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Self-assembled urea-amine compound as vapor phase corrosion inhibitor for mild steel
Surface & Coatings Technology 204 (2010) 1646–1650
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Self-assembled urea-amine compound as vapor phase corrosion inhibitor for mild steel Da-Quan Zhang ⁎, Li-Xin Gao, Guo-Ding Zhou Department of Environmental Engineering, Shanghai University of Electric Power, Shanghai 200090, PR China
a r t i c l e
i n f o
Article history: Received 1 June 2009 Accepted in revised form 30 October 2009 Available online 10 November 2009 Keywords: Surface properties Adsorption Corrosion Electrochemical techniques
a b s t r a c t VPIs (vapor phase corrosion inhibitors) extend their corrosion-inhibiting properties by volatilization and condensation to form a protective film on metal surface. Vapor phase self-assembled films of VPIs have become an attractive means to prevent metal from corrosion. This new methods meet the rust-proof challenges for the steel materials with many advantages. In this paper, bis-(cyclohexylaminomethyl)-urea (BCMU) was developed as VPIs for mild steel. Its corrosion inhibition property was studied by volatile inhibiting sieve test (VIS) and electrochemical measurements. BCMU has good protection effect for mild steel. It suppresses the anodic corrosion reaction and renders the corrosion potential to more positive values. Its interaction with steel surface was investigated by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The results suggest that the BCMU interacts with the ferric ions via N and O atoms in its molecule. BCMU inhibits the corrosion processes by blocking the active iron dissolution. It can stabilize the oxide layer and decrease the surface roughness. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved.
1. Introduction A vapor phase corrosion inhibitor (VPI) is a compound that has the ability to vaporize and condense on a metallic surface to make it less susceptible to corrosion. The main advantage of VPIs compared with conventional corrosion control methods stems from their gas-phase transport. A VPI reaches the metallic surfaces without contacting the surface directly. The efficacy, convenience, and cost effectiveness of VPIs made their application for rust control almost universal in automotivemanufacturing, steel-making, ship-building, power generation, and defense production. As with all industrial chemical products, however, the increased usage of VPIs has also raised significant scientific interest as to their safety. Cumulative research over the last 30 years has, unfortunately, proven that certain specific VPI formulations can, in fact, be toxic [1]. For example, dicyclohexyl ammonium nitrite (DICHAN) has been found to be most effective for inhibiting the atmospheric corrosion of steel, and gained industrial application for several decades [2]. Research confirmed that some N-nitrosoamines, including those generated by DICHAN, were not only carcinogenic, but also genotoxic as well. Thus, alternative of environmental-friendly VPIs is under consideration [3–6]. Urea is one of the main ingredients of corrosion inhibitor formulations for many volatile corrosion-inhibiting papers. It is cheap and has been used for many years as a feed additive for animals. But the disadvantage is its too high vapor pressure to give lasting rust prevention [7].
⁎ Corresponding author. Tel./fax: +86 21 65700719. E-mail address: [email protected] (D.-Q. Zhang).
On the other hand, mechanistic information on corrosion and inhibition processes is very important for proper selection of inhibitors. In general, the inhibition mechanism proposed for VPIs, is mainly based on its adsorption on the metal surface whereby a thin monomolecular barrier film should be formed [8]. The metal surface is usually covered with a thin oxide layer under near neutral atmospheric conditions. It is an irregular physical surface with grains of different sizes and orientations, grain boundaries and defects, and most important, a chemistry that is far from that of a pure metal [9]. Thus, the adsorption of VPIs on a metal surface may not be uniform and therefore the protection mechanism of VPIs may involve a combination of passivation and adsorption. In this paper, a urea derivative, bis-(cyclohexylaminomethyl)-urea (BCMU), was developed as a novel VPIs for mild steel. BCMU has one urea moiety and two cyclohexamine moieties, which molecular structures are shown in Fig. 1. Its inhibition effectiveness was investigated by volatile inhibiting sieve test (VIS) and electrochemical measurements. The interactions of BCMU with the steel were characterized by X-ray photoelectron spectroscopy (XPS) analysis. The vapor phase selfassembled film on the steel surface was studied by AFM measurement. 2. Experimental details 2.1. Materials and apparatus Steel specimens (50 mm × 25 mm × 1.5 mm) were used for volatile inhibiting sieve test (VIS). BCMU was synthesized by using cyclohexamine, urea, and formaldehyde solutions in the laboratory according
0257-8972/$ – see front matter. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.10.054
D.-Q. Zhang et al. / Surface & Coatings Technology 204 (2010) 1646–1650
1647
Fig. 1. Molecular structures of bis-(cyclohexylaminomethyl)-urea (BCMU).
to a procedure reported previously [10]. The melting of platelets was at 214 °C–216 °C. A volatile corrosion inhibitor monitor cell (VCIM) consisted of 20 foils of mild steel (80 mm × 8.0 mm × 0.5 mm), with each steel plate separated by a 170 μm thick layer of an insulating sheet [11]. Electrochemical measurement was performed using a PARC M283 potentiostat (EG&G) and PARC Model 1025 frequency response analyzer. The XPS experiments were performed on a PHI 550 ESCA/SAM spectrometer (Perkin-Elmer). A commercial AFM with contacting mode (Zhejiang University of China, AFM&STM-II) was used to characterize the surface morphology and roughness before and after VPI adsorption.
Fig. 2. Potentiodynamic polarization curves for steel electrode after 30 min immersion in simulated atmospheric corrosion solution (1— blank, 2— 0.2% BCMU).
3. Results and discussion 2.2. Procedures 3.1. Inhibition of BCMU film on the corrosion of mild steel 2.2.1. Volatile corrosion inhibition test The volatile inhibiting sieve test (VIS) was conducted to evaluate the inhibition effect of BCMU. The test process was reported in an earlier publication [6]. After corrosion tests, samples were removed for visual inspection and weight-loss determination. Corrosion rates and inhibitor effectiveness were calculated.
2.2.2. Electrochemical measurements Electrochemical measurements were conducted both in stimulated atmospheric corrosion solution and under thin electrolyte layer. A three-electrode cell was used for solution electrochemical measurements. The stimulated atmospheric corrosion solution was prepared by using double-distilled water and containing Cl− 100 mg/L, HCO− 3 100 mg/L kg/m3, SO2− 4 100 mg/L, respectively. The working electrode was polarized at a speed of 1 mV/s. The EIS experiments were performed at open circuit potential over a frequency range of 0.05 Hz to 100 kHz. The sinusoidal potential perturbation was 5 mV in amplitude. The potential values reported here were versus SCE. The cell was open to the laboratory air and the measurement was conducted without agitation at room temperature (25 °C). A volatile corrosion inhibitor monitor cell (VCIM) was used to obtain electrochemical impedance data under thin electrolyte layer [12]. The VCIM was placed upon a 300 mL-beaker, which contained a lid of 5 g inhibitors. The edges of the polished steel foils faced down to the lid. After a specific period of film-forming time of the inhibitor, the VCIM was then covered with a filter paper saturated by stimulated atmospheric solution. The VCIM was placed in a vessel at a relative humidity of 90%.
After the volatile inhibiting sieve test, the specimens treated by BCMU were bright in almost all areas. The corrosion rate and inhibition effectiveness for the BCMU film-forming specimens were 28.57 mg m− 2 h− 1 and 90.84%, respectively. This shows that BCMU can be volatile and adsorbed on the mild steel surface to protect the steel. 3.2. Electrochemical measurements in stimulated atmospheric corrosion solution Fig. 2 shows the polarization curves for freshly polished mild steel electrode after 30 min immersion in stimulated atmospheric corrosion solution. The anodic curve for the steel electrode exhibits an active–passive behavior. The cathodic portion of the polarization curve for blank is a composite and represents oxygen reduction. The addition of 0.2% BCMU has no special effect for the pH values of electrolyte solution. It renders Ecorr more noble to −497.0 mV (SCE). The anodic curve for the steel electrode containing BCMU shows an initial region of anodic active behavior and then a passive region. Inhibitor-induced passive behavior was confirmed by a marked decrease in anodic currents compared with the blank solution [14]. In the presence of BCMU, both the cathodic and anodic current densities were greatly decreased near Ecorr region. BCMU decreases the anodic reaction rate more strongly than the cathodic reaction rate. The free corrosion potential shifts to more positive values. Fig. 3 shows the impedance Nyquist plots of a steel electrode in stimulated atmospheric corrosion solution with and without BCMU.
2.3. XPS and AFM measurements As for XPS measurement, the specimens were mounted on a steel stopper using a molybdenum mask to assure good electrical contact. The analyses were done using a Mg Kα X-ray source at 10 kV and 400 W. All analyses were performed at a pressure below 1.33 × 10− 6 Pa. Charge referencing was determined by setting the main C 1s component at 285.0 eV. The AFM apparatus setup was described in an earlier research [13]. The morphology of the specimens was observed by a contacted AFM with a silicon nitride (Si3N4) probe. A 10 mW laser diode was used on the AFM unit at a wavelength of 660 nm. The roughness (Ra) calculated by the software package was adopted to compare the surface quality.
Fig. 3. Nyquist plots for steel electrode after 30 min immersion in simulated atmospheric corrosion solution (1— blank, 2— 0.2% BCMU).
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D.-Q. Zhang et al. / Surface & Coatings Technology 204 (2010) 1646–1650
Fig. 5 represents the variation of both ROX and COX with the immersion time. It is obvious that the ROX values increased sharply from 23.03 kΩ cm2 to 82.5 kΩ cm2 during the initial 8 days and remained slightly increasing afterward. At the same time, the capacitance values were reduced drastically from 88.3 µF cm− 2 to 48.2 µF cm− 2. These results demonstrate that the formation of the ferric-inhibitor complexes was relatively slow and the completed plugging in the defects reaches within 5 days. Fig. 4. The equivalent circuit used to fit the EIS data for steel in simulated atmospheric corrosion solution.
The equivalent circuit model employed for this system is shown in Fig. 4. This is a two-time constant model due to the presence of both passive and corroding regions on these surfaces [9]. The passive regions refer to the oxide layer on steel surface, while the corroding regions refer to the oxide layer breakdown. RS is the electrolyte resistance, COX and ROX can be attributed to the capacitance and resistance of the passive surface, respectively. Cad and Rad are the capacitance and resistance of corroding regions. Since the electrochemical systems show various types of inhomogeneities, COX can be better substituted by a constant phase element (CPE, Q). The CPE element was introduced formally only for fitting impedance data. It was found that the presence of BCMU enhances the ROX values from 2.531 to 23.03 kΩ cm2 and reduces the Q values from 161 μF cm− 2 to 88.3 μF cm− 2. The decrease in Q may be due to the adsorption of BCMU in the defects of the metal surface oxide layer. The increases of ROX values may be due to BCMU's blocking of the surface sites of active iron dissolution to stabilize the oxide layer [9].
3.3. Electrochemical impedance measurements under thin electrolyte layer The atmospheric corrosion of mild steel is an electrochemical process that was conducted under thin electrolyte layer. The thin
Fig. 5. Variation of both ROX and QOX with immersed time in simulated atmospheric corrosion solution.
Fig. 6. Nyquist plots for a VCIM under thin corrosion water layer (1— blank, 2— after 8 h BCMU film-forming).
Fig. 7. XPS plots of adsorption of main elements in complex film after the adsorption of BCMU on the steel surface: (a) Fe 2p3/2; (b) N 1s; and (c) O 1s.
D.-Q. Zhang et al. / Surface & Coatings Technology 204 (2010) 1646–1650 Table 1 Binding energies (eV) of the inhibition film on steel surface. Fe 2p3/2
BCMU
N 1s
O 1s
1
2
1
2
1
2
3
708.24
710.56
398.11
399.70
529.44
531.52
532.87
1649
electrolyte layer was formed by adsorption, condensation or precipitation of water in humid air. Fig. 6 shows Nyquist plots for a VCIM obtained with and without a period of BCMU film-forming. The VCIM electrode impedance with BCMU film-forming was higher than the blank electrode. The corrosion protectiveness of BCMU was also proved under thin electrolyte layer. 3.4. XPS and AFM measurements In spite of the fact that VPIs have been widely used for inhibiting atmospheric corrosion, the action mechanism of these compounds is not completely clear. The adsorption of VPIs on a metal surface can be determined by XPS analysis. Fig. 7 shows the Fe 2p3/2, N 1s, and O 1s XPS peak measured on the surface of steel samples after exposure to 48 h of VPI film-forming periods. The results of bonding energy are summarized in Table 1. It has been reported that the Fe 2p3/2 peak at 710.56 eV corresponded to the oxide state and the Fe 2p3/2 peak at 708.24 eV corresponded to the metallic Fe atom [15]. A shift of binding energy can be explained by the interaction of Fe and N atom. Thus, the Fe 2p3/2 peak at 708.24 eV may be attributed to the Fe atom, which is bonded to the VPI molecule. Two kinds of N 1s peaks are present in the XPS spectra, which indicate that a BCMU film is present on the surface of the steel. The N 1s peak at 398.11 eV corresponded to the N atom in the organic VPI molecule, and the peak at 399.70 eV to the N atom interacted with the Fe surface [16,17]. The bonding energy of the O 1s peak on the steel surface was 529.0–532.9 eV, which could correspond to O in the ferrous oxide (FeO), O in the inhibitor, and O in the VPI molecule that interacted with the Fe surface. Thus, BCMU acts to block the active iron dissolution by forming insoluble ferricinhibitor complexes via the N and O heteroatoms. The protection due to the action of BCMU was monitored in situ with an AFM while varying the exposure time. AFM images are shown in Fig. 8. The surface morphology of the steel sample before exposure to BCMU volatilization indicates there were a few scratches from the mechanical polishing treatment. The roughness is 2.7 nm. It shows a thin covering surface film composed of many particles after exposure to BCMU volatilization. The steel surface after 2 h BCMU exposure was rougher than that before BCMU film-forming. As the exposure time increased from 2 h to 48 h, the roughness of the steel surface decreased from 3.7 nm to 1.8 nm. This can be attributed to more BCMU molecular aggregators formed on the steel surface. The investigation by Leng and Stratmann [18] suggested that adsorption of VPI may form a few protective monolayers. However, the oxide layer covered on the steel surface is seldom homogeneous and has many defects. BCMU is able to suppress the corrosion reaction at the defect sites. Thereby, the effect of the oxide layer stabilization can be caused by an adsorption of BCMU on active surface sites. 4. Conclusions Bis-cyclohexylaminomethyl-urea (BCMU) has been proved to be a good volatile corrosion inhibitor for the steel corrosion. BCMU suppressed the anodic reaction of steel electrode and renders the corrosion potential to more noble direction. XPS measurements suggest that the BCMU interacts with the ferric ions via N and O atoms in its molecule. AFM images have shown that BCMU can stabilize the oxide layer and decrease the surface roughness. Acknowledgments
Fig. 8. AFM images of the steel surface: (a) initial surface; (b) after 2 h exposure for BCMU film-forming; and (c) after 48 h exposure for BCMU film-forming.
Supports from the National Natural Science Foundation of China (20576069 and 20911140272) and from the New Century Excellent Talents Program (NCET-08-0895) from the Ministry of Education in China are gratefully acknowledged. This work was carried out in the Key Laboratory of Shanghai Colleges and Universities for Electric Power Corrosion Control and Applied Electrochemistry.
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References [1] G.P. Lynch, A.J. Henderson, Metal Finish. 11 (2004) 1. [2] A. Subramanian, M. Natesan, V.S. Muralidharan, K. Balakrishnan, T. Vasudevan, Corrosion 56 (2000) 144. [3] D.Q. Zhang, L.X. Gao, Mater. Perform. 42 (2003) 40. [4] L.R.M. Estevao, R.S.V. Nascimento, Corros. Sci. 43 (2001) 1133. [5] M.A. Quraishi, D. Jamal, Corrosion 58 (2002) 387. [6] D.Q. Zhang, Z.X. An, Q.Y. Pan, L.X. Gao, G.D. Zhou, Corros. Sci. 48 (2006) 1437. [7] E. Vuorinen, P. Ngobent, G.H. Vander, Klashorst, W. Skinner, E. Dewet, W.S. Ernst, Br. Corros. J. 29 (1994) 120. [8] A. Subramanian, A.R.S. Priya, T. Vasudevan, Mater. Chem. Phys. 100 (2006) 193.
[9] U. Rammelt, S. Koehler, G. Reinhard, Corros. Sci. 51 (2009) 921. [10] Z.X. An, Study on inhibition performance and mechanism of organic polyamine volatile corrosion inhibitors, Master Degree thesis, Shanghai University, Shanghai, China, 2004. [11] J.M. Bastidas, E.M. Mora, S. Feliu, Werkst. Korros. 41 (1990) 343. [12] F. Zucci, M. Fonsati, G. Trabanelli, Corros. Sci. 40 (1997) 1927. [13] D.Q. Zhang, Z.X. An, Q.Y. Pan, L.X. Gao, G.D. Zhou, Appl. Surf. Sci 253 (2006) 1343. [14] D. Tromans, J.C. Silva, J. Electrochem. Soc. 143 (1996) 458. [15] X.Y. Wang, Y.S. Wu, L. Zhang, Z.Y. Zhang, Corrosion 57 (2001) 540. [16] D.Q. Zhang, L.X. Gao, G.D. Zhou, J. Appl. Electrochem. 33 (2003) 361. [17] F. Bentiss, M. Lagrence, M. Traisnel, J.C. Hornez, Corros. Sci. 41 (1999) 789. [18] A. Leng, M. Stratmann, Corros. Sci. 34 (1993) 1657.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Self-assembled urea-amine compound as vapor phase corrosion inhibitor for mild steel Da-Quan Zhang ⁎, Li-Xin Gao, Guo-Ding Zhou Department of Environmental Engineering, Shanghai University of Electric Power, Shanghai 200090, PR China
a r t i c l e
i n f o
Article history: Received 1 June 2009 Accepted in revised form 30 October 2009 Available online 10 November 2009 Keywords: Surface properties Adsorption Corrosion Electrochemical techniques
a b s t r a c t VPIs (vapor phase corrosion inhibitors) extend their corrosion-inhibiting properties by volatilization and condensation to form a protective film on metal surface. Vapor phase self-assembled films of VPIs have become an attractive means to prevent metal from corrosion. This new methods meet the rust-proof challenges for the steel materials with many advantages. In this paper, bis-(cyclohexylaminomethyl)-urea (BCMU) was developed as VPIs for mild steel. Its corrosion inhibition property was studied by volatile inhibiting sieve test (VIS) and electrochemical measurements. BCMU has good protection effect for mild steel. It suppresses the anodic corrosion reaction and renders the corrosion potential to more positive values. Its interaction with steel surface was investigated by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). The results suggest that the BCMU interacts with the ferric ions via N and O atoms in its molecule. BCMU inhibits the corrosion processes by blocking the active iron dissolution. It can stabilize the oxide layer and decrease the surface roughness. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved.
1. Introduction A vapor phase corrosion inhibitor (VPI) is a compound that has the ability to vaporize and condense on a metallic surface to make it less susceptible to corrosion. The main advantage of VPIs compared with conventional corrosion control methods stems from their gas-phase transport. A VPI reaches the metallic surfaces without contacting the surface directly. The efficacy, convenience, and cost effectiveness of VPIs made their application for rust control almost universal in automotivemanufacturing, steel-making, ship-building, power generation, and defense production. As with all industrial chemical products, however, the increased usage of VPIs has also raised significant scientific interest as to their safety. Cumulative research over the last 30 years has, unfortunately, proven that certain specific VPI formulations can, in fact, be toxic [1]. For example, dicyclohexyl ammonium nitrite (DICHAN) has been found to be most effective for inhibiting the atmospheric corrosion of steel, and gained industrial application for several decades [2]. Research confirmed that some N-nitrosoamines, including those generated by DICHAN, were not only carcinogenic, but also genotoxic as well. Thus, alternative of environmental-friendly VPIs is under consideration [3–6]. Urea is one of the main ingredients of corrosion inhibitor formulations for many volatile corrosion-inhibiting papers. It is cheap and has been used for many years as a feed additive for animals. But the disadvantage is its too high vapor pressure to give lasting rust prevention [7].
⁎ Corresponding author. Tel./fax: +86 21 65700719. E-mail address: [email protected] (D.-Q. Zhang).
On the other hand, mechanistic information on corrosion and inhibition processes is very important for proper selection of inhibitors. In general, the inhibition mechanism proposed for VPIs, is mainly based on its adsorption on the metal surface whereby a thin monomolecular barrier film should be formed [8]. The metal surface is usually covered with a thin oxide layer under near neutral atmospheric conditions. It is an irregular physical surface with grains of different sizes and orientations, grain boundaries and defects, and most important, a chemistry that is far from that of a pure metal [9]. Thus, the adsorption of VPIs on a metal surface may not be uniform and therefore the protection mechanism of VPIs may involve a combination of passivation and adsorption. In this paper, a urea derivative, bis-(cyclohexylaminomethyl)-urea (BCMU), was developed as a novel VPIs for mild steel. BCMU has one urea moiety and two cyclohexamine moieties, which molecular structures are shown in Fig. 1. Its inhibition effectiveness was investigated by volatile inhibiting sieve test (VIS) and electrochemical measurements. The interactions of BCMU with the steel were characterized by X-ray photoelectron spectroscopy (XPS) analysis. The vapor phase selfassembled film on the steel surface was studied by AFM measurement. 2. Experimental details 2.1. Materials and apparatus Steel specimens (50 mm × 25 mm × 1.5 mm) were used for volatile inhibiting sieve test (VIS). BCMU was synthesized by using cyclohexamine, urea, and formaldehyde solutions in the laboratory according
0257-8972/$ – see front matter. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.10.054
D.-Q. Zhang et al. / Surface & Coatings Technology 204 (2010) 1646–1650
1647
Fig. 1. Molecular structures of bis-(cyclohexylaminomethyl)-urea (BCMU).
to a procedure reported previously [10]. The melting of platelets was at 214 °C–216 °C. A volatile corrosion inhibitor monitor cell (VCIM) consisted of 20 foils of mild steel (80 mm × 8.0 mm × 0.5 mm), with each steel plate separated by a 170 μm thick layer of an insulating sheet [11]. Electrochemical measurement was performed using a PARC M283 potentiostat (EG&G) and PARC Model 1025 frequency response analyzer. The XPS experiments were performed on a PHI 550 ESCA/SAM spectrometer (Perkin-Elmer). A commercial AFM with contacting mode (Zhejiang University of China, AFM&STM-II) was used to characterize the surface morphology and roughness before and after VPI adsorption.
Fig. 2. Potentiodynamic polarization curves for steel electrode after 30 min immersion in simulated atmospheric corrosion solution (1— blank, 2— 0.2% BCMU).
3. Results and discussion 2.2. Procedures 3.1. Inhibition of BCMU film on the corrosion of mild steel 2.2.1. Volatile corrosion inhibition test The volatile inhibiting sieve test (VIS) was conducted to evaluate the inhibition effect of BCMU. The test process was reported in an earlier publication [6]. After corrosion tests, samples were removed for visual inspection and weight-loss determination. Corrosion rates and inhibitor effectiveness were calculated.
2.2.2. Electrochemical measurements Electrochemical measurements were conducted both in stimulated atmospheric corrosion solution and under thin electrolyte layer. A three-electrode cell was used for solution electrochemical measurements. The stimulated atmospheric corrosion solution was prepared by using double-distilled water and containing Cl− 100 mg/L, HCO− 3 100 mg/L kg/m3, SO2− 4 100 mg/L, respectively. The working electrode was polarized at a speed of 1 mV/s. The EIS experiments were performed at open circuit potential over a frequency range of 0.05 Hz to 100 kHz. The sinusoidal potential perturbation was 5 mV in amplitude. The potential values reported here were versus SCE. The cell was open to the laboratory air and the measurement was conducted without agitation at room temperature (25 °C). A volatile corrosion inhibitor monitor cell (VCIM) was used to obtain electrochemical impedance data under thin electrolyte layer [12]. The VCIM was placed upon a 300 mL-beaker, which contained a lid of 5 g inhibitors. The edges of the polished steel foils faced down to the lid. After a specific period of film-forming time of the inhibitor, the VCIM was then covered with a filter paper saturated by stimulated atmospheric solution. The VCIM was placed in a vessel at a relative humidity of 90%.
After the volatile inhibiting sieve test, the specimens treated by BCMU were bright in almost all areas. The corrosion rate and inhibition effectiveness for the BCMU film-forming specimens were 28.57 mg m− 2 h− 1 and 90.84%, respectively. This shows that BCMU can be volatile and adsorbed on the mild steel surface to protect the steel. 3.2. Electrochemical measurements in stimulated atmospheric corrosion solution Fig. 2 shows the polarization curves for freshly polished mild steel electrode after 30 min immersion in stimulated atmospheric corrosion solution. The anodic curve for the steel electrode exhibits an active–passive behavior. The cathodic portion of the polarization curve for blank is a composite and represents oxygen reduction. The addition of 0.2% BCMU has no special effect for the pH values of electrolyte solution. It renders Ecorr more noble to −497.0 mV (SCE). The anodic curve for the steel electrode containing BCMU shows an initial region of anodic active behavior and then a passive region. Inhibitor-induced passive behavior was confirmed by a marked decrease in anodic currents compared with the blank solution [14]. In the presence of BCMU, both the cathodic and anodic current densities were greatly decreased near Ecorr region. BCMU decreases the anodic reaction rate more strongly than the cathodic reaction rate. The free corrosion potential shifts to more positive values. Fig. 3 shows the impedance Nyquist plots of a steel electrode in stimulated atmospheric corrosion solution with and without BCMU.
2.3. XPS and AFM measurements As for XPS measurement, the specimens were mounted on a steel stopper using a molybdenum mask to assure good electrical contact. The analyses were done using a Mg Kα X-ray source at 10 kV and 400 W. All analyses were performed at a pressure below 1.33 × 10− 6 Pa. Charge referencing was determined by setting the main C 1s component at 285.0 eV. The AFM apparatus setup was described in an earlier research [13]. The morphology of the specimens was observed by a contacted AFM with a silicon nitride (Si3N4) probe. A 10 mW laser diode was used on the AFM unit at a wavelength of 660 nm. The roughness (Ra) calculated by the software package was adopted to compare the surface quality.
Fig. 3. Nyquist plots for steel electrode after 30 min immersion in simulated atmospheric corrosion solution (1— blank, 2— 0.2% BCMU).
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D.-Q. Zhang et al. / Surface & Coatings Technology 204 (2010) 1646–1650
Fig. 5 represents the variation of both ROX and COX with the immersion time. It is obvious that the ROX values increased sharply from 23.03 kΩ cm2 to 82.5 kΩ cm2 during the initial 8 days and remained slightly increasing afterward. At the same time, the capacitance values were reduced drastically from 88.3 µF cm− 2 to 48.2 µF cm− 2. These results demonstrate that the formation of the ferric-inhibitor complexes was relatively slow and the completed plugging in the defects reaches within 5 days. Fig. 4. The equivalent circuit used to fit the EIS data for steel in simulated atmospheric corrosion solution.
The equivalent circuit model employed for this system is shown in Fig. 4. This is a two-time constant model due to the presence of both passive and corroding regions on these surfaces [9]. The passive regions refer to the oxide layer on steel surface, while the corroding regions refer to the oxide layer breakdown. RS is the electrolyte resistance, COX and ROX can be attributed to the capacitance and resistance of the passive surface, respectively. Cad and Rad are the capacitance and resistance of corroding regions. Since the electrochemical systems show various types of inhomogeneities, COX can be better substituted by a constant phase element (CPE, Q). The CPE element was introduced formally only for fitting impedance data. It was found that the presence of BCMU enhances the ROX values from 2.531 to 23.03 kΩ cm2 and reduces the Q values from 161 μF cm− 2 to 88.3 μF cm− 2. The decrease in Q may be due to the adsorption of BCMU in the defects of the metal surface oxide layer. The increases of ROX values may be due to BCMU's blocking of the surface sites of active iron dissolution to stabilize the oxide layer [9].
3.3. Electrochemical impedance measurements under thin electrolyte layer The atmospheric corrosion of mild steel is an electrochemical process that was conducted under thin electrolyte layer. The thin
Fig. 5. Variation of both ROX and QOX with immersed time in simulated atmospheric corrosion solution.
Fig. 6. Nyquist plots for a VCIM under thin corrosion water layer (1— blank, 2— after 8 h BCMU film-forming).
Fig. 7. XPS plots of adsorption of main elements in complex film after the adsorption of BCMU on the steel surface: (a) Fe 2p3/2; (b) N 1s; and (c) O 1s.
D.-Q. Zhang et al. / Surface & Coatings Technology 204 (2010) 1646–1650 Table 1 Binding energies (eV) of the inhibition film on steel surface. Fe 2p3/2
BCMU
N 1s
O 1s
1
2
1
2
1
2
3
708.24
710.56
398.11
399.70
529.44
531.52
532.87
1649
electrolyte layer was formed by adsorption, condensation or precipitation of water in humid air. Fig. 6 shows Nyquist plots for a VCIM obtained with and without a period of BCMU film-forming. The VCIM electrode impedance with BCMU film-forming was higher than the blank electrode. The corrosion protectiveness of BCMU was also proved under thin electrolyte layer. 3.4. XPS and AFM measurements In spite of the fact that VPIs have been widely used for inhibiting atmospheric corrosion, the action mechanism of these compounds is not completely clear. The adsorption of VPIs on a metal surface can be determined by XPS analysis. Fig. 7 shows the Fe 2p3/2, N 1s, and O 1s XPS peak measured on the surface of steel samples after exposure to 48 h of VPI film-forming periods. The results of bonding energy are summarized in Table 1. It has been reported that the Fe 2p3/2 peak at 710.56 eV corresponded to the oxide state and the Fe 2p3/2 peak at 708.24 eV corresponded to the metallic Fe atom [15]. A shift of binding energy can be explained by the interaction of Fe and N atom. Thus, the Fe 2p3/2 peak at 708.24 eV may be attributed to the Fe atom, which is bonded to the VPI molecule. Two kinds of N 1s peaks are present in the XPS spectra, which indicate that a BCMU film is present on the surface of the steel. The N 1s peak at 398.11 eV corresponded to the N atom in the organic VPI molecule, and the peak at 399.70 eV to the N atom interacted with the Fe surface [16,17]. The bonding energy of the O 1s peak on the steel surface was 529.0–532.9 eV, which could correspond to O in the ferrous oxide (FeO), O in the inhibitor, and O in the VPI molecule that interacted with the Fe surface. Thus, BCMU acts to block the active iron dissolution by forming insoluble ferricinhibitor complexes via the N and O heteroatoms. The protection due to the action of BCMU was monitored in situ with an AFM while varying the exposure time. AFM images are shown in Fig. 8. The surface morphology of the steel sample before exposure to BCMU volatilization indicates there were a few scratches from the mechanical polishing treatment. The roughness is 2.7 nm. It shows a thin covering surface film composed of many particles after exposure to BCMU volatilization. The steel surface after 2 h BCMU exposure was rougher than that before BCMU film-forming. As the exposure time increased from 2 h to 48 h, the roughness of the steel surface decreased from 3.7 nm to 1.8 nm. This can be attributed to more BCMU molecular aggregators formed on the steel surface. The investigation by Leng and Stratmann [18] suggested that adsorption of VPI may form a few protective monolayers. However, the oxide layer covered on the steel surface is seldom homogeneous and has many defects. BCMU is able to suppress the corrosion reaction at the defect sites. Thereby, the effect of the oxide layer stabilization can be caused by an adsorption of BCMU on active surface sites. 4. Conclusions Bis-cyclohexylaminomethyl-urea (BCMU) has been proved to be a good volatile corrosion inhibitor for the steel corrosion. BCMU suppressed the anodic reaction of steel electrode and renders the corrosion potential to more noble direction. XPS measurements suggest that the BCMU interacts with the ferric ions via N and O atoms in its molecule. AFM images have shown that BCMU can stabilize the oxide layer and decrease the surface roughness. Acknowledgments
Fig. 8. AFM images of the steel surface: (a) initial surface; (b) after 2 h exposure for BCMU film-forming; and (c) after 48 h exposure for BCMU film-forming.
Supports from the National Natural Science Foundation of China (20576069 and 20911140272) and from the New Century Excellent Talents Program (NCET-08-0895) from the Ministry of Education in China are gratefully acknowledged. This work was carried out in the Key Laboratory of Shanghai Colleges and Universities for Electric Power Corrosion Control and Applied Electrochemistry.
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