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Electrochimica Acta
j o u r n a l h o m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / e l e c t a c t a
Adsorption and inhibition effect of 6-benzylaminopurine on
cold rolled steel in 1.0 M HCl
Xianghong Li
a,∗, Shuduan Deng
b, Hui Fu
a, Taohong Li
aaDepartment of Fundamental Courses, Southwest Forestry University, Bai Long Street, Kunming 650224, PR China bDepartment of Wood Science and Technology, Southwest Forestry University, Kunming 650224, PR China
a r t i c l e i n f o
Article history:
Received 14 November 2008
Received in revised form 9 February 2009 Accepted 25 February 2009
Available online 9 March 2009
Keywords:
Cold rolled steel 6-Benzylaminopurine Corrosion inhibitor Polarization AFM
a b s t r a c t
The adsorption and inhibition effect of 6-benzylaminopurine (BAP) on cold rolled steel (CRS) in 1.0 M HCl at 25–50◦C was studied by weight loss and potentiodynamic polarization methods. The results
showed that BAP was a good inhibitor in 1.0 M HCl, and the inhibition efficiency (IE) increased with the inhibitor concentration. The adsorption of BAP on the CRS surface obeyed the Langmuir adsorption isotherm equation. Both thermodynamic parameters (adsorption heatH0, adsorption free energyG0
and adsorption entropyS0) and kinetic parameters (apparent activation energy Eaand pre-exponential
factor A) were calculated and discussed. Polarization curves showed that BAP acted as a mixed-type inhibitor in hydrochloric acid. Good agreement between weight loss and polarization methods was obtained. The adsorbed film on CRS surface containing optimum dose of BAP was investigated by Fourier transform infrared spectroscopy (FTIR) and atomic force microscope (AFM). Quantum chemical calcula-tion was applied to elucidate the adsorpcalcula-tion mode of the inhibitor molecule onto steel surface. Depending on the results, the inhibitive mechanism was proposed from the viewpoint of adsorption theory.
© 2009 Elsevier Ltd. All rights reserved.
1. Introduction
Corrosion is a fundamental process playing an important role in economics and safety, particularly for metals and alloys. Steel has found wide application in a broad spectrum of industries and machinery; however its tendency to corrosion. The corrosion of steel is a fundamental academic and industrial concern that has received a considerable amount of attention[1]. Using inhibitors is an important method of protecting materials against deterioration due to corrosion, especially in acidic media[2]. Acid solutions are widely used in industry, some of the important fields of application being acid pickling of iron and steel, chemical cleaning and pro-cessing, ore production and oil well acidification. As acidic media, the use of hydrochloric acid in pickling of metals, acidization of oil wells and in cleaning of scales is more economical, efficient and trouble-free, compared to other mineral acids[3]. Because of the general aggression of acid solutions, inhibitors are commonly used to reduce the corrosive attack on metallic materials.
The review including extensive listing of various types of organic inhibitors has been published[4]. Most well-known acid inhibitors are organic compounds containing nitrogen, sulfur, and oxygen atoms. Among them, nitrogen-containing heterocyclic compounds
∗ Corresponding author. Tel.: +86 871 3861218; fax: +86 871 3863150.
E-mail address:xianghong-li@163.com(X. Li).
are considered to be effective corrosion inhibitors on steel in acid media[5]. N-heterocyclic compound inhibitors act by adsorption on the metal surface, and the adsorption of N-heterocyclic inhibitor takes place through nitrogen heteroatom, as well as those with triple or conjugated double bonds or aromatic rings in their molec-ular structures.
Up to now, many N-heterocyclic compounds, such as imida-zoline derivatives[6], 1,2,3-trizaole derivatives[7], 1,2,4-trizaole derivatives[5,8–12], tetrazole derivatives[13,14], pyrrole[15], pyri-dine derivatives[16–18], pyrazole derivatives[19–22], bipyrazole derivatives[23,24], pyrimidine derivatives[25], pyridazine deriva-tives [26], indole derivatives[27–29], benzimidazole derivatives [30–34], and quinoline derivatives[35]have been used for the cor-rosion inhibition of iron or steel in acidic media. Though the existing data show that numerous N-heterocyclic organic compounds have good anticorrosive activity, some of them are highly toxic to both human beings and environment. The safety and environmental issues of corrosion inhibitors arisen in industries have always been a global concern. These toxic effects have led to the use of eco-friendly and harmless N-heterocyclic compounds as inhibitors.
As an important N-heterocyclic compound, purine (PU) and purine derivatives are nontoxic and biodegradable; this makes the investigation of their inhibiting properties significant in the context of the current priority to produce eco-friendly inhibitors. Recently, Scendo[36–38]has studied the inhibition of purine and adenine (AD) on the corrosion of copper in neutral 1.0 M NaCl, 0.5 M Na2SO4 and 0.5 M NaNO3solutions. Yan et al.[39]also studied the corrosion 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
inhibition of five purine derivatives (guanine, adenine, 2,6-diaminopurine, 6-thioguanine and 2,6-dithiopurine) on mild steel in 1.0 M HCl, and the results showed that the inhibition efficiency (IE) of these purines follows the order: 2,dithiopurine > 6-thioguanine > 2,6-diaminopurine > adenine > guanine, and the maximum IEs (using potentiodynamic polarization method) are 85.9%, 77.9%, 73.5%, 69.6% and 68.3% when the inhibitor concentra-tion is 1.0× 10−3M, respectively. However, data regarding the use of purine derivatives as corrosion inhibitors are still poor. In addi-tion, as another purine derivative, 6-benzylaminopurine (BAP) has not yet been investigated as an inhibitor for steel in acid solution.
For these reasons, in order to extend N-heterocyclic inhibitors of purine derivatives, the objective of present work is to investigate the corrosion inhibition by BAP in 1.0 M HCl. Weight loss and polariza-tion methods were employed to evaluate corrosion rate of steel and inhibition efficiency of inhibitors. The steel surface was examined by Fourier transform infrared spectroscopy (FTIR) and atomic force microscope (AFM). Meanwhile, quantum chemical calculation was applied to elucidate the adsorption mode of the inhibitor molecule onto steel surface. It is expected to get general information on the adsorption and inhibition effect of purine derivative on steel in acid solution.
2. Experimental
2.1. Materials
Tests were performed on cold rolled steel (CRS) of the following composition (wt.%): 0.07% C, 0.3% Mn, 0.022% P, 0.010% S, 0.01% Si, 0.030% Al, and bal. Fe.
2.2. Inhibitor
The purine derivative of 6-benzylaminopurine was obtained from Shanghai Chemical Reagent Company of China.Fig. 1shows the molecular structure of BAP. Clearly, BAP is a N-heterocyclic compound containing five nitrogen atoms, which could be easily protonated in acid solution, and a great deal of-electrons exist in this molecule.
2.3. Solutions
The aggressive solutions, 1.0 M HCl were prepared by dilution of analytical grade 37% HCl with distilled water. Accurately weighed 1.0000 g BAP, and was dissolved with 20 ml 0.01 M NaOH, and then diluted to 1000 ml volumetric flask with distilled water as stock solution (1000 mg l−1). The stock solution was diluted to a certain concentration of BAP. The concentration range of BAP used was 5–100 mg l−1.
Fig. 1. Chemical molecular structure of 6-benzylaminopurine (BAP).
2.4. Weight loss measurements
The cold rolled steel sheets of 2.5 cm× 2.0 cm × 0.06 cm were abraded with a series of emery paper (grade 320-500-800) and then washed with distilled water and acetone. After weighing accurately, the specimens were immersed in 250 ml beaker, which contained 250 ml 1.0 M HCl without and with addition of different concentra-tions of BAP. All the aggressive acid soluconcentra-tions were open to air. After 6 h, the specimens were taken out, washed, dried, and weighed accurately. In order to get good reproducibility, experiments were carried out in triplicate, and the average weight loss of three parallel CRS sheets was reported. Then the tests were repeated at differ-ent temperatures. The corrosion rate (
v
) was calculated from the following equation[40,41]:v
= WSt (1)where W is the average weight loss of three parallel CRS sheets, S the total area of one CRS sheet, and t is immersion time (6 h). With the calculated corrosion rate, the inhibition efficiency (IE) of BAP on the corrosion of CRS was calculated as follows[40,41]:
IE%=
v
0−v
v
0100 (2)
where
v
0andv
are the values of the corrosion rate without and with addition of the inhibitor, respectively.2.5. Polarization measurements
Polarization experiments were carried out in a conventional three-electrode cell with a platinum counter electrode (CE) and a saturated calomel electrode (SCE) coupled to a fine Luggin capillary as the reference electrode. To minimize the ohmic contribution, the Luggin capillary was kept close to the working electrode. The work-ing electrode (WE) was in the form of a square CRS embedded in PVC holder using epoxy resin so that the flat surface was the only sur-face in the electrode. The working sursur-face area was 1.0 cm× 1.0 cm, polished with emery paper (grade 320-500-800) on the test face, rinsed with distilled water, degreased with acetone, and dried with a cold air stream.
Before measurement, the electrode was immersed in test solu-tion at open circuit potential for 2 h until a steady state was reached. All polarization curves were recorded by a PARSTAT 2263 Potentio-stat/Galvanostat (Princeton Applied Research, USA) at 25◦C. The potential increased with a speed of 30 mV min−1and started from potential of−250 mV to +250 mV vs. corrosion potential (Ecorr). Each experiment was repeated at least three times to check the reproducibility. IE% was defined as
IE%= Icorr− Icorr(inh)Icorr 100 (3)
where Icorr and Icorr(inh) represent the corrosion current density values without and with inhibitor, respectively.
2.6. Fourier transform infrared spectroscopy
FTIR spectra were recorded in an AVATAR-FTIR-360 spectropho-tometer (Thermo Nicolet Company, USA), which extended from 4000 to 400 cm−1, using the KBr disk technique. The pure BAP was mixed with KBr and made the disk. The CRS specimen of size 2.5 cm× 2.0 cm × 0.06 cm was prepared as described above (Sec-tion2.4). After immersion in 1.0 M HCl with addition of 100 mg l−1 BAP at 25◦C for 6 h, the specimen was cleaned with distilled water, dried with a cold air blaster. Then the thin adsorption layer formed on steel surface was rubbed with a small amount of KBr powder in
Table 1
Corrosion rate values for the corrosion of cold rolled steel in 1.0 M HCl containing different concentrations of BAP at different temperatures.
c (mg l−1) Corrosion ratev(g m−2h−1) 20◦C 30◦C 40◦C 50◦C 0 10.05 16.83 32.62 64.15 5 1.65 3.02 9.00 26.78 10 1.18 2.32 4.93 15.20 20 1.02 1.47 3.10 9.42 30 0.80 1.27 2.57 7.12 40 0.68 1.20 2.22 5.80 50 0.65 1.07 1.85 5.67 70 0.62 0.92 1.65 4.40 100 0.52 0.78 1.38 3.02
an agate mortar in a dry box and a KBr disk was prepared using this powder.
2.7. Atomic force microscope
The CRS specimens of size 1.5 cm× 1.0 cm × 0.06 cm were pre-pared as described above (Section2.4). After immersion in 1.0 M HCl without and with addition of 100 mg l−1BAP at 25◦C for 6 h, the specimen was cleaned with distilled water, dried with a cold air blaster, and then used for a Japan instrument model SPA-400 SPM Unit atomic force microscope examinations.
2.8. Quantum chemical calculations
Quantum chemical calculations were carried out using density function theory (DFT) method B3LYP with all electron basis set 6-31G* on all atoms[42]. This theoretical level is denoted as B3LYP/6-31G*. All calculations were performed with Gaussian98 program package[43].
3. Results and discussion
3.1. Weight loss measurements 3.1.1. Effect of BAP on the corrosion rate
The corrosion rate values of CRS with the addition of BAP in 1.0 M HCl at various temperatures are listed inTable 1. The corro-sion rate values (g m−2h−1) of CRS in 1.0 M HCl solution containing BAP decrease as the concentration of the inhibitor increases, i.e. the corrosion inhibition enhances with the inhibitor concentra-tion. This behavior is the result of the fact the adsorption amount and the coverage of inhibitor on CRS surface increases with the inhibitor concentration [44]. It should be noted that when the inhibitor concentration reaches approximately 30 mg l−1, the cor-rosion rate values reach certain values and do not change obviously. Also, the values inTable 1show that the corrosion rate increases with increasing temperature both in uninhibited and inhibited solutions. The corrosion rate increases more rapidly with tem-perature in the absence of the inhibitor. These results confirm that BAP acts as an efficient inhibitor in the range of temperature studied.
3.1.2. Effect of BAP concentration and temperature on inhibition efficiency
The values of inhibition efficiencies obtained from the weight loss for different inhibitor concentrations in 1.0 M HCl are shown inFig. 2. The results show that IE increases as the concentration of inhibitor increases from 5 to 100 mg l−1. The maximum IE was about 95.8% at 100 mg l−1(about 4.4× 10−4M), and the inhibition was estimated to be 83% at 25◦C even at very low concentration (5 mg l−1), and at 30 mg l−1concentration its protection was >90%
Fig. 2. Relationship between inhibition efficiency and concentration of BAP in 1.0 M
HCl. () 25◦C; (䊉) 30◦C; () 40◦C; () 50◦C.
(25–40◦C), which indicated that BAP was a good inhibitor in 1.0 M HCl.
Fig. 2 also shows that IE decreases with the experimental temperature at low inhibitor concentration (5–20 mg l−1), which indicates that the higher temperatures might cause desorption of BAP from the steel surface. It is of interest to note that when the inhibitor concentration is 20–80 mg l−1, IE values are almost same at three temperatures (25, 30, and 40◦C). When the inhibitor concentration is 80–100 mg l−1, IE values are almost same at four temperatures.
3.1.3. Adsorption isotherm
Basic information on the interaction between the inhibitor and the steel surface can be provided by the adsorption isotherm. Attempts were made to fit to various isotherms including Frumkin, Langmuir, Temkin, Freundlich, Bockris-Swinkels and Flory-Huggins isotherms. By far the results were best fitted by Langmuir adsorp-tion isotherm equaadsorp-tion[40]:
c =
1
K + c (4)
where c is the concentration of inhibitor, K the adsorptive equilib-rium constant and is the surface coverage. was calculated by the Sekine and Hirakawa’s method[45]:
=
v
0−v
v
0−v
m (5)where
v
mis the smallest corrosion rate.From the values of surface coverage, the linear regressions between c/ and c were calculated by the computer, and the parameters were listed in Table 2. Fig. 3 is the relationship between c/ and c at 25◦C. These results show that all the lin-ear correlation coefficients (r) are almost equal to 1 and all the slopes are very close to 1, which indicates the adsorption
Table 2
Parameters of the linear regression between c/ and c. Temperature (◦C) Linear correlation
coefficient (r) Slope Intercept K (l mg−1) 25 1.0000 0.9937 0.9699 1.0311 30 1.0000 0.9906 0.9906 1.0095 40 1.0000 0.9849 1.5763 0.6344 50 0.9999 0.9732 3.0743 0.3253
Fig. 3. The relationship between c/ and c at 25◦C in 1.0 M HCl.
of inhibitor onto steel surface obeys the Langmuir adsorption isotherm.
Table 2also shows that the adsorptive equilibrium constant (K) value (in l mg−1) decreases with increasing temperature, which indicates that, it is easily and strongly adsorbed onto the steel sur-face for the inhibitor at relatively lower temperature. But when the temperature was relatively higher, the adsorbed inhibitors tended to desorb from the steel surface.
3.1.4. Thermodynamic parameters of BAP on the CRS surface
Thermodynamic parameters are important to study the inhibitive mechanism. The adsorption heat (H) could be calcu-lated according to the Van’t Hoff equation[40]:
lnK = −HRT + constant (6)
where R is the gas constant (8.314 J K−1mol−1), T the absolute tem-perature (K), respectively.
Before calculating the parameters, the adsorptive equilibrium constant K unit l mg−1should be changed into l mol−1(or M−1) in order to agree with the basic unit of SI. The molecular weight of BAP is 225.25 g mol−1, the changed K in unit M−1is used to calculate the thermodynamic parameters.
It should be noted that−H/R is the slope of the straight line ln K vs. 1/T according to Eq. (6). To obtain the adsorption heat, the linear regression between ln K and 1/T was dealt with.Fig. 4 is the straight line of ln K vs. 1/T. The adsorption heat (H) could be approximately regarded as the standard adsorption heat (H0) under experimental conditions[40,41,44]. The standard adsorption
Fig. 4. The relationship between ln K and 1/T.
Table 3
The thermodynamic parameters of adsorption of BAP on the CRS surface. Temperature (◦C) G0(kJ mol−1) H0(kJ mol−1) S0(J mol−1K−1)
25 −40.58 −37.95 8.82
30 −41.21 −37.95 10.75
40 −41.36 −37.95 10.89
50 −40.89 −37.95 9.10
free energy (G0) was obtained according to[12,46]:
K = 551.5exp
−G0 RT (7) where the value 55.5 is the concentration of water in solution expressed in M (mol l−1)[12,46]. The unit ofG0 is J mol−1 (or kJ mol−1). Obviously, the adsorptive equilibrium constant K unit is l mol−1(M−1) in Eq.(7) [12,40,41]. Thus, the adsorptive equilibrium constant K unit l mg−1 (Table 2) should be changed into l mol−1 (M−1)[47,48]. Then the standard adsorption entropy (S0) can be obtained by the thermodynamic basic equation:S0=H0− G0
T (8)
All the calculated thermodynamic parameters are listed in Table 3. The negative sign ofH0 shows that the adsorption of inhibitor is an exothermic process[47], which indicates that IEs decrease with the temperature. Such behavior can be interpreted on the basis that increasing temperature resulted in desorption of the some adsorbed inhibitor molecules from the metal surface. The negative values ofG0suggest that the adsorption of inhibitor molecule onto steel surface is a spontaneous process. Generally, val-ues ofG0 up to−20 kJ/mol are consistent with the electrostatic interaction between the charged molecules and the charged metal (physical adsorption) while those more negative than−40 kJ/mol involve sharing or transfer of electrons from the inhibitor molecules to the metal surface to form a coordinate type of bond (chemisorp-tion)[12,49,50].Table 3shows that the value ofG0 is slightly negative than −40 kJ mol−1; probably mean that the adsorption mechanism of the BAP on steel in 1.0 M HCl solution involves chemisorption. The unshared electron pairs in nitrogen atoms may interact with d-orbitals of Fe to provide a protective chemisorbed film[50].
As for the value ofS0 inTable 3, the sign ofS0 is positive. This is opposite to what would be expected, since adsorption is an exothermic process and always accompanied by a decrease of entropy. The reason could be explained as follows: the adsorption of organic inhibitor molecules from the aqueous solution can be regarded as a quasi-substitution process between the organic com-pound in the aqueous phase [Org(sol)] and water molecules at the electrode surface [H2O(ads)][51].
Org(sol)+ xH2O(ads)↔ Org(ads)+ xH2O(sol) (9)
where x is the size ratio, that is, the number of water molecules replaced by one organic inhibitor. In this situation, the adsorption of BAP is accompanied by desorption of water molecules from the sur-face. Thus, while the adsorption process for the inhibitor is believed to be exothermic and associated with a decrease in entropy of the solute, the opposite is true for the solvent. The thermodynamic val-ues obtained are the algebraic sum of the adsorption of organic molecules and the desorption of water molecules[52]. Therefore, the gain in entropy is attributed to the increase in solvent entropy and to more positive water desorption enthalpy[52,53]. The posi-tive values ofS0also mean that an increasing in disordering takes place in going form reactants to the metal/solution interface[54], which is the driving force for the adsorption of inhibitor onto the steel surface[41].
Table 4
Parameters of the regression between lnvand 1/T.
c (mg l−1) Ea(kJ mol−1) A (g m−2h−1) Linear regression coefficient (r)
0 58.04 1.57× 1011 0.9977 5 88.92 6.25× 1015 0.9999 10 78.76 7.73× 1013 0.9952 20 70.45 2.07× 1012 0.9920 30 68.47 7.72× 1011 0.9955 40 67.37 4.25× 1011 0.9972 50 65.57 2.00× 1011 0.9817 70 57.60 8.17× 109 0.9839 100 55.06 2.30× 109 0.9969
3.1.5. Apparent activation energy (Ea) and pre-exponential factor
(A)
It has been reported by a number of authors that for the acid corrosion of steel, the natural logarithm of the corrosion rate
v
(in g m−2h−1) is a linear function with 1/T (following Arrhenius equation)[40,41,47,55,56]:ln
v
=−EaRT + ln A (10)
where Eaand A represent the apparent activation energy and the
pre-exponential factor, respectively.
The regression between ln
v
and 1/T was calculated by computer, and the parameters were calculated and given inTable 4. Arrhenius plots of lnv
vs. 1/T for the blank and different concentrations of BAP were shown inFig. 5.Table 4shows that the apparent activa-tion energy, at relative lower concentraactiva-tion (0–5 mg l−1), increases with the concentration of BAP, while at a range from 5 to 100 mg l−1, decreases with increasing concentration of BAP, and when the BAP concentration is 70–100 mg l−1, the apparent activation energy value is lower than that of blank. That is to say, in the present system there was a “peak-like” value for Ea. The similar result isalso reported for N heterocyclic inhibitor in HCl[57]. The increase in the apparent activation energy Ea at low inhibitor
concentra-tions may be interpreted as physical adsorption that occurs in the first stage[58–60]. A drop in Ea, with respect to the uninhibited
solution, observed at the higher inhibitor concentration indicates chemisorption[50,58,61,62]. Such behavior may be considered in line with the suggestion that physisorption at lower concentration and chemisorption at higher concentration for a given inhibitor [63].
According to Eq.(10), both Eaand A affect the corrosion rate
(
v
). Generally speaking, the influence of Eaon the steel corrosionFig. 5. Arrhenius plots related to the corrosion rate of CRS for various
concentra-tions of BAP in 1.0 M HCl. () Blank; () 5 mg l−1; (䊉) 10 mg l−1; () 20 mg l−1; () 30 mg l−1; () 50 mg l−1; () 100 mg l−1.
Fig. 6. Polarization curves for CRS in 1.0 M HCl containing different concentrations
of BAP at 25◦C. () Blank; (䊉) 10 mg l−1; () 50 mg l−1; () 100 mg l−1.
was bigger than that of A on the steel corrosion. However, if the variance of A was drastically bigger than that of Ea, the value of A might be the dominant factor to determine the steel corrosion.
For the present study, Ea decreases with the BAP concentration
in the range of 70–100 mg l−1 (the lower Ealeads to higher
cor-rosion rate). But because the pre-exponential factor significantly decreased with increasing the inhibitor concentration (the decrease in A reduced the corrosion rate of steel). As a result, the corrosion rate of steel decreased with increasing the inhibitor concentration. So, it was clear that in this case the reduction of pre-exponential fac-tor (Table 4) was a decisive factor affect the corrosion rate of steel in 1.0 M hydrochloric acid. Influenced by the cumulative effect of the magnitudes of Eaand A, the corrosion rate decreases with the
inhibitor concentration.
3.2. Polarization studies
Polarization curves for CRS in 1.0 M HCl at various concentra-tions of BAP are shown in Fig. 6. It shows that the presence of inhibitor causes a prominent decrease in the corrosion rate, i.e. shifts the anodic curves to more positive potentials and the cathodic curves to more negative potentials, and to lower values of current densities. Namely, both cathodic and anodic reactions of CRS elec-trode corrosion are inhibited by BAP in 1.0 M HCl. This may be ascribed to adsorption of inhibitor over the corroded surface[64]. The values of corrosion current densities (Icorr), corrosion poten-tial (Ecorr), the cathodic Tafel slope (bc), anodic Tafel slope (ba), and the inhibition efficiency (IE) as functions of BAP concentra-tion, were calculated from the curves ofFig. 6and given inTable 5. It reveals that the corrosion current (Icorr) decreases prominently and IE increases with the inhibitor concentration. The presence of BAP does not remarkably shift the corrosion potential (Ecorr), there-fore, BAP can be arranged as mixed-type inhibitor in 1.0 M HCl, and the inhibition of BAP on CRS is caused by geometric blocking effect[65]. Namely, the inhibition effect comes from the reduction of the reaction area on the surface of the corroding metal[65]. Both the anodic and cathodic Tafel slopes slightly change upon addition of BAP (Table 5), which means that the BAP molecules are adsorbed on both the anodic and cathodic sites resulting in an inhibition of both anodic dissolution and cathodic reduction reactions.
As is evident fromFig. 6, the cathodic polarization curves indi-cate that the hydrogen evolution reaction is activation controlled
Table 5
Potentiodynamic polarization parameters for the corrosion of CRS in 1.0 M HCl containing different concentrations of BAP at 25◦C.
c (mg l−1) Ecorr(mV vs. SCE) Icorr(A cm−2) −bc(mV dec−1) ba(mV dec−1) IE (%)
0 −451.3 200.6 112 78 –
10 −449.6 38.1 119 64 81.0
50 −453.8 30.9 126 67 84.6
100 −452.4 9.3 106 54 95.4
and the presence of the BAP does not affect the mechanism of hydrogen reaction.
It is important to note that in anodic domain, for potential higher than−300 mV vs. SCE, the presence of BAP did not change the current-vs.-potential characteristics (Fig. 6). This potential can be defined as the desorption potential. The same results have been reported with N-heterocyclic compound inhibitor in 1.0 M HCl [66]. This fact means that the inhibition mode of BAP depends on electrode potential. In this case, where the inhibition of corro-sion depends on the potential of electrode, the observed inhibition phenomenon is generally associated to the formation of a bidimen-tional layer of adsorbed species on the surface of the electrode[66]. The behavior of BAP at potentials greater than−300 mV vs. SCE could be associated to the significant dissolution of steel. This dis-solution results in desorption of the adsorbed film of BAP on the surface of the electrode in 1.0 M HCl media. In this case desorption rate of BAP is raised more than its adsorption. However, BAP influ-ences anodic reaction at potentials lower than−300 mV vs. SCE. This result shows clearly that the inhibition of the steel corrosion is under cathodic and anodic control.
FromTable 5it can be concluded that inhibition efficiencies obtained from weight loss and electrochemical polarization curves are in good agreement.
3.3. Fourier transform infrared spectroscopy
Several researchers[67–69]have confirmed that FTIR spectrom-eter is a powerful instrument that can be used to dspectrom-etermine the type of bonding for organic inhibitors absorbed on the metal surface. In this paper, FTIR spectrometer was used to identify whether there was adsorption and to provide new bonding information on the steel surface after immersion inhibited HCl solution.
The Fourier transform infrared spectroscopy of pure BAP is shown inFig. 7a. The weak band at 3447 cm−1and the bands at 3260 and 3207 cm−1are attributed to N–H vibrations. The band at 3067 cm−1is attributed to C–H stretching vibration in benzene. The bands at 2990 and 2819 cm−1 are attributed to the aliphatic C–H asymmetric and symmetric stretching vibrations, respectively. The
Fig. 7. FTIR spectra of (a) BAP and (b) adsorption layer formed on the CRS surface
after immersion in 1.0 M HCl + 100 mg l−1BAP for 6 h at 25◦C.
strong band at 1621 cm−1is attributed to C N stretching vibration. The band at 1452 cm−1is attributed to C–H bending vibration of the –CH2. The absorption bands at 1337, 1299 and 1254 cm−1is due to the framework vibration of N-heteroaromatic purine ring. Besides these, there are absorption bands at 1149 and 1077 cm−1, which can be assigned to the C–N stretching vibration. The bands at 892 and 704 cm−1, which can be assigned to the C–H bending vibration in benzene.
The FTIR spectrum of adsorbed protective layer formed CRS sur-face after immersion in 1.0 M HCl containing 100 mg l−1 BAP is shown inFig. 7b. The weak bands at 3861 and 3754 cm−1which do not appear inFig. 7a are assigned to Fe–O bending[68], and the bands at 771, 674 and 585 cm−1arise form FeOOH and Fe2O3 [70], which reveal the fact that the adsorbed protective film is oxi-dized by O2 and H2O in air. The band at 3447 cm−1 is attributed to O–H stretching, which further indicate the protective film con-tains H2O. The weak bands at 2926 and 2854 cm−1are attributed to the aliphatic C–H asymmetric and symmetric stretching vibrations, respectively. The disappearance of N–H stretching vibrations (from 3300 to 3000 cm−1) and negligible of the framework vibration of N-heteroaromatic purine ring are due to the fact BAP gets proto-nated in acidic solutions. ComparingFig. 7(a) with (b), it could be suggested that BAP can be absorbed on the CRS surface after being protonated. The similar result is also reported to that benzotriazole (BTA) as corrosion inhibitor for steel in HCl solution[68]. The C N stretching vibration at 1637 cm−1shifting to higher wavenumbers may be due to formation of the complex of Fe2+–BAP and adsorb on steel surface. The band at 1460 cm−1is attributed to –CH2, and the band at 1394 cm−1 is due to C–H. In addition, it should be noted that the strong band at 1090 cm−1 is the stretching vibra-tion in C–N. The bands at 881 and 473 cm−1are due to Fe–N–H and Fe–N stretching vibration, respectively[71]. The results may sug-gest the presence of a trace of the BAP complex with Fe2+on the surface.
3.4. Atomic force microscope surface examination
The atomic force microscope provides a powerful means of characterizing the microstructure[72]. The two-dimensional AFM images of CRS surface are shown in Fig. 8. As can be seen from Fig. 8(a), the CRS surface before immersion seems smooth and shows no obvious corrosion products on the surface. However, it is not absolute smooth and uniform, and appears small crevices and covered with grains, which may be attributed to the defect of steel, and probably an oxide inclusion or carbides[73]. As for Fig. 8(b), the CRS surface after immersion in uninhibited 1.0 M HCl for 6 h was damaged strongly comparingFig. 8(a), and covered with the corrosion products. On the other hand, in the presence of BAP inhibitor,Fig. 8(c) shows that the steel surface appears the more flat, homogeneous and uniform, which indicates that BAP inhibitor, shows an appreciable resistance to corrosion. In addition,Fig. 8(c) shows some compact spherical or bread-like particles distribute on the steel surface in the presence of 100 mg l−1BAP, which do not exist inFig. 8(a) and (b). The similar results are also reported in our recent studies for organic inhibitor in acid[41,47,73]. Therefore, it might be concluded that these particles are the adsorption film of the inhibitor, which efficiently inhibits the corrosion of CRS[47].
Fig. 8. AFM two-dimensional images of CRS surface: (a) before immersion; (b) after 6 h of immersion at 25◦C in 1.0 M HCl; (c) after 6 h of immersion at 25◦C in 100 mg l−1 BAP + 1.0 M HCl.
Fig. 9shows the CRS surface topography. It can be seen from Fig. 9(a) that the micrograph of CRS surface before immersion appears small granule particles adsorbed on stripe tube surface. Fig. 9(b) shows that the steel surface after immersion in uninhib-ited 1.0 M HCl shows the uneven and potholed corrosion products covers layer upon layer.Fig. 9(c) shows that some particles decorat-ing the steel surface orderly and the particles are relative smaller, and the inhibitor layer becomes more compact and even and covers almost the steel surface.
Fig. 10illustrates the height profiles, which are made along the lines marked in correspondingFig. 9. The surface roughness of the CRS before immersion is 10.70 nm fromFig. 10(a).Fig. 10(b) indicates that the surface roughness of the CRS after immersion in uninhibited 1.0 M HCl is up to 57.17 nm, while in the presence of BAP inhibitor, the roughness decreases to 34.18 nm (Fig. 10(c)). In general, the result of the surface roughness agrees with that of Figs. 8 and 9.
3.5. Quantum chemical calculations
In order to investigate the adsorption mode through light on the BAP molecular structure, quantum chemical calculations were carried out.Fig. 11shows a full geometry optimization of the BAP molecule. It shows that the purine ring and substitution –NH group are in one plane, while benzene ring in another plane. Mulliken charges of the atoms in BAP molecule are also shown inFig. 11.
It has been reported that inhibitor can form coordination bonds between the unshared electron pair of N atom and the unoccupied d electronic of Fe[1]. The larger negative charge of the N atom, the better is the action as an electronic donor. By careful examination of the values of Mulliken charges, the larger negative N atoms are found in N1 and N7 of purine ring and substitution –NH group, which connect to the donating electrons, which could donate its lone electron pair to the unfilled orbit of the metal atom, the BAP molecule can be adsorbed on the meal surface.
The dipole moment is 7.2274 Debye (D) (2.4108× 10−29C m). According to the quantum chemical calculation results of five purine derivatives[39], the dipole moment of guanine, adenine, 2,6-diaminopurine, 6-thioguanine and 2,6-dithiopurine are 6.796, 2.546, 1.194, 4.083, and 5.841 D (2.2669× 10−29, 8.4925× 10−30, 3.9828× 10−30, 1.3619× 10−29, and 1.9484× 10−29C m), respec-tively. So the dipole moment of BAP is relative larger. The large value of dipole moment probably increases the inhibitor adsorp-tion and increases inhibiadsorp-tion efficiency[74]. In the present study, BAP shows good inhibition performance, which might be attributed to the large dipole moment to some extent.
The electric/orbital density distribution of HOMO (the high-est occupied molecular orbital) and LUMO (the lowhigh-est unoccupied molecular orbital) for BAP was shown inFig. 12. It is found that that the electron density of both HOMO and LUMO are localized princi-pally on the purine ring and –NH substitution. It should be noted that HOMO density is absent on N9 atom of purine ring, whereas,
Fig. 9. AFM images of the cold rolled steel (CRS) surface topography: (a) before immersion; (b) after 6 h of immersion at 25◦C in 1.0 M HCl; (c) after 6 h of immersion at 25◦C in 100 mg l−1BAP + 1.0 M HCl.
Fig. 10. Height profiles of the CRS surface: (a) before immersion; (b) after 6 h of
immersion at 25◦C in 1.0 M HCl; (c) after 6 h of immersion at 25◦C in 100 mg l−1 BAP + 1.0 M HCl.
while LUMO density is absent on C2 and C5 atoms. It had reported that the EHOMOoften associated with the electron donating abil-ity of molecule. High values of EHOMO indicate a tendency of the molecule to donate electrons to act with acceptor molecules with low-energy, empty molecular orbital[39]. Similarly, the ELUMO rep-resents the ability of the molecule to accept electrons, and the lower value of ELUMOsuggests the molecule accepts electrons more prob-able[39]. The calculations of five purine derivatives[39]showed that the 2,6-dithiopuine with highest inhibition efficiency (85.9% at 1.0× 10−3M) has HOMO level at−8.639 eV, and lowest LUMO level at 2.517 eV. For BAP, the values of EHOMOand ELUMOare−6.0242
Fig. 12. The frontier molecular orbital density distribution of BAP: (a) HOMO; (b) LUMO.
and−0.6898 eV, respectively. Comparing with 2,6-dithiopuine, BAP has higher EHOMOvalue and lower ELUMO, which suggests the BAP could be both the acceptor of the electron and the donor of the electron. That is, there is electron transferring in the interaction between the inhibitor molecule and metal surface. This may explain the good inhibition efficiency of BAP molecule is due to increasing energy of the HOMO and the decreasing energy of the LUMO.
The separation energy (energy gap)E (ELUMO− EHOMO) is an important parameter as a function of reactivity of the inhibitor molecule towards the adsorption on metallic surface. AsE value decreases, the reactivity of the molecule increases leading to increase the inhibition of the molecule[74]. The results of five purines calculated by Yan et al. [39] indicate that theE value change regularly, the lowerE value, and the higher IE value. The 2,6-dithiopuine molecule has the lowest value of E, 11.155 eV among five purines[39]. For BAP, theE has the value of 5.3344 eV, which can facilitate its adsorption on the metal surface and accord-ing has higher inhibition efficiency.
In addition, the adsorption of BAP is trough the hyper plane con-jugating system constituted of purine ring and –NH substitution, i.e. the interactions between the inhibitor and the metal surface might be ascribed to the hyper conjugation interactions- stacking[39].
It can be observed fromFigs. 11 and 12that benzene ring and purine ring are not in the same plane. The results may deduce that the purine ring adsorbs on the CRS surface while the benzene ring may extends to the solution face to form a hydrophobic barrier.
3.6. Explanation for adsorption and inhibition
It has been assumed that organic inhibitor molecule establish its inhibition action via the adsorption of the inhibitor onto the metal surface. The adsorption process is affected by the chemical struc-tures of the inhibitors, the nature and charged surface of the metal and the distribution of charge over the whole inhibitor molecule. In general, owing to the complex nature of adsorption and inhibi-tion of a given inhibitor, it is impossible for single adsorpinhibi-tion mode between inhibitor and metal surface.
The adsorption and inhibition effect of BAP in HCl solution can be explained as follows: BAP might be protonated in the acid solution as following:
BAP+ xH+↔ [BAPHx]x+ (11)
Thus, in aqueous acidic solutions, the BAP exists either as neutral molecules or in the form of cations (protonated BAP). Generally, two modes of adsorption could be considered. According to the quan-tum chemical calculations results of Section3.6, the neutral BAP may be adsorbed on the metal surface via the chemisorption mech-anism, involving the displacement of water molecules from the metal surface and the sharing electrons between the N atoms and iron. The BAP molecules can be also adsorbed on the metal surface on the basis of donor–acceptor interactions between-electrons
of the heterocycle and vacant d-orbitals of iron. In another hand, it is well known that the steel surface charges positive charge in acid solution[75], so it is difficult for the protonated BAP to approach the positively charged steel surface (H3O+/metal interface) due to the electrostatic repulsion. Since chloride ions have a smaller degree of hydration, being specifically adsorbed, they create an excess neg-ative charge towards the solution and favor more adsorption of the cations[1], the protonated BAP may adsorb through electro-static interactions between the positively charged molecules and the negatively charged metal surface, i.e. there may be a synergism between Cl−and protonated BAP. The schematic illustration of syn-ergistic inhibition between chloride ion and protonated BAP for the corrosion of steel in HCl is shown in Fig. 13. It should be noted that the molecular structure of protonated BAP remains unchanged with respect to its neutral form, the N atoms on the ring remain-ing strongly blocked, so when protonated BAP adsorbed on metal surface, coordinate bond may be formed by partial transference of electrons from the polar atom (N atom) to the metal surface. In addition, owing to lone-pair electrons among in BAP molecule, BAP could also be seemed as a good ligand, and some metal complexes with BAP has been prepared[76]. Thus, BAP or protonated BAP may combine with freshly generated Fe2+ions on steel surface forming metal inhibitor complexes:
Fe→ Fe2++ 2e (12)
BAP+ Fe2+→ [BAP–Fe]2+ (13)
[BAPHx]x++ Fe2+→ [BAPHx–Fe](2+x)+ (14)
Fig. 13. The schematic illustration of synergistic inhibition between chloride ion
These complexes might adsorb onto steel surface by the van der Waals force to form a protective film to keep CRS from corrosion. This assumption could be further confirmed by the FTIR results. AFM results show that the inhibitor could adsorb onto steel surface to form a denser and more tightly protective film, which drastically decrease the steel surface roughness. The film covers both anodic and cathodic reactive sites on the steel surface, and inhibited the both reactions at the same time.
4. Conclusions
(1) BAP acts as a good inhibitor for the corrosion of CRS in 1.0 M HCl. IE values increase with the inhibitor concentration, and the maximum IE is about 95.8%.
(2) The adsorption of the BAP on the CRS surface obeys the Langmuir adsorption isotherm. The adsorption process is a spontaneous and exothermic process accompanied by an increase in entropy.
(3) The values of both apparent activation energy (Ea) and
pre-exponential factor (A) depend on BAP concentration. A “peak” value of apparent activation energy appears with an increase in concentration of BAP.
(4) BAP acts as a mixed-type inhibitor in 1.0 M HCl, and the inhibi-tion of BAP on CRS is caused by geometric blocking effect. The weight loss and polarization curves are in good agreement. (5) The introduction of BAP into 1.0 M HCl solution results in the
formation of an adsorptive film on the CRS surface, which causes the decrease the steel surface roughness and effectively protects steel from corrosion.
Acknowledgement
This work was carried out in the frame of research project funded by the Southwest Forestry University Foundation.
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