2 experimental and theoretical study of corrosion inhibition of 3- for mild steel in hydrochloric acid_石泽民_W

1、Corrosion Science 74 (2013) 206213Contents lists available at SciVerse ScienceDirectCorrosion Sciencejournal homepage: /locate/corsciExperimental and theoretical study of corrosion inhibition of 3-pyridinecarbozalde thiosemicarbazone for mild steel in hydrochloric acidBin Xu a, Ying

2、Liu a, Xiaoshuang Yin a, Wenzhong Yang a, Yizhong Chen ba School of Science, Nanjing University of Technology, Nanjing 210009, PR Chinab School of Environmental and Safety Engineering, Jiangsu Polytechnic University, Changzhou 213164, PR Chinaa r t i c l e i n f oArticle history:Received 13 December

3、 2012Accepted 25 April 2013Available online 3 May 2013Keywords:A. Mild steelB. Weight lossB. EISC. Acid corrosiona b s t r a c tThe corrosion inhibition effect of 3-pyridinecarboxaldehyde thiosemicarbazone (3-PCT) on mild steel was studied in 1 M HCl solution by means of weight loss, potentiodynamic

4、 polarization and electrochemical impedance spectroscopy (EIS) measurements. The surface morphology of mild steel was examined with scanning electron microscopy (SEM) and the mechanism of inhibition was determined by the potential of zero charge (PZC) measurement at the metalsolution interface. Then

5、 molecular dynamics simulationswere also executed for 3-PCT. The results show that 3-PCT is a good corrosion inhibitor, retarding both cathodic and anodic reactions in hydrochloric acid. The adsorption of 3-PCT on the mild steel surfaceobeys the Langmuir isotherm, and the thermodynamic parameters K

6、ads ; Ea; DG0 ads were also deter- mined and discussed.2013 Elsevier Ltd. All rights reserved.1. IntroductionMild steel is a well-known material used in various industries and highly susceptible to corrosion in acid media 1. Acid solu- tions, especially hydrochloric acid, are widely used in industri

7、es for acid picking, acid cleaning, acid descaling and oil well acidizing 24. The acid solutions may cause metal corrosion leading to huge economic losses. One of the most important methods in cor- rosion protection is to use inhibitor 5. Nowadays, the study of material corrosion process and the ads

8、orption mechanism of or- ganic inhibitors is a hot research 6. Many efcient corrosion inhibitors are the organic compounds rich in hetero-atoms such as nitrogen, oxygen, sulfur and p-bonds 79. Schiffs bases, con- densation products of aldehydes or ketones with amines, have re- ceived a considerable

9、amount attention recently, due to the presence of CNA group, exhibiting perfect inhibition ability 1,1015. The lone pair of electrons on the N atom and the planar- ity structure are important in determining the adsorption of the molecules on the metal surface 1. Many kinds of nitrogen-con- taining c

10、ompounds also attract attention on their inhibition prop- erties for metal corrosion, for example, pyridine and its derivatives 1618. The nitrogen atom, high electron density at the pyridine ring and the electron donating power to the metal ions are likely to facilitate the adsorption action on the

11、metal surface. Corresponding author. Tel./fax: +86 25 83172359.E-mail address: (W. Yang).In recent years, the molecular dynamics (MD) simulations have become a powerful tool for studying the mechanism of corrosion inhibition. The MD simulations can provide insights into the design o

12、f inhibitor systems with superior properties and elucidate the adsorption process at molecular level 1921.In the present work, the corrosion inhibition efciency of a newly synthesized pyridine derivative, 3-pyridinecarboxaldehyde thiosemicarbazone (3-PCT, Fig. 1), on mild steel in hydrochloric acid

13、solution was investigated utilizing weight loss measurement, electrochemical techniques and SEM method. Besides, the molecu- lar dynamics simulations were performed to study the electronic structure and the adsorption of 3-PCT at molecular level, providing insights into the design of inhibitor syste

14、ms with superior proper- ties. Finally, an attempt was made to elucidate the inhibition mechanism by studying the potential of zero charge (PZC).2. Experimental2.1. Materials and sample preparationThe inhibitor, namely 3-pyridinecarboxaldehyde thiosemicar- bazone (3-PCT) has been synthesized in labo

15、ratory according to the following procedures. Thiosemicarbazide (0.01 mol) was slowly dropwise to a mixture of 3-pyridinecarboxaldehyde (0.01 mol), 40 mL ethanol and a catalytic amount of concentrated sulfuric acid (three drops), then the mixture solution was heated to 353.15 K. The reaction solutio

16、n was stirred under reux for 4 h0010-938X/$ - see front matter 2013 Elsevier Ltd. All rights reserved. /10.1016/j.corsci.2013.04.044B. Xu et al. / Corrosion Science 74 (2013) 2062132072.4. Measurement of the potential of zero chargeFig. 1. Chemical structure of the synthesized inhibi

17、tor.and then cooled to room temperature. The obtained precipitate was collected and recrystallized by ethanol. The structure of the compound was conrmed by 1H NMR and FT-IR spectroscopic methods. 1H NMR (DMSO-d6, 500 MHz), d: 7.427.45 (m, 1H, PyH), 8.11 (s, 1H, NCH), 8.15 (s, 1H, NH2), 8.30 (s, 1H,

18、NH2),8.27 (d, 1H, PyH), 8.58 (d, 1H, PyH), 8.94 (s, 1H, PyH), 11.59 (s,1H, NNHCS); IR (KBr) t: 3414, 3249, 3148, 1593, 1543, 1100,875, 627, 425 cm 1. The molecular formula of 3-PCT is shown in Fig. 1.The aggressive solutions of 1 M HCl were prepared by dilution of an analytical grade 37% HCl with do

19、uble distilled water. The con- centration range of the Schiff base inhibitor employed was1.5 10 4 to 1.5 10 3 mol/L, and the solution in the absence of inhibitor was prepared for comparison. Mild steel samples (0.17% C, 0.37% Mn, 0.20% Si, 0.03% S, 0.01% P and the remainder Fe) were mechanically cut

20、 into 5.00 cm 2.50 cm 0.20 cm dimensions for weight loss experiments. For electrochemical experiments, the steel specimens were embedded in epoxy resin with an exposed area of 0.5 cm2 to the electrolyte. Prior to all measurements, the samples were mechanically in ethanol and acetone, nally dried in

21、room temperature.2.2. Weight loss experimentsThe weight loss measurements were carried out in a double walled glass cell equipped with a thermostat-cooling condenser. The solution volume was 500 mL. The mild steel specimens, in trip- licate, were immersed in the acid solutions for 24 h both with and

22、 without the inhibitor at (303 1) K. After the corrosion tests, the specimens were removed, carefully washed in bidistilled water in desiccators and then weighed. The rinse removed loose segments of the lm on the corroded samples. The loss in weight was deter- mined by analytical balance and the mea

23、n value of the weight loss is reported.2.3. Electrochemical experimentsThe electrochemical measurements were performed with ZAH- NER IM6ex electrochemical workstation, which was controlled by ZAHNER THALES software. A conventional three-electrodes cell system with a platinum counter electrode and a

24、saturated calomel electrode (SCE) coupled to a ne Luggin capillary as the reference electrode was employed. The mild steel cylinder, with surface pre- pared as described in the weight loss experimental method, served as the working electrode (WE). For potentiodynamic polarization measurements, the p

25、otential was swept at the rate of 1 mV s 1, primarily from 700 to 300 mV (vs. SCE). The electrochemical impedance spectroscopy (EIS) measurements were performed over the frequency range 0.01 Hz100 kHz at the open circuit potential by superimposing a sinusoidal AC signal of small amplitude, 5 mV, aft

26、er immersion for 2 h in the corrosive media. All electrochemical tests have been performed in nondeaerated solutions under un- stirred conditions, and the cell temperature was controlled by using a thermostatic water bath. Each electrochemical measure- ment has been repeated three times under the sa

27、me conditions, meanwhile the mean values and standard deviations of some elec- trochemical parameters are reported.The electrochemical impedance spectra were recorded at the AC amplitude of 5 mV and by applying different potentials. The values of double layer capacitance were plotted against the pot

28、entials to determine the PZC after mild steel immersed in 1 M HCl solutions containing 1.5 10 3 mol/L 3-PCT.2.5. Scanning electron microscopy (SEM) studiesThe surface morphology of specimens after exposure to 1 M HCl in the absence and presence of 1.5 10 3 mol/L 3-PCT for 24 h were examined by SEM.

29、The SEM images were conducted using a Quanta200 scanning electronic microscope at high vacuum and20.0 kV EHT.2.6. Molecular dynamics simulationsThe molecular dynamics (MD) simulations were performed using the software, Material Studio 5.5, Discover module. Fe (110) surface was chosen for the simulat

30、ion study. The MD calcu- lation of the simulation of the interaction between molecule 3-PCT and the Fe (110) surface was carried out in a simulation box (19.86 19.86 38.10 ) with periodic boundary conditions to model a representative part of the interface devoid of any arbi- trary boundary effects.

31、The Fe crystal was cleaved along the (110) plane with the uppermost and the lowest layers released and the inner layer xed. The addition of the 3-PCT molecule near to the surface was carried out and using the PCFF force eld to simulate the behavior of the 3-PCT molecule on the Fe (110) surface. The

32、MD simulation was performed under 298 K, NVT ensemble, with a time step of 0.1 fs and simulation time of 50 ps. The interaction energy Einteraction of the Fe surface with the 3-PCT molecule was cal- culated according to the following equation:Einteraction Etotal Esurface E3-PCT1where Etotal is the t

33、otal energy of Fe crystal together with the ad- sorbed 3-PCT molecule, Esurface and E3-PCT are the energy of the Fe crystal and free 3-PCT molecule, respectively. And the binding en- ergy is the negative value of the interaction energy, Ebinding = Einteraction.3. Results and discussions3.1. Weight l

34、oss measurementsThe inhibition efciency values obtained from weight loss mea- surements for different concentrations of 3-PCT in 1 M HCl solu- tions are presented in Table 1. The inhibition efciency (g) is determined by the following relation:W0Wg W0100%2Table 1Weight loss results of mild steel in 1

35、 M HCl without and with different concentrations of 3-PCT at 30 C.Conc. (mol/L)v (mg cm 2 h 1)g (%)Blank5.541.5 10 41.61712.5 10 40.51915 10 40.21961.0 10 30.21961.5 10 30.1497208B. Xu et al. / Corrosion Science 74 (2013) 206213where W0 and W represent the corrosion rates of the mild steel in the ab

36、sence and presence of the inhibitors, respectively.with the increase in inhibitor concentrations in 1 M HCl and theConc.Ecorr (mVIcorrbcbaginhibition efciency reaches its maximum value at 1.5 mmol/L.(mmol/L)vs.SCE)(mA cm 2)(mV dec 1)(mV dec 1)(%)This behavior can be explained based on the strong int

37、eraction ofBlank461.9 6.41.13 0.09133.7 5.363.3 2.7the inhibitor molecules with the metal surface resulting in adsorp-0.15498.7 1.10.29 0.01116.3 3.359.5 6.874It is apparent that the inhibition efciency increases noticeablyTable 2Potentiodynamic polarization parameters for mild steel in 1 M HCl with

38、 and without different concentrations of 3-PCT at 30 C.tion 22. The results indicate that 3-PCT is a good inhibitor for mild steel in hydrochloride acid. It should be noted that when the inhibitor concentration reaches 0.5 mmol/L, the inhibition ef- ciency does not increase obviously.3.2. Potentiody

39、namic polarization measurementsFig. 2 shows the polarization curves for mild steel in 1 M HCl in the absence and presence of inhibitor 3-PCT. It can be noticed that the addition of 3-PCT causes a remarkable decrease in the corro- sion rate, shifting both anodic and cathodic Tafel curves to lower cur

40、rent densities. This phenomenon indicates that both anodic and cathodic reactions are suppressed and the suppression effect becomes more pronounced with the increase of the concentration of 3-PCT. The electrochemical parameters such as corrosion poten- tial (Ecorr), cathodic and anodic Tafel slope (

41、bc and ba), and corro- sion current density (Icorr) obtained by extrapolating the Tafel line are given in Table 2. The corrosion inhibition efciency (g) of 3-PCT is calculated using the relationship:I0 Icorr0.25506.4 2.00.20 0.016 122.3 4.3 54.2 1.8820.5506.6 2.00.17 0.002 124.0 4.0 61.8 5.6851.0501

42、.5 6.50.14 0.01128.5 3.5 67.7 1.087 1.5507.3 5.30.13 0.007 133.7 4.3 71.3 1.788steel surface 24. For anodic polarization curves of mild steel with 3-PCT, it is apparent that the presence of the 3-PCT does not mod- ify the current vs. potential characteristics obviously. The data in Table 2 reveals t

43、hat when the concentration of 3-PCT increased, the inhibition efciency increases and the corrosion current den- sity decreases sharply. This may be due to the adsorption layer of the inhibitor on the metal surface.3.3. Electrochemical impedance spectroscopy (EIS)The Nyquist plots of mild steel in 1

44、M HCl in the presence and absence of 3-PCT are shown in Fig. 3. It can be observed that all the impedance spectra obtained show a single depressed capaci- tive loop which is related to charge transfer of the corrosion pro-g corr 0Icorrcorrwhere I0100%3and Icorr represent the current density values o

45、f the mildcess and the diameters of the capacitive loops increase sharply with increasing 3-PCT concentration. It is noticed that the imped- ance loops do not yield perfect semicircle as studied from the EIStheory and the centers of the loops are under the real axis. Suchsteel in the absence and in

46、the presence of 3-PCT, respectively.We can classify an inhibitor as cathodic or anodic type if the dis-placement in corrosion potential is more than 85 mV with respect to corrosion potential of the blank 23. In the presence of 3-PCT, the corrosion potential of mild steel shifted to the negative side

47、 only 45 mV (vs. SCE). This can be interpreted that 3-PCT acts as a mixed type inhibitor and shows more pronounced inuence in the cathodic polarization plots compared to that in the anodic plots. The parallel cathodic currentpotential curves suggest that the addition of the 3-PCT does not modify the

48、 hydrogen evolution mechanism and the hydrogen evolution is activation-controlled. This suppression of the corrosion process may be attributed to the covering of adsorbed 3-PCT inhibitor molecules on the mildphenomenon is known as the dispersing effect, related to the roughness and in-homogeneity of

49、 the solid surfaces and adsorption of inhibitors 25. The related equivalent circuit model used to model the EIS results is shown in Fig. 4 containing electrolyte resis- tance (Rs), polarization resistance (Rct) including charge transfer resistance, diffuse layer resistance, lm resistance, etc. 26 an

50、d constant phase element (CPE). The impedance of CPE is repre- sented by the expression:ZCPE Y 1jx n4where Y is a proportional factor; x is the angular frequency; n, a deviation parameter, shows the phase shift which can be explainedFig. 2. Potentiodynamic polarization curves for mild steel in 1 M H

51、Cl in the absence and presence of different concentrations of 3-PCT at 30 C.Fig. 3. Nyquist diagrams for mild steel in 1 M HCl in the absence and presence of different concentrations of 3-PCT at 30 C.B. Xu et al. / Corrosion Science 74 (2013) 206213209Fig. 4. Equivalent circuit model used to t the E

52、IS experiment data.as the degree of surface in-homogeneity 27. For n = 0, the CPE means a pure resistor, for n = 1, CPE is an inductor and for n = +1, CPE represents a pure capacitor 28. The values of the dou- ble layer capacitance (Q) and inhibition efciency (g) are calculated as follow:As seen fro

53、m Table 4 and Fig. 6, it is apparent that the increase of corrosion current density is pronounced with the rise of temper- ature in both uninhibited and inhibited solutions. As depicted in Fig. 6, all the regression coefcients are almost close to1, indicating the good relationship between ln icorr a

54、nd 1/T. Inspection of Table 4 showed that the values of Ea determined in 1 M HCl containing 3- PCT is higher (65.0 kJ mol 1) than that for uninhibited solution (41.2 kJ mol 1). The higher Ea value in the inhibited solution can be correlated with the existence of physical adsorption phenome- non by f

55、orming an adsorptive lm of an electrostatic character on mild steel surface 33.3.5. Adsorption isothermsBasic information about the corrosion inhibitive action betweenmQ Yx00 n 15RR0the 3-PCT molecules and mild steel surface can be provided by the type of adsorption isotherm. The adsorption process

56、depends on the structure of the inhibitor molecules, the electrochemical poten-g ctctRct100%6tial, and the temperature during the experiments. At the metal/ solution interface, the adsorption process of organic inhibitorsmctwhere x00 is the angular frequency at the maximum value of the imaginary par

57、t of the impedance spectrum. Rct and R0 are the polar- ization resistance in the presence and absence of 3-PCT, respectively.Inspection of Table 3 reveals that the value of Rctincreases prominently with the increase in 3-PCT concentration, while the value of Q is brought down. This indicates an increase in the sur- face coverage by