Qin Hu ,Yubing Qiu ,2,Guoan Zhang ,2,Xingpeng Guo ,2,*

1 School of Chemistry and Chemical Engineering,Huazhong University of Science and Technology,Wuhan 430074,China

2 Hubei Key Laboratory of Materials Chemistry and Service Failure,Wuhan 430074,China

Keywords:Biomaterial Corrosion inhibitor Electrochemical technique SEM Raman spectroscopy

ABSTRACT The corrosion inhibition effect of Capsella bursa-pastoris extracts(CBE)for Q235 carbon steels in 1 mol·L?1 hydrochloric acid solution was studied using electrochemical methods,environmental scanning electron microscopy(SEM)and Raman microscopy analysis.The polarization plots indicate that CBE serves as an effective,mixedtype inhibitor.Linear polarization resistance shows that increasing CBE concentration and temperature results in increased inhibition efficiency.The highest inhibition efficiency can reach 97%when adding 60 mg·L?1 CBE,which is better than some reported plant extracts under the similar environment.The adsorption of CBE molecules is found to obey the Langmuir adsorption isotherm.Some thermodynamic and kinetic parameters for the adsorption process,such as the adsorption equilibrium constant(K),free energy of adsorption(ΔG ads),activation energy of corrosion reaction(E a)and the heat of adsorption(Q ads),are calculated and discussed.SEM and Raman microscopy analysis also demonstrate the formation of a CBE inhibition film on the metal surface.

1.Introduction

Corrosion is a common problem and corrosion control of metals is also an expensive process.The use of corrosion inhibitors is more popular than barrier protection,galvanization and cathodic protection methods.Corrosion inhibitors decrease the rate of attack by the environment on the metal when added in small amounts to an environment.Heterocyclic compounds are considered to be effective corrosion inhibitors.The adsorption takes place through nitrogen,oxygen,phosphorus and sulphur atoms or triple bonds or aromatic rings,displacing water molecules on the surface and forming a compact barrier film[1].However,most of the synthetic inhibitors are highly toxic to living systems such as azole,hydrazine,pyridine,benzotriazol,carbaryl and oximes[2–5].

As we know,plant extracts generally have low toxicity,are inexpensive and can be obtained through simple extraction processes.More importantly,extracts of plants contain mixtures of organic compounds in nature,which have nitrogen,sulfur,triple bonds and aromatic rings as effective inhibition function groups.To find environmentally safe and readily available corrosion inhibitors with good efficiency,there has been a growing trend to use natural products[6–16].Oguzie[17]studied the inhibitive effect of Sansevieria trifasciata and Occium viridis extract on the acid corrosion of mild steel.Abiola et al.[18]studied the corrosion inhibition of Al in acidic media using Delonix Regia extract.Abdel-Gaber et al.[19]studied the inhibitive action of Chamomile,Halfabar,Black cumin and Kidney bean extracts on the corrosion of steel in acidic media.

The chemical composition of Capsella bursa-pastoris consists of 28 and 38 amino acids[20],fatty acids,and flavonoids which were considered to be efficient inhibition groups[21].However,Capsella bursa-pastoris,commonly known as shepherd's purse,is a wild plant whose young leaves and roots are edible,eaten raw or cooked in some parts of China[22].In recent years,Japan,America and the European Union have been studying Capsella bursa-pastoris as a safe raw material with wide potential uses for its useful biological and medicinal properties[23–25].And this work aims to investigate Capsella bursapastoris as a new natural inhibitor for Q235 mild steel in 1 mol·L?1HCl solution.

2.Material and Methods

2.1.Electrodes for corrosion studies

Q235 carbon steel was used as the studied material with the chemical composition(%,by mass):P 0.016,Si 0.07%,Ni 0.01,Mn 0.28,C 0.18%,S 0.006,Cr 0.02,Cu 0.05,Fe balance.Coupons(1.0 cm×1.0 cm×0.6 cm)were used as working electrodes for electrochemical tests,which were sealed by epoxy resin with 1.0 cm2of working area.The exposed surface area of the electrode was polished with silicon carbide abrasive paper of up to 1200 grits,then cleaned with double-distilled water and degreased with ethanol and acetone before each experiment.

2.2.Corrosion inhibitors

The dried aerial parts of Capsella bursa-pastoris were bought from a plant distributor in Rongcheng,Shandong,China.The plant material was powdered(particle sizes＜0.3 mm)for the extraction.The extraction conditions were optimized by orthogonal experiments.AR grade HCl and double-distilled water were used to prepare the aggressive solution.Inhibitor solutions were made by adding different volume of Capsella bursa-pastoris extract(CBE)to 1 mol·L?1HCl solutions.

2.3.Electrochemical studies

A conventional three-electrode system in a single compartment glass cell was used as the test system.A platinum foil was used as the counter electrode and a saturated calomel electrode(SCE)as reference electrode,which was connected to the test cell through a glass salt bridge with a Luggin capillary tube.The glass cell was filled with 200 ml of test solution.All potentials reported referred to the SCE and all the electrochemical tests were performed using a CS350 electrochemical workstation.

Electrochemical impedance spectroscopy(EIS)was recorded over a frequency range of 100 kHz to 10 mHz at the stable corrosion potential(Ecorr)of each test condition(changing rate＜±1 mV·min?1).The amplitude of the AC perturbation was 10 mV.The capacitance of electrical double layer(Cdl)and charge transfer resistance(Rt)were calculated to investigate the adsorption process.The anodic and cathodic polarization curves were measured with a constant scan rate of 0.5 mV·s?1,ranging from?0.25 to 0.25 V vs.Ecorr.Linear polarization curves were recorded with a constant scan rate of 0.2 mV·s?1in the potential range of?0.01–0.01 V vs.Ecorrso as to calculate the linear-polarisation resistance(Rp)of each test condition.

2.4.Surface analysis

After the experiments,all specimens were washed with distilled water and dried in nitrogen.The surface morphology of the specimens before and after immersion in 1 mol·L?1HCl with or without CBE was investigated using a Quanta 200 environmental scanning electron microscope(SEM).Raman spectra analysis for the specimen surface was performed using a DXR Raman microscope.

3.Results

3.1.Optimization of extraction conditions

Some extraction methods,such as reflux extraction,ultrasound extraction and soak extraction,were employed to obtain the CBE and ethyl alcohol(EtOH)was used as extraction solvent.It is found that extraction methods,the concentration of EtOH and extraction time are key factors affecting the extraction effect.These extraction conditions were optimized with a L9(34)orthogonal design method,in which the selected factors and levels are shown in Table 1 and the details of the L9(34)table are presented in Table 2,where Mjkrepresents the sum of all the experimental results of j level in k column,mjkrepresentsthe corresponding average of Mjk[see Eq.(1)],and Rkis the range of the maximum and minimum values of Mjk[see Eq.(2)]

Table 1 Factors and levels of the orthogonal experiment

Table 2 L9(34)orthogonal design table for the optimization of extraction conditions

The content in Table 2 is the inhibition efficiency(η)of CBE calculated with Eq.(3),where icorrand i′corrare the uninhibited and inhibited corrosion current densities,respectively,which are calculated from the potentiodynamic polarization curves measured with 60× 10?6CBE in 1 mol·L?1HCl at 25 °C(scan rate=0.5 mV·s?1).Some electrochemical parameters obtained from these polarization curves are also listed in Table 3,where baand bcrepresent Tafel slope of anodic and cathodic reactions,respectively.

According to the results in Table 2,there is R2＞R1＞R3＞R4,indicating that the factor B(concentration of EtOH)is more important than the factor A(extraction methods)and the factor C(extraction time).From the data of mjk,it is observed that the level 2 of A,the level 1 of B,and the level 3 of C is better.Therefore,the optimized extraction conditions for the CBE are determined as follows:ultrasonic extraction method,50%(by mass)ethanol+50%(by mass)water EtOH solution and 45 min extraction time.In this paper,the CBE was prepared with this optimized extraction method for other studies.

3.2.Potentiodynamic polarization curve measurements

Fig.1 shows the potentiodynamic polarization curves for Q235 mild steel in 1 mol·L?1HCl with varying concentrations of CBE at25 °C.It can be seen that the extract retards both anodic and cathodic reactions[26],and the level increases with the amount of the inhibitor.Anodic polarization also indicates that the inhibitor has less effect when the potential is more positive than?300 mV vs.SCE.This is probably because the surface area increases as the mild steel dissolves and desorbs the plant extract[27].

Electrochemical parameters and inhibition efficiencies are given in Table 4.An inhibitor can be classified as cathodic or anodic only when the corrosion potential value differs by more than 85 mV with respect to the Ecorrof the blank[28].The presence of CBE produces a positive shift in Ecorrfrom 22 to 44 mV vs.the Ecorrofthe blank,suggesting that CBE functions as an inhibitor according to a mixed-inhibitor mechanism.The action coefficients of the anodic and cathodic reactions(faand fc)can be calculated with Eqs.(4)and(5):

Table 3 Electrochemical parameters obtained from the polarization curves in 1 mol·L?1 HCl without or with CBE prepared under different extraction conditions(60 × 10?6,scan rate=0.5 mV·s?1,at 25 °C)

Fig.1.Potentiodynamic polarization curves for Q235 mild steel in 1 mol·L?1 HCl with different concentrations of CBE at 25 °C(scan rate=0.5 mV·s?1).

where ΔEcorris the range of the uninhibited and inhibited corrosion potentials.The values of faand fcin Table 5 show that faand fcare smaller than unit,indicating that the adsorbed inhibitor molecules affect both the anodic and cathodic reactions[29].The fcis greater than fain a low concentration(10–20 mg·L-1),but equal with each other in a high concentration(40–60 mg·L-1),suggesting that the CBE as a mixed-type corrosion inhibitor inhibits anodic reaction more than the cathodic reaction in low concentrations.

Table 4 Polarization parameters for Q235 mild steel in 1 mol·L?1 HCl solution with different concentrations of CBE at 25°C

Table 5 Action coefficients for Q235 mild steel in 1 mol·L?1 HCl with different concentrations of CBE at 25°C

3.3.Linear polarization

The linear polarization resistance method was conducted at 25°C,35 °C and 45 °C.The inhibition efficiencies(η)are calculated using the following Eq.(6):

where Rpand R′pare respectively the uninhibited and inhibited linear polarization resistances,and corresponding values are presented in Table 6.The surface fraction occupied by adsorbed molecules(θ),i.e.the surface coverage of CBE,is obtained by the ratio(η/100)and listed in Table 6 too.It is observed that at constant temperature,the value of Rpincreases with increasing CBE concentration.The increase of Rpoffers a higher level of protection on the interface against hydrogen reduction and iron dissolution,which is attributed to a film of the active substance and corrosion products forming over the surface of the electrode.At constant concentrations of CBE,the Rpdecreases with increasing temperature,suggesting that high temperature accelerates the corrosion of mild steel.However,the inhibition efficiency is found to increase with increasing temperature,similar to the effects of concentrationchange.In a similar environment,inhibition performance of CBE is superior to some other plants,such as Telfaria occidentalis,Chamomile,Halfabar extract and so on[19,30].

Table 6 The corresponding values from linear polarization resistance measurement at 25 °C,35 °C and 45°C

3.4.Electrochemical impedance spectroscopy

EIS was also used to investigate the adsorption process by calculating the double layer capacitance(Cdl)and the charge transfer resistances(Rt)under different conditions[31].Fig.2 gives the Nyquist plots for Q235 mild steel in 1 mol·L?1HCl solution with different concentrations of CBE.It is shown that the impedance response for mild steel in 1 mol·L?1HCl solution changes significantly with increasing the inhibitor concentration.

Fig.2.Nyquist plots at E corr for Q235 mild steel in 1 mol·L?1 HCl solution with different concentrations of CBE at 25°C.

All the Nyquist plots are characterized by one depressed capacitive semicircle,which should be attributed to the heterogeneity of electrode surface and the time constant of the charge transfer and the double layer capacitance[32].This suggests that the charge transfer process controls the corrosion of mild steel[33,34].An equivalent circuit,shown in Fig.3,was selected to fit the EIS results in Fig.2 so as to gain the values of Cdland Rt.The inhibition efficiencies(η)are also calculated using the following Eq.(7):）

where Rtand R′tare the uninhibited and inhibited charge transfer resistance,respectively.All the parameters calculated from the EIS data are listed in Table 7.Fig.4 shows the relations of these values to different concentrations of CBE.The addition of CBE further enhances the Rtvalues and reduces Cdl.This can be attributed to the adsorption of CBE,with molecules that can adsorb directly onto the metal/solution interface instead of H2O molecules[35].

Fig.3.Equivalent electrical circuit of Q235 mild steel in different concentrations of CBE.

Table 7 EIS parameters for the Q235 mild steel in 1 mol·L?1 HCl with different concentrations of CBE at 25°C

Fig.4.Changes of C dl and R t with the concentration of CBE for Q235 mild steel in 1 mol·L?1 HCl.

4.Discussions

4.1.Adsorption

Inhibition is due to the adsorption of CBE molecular species[36].Linear polarization data listed in Table 6 was used to analyze the adsorption mechanism.The dependence of the surface coverage(θ)was found as a function of the concentration of the inhibitor(c).A direct relationship is assumed between CBE concentration and the surface coverage of the inhibitor may be expressed as following Eq.(8):

Fig.5.Langmuir adsorption isotherm model for CBE.

where K is the adsorption equilibrium constant.Fig.5 illustrates that the plots of c/θ against c for CBE at different temperatures are linear with a slope of 1.02 and a high regression correlation coefficient(0.99).This suggests that the adsorption of CBE molecules on the carbon steel surface follows the Langmuir isotherm[37].

4.2.Thermodynamic and kinetic parameters

The equilibrium constant of adsorption(K)is related to the free energy of adsorption,ΔGadsby

where R is the gas constant(8.314 J·mol?1·K?1),T is the absolute temperature(K),and csolventrepresents the concentration of water in the solution with a value of approximately 1.0×106,which results from the unit of K(L·mg?1)[12,38].Table 8 gives the K values calculated from the results in Fig.5 and the ΔGadsvalues calculated from Eq.(9).From Table 8,it can be found that the ΔGadsvalues were negative,ensuring spontaneous adsorption processes and a stable adsorbed layer on the mild steel surface.Moreover,the adsorption is said to be a physisorption if|ΔGads|＜ 20 kJ·mol?1.If|ΔGads|＞ 40 kJ·mol?1,the adsorption is said to be chemisorption[39,40].For the|ΔGads|in Table 7 are between 20 kJ·mol?1and 40 kJ·mol?1and increase with temperature,it confirms that both the chemisorption and physisorption of CBE exist at the same time and physisorption may transform to chemisorption with increasing temperature so as to play a more important role.

Table 8 Dates of Langmuir adsorption isotherm and ΔG ads

The values of the activation energy(Ea)and the heat of adsorption(Qads)were calculated with Eqs.(10)–(11)[11,41]

where θ1and θ2represent the degree of surface coverage at temperatures T1and T2,respectively.Fig.6 shows the Arrhenius plots for the corrosion of Q235 mild steel in 1 mol·L?1HCl in the absence and presence of different concentrations of CBE,respectively.

Fig.6.Arrhenius slopes for Q235 mild steel in 1 mol·L?1 HCl in the absence and presence of CBE.

From the plot slopes in Fig.6,Eavalues were calculated and presented in Table 9.It is found that the Eavalues of the inhibited systems are lower than the value for the uninhibited system and increase with temperature.The decrease of Eain the presence of the inhibitor may be attributed to the chemisorption of the inhibitor onto the steel surface[42–44].With an increase in temperature,some chemical changes occur in the inhibitor molecules,leading to incremental changes in the electron densities at the adsorption centre of the molecule,which can improve the inhibitor efficiency.The Qadsvalues in Table 9 are positive for all cases,indicating that the adsorption is an endothermic reaction and subsequent inhibition efficiency increases with increased temperature.This supports the proposed chemical adsorption mechanism[45].

Table 9 Q ads(kJ·mol?1)and E a(kJ·mol?1)values for Q235 mild steel 1 mol·L?1 HCl solution in the presence of CBE

4.3.ESEM spectra

The protection provided by CBE for Q235 mild steel in 1 mol·L?1HCl solutions was retained by environmental scanning electron microscopy,as shown in Fig.7.Fig.7(a)shows that the finely polished surface of the specimen before test just reveals some scratches.Fig.7(b)shows a rough surface,characteristic of the corrosion of carbon steel in acid.This indicates that the mild steel electrode surface is highly corroded in 1 mol·L?1HCl,caused by the direct attack of aggressive acids[46].Fig.7(c)shows the relatively smooth surface due to the protective CBE film on the metal surface that significantly inhibits the corrosion in the acid medium.

Notable differences were also observed in the energy-dispersive analysis microscope spectrum(EDS)in Fig.8,indicating a possible chemical change of the film on the steel surface.The Q235 mild steel surface corroded in 1 mol·L?1HCl medium has high C and O concentrations compared with the surface before the test.And many granules of phytochemicals present in CBE contain Ca and K[47]and notably higher C and O content were discovered on the surface after immersing in 1 mol·L?1HCl in the presence of CBE.This observation may be due to the adsorption of the organic chemicals of CBE onto the surface,and proves that the inhibition is a result of the formation of a stable film of CBE on the steel surface.

4.4.DXR Raman microscope

DXR Raman microscopy was carried out to characterize the Q235 steel surfaces under different conditions,as shown in Fig.9.Table 10 shows all the characteristic bands that correspond to the functional groups that are present in iron oxide and organic compounds.All spectra were obtained in duplicate or more from different specimens,with minor changes to the band position or intensity.The spectra of Q235 steel in 1 mol·L?1HCl were recorded over the Raman shift range of 1553–180 cm?1,which confirms the presence of iron oxide such as γ-Fe2O3[48].The spectrum of the Q235 steel in 1 mol·L?1HCl with the CBE film shows major spectroscopic features indicating organic discrimination.In particular,the key reference bands of aromatic compounds are featured at 1600,1580,1500 and 1450 cm?1.The spectrum in the fingerprint region of 1000–650 cm?1is related to the number of substituent groups and their position[49].The exposed specimen was examined with a flat pro file,indicating that there is less corrosion product and CBE presents on the steel surface.This further confirms that CBE inhibits the corrosion of Q235 steel in HCl solution.

Fig.7.SEM photographs for the Q235 mild steel(a)before test,(b)after immersion in 1 mol·L?1 HCl and(c)after immersion in 1 mol·L?1 HCl with CBE.

Fig.8.EDS spectrum for the Q235 mild steel(a)before test,(b)after immersion in 1 mol·L?1 HCl and(c)after immersion in 1 mol·L?1 HCl with CBE.

Fig.9.DXR Raman spectrum for the Q235 mild steel:(a)after polarization test in 1 mol·L?1 HCl,(b)after polarization test in 1 mol·L?1 HCl with CBE and(c)before immersion.

4.5.Inhibitive mechanism

As mentioned above,CBE consists of amino acids,fatty acids and flavonoids.These chemical compounds contain oxygen and nitrogen atoms which may be adsorbed on metal surface.The results we gained above have shown that CBE obeys Langmuir adsorption isotherm and its adsorption process is spontaneous(ΔGads＜ 0)and endothermic(Qads＞0)with lower activation energy(Ea).These parameters combined with the results of electrochemical tests,SEM,EDS and Raman spectrum analysis indicate that CBE can be adsorbed on Q235 steel surface to form a protective CBE film so as to inhibit both the anodic and the cathodic processes on the metal by geometrical covering effect.So the significant reduction of corrosion rate is due to the formation of the protective CBE film on steel surface[50].

5.Conclusions

This study proves that Capsella bursa-pastoris extracts(CBE)can be extracted with a simple and convenient method and used in chemical cleaning or picking processes as an eco-friendly inhibitor.The main conclusions were drawn as following:

·Increasing the concentration of CBE and temperature causes the inhibition efficiency to increase.

·The CBE act as an eco-friendly,mixed-type inhibitor,and the inhibition performance is superior to some other plants in a similar environment.

·The adsorption of CBE obeys Langmuir isotherm.Thermodynamic and kinetic parameters indicate a spontaneous process,and chemisorption plays a key role.

Table 10 Raman shift of iron oxide and organic compound

·SEM,EDS and Raman spectra were recorded to confirm the presence of a protective Capsella bursa-pastoris film on the metal surface.

Nomenclature

baTafel slope of anodic reaction

bcTafel slope of cathode reaction

Cdldouble layer capacitance

Eaapparent activation energy

Ecorrcorrosion potential

fathe action coefficients of anodic reaction

fcthe action coefficients of cathodic reaction

icorruninhibited corrosion current density

i'corrinhibited corrosion current density

K adsorption equilibrium constant

Mijsum of all the experimental results of i level in j column in the table

mijcorresponding average of Mij

Qadsheat of adsorption

Rjrange of the max and min value

Rpuninhibited linear polarization resistance

R'pinhibited linear polarization resistance

Rssolution resistance

Rtcharge transfer resistance

η corrosion inhibition efficiency

θ dependence of the surface fraction

ΔGadsadsorption free energy change

Subscripts

a anodic

ads adsorption

c cathode

corr current

p polarization