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Electrochemical investigation of the inhibition effect of CeO2 nanoparticles on the corrosion of mild steel
Electrochimica Acta 131 (2014) 71–78
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical investigation of the inhibition effect of CeO2 nanoparticles on the corrosion of mild steel M. Fedel a , A. Ahniyaz b , L.G. Ecco a , F. Deflorian a,∗ a b
Department of Industrial Engineering, University of Trento, Via Mesiano, 77, 38123, Trento, Italy SP Chemistry, Materials and Surfaces, Drottning Kristinas vag 45, Box 5607, SE-114, Stockholm, Sweden
a r t i c l e
i n f o
Article history: Received 24 July 2013 Received in revised form 22 November 2013 Accepted 27 November 2013 Available online 12 December 2013 Keywords: Cerium oxides Corrosion inhibition EIS Steel
s u m m a r y The paper aims to provide some insight into the fundamental mechanisms for the behavior of cerium oxides nanoparticles as corrosion inhibitors for steel. The work was carried out on mild steel water based dispersions of cerium oxides nanoparticles in presence of both, sulfates and chlorides. Ceria nanoparticles were produced via precipitation of cerium Ce(NO3 )3 .6H2 O in water, particles of about 70 nm hydrodynamic diameter were obtained. The analysis of the effectiveness of the ceria nanoparticles as corrosion inhibitors was performed by means of electrochemical techniques such as electrochemical impedance spectroscopy (EIS) and open circuit potential (OCP) versus time measurements. The experimental measurements suggested that cerium oxide affects the electrochemical properties of mild steel surface; they promoted an ennoblement effect and strong modifications in the impedance response. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Due to the health and environmental issues related to the use of cancerogenic or toxic compounds, in the last decades the need of novel corrosion inhibitor compounds has become more and more stringent [1]. Anionic oxygen-dependent compounds, such as molybdates, permanganates, vanadates and tungstates which act as passivating agents, have been widely studied as alternatives to the well-known chromium IV based materials [2,3]. In the field of anti-corrosion pigments for organic coatings, these materials have been recently investigated in conjunction with ions exchange particles such as hydrotalcites and bentonites [4,5] which act as reservoirs for corrosion inhibitors. Based on present literature, several different strategies and materials were exploited for storage and prolonged release of corrosion inhibitors [6–9]. Among the novel materials developed as corrosion inhibitors, the interest for the use of lanthanide compounds is growing over the last years. The success of these compounds is attributed to the corrosion potential efficiency of cerium salts, in particular chlorides and nitrites. It has been demonstrated that they promoted protection to a significant number of metals and alloys, for example steel, hot dip galvanized steel, tin, aluminum and their alloys, etc. [10–14]. In fact, because of the local pH increase at the cathodic sites due to the generation of OH− ions (2H2 O + O2 + 2e− → 4OH− ), which
∗ Corresponding author. E-mail address: flavio.defl[email protected] (F. Deflorian). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.11.164
occurs when a corrosion process takes place, insoluble cerium compounds such as Ce(OH)4 and CeO2 ·xH2 O are formed [15,16]. These compounds are believed to precipitate on the surface of the metal, reducing the cathodic reaction rate and, thus, the overall corrosion extent [17]. Following the success of soluble cerium salts as corrosion inhibitors, cerium oxides particles have been likewise studied as pigments inside protective coatings for corrosion mitigation purposes [18–20]. Cerium oxides are widely used as a reducible oxide support material in the field of emission control catalysis for the purification of exhaust gases for different combustion systems [21], mainly due to their oxygen storage capacity (OSC) [22]. Owing to the ease of the Ce4+ ↔Ce3+ redox shuttle, the exchange process 2CeO2 ↔ Ce2 O3 +1/2O2 is favored [23]. Thus, the material is suitable for storing and releasing oxygen under conditions fluctuating between oxidizing and reducing state of cerium ions [24]. In fact, the Ce3+ ↔ Ce4+ shift leads to a high oxygen mobility in the ceria lattice that is responsible for its high catalytic activity [25]. The particular behavior of the cerium oxides was attributed to the unstable fluorite structure of the lattice where some Ce4+ have a tendency to be reduced to Ce3+ , which has a larger ionic size than Ce4+ , followed by oxygen molecules release with subsequent formation of oxygen vacancies [26,27]. Moreover, greater the number of vacancies, easier is the oxygen mobility around the crystal, allowing ceria to promote the reduction and oxidation of molecules. It is believed that the previously discussed properties, which are related to the bulk material, are maintained also when cerium oxides are produced as nano-sized particles. In addition, consider that under these conditions the chemical potential
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of the material increases according to the Young-Laplace equation [28] leading to a more reactive surface of the cerium oxide [25]. Due to the capability to shift the oxidation state from Ce4+ ↔ Ce3+ as a function of oxidative/reduction conditions, in this study the potential of cerium oxides nanoparticles as a corrosion inhibitor for steel is investigated. Despite the presence of a few works which proved the beneficial effect of the cerium oxides for corrosion protection, the mechanisms by which ceria is able to inhibit the corrosion process on a metal substrate are still unclear and remains as a subject of debate. For this reason, the aim of this work is to provide some insight into the fundamental mechanisms for the behavior of the investigated cerium oxides nanoparticles. The experimental work reported here has been carried out on mild steel substrates immersed in water based dispersions containing cerium oxides nanoparticles with diverse concentrations. The anticorrosion effectiveness has been investigated by means of electrochemical techniques such as electrochemical impedance spectroscopy (EIS) and open circuit potential (OCP) versus time. Supplementary information about the surface of the mild steel after exposure has been analyzed by means of An environment scanning electron microscopy (ESEM). Fig. 1. TEM Image of the ceria nanoparticles and corresponding SAED.
2. Experimental The ceria nanoparticles were produced by the precipitation of cerium Ce(NO3 )3 .6H2 O with NH4 OH in the presence of H2 O2 and acetic acid at elevated temperature. First, 30 g of Ce(NO3 )3 ·6H2 O and 2.6 g of acetic acid were dissolved in 2000 ml preheated water at 80 ◦ C; 28% ammonia was slowly added drop-wise to the above solution until pH reached 9 and then heated another 1 hour under stirring with Ultra Turrax at 10000 rpm. Then, reaction was cooled down by turning off the heat source. The resultant precipitate was collected by centrifugation at 4000 rpm for 20 minutes. To remove all the free-ions, such as acetate, nitrate ions in the nanoparticle dispersions, nanoparticles were extensively washed with double distilled water and re-dispersed in distilled water. Then, the pH was adjusted to 6-8 using diluted HNO3 acid and more water was added. The resulting solution was sonicated for about 10 minutes to obtain 10wt% ceria nanoparticle dispersion. The original solution containing the nanoparticles was modified adding Na2 SO4 (final concentration in the solution: 0.3wt%) or NaCl (final concentration in the solution 0.1 M) to perform the electrochemical measurements. In this way, it was possible to control the electrical conductivity of the aqueous media. Whenever necessary the solution containing the ceria nanoparticles was diluted to obtain different concentrations. For this purpose 0.3wt% Na2 SO4 or 0.1 M NaCl solutions were used. Mild steel sheets (Q-Panels) were used as substrate. The metal sheets were ultrasonically degreased with acetone, rinsed with de-mineralized water and then air blowing dried before the experimental measurements. Dynamic light scattering study was carried out using a Zetasizer (Nano ZS, 2003, Malvern Instruments, UK). Zeta potential of cerium oxide nanoparticle was measured using Zetasizer in 10 mM NaCl solution at pH 6. Transmission Electron Microscopy (TEM) images and Selected Area Electron Diffraction (SAED) patterns were obtained using a JEOL JEM-3010 microscope operating at 300 kV (Cs = 0.6 mm, Point resolution 0.17 nm). Images were recorded with a CCD camera (MultiScan model 794, Gatan, 1024 × 1024 m). TEM images were obtained from the very diluted and pH adjusted (to pH 6) dispersion of cerium oxide nanoparticles which was sonicated for additional 30 min during the TEM sample preparation. In order to avoid the aggregation of ceria nanoparticle during the drying process, hydrophobic carbon-coated TEM grid modified with 0,5% SDS
(sodium dodecyl sulfonic acid) was used. Due to the hydrophilic nature of the modified TEM grid surface, drying and induced evaporation, aggregation of cerium oxide nanoparticles was avoided. Electrochemical analyses were carried out on the mild steel electrodes immersed in different solutions containing the particles. The OCP was measured employing a traditional two electrodes arrangement using an AMEL Ammeter/Electrometer (Model: 668/RM). The potential of the mild steel as working electrode with respect to the Ag/AgCl (+205 mV vs SHE) reference electrode was monitored during immersion time. EIS measurements were performed at the free corrosion potential using an Autolab 302 N Potentiostat/Frequency Response Analyser. The selected signal amplitude was 5 mV, 105 –10−2 Hz frequency range and about 0.6 cm2 testing area was employed. A classical three electrodes arrangement was used: a Ag/AgCl reference electrode (+205 mV vs SHE) and platinum ring counter electrode were employed while the working electrode was the mild steel sheet. The measurements were performed in the solutions containing the nanoparticles. The solution consisted in 0.3wt% Na2 SO4 and 0.1 M NaCl containing different amounts of ceria nanoparticles. The impedance data were analysed with the software ZSimpWin 3.22. The different electrolytes were employed to assess the effectiveness of the ceria nanoparticles both in a mild (i.e. sulfates solution) and in a more aggressive solution (i.e. chlorides solution). All the measurements were carried out at room temperature and the reported data hold consistent repeatability. A Philips XL30 Environmental Scanning Electron Microscope was exploited to investigate the surface of the steel electrode after immersion in the different electrolytes. 3. Results and discussion Dynamic light scattering (DLS) study revealed that the average size of ceria particles in the dispersion is around 70 nm and zeta potential is -22 mV. It has been checked that z-potential did not change significantly in between 6 to 9 pH range. TEM study (Fig. 1) indicated that primary particle size of cerium oxides nanoparticles is around 5-20 nm. The selected area diffraction pattern (SAED) of cerium oxide nanoparticles is also reported. As far as Fig. 1 is concerned, the ring electron diffraction pattern corresponds to randomly distributed crystals of the nanoparticles. Comparing TEM
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For example, Fig. 3 displays the evolution of the impedance modulus versus the time of exposure for a mild steel electrode exposed to a solution containing 1 wt% of the cerium oxide nanoparticles dispersed in 0.3wt% Na2 SO4 . One can notice that the electrode immersed in the sodium sulphate solution containing 1.0 wt% of ceria nanoparticles showed relatively high and stable values of the low frequency impedance (|Z|0.01 ) during time, in the order of 106 .cm2 . It is worth to observe that the electrode maintained a stable impedance value for over than 500 hours of analysis. It seems that when about 1.0 wt% of cerium oxide nanoparticles is added to the electrolyte, high impedance values are recorded by the equipment. EIS were carried out also for 5 and 10 wt% ceria containing solution, as shown in Figs. 4 and 5, respectively. As far as the 0.3wt% Na2 SO4 solution is concerned, the evolution of the corrosion behaviour seems not to be affected by different concentrations of cerium oxide. The EIS spectra related of the steel electrodes immersed in the different solutions are quite similar to those with only 1.0 wt% of cerium oxide nanoparticles. The impedance modulus in the low frequency range is about 106 .cm2 for all the investigated solutions and remained constant with significant stability for over than 500 h of immersion. In order to evaluate the effect of the cerium containing solutions on the steel electrodes, the morphology of the metal surface was observed after 500 hours of exposure. Figs. 6(a) and (b) show the metal surfaces exposed to the sodium sulphate solution containing
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and DLS findings, as one may expect, agglomeration of the nanoparticles occurred when immersed into a water-based solution. As described in the previous section, cerium oxide nanoparticles were dispersed in a 0.1 M NaCl solution. Fig. 2 shows the OCP evolution of the mild steel electrode immersed in 0.1 M sodium chloride solution containing diverse concentrations of the cerium oxide. Similarly, the evolution of a mild steel electrode immersed in a blank solution is reported. The pH of the different solutions containing the nanoparticles, measured prior the starting of the measurements, was found to be around 8 for all the investigated solutions. For this reason, the pH of the blank 0.1 M NaCl solution was adjusted to 8 by adding sodium hydroxide in order to have a consistent comparison among the studied samples. As far as electrode immersed in the blank solution is concerned, notice that the (OCP) dropped from -0.43 to -0.67 V in less than one hour, according to the free corrosion potential value of mild steel [29]. Afterward, the potential value remained stable during immersion time, for over 300 hours. On the other hand, in presence of 1 and 2 wt% of cerium oxide nanoparticles a higher initial OCP value of the steel electrode (near -0.15 V) is observed. However, the ennoblement lasted for 3 hours of immersion. After the third hour, a reduction of the free corrosion potential down to about -0.75 V is observed. Still more relevant is the ennoblement promoted by contents equal to or higher than 3 wt% of cerium oxide nanoparticles. When the amounts of 3 or 5 wt% of ceria particles have been added into the solution a dramatic increase of the OCP is observed. Approximately the same ennoblement, near -0.15 V, accompanied by a remarkable stability, is provided by the 3 and 5 wt% ceria nanoparticles containing solutions, which lasted for about 150 and 350 hours, respectively. Based on the present results, ceria nanoparticles seems to be able to promote an increase of the free corrosion potential value of the steel electrode from ∼ -0.65 V (free corrosion potential of steel) to ∼ -0.05 ÷ 0.10 V. As far as Fig. 2 is concerned, itseems that the presence of any amount of ceria nanoparticles leads to an increase of the free corrosion potential of the steel electrodes from ∼ -0.65 V to about 0.0 V. In addition, one can notice that the higher the amount of ceria nanoparticles into the chloride solution, longer the ennoblement effect lasts. To better investigate the effectiveness of the ceria nanoparticles, electrochemical impedance spectroscopy (EIS) spectra were acquired during immersion time in a mild electrolyte such as 0.3wt% Na2 SO4. Also in presence of sulphates the pH of the different solutions containing the nanoparticles, acquired before the electrochemical tests, was found to be around 8. Following a similar procedure, the pH of the blank 0.3wt% Na2 SO4 solution was adjusted to 8.
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1 and 5 wt% of cerium oxides for about 500 hours, evaluated by means of environmental scanning electron microscopy (ESEM). The topographies depicted in Figs. 6(a) and 6(b) appears to be similar, characterized by the presence of only very limited corrosion extent. In this sense, while in contact with steel, it seems that both solutions protected steel against corrosion, as only a slight corrosion attack is observed. In fact, the steel electrodes immersed in a sodium sulphate solution containing about 1 wt% of ceria nanoparticles revealed the presence of small corrosion attack, as it is shown in Fig. 7. No similar corrosion products were found in the steel surface exposed to the 5 and 10 wt% ceria solutions. The corrosion protection potential of the ceria particles were further investigated in presence of sodium chloride solution. Chlorides are recognized to promote the corrosion of steel in neutral environment by forming complexes which tend to be unstable and soluble [30]. The electrochemical impedance evolution during immersion time of the steel electrodes immersed in the 0.1 M NaCl solutions containing 1 wt% and 3 wt% of CeO2 nanoparticles is reported in Figs. 8 and 9, respectively. Considering Fig. 8, one can observe that the overall impedance decreased during immersion time after the first hours of immersion. The low frequency impedance (|Z|0.01 ) showed a reduction
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from about 105 *cm2 , from the beginning, to 103 *cm2 after about 360 hours. As far as Fig. 9 is concerned, it is possible to appreciate a remarkable stability of the impedance spectrum of the steel immersed in the 0.1 M NaCl solution containing 3 wt% of CeO2 particles. In fact, |Z|0.01 remained stable around 106 *cm2 during the 312 hours of immersion (Fig. 9a). Furthermore, one can notice that the impedance phase in the low frequency range is shifted towards higher angles after 1 hour of immersion. As one can appreciate from Fig. 9(b), the very first acquisition (t = 0 h) showed one time constant for the phase angle spectrum. However, during time the system evolved and the phase spectrum exhibited an approximately horizontal line positioned at about 85÷86 degrees in the low frequency
0h 5h 24h 48h 120h 336h 408h 528h
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Fig. 6. SEM topography of mild steel after 500 hours of exposure in the solution containing 1wt% of ceria nanoparticles (a) and 5wt% of ceria nanoparticles (b).
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Fig. 7. SEM topography of mild steel after 500 hours of exposure to the sodium sulfate solution containing 1wt% of ceria.
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range. The resulting shape of the impedance spectrum seems to be in accordance with the electrochemical impedance response of a “blocking electrode” (described elsewhere in literature [31]) corresponding to an electrical circuit where no current flows when the dc limit is reached. The same behavior is observed for the steel electrodes immersed in the 0.1 M NaCl solutions containing 5 and 10 wt% of CeO2 particles. A comparison among electrochemical evolution within the first and the fifth hours of immersion of the mild steel electrodes exposed to the solutions is shown in Figs. 10 and 11, where the EIS spectra are reported after 0 and 5 hours of exposure, respectively. The EIS spectra at time = 0 h (Fig. 10) highlight that when ceria nanoparticles have been added into the 0.1 M NaCl solution a strong increase of the impedance in the middle-low frequency range is observed. All the electrodes immersed in the different solutions presented a |Z|0.01 which exceeded the value of 105 *cm2 , remarkably higher than the 103 *cm2 measured for the samples immersed in the blank 0.1 M NaCl solution. The impedance modulus in the low frequency range for the steel electrode immersed in the 1 wt% ceria containing solution dropped from 105 *cm2 down to 103 *cm2 within 5 hours of exposure (Fig. 11). Higher contents of cerium oxide promoted higher initial
values for |Z| at low frequency, quite close to 106 *cm2 , along with a remarkable stability. On the contrary, from Fig. 10 it is possible to appreciate that the electrodes immersed in the solution containing 3, 5 and 10 wt% of ceria nanoparticles showed an increase of the impedance during the first hours of immersion. The evolution
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Fig. 13. Circuit used for fitting the experimental data: RQ (a) and R(QR) (b).
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with immersion time of the different electrodes was investigated for longer periods of exposure. Fig. 12 shows the EIS spectra for steel sheets immersed in the different solutions after about 400 h. It is worth noticing that the mild steel electrodes immersed in a sodium chloride solution containing at least 3 wt% of ceria nanoparticles showed high values of the impedance even after long immersion time (about 400 hours). On the other hand, the steel electrode immersed in the solution containing 1 wt% of ceria nanoparticles shows an impedance spectra which is comparable to that of the electrode in the blank solution. It seems that the presence of a certain amount of ceria nanoparticles is needed to maintain high impedance values during immersion time. Although not unequivocally proven by the experimental results, EIS and OCP measurements are suggesting the formation of a sort of passive or conversion layer on the steel electrodes by the cerium oxide nanoparticles, which have led to an increase of the absolute values of the impedance and to a relative stability, accompanied by an ennoblement of the potential. A deeper investigation of the evolution of the electrochemical properties of the steel electrodes immersed in the different solutions has been done by fitting the experimental spectra. The EIS spectra were modelled using the electrical equivalent circuits
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depicted in Fig. 13. During the first hours of immersion the spectra were fitted using a Rel (Qdl Rct ) circuit, where Rel stands for the electrolyte resistance while (Qdl Rct ) is the time constant attributed to the faradic process occurring on the electrode surface [32]. Qdl is a constant phase element (CPE) related to the double layer capacitance of the steel surface, while Rct represents the charge transfer resistance of the faradic process. The electrical equivalent circuit depicted in Fig. 13 (a) consists of a Rel Q system. Since it represents the behaviour of a “blocking electrode” [33], according to similar approach for passive metals, the meaning of the Q is likely to be related to the contribution of the passive oxide on the surface of steel. It is worth emphasizing that in cases where the Rel Q circuit was exploited it was not possible to use the Rel (Rct Qdl ) circuit without a dramatic increase of the fitting relative error. The EIS spectra of the steel electrode immersed in the 0.1 M NaCl solutions containing 1 wt% of CeO2 particles were fitted using the circuit reported in Fig. 13 (b), since one time constant was clearly visible throughout immersion time. On the other hand, the steel electrodes immersed in the 0,1 M NaCl solutions containing 3, 5 and 10 wt% of CeO2 , were fitted using the circuit depicted in Fig. 13 (b) until an “active behavior” was observed and using the circuit depicted in Fig. 13 (a) when a sort of “blocking electrode” impedance response was recognized. The switch from “active” to “blocking” behavior was observed after a certain lapse of time, in the order of a few hours, different from sample to sample. The chi-squared error parameter related to the fitting procedure of the reported data was in the 10−5 ÷10−4 Range. Fig. 14 (a) shows the evolution during immersion time of the capacitance of the Y0 value related to the CPE (|Z|CPE = 1/(Y0 ·(j)n ) [34] used to fit the experimental data. Fig. 14 (b) shows the evolution of the corresponding exponent “n” value. Considering Fig. 14 (a), one can notice that the electrode immersed in the solution containing 1 wt% of cerium oxides showed a completely different trend of the Y0 values during immersion time. For this sample it was possible to appreciate a continuous increase of Y0 followed by a simultaneous shift toward lower values of the exponent “n” value (Fig. 14b). On the other hand, the steel electrodes immersed in the solution containing 3, 5 and 10 wt% of CeO2 showed a slight decrease of the Y0 values during the first hours of immersion to reach a sort of steady state around 10−5 S·sn ·cm−2 . At the same time, the exponent “n” values remained constant around 0.95. It is worth noticing that after a transient of about 24 hours, the electrodes immersed in the solutions containing 3, 5 and 10 wt% of CeO2 reached a constant value of Y0 and of the corresponding “n” exponent. Simultaneously, the shape of the phase angle spectra indicated that the behavior of the steel electrodes is changing from “active electrode” to “blocking electrode”. Considering the Rct values, Fig. 15 shows a comparison among the different samples.
M. Fedel et al. / Electrochimica Acta 131 (2014) 71–78
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As one can notice, the Rct values for the electrodes immersed in the solution containing 3, 5 and 10 wt% of CeO2 are reported only for the first few hours of exposure. After this period of time, the Rel Q circuit was used to model the experimental data and the Rct was no longer measured. The electrode immersed in the different solutions presented very high charge transfer resistance values, over 105 *cm2 , regardless of the concentration of CeO2 . However,
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Time / h Fig. 15. Evolution of the charge transfer resistance (Rct ) value during immersion time.
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the solution containing 1 wt% of CeO2 showed a quick drop to 103 *cm2 after a few hours of immersion. On the other hand, when immersed in a solution containing at least 3 wt% of CeO2 the steel electrode exhibited an increase of the Rct reaching about 107 *cm2 before being no more possible to measure the charge transfer resistance value. Notice that the steel electrode immersed in a 10 wt% CeO2 solution behaves like an “active electrode” only in the very first hours of immersion. Therefore, after 3 hours of immersion the charge transfer resistance was no longer measured, as a Rel Q circuit was exploited to fit the experimental data. Considering the outcome of the experimental measurements, it was found that the electrochemical response of the steel electrode immersed in a solution containing a certain amount of ceria nanoparticles exhibits; (1) a noticeable ennoblement of the corrosion potential; (2) a stabilization around relatively high value of the low frequency impedance during immersion time. In particular it was found that, depending on the amount of ceria nanoparticles in the testing solution, the electrochemical impedance response of the steel electrodes is similar to a “blocking electrode”. In that cases the fitting of the experimental curves using a Rel (Rct Qdl ) equivalent circuit was no more possible and a Rel Q equivalent circuit was employed. By modeling the experimental curves it was shown that the Rct keeps increasing immediately after the immersion of the steel electrodes in the solutions containing the ceria nanoparticles. After a certain time of exposure, it is no more possible to extrapolate Rct values as the shape of the curves indicates that the steel coupon behaves as a blocking electrode. Despite only indirect proofs were provided, this phenomenon, accompanied by a strong increase of the corrosion potential of the electrodes, is in agreement with the formation of a sort of passive or conversion layer on the metal surface. In principle, it seems that the presence of the particles inhibits the corrosion processes, probably by forming or inducing the formation of a passive or conversion layer on steel. The reason for this particular behavior is still under investigation, but it is believed to be related to the capability to shift the oxidation state from Ce4+ ↔ Ce3+ as a function of oxidative/reduction conditions.
4. Conclusions The electrochemical properties of mild steel electrode immersed in water based solution containing different amounts of ceria nanoparticles were investigated. The electrochemical characterization revealed that the presence of the particles leads to a remarkable ennoblement of the potential of the mild steel electrode. The EIS measurement revealed that the presence of the particles in the electrolyte exploited for the investigation (sulfates and chlorides containing solutions) have led to a noticeable increase of the impedance in the low frequency range. Simultaneously, the steel electrodes immersed in the cerium oxides containing solutions showed a switch from an “active electrode” behavior to a “blocking electrode” behavior. A further investigation performed in 0.1 M NaCl solutions revealed that a minimum value (around 3 wt%) of particles inside the solution was needed to guarantee a long term stability of the steel electrodes surface in such a way that the corrosion processes are delayed. Although not unequivocally proven by the experimental results, the shift of the corrosion potential as well as the switch of the impedance response of the steel electrodes immersed in the solutions was associated to the formation of a sort of passive or conversion layer on the surface of the metal electrode. This hypothesis would explain the strong reductions of the corrosion rate of the metal substrate in the ceria nanoparticles containing electrolytes as the ennoblement of the corrosion potential.
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Acknowledgments This work was supported by the SteelCoat Project (NMP1910.1.2-2 Substitution of materials or components utilizing green nanotechnology. Project Number: 263262) funded by FP7 program of European Commission References [1] C.M. Caruana, Met. Finish. 104 (2006) 44. [2] J. Sinko, Prog. Org. Coat. 42 (2001) 267. [3] E. Almeida, T.C. Diamantino, M.O. Figueiredo, C. Sà, Surf. Coat. Technol. 106 (1998) 8. [4] X. Yu, J. Wang, M. Zhang, P. Yang, L. Yang, D. Cao, J. Li, Solid State Sci. 11 (2009) 376. [5] C. Motte, M. Poelman, A. Roobroeck, M. Fedel, F. Deflorian, M.-G. Olivier, Prog. Org. Coat. 74 (2012) 326. [6] M.L. Zheludkevich, R. Serra, M.F. Montemor, M.G.S. Ferreira, Electrochem. Communic. 7 (2005) 836. [7] D. Snihirova, S.V. Lamaka, M.F. Montemor, Electrochim. Acta 83 (2012) 439. [8] A.M. Atta, O.E. El-Azabawy, H.S. Ismail, M.A. Hegazy, Corros. Sci. 53 (2011) 1680. [9] M.L. Zheludkevich, R. Serra, M.F. Montemor, K.A. Yasakau, I.M. Miranda Salvado, M.G.S. Ferreira, Electrochim. Acta 51 (2005) 208. [10] Y.C. Lu, M.B. Ives, Corros. Sci. 37 (1995) 145. [11] C. Motte, N. Maury, M.-G. Olivier, J.-P. Petitjean, J.-F. Willem, Surf. Coat. Technol. 200 (2005) 2366. [12] M.F. Montemor, A.M. Simões, M.G.S. Ferreira, M.J. Carmezim, App. Surf. Sci. 254 (2008) 1806. [13] M.A. Arenas, J. de Damborenea, Surf. Coat. Technol. 200 (2005) 2085. [14] A. Conde, M.A. Arenas, A. de Frutos, J. de Damborenea, Electrochim. Acta 53 (2008) 7760. [15] P. Yu, S.A. Hayes, T.J. O’Keefe, M.J. O’Keefe, J.O. Stoffer, J. Electrochem. Soc. 153 (2006) C74.
[16] S.A. Hayes, P. Yu, T.J. O’Keefe, M.J. O’Keefe, J.O. Stoffer, J. Electrochem. Soc. 149 (2002) C623. [17] F. Andreatta, M.-E. Druart, A. Lanzutti, M. Lekka, D. Cossement, M.-G. Olivier, L. Fedrizzi, Corros. Sci. 65 (2012) 376. [18] M. Schem, T. Schmidt, J. Gerwann, M. Wittmar, M. Veith, G.E. Thompson, I.S. Molchan, T. Hashimoto, P. Skeldon, A.R. Phani, S. Santucci, M.L. Zheludkevich, Corros. Sci. 51 (2009) 2304. [19] F. Deflorian, M. Fedel, S. Rossi, P. Kamarchik, Electrochim. Acta 56 (2011) 7833. [20] M. Fedel, F. Deflorian, S. Rossi, P. Kamarchik, Prog. Org. Coat. 74 (2012) 36. [21] K. Leistnera, A. Nicolle, P. Da Costa, Appl. Catal., B 111–112 (2012) 415. [22] P.S. Lambrou, C.N. Costa, S.Y. Christou, A.M. Efstathiou, Appl. Catal., B 54 (2004) 237. [23] H. Shinjoh, J. Alloys Compd. 408–412 (2006) 1061. [24] V.M. Shinde, G. Madras, Appl. Catal., B 123–124 (2012) 367. [25] N. J. Lawrence, Synthesis and Catalytic Activity of Nanostructured Cerium Oxide, Master thesis, University of Nebraska, 2010. On website: http://digitalcommons.unl.edu/chemistrydiss/16 [26] R. Li, S. Yabe, M. Yamashita, S. Momose, S. Yoshida, S. Yin, T. Sato, Solid State Ionics 151 (2002) 235. [27] N.V. Skorodumova, S.I. Simak, B.I. Lundqvist, I.A. Abrikosov, B. Johansson, Phys. Rev. Lett. 89 (2002) 166601–166602. [28] G.Z. Cao, Y. Wang, Nanostructures and Nanomaterials: Synthesis, Properties, and Applications, 2nd Ed, World Scientific Publishing, Toh Tuk Link, Singapore, 2011. [29] M.G. Fontana, N.D. Green, Corrosion Engineering, McGraw-Hill International, USA, 1978. [30] Uhlig’s Corrosion Handbook, R. Winston Revie (Ed.) John Wiley & Sons (2000), New York, USA. [31] V.F. Lvovich, Impedance Spectroscopy Applications to Electrochemical and Dielectric Phenomena, John Wiley & Sons, Hoboken, USA, 2012. [32] J.R. Scully, D.C. Silverman, M.W. Kendig, Electrochemical impedance–Analysis and interpretation, ASTM, Philadelphia, USA, 1993. [33] M.E. Orazem, N. Pébére, B. Tribollet, J. Electrochem. Soc. 153 (2006) B129. [34] J.-B. Jorcin, M.E. Orazem, N. Pébére, B. Tribollet, Electrochim. Acta 51 (2006) 1473.
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electrochemical investigation of the inhibition effect of CeO2 nanoparticles on the corrosion of mild steel M. Fedel a , A. Ahniyaz b , L.G. Ecco a , F. Deflorian a,∗ a b
Department of Industrial Engineering, University of Trento, Via Mesiano, 77, 38123, Trento, Italy SP Chemistry, Materials and Surfaces, Drottning Kristinas vag 45, Box 5607, SE-114, Stockholm, Sweden
a r t i c l e
i n f o
Article history: Received 24 July 2013 Received in revised form 22 November 2013 Accepted 27 November 2013 Available online 12 December 2013 Keywords: Cerium oxides Corrosion inhibition EIS Steel
s u m m a r y The paper aims to provide some insight into the fundamental mechanisms for the behavior of cerium oxides nanoparticles as corrosion inhibitors for steel. The work was carried out on mild steel water based dispersions of cerium oxides nanoparticles in presence of both, sulfates and chlorides. Ceria nanoparticles were produced via precipitation of cerium Ce(NO3 )3 .6H2 O in water, particles of about 70 nm hydrodynamic diameter were obtained. The analysis of the effectiveness of the ceria nanoparticles as corrosion inhibitors was performed by means of electrochemical techniques such as electrochemical impedance spectroscopy (EIS) and open circuit potential (OCP) versus time measurements. The experimental measurements suggested that cerium oxide affects the electrochemical properties of mild steel surface; they promoted an ennoblement effect and strong modifications in the impedance response. © 2013 Elsevier Ltd. All rights reserved.
1. Introduction Due to the health and environmental issues related to the use of cancerogenic or toxic compounds, in the last decades the need of novel corrosion inhibitor compounds has become more and more stringent [1]. Anionic oxygen-dependent compounds, such as molybdates, permanganates, vanadates and tungstates which act as passivating agents, have been widely studied as alternatives to the well-known chromium IV based materials [2,3]. In the field of anti-corrosion pigments for organic coatings, these materials have been recently investigated in conjunction with ions exchange particles such as hydrotalcites and bentonites [4,5] which act as reservoirs for corrosion inhibitors. Based on present literature, several different strategies and materials were exploited for storage and prolonged release of corrosion inhibitors [6–9]. Among the novel materials developed as corrosion inhibitors, the interest for the use of lanthanide compounds is growing over the last years. The success of these compounds is attributed to the corrosion potential efficiency of cerium salts, in particular chlorides and nitrites. It has been demonstrated that they promoted protection to a significant number of metals and alloys, for example steel, hot dip galvanized steel, tin, aluminum and their alloys, etc. [10–14]. In fact, because of the local pH increase at the cathodic sites due to the generation of OH− ions (2H2 O + O2 + 2e− → 4OH− ), which
∗ Corresponding author. E-mail address: flavio.defl[email protected] (F. Deflorian). 0013-4686/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2013.11.164
occurs when a corrosion process takes place, insoluble cerium compounds such as Ce(OH)4 and CeO2 ·xH2 O are formed [15,16]. These compounds are believed to precipitate on the surface of the metal, reducing the cathodic reaction rate and, thus, the overall corrosion extent [17]. Following the success of soluble cerium salts as corrosion inhibitors, cerium oxides particles have been likewise studied as pigments inside protective coatings for corrosion mitigation purposes [18–20]. Cerium oxides are widely used as a reducible oxide support material in the field of emission control catalysis for the purification of exhaust gases for different combustion systems [21], mainly due to their oxygen storage capacity (OSC) [22]. Owing to the ease of the Ce4+ ↔Ce3+ redox shuttle, the exchange process 2CeO2 ↔ Ce2 O3 +1/2O2 is favored [23]. Thus, the material is suitable for storing and releasing oxygen under conditions fluctuating between oxidizing and reducing state of cerium ions [24]. In fact, the Ce3+ ↔ Ce4+ shift leads to a high oxygen mobility in the ceria lattice that is responsible for its high catalytic activity [25]. The particular behavior of the cerium oxides was attributed to the unstable fluorite structure of the lattice where some Ce4+ have a tendency to be reduced to Ce3+ , which has a larger ionic size than Ce4+ , followed by oxygen molecules release with subsequent formation of oxygen vacancies [26,27]. Moreover, greater the number of vacancies, easier is the oxygen mobility around the crystal, allowing ceria to promote the reduction and oxidation of molecules. It is believed that the previously discussed properties, which are related to the bulk material, are maintained also when cerium oxides are produced as nano-sized particles. In addition, consider that under these conditions the chemical potential
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of the material increases according to the Young-Laplace equation [28] leading to a more reactive surface of the cerium oxide [25]. Due to the capability to shift the oxidation state from Ce4+ ↔ Ce3+ as a function of oxidative/reduction conditions, in this study the potential of cerium oxides nanoparticles as a corrosion inhibitor for steel is investigated. Despite the presence of a few works which proved the beneficial effect of the cerium oxides for corrosion protection, the mechanisms by which ceria is able to inhibit the corrosion process on a metal substrate are still unclear and remains as a subject of debate. For this reason, the aim of this work is to provide some insight into the fundamental mechanisms for the behavior of the investigated cerium oxides nanoparticles. The experimental work reported here has been carried out on mild steel substrates immersed in water based dispersions containing cerium oxides nanoparticles with diverse concentrations. The anticorrosion effectiveness has been investigated by means of electrochemical techniques such as electrochemical impedance spectroscopy (EIS) and open circuit potential (OCP) versus time. Supplementary information about the surface of the mild steel after exposure has been analyzed by means of An environment scanning electron microscopy (ESEM). Fig. 1. TEM Image of the ceria nanoparticles and corresponding SAED.
2. Experimental The ceria nanoparticles were produced by the precipitation of cerium Ce(NO3 )3 .6H2 O with NH4 OH in the presence of H2 O2 and acetic acid at elevated temperature. First, 30 g of Ce(NO3 )3 ·6H2 O and 2.6 g of acetic acid were dissolved in 2000 ml preheated water at 80 ◦ C; 28% ammonia was slowly added drop-wise to the above solution until pH reached 9 and then heated another 1 hour under stirring with Ultra Turrax at 10000 rpm. Then, reaction was cooled down by turning off the heat source. The resultant precipitate was collected by centrifugation at 4000 rpm for 20 minutes. To remove all the free-ions, such as acetate, nitrate ions in the nanoparticle dispersions, nanoparticles were extensively washed with double distilled water and re-dispersed in distilled water. Then, the pH was adjusted to 6-8 using diluted HNO3 acid and more water was added. The resulting solution was sonicated for about 10 minutes to obtain 10wt% ceria nanoparticle dispersion. The original solution containing the nanoparticles was modified adding Na2 SO4 (final concentration in the solution: 0.3wt%) or NaCl (final concentration in the solution 0.1 M) to perform the electrochemical measurements. In this way, it was possible to control the electrical conductivity of the aqueous media. Whenever necessary the solution containing the ceria nanoparticles was diluted to obtain different concentrations. For this purpose 0.3wt% Na2 SO4 or 0.1 M NaCl solutions were used. Mild steel sheets (Q-Panels) were used as substrate. The metal sheets were ultrasonically degreased with acetone, rinsed with de-mineralized water and then air blowing dried before the experimental measurements. Dynamic light scattering study was carried out using a Zetasizer (Nano ZS, 2003, Malvern Instruments, UK). Zeta potential of cerium oxide nanoparticle was measured using Zetasizer in 10 mM NaCl solution at pH 6. Transmission Electron Microscopy (TEM) images and Selected Area Electron Diffraction (SAED) patterns were obtained using a JEOL JEM-3010 microscope operating at 300 kV (Cs = 0.6 mm, Point resolution 0.17 nm). Images were recorded with a CCD camera (MultiScan model 794, Gatan, 1024 × 1024 m). TEM images were obtained from the very diluted and pH adjusted (to pH 6) dispersion of cerium oxide nanoparticles which was sonicated for additional 30 min during the TEM sample preparation. In order to avoid the aggregation of ceria nanoparticle during the drying process, hydrophobic carbon-coated TEM grid modified with 0,5% SDS
(sodium dodecyl sulfonic acid) was used. Due to the hydrophilic nature of the modified TEM grid surface, drying and induced evaporation, aggregation of cerium oxide nanoparticles was avoided. Electrochemical analyses were carried out on the mild steel electrodes immersed in different solutions containing the particles. The OCP was measured employing a traditional two electrodes arrangement using an AMEL Ammeter/Electrometer (Model: 668/RM). The potential of the mild steel as working electrode with respect to the Ag/AgCl (+205 mV vs SHE) reference electrode was monitored during immersion time. EIS measurements were performed at the free corrosion potential using an Autolab 302 N Potentiostat/Frequency Response Analyser. The selected signal amplitude was 5 mV, 105 –10−2 Hz frequency range and about 0.6 cm2 testing area was employed. A classical three electrodes arrangement was used: a Ag/AgCl reference electrode (+205 mV vs SHE) and platinum ring counter electrode were employed while the working electrode was the mild steel sheet. The measurements were performed in the solutions containing the nanoparticles. The solution consisted in 0.3wt% Na2 SO4 and 0.1 M NaCl containing different amounts of ceria nanoparticles. The impedance data were analysed with the software ZSimpWin 3.22. The different electrolytes were employed to assess the effectiveness of the ceria nanoparticles both in a mild (i.e. sulfates solution) and in a more aggressive solution (i.e. chlorides solution). All the measurements were carried out at room temperature and the reported data hold consistent repeatability. A Philips XL30 Environmental Scanning Electron Microscope was exploited to investigate the surface of the steel electrode after immersion in the different electrolytes. 3. Results and discussion Dynamic light scattering (DLS) study revealed that the average size of ceria particles in the dispersion is around 70 nm and zeta potential is -22 mV. It has been checked that z-potential did not change significantly in between 6 to 9 pH range. TEM study (Fig. 1) indicated that primary particle size of cerium oxides nanoparticles is around 5-20 nm. The selected area diffraction pattern (SAED) of cerium oxide nanoparticles is also reported. As far as Fig. 1 is concerned, the ring electron diffraction pattern corresponds to randomly distributed crystals of the nanoparticles. Comparing TEM
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For example, Fig. 3 displays the evolution of the impedance modulus versus the time of exposure for a mild steel electrode exposed to a solution containing 1 wt% of the cerium oxide nanoparticles dispersed in 0.3wt% Na2 SO4 . One can notice that the electrode immersed in the sodium sulphate solution containing 1.0 wt% of ceria nanoparticles showed relatively high and stable values of the low frequency impedance (|Z|0.01 ) during time, in the order of 106 .cm2 . It is worth to observe that the electrode maintained a stable impedance value for over than 500 hours of analysis. It seems that when about 1.0 wt% of cerium oxide nanoparticles is added to the electrolyte, high impedance values are recorded by the equipment. EIS were carried out also for 5 and 10 wt% ceria containing solution, as shown in Figs. 4 and 5, respectively. As far as the 0.3wt% Na2 SO4 solution is concerned, the evolution of the corrosion behaviour seems not to be affected by different concentrations of cerium oxide. The EIS spectra related of the steel electrodes immersed in the different solutions are quite similar to those with only 1.0 wt% of cerium oxide nanoparticles. The impedance modulus in the low frequency range is about 106 .cm2 for all the investigated solutions and remained constant with significant stability for over than 500 h of immersion. In order to evaluate the effect of the cerium containing solutions on the steel electrodes, the morphology of the metal surface was observed after 500 hours of exposure. Figs. 6(a) and (b) show the metal surfaces exposed to the sodium sulphate solution containing
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and DLS findings, as one may expect, agglomeration of the nanoparticles occurred when immersed into a water-based solution. As described in the previous section, cerium oxide nanoparticles were dispersed in a 0.1 M NaCl solution. Fig. 2 shows the OCP evolution of the mild steel electrode immersed in 0.1 M sodium chloride solution containing diverse concentrations of the cerium oxide. Similarly, the evolution of a mild steel electrode immersed in a blank solution is reported. The pH of the different solutions containing the nanoparticles, measured prior the starting of the measurements, was found to be around 8 for all the investigated solutions. For this reason, the pH of the blank 0.1 M NaCl solution was adjusted to 8 by adding sodium hydroxide in order to have a consistent comparison among the studied samples. As far as electrode immersed in the blank solution is concerned, notice that the (OCP) dropped from -0.43 to -0.67 V in less than one hour, according to the free corrosion potential value of mild steel [29]. Afterward, the potential value remained stable during immersion time, for over 300 hours. On the other hand, in presence of 1 and 2 wt% of cerium oxide nanoparticles a higher initial OCP value of the steel electrode (near -0.15 V) is observed. However, the ennoblement lasted for 3 hours of immersion. After the third hour, a reduction of the free corrosion potential down to about -0.75 V is observed. Still more relevant is the ennoblement promoted by contents equal to or higher than 3 wt% of cerium oxide nanoparticles. When the amounts of 3 or 5 wt% of ceria particles have been added into the solution a dramatic increase of the OCP is observed. Approximately the same ennoblement, near -0.15 V, accompanied by a remarkable stability, is provided by the 3 and 5 wt% ceria nanoparticles containing solutions, which lasted for about 150 and 350 hours, respectively. Based on the present results, ceria nanoparticles seems to be able to promote an increase of the free corrosion potential value of the steel electrode from ∼ -0.65 V (free corrosion potential of steel) to ∼ -0.05 ÷ 0.10 V. As far as Fig. 2 is concerned, itseems that the presence of any amount of ceria nanoparticles leads to an increase of the free corrosion potential of the steel electrodes from ∼ -0.65 V to about 0.0 V. In addition, one can notice that the higher the amount of ceria nanoparticles into the chloride solution, longer the ennoblement effect lasts. To better investigate the effectiveness of the ceria nanoparticles, electrochemical impedance spectroscopy (EIS) spectra were acquired during immersion time in a mild electrolyte such as 0.3wt% Na2 SO4. Also in presence of sulphates the pH of the different solutions containing the nanoparticles, acquired before the electrochemical tests, was found to be around 8. Following a similar procedure, the pH of the blank 0.3wt% Na2 SO4 solution was adjusted to 8.
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1 and 5 wt% of cerium oxides for about 500 hours, evaluated by means of environmental scanning electron microscopy (ESEM). The topographies depicted in Figs. 6(a) and 6(b) appears to be similar, characterized by the presence of only very limited corrosion extent. In this sense, while in contact with steel, it seems that both solutions protected steel against corrosion, as only a slight corrosion attack is observed. In fact, the steel electrodes immersed in a sodium sulphate solution containing about 1 wt% of ceria nanoparticles revealed the presence of small corrosion attack, as it is shown in Fig. 7. No similar corrosion products were found in the steel surface exposed to the 5 and 10 wt% ceria solutions. The corrosion protection potential of the ceria particles were further investigated in presence of sodium chloride solution. Chlorides are recognized to promote the corrosion of steel in neutral environment by forming complexes which tend to be unstable and soluble [30]. The electrochemical impedance evolution during immersion time of the steel electrodes immersed in the 0.1 M NaCl solutions containing 1 wt% and 3 wt% of CeO2 nanoparticles is reported in Figs. 8 and 9, respectively. Considering Fig. 8, one can observe that the overall impedance decreased during immersion time after the first hours of immersion. The low frequency impedance (|Z|0.01 ) showed a reduction
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from about 105 *cm2 , from the beginning, to 103 *cm2 after about 360 hours. As far as Fig. 9 is concerned, it is possible to appreciate a remarkable stability of the impedance spectrum of the steel immersed in the 0.1 M NaCl solution containing 3 wt% of CeO2 particles. In fact, |Z|0.01 remained stable around 106 *cm2 during the 312 hours of immersion (Fig. 9a). Furthermore, one can notice that the impedance phase in the low frequency range is shifted towards higher angles after 1 hour of immersion. As one can appreciate from Fig. 9(b), the very first acquisition (t = 0 h) showed one time constant for the phase angle spectrum. However, during time the system evolved and the phase spectrum exhibited an approximately horizontal line positioned at about 85÷86 degrees in the low frequency
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Fig. 7. SEM topography of mild steel after 500 hours of exposure to the sodium sulfate solution containing 1wt% of ceria.
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range. The resulting shape of the impedance spectrum seems to be in accordance with the electrochemical impedance response of a “blocking electrode” (described elsewhere in literature [31]) corresponding to an electrical circuit where no current flows when the dc limit is reached. The same behavior is observed for the steel electrodes immersed in the 0.1 M NaCl solutions containing 5 and 10 wt% of CeO2 particles. A comparison among electrochemical evolution within the first and the fifth hours of immersion of the mild steel electrodes exposed to the solutions is shown in Figs. 10 and 11, where the EIS spectra are reported after 0 and 5 hours of exposure, respectively. The EIS spectra at time = 0 h (Fig. 10) highlight that when ceria nanoparticles have been added into the 0.1 M NaCl solution a strong increase of the impedance in the middle-low frequency range is observed. All the electrodes immersed in the different solutions presented a |Z|0.01 which exceeded the value of 105 *cm2 , remarkably higher than the 103 *cm2 measured for the samples immersed in the blank 0.1 M NaCl solution. The impedance modulus in the low frequency range for the steel electrode immersed in the 1 wt% ceria containing solution dropped from 105 *cm2 down to 103 *cm2 within 5 hours of exposure (Fig. 11). Higher contents of cerium oxide promoted higher initial
values for |Z| at low frequency, quite close to 106 *cm2 , along with a remarkable stability. On the contrary, from Fig. 10 it is possible to appreciate that the electrodes immersed in the solution containing 3, 5 and 10 wt% of ceria nanoparticles showed an increase of the impedance during the first hours of immersion. The evolution
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Frequency / Hz Fig. 10. Impedance spectra at time = 0 hour for diverse amounts of ceria nanoparticles in 0.1 M NaCl solution.
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blank + 10.0wt.% CeO2
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Fig. 13. Circuit used for fitting the experimental data: RQ (a) and R(QR) (b).
-2
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Frequency / Hz Fig. 11. Impedance spectra at time = 5 hours for diverse amounts of ceria nanoparticles in 0.1 M NaCl solution.
with immersion time of the different electrodes was investigated for longer periods of exposure. Fig. 12 shows the EIS spectra for steel sheets immersed in the different solutions after about 400 h. It is worth noticing that the mild steel electrodes immersed in a sodium chloride solution containing at least 3 wt% of ceria nanoparticles showed high values of the impedance even after long immersion time (about 400 hours). On the other hand, the steel electrode immersed in the solution containing 1 wt% of ceria nanoparticles shows an impedance spectra which is comparable to that of the electrode in the blank solution. It seems that the presence of a certain amount of ceria nanoparticles is needed to maintain high impedance values during immersion time. Although not unequivocally proven by the experimental results, EIS and OCP measurements are suggesting the formation of a sort of passive or conversion layer on the steel electrodes by the cerium oxide nanoparticles, which have led to an increase of the absolute values of the impedance and to a relative stability, accompanied by an ennoblement of the potential. A deeper investigation of the evolution of the electrochemical properties of the steel electrodes immersed in the different solutions has been done by fitting the experimental spectra. The EIS spectra were modelled using the electrical equivalent circuits
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blank + 3.0wt.% CeO2 blank + 5.0wt.% CeO2
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Frequency / Hz Fig. 12. Impedance spectra after 400 hours for several amounts of CeO2 in 0.1 M NaCl solution.
depicted in Fig. 13. During the first hours of immersion the spectra were fitted using a Rel (Qdl Rct ) circuit, where Rel stands for the electrolyte resistance while (Qdl Rct ) is the time constant attributed to the faradic process occurring on the electrode surface [32]. Qdl is a constant phase element (CPE) related to the double layer capacitance of the steel surface, while Rct represents the charge transfer resistance of the faradic process. The electrical equivalent circuit depicted in Fig. 13 (a) consists of a Rel Q system. Since it represents the behaviour of a “blocking electrode” [33], according to similar approach for passive metals, the meaning of the Q is likely to be related to the contribution of the passive oxide on the surface of steel. It is worth emphasizing that in cases where the Rel Q circuit was exploited it was not possible to use the Rel (Rct Qdl ) circuit without a dramatic increase of the fitting relative error. The EIS spectra of the steel electrode immersed in the 0.1 M NaCl solutions containing 1 wt% of CeO2 particles were fitted using the circuit reported in Fig. 13 (b), since one time constant was clearly visible throughout immersion time. On the other hand, the steel electrodes immersed in the 0,1 M NaCl solutions containing 3, 5 and 10 wt% of CeO2 , were fitted using the circuit depicted in Fig. 13 (b) until an “active behavior” was observed and using the circuit depicted in Fig. 13 (a) when a sort of “blocking electrode” impedance response was recognized. The switch from “active” to “blocking” behavior was observed after a certain lapse of time, in the order of a few hours, different from sample to sample. The chi-squared error parameter related to the fitting procedure of the reported data was in the 10−5 ÷10−4 Range. Fig. 14 (a) shows the evolution during immersion time of the capacitance of the Y0 value related to the CPE (|Z|CPE = 1/(Y0 ·(j)n ) [34] used to fit the experimental data. Fig. 14 (b) shows the evolution of the corresponding exponent “n” value. Considering Fig. 14 (a), one can notice that the electrode immersed in the solution containing 1 wt% of cerium oxides showed a completely different trend of the Y0 values during immersion time. For this sample it was possible to appreciate a continuous increase of Y0 followed by a simultaneous shift toward lower values of the exponent “n” value (Fig. 14b). On the other hand, the steel electrodes immersed in the solution containing 3, 5 and 10 wt% of CeO2 showed a slight decrease of the Y0 values during the first hours of immersion to reach a sort of steady state around 10−5 S·sn ·cm−2 . At the same time, the exponent “n” values remained constant around 0.95. It is worth noticing that after a transient of about 24 hours, the electrodes immersed in the solutions containing 3, 5 and 10 wt% of CeO2 reached a constant value of Y0 and of the corresponding “n” exponent. Simultaneously, the shape of the phase angle spectra indicated that the behavior of the steel electrodes is changing from “active electrode” to “blocking electrode”. Considering the Rct values, Fig. 15 shows a comparison among the different samples.
M. Fedel et al. / Electrochimica Acta 131 (2014) 71–78
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Time / h Fig. 14. Evolution of the Y0 value (a) and exponent “n” value (b) during immersion time.
As one can notice, the Rct values for the electrodes immersed in the solution containing 3, 5 and 10 wt% of CeO2 are reported only for the first few hours of exposure. After this period of time, the Rel Q circuit was used to model the experimental data and the Rct was no longer measured. The electrode immersed in the different solutions presented very high charge transfer resistance values, over 105 *cm2 , regardless of the concentration of CeO2 . However,
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Time / h Fig. 15. Evolution of the charge transfer resistance (Rct ) value during immersion time.
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the solution containing 1 wt% of CeO2 showed a quick drop to 103 *cm2 after a few hours of immersion. On the other hand, when immersed in a solution containing at least 3 wt% of CeO2 the steel electrode exhibited an increase of the Rct reaching about 107 *cm2 before being no more possible to measure the charge transfer resistance value. Notice that the steel electrode immersed in a 10 wt% CeO2 solution behaves like an “active electrode” only in the very first hours of immersion. Therefore, after 3 hours of immersion the charge transfer resistance was no longer measured, as a Rel Q circuit was exploited to fit the experimental data. Considering the outcome of the experimental measurements, it was found that the electrochemical response of the steel electrode immersed in a solution containing a certain amount of ceria nanoparticles exhibits; (1) a noticeable ennoblement of the corrosion potential; (2) a stabilization around relatively high value of the low frequency impedance during immersion time. In particular it was found that, depending on the amount of ceria nanoparticles in the testing solution, the electrochemical impedance response of the steel electrodes is similar to a “blocking electrode”. In that cases the fitting of the experimental curves using a Rel (Rct Qdl ) equivalent circuit was no more possible and a Rel Q equivalent circuit was employed. By modeling the experimental curves it was shown that the Rct keeps increasing immediately after the immersion of the steel electrodes in the solutions containing the ceria nanoparticles. After a certain time of exposure, it is no more possible to extrapolate Rct values as the shape of the curves indicates that the steel coupon behaves as a blocking electrode. Despite only indirect proofs were provided, this phenomenon, accompanied by a strong increase of the corrosion potential of the electrodes, is in agreement with the formation of a sort of passive or conversion layer on the metal surface. In principle, it seems that the presence of the particles inhibits the corrosion processes, probably by forming or inducing the formation of a passive or conversion layer on steel. The reason for this particular behavior is still under investigation, but it is believed to be related to the capability to shift the oxidation state from Ce4+ ↔ Ce3+ as a function of oxidative/reduction conditions.
4. Conclusions The electrochemical properties of mild steel electrode immersed in water based solution containing different amounts of ceria nanoparticles were investigated. The electrochemical characterization revealed that the presence of the particles leads to a remarkable ennoblement of the potential of the mild steel electrode. The EIS measurement revealed that the presence of the particles in the electrolyte exploited for the investigation (sulfates and chlorides containing solutions) have led to a noticeable increase of the impedance in the low frequency range. Simultaneously, the steel electrodes immersed in the cerium oxides containing solutions showed a switch from an “active electrode” behavior to a “blocking electrode” behavior. A further investigation performed in 0.1 M NaCl solutions revealed that a minimum value (around 3 wt%) of particles inside the solution was needed to guarantee a long term stability of the steel electrodes surface in such a way that the corrosion processes are delayed. Although not unequivocally proven by the experimental results, the shift of the corrosion potential as well as the switch of the impedance response of the steel electrodes immersed in the solutions was associated to the formation of a sort of passive or conversion layer on the surface of the metal electrode. This hypothesis would explain the strong reductions of the corrosion rate of the metal substrate in the ceria nanoparticles containing electrolytes as the ennoblement of the corrosion potential.
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Acknowledgments This work was supported by the SteelCoat Project (NMP1910.1.2-2 Substitution of materials or components utilizing green nanotechnology. Project Number: 263262) funded by FP7 program of European Commission References [1] C.M. Caruana, Met. Finish. 104 (2006) 44. [2] J. Sinko, Prog. Org. Coat. 42 (2001) 267. [3] E. Almeida, T.C. Diamantino, M.O. Figueiredo, C. Sà, Surf. Coat. Technol. 106 (1998) 8. [4] X. Yu, J. Wang, M. Zhang, P. Yang, L. Yang, D. Cao, J. Li, Solid State Sci. 11 (2009) 376. [5] C. Motte, M. Poelman, A. Roobroeck, M. Fedel, F. Deflorian, M.-G. Olivier, Prog. Org. Coat. 74 (2012) 326. [6] M.L. Zheludkevich, R. Serra, M.F. Montemor, M.G.S. Ferreira, Electrochem. Communic. 7 (2005) 836. [7] D. Snihirova, S.V. Lamaka, M.F. Montemor, Electrochim. Acta 83 (2012) 439. [8] A.M. Atta, O.E. El-Azabawy, H.S. Ismail, M.A. Hegazy, Corros. Sci. 53 (2011) 1680. [9] M.L. Zheludkevich, R. Serra, M.F. Montemor, K.A. Yasakau, I.M. Miranda Salvado, M.G.S. Ferreira, Electrochim. Acta 51 (2005) 208. [10] Y.C. Lu, M.B. Ives, Corros. Sci. 37 (1995) 145. [11] C. Motte, N. Maury, M.-G. Olivier, J.-P. Petitjean, J.-F. Willem, Surf. Coat. Technol. 200 (2005) 2366. [12] M.F. Montemor, A.M. Simões, M.G.S. Ferreira, M.J. Carmezim, App. Surf. Sci. 254 (2008) 1806. [13] M.A. Arenas, J. de Damborenea, Surf. Coat. Technol. 200 (2005) 2085. [14] A. Conde, M.A. Arenas, A. de Frutos, J. de Damborenea, Electrochim. Acta 53 (2008) 7760. [15] P. Yu, S.A. Hayes, T.J. O’Keefe, M.J. O’Keefe, J.O. Stoffer, J. Electrochem. Soc. 153 (2006) C74.
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