- Email: [email protected]
A multi-electrode cell for high-throughput SVET screening of corrosion inhibitors
Corrosion Science 52 (2010) 3146–3149
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Short Communication
A multi-electrode cell for high-throughput SVET screening of corrosion inhibitors S. Kallip *, A.C. Bastos, M.L. Zheludkevich, M.G.S. Ferreira DECV/CICECO, University of Aveiro, 3810-193 Aveiro, Portugal
a r t i c l e
i n f o
Article history: Received 21 December 2009 Accepted 18 May 2010 Available online 23 May 2010 Keywords: B. SVET C. Corrosion C. Corrosion protection C. Inhibition
a b s t r a c t This communication introduces a novel approach for high-throughput screening of corrosion inhibitors using a multi-electrode cell combined with SVET (Scanning Vibrating Electrode Technique). The method has the advantage of saving time and materials. The cell is constituted by wire electrodes composed by the different metallic materials to be tested. SVET is used as the electrochemical tool to assess the corrosion on the metal wires in the multi-electrode cell, allowing to measure the localized cathodic and anodic currents. When inhibitive species are added to the test electrolyte their protection efficiency can be evaluated for the different materials in single experiment. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The need for active corrosion protection of different metallic structures calls for the development of more efficient corrosion inhibitors compatible with protective barrier polymer coatings. Moreover, corrosion inhibitors efficient for multi-material combinations present in the same structure are of great interest for many industrial applications. Given the high number of alloys and metal combinations used in the different areas of engineering and the nearly unlimited number of potential corrosion inhibitors, the selection process of the best inhibitors along with the conditions for best performance, can be tedious and time-consuming. Application of multi-electrode cells for fast screening of the corrosion inhibitors was recently suggested. Mol et al. have used potentiodynamic polarization as a method to test inhibition efficiency of corrosion inhibitors for different metals in multi-electrode cells [1]. However, the DC polarization is destructive and leads to serious cross-contamination. The concept of wire beam electrode (WBE) was developed by Tan et al. [2–4] The WBE is a multi-piece electrode constructed with a variable number of metal wires embedded in insulating material to simulate one-piece electrode but enabling the determination of local electrochemical parameters. The WBE was used together with SVET to emulate the behaviour of 2024-T3 aluminium alloy [5]. WBE concept relies on the assumption that the behaviour of a continuous surface under corrosion can be emulated using a multi-piece electrode. In this short communication a multi-electrode cell for fast screening of corrosion inhibitors is presented. In contrast to the
* Corresponding author. Fax: +351 234 378 146. E-mail address: [email protected] (S. Kallip). 0010-938X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2010.05.018
WBE, the metal wires in our multi-electrode cell are usually not electrically connected. In this case each wire corrodes free from the action of the others. SVET is used to monitor the local currents in solution above each metal wire. The SVET method was firstly used in life sciences [6–9], being introduced later in corrosion field [10–13]. With SVET it becomes possible to follow, for each metal, the evolution in space and time of the anodic and cathodic processes. The conditions of the SVET experiments are close to the undisturbed environment since the corrosion processes are not stimulated by any applied polarization. Different metals and corrosion inhibitors were tested in the present work showing the applicability of the method.
2. Experimental 2.1. The multi-electrode cell The multi-electrode cell was made using pure metal wires from Goodfellow (Cambridge, UK): Al, Zn and Cu with diameter 1 mm, 316L austenitic stainless steel (SS), Mg wires with diameters of 0.5 and 0.125 mm, respectively. The Fe wire electrode was machined from a 2 mm thick foil (Goodfellow). The wires were assembled into inert epoxy resin (Epo-Kwick TM (Buehler)). The epoxy mount was prepared in a way that fits in the SVET measurement system. Application of scotch-tape around the epoxy mount creates the container for the testing medium (Fig. 1). The surface of the multi-electrode array was renewed routinely before experiments with 1200 and 2500 grit silicon carbide paper. After polishing the cell was rinsed with distilled water, MilliporeTM deionised water and dried with pure ethanol before every exposure in the corrosive medium.
S. Kallip et al. / Corrosion Science 52 (2010) 3146–3149
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2.3. Calculation of corrosion parameters Five parameters were used to estimate the inhibition efficiency (IE) [15]: the maximal anodic (Imax AN) and cathodic (Imax CAT) current densities and the integrated anodic (Iint AN), cathodic (Iint CAT) and overall (Iint OV) ionic currents. Iint OV was calculated by the summation of the absolute values of Iint AN and Iint CAT. The integrated ionic currents Iint AN, Iint CAT and Iint OV are independent from the number of observed data points, calculated by:
Iint ¼
N X
in Sn
ð1Þ
n¼1
Fig. 1. The diagram of proposed muli-electrode setup for SVET measurements.
2.2. Electrochemical measurements In this work five solutions were tested. The corrosive medium (0.05 M NaCl) and the same solution with 0.01 M addition of different corrosion inhibitors - NaNO3, 1H-Benzotriazole (BTA), CeCl3 and Ce(NO3)3. The solution was naturally aerated and experiments were conducted at ambient temperature. The cell was filled with the testing medium and the electrodes were left at open circuit potential, electrically isolated from the others. The SVET measurements were performed using Applicable Electronics Inc. (USA) instrumentation and controlled with the ASET software from ScienceWares (USA). SVET measures potential gradients in solution that are converted to current densities (normalized for 1 cm2) after a calibration [11,12]. The calibration takes into account the different conductivity of the various solutions used. The vibrating microelectrode had a 10–20 lm spherical platinum black tip and vibrated with 20 lm amplitude in two directions (normal and parallel to surface) at an average distance of 100 lm above the surface of the sample. The WSxMTM free software [14] has been used for processing the SVET maps.
where in is the SVET current density measured in point n (at 100 lm above the surface), Sn is the surface area (cm2) corresponding to one data point and N is the number of data points (anodic, cathodic or overall) considered for calculation. For all the above I parameters, values of the inhibition efficiency (IE) were calculated by:
IEð%Þ ¼
I0 Iinh 100 I0
ð2Þ
where I0 is the measured current in 0.05 M NaCl solution and Iinh is the current in the presence of inhibitor. 3. Results and Discussion Fig. 2a shows the multi-electrode array constituted by wires of pure Fe, Zn, Al, Cu, Mg and 316L austenitic stainless steel before immersion in 0.05 M NaCl and Fig. 2b shows the same after 6 h of immersion. Fig. 2c presents the SVET map taken in 0.05 M NaCl using the multi-electrode cell after 6 h of immersion. Well defined anodic and cathodic currents are visible in different zones of the cell. The most active metals are Mg, Zn and Fe, as expected. The anodic currents are generated by the flux of cations formed during metal dissolution:
Me ! Menþ þ ne
ð3Þ
where Me designates the respective metals.
Fig. 2. Multi-electrode cell in a corrosive medium (0.05 M NaCl). Surface photograph before immersion (a), surface photograph after 6 h of immersion (b), SVET map after 6 h of immersion (c), and selected current profiles from SVET map for Mg, Zn and Fe electrodes (d).
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S. Kallip et al. / Corrosion Science 52 (2010) 3146–3149
Fig. 3. Multi-electrode cell in a corrosive medium + inhibitor (0.05 M NaCl + 0.01 M Ce(NO3)3) after 6 h of immersion. SVET map (a), surface photograph (b) and selected current profiles from SVET map for Mg and Fe electrodes (c).
The cathodic currents result from the following reduction reactions:
O2 þ 2H2 O þ 4e ! 4OH
ð4Þ
2H2 O þ 2e ! 2OH þ H2
ð5Þ
Ionic current profiles obtained after 6 h in NaCl solution are presented in Fig. 2d for Mg, Zn and Fe, showing differences in corrosion activity of the tested metals. The other metals, Al, Cu and SS 316L, showed no detectable activity as can be anticipated form the diagram in Fig. 2c. A remarkable decrease of corrosion activity on all metals occurs when 0.01 M Ce(NO3)3 is added to the electrolyte, Fig. 3. No corrosion processes are observed on Zn electrode and only small anodic currents (iz < 6 lA cm2) can be detected on Fe and Mg electrodes. For this particular inhibitor the possible mechanism of the corrosion inhibition seems to be based on the blocking of cathodic sites by precipitated cerium hydroxide. At the cathodic areas, due to the local pH increase originated in reactions (4) and (5), the deposition of cerium hydroxide may occur [16,17]:
Ce3þ þ 3OH ! CeðOHÞ3 #
ð6Þ
Other inhibitors, CeCl3, NaNO3 and n-benzotriazole, were also tested in the same manner in order to estimate their inhibition properties for the different metals used in the multi-electrode cell.
Table 1 Observed maximal anodic and cathodic currents (Imax efficiencies (IE) from SVET data. Metal
Zn
Fe
Solution
NaCl NaNO3 BTA CeCl3 Ce(NO3)3 NaCl NaNO3 BTA CeCl3 Ce(NO3)3
AN
and Imax
CAT),
CeCl3 and NaNO3 were selected in order to evaluate the role of Ce3+ and NO 3 ions in the inhibiting effect of cerium nitrate. n-Benzotriazole was chosen to show the applicability of the suggested method for organic inhibitors which work via adsorption mechanism [18]. The diagrams are not shown for these inhibitors. The observed maximal ionic current values and the integrated ionic currents are displayed in Table 1 for all the Zn and Fe systems and were used to calculate the inhibition efficiencies. Only the parameters for Zn and Fe electrodes are presented since copper, stainless steel and aluminium showed only negligible corrosion activity in the used solutions. The inhibition efficiency for Mg cannot be monitored using the current design of the cell because of the low reproducibility. Due to the small diameter of the wire and the fast corrosion of Mg, the electrode dissolves rapidly and corrosion products cover the surface preventing SVET detection. The diameter of the Mg electrode was selected intentionally smaller than the others to avoid its influence over the rest of the electrodes (deposition of corrosion products and change in pH of testing solution). The results presented in Table 1 show a reasonable correlation between the corrosion efficiency obtained using the maximal anodic and cathodic currents and the integrated parameters. The BTA and both Ce3+ containing systems showed high inhibition efficiencies for Zn and Fe electrodes while the NaNO3 showed only minor inhibition for Zn. In the case of Fe the addition of NaNO3 raised remarkably both the anodic and cathodic activities. These results
integrated anodic, cathodic and overall (Iint
Anodic
AN, Iint CAT
and Iint
OV)
ionic currents and calculated inhibition
Cathodic
Overall
imax AN lAcm2
IE %
Iint AN nA
IE %
imax CAT lAcm2
IE %
Iint CAT nA
IE %
Iint OV nA
IE %
271.6 208.3 1.9 14.7 1.3 37.4 144.1 12.3 3.5 5.3
– 23.3 99.3 94.6 99.5 – 285 67.2 90.5 85.8
51.2 46.4 0.009 0.1 0.008 8.4 32.7 0.2 0.03 0.02
– 9.3 99.9 99.8 99.9 – 290 97.5 99.7 99.7
70.7 74.3 3.1 3.6 1.3 56.5 56.9 3.9 2.6 1.2
– 5.1 95.6 94.9 98.1 – 0.9 93.1 95.4 97.7
62.2 50.2 0.07 0.12 0.005 8.9 32.8 0.2 0.03 0.02
– 19.2 99.9 99.8 99.9 – 268 97.7 99.6 99.8
113.4 96.6 0.08 0.2 0.01 17.3 65.5 0.4 0.05 0.04
– 14.7 99.9 99.8 99.9 – 279 97.6 99.7 99.8
S. Kallip et al. / Corrosion Science 52 (2010) 3146–3149
also make evident, that the main inhibition effect of Ce(NO3)3 comes from Ce3+ cations and not from nitrate. There are some discrepancies between the different ways of calculation of IE (Table 1). In BTA and Ce3+ containing systems for Fe, when the calculation is done using the integrated anodic currents (Iint AN) the higher efficiency is obtained, comparing to IE calculated from the maximal anodic currents (Imax AN). It shows that there are still some small areas anodically active, even if the main part of electrode surface is passive. For instance, the BTA inhibited Fe showed Imax AN 12.3 lA cm2 and the IE of 67.2%. But since the active area is very small, the integrated current parameter Iint AN leads to higher inhibition efficiency (97.5%). The comparison of both ways of inhibition efficiency evaluation concludes that the use of the integrated values of ionic currents is in principle more reliable than the maximal currents. In the cases of low uniform and strongly localized corrosion, the maximal anodic ionic current can give additional information such as the degree of localization of the attack. The presented results show the applicability of the multi-electrode cell approach for high-throughput screening of inhibition efficiency for different metals. This cell is intended for fast preliminary selection when a large number of metals/inhibitors/concentrations have to be analysed. After a primer selection is made, the work should continue with deeper studies for the selected effective metal – inhibitor systems. In comparison to the method based on DC polarization as screening technique [1] the present approach has the advantage of being less destructive, since the samples are kept at the open circuit potential. However there are still a number of open questions to be answered in future work and a number of improvements to be done before the approach can be used as a reliable routine technique. The questions currently under investigation are the following: (a) Is the size of electrodes important for SVET measurements? We assumed that the results should not be strongly affected by the size of electrode. But the scattering of observed values for the small Mg electrode leads to the need of a deeper study. (b) Can the corrosion of some metal wires affect the behaviour of other systems by modification of the testing medium or by production of metallic cations and corrosion products that affect the processes taking place on other electrodes? (c) What are the best parameters to estimate/calculate the inhibition efficiency? Additionally to the compared maximal and integrated ionic currents also some statistical functions can be considered in future work to obtain more reliable parameters.
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(d) How trustful is the inhibition efficiency estimated by SVET? 4. Conclusions The multi-electrode cell presented in this short communication permits to assess the corrosion susceptibility and corrosion inhibition of different metals and alloys simultaneously. It has the advantage of reducing time and saving the amount of materials (reagents and metals) needed for measurement. This cell is intended for fast preliminary testing, when many metals and corrosion inhibitors have to be tested. Limitations of the proposed method are currently under investigation. Acknowledgements European FP7 project ‘‘MUST” NMP3-LA-2008-214261 and projects PTDC/CTM/108446/2008 and PTDC/CTM/66041/2006 (FCT, Portugal) are greatly acknowledged. S. Kallip and A.C. Bastos thank FCT for post-doctoral grants. References [1] T.H. Muster, A.E. Hughes, S.A. Furman, T. Harvey, N. Sherman, S. Hardin, P. Corrigan, D. Lau, F.H. Scholes, P.A. White, M. Glenn, J. Mardel, S.J. Garcia, J.M.C. Mol, Electrochim. Acta 54 (2009) 3402. [2] Y.-J. Tan, Prog. Org. Coat. 19 (1991) 89. [3] Y.-J. Tan, Corros. Sci. 41 (1999) 229. [4] Y.-J. Tan, S. Bailey, B. Kinsella, Corros. Sci. 43 (2001) 1905. [5] D. Battocchi, J. He, G.P. Bierwagen, D.E. Tallman, Corros. Sci. 47 (2005) 1165. [6] O. Bluh, B. Scott, Rev. Sci. Instr. 21 (1950) 867. [7] L.F. Jaffe, R. Nuccitelli, J. Cell Biol. 63 (1974) 614. [8] C. Scheffey, Rev. Sci. Instrum. 59 (1988) 787. [9] P. Somieski, W. Nagel, J. Exp. Biol. 201 (1998) 2489. [10] H.S. Isaacs, G. Kissel, J. Electrochem. Soc. 119 (1972) 1628. [11] H.S. Isaacs, Y. Ishikawa, Applications of the vibrating probe to localized current measurements, in: R. Baboian (Ed.), Electrochemical Techniques for Corrosion Engineering, NACE, Houston, 1986, pp. 56–76. [12] H.S. Isaacs, Corros. Sci. 28 (1988) 547. [13] K. Ogle, V. Baudu, L. Garrides, X. Philippe, J. Electrochem. Soc. 147 (2000) 3654. [14] I. Horcas, R. Fernandez, J.M. Gomez-Rodriguez, J. Colchero, J. Gomez-Herrero, A.M. Baro, Rev. Sci. Instruments 78 (2007) 013705. [15] A.C. Bastos, M.L. Zheludkevich, M.G.S. Ferreira, Portugaliae Electrochim. Acta 26 (2008) 47. [16] A.J. Aldykiewicz Jr, A.J. Davenport, H.S. Isaacs, J. Electrochem. Soc. 143 (1996) 147. [17] K.A. Yasakau, M.L. Zheludkevich, M.G.S. Ferreira, J. Electrochem. Soc. 155 (2008) C169. [18] M.L. Zheludkevich, K.A. Yasakau, S.K. Poznyak, M.G.S. Ferreira, Corros. Sci. 47 (2005) 3368.
Contents lists available at ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Short Communication
A multi-electrode cell for high-throughput SVET screening of corrosion inhibitors S. Kallip *, A.C. Bastos, M.L. Zheludkevich, M.G.S. Ferreira DECV/CICECO, University of Aveiro, 3810-193 Aveiro, Portugal
a r t i c l e
i n f o
Article history: Received 21 December 2009 Accepted 18 May 2010 Available online 23 May 2010 Keywords: B. SVET C. Corrosion C. Corrosion protection C. Inhibition
a b s t r a c t This communication introduces a novel approach for high-throughput screening of corrosion inhibitors using a multi-electrode cell combined with SVET (Scanning Vibrating Electrode Technique). The method has the advantage of saving time and materials. The cell is constituted by wire electrodes composed by the different metallic materials to be tested. SVET is used as the electrochemical tool to assess the corrosion on the metal wires in the multi-electrode cell, allowing to measure the localized cathodic and anodic currents. When inhibitive species are added to the test electrolyte their protection efficiency can be evaluated for the different materials in single experiment. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction The need for active corrosion protection of different metallic structures calls for the development of more efficient corrosion inhibitors compatible with protective barrier polymer coatings. Moreover, corrosion inhibitors efficient for multi-material combinations present in the same structure are of great interest for many industrial applications. Given the high number of alloys and metal combinations used in the different areas of engineering and the nearly unlimited number of potential corrosion inhibitors, the selection process of the best inhibitors along with the conditions for best performance, can be tedious and time-consuming. Application of multi-electrode cells for fast screening of the corrosion inhibitors was recently suggested. Mol et al. have used potentiodynamic polarization as a method to test inhibition efficiency of corrosion inhibitors for different metals in multi-electrode cells [1]. However, the DC polarization is destructive and leads to serious cross-contamination. The concept of wire beam electrode (WBE) was developed by Tan et al. [2–4] The WBE is a multi-piece electrode constructed with a variable number of metal wires embedded in insulating material to simulate one-piece electrode but enabling the determination of local electrochemical parameters. The WBE was used together with SVET to emulate the behaviour of 2024-T3 aluminium alloy [5]. WBE concept relies on the assumption that the behaviour of a continuous surface under corrosion can be emulated using a multi-piece electrode. In this short communication a multi-electrode cell for fast screening of corrosion inhibitors is presented. In contrast to the
* Corresponding author. Fax: +351 234 378 146. E-mail address: [email protected] (S. Kallip). 0010-938X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2010.05.018
WBE, the metal wires in our multi-electrode cell are usually not electrically connected. In this case each wire corrodes free from the action of the others. SVET is used to monitor the local currents in solution above each metal wire. The SVET method was firstly used in life sciences [6–9], being introduced later in corrosion field [10–13]. With SVET it becomes possible to follow, for each metal, the evolution in space and time of the anodic and cathodic processes. The conditions of the SVET experiments are close to the undisturbed environment since the corrosion processes are not stimulated by any applied polarization. Different metals and corrosion inhibitors were tested in the present work showing the applicability of the method.
2. Experimental 2.1. The multi-electrode cell The multi-electrode cell was made using pure metal wires from Goodfellow (Cambridge, UK): Al, Zn and Cu with diameter 1 mm, 316L austenitic stainless steel (SS), Mg wires with diameters of 0.5 and 0.125 mm, respectively. The Fe wire electrode was machined from a 2 mm thick foil (Goodfellow). The wires were assembled into inert epoxy resin (Epo-Kwick TM (Buehler)). The epoxy mount was prepared in a way that fits in the SVET measurement system. Application of scotch-tape around the epoxy mount creates the container for the testing medium (Fig. 1). The surface of the multi-electrode array was renewed routinely before experiments with 1200 and 2500 grit silicon carbide paper. After polishing the cell was rinsed with distilled water, MilliporeTM deionised water and dried with pure ethanol before every exposure in the corrosive medium.
S. Kallip et al. / Corrosion Science 52 (2010) 3146–3149
3147
2.3. Calculation of corrosion parameters Five parameters were used to estimate the inhibition efficiency (IE) [15]: the maximal anodic (Imax AN) and cathodic (Imax CAT) current densities and the integrated anodic (Iint AN), cathodic (Iint CAT) and overall (Iint OV) ionic currents. Iint OV was calculated by the summation of the absolute values of Iint AN and Iint CAT. The integrated ionic currents Iint AN, Iint CAT and Iint OV are independent from the number of observed data points, calculated by:
Iint ¼
N X
in Sn
ð1Þ
n¼1
Fig. 1. The diagram of proposed muli-electrode setup for SVET measurements.
2.2. Electrochemical measurements In this work five solutions were tested. The corrosive medium (0.05 M NaCl) and the same solution with 0.01 M addition of different corrosion inhibitors - NaNO3, 1H-Benzotriazole (BTA), CeCl3 and Ce(NO3)3. The solution was naturally aerated and experiments were conducted at ambient temperature. The cell was filled with the testing medium and the electrodes were left at open circuit potential, electrically isolated from the others. The SVET measurements were performed using Applicable Electronics Inc. (USA) instrumentation and controlled with the ASET software from ScienceWares (USA). SVET measures potential gradients in solution that are converted to current densities (normalized for 1 cm2) after a calibration [11,12]. The calibration takes into account the different conductivity of the various solutions used. The vibrating microelectrode had a 10–20 lm spherical platinum black tip and vibrated with 20 lm amplitude in two directions (normal and parallel to surface) at an average distance of 100 lm above the surface of the sample. The WSxMTM free software [14] has been used for processing the SVET maps.
where in is the SVET current density measured in point n (at 100 lm above the surface), Sn is the surface area (cm2) corresponding to one data point and N is the number of data points (anodic, cathodic or overall) considered for calculation. For all the above I parameters, values of the inhibition efficiency (IE) were calculated by:
IEð%Þ ¼
I0 Iinh 100 I0
ð2Þ
where I0 is the measured current in 0.05 M NaCl solution and Iinh is the current in the presence of inhibitor. 3. Results and Discussion Fig. 2a shows the multi-electrode array constituted by wires of pure Fe, Zn, Al, Cu, Mg and 316L austenitic stainless steel before immersion in 0.05 M NaCl and Fig. 2b shows the same after 6 h of immersion. Fig. 2c presents the SVET map taken in 0.05 M NaCl using the multi-electrode cell after 6 h of immersion. Well defined anodic and cathodic currents are visible in different zones of the cell. The most active metals are Mg, Zn and Fe, as expected. The anodic currents are generated by the flux of cations formed during metal dissolution:
Me ! Menþ þ ne
ð3Þ
where Me designates the respective metals.
Fig. 2. Multi-electrode cell in a corrosive medium (0.05 M NaCl). Surface photograph before immersion (a), surface photograph after 6 h of immersion (b), SVET map after 6 h of immersion (c), and selected current profiles from SVET map for Mg, Zn and Fe electrodes (d).
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Fig. 3. Multi-electrode cell in a corrosive medium + inhibitor (0.05 M NaCl + 0.01 M Ce(NO3)3) after 6 h of immersion. SVET map (a), surface photograph (b) and selected current profiles from SVET map for Mg and Fe electrodes (c).
The cathodic currents result from the following reduction reactions:
O2 þ 2H2 O þ 4e ! 4OH
ð4Þ
2H2 O þ 2e ! 2OH þ H2
ð5Þ
Ionic current profiles obtained after 6 h in NaCl solution are presented in Fig. 2d for Mg, Zn and Fe, showing differences in corrosion activity of the tested metals. The other metals, Al, Cu and SS 316L, showed no detectable activity as can be anticipated form the diagram in Fig. 2c. A remarkable decrease of corrosion activity on all metals occurs when 0.01 M Ce(NO3)3 is added to the electrolyte, Fig. 3. No corrosion processes are observed on Zn electrode and only small anodic currents (iz < 6 lA cm2) can be detected on Fe and Mg electrodes. For this particular inhibitor the possible mechanism of the corrosion inhibition seems to be based on the blocking of cathodic sites by precipitated cerium hydroxide. At the cathodic areas, due to the local pH increase originated in reactions (4) and (5), the deposition of cerium hydroxide may occur [16,17]:
Ce3þ þ 3OH ! CeðOHÞ3 #
ð6Þ
Other inhibitors, CeCl3, NaNO3 and n-benzotriazole, were also tested in the same manner in order to estimate their inhibition properties for the different metals used in the multi-electrode cell.
Table 1 Observed maximal anodic and cathodic currents (Imax efficiencies (IE) from SVET data. Metal
Zn
Fe
Solution
NaCl NaNO3 BTA CeCl3 Ce(NO3)3 NaCl NaNO3 BTA CeCl3 Ce(NO3)3
AN
and Imax
CAT),
CeCl3 and NaNO3 were selected in order to evaluate the role of Ce3+ and NO 3 ions in the inhibiting effect of cerium nitrate. n-Benzotriazole was chosen to show the applicability of the suggested method for organic inhibitors which work via adsorption mechanism [18]. The diagrams are not shown for these inhibitors. The observed maximal ionic current values and the integrated ionic currents are displayed in Table 1 for all the Zn and Fe systems and were used to calculate the inhibition efficiencies. Only the parameters for Zn and Fe electrodes are presented since copper, stainless steel and aluminium showed only negligible corrosion activity in the used solutions. The inhibition efficiency for Mg cannot be monitored using the current design of the cell because of the low reproducibility. Due to the small diameter of the wire and the fast corrosion of Mg, the electrode dissolves rapidly and corrosion products cover the surface preventing SVET detection. The diameter of the Mg electrode was selected intentionally smaller than the others to avoid its influence over the rest of the electrodes (deposition of corrosion products and change in pH of testing solution). The results presented in Table 1 show a reasonable correlation between the corrosion efficiency obtained using the maximal anodic and cathodic currents and the integrated parameters. The BTA and both Ce3+ containing systems showed high inhibition efficiencies for Zn and Fe electrodes while the NaNO3 showed only minor inhibition for Zn. In the case of Fe the addition of NaNO3 raised remarkably both the anodic and cathodic activities. These results
integrated anodic, cathodic and overall (Iint
Anodic
AN, Iint CAT
and Iint
OV)
ionic currents and calculated inhibition
Cathodic
Overall
imax AN lAcm2
IE %
Iint AN nA
IE %
imax CAT lAcm2
IE %
Iint CAT nA
IE %
Iint OV nA
IE %
271.6 208.3 1.9 14.7 1.3 37.4 144.1 12.3 3.5 5.3
– 23.3 99.3 94.6 99.5 – 285 67.2 90.5 85.8
51.2 46.4 0.009 0.1 0.008 8.4 32.7 0.2 0.03 0.02
– 9.3 99.9 99.8 99.9 – 290 97.5 99.7 99.7
70.7 74.3 3.1 3.6 1.3 56.5 56.9 3.9 2.6 1.2
– 5.1 95.6 94.9 98.1 – 0.9 93.1 95.4 97.7
62.2 50.2 0.07 0.12 0.005 8.9 32.8 0.2 0.03 0.02
– 19.2 99.9 99.8 99.9 – 268 97.7 99.6 99.8
113.4 96.6 0.08 0.2 0.01 17.3 65.5 0.4 0.05 0.04
– 14.7 99.9 99.8 99.9 – 279 97.6 99.7 99.8
S. Kallip et al. / Corrosion Science 52 (2010) 3146–3149
also make evident, that the main inhibition effect of Ce(NO3)3 comes from Ce3+ cations and not from nitrate. There are some discrepancies between the different ways of calculation of IE (Table 1). In BTA and Ce3+ containing systems for Fe, when the calculation is done using the integrated anodic currents (Iint AN) the higher efficiency is obtained, comparing to IE calculated from the maximal anodic currents (Imax AN). It shows that there are still some small areas anodically active, even if the main part of electrode surface is passive. For instance, the BTA inhibited Fe showed Imax AN 12.3 lA cm2 and the IE of 67.2%. But since the active area is very small, the integrated current parameter Iint AN leads to higher inhibition efficiency (97.5%). The comparison of both ways of inhibition efficiency evaluation concludes that the use of the integrated values of ionic currents is in principle more reliable than the maximal currents. In the cases of low uniform and strongly localized corrosion, the maximal anodic ionic current can give additional information such as the degree of localization of the attack. The presented results show the applicability of the multi-electrode cell approach for high-throughput screening of inhibition efficiency for different metals. This cell is intended for fast preliminary selection when a large number of metals/inhibitors/concentrations have to be analysed. After a primer selection is made, the work should continue with deeper studies for the selected effective metal – inhibitor systems. In comparison to the method based on DC polarization as screening technique [1] the present approach has the advantage of being less destructive, since the samples are kept at the open circuit potential. However there are still a number of open questions to be answered in future work and a number of improvements to be done before the approach can be used as a reliable routine technique. The questions currently under investigation are the following: (a) Is the size of electrodes important for SVET measurements? We assumed that the results should not be strongly affected by the size of electrode. But the scattering of observed values for the small Mg electrode leads to the need of a deeper study. (b) Can the corrosion of some metal wires affect the behaviour of other systems by modification of the testing medium or by production of metallic cations and corrosion products that affect the processes taking place on other electrodes? (c) What are the best parameters to estimate/calculate the inhibition efficiency? Additionally to the compared maximal and integrated ionic currents also some statistical functions can be considered in future work to obtain more reliable parameters.
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(d) How trustful is the inhibition efficiency estimated by SVET? 4. Conclusions The multi-electrode cell presented in this short communication permits to assess the corrosion susceptibility and corrosion inhibition of different metals and alloys simultaneously. It has the advantage of reducing time and saving the amount of materials (reagents and metals) needed for measurement. This cell is intended for fast preliminary testing, when many metals and corrosion inhibitors have to be tested. Limitations of the proposed method are currently under investigation. Acknowledgements European FP7 project ‘‘MUST” NMP3-LA-2008-214261 and projects PTDC/CTM/108446/2008 and PTDC/CTM/66041/2006 (FCT, Portugal) are greatly acknowledged. S. Kallip and A.C. Bastos thank FCT for post-doctoral grants. References [1] T.H. Muster, A.E. Hughes, S.A. Furman, T. Harvey, N. Sherman, S. Hardin, P. Corrigan, D. Lau, F.H. Scholes, P.A. White, M. Glenn, J. Mardel, S.J. Garcia, J.M.C. Mol, Electrochim. Acta 54 (2009) 3402. [2] Y.-J. Tan, Prog. Org. Coat. 19 (1991) 89. [3] Y.-J. Tan, Corros. Sci. 41 (1999) 229. [4] Y.-J. Tan, S. Bailey, B. Kinsella, Corros. Sci. 43 (2001) 1905. [5] D. Battocchi, J. He, G.P. Bierwagen, D.E. Tallman, Corros. Sci. 47 (2005) 1165. [6] O. Bluh, B. Scott, Rev. Sci. Instr. 21 (1950) 867. [7] L.F. Jaffe, R. Nuccitelli, J. Cell Biol. 63 (1974) 614. [8] C. Scheffey, Rev. Sci. Instrum. 59 (1988) 787. [9] P. Somieski, W. Nagel, J. Exp. Biol. 201 (1998) 2489. [10] H.S. Isaacs, G. Kissel, J. Electrochem. Soc. 119 (1972) 1628. [11] H.S. Isaacs, Y. Ishikawa, Applications of the vibrating probe to localized current measurements, in: R. Baboian (Ed.), Electrochemical Techniques for Corrosion Engineering, NACE, Houston, 1986, pp. 56–76. [12] H.S. Isaacs, Corros. Sci. 28 (1988) 547. [13] K. Ogle, V. Baudu, L. Garrides, X. Philippe, J. Electrochem. Soc. 147 (2000) 3654. [14] I. Horcas, R. Fernandez, J.M. Gomez-Rodriguez, J. Colchero, J. Gomez-Herrero, A.M. Baro, Rev. Sci. Instruments 78 (2007) 013705. [15] A.C. Bastos, M.L. Zheludkevich, M.G.S. Ferreira, Portugaliae Electrochim. Acta 26 (2008) 47. [16] A.J. Aldykiewicz Jr, A.J. Davenport, H.S. Isaacs, J. Electrochem. Soc. 143 (1996) 147. [17] K.A. Yasakau, M.L. Zheludkevich, M.G.S. Ferreira, J. Electrochem. Soc. 155 (2008) C169. [18] M.L. Zheludkevich, K.A. Yasakau, S.K. Poznyak, M.G.S. Ferreira, Corros. Sci. 47 (2005) 3368.