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Synergism between rare earth cerium(IV) ion and vanillin on the corrosion of steel in H2SO4 solution: Weight loss, electrochemical, UV–vis, FTIR, XPS, and AFM approaches
Applied Surface Science 254 (2008) 5574–5586
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Synergism between rare earth cerium(IV) ion and vanillin on the corrosion of steel in H2SO4 solution: Weight loss, electrochemical, UV–vis, FTIR, XPS, and AFM approaches Xianghong Li a,*, Shuduan Deng b, Hui Fu a, Guannan Mu c, Ning Zhao a a b c
Department of Fundamental Courses, Southwest Forestry University, Kunming 650224, PR China Department of Wood Science and Technology, Southwest Forestry University, Kunming 650224, PR China Department of Chemistry, Yunnan University, Kunming 650091, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 22 July 2007 Received in revised form 27 February 2008 Accepted 2 March 2008 Available online 13 March 2008
The synergism between rare earth cerium(IV) ion and vanillin (4-hydroxy-3-methoxy-benzaldehyde) on the corrosion of cold rolled steel (CRS) in 1.0 M H2SO4 solution at five temperatures ranging from 20 to 60 8C was first studied by weight loss and potentiodynamic polarization methods. The inhibited solutions were analyzed by ultraviolet and visible spectrophotometer (UV–vis). The adsorbed film of CRS surface containing optimum doses of the blends Ce4+–vanillin was investigated by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM). The results revealed that vanillin had a moderate inhibitive effect, and the inhibition efficiency (IE) increased with the vanillin concentration. The adsorption of vanillin obeyed Temkin adsorption isotherm. Polarization curves showed that vanillin was a mixed-type inhibitor in sulfuric acid, while prominently inhibited the cathodic reaction. For the cerium(IV) ion, it had a negligible effect, and the maximum IE was only about 20%. However, incorporation of Ce4+ with vanillin improved significantly the inhibition performance. The IE for Ce4+ in combination with vanillin was higher than the summation of IE for single Ce4+ and single vanillin, which was synergism in nature. A high inhibition efficiency, 98% was obtained by a mixture of 25–200 mg l1 vanillin and 300–475 mg l1 Ce4+. UV–vis showed that the new complex of Ce4+–vanillin was formed in 1.0 M H2SO4 for Ce4+ combination with vanillin. Polarization studies showed that the complex of Ce4+–vanillin acted as a mixed-type inhibitor, which drastically inhibits both anodic and cathodic reactions. FTIR and XPS revealed that a protective film formed in the presence of both vanillin and Ce4+ was composed of cerium oxide and the complex of Ce4+–vanillin. The synergism between Ce4+ and vanillin could also be evidenced by AFM images. Depending on the results, the synergism mechanism was discussed from the viewpoint of adsorption theory. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Cold rolled steel Sulfuric acid Vanillin Rare earth Cerium(IV) ion UV–vis FTIR XPS AFM Corrosion inhibitor Adsorption Synergism
1. Introduction Synergism (synergistic inhibition effect) is a combined action of compounds greater in total effect than the sum of the individual effect. Synergism of corrosion inhibitors is either due to the interaction between components of the inhibitor composition or due to interaction between the inhibitor and one of the ions present in the aqueous solution [1]. Synergism is an effective means to improve the inhibitive force of inhibitor, to decrease the amount of usage, to diversify the application of inhibitor in acidic media. It plays an important role not only in theoretical research on corrosion inhibitors but also in practical work [2]. In previous investigations, synergism between organic inhibitors and halide ions on metal * Corresponding author. E-mail address: [email protected] (X. Li). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.03.026
corrosion in acidic solution has been studied by many authors [2– 13]. The synergistic inhibition effects of organic inhibitor/metallic ion mixture [14–17], organic inhibitor/organic inhibitor mixture [18–22] and inorganic inhibitor/inorganic inhibitor mixture [23] on corrosion of metal in acidic media have also been studied. Forsyth et al. [24] have studied the synergistic corrosion inhibition of mild steel by rare earth (RE) Ce3+ and sodium salicylate in 0.1 M NaCl neutral media. Aramaki [25,26] has investigated the synergism of RE Ce3+/inorganic compound Na2Si2O5 mixture and Ce3+/organic compound C8H17S(CH2)2COONa (NaOTP) mixture on the corrosion of zinc in 0.5 M NaCl. However, up to now, there is little report in the literature about the synergism between RE ions and other compounds on metal corrosion in strong acidic media. In our laboratory, much work has been conducted to study the synergism between RE ions and other compounds on the corrosion of some metals and alloys in strong acidic media. In 1996, the
X. Li et al. / Applied Surface Science 254 (2008) 5574–5586
synergism between anionic surfactant (sodium dodecyl sulfonate, SDS) and RE La3+ for mild steel corrosion in 1.0 M HCl was reported [17]. Recently, the synergistic effect of inorganic compound Na2MoO4 and Ce4+ on cold rolled steel (CRS) corrosion inhibition in 1.0 M HCl was also investigated [27]. In the present work, the synergism between rare earth Ce4+ and organic compound vanillin on CRS in sulfuric acid is first studied by weight loss and potentiodynamic polarization methods. The inhibited solutions were analyzed by ultraviolet and visible spectrophotometer (UV– vis). The steel surface was also examined by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM). It is expected to get general information on the synergism between RE ion and organic inhibitor on the corrosion of steel in strong acid solution.
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calculated corrosion rate, the inhibition efficiency (IE) on the corrosion of CRS was calculated as follows [2]: IE% ¼
v0 v 100 v
(2)
where v0 and v are the values of the corrosion rate without and with addition of the inhibitor, respectively. IE of potentiodynamic polarization measurement was defined as: IE% ¼
Icorr IcorrðinhÞ 100 Icorr
(3)
where Icorr and Icorr(inh) are the uninhibited and inhibited corrosion current density values, respectively, determined by extrapolation of Tafel lines to the corrosion potential.
2. Experimental method 2.5. Ultraviolet and visible spectrophotometer (UV–vis) 2.1. Materials Tests were performed on a cold rolled steel of the following composition (given in wt.%): 0.07% C, 0.3% Mn, 0.022% P, 0.010% S, 0.01% Si, 0.030% Al, and bal. Fe.
A UV-2401PC spectrophotometer (Shimadzu Company, Japan) was used for UV–vis spectral measurements. A series of inhibited solutions were prepared with a fixed concentration of vanillin and Ce4+ in 1.0 M H2SO4. The absorption spectra of these solutions were determined with uninhibited 1.0 M H2SO4 as reference.
2.2. Inhibitors 2.6. Fourier transform infrared spectroscopy (FTIR) Both vanillin (4-hydroxy-3-methoxy-benzaldehyde) and cerium(IV) sulfate tetrahydrate (Ce(SO4)24H2O) were obtained from Shanghai Chemical Reagent Company of China, and of analyticalreagent grade. Fig. 1 shows the molecular structure of vanillin.
FTIR spectra were recorded in a AVATAR-FTIR-360 spectrophotometer (Thermo Nicolet Company, USA). In order not to damage the corrosion layer or protective film of the CRS surfaces, the FTIR reflectance accessory was applied to measure the CRS surfaces.
2.3. Solutions 2.7. X-ray photoelectron spectroscopy (XPS) The aggressive solutions, 1.0 M H2SO4 were prepared by dilution of analytical grade 98% H2SO4 with distilled water. The concentration range of inhibitors used was 25–500 mg l1. 2.4. Weight loss and polarization measurements The weight loss and potentiodynamic polarization measurements have been described in detail in the earlier reports [28,29]. The immersion time is 6 h, and the experimental temperature is 20–60 8C. All polarization curves were recorded at 20 8C, and the electrode was immersed in tested solution at natural potential for 2 h until a steady state was reached before measurement. The value of corrosion rate (v) of weight loss measurement was calculated from the following equation [2]: v¼
W St
(1)
where W is the average weight loss of three parallel CRS sheets, S the total area of the specimen, and t is immersion time. With the
Samples of dimension 1.0 cm 1.0 cm 0.06 cm were abraded with emery paper (grade 320-500-800) and then washed with distilled water and acetone. The steel was exposed to 250 ml 1.0 M H2SO4 with 100 mg l1 vanillin + 400 mg l1 Ce4+ at 20 8C for 6 h. After the specimen was rinsed with distilled water, and then dried in vacuum overnight. X-ray photoelectron spectra of components, Fe 2p, O 1s, C 1s, Ce 3d, S 2p for the CRS surfaces were recorded on an X-ray photoelectron spectrometer (PHI-5500 ESCA, USA) with 200 W Mg Ka radiation as the source. The binding energy (BE) internal calibration was referenced to the C 1s energy at 284.6 eV. 2.8. Atomic force microscope (AFM) The CRS specimens of size 1.0 cm 1.0 cm 0.06 cm were prepared as described above (2.7). After immersion in 1.0 M H2SO4 without and with addition of 100 mg l1 vanillin, 400 mg l1 Ce4+ and 100 mg l1 vanillin + 400 mg l1 Ce4+ at 20 8C for 6 h, the specimens were 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. 3. Experimental results and discussion 3.1. Weight loss measurements
Fig. 1. Chemical molecular structure of vanillin.
3.1.1. Effect of single vanillin on the corrosion rate The corrosion rate curves of CRS with the addition of vanillin in 1.0 M H2SO4 at various temperatures are shown in Fig. 2. The curves show that the corrosion rate values (g m2 h1) of CRS in 1.0 M H2SO4 solution containing vanillin decrease as the concentrations of the inhibitor increase, i.e. the corrosion inhibition enhances with the inhibitor concentration. This trend, it may result
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Fig. 2. Relationship between corrosion rate (v) and concentration of vanillin (C) in 1.0 M H2SO4.
from the fact that adsorption amount and the coverage of inhibitor on the CRS increase with the inhibitor concentration, thus the CRS surface is efficiently separated from the medium [30,31]. Also, the curves in Fig. 2 show that the corrosion rate values increase with the temperature. 3.1.2. Effect of vanillin concentration and temperature on inhibition efficiency The values of inhibition efficiencies for different vanillin concentrations in 1.0 M H2SO4 are shown in Fig. 3. The results show that inhibition efficiency increases with the inhibitor concentration ranges from 25 to 500 mg l1. The maximum IE was about 86% for vanillin at the concentration of 500 mg l1 and 60 8C. The inhibition was estimated to be moderate (60–70%) at 20–50 8C and 500 mg l1. These moderate inhibition performances could be explained as follows: Fig. 1 shows that vanillin is an aromatic aldehyde containing carbonyl, methoxy and hydroxyl groups arranged around the aromatic ring. The adsorption of vanillin on steel surface would take place through all these functional groups. The simultaneous adsorption of the three functional groups forces the vanillin molecule to be horizontally oriented at the metal surface [32,33]. As the inhibitor concentra-
tion increases, the part of the metal surface covered by inhibitor molecule increases leading to an increase in IE. Fig. 3 also gives that the IE decreases with the experimental temperatures ranging from 20 to 40 8C at first, which indicates that the higher temperatures might cause the desorption of vanillin from the steel surface. However, the IE increases with the experimental temperatures range from 40 to 60 8C. According to Oguzie et al. [11] and El Rehim et al. [34], a decrease in IE with rise in temperature suggests that inhibitor molecules are physically adsorbed on the metal surface, while the reverse behaviour suggests chemisorption. Accordingly, the results may be deduced that the adsorption of vanillin is mainly the physical adsorption at 20–40 8C, while mainly the chemisorption at 40–60 8C. In this experiment, it was observed a black insoluble and uniform layer covering on the steel surface in inhibited acid solution at 50 and 60 8C. The result might be ascribed to that the two closely spaced groups, methoxy and hydroxyl, may form easily an intramolecular hydrogen bond at relative high temperatures (50 and 60 8C). Under the circumstances, the inhibition efficiency would be improved. 3.1.3. Adsorption isotherm of vanillin on the CRS surface It is generally accepted that organic molecules inhibit corrosion by adsorption at the metal/solution interface and that the adsorption depends on the molecule’s chemical composition, the temperature and the electrochemical potential at the metal/ solution interface. Basic information on the adsorption of inhibitor on metals surface can be provided by 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 Temkin adsorption isotherm equation [31]: 1 u¼ lnðKCÞ (4) f where C is the concentration of inhibitor, K the adsorptive equilibrium constant, f the heterogeneous factor of the metal surface describing the molecular interactions in the adsorption layer and the heterogeneity of the metal surface, and u is the surface coverage, and could be calculated by the following relationship [31]: u¼
ðv0 vÞ V0
(5)
From the values of surface coverage, the linear regressions between u and ln C were calculated by the computer, and the parameters were listed in Table 1. Fig. 4 is the relationship between u and ln C at different temperatures. These results show that all the linear correlation coefficients (r) are almost equal to 1, which indicates the adsorption of vanillin onto steel surface obeys the Temkin adsorption isotherm. It can be deduced that the repulsion exists in the adsorption layer due to f > 0. Table 1 also indicates that the adsorptive equilibrium constant (K) values (in l mg1) decrease with increasing temperature range from 20 to 40 8C at first, then increase with the temperature range from 40 to 60 8C. Large values of K mean better inhibition efficiency of a given inhibitor. The conclusions agree with those of Section 3.1.2. Table 1 The linear regression parameters of u ln C in 1.0 M H2SO4
Fig. 3. Relationship between inhibition efficiency (IE) and concentration of vanillin (C) in 1.0 M H2SO4.
Temperature (8C)
r
Slope
Intercept
f
K (l mg1)
Maximum IE (%)
20 30 40 50 60
0.9966 0.9934 0.9971 0.9979 0.9917
0.1251 0.1562 0.1875 0.1989 0.1922
0.03874 0.2777 0.4663 0.4590 0.3437
7.99 6.40 5.33 5.03 5.26
0.73 0.17 0.083 0.099 0.17
75.3 72.7 69.6 77.5 86.4
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Fig. 4. The relationship between u and ln C in 1.0 M H2SO4.
3.1.4. Effect of single rare earth cerium(IV) ion on inhibition efficiency Fig. 5 shows the influence of concentration of cerium(IV) ion on IE of CRS in 1.0 M H2SO4. It can be seen that IE increases with the cerium(IV) ion concentration. The inhibition might be attributed to a consequence of adsorption of the rare earth cation in the hydrated state, namely, Ce4+ is incorporated with H2O because of affinity to the H2O in H2O/metal interface. However, the maximum IE is only about 20%. The poor inhibitive ability may be explained as follows: it is well known that the steel surface charges positive charge in H2SO4 because of Ecorr Eq = 0 (zero charge potential) > 0 [35], so it is difficult for the cerium(IV) ion to approach the positively charged steel surface (H3O+/metal interface) due to the electrostatic repulsion. In addition, the standard electrode potential values (vs. standard hydrogen electrode) of Ce4+/Ce3+, Fe3+/Fe2+ and Fe2+/Fe are 1.61 V, 0.771 V, and 0.441 V, respectively. And the main oxidizing-reducing reactions are showed as follows: Ce4þ þ Fe2þ ! Ce3þ þ Fe3þ
(6)
2Fe3þ þ Fe ! 3Fe2þ
(7)
2Ce4þ þ Fe ! 2Ce3þ þ Fe2þ
(8)
Fig. 6. The relationship between inhibition efficiency and different concentration ratios of vanillin and Ce4+ at a total blend concentration of 500 mg lS1 in 1.0 M H2SO4.
The reactions indicated that the rare earth ion Ce4+ and Fe3+ (produced as a result of the above reaction (6)) had a depolarizing effect on the anodic reaction rate and accelerated the dissolution of base metal in acid. Furthermore, the standard electrode potential of Ce3+/Ce (2.48 V) is much lower than that of Fe2+/Fe (0.441 V), so it is impossible for the cerium ion (Ce4+ and Ce3+) to precipitate onto the steel surface [36]. 3.1.5. Synergism between cerium(IV) ion and vanillin Fig. 6 shows the inhibition efficiency in 1.0 M H2SO4 with different concentration ratios of vanillin and Ce4+ at a total blend concentration of 500 mg l1 at different temperatures. It can be seen that the synergism between cerium(IV) ion and vanillin is exhibited. Namely, the IE for Ce4+ in combination with vanillin is higher than the summation of IE for single Ce4+ and single vanillin, which is synergism in nature. For example, at 30 8C the IEs of 25 mg l1 vanillin and 475 mg l1 Ce4+ are only 24.1% and 17.5%, respectively, while the IE their complex is 97.8% at the same conditions. A high inhibition efficiency, 98% was obtained by a mixture of 25–200 mg l1 vanillin and 300–475 mg l1 Ce4+ at 20–30 8C. To further judge whether synergism is taking place, one has to calculate the synergism parameter (s), as initially proposed by Murakwa et al. [37] for describing the combined inhibition behaviour of amines and halide ions. Generally, for the interaction of inhibitors A and B this synergism parameter (s) is defined as follows: s¼
Fig. 5. Relationship between inhibition efficiency (IE) and concentration of cerium(IV) ion (C) in 1.0 M H2SO4.
1 IEA IEB þ IEA IEB 1 IEAB
(9)
where IEA and IEB are the inhibition efficiencies observed with compound A, respectively, B, acting alone, and IEAB is the experimentally observed inhibition efficiency for the mixture A + B (CA and CB in the mixture should be the same as in the corresponding separate situations). The expression actually compares the theoretically expected corrosion rate (numerator), based on the known rates when either A or B are present and on the condition that they do not interact, with the experimentally observed rate in the presence of the inhibitor mixture (dominator). Consequently, in the case where inhibitors A and B have no effect on each other and adsorb at the metal/solution interface independently, s = 1 as in that case the predicted behaviour is experimentally confirmed. Alternatively,
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Table 2 The synergism parameters of vanillin and rare earth Ce4+ on the corrosion of CRS in 1.0 M H2SO4 solution at different temperatures C(vanillin) (mg l1) 25 50 100 150 200 250 300 350 400 450 475
C(Ce4+) (mg l1) 475 450 400 350 300 250 200 150 100 50 25
Synergism parameter (s) 20 8C
30 8C
40 8C
50 8C
60 8C
6.51 10.12 11.35 7.16 6.29 2.62 2.43 2.01 1.89 1.72 1.52
26.42 19.66 27.70 13.70 11.90 5.60 2.34 2.14 2.10 1.79 1.65
2.32 3.23 4.62 3.70 3.26 2.62 1.94 1.53 1.32 1.24 1.19
1.37 1.99 2.35 2.35 2.06 1.76 1.71 1.62 1.50 1.22 1.14
1.11 1.29 1.48 1.50 1.34 1.29 1.09 1.15 1.05 1.01 0.99
synergistic effects manifest themselves if s > 1 and antagonistic effects if s < 1. Table 2 indicates that the values of s for all investigated concentrations of vanillin and Ce4+ in 1.0 M H2SO4 are bigger than the unity except for the complex of 475 mg l1 vanillin—25 mg l1 Ce4+ at 60 8C, which suggests that there is a true synergism between vanillin and Ce4+ in 1.0 M H2SO4. It should be noted that the values of s are higher than 5.0 for the mixture of 25–200 mg l1 vanillin and 300–475 mg l1 Ce4+ at 20–30 8C, which can be seemed the optimized synergistic conditions. Table 2 also shows that the synergism degree at different temperatures follows the general order: 30 8C > 20 8C > 40 8C > 50 8C > 60 8C.
Table 3 Potentiodynamic polarization parameters for the corrosion of CRS in 1.0 M H2SO4 containing different concentrations of vanillin at 20 8C C (mg l1)
Ecorr (mV)
Icorr (mA cm2)
bc (mV dec1)
ba (mV dec1)
IE (%)
0 25 100 250 500
443.3 448.9 434.2 431.6 434.4
828.0 478.9 400.7 317.1 262.5
125 110 124 130 133
46 39 37 36 39
42.3 51.6 61.7 68.3
The values of corrosion current density (Icorr), corrosion potential (Ecorr), the cathodic Tafel slope (bc), anodic Tafel slope (ba), and the inhibition efficiency as functions of vanillin concentration, were calculated from the curves Fig. 7 and given in Table 3. Table 3 reveals that the corrosion current (Icorr) decreases and IE increases with the inhibitor concentration. The maximum IE is 68.3%, which also indicates vanillin has a moderate inhibitive effect. Good agreement between weight loss and polarization curve is obtained. Both the cathodic Tafel slopes (bc) and the anodic Tafel slopes (ba) do not change remarkably, which indicates that the mechanism of the corrosion reaction does not change and the corrosion reaction is inhibited by simple adsorption mode [39]. In the presence of vanillin, the corrosion potential (Ecorr) does not change remarkably, which indicates that vanillin acts as a mixed-type inhibitor [39].
3.2.1. Polarization curve of single vanillin The polarization behaviour of CRS in 1.0 M H2SO4 in the absence and presence of different concentrations of vanillin at 20 8C is shown in Fig. 7. It is clear that the inhibitor causes a decrease in the corrosion rate, i.e. shifts the anodic curves to positive potentials and the cathodic curves to negative potentials. This may be ascribed to adsorption of inhibitor over the corroded surface [38]. As the concentration of vanillin increases, the shift in the cathodic line increases. The inhibitor has an inhibitive effect on the rate of hydrogen evolution reaction. On the other hand, the inhibitor has slight effect on the anodic curves.
3.2.2. Polarization curve of single Ce4+ Both anodic and cathodic polarization curves for CRS in 1.0 M H2SO4 at various concentrations of Ce4+ are shown in Fig. 8. It can be seen that both the anodic and cathodic reactions of electrode are not inhibited by Ce4+, and the corrosion potentials do not change remarkably, either, compared with the blank. Although Fig. 8 shows that the cathodic curve in the presence of Ce4+ changes compared with the blank, the changes exhibited by the cathodic branch are not significant enough to indicate that Ce4+ inhibits the cathodic reaction of steel. The potentiodynamic polarization parameters are listed in Table 4. Table 4 shows that the corrosion current (Icorr) decreases and IE increases with Ce4+ concentration. However, the maximum IE was only 28.3%. In the presence of Ce4+, the corrosion potential (Ecorr), cathodic Tafel slopes (bc) and anodic Tafel slopes (ba) change slightly, which indicates that the corrosion mechanism of steel does not change. From Table 4, it can be concluded that IEs obtained from weight loss, electrochemical polarization curves are
Fig. 7. Potentiodynamic polarization curves for CRS in 1.0 M H2SO4 containing different concentrations of vanillin at 20 8C.
Fig. 8. Potentiodynamic polarization curves for CRS in 1.0 M H2SO4 containing different concentrations of cerium(IV) ion at 20 8C.
3.2. Polarization studies
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Table 4 Potentiodynamic polarization parameters for the corrosion of CRS in 1.0 M H2SO4 containing different concentrations of rare earth Ce4+ at 20 8C C (mg l1)
Ecorr (mV)
Icorr (mA cm2)
bc (mV dec1)
ba (mV dec1)
IE (%)
0 25 100 250 500
443.3 441.2 441.9 439.7 440.8
828.0 742.7 698.8 648.3 593.7
125 123 119 114 122
46 49 50 45 49
10.3 15.6 21.7 28.3
Fig. 10. UV–vis spectra of the inhibitors for CRS in 1.0 M H2SO4 (vs. 1.0 M H2SO4).
Fig. 9. Potentiodynamic polarization curves for CRS in 1.0 M H2SO4 containing different concentrations of vanillin and Ce4+ at 20 8C.
in good agreement. Namely, cerium(IV) ion has a negligible effect on the corrosion rate of CRS in 1.0 M H2SO4. 3.2.3. Polarization curve for synergistic inhibition between vanillin and Ce4+ Fig. 9 shows the polarization curves for CRS in 1.0 M H2SO4 at various concentrations of vanillin and Ce4+ at a total blend concentration of 500 mg l1 at 20 8C. Clearly, both anodic and cathodic reactions are drastically inhibited, and the phenomenon is evident in the presence of the complex 25 mg l1 vanillin – 475 mg l1 Ce4+ and 100 mg l1 vanillin – 400 mg l1 Ce4+. On the other hand, the complex of 250 mg l1 vanillin – 250 mg l1 Ce4+ and 400 mg l1 vanillin – 100 mg l1 Ce4+ have slight effect on the anodic curves, thus the synergistic degree decrease. The reason may be explained as follows: The new complex of Ce4+–vanillin could be formed and covering both anodic and cathodic reactive sites, which inhibited the anodic and cathodic reactions of steel corrosion. It should be noted that some of the vanillin molecules are still free in the acid solution. So it is expected that there may be a competition between vanillin and the Ce4+– vanillin complex to form a protective film onto the steel surface. Owing to the new complex of Ce4+–vanillin covers a large fraction steel surface as compared to the case of single vanillin additive, the IE of Ce4+–vanillin complex is higher than IE of single vanillin.
When Ce4+ concentration is relatively high (>400 mg l1), the new complex amount is high, and efficiently inhibits the anodic reaction (shifting the anodic curves to more positive potentials). In contrast, when Ce4+ concentration is relatively low (<250 mg l1), the new complex amount is low and does not efficiently inhibits the anodic reaction (slight effect on the anodic curves). This finding might suggest the CRS surface is protected by film most probably composed from adsorbed Ce4+–vanillin complex. The electrochemical parameters have been calculated and listed in Table 5. The complexes of 25 mg l1 vanillin – 475 mg l1 Ce4+ and 100 mg l1 vanillin – 400 mg l1 Ce4+ (the strong synergistic inhibition blended concentration) make the corrosion potential (Ecorr) shift to anodic direction. Both cathodic Tafel slopes (bc) and anodic Tafel slopes (ba) change remarkably, and the corrosion current (Icorr) decreases remarkably, which means that both anodic and cathodic reactions are drastically inhibited and has a satisfactory IE. For example, the IE calculated from corrosion current density for the complex 25 mg l1 vanillin— 475 mg l1 Ce4+ inhibitor reaches a considerable value (98.9%). The similar results were also reported with the synergism between organic inhibitor and chloride in H2SO4 media [40]. 3.2.4. Ultraviolet and visible spectrophotometer (UV–vis) The UV–vis absorption spectra of 25 mg l1 vanillin, 25 mg l1 4+ Ce , and the mixture of 25 mg l1 vanillin and 475 mg l1 Ce4+ (without CRS immersion) in 1.0 M H2SO4 were shown in Fig. 10. For single Ce4+, a faint wide peak appears at 276.50 nm. For single vanillin, the spectra absorption maximum wavelength is at 203.00 nm, and other three speaks are at 229.50, 278.50 and 309.00 nm, respectively. But for the mixture of 25 mg l1 vanillin and 475 mg l1 Ce4+, the absorption maximum wavelength appears at 254.40 nm with a shoulder at 222.80 nm. Consequently, the absorption spectra of the mixture are quite different from those of single vanillin or single Ce4+. Thus, the new complex of vanillin–Ce4+ is formed in 1.0 M H2SO4.
Table 5 Potentiodynamic polarization parameters for the corrosion of CRS in 1.0 M H2SO4 containing different concentrations of vanillin and Ce4+ at 20 8C C (vanillin) (mg l1)
C (Ce4+) (mg l1)
Ecorr (mV)
Icorr (mA cm2)
bc (mV dec1)
ba (mV dec1)
IE (%)
0 25 100 250 400
0 475 400 250 100
443.3 413.7 428.3 438.7 440.8
828.0 9.2 27.8 58.2 76.2
125 98 166 136 131
46 26 41 39 42
98.9 96.6 93.0 90.8
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Fig. 11. FTIR spectra of CRS specimens after immersion in various solutions (a) after immersion in 1.0 M H2SO4; (b) after immersion in 1.0 M H2SO4 + 100 mg lS1 vanillin + 400 mg lS1 Ce4+.
3.3. FTIR spectra Fourier transform infrared spectroscopy is a well-established characterization tool offering a ‘fingerprint’ for chemical compounds. The FTIR reflectance spectrum of corrosion layer formed on the CRS surface after immersion in 1.0 M H2SO4 is shown in Fig. 11(a). The band at 3395 cm1 is attributed to O–H stretching and 1659 cm1 is for O–H bending. There is a wide peak at 1032 cm1, which is attributed to the S O stretching of SO42. According to the literature [41], the S O stretching frequency of SO42 absorbs at
1024 cm1 for the corrosion layer formed on steel surface. The SO42 in CRS corrosion layer comes from the corrosion product of FeSO4. The strong band at 822 cm1 arises from FeOOH [42]. The FTIR reflectance spectrum of the protective film formed on the CRS surface after immersion in 1.0 M H2SO4 containing 100 mg l1 vanillin and 400 mg l1 Ce4+ is shown in Fig. 11(b). The O–H stretching has the band at 3368 cm1, and O–H bending at 1655 cm1. The weak band at 3938 cm1 is attributed to Fe–O bending, which indicates the coordinate bond formed by partial transference of electrons from the polar atom (O atom) of vanillin to the metal surface. The band at 1294 cm1 is the characteristic band Ce–O bond [43], which may indicate the adsorption film of Ce4+–vanillin complex. The bond at 1104 cm1 is attributed to C–O bond in methoxy (–OCH3) and 738 cm1 is for C–H bending in – CH3. The result may indicate that the organic inhibitor vanillin could adsorb onto the CRS surface in 1.0 M H2SO4. FeOOH was also found for the band at 844 cm1 [44]. Furthermore, due to the spectra in the experiment were obtained in the region from 650 to 4000 cm1, and Fe3O4, FeO and a-Fe2O3 only have the bands in the region from 300 to 600 cm1 corresponds to the steel-oxygen bond, so the ferrous oxides of Fe3O4, FeO and aFe2O3 can not be detected by using FTIR in this paper [42]. 3.4. X-ray photoelectron spectroscopy (XPS) The XPS survey scan of CRS immersed in 1.0 M H2SO4 inhibited by 100 mg l1 vanillin and 400 mg l1 Ce4+ for 6 h at 20 8C is shown in Fig. 12. It contains Fe, O, C, Ce, S and adventious contaminates such as Ca. High resolution spectra of the C 1s peak is shown in
Fig. 12. Survey spectra of XPS analysis for CRS surface exposed to a (1.0 M H2SO4 + 100 mg lS1 vanillin + 400 mg lS1 Ce4+) solution for 6 h at 20 8C.
Fig. 13. High resolution C 1s spectra of XPS analysis for CRS surface exposed to a (1.0 M H2SO4 + 100 mg lS1 vanillin + 400 mg lS1 Ce4+) solution for 6 h at 20 8C.
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Fig. 14. High resolution S 2p spectra of XPS analysis for CRS surface exposed to a (1.0 M H2SO4 + 100 mg lS1 vanillin + 400 mg lS1 Ce4+) solution for 6 h at 20 8C.
Fig. 15. High resolution O 1s spectra of XPS analysis for CRS surface exposed to a (1.0 M H2SO4 + 100 mg lS1 vanillin + 400 mg lS1 Ce4+) solution for 6 h at 20 8C.
Fig. 13. Four peaks are evident independently of the microstructure: 284.8, 286.4, 288.2 and 289.6 eV. The peak of binding energy at 284.8 eV is related to the presence of cementite (Fe3C) [45]. The BE at 286.4 eV corresponds to residual or adventious carbon [46,47]. The peak at 288.2 eV is due to C O and the peak at the highest binding energy of 289.6 eV might be due to O–C O or Ce C O, which may be indicated that the complex of Ce4+– vanillin was adsorbed on the steel surface. Fig. 14 presents the S 2p high resolution XPS spectra. There is only one peak of BE at 169.4 eV, which is attributed to the characteristic peak of SO42 [47].
Fig. 15 shows the O 1s high resolution XPS spectra. Three distinct peaks are observed: the one at the lowest binding energy at 529.6 eV presents the oxide bond (MO), the 531.1 eV peak is due to metal hydroxide (MOH) and the peak at the highest binding energy of 532.8 eV is due to metal water bonds (MH2O) [48]. The high resolution spectra of Ce 3d peak are shown in Fig. 16. Three peaks are evident independently of the microstructure: 881.8, 885.7, and 888.4 eV. The peak of binding energy at 881.8 eV is related to the presence of CeO2 [47]. In addition, a slight yellow film was observed on the CRS surface, which may be indicated the Ce4+ compounds were in the presence of the film. The BE at
Fig. 16. High resolution Ce 3d spectra of XPS analysis for CRS surface exposed to a (1.0 M H2SO4 + 100 mg lS1 vanillin + 400 mg lS1 Ce4+) solution for 6 h at 20 8C.
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Fig. 17. Fe 2p spectra for: (a) cold rolled steel before immersion (b) oxidized steel surface exposed to a solution (1.0 M H2SO4 + 100 mg lS1 vanillin + 400 mg lS1 Ce4+) for 6 h at 20 8C.
885.7 eV is a characteristic of Ce3+ compounds such as Ce2O3, Ce(OH)3 [47], but the compounds of Ce2O3 and Ce(OH)3 could not be produced in the strong acidic solution [49], so the Ce3+ compounds of Ce2O3 and Ce(OH)3 may be attributed to the reduction process of the film in the air form Ce4+ compounds. The peak at 888.4 eV may be ascribed to the complex of Ce4+–vanillin. The XPS spectra for Fe 2p region are shown in Fig. 17. Fig. 17(a) shows that a peak for Fe 2p at BE 707.4 eV characteristic of Fe0, but the substrate covered with the blend of vanillin and rare earth Ce4+ exhibited a peak for Fe 2p at BE 710.6 eV (Fe 2p3/2) and a peak at 723.9 eV (Fe 2p1/2) characteristic of Fe3+, showing the subsequent oxidation of the steel surface (Fig. 17(b)). The increase in Fe 2p signal confirmed the adsorption and film formation on the steel surface [50]. 3.5. Atomic force microscope (AFM) surface examination The atomic force microscope provides a powerful means of characterizing the microstructure [51–54]. The two-dimensional AFM images of CRS surface in 1.0 M H2SO4 are shown in Fig. 18. Fig. 18(a) shows clearly the CRS surface before immersion seems smooth and shows no evident corrosion products on the surface. However, it is not absolute smooth and uniform, and appears black holes and crevices, which may be attributed to the defect of steel, and probably an oxide inclusion. As for Fig. 18(b), the CRS surface after immersion in uninhibited 1.0 M H2SO4 for 6 h was damaged strongly comparing with Fig. 18(a), and covered with the corrosion products. It appeared to be relatively uniform in general, and there were no evident black holes or crevices because of the corrosion products precipitated in the defect. The image is quite different from the pitting corrosion image [53] or the corrosion in HClO4–CH3COOH solution [55]. Fig. 18(c) shows that there are some spherical or bread-like particles on the steel surface in the presence of 100 mg l1 vanillin, which do not exist in the Fig. 18(b). Therefore, it may be concluded that these particles are the adsorption film of the inhibitor, which inhibits the corrosion of CRS [28,29]. Fig. 18(d) also shows that the steel surface in the presence of Ce4+ appears some a layer image, but the roughness of the surface layer is high compared to the roughness of the film formed in the presence of vanillin. In contrast, Fig. 18(e) shows that the steel surface in the presence of both vanillin and Ce4+ is fully and orderly covered with unique particles, which are quite different form the corrosion product particles (Fig. 18(b)) and do not exist in the Figs. 18(c) and (d), and could be ascribed to the adsorption of the complex Ce4+–vanillin. The particles are more distributed and rather
uniform, and the outer surface of the particles is enwrapped with granular aggregates tightly and regularly. Fig. 18(e) also shows that the film on the CRS surface becomes denser and more tightly comparing with Fig. 18(c) and (d); so, it can efficiently protect CRS from corrosion comparing single vanillin or Ce4+. Fig. 19 shows the CRS surface topography. It can be seen from Fig. 19(a) that the micrograph of CRS surface before immersion shows no evident particles on the surface. Fig. 19(b) shows that the steel surface after immersion in uninhibited 1.0 H2SO4 shows the main characteristic of uniform corrosion in acidic media [28,29]. Fig. 19(c) and (e) shows that some spherical particles decorating the steel surface and the retention of surface grinding marks, which indicate a macroscopically thin film, and the particles on the steel surface are the smallest for the presence of both vanillin and Ce4+. Fig. 19(d) shows that the particles are relative bigger. Fig. 20 illustrates the height profiles, which are made along the lines marked in corresponding Fig. 19. The surface roughness of the CRS before immersion is 134.45 nm from Fig. 20(a). Fig. 20(b) indicates that the surface roughness of the CRS after immersion in uninhibited 1.0 H2SO4 is up to 463.08 nm, while in the presence of single vanillin, the roughness decreases to 303.74 nm (Fig. 20(c)). In the presence of single Ce4+, the roughness increases to 578.56 nm (Fig. 20(d)), which is higher than that of immersion in uninhibited H2SO4. Fig. 20(e) shows that the surface roughness is only 206.59 nm in the presence of both vanillin and Ce4+. In general, the result of the surface roughness agrees with that of Fig. 18. 3.6. Explanation for synergism It is well known that steel corrosion reaction in 1.0 M H2SO4 may be split into the anodic partial reaction (steel dissolution) and the cathodic partial reaction (hydrogen evolution). Vanillin (4-hydroxy-3-methoxy-benzaldehyde) contains oxygen atoms with lone-pair electrons, so it can adsorb onto the steel surface through the lone pair of unshared electrons available in the oxygen atom of vanillin, which inhibits both anodic and cathodic reactions of steel, while prominently inhibits the cathodic reaction. However, there is a repulsion force among the adsorbed vanillin inhibitors onto the steel surface due to f > 0 (Table 1). As a result, the inhibition efficiency is not high for single vanillin. In the case of addition of rare earth Ce4+ + vanillin mixture, the results demonstrated the inhibition efficiency was higher than the
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Fig. 18. AFM two-dimensional images of CRS surface: (a) before immersion; (b) after 6 h of immersion at 20 8C in 1.0 M H2SO4; (c) after 6 h of immersion at 20 8C in 100 mg lS1 vanillin + 1.0 M H2SO4; (d) after 6 h of immersion at 20 8C in 400 mg lS1 Ce4+ + 1.0 M H2SO4; (e) after 6 h of immersion at 20 8C in 100 mg lS1 vanillin + 400 mg lS1 Ce4+ + 1.0 M H2SO4.
summation of inhibition efficiency for single Ce4+ and single vanillin. The values of synergistic parameter (s) are higher than unity (Table 2), which indicates there is a true synergism between vanillin and Ce4+ in 1.0 M H2SO4. In order to explain the fact, the following synergism mechanism is proposed: Rare earth Ce4+ belongs to lanthanide in periodic table of chemical elements, and it has a lot of vacant orbits (4f, 5d and 6s). There are lone-pair electrons among vanillin molecule. So, when vanillin was mixed with Ce4+, the new complex of Ce4+–
vanillin was easily formed (Fig. 10), and the complex played an important role in the enhancement of the protection of CRS against corrosion. It should be noted that some of the vanillin molecules are still free in the acid solution when the ratio of vanillin is high. Though there is no reference available in the literature on the exact structure of the complex of Ce4+–vanillin, it is doubtless that the higher ratio of vanillin, the more free vanillin molecules in the acid solution. So it is expected that there is a competition between vanillin and the Ce4+–vanillin
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Fig. 19. AFM images of CRS surface topography: (a) before immersion; (b) after 6 h of immersion at 20 8C in 1.0 M H2SO4; (c) after 6 h of immersion at 20 8C in 100 mg lS1 vanillin + 1.0 M H2SO4; (d) after 6 h of immersion at 20 8C in 400 mg lS1 Ce4+ + 1.0 M H2SO4; (e) after 6 h of immersion at 20 8C in 100 mg lS1 vanillin + 400 mg lS1 Ce4+ + 1.0 M H2SO4.
complex to form a protective film onto the steel surface. This competition may slow down the rate of film formation and consequently reduce the percentage inhibition efficiency [56]. Although the experiment results (Figs. 6 and 9) revealed that IE value decreases when the ration of vanillin is higher than the ratio of Ce4+, its value is still high as compared with that obtained in case of using different concentration of vanillin separately. This finding might suggest the CRS surface is protected by film most probably composed from adsorbed Ce4+–vanillin complex. This assumption could be further confirmed by the FTIR and XPS
results. Owing to the new complex of Ce4+–vanillin covering a large fraction steel surface as compared with the case of single vanillin additive, the IE of Ce4+–vanillin complex is higher than IE of single vanillin. The complex could adsorb onto steel surface by the van der Waals force 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. In this case, vanillin molecules may have played a role in bridging the access of rare earth Ce4+ to the metal surface from
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Fig. 20. Height profiles of the CRS surface: (a) before immersion; (b) after 6 h of immersion at 20 8C in 1.0 M H2SO4; (c) after 6 h of immersion at 20 8C in 100 mg lS1 vanillin + 1.0 M H2SO4; (d) after 6 h of immersion at 20 8C in 400 mg lS1 Ce4+ + 1.0 M H2SO4; (e) after 6 h of immersion at 20 8C in 100 mg lS1 vanillin + 400 mg lS1 Ce4+ + 1.0 M H2SO4.
the solution. So, it is concluded that Ce4+ and vanillin demonstrated a strong inhibition synergism for CRS. 4. Conclusion 1. Vanillin acts as a moderate inhibitor for the corrosion of CRS in 1.0 M H2SO4, and the adsorption obeys the Temkin adsorption isotherm. Rare earth cerium(IV) ion has a negligible effect on the corrosion rate of CRS, and the maximum IE is only about 20%. 2. There is a strong synergism between vanillin and Ce4+ in 1.0 M H2SO4. Namely, the IE for Ce4+ in combination with vanillin is
higher than the summation of IE for single Ce4+ and single vanillin. The values of synergism parameter (s) are higher than unity. 3. Vanillin acts as a mixed-type inhibitor in sulfuric acid, while prominently inhibits cathodic reaction. Ce4+ has a negligible effect on both anodic and cathodic reactions. However, the vanillin–Ce4+ mixture behave as a mixed-type inhibitor, which drastically inhibits both anodic and cathodic reactions. 4. The new complex of Ce4+–vanillin was formed in 1.0 M H2SO4 for Ce4+ in combination with vanillin, which played an important role in the synergism mechanism. The protective
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film on the steel surface formed in the presence of both vanillin and Ce4+ containing Fe, O, C, Ce and S elements, and the complex of Ce4+–vanillin. 5. The film formed after immersion in the presence of both vanillin and Ce4+ appears denser and more tightly protective, which decrease greatly the steel surface roughness and effectively protects steel from corrosion. References [1] E. Kalman, I. Lukovits, G. Palinkas, ACH—Models Chem. 132 (1995) 527. [2] G.N. Mu, X.M. Li, F. Li, Mater. Chem. Phys. 86 (2004) 59. [3] F. Bentiss, M. Bouanis, B. Mernari, M. Traisnel, M. Lagrene´e, J. Appl. Electrochem. 32 (2002) 671. [4] E.E. Ebenso, Mater. Chem. Phys. 79 (2003) 58. [5] Y. Feng, K.S. Siow, W.K. Teo, A.K. Hsieh, Corros. Sci. 41 (1999) 829. [6] T. Du, J. Chen, D. Cao, Br. Corros. J. 35 (2000) 229. [7] D.Q. Zhang, L.X. Gao, G.D. Zhou, J. Appl. Electrochem. 33 (2003) 361. [8] G.K. Gomma, Mater. Chem. Phys. 55 (1998) 241. [9] R.M. Hudson, C.J. Warning, Corros. Sci. 10 (1970) 121. [10] N.I. Podobacv, L.N. Zimova, G.F. Semikolenov, Zashch. Metall. 13 (1997) 600. [11] E.E. Oguzie, C. Unaegbu, C.N. Ogukwe, B.N. Okolue, A.I. Onuchukwu, Mater. Chem. Phys. 84 (2004) 363. [12] E.E. Ebenso, U.J. Ekpe, S.A. Umoren, E. Jackson, O.K. Abiola, N.C. Oforka, J. Appl. Polym. Sci. 100 (2006) 2889. [13] M. Bouklah, B. Hammouti, A. Aouniti, M. Benkaddour, A. Bouyanzer, Appl. Surf. Sci. 252 (2006) 6236. [14] I. Sekine, Y. Hirakawa, Corrosion 42 (1986) 272. [15] I. Singh, M. Singh, Corrosion 43 (1987) 425. [16] D.D.N. Singh, T.B. Singh, B. Gaur, Corros. Sci. 37 (1995) 1005. [17] G.N. Mu, T.P. Zhao, M. Liu, T. Gu, Corrosion 52 (1996) 853. [18] M. Hosseini, S.F.L. Mertens, M.R. Arshadi, Corros. Sci. 45 (2003) 1473. [19] R.F.V. Villamil, G.G.O. Cordeiro, J. Matos, E. Delia, S.M.L. Agostinho, Mater. Chem. Phys. 78 (2002) 448. [20] R.F.V. Villamil, P. Corio, J.C. Rubim, S.M.L. Agostinho, J. Electroanal. Chem. 472 (1999) 112. [21] R.F.V. Villamill, P. Corio, J.C. Rubim, S.M.L. Agostinho, J. Electroanal. Chem. 535 (2002) 75. [22] M.N. Shalaby, M.M. Osman, Anti-Corr. Meth. Mater. 48 (2001) 309. [23] G.N. Mu, T.P. Zhao, J. Yunnan Univ. (China) 21 (1999) 279.
[24] M. Forsyth, C.M. Forsyth, K. Wilson, T. Behrsing, G.B. Deacon, Corros. Sci. 44 (2002) 2651. [25] K. Aramaki, Corros. Sci. 44 (2002) 1361. [26] K. Aramaki, Corros. Sci. 44 (2002) 871. [27] G.N. Mu, X.H. Li, Q. Qu, J. Zhou, Acta Chim. Sinca 62 (24) (2004) 2386 (in Chinese). [28] X.H. Li, G.N. Mu, Appl. Surf. Sci. 252 (2005) 1254. [29] G.N. Mu, X.H. Li, J. Colloid Interface Sci. 289 (2005) 184. [30] T.P. Zhao, G.N. Mu, Corros. Sci. 41 (1999) 1937. [31] A.K. Maaya, N.A.F. Al-Rawashdeh, Corros. Sci. 46 (2004) 1129. [32] A.Y. El-Etre, Corros. Sci. 43 (2001) 1031. [33] K.C. Emregu¨l, M. Hayval1, Mater. Chem. Phys. 83 (2004) 209. [34] S.S. Abd El Rehim, M.A.M. Ibrahim, K.E. Khalid, Mater. Chem. Phys. 70 (2001) 269. [35] D.P. Schweinsberg, V. Ashworth, Corros. Sci. 28 (1988) 539. [36] X.H. Li, S.D. Deng, G.N. Mu, Q. Qu, Mater. Lett. 61 (2007) 2514. [37] T. Murakwa, S. Nagaura, N. Hackerman, Corros. Sci. 7 (1967) 79. [38] G.N. Mu, X.H. Li, Q. Qu, J. Zhou, Corros. Sci. 48 (2006) 445. [39] C.N. Cao, Corrosion Electrochemistry Mechanism, Chemical Industrial Engineering Press, Beijing, 2004, p. 235 (in Chinese). [40] G.N. Mu, X.M. Li, G.H. Liu, Corros. Sci. 47 (2005) 1932. [41] J.G. Wu, Techniques and Applications of Infrared Fourier Transform Spectroscopy, vol. 2, Science and Technology Information Press, Beijing, 1994 (in Chinese). [42] A. Raman, B. Kuban, A. Razvan, Corros. Sci. 32 (1991) 1295. [43] C. Yang, Q.N. Dong, J. Ren, Y.H. Sun, Spectrosc. Spectral Anal. 24 (2004) 810 (in Chinese). [44] X.M. Yang, J. Ningxia Univ. 20 (1) (1999) 47 (in Chinese). [45] E. Park, J. Zhang, S. Thomson, O. Ostrousky, R. Howe, Metall. Trans. B 32 (2001) 839. [46] D.A. Lo´pez, W.H. Schreiner, S.R. de Sa´nchez, S.N. Simison, Appl. Surf. Sci. 236 (2004) 77. [47] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg (Eds.), Handbook of X-ray Photoelectron Spectroscopy, Perkin Elmer Corporation (Physical Electronics), Eden Prairie, Minnesota, 1979. [48] I.M. Baghmi, S.B. Lyon, B. Ding, Surf. Coat. Technol. 185 (2004) 194. [49] M.H. Shao, R.S. Huang, Y. Fu, C.J. Lin, J. Rare Earths 20 (2002) 640. [50] F. Bentiss, M. Lagrene´e, M. Traisnel, Corrosion 56 (2000) 733. [51] A.A. Gewirth, B.K. Niece, Chem. Rev. 97 (1997) 1129. [52] I.C. Oppenherm, D. Trevor, C.E.D. Chidsey, P.L. Trevor, K. Sieradzki, Science 254 (1991) 688. [53] J. Li, D. Lampner, Colloids Surf. A 154 (1999) 227. [54] H.H. Teng, P.M. Dove, C.A. Orme, J.J. De Yoreo, Science 282 (1998) 724. [55] C.A. Huang, W. Lin, S.C. Lin, Corros. Sci. 45 (2003) 2627. [56] M. Abdallah, M.M. El-Naggar, Mater. Chem. Phys. 71 (2001) 291.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synergism between rare earth cerium(IV) ion and vanillin on the corrosion of steel in H2SO4 solution: Weight loss, electrochemical, UV–vis, FTIR, XPS, and AFM approaches Xianghong Li a,*, Shuduan Deng b, Hui Fu a, Guannan Mu c, Ning Zhao a a b c
Department of Fundamental Courses, Southwest Forestry University, Kunming 650224, PR China Department of Wood Science and Technology, Southwest Forestry University, Kunming 650224, PR China Department of Chemistry, Yunnan University, Kunming 650091, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 22 July 2007 Received in revised form 27 February 2008 Accepted 2 March 2008 Available online 13 March 2008
The synergism between rare earth cerium(IV) ion and vanillin (4-hydroxy-3-methoxy-benzaldehyde) on the corrosion of cold rolled steel (CRS) in 1.0 M H2SO4 solution at five temperatures ranging from 20 to 60 8C was first studied by weight loss and potentiodynamic polarization methods. The inhibited solutions were analyzed by ultraviolet and visible spectrophotometer (UV–vis). The adsorbed film of CRS surface containing optimum doses of the blends Ce4+–vanillin was investigated by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM). The results revealed that vanillin had a moderate inhibitive effect, and the inhibition efficiency (IE) increased with the vanillin concentration. The adsorption of vanillin obeyed Temkin adsorption isotherm. Polarization curves showed that vanillin was a mixed-type inhibitor in sulfuric acid, while prominently inhibited the cathodic reaction. For the cerium(IV) ion, it had a negligible effect, and the maximum IE was only about 20%. However, incorporation of Ce4+ with vanillin improved significantly the inhibition performance. The IE for Ce4+ in combination with vanillin was higher than the summation of IE for single Ce4+ and single vanillin, which was synergism in nature. A high inhibition efficiency, 98% was obtained by a mixture of 25–200 mg l1 vanillin and 300–475 mg l1 Ce4+. UV–vis showed that the new complex of Ce4+–vanillin was formed in 1.0 M H2SO4 for Ce4+ combination with vanillin. Polarization studies showed that the complex of Ce4+–vanillin acted as a mixed-type inhibitor, which drastically inhibits both anodic and cathodic reactions. FTIR and XPS revealed that a protective film formed in the presence of both vanillin and Ce4+ was composed of cerium oxide and the complex of Ce4+–vanillin. The synergism between Ce4+ and vanillin could also be evidenced by AFM images. Depending on the results, the synergism mechanism was discussed from the viewpoint of adsorption theory. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Cold rolled steel Sulfuric acid Vanillin Rare earth Cerium(IV) ion UV–vis FTIR XPS AFM Corrosion inhibitor Adsorption Synergism
1. Introduction Synergism (synergistic inhibition effect) is a combined action of compounds greater in total effect than the sum of the individual effect. Synergism of corrosion inhibitors is either due to the interaction between components of the inhibitor composition or due to interaction between the inhibitor and one of the ions present in the aqueous solution [1]. Synergism is an effective means to improve the inhibitive force of inhibitor, to decrease the amount of usage, to diversify the application of inhibitor in acidic media. It plays an important role not only in theoretical research on corrosion inhibitors but also in practical work [2]. In previous investigations, synergism between organic inhibitors and halide ions on metal * Corresponding author. E-mail address: [email protected] (X. Li). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.03.026
corrosion in acidic solution has been studied by many authors [2– 13]. The synergistic inhibition effects of organic inhibitor/metallic ion mixture [14–17], organic inhibitor/organic inhibitor mixture [18–22] and inorganic inhibitor/inorganic inhibitor mixture [23] on corrosion of metal in acidic media have also been studied. Forsyth et al. [24] have studied the synergistic corrosion inhibition of mild steel by rare earth (RE) Ce3+ and sodium salicylate in 0.1 M NaCl neutral media. Aramaki [25,26] has investigated the synergism of RE Ce3+/inorganic compound Na2Si2O5 mixture and Ce3+/organic compound C8H17S(CH2)2COONa (NaOTP) mixture on the corrosion of zinc in 0.5 M NaCl. However, up to now, there is little report in the literature about the synergism between RE ions and other compounds on metal corrosion in strong acidic media. In our laboratory, much work has been conducted to study the synergism between RE ions and other compounds on the corrosion of some metals and alloys in strong acidic media. In 1996, the
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synergism between anionic surfactant (sodium dodecyl sulfonate, SDS) and RE La3+ for mild steel corrosion in 1.0 M HCl was reported [17]. Recently, the synergistic effect of inorganic compound Na2MoO4 and Ce4+ on cold rolled steel (CRS) corrosion inhibition in 1.0 M HCl was also investigated [27]. In the present work, the synergism between rare earth Ce4+ and organic compound vanillin on CRS in sulfuric acid is first studied by weight loss and potentiodynamic polarization methods. The inhibited solutions were analyzed by ultraviolet and visible spectrophotometer (UV– vis). The steel surface was also examined by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM). It is expected to get general information on the synergism between RE ion and organic inhibitor on the corrosion of steel in strong acid solution.
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calculated corrosion rate, the inhibition efficiency (IE) on the corrosion of CRS was calculated as follows [2]: IE% ¼
v0 v 100 v
(2)
where v0 and v are the values of the corrosion rate without and with addition of the inhibitor, respectively. IE of potentiodynamic polarization measurement was defined as: IE% ¼
Icorr IcorrðinhÞ 100 Icorr
(3)
where Icorr and Icorr(inh) are the uninhibited and inhibited corrosion current density values, respectively, determined by extrapolation of Tafel lines to the corrosion potential.
2. Experimental method 2.5. Ultraviolet and visible spectrophotometer (UV–vis) 2.1. Materials Tests were performed on a cold rolled steel of the following composition (given in wt.%): 0.07% C, 0.3% Mn, 0.022% P, 0.010% S, 0.01% Si, 0.030% Al, and bal. Fe.
A UV-2401PC spectrophotometer (Shimadzu Company, Japan) was used for UV–vis spectral measurements. A series of inhibited solutions were prepared with a fixed concentration of vanillin and Ce4+ in 1.0 M H2SO4. The absorption spectra of these solutions were determined with uninhibited 1.0 M H2SO4 as reference.
2.2. Inhibitors 2.6. Fourier transform infrared spectroscopy (FTIR) Both vanillin (4-hydroxy-3-methoxy-benzaldehyde) and cerium(IV) sulfate tetrahydrate (Ce(SO4)24H2O) were obtained from Shanghai Chemical Reagent Company of China, and of analyticalreagent grade. Fig. 1 shows the molecular structure of vanillin.
FTIR spectra were recorded in a AVATAR-FTIR-360 spectrophotometer (Thermo Nicolet Company, USA). In order not to damage the corrosion layer or protective film of the CRS surfaces, the FTIR reflectance accessory was applied to measure the CRS surfaces.
2.3. Solutions 2.7. X-ray photoelectron spectroscopy (XPS) The aggressive solutions, 1.0 M H2SO4 were prepared by dilution of analytical grade 98% H2SO4 with distilled water. The concentration range of inhibitors used was 25–500 mg l1. 2.4. Weight loss and polarization measurements The weight loss and potentiodynamic polarization measurements have been described in detail in the earlier reports [28,29]. The immersion time is 6 h, and the experimental temperature is 20–60 8C. All polarization curves were recorded at 20 8C, and the electrode was immersed in tested solution at natural potential for 2 h until a steady state was reached before measurement. The value of corrosion rate (v) of weight loss measurement was calculated from the following equation [2]: v¼
W St
(1)
where W is the average weight loss of three parallel CRS sheets, S the total area of the specimen, and t is immersion time. With the
Samples of dimension 1.0 cm 1.0 cm 0.06 cm were abraded with emery paper (grade 320-500-800) and then washed with distilled water and acetone. The steel was exposed to 250 ml 1.0 M H2SO4 with 100 mg l1 vanillin + 400 mg l1 Ce4+ at 20 8C for 6 h. After the specimen was rinsed with distilled water, and then dried in vacuum overnight. X-ray photoelectron spectra of components, Fe 2p, O 1s, C 1s, Ce 3d, S 2p for the CRS surfaces were recorded on an X-ray photoelectron spectrometer (PHI-5500 ESCA, USA) with 200 W Mg Ka radiation as the source. The binding energy (BE) internal calibration was referenced to the C 1s energy at 284.6 eV. 2.8. Atomic force microscope (AFM) The CRS specimens of size 1.0 cm 1.0 cm 0.06 cm were prepared as described above (2.7). After immersion in 1.0 M H2SO4 without and with addition of 100 mg l1 vanillin, 400 mg l1 Ce4+ and 100 mg l1 vanillin + 400 mg l1 Ce4+ at 20 8C for 6 h, the specimens were 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. 3. Experimental results and discussion 3.1. Weight loss measurements
Fig. 1. Chemical molecular structure of vanillin.
3.1.1. Effect of single vanillin on the corrosion rate The corrosion rate curves of CRS with the addition of vanillin in 1.0 M H2SO4 at various temperatures are shown in Fig. 2. The curves show that the corrosion rate values (g m2 h1) of CRS in 1.0 M H2SO4 solution containing vanillin decrease as the concentrations of the inhibitor increase, i.e. the corrosion inhibition enhances with the inhibitor concentration. This trend, it may result
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Fig. 2. Relationship between corrosion rate (v) and concentration of vanillin (C) in 1.0 M H2SO4.
from the fact that adsorption amount and the coverage of inhibitor on the CRS increase with the inhibitor concentration, thus the CRS surface is efficiently separated from the medium [30,31]. Also, the curves in Fig. 2 show that the corrosion rate values increase with the temperature. 3.1.2. Effect of vanillin concentration and temperature on inhibition efficiency The values of inhibition efficiencies for different vanillin concentrations in 1.0 M H2SO4 are shown in Fig. 3. The results show that inhibition efficiency increases with the inhibitor concentration ranges from 25 to 500 mg l1. The maximum IE was about 86% for vanillin at the concentration of 500 mg l1 and 60 8C. The inhibition was estimated to be moderate (60–70%) at 20–50 8C and 500 mg l1. These moderate inhibition performances could be explained as follows: Fig. 1 shows that vanillin is an aromatic aldehyde containing carbonyl, methoxy and hydroxyl groups arranged around the aromatic ring. The adsorption of vanillin on steel surface would take place through all these functional groups. The simultaneous adsorption of the three functional groups forces the vanillin molecule to be horizontally oriented at the metal surface [32,33]. As the inhibitor concentra-
tion increases, the part of the metal surface covered by inhibitor molecule increases leading to an increase in IE. Fig. 3 also gives that the IE decreases with the experimental temperatures ranging from 20 to 40 8C at first, which indicates that the higher temperatures might cause the desorption of vanillin from the steel surface. However, the IE increases with the experimental temperatures range from 40 to 60 8C. According to Oguzie et al. [11] and El Rehim et al. [34], a decrease in IE with rise in temperature suggests that inhibitor molecules are physically adsorbed on the metal surface, while the reverse behaviour suggests chemisorption. Accordingly, the results may be deduced that the adsorption of vanillin is mainly the physical adsorption at 20–40 8C, while mainly the chemisorption at 40–60 8C. In this experiment, it was observed a black insoluble and uniform layer covering on the steel surface in inhibited acid solution at 50 and 60 8C. The result might be ascribed to that the two closely spaced groups, methoxy and hydroxyl, may form easily an intramolecular hydrogen bond at relative high temperatures (50 and 60 8C). Under the circumstances, the inhibition efficiency would be improved. 3.1.3. Adsorption isotherm of vanillin on the CRS surface It is generally accepted that organic molecules inhibit corrosion by adsorption at the metal/solution interface and that the adsorption depends on the molecule’s chemical composition, the temperature and the electrochemical potential at the metal/ solution interface. Basic information on the adsorption of inhibitor on metals surface can be provided by 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 Temkin adsorption isotherm equation [31]: 1 u¼ lnðKCÞ (4) f where C is the concentration of inhibitor, K the adsorptive equilibrium constant, f the heterogeneous factor of the metal surface describing the molecular interactions in the adsorption layer and the heterogeneity of the metal surface, and u is the surface coverage, and could be calculated by the following relationship [31]: u¼
ðv0 vÞ V0
(5)
From the values of surface coverage, the linear regressions between u and ln C were calculated by the computer, and the parameters were listed in Table 1. Fig. 4 is the relationship between u and ln C at different temperatures. These results show that all the linear correlation coefficients (r) are almost equal to 1, which indicates the adsorption of vanillin onto steel surface obeys the Temkin adsorption isotherm. It can be deduced that the repulsion exists in the adsorption layer due to f > 0. Table 1 also indicates that the adsorptive equilibrium constant (K) values (in l mg1) decrease with increasing temperature range from 20 to 40 8C at first, then increase with the temperature range from 40 to 60 8C. Large values of K mean better inhibition efficiency of a given inhibitor. The conclusions agree with those of Section 3.1.2. Table 1 The linear regression parameters of u ln C in 1.0 M H2SO4
Fig. 3. Relationship between inhibition efficiency (IE) and concentration of vanillin (C) in 1.0 M H2SO4.
Temperature (8C)
r
Slope
Intercept
f
K (l mg1)
Maximum IE (%)
20 30 40 50 60
0.9966 0.9934 0.9971 0.9979 0.9917
0.1251 0.1562 0.1875 0.1989 0.1922
0.03874 0.2777 0.4663 0.4590 0.3437
7.99 6.40 5.33 5.03 5.26
0.73 0.17 0.083 0.099 0.17
75.3 72.7 69.6 77.5 86.4
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Fig. 4. The relationship between u and ln C in 1.0 M H2SO4.
3.1.4. Effect of single rare earth cerium(IV) ion on inhibition efficiency Fig. 5 shows the influence of concentration of cerium(IV) ion on IE of CRS in 1.0 M H2SO4. It can be seen that IE increases with the cerium(IV) ion concentration. The inhibition might be attributed to a consequence of adsorption of the rare earth cation in the hydrated state, namely, Ce4+ is incorporated with H2O because of affinity to the H2O in H2O/metal interface. However, the maximum IE is only about 20%. The poor inhibitive ability may be explained as follows: it is well known that the steel surface charges positive charge in H2SO4 because of Ecorr Eq = 0 (zero charge potential) > 0 [35], so it is difficult for the cerium(IV) ion to approach the positively charged steel surface (H3O+/metal interface) due to the electrostatic repulsion. In addition, the standard electrode potential values (vs. standard hydrogen electrode) of Ce4+/Ce3+, Fe3+/Fe2+ and Fe2+/Fe are 1.61 V, 0.771 V, and 0.441 V, respectively. And the main oxidizing-reducing reactions are showed as follows: Ce4þ þ Fe2þ ! Ce3þ þ Fe3þ
(6)
2Fe3þ þ Fe ! 3Fe2þ
(7)
2Ce4þ þ Fe ! 2Ce3þ þ Fe2þ
(8)
Fig. 6. The relationship between inhibition efficiency and different concentration ratios of vanillin and Ce4+ at a total blend concentration of 500 mg lS1 in 1.0 M H2SO4.
The reactions indicated that the rare earth ion Ce4+ and Fe3+ (produced as a result of the above reaction (6)) had a depolarizing effect on the anodic reaction rate and accelerated the dissolution of base metal in acid. Furthermore, the standard electrode potential of Ce3+/Ce (2.48 V) is much lower than that of Fe2+/Fe (0.441 V), so it is impossible for the cerium ion (Ce4+ and Ce3+) to precipitate onto the steel surface [36]. 3.1.5. Synergism between cerium(IV) ion and vanillin Fig. 6 shows the inhibition efficiency in 1.0 M H2SO4 with different concentration ratios of vanillin and Ce4+ at a total blend concentration of 500 mg l1 at different temperatures. It can be seen that the synergism between cerium(IV) ion and vanillin is exhibited. Namely, the IE for Ce4+ in combination with vanillin is higher than the summation of IE for single Ce4+ and single vanillin, which is synergism in nature. For example, at 30 8C the IEs of 25 mg l1 vanillin and 475 mg l1 Ce4+ are only 24.1% and 17.5%, respectively, while the IE their complex is 97.8% at the same conditions. A high inhibition efficiency, 98% was obtained by a mixture of 25–200 mg l1 vanillin and 300–475 mg l1 Ce4+ at 20–30 8C. To further judge whether synergism is taking place, one has to calculate the synergism parameter (s), as initially proposed by Murakwa et al. [37] for describing the combined inhibition behaviour of amines and halide ions. Generally, for the interaction of inhibitors A and B this synergism parameter (s) is defined as follows: s¼
Fig. 5. Relationship between inhibition efficiency (IE) and concentration of cerium(IV) ion (C) in 1.0 M H2SO4.
1 IEA IEB þ IEA IEB 1 IEAB
(9)
where IEA and IEB are the inhibition efficiencies observed with compound A, respectively, B, acting alone, and IEAB is the experimentally observed inhibition efficiency for the mixture A + B (CA and CB in the mixture should be the same as in the corresponding separate situations). The expression actually compares the theoretically expected corrosion rate (numerator), based on the known rates when either A or B are present and on the condition that they do not interact, with the experimentally observed rate in the presence of the inhibitor mixture (dominator). Consequently, in the case where inhibitors A and B have no effect on each other and adsorb at the metal/solution interface independently, s = 1 as in that case the predicted behaviour is experimentally confirmed. Alternatively,
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Table 2 The synergism parameters of vanillin and rare earth Ce4+ on the corrosion of CRS in 1.0 M H2SO4 solution at different temperatures C(vanillin) (mg l1) 25 50 100 150 200 250 300 350 400 450 475
C(Ce4+) (mg l1) 475 450 400 350 300 250 200 150 100 50 25
Synergism parameter (s) 20 8C
30 8C
40 8C
50 8C
60 8C
6.51 10.12 11.35 7.16 6.29 2.62 2.43 2.01 1.89 1.72 1.52
26.42 19.66 27.70 13.70 11.90 5.60 2.34 2.14 2.10 1.79 1.65
2.32 3.23 4.62 3.70 3.26 2.62 1.94 1.53 1.32 1.24 1.19
1.37 1.99 2.35 2.35 2.06 1.76 1.71 1.62 1.50 1.22 1.14
1.11 1.29 1.48 1.50 1.34 1.29 1.09 1.15 1.05 1.01 0.99
synergistic effects manifest themselves if s > 1 and antagonistic effects if s < 1. Table 2 indicates that the values of s for all investigated concentrations of vanillin and Ce4+ in 1.0 M H2SO4 are bigger than the unity except for the complex of 475 mg l1 vanillin—25 mg l1 Ce4+ at 60 8C, which suggests that there is a true synergism between vanillin and Ce4+ in 1.0 M H2SO4. It should be noted that the values of s are higher than 5.0 for the mixture of 25–200 mg l1 vanillin and 300–475 mg l1 Ce4+ at 20–30 8C, which can be seemed the optimized synergistic conditions. Table 2 also shows that the synergism degree at different temperatures follows the general order: 30 8C > 20 8C > 40 8C > 50 8C > 60 8C.
Table 3 Potentiodynamic polarization parameters for the corrosion of CRS in 1.0 M H2SO4 containing different concentrations of vanillin at 20 8C C (mg l1)
Ecorr (mV)
Icorr (mA cm2)
bc (mV dec1)
ba (mV dec1)
IE (%)
0 25 100 250 500
443.3 448.9 434.2 431.6 434.4
828.0 478.9 400.7 317.1 262.5
125 110 124 130 133
46 39 37 36 39
42.3 51.6 61.7 68.3
The values of corrosion current density (Icorr), corrosion potential (Ecorr), the cathodic Tafel slope (bc), anodic Tafel slope (ba), and the inhibition efficiency as functions of vanillin concentration, were calculated from the curves Fig. 7 and given in Table 3. Table 3 reveals that the corrosion current (Icorr) decreases and IE increases with the inhibitor concentration. The maximum IE is 68.3%, which also indicates vanillin has a moderate inhibitive effect. Good agreement between weight loss and polarization curve is obtained. Both the cathodic Tafel slopes (bc) and the anodic Tafel slopes (ba) do not change remarkably, which indicates that the mechanism of the corrosion reaction does not change and the corrosion reaction is inhibited by simple adsorption mode [39]. In the presence of vanillin, the corrosion potential (Ecorr) does not change remarkably, which indicates that vanillin acts as a mixed-type inhibitor [39].
3.2.1. Polarization curve of single vanillin The polarization behaviour of CRS in 1.0 M H2SO4 in the absence and presence of different concentrations of vanillin at 20 8C is shown in Fig. 7. It is clear that the inhibitor causes a decrease in the corrosion rate, i.e. shifts the anodic curves to positive potentials and the cathodic curves to negative potentials. This may be ascribed to adsorption of inhibitor over the corroded surface [38]. As the concentration of vanillin increases, the shift in the cathodic line increases. The inhibitor has an inhibitive effect on the rate of hydrogen evolution reaction. On the other hand, the inhibitor has slight effect on the anodic curves.
3.2.2. Polarization curve of single Ce4+ Both anodic and cathodic polarization curves for CRS in 1.0 M H2SO4 at various concentrations of Ce4+ are shown in Fig. 8. It can be seen that both the anodic and cathodic reactions of electrode are not inhibited by Ce4+, and the corrosion potentials do not change remarkably, either, compared with the blank. Although Fig. 8 shows that the cathodic curve in the presence of Ce4+ changes compared with the blank, the changes exhibited by the cathodic branch are not significant enough to indicate that Ce4+ inhibits the cathodic reaction of steel. The potentiodynamic polarization parameters are listed in Table 4. Table 4 shows that the corrosion current (Icorr) decreases and IE increases with Ce4+ concentration. However, the maximum IE was only 28.3%. In the presence of Ce4+, the corrosion potential (Ecorr), cathodic Tafel slopes (bc) and anodic Tafel slopes (ba) change slightly, which indicates that the corrosion mechanism of steel does not change. From Table 4, it can be concluded that IEs obtained from weight loss, electrochemical polarization curves are
Fig. 7. Potentiodynamic polarization curves for CRS in 1.0 M H2SO4 containing different concentrations of vanillin at 20 8C.
Fig. 8. Potentiodynamic polarization curves for CRS in 1.0 M H2SO4 containing different concentrations of cerium(IV) ion at 20 8C.
3.2. Polarization studies
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Table 4 Potentiodynamic polarization parameters for the corrosion of CRS in 1.0 M H2SO4 containing different concentrations of rare earth Ce4+ at 20 8C C (mg l1)
Ecorr (mV)
Icorr (mA cm2)
bc (mV dec1)
ba (mV dec1)
IE (%)
0 25 100 250 500
443.3 441.2 441.9 439.7 440.8
828.0 742.7 698.8 648.3 593.7
125 123 119 114 122
46 49 50 45 49
10.3 15.6 21.7 28.3
Fig. 10. UV–vis spectra of the inhibitors for CRS in 1.0 M H2SO4 (vs. 1.0 M H2SO4).
Fig. 9. Potentiodynamic polarization curves for CRS in 1.0 M H2SO4 containing different concentrations of vanillin and Ce4+ at 20 8C.
in good agreement. Namely, cerium(IV) ion has a negligible effect on the corrosion rate of CRS in 1.0 M H2SO4. 3.2.3. Polarization curve for synergistic inhibition between vanillin and Ce4+ Fig. 9 shows the polarization curves for CRS in 1.0 M H2SO4 at various concentrations of vanillin and Ce4+ at a total blend concentration of 500 mg l1 at 20 8C. Clearly, both anodic and cathodic reactions are drastically inhibited, and the phenomenon is evident in the presence of the complex 25 mg l1 vanillin – 475 mg l1 Ce4+ and 100 mg l1 vanillin – 400 mg l1 Ce4+. On the other hand, the complex of 250 mg l1 vanillin – 250 mg l1 Ce4+ and 400 mg l1 vanillin – 100 mg l1 Ce4+ have slight effect on the anodic curves, thus the synergistic degree decrease. The reason may be explained as follows: The new complex of Ce4+–vanillin could be formed and covering both anodic and cathodic reactive sites, which inhibited the anodic and cathodic reactions of steel corrosion. It should be noted that some of the vanillin molecules are still free in the acid solution. So it is expected that there may be a competition between vanillin and the Ce4+– vanillin complex to form a protective film onto the steel surface. Owing to the new complex of Ce4+–vanillin covers a large fraction steel surface as compared to the case of single vanillin additive, the IE of Ce4+–vanillin complex is higher than IE of single vanillin.
When Ce4+ concentration is relatively high (>400 mg l1), the new complex amount is high, and efficiently inhibits the anodic reaction (shifting the anodic curves to more positive potentials). In contrast, when Ce4+ concentration is relatively low (<250 mg l1), the new complex amount is low and does not efficiently inhibits the anodic reaction (slight effect on the anodic curves). This finding might suggest the CRS surface is protected by film most probably composed from adsorbed Ce4+–vanillin complex. The electrochemical parameters have been calculated and listed in Table 5. The complexes of 25 mg l1 vanillin – 475 mg l1 Ce4+ and 100 mg l1 vanillin – 400 mg l1 Ce4+ (the strong synergistic inhibition blended concentration) make the corrosion potential (Ecorr) shift to anodic direction. Both cathodic Tafel slopes (bc) and anodic Tafel slopes (ba) change remarkably, and the corrosion current (Icorr) decreases remarkably, which means that both anodic and cathodic reactions are drastically inhibited and has a satisfactory IE. For example, the IE calculated from corrosion current density for the complex 25 mg l1 vanillin— 475 mg l1 Ce4+ inhibitor reaches a considerable value (98.9%). The similar results were also reported with the synergism between organic inhibitor and chloride in H2SO4 media [40]. 3.2.4. Ultraviolet and visible spectrophotometer (UV–vis) The UV–vis absorption spectra of 25 mg l1 vanillin, 25 mg l1 4+ Ce , and the mixture of 25 mg l1 vanillin and 475 mg l1 Ce4+ (without CRS immersion) in 1.0 M H2SO4 were shown in Fig. 10. For single Ce4+, a faint wide peak appears at 276.50 nm. For single vanillin, the spectra absorption maximum wavelength is at 203.00 nm, and other three speaks are at 229.50, 278.50 and 309.00 nm, respectively. But for the mixture of 25 mg l1 vanillin and 475 mg l1 Ce4+, the absorption maximum wavelength appears at 254.40 nm with a shoulder at 222.80 nm. Consequently, the absorption spectra of the mixture are quite different from those of single vanillin or single Ce4+. Thus, the new complex of vanillin–Ce4+ is formed in 1.0 M H2SO4.
Table 5 Potentiodynamic polarization parameters for the corrosion of CRS in 1.0 M H2SO4 containing different concentrations of vanillin and Ce4+ at 20 8C C (vanillin) (mg l1)
C (Ce4+) (mg l1)
Ecorr (mV)
Icorr (mA cm2)
bc (mV dec1)
ba (mV dec1)
IE (%)
0 25 100 250 400
0 475 400 250 100
443.3 413.7 428.3 438.7 440.8
828.0 9.2 27.8 58.2 76.2
125 98 166 136 131
46 26 41 39 42
98.9 96.6 93.0 90.8
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Fig. 11. FTIR spectra of CRS specimens after immersion in various solutions (a) after immersion in 1.0 M H2SO4; (b) after immersion in 1.0 M H2SO4 + 100 mg lS1 vanillin + 400 mg lS1 Ce4+.
3.3. FTIR spectra Fourier transform infrared spectroscopy is a well-established characterization tool offering a ‘fingerprint’ for chemical compounds. The FTIR reflectance spectrum of corrosion layer formed on the CRS surface after immersion in 1.0 M H2SO4 is shown in Fig. 11(a). The band at 3395 cm1 is attributed to O–H stretching and 1659 cm1 is for O–H bending. There is a wide peak at 1032 cm1, which is attributed to the S O stretching of SO42. According to the literature [41], the S O stretching frequency of SO42 absorbs at
1024 cm1 for the corrosion layer formed on steel surface. The SO42 in CRS corrosion layer comes from the corrosion product of FeSO4. The strong band at 822 cm1 arises from FeOOH [42]. The FTIR reflectance spectrum of the protective film formed on the CRS surface after immersion in 1.0 M H2SO4 containing 100 mg l1 vanillin and 400 mg l1 Ce4+ is shown in Fig. 11(b). The O–H stretching has the band at 3368 cm1, and O–H bending at 1655 cm1. The weak band at 3938 cm1 is attributed to Fe–O bending, which indicates the coordinate bond formed by partial transference of electrons from the polar atom (O atom) of vanillin to the metal surface. The band at 1294 cm1 is the characteristic band Ce–O bond [43], which may indicate the adsorption film of Ce4+–vanillin complex. The bond at 1104 cm1 is attributed to C–O bond in methoxy (–OCH3) and 738 cm1 is for C–H bending in – CH3. The result may indicate that the organic inhibitor vanillin could adsorb onto the CRS surface in 1.0 M H2SO4. FeOOH was also found for the band at 844 cm1 [44]. Furthermore, due to the spectra in the experiment were obtained in the region from 650 to 4000 cm1, and Fe3O4, FeO and a-Fe2O3 only have the bands in the region from 300 to 600 cm1 corresponds to the steel-oxygen bond, so the ferrous oxides of Fe3O4, FeO and aFe2O3 can not be detected by using FTIR in this paper [42]. 3.4. X-ray photoelectron spectroscopy (XPS) The XPS survey scan of CRS immersed in 1.0 M H2SO4 inhibited by 100 mg l1 vanillin and 400 mg l1 Ce4+ for 6 h at 20 8C is shown in Fig. 12. It contains Fe, O, C, Ce, S and adventious contaminates such as Ca. High resolution spectra of the C 1s peak is shown in
Fig. 12. Survey spectra of XPS analysis for CRS surface exposed to a (1.0 M H2SO4 + 100 mg lS1 vanillin + 400 mg lS1 Ce4+) solution for 6 h at 20 8C.
Fig. 13. High resolution C 1s spectra of XPS analysis for CRS surface exposed to a (1.0 M H2SO4 + 100 mg lS1 vanillin + 400 mg lS1 Ce4+) solution for 6 h at 20 8C.
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Fig. 14. High resolution S 2p spectra of XPS analysis for CRS surface exposed to a (1.0 M H2SO4 + 100 mg lS1 vanillin + 400 mg lS1 Ce4+) solution for 6 h at 20 8C.
Fig. 15. High resolution O 1s spectra of XPS analysis for CRS surface exposed to a (1.0 M H2SO4 + 100 mg lS1 vanillin + 400 mg lS1 Ce4+) solution for 6 h at 20 8C.
Fig. 13. Four peaks are evident independently of the microstructure: 284.8, 286.4, 288.2 and 289.6 eV. The peak of binding energy at 284.8 eV is related to the presence of cementite (Fe3C) [45]. The BE at 286.4 eV corresponds to residual or adventious carbon [46,47]. The peak at 288.2 eV is due to C O and the peak at the highest binding energy of 289.6 eV might be due to O–C O or Ce C O, which may be indicated that the complex of Ce4+– vanillin was adsorbed on the steel surface. Fig. 14 presents the S 2p high resolution XPS spectra. There is only one peak of BE at 169.4 eV, which is attributed to the characteristic peak of SO42 [47].
Fig. 15 shows the O 1s high resolution XPS spectra. Three distinct peaks are observed: the one at the lowest binding energy at 529.6 eV presents the oxide bond (MO), the 531.1 eV peak is due to metal hydroxide (MOH) and the peak at the highest binding energy of 532.8 eV is due to metal water bonds (MH2O) [48]. The high resolution spectra of Ce 3d peak are shown in Fig. 16. Three peaks are evident independently of the microstructure: 881.8, 885.7, and 888.4 eV. The peak of binding energy at 881.8 eV is related to the presence of CeO2 [47]. In addition, a slight yellow film was observed on the CRS surface, which may be indicated the Ce4+ compounds were in the presence of the film. The BE at
Fig. 16. High resolution Ce 3d spectra of XPS analysis for CRS surface exposed to a (1.0 M H2SO4 + 100 mg lS1 vanillin + 400 mg lS1 Ce4+) solution for 6 h at 20 8C.
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Fig. 17. Fe 2p spectra for: (a) cold rolled steel before immersion (b) oxidized steel surface exposed to a solution (1.0 M H2SO4 + 100 mg lS1 vanillin + 400 mg lS1 Ce4+) for 6 h at 20 8C.
885.7 eV is a characteristic of Ce3+ compounds such as Ce2O3, Ce(OH)3 [47], but the compounds of Ce2O3 and Ce(OH)3 could not be produced in the strong acidic solution [49], so the Ce3+ compounds of Ce2O3 and Ce(OH)3 may be attributed to the reduction process of the film in the air form Ce4+ compounds. The peak at 888.4 eV may be ascribed to the complex of Ce4+–vanillin. The XPS spectra for Fe 2p region are shown in Fig. 17. Fig. 17(a) shows that a peak for Fe 2p at BE 707.4 eV characteristic of Fe0, but the substrate covered with the blend of vanillin and rare earth Ce4+ exhibited a peak for Fe 2p at BE 710.6 eV (Fe 2p3/2) and a peak at 723.9 eV (Fe 2p1/2) characteristic of Fe3+, showing the subsequent oxidation of the steel surface (Fig. 17(b)). The increase in Fe 2p signal confirmed the adsorption and film formation on the steel surface [50]. 3.5. Atomic force microscope (AFM) surface examination The atomic force microscope provides a powerful means of characterizing the microstructure [51–54]. The two-dimensional AFM images of CRS surface in 1.0 M H2SO4 are shown in Fig. 18. Fig. 18(a) shows clearly the CRS surface before immersion seems smooth and shows no evident corrosion products on the surface. However, it is not absolute smooth and uniform, and appears black holes and crevices, which may be attributed to the defect of steel, and probably an oxide inclusion. As for Fig. 18(b), the CRS surface after immersion in uninhibited 1.0 M H2SO4 for 6 h was damaged strongly comparing with Fig. 18(a), and covered with the corrosion products. It appeared to be relatively uniform in general, and there were no evident black holes or crevices because of the corrosion products precipitated in the defect. The image is quite different from the pitting corrosion image [53] or the corrosion in HClO4–CH3COOH solution [55]. Fig. 18(c) shows that there are some spherical or bread-like particles on the steel surface in the presence of 100 mg l1 vanillin, which do not exist in the Fig. 18(b). Therefore, it may be concluded that these particles are the adsorption film of the inhibitor, which inhibits the corrosion of CRS [28,29]. Fig. 18(d) also shows that the steel surface in the presence of Ce4+ appears some a layer image, but the roughness of the surface layer is high compared to the roughness of the film formed in the presence of vanillin. In contrast, Fig. 18(e) shows that the steel surface in the presence of both vanillin and Ce4+ is fully and orderly covered with unique particles, which are quite different form the corrosion product particles (Fig. 18(b)) and do not exist in the Figs. 18(c) and (d), and could be ascribed to the adsorption of the complex Ce4+–vanillin. The particles are more distributed and rather
uniform, and the outer surface of the particles is enwrapped with granular aggregates tightly and regularly. Fig. 18(e) also shows that the film on the CRS surface becomes denser and more tightly comparing with Fig. 18(c) and (d); so, it can efficiently protect CRS from corrosion comparing single vanillin or Ce4+. Fig. 19 shows the CRS surface topography. It can be seen from Fig. 19(a) that the micrograph of CRS surface before immersion shows no evident particles on the surface. Fig. 19(b) shows that the steel surface after immersion in uninhibited 1.0 H2SO4 shows the main characteristic of uniform corrosion in acidic media [28,29]. Fig. 19(c) and (e) shows that some spherical particles decorating the steel surface and the retention of surface grinding marks, which indicate a macroscopically thin film, and the particles on the steel surface are the smallest for the presence of both vanillin and Ce4+. Fig. 19(d) shows that the particles are relative bigger. Fig. 20 illustrates the height profiles, which are made along the lines marked in corresponding Fig. 19. The surface roughness of the CRS before immersion is 134.45 nm from Fig. 20(a). Fig. 20(b) indicates that the surface roughness of the CRS after immersion in uninhibited 1.0 H2SO4 is up to 463.08 nm, while in the presence of single vanillin, the roughness decreases to 303.74 nm (Fig. 20(c)). In the presence of single Ce4+, the roughness increases to 578.56 nm (Fig. 20(d)), which is higher than that of immersion in uninhibited H2SO4. Fig. 20(e) shows that the surface roughness is only 206.59 nm in the presence of both vanillin and Ce4+. In general, the result of the surface roughness agrees with that of Fig. 18. 3.6. Explanation for synergism It is well known that steel corrosion reaction in 1.0 M H2SO4 may be split into the anodic partial reaction (steel dissolution) and the cathodic partial reaction (hydrogen evolution). Vanillin (4-hydroxy-3-methoxy-benzaldehyde) contains oxygen atoms with lone-pair electrons, so it can adsorb onto the steel surface through the lone pair of unshared electrons available in the oxygen atom of vanillin, which inhibits both anodic and cathodic reactions of steel, while prominently inhibits the cathodic reaction. However, there is a repulsion force among the adsorbed vanillin inhibitors onto the steel surface due to f > 0 (Table 1). As a result, the inhibition efficiency is not high for single vanillin. In the case of addition of rare earth Ce4+ + vanillin mixture, the results demonstrated the inhibition efficiency was higher than the
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Fig. 18. AFM two-dimensional images of CRS surface: (a) before immersion; (b) after 6 h of immersion at 20 8C in 1.0 M H2SO4; (c) after 6 h of immersion at 20 8C in 100 mg lS1 vanillin + 1.0 M H2SO4; (d) after 6 h of immersion at 20 8C in 400 mg lS1 Ce4+ + 1.0 M H2SO4; (e) after 6 h of immersion at 20 8C in 100 mg lS1 vanillin + 400 mg lS1 Ce4+ + 1.0 M H2SO4.
summation of inhibition efficiency for single Ce4+ and single vanillin. The values of synergistic parameter (s) are higher than unity (Table 2), which indicates there is a true synergism between vanillin and Ce4+ in 1.0 M H2SO4. In order to explain the fact, the following synergism mechanism is proposed: Rare earth Ce4+ belongs to lanthanide in periodic table of chemical elements, and it has a lot of vacant orbits (4f, 5d and 6s). There are lone-pair electrons among vanillin molecule. So, when vanillin was mixed with Ce4+, the new complex of Ce4+–
vanillin was easily formed (Fig. 10), and the complex played an important role in the enhancement of the protection of CRS against corrosion. It should be noted that some of the vanillin molecules are still free in the acid solution when the ratio of vanillin is high. Though there is no reference available in the literature on the exact structure of the complex of Ce4+–vanillin, it is doubtless that the higher ratio of vanillin, the more free vanillin molecules in the acid solution. So it is expected that there is a competition between vanillin and the Ce4+–vanillin
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Fig. 19. AFM images of CRS surface topography: (a) before immersion; (b) after 6 h of immersion at 20 8C in 1.0 M H2SO4; (c) after 6 h of immersion at 20 8C in 100 mg lS1 vanillin + 1.0 M H2SO4; (d) after 6 h of immersion at 20 8C in 400 mg lS1 Ce4+ + 1.0 M H2SO4; (e) after 6 h of immersion at 20 8C in 100 mg lS1 vanillin + 400 mg lS1 Ce4+ + 1.0 M H2SO4.
complex to form a protective film onto the steel surface. This competition may slow down the rate of film formation and consequently reduce the percentage inhibition efficiency [56]. Although the experiment results (Figs. 6 and 9) revealed that IE value decreases when the ration of vanillin is higher than the ratio of Ce4+, its value is still high as compared with that obtained in case of using different concentration of vanillin separately. This finding might suggest the CRS surface is protected by film most probably composed from adsorbed Ce4+–vanillin complex. This assumption could be further confirmed by the FTIR and XPS
results. Owing to the new complex of Ce4+–vanillin covering a large fraction steel surface as compared with the case of single vanillin additive, the IE of Ce4+–vanillin complex is higher than IE of single vanillin. The complex could adsorb onto steel surface by the van der Waals force 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. In this case, vanillin molecules may have played a role in bridging the access of rare earth Ce4+ to the metal surface from
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Fig. 20. Height profiles of the CRS surface: (a) before immersion; (b) after 6 h of immersion at 20 8C in 1.0 M H2SO4; (c) after 6 h of immersion at 20 8C in 100 mg lS1 vanillin + 1.0 M H2SO4; (d) after 6 h of immersion at 20 8C in 400 mg lS1 Ce4+ + 1.0 M H2SO4; (e) after 6 h of immersion at 20 8C in 100 mg lS1 vanillin + 400 mg lS1 Ce4+ + 1.0 M H2SO4.
the solution. So, it is concluded that Ce4+ and vanillin demonstrated a strong inhibition synergism for CRS. 4. Conclusion 1. Vanillin acts as a moderate inhibitor for the corrosion of CRS in 1.0 M H2SO4, and the adsorption obeys the Temkin adsorption isotherm. Rare earth cerium(IV) ion has a negligible effect on the corrosion rate of CRS, and the maximum IE is only about 20%. 2. There is a strong synergism between vanillin and Ce4+ in 1.0 M H2SO4. Namely, the IE for Ce4+ in combination with vanillin is
higher than the summation of IE for single Ce4+ and single vanillin. The values of synergism parameter (s) are higher than unity. 3. Vanillin acts as a mixed-type inhibitor in sulfuric acid, while prominently inhibits cathodic reaction. Ce4+ has a negligible effect on both anodic and cathodic reactions. However, the vanillin–Ce4+ mixture behave as a mixed-type inhibitor, which drastically inhibits both anodic and cathodic reactions. 4. The new complex of Ce4+–vanillin was formed in 1.0 M H2SO4 for Ce4+ in combination with vanillin, which played an important role in the synergism mechanism. The protective
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