 200 mg l�1 of DSCLE which presents Rt value of513.6 X cm2 in 1.0 M HCl, and 218.6 X cm2 in 0.5 M H2SO4; Cdl va-lue of 39.3 lF cm�2 in 1.0 M HCl, and 66.7 lF cm�2 in 0.5 M H2SO4.A large charge transfer resistance is associated with a less corrod-ing system. On the contrary, better protection provided by aninhibitor can be associated with a decrease in capacitance of themetal. According to Helmholtz model [73]:

Cdl ¼e0ed

A ð14Þ

where e0 is the permittivity of air, e the local dielectric constant, dthe thickness of the film and A is the surface area of the electrode.Accordingly, the small in Cdl in comparing with that in blank solu-tion (without inhibitor), which can result from a decrease in localdielectric constant and/or an increase in the thickness of the electri-cal double layer, suggests that the inhibitor molecules function byadsorption at the metal/solution interface [67]. gR increases withthe concentration of DSCLE, and follows the order: gR (HCl) > gR(H2SO4). The maximum gR values are 87.4% and 78.8% in 1.0 MHCl and 0.5 M H2SO4, respectively. These results again confirm that

Fig. 20. AFM three-dimensional images of CRS surface: (a) after 6 h of immersion at 20 �Cafter 6 h of immersion at 20 �C in 0.5 M H2SO4; (d) after 6 h of immersion at 20 �C in 20

DSCLE exhibits good inhibitive performance for CRS in both acidsolutions, and it is more efficient to inhibit the corrosion of steelin 1.0 M HCl than 0.5 M H2SO4.

Inhibition efficiencies obtained from weight loss (gw), potentio-dynamic polarization curves (gp) and EIS (gR) are in good reason-ably agreement.

3.5. Atomic force microscope (AFM) surface examination

The atomic force microscope (AFM) provides a powerful tool ofcharacterizing the microstructure [74]. Fig. 20 shows that thethree-dimensional AFM images of steel surface in both 1.0 MHCl and 0.5 M H2SO4 solutions without and with DSCLE at 20 �C(immersion time is 6 h). Fig. 20(a) and (c) reveal that the steelsurfaces after immersion in uninhibited 1.0 M HCl and 0.5 MH2SO4 solutions appear an aggressive attack on steel surface bythe corroding medium. There are no evident black holes or cre-vices on the surface owing to general uniform corrosion of steelin strong acid media. Clearly, the corrosion of steel in 0.5 MH2SO4 is stronger than that in 1.0 M HCl. In the experiment, a

in 1.0 M HCl; (b) after 6 h of immersion at 20 �C in 200 mg l�1 DSCLE + 1.0 M HCl; (c)0 mg l-1 DSCLE + 0.5 M H2SO4.

X. Li et al. / Corrosion Science 62 (2012) 163–175 173

slight yellow film was observed on CRS surface in the presence of200 mg l�1 DSCLE. AFM microstructures of Fig. 20(b) as well as (d)shows that there is a wrapping zonal film adsorbed on CRS sur-face in either 1.0 M HCl or 0.5 M H2SO4 with addition of200 mg l�1 DSCLE, which does not exist in correspondingFig. 20(a) and (c). In accordance, it could be concluded that theadsorption film can efficiently protect the steel from corrosion.

O

OH

OH

OH

HO

glu O xyl

a b

O

OH

OCH3

OH

HO

c O

glu

d

H2N

NH2

OHe

OH

O

O

O

OO

OO

O

g

Fig. 21. The chemical structures of (a) orientiu-xyloside; (b) lsoorient; (c) vitexin-40-

It should be noted that AFM has examined only the microstruc-ture; we focus on only the comparison of the steel surface corro-sion before and after adding inhibitor. Some limitations of thisstudy are that it could not provide the chemical composition ofadsorption film. Despite its limitation, this study can clearly indi-cate that the steel corrosion is retarded after adding inhibitor inboth acids.

O

OH

OH

OH

HO

glu

O

OO

OH

OH

OH O

glu

f

OH

OH

HO

O

HO

HN

O

O

O

OCH3; (d) luteolin-7-O-glucosioe; (e) d–hydroxyl lysine; (f) xylose; (g) galactose.

174 X. Li et al. / Corrosion Science 62 (2012) 163–175

3.6. Explanation for inhibition

Bamboo leaf extract (BLE) is composed of numerous naturallyoccurring organic compounds. Accordingly, the inhibitive actionof BLE could be attributed to the adsorption of its components onthe steel surface. The ethanol/water extract of bamboo leaf mainlycontains flavonoids, amino acid and amalose that have been wellcharacterized [75,76], and the chemical structures is shown inFig. 21. Inspection of Fig. 21 reveals that these compounds containmany O, N atoms in functional groups (O–H, C@O, C–O, N–H) andO–herterocyclic rings, which meets the general characteristics oftypical corrosion inhibitors. Thus, it is reasonable to deduce thatthe flavonoids, amino acid and amalose in DSCLE exhibit the inhibi-tion performance. In the present study, our results are lack of isolat-ing these compounds, thus it is not possible to determine whatcomponents present in BLF created their relatively high ability toinhibit corrosion.

These main chemical compounds as shown in Fig. 21 might be pro-tonated in acid media. The charge of the metal surface is determinedby the minus value of Ecorr – Eq=0 (zero charge potential) [73]. The Eq=0of iron is�530 mV vs. SCE in HCl [77]; and�550 mV vs. SCE in H2SO4[78]. In the present system, the values of Ecorr in 1.0 M HCl and 0.5 MH2SO4 are�455 mV and�481 mV vs. SCE, respectively. The steel sur-face charges positive in both 1.0 M HCl and 0.5 M H2SO4 solutions be-cause of Ecorr – Eq=0 > 0. The acid anions of Cl– and SO42� could befirstly adsorbed; they create excess negative charge toward the solu-tion, and favor more adsorption of the cations [79]. Then the proton-ated inhibitor may adsorb on the negatively charged metal surfacethrough electrostatic interactions. In other words, there could be asynergism between anions (Cl�, SO42�) and protonated inhibitor.The inhibitive ability of DSCLE in HCl is greater than that in H2SO4,which implies that the adsorption of inhibitor could be influencedby the nature of anions in acidic solutions. It is well known that Cl�

ions have stronger tendency to adsorb than do SO42� ions [58], andthe electrostatic influence on the inhibitor adsorption may be the rea-son for an increased protective effect in halide-containing solution[59]. Moreover, the lesser interference of SO42� ions with the adsorbedprotonated cations may lead to lower adsorption [60]. So, the adsorp-tion of DSCLE on steel surface in HCl solution is stronger than that inH2SO4 solution, which leads to higher inhibition performance in HClthan that in H2SO4. When protonated chemical molecules in DSCLEare adsorbed on steel surface, a coordinate bond may be formed bypartial transference of electrons from O, N atoms to vacant d orbitsof Fe. Owing to lone-pair electrons of O, N atoms in DSCLE, DSCLEmay combine with freshly generated Fe2+ ions on steel surface to formthe metal inhibitor complexes. These complexes might get adsorbedonto steel surface by van der Waals force to form a protective filmwhich keeps CRS from corrosion.

4. Conclusions

(1) DSCLE acts as a good inhibitor for the corrosion of CRS in1.0 M HCl and 0.5 M H2SO4 solutions. Inhibition efficiency(gw) increases with the inhibitor concentration, and themaximum gw of 200 mg l�1 DSCLE are higher than 89%and 79% at 20–50 �C, respectively.

(2) The adsorption of DSCLE on CRS surface obeys Langmuiradsorption isotherm. The adsorption process is an exother-mic process accompanied by a decrease in entropy,involves in both physisorption and chemisorption.

(3) There was a ‘‘peak-like’’ changed rule for Ea changing withthe concentration in both HCl and H2SO4 solutions. Theparameter of Ea indicates that the adsorption of DSCLEmanifests mainly physisorption at 10–80 mg l�1 whilechemisorption at 90–200 mg l�1.

(4) The corrosion rate of steel in HCl and H2SO4 solutions with-out and with DSCLE acts as a function of immersion time.gw increases with immersion time from 6 to 36 h, and thenremains constant (95% in 1.0 M HCl, 86% in 0.5 M H2SO4)prolonging to 160 h.

(5) Inhibition efficiency decreases gradually with acid concen-tration (1.0–5.0 M HCl; 0.5–5.0 M H2SO4). The corrosion ofCRS in HCl and H2SO4 solutions without and with inhibitorsupports the kinetic equation proposed by Mathur andVasudevan. The rate constant (k) decreases markedly afteradding DSCLE, while the reaction constant (B) increases.

(6) ]DSCLE acts as a mixed-type inhibitor in 1.0 M HCl and0.5 M H2SO4 solutions. The inhibition efficiency valuesobtained from weight loss, polarization curves and EIS arein good agreement.

(7) The introduction of DSCLE into HCl and H2SO4 solutionsresults in the formation of a film on the CRS surface, whicheffectively protects steel from corrosion.

Acknowledgement

This work was carried out in the frame of research projectsfunded by Chinese Natural Science Foundation under the grantNo. 51161023. The electrochemical measurements were carriedout using PARSTAT 2273 advanced electrochemical system (Prince-ton Applied Research) provided by Advanced Science InstrumentSharing Center of Southwest Forestry University.

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Inhibition of the corrosion of steel in HCl, H2SO4 solutions by bamboo leaf extract1 Introduction2 Experimental2.1 Materials2.2 Characterization of DSCLE2.3 Weight loss measurements2.4 Electrochemical measurements2.5 Atomic force microscope (AFM)

3 Results and discussion3.1 FTIR and UV of DSCLE3.2 Weight loss measurements3.2.1 Effect of DSCLE on corrosion rate3.2.2 Effect of DSCLE on inhibition efficiency3.2.3 Adsorption isotherm3.2.4 Thermodynamic parameters3.2.5 Apparent activation energy (Ea) and pre-exponential factor (A)3.2.6 Effect of immersion time on corrosion inhibition3.2.7 Effect of acid concentration on corrosion inhibition

3.3 Potentiodynamic polarization curves3.4 Electrochemical impedance spectroscopy (EIS)3.5 Atomic force microscope (AFM) surface examination3.6 Explanation for inhibition

4 ConclusionsAcknowledgementReferences