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Novel corrosion experiments using the wire beam electrode. (IV) Studying localised anodic dissolution of aluminium
Corrosion Science 48 (2006) 67–78 www.elsevier.com/locate/corsci
Novel corrosion experiments using the wire beam electrode. (IV) Studying localised anodic dissolution of aluminium Tie Liu, Yong-Jun Tan *, Bernice Zee Mei Lin, Naing Naing Aung School of Materials Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore Received 12 January 2004; accepted 6 November 2004 Available online 19 February 2005
Abstract An electrochemically integrated multi-electrode array namely the wire beam electrode (WBE) has been applied in novel experiments to study the anodic dissolution behaviour of aluminium (AA1100), which was exposed to corrosive media with and without the presence of corrosion inhibitor potassium dichromate. The objective of this work is to demonstrate the applicability of the WBE for investigating corrosion processes under anodic polarisation. Anodic current measurements and mapping have been made, for the first time, with the WBE surface being anodically polarised. Pitting potential as well as anodic dissolution profile has been successfully determined by mapping anodic dissolution currents over the anodically polarised WBE surface. The pitting potential determined using the WBE method was found to correlate well with that determined using the conventional pitting scan method; and the anodic dissolution profile determined using the WBE method was found to correlate with maps obtained using the scanning reference electrode technique (SRET). Potassium dichromate was found to significantly affect the pitting potential, anodic dissolution profile and pitting initiation characteristics of aluminium. Two mechanisms of localised corrosion initiation have been identified. For WBE surface under free corrosion or low anodic polarisation conditions, the initiation of localised corrosion was found to be due to the disappearance of minor anodes, *
Corresponding author. Tel.: +65 67905175; fax: +65 67909081. E-mail address: [email protected] (Y.-J. Tan).
0010-938X/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2004.11.022
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which lead to accelerated dissolution of a few major anodes. For WBE surface under large anodic polarisation, the initiation of localised corrosion was found to be due to the formation of active new anodic sites. This work suggests that the WBE method is useful for understanding the electrochemical behaviour of localised anodic processes, and their dependence on externally controllable variables. 2005 Elsevier Ltd. All rights reserved. Keywords: Localised corrosion; Wire beam electrode; Scanning reference electrode technique; Aluminium; Corrosion inhibitor
1. Introduction The corrosion resistance of many important engineering materials such as aluminium relies upon a passive film that protects its surface. In order to prevent this passive film from breakdown in aggressive media, corrosion inhibitors are widely added to the environment. Corrosion inhibitors such as chromate or dichromate are believed to enhance the passive film, prevent the initiation of localised anodes, and repair any damage in the passive film. However some important issues regarding aluminium localised corrosion and its inhibition have not been fully understood, for instance, the mechanisms of localised passive film breakdown, and the means by which chromates interact with the passive film are still controversial. Frankel et al. [1] recently questioned if chromate is an anodic inhibitor that prevents corrosion by significantly inhibiting anodic dissolution process. Their results suggested that the mechanism of localised corrosion inhibition of aluminium alloys by chromate must be something other than inhibition of anodic dissolution in an active pit or crevice [1]. These issues have received increasingly more attention during recent years, primarily due to the search for environmentally friendly replacements of traditional toxic corrosion inhibitors by mimicking the inhibition mechanism of the effective hexavalent chromium-containing inhibitors. Obviously the success of this research approach depends heavily upon the understanding of these issues. This present work aims to contribute to this research effort by introducing a new experimental method that could be used to achieve better understanding of the mechanisms of passive film breakdown and anodic dissolution processes, and the interaction between passive film and corrosion inhibitors. The key strategy of this work is to apply an electrochemically integrated multi-piece electrode, namely the wire beam electrode (WBE) [2–6], in a novel experimental set-up. The WBE is an array of mini-electrodes, which constitutes a unique integrated multi-electrode system that effectively could simulate a conventional one-piece electrode surface in electrochemical corrosion behaviour. Its addressable multi-electrode structure is a special feature that could allow the measurement of local electrochemical changes such as local currents flowing between corrosion anodes and cathodes located over its working surface. The WBE was originally developed for the study of the electrochemical heterogeneity of organic coatings, and later its application has been extended to the study of localised corrosion and various other nonuniform electrode processes. In this work, the WBE has been utilised, for the first time, under external anodic
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polarisation to enable the monitoring of the initiation of localised anodes, and also the distribution of anodic dissolution currents. The scanning reference electrode technique (SRET) [7–9] and the conventional cyclic potentiodynamic polarisation method have also been applied in this work, in conjunction with the WBE method. The objective of this preliminary work is only to demonstrate the applicability of the WBE method in studying the fundamental processes that underlie the localised corrosion of aluminium and its interaction with corrosion inhibitors. This work will be followed by more detailed investigation on aluminium corrosion processes and mechanism.
2. Experimental Fig. 1 shows an experimental set-up used in this work, which incorporates a threeelectrode electrochemical cell with aluminium WBE as working electrode, a saturated calomel electrode (SCE) as reference electrode and a platinum electrode as
Fig. 1. The schematic set-up of experiment using the WBE in combination with SRET, and photos showing the WBE and its working surface.
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auxiliary electrode. The WBE used in this work was made of one hundred aluminium (AA1100) wires that were pre-coated with epoxy coating and bound together using epoxy. Each aluminium wire had a surface area of about 1.17 mm2 and the WBE array had a dimension of 15 mm · 15 mm. The working surface of the WBE was polished with 320, 800 and 1000 grit silicon carbide paper and cleaned with deionised water and ethanol before being installed in the electrolyte tank of a SRET instrument (SP100 SRET system, EG&Company, USA), which functioned as an electrochemical cell. A solution containing 0.5 M NaCl was used in all experiment as the base corrosive media. Sodium dichromate (K2Cr2O7) was added into the base solution for studying inhibition effects. In order to study anodic dissolution processes, the WBE surface was polarised anodically under potentiostatic control, which was realised by connecting the WBE to the ÔWEÕ terminal of an AutoAC (ACM Instruments, UK) functioning as a potentiostat. The impressed anodic polarisation current for each wire (e.g. Ik for wire k) was measured by connecting another GillAC, which functioned as a zero resistance ammeter, in sequence between the chosen wire terminal and all other terminals (shorted together) using a computer controlled automatic switch (custom made). This was repeated for all 100 wires so that an anodic polarisation current distribution map that represents the current distribution across the WBE surface was generated. More details regarding the data analysis have been described previously [2,3,5,6]. SRET measurements were also carried out, alternately with WBE measurements, over the anodically polarised WBE surface, to map current distribution in the electrolytic phase. To avoid the probe tip contacting the specimen surface during the SRET measurements, the WBE surface was adjusted with a spirit level to ensure that its surface was flat and vertical to the probe tip. The distance between the probe tip and the WBE surface was carefully adjusted to approximately 100 lm, by moving the probe tip away from the specimen surface under computer control with the aid of a video camera. The ÔgainÕ of the SRET system was set at 2000 and the scan speed was set at 20 mm/s. The scanning ranges for X-direction and Y-direction were set as 15,000 lm. Each SRET measurement produces a map containing 384 scan lines with 512 data points per line. WBE and SRET measurements were repeated regularly, at certain desired intervals, during the duration of electrode exposure under anodic polarisation. During no measurement periods, all the wire terminals of the WBE were connected together to allow electrons to move freely between wires, in a similar way as would be the case with a single-piece mild steel electrodes with a larger surface area. All the experiments were conducted at air-conditioned room temperature (approximately 20 C).
3. Results and discussions Localised aluminium corrosion is believed to initiate due to the polarisation of aluminium above its pitting potential (Epit) by external or internal factors, such as the galvanic effect of a noble particle in the passive film or the presence of local
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chemistry that causes a higher local electrode potential [10]. In order to simulate and study such processes, in this work, external anodic polarisation was applied upon aluminium WBE surfaces that were exposed to corrosive media. WBE and SRET measurements were carried out to study the anodic dissolution by mapping current distributions in the metallic and electrolytic phases. It is expected that when the electrode is polarised to a potential above its Epit, according to conventional understanding on localised corrosion, WBE and SRET measurements should detect phenomena that are related to the initiation of localised anodic sites. The presence of corrosion inhibitor potassium dichromate should prevent the formation of such anodic sites. 3.1. Aluminium corrosion in 0.5 M NaCl solution Fig. 2 shows maps measured from a WBE surface before and after applying external anodic polarisation. Under this corrosion environment, as shown in
Fig. 2. WBE and SRET maps measured from aluminium WBE exposed to 0.5 M NaCl solution before and after applying external anodic polarisation.
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Fig. 2 (continued )
Fig. 3, aluminium electrode had an open circuit potential of around 900 mV (vs. SCE) and a pitting potential of approximately 760 mV. Before external polarisation was applied, as shown in WBE map of Fig. 2a, aluminium was under localised corrosion with anodic sites distributing over the WBE surface in a random manner. The anodic current values were low with the maximum anodic current density of only 0.048 mA/cm2. SRET were not able to detect such currents, and thus the map obtained was ÔemptyÕ. This result indicates that the SRET had a limitation in detecting this type of slow aluminium corrosion that produced only small currents flowing between local anodes and cathodes. It is well know that the sensitivity of SRET measurement is dependent heavily upon the ionic current value, the conductivity of the electrolyte, the distance between the probe tip and the specimen and also the distribution of anodes and cathodes on the specimen surface [6].
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Fig. 3. Cyclic potentiodynamic polarisation measurement (pitting scan) of aluminium WBE exposed to 0.5 M NaCl with and without inhibitor.
When the WBE surface was anodically polarised to 767 mV, which is above the open circuit potential but 7 mV below the pitting potential, the WBE map in Fig. 2b indicates that anodic dissolution was accelerated with the maximum anodic dissolution current density increased from 0.048 to 0.28 mA/cm2. An interesting observation is that anodic dissolution sites remained almost unchanged after the application of anodic polarisation. The SRET was still unable to detect any ionic current in the electrolytic phase.
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When anodic polarisation of the WBE surface was further increased to 747 mV, which is 13 mV above the pitting potential, as shown in the WBE map of Fig. 2c, anodic dissolution currents were further increased to a maximum value of 0.718 mA/cm2. SRET measurements were able to detect anodic dissolution processes, as shown in Fig. 2c, revealing similar anodic dissolution behaviour to that in the corresponding WBE map. Again the anodic dissolution sites were found to remain almost unchanged and that the anodic dissolution current density increased after the application of higher anodic polarisation. An unexpected phenomenon observable in Fig. 2 is that the number of main anodic sites over the WBE surface decreased with the increase in polarisation voltage. For instance, the numbers of main anodes shown in Fig. 2a and c are 9 and 5, respectively. This result is surprising, since conventional understanding on localised corrosion initiation expects that new localised anodic sites should initiate when the electrode is polarised to a potential above its Epit. This result suggests a mechanism of localised corrosion initiation—a process that minor anodes disappear, and anodic dissolution currents concentrate on a few main anodes. Similar pitting initiation mechanism has been observed in other experiments of studying localised corrosion related electrochemical noise [11]. The SRET maps in Fig. 2 also reveal another phenomenon that SRET is able to effectively detect ionic currents and their distribution when a WBE surface is polarised above its Epit. This result suggests that the SRET could be used to detect Epit, and more importantly the profile of pitting. Similar phenomena have been observed repeatedly in other experiments using WBE or conventional one-piece electrodes. When anodic polarisation was removed and thus aluminium WBE corrosion returned to free corrosion status, as shown in Fig. 2d, the WBE patterns became similar to that in Fig. 2a and the SRET map became ÔemptyÕ again. A difference between Fig. 2d and c is that some new major cathodes formed during the anodic polarisation processes. The mechanism behind this change is not clear and requires further investigation. 3.2. Aluminium corrosion in 0.5 M NaCl solution containing 0.5 M K2Cr2O7 When corrosion inhibitor potassium dichromate was added to the corrosive solution, as shown in Fig. 3b, pitting potential of aluminium became much less negative (approximately 650 mV vs. SCE). When the WBE specimen was exposed to such a corrosion condition without any external polarisation, as shown in Fig. 4a, localised corrosion occurred with anodic sites distributing over the WBE surface in a random manner. The corrosion behaviour was similar to that in Fig. 2a, except that the anodic current density values (maximum 0.025 mA/cm2) were lower than that in Fig. 2a (maximum 0.048 mA/cm2). These lower anodic dissolution currents suggest that potassium dichromate inhibited anodic dissolution. Again, SRET were not able to detect such currents flowing over the WBE surface, and thus the map obtained was ÔemptyÕ. The SERT measurements were not able to detect ionic current in the solution even when the WBE was polarised to 611 mV, 39 mV above its pitting potential. This
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Fig. 4. WBE and SRET maps measured from aluminium WBE exposed to 0.5 M NaCl solution with 0.5 M K2Cr2O7 added, before and after applying external anodic polarisation.
result clearly demonstrates that potassium dichromate effectively inhibited anodic dissolution. The WBE map in Fig. 4b indicates that anodic dissolution was
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Fig. 4 (continued )
accelerated with the maximum anodic dissolution current density increased from 0.025 to 0.164 mA/cm2. Similar to the observations made in the previous experiment, the anodic dissolution sites remained almost unchanged. However, when polarisation was extended to 30 min and 1 h, as shown in Fig. 4c and d, major changes occurred to WBE and SRET maps. Major anodic dissolution anodes, which are different from the major anodic sites in Fig. 4a and b, are observable form both the WBE and SRET maps. Anodic dissolution currents increased significantly to a maximum value of 4.733 and later 5.346 mA/cm2. SRET measurements were able to detect anodic dissolution processes, as shown in Fig. 4c and d, revealing similar anodic dissolution behaviour to that in the corresponding WBE map. This result suggests another anodic dissolution mechanism: The initiation of localised corrosion due to the initiation of new localised anodic sites. This process requires a higher anodic polarisation voltage and also an initiation period. This
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mechanism appears to agree with conventional understanding on localised corrosion initiation [10,12]. When polarisation voltage was removed and thus the aluminium WBE returned to free corrosion status, as shown in Fig. 4e, the WBE map became similar to that in Fig. 4a, and SRET map became ÔemptyÕ again. The newly formed anodic dissolution sites in Fig. 4c and 4d ceased to exist. The mechanism behind this phenomenon is not clear and requires further investigation.
4. Conclusions The wire beam electrode and the scanning reference electrode technique have been used in combination to study the anodic dissolution of aluminium in 0.5 M NaCl solution, and the inhibiting effects of potassium dichromate. Under polarisation conditions, SRET and WBE techniques were used successfully in determining corrosion behaviour and profile below and above the pitting potential (Epit), and the effects of inhibitors. It was found that with inhibitors, larger polarisation were required for pitting to initiate, in particular, long duration was required for pit to grow. It was also found that the SRET was not sensitive enough to monitor free corrosion process occurring on the surface of aluminium exposed to relatively mild corrosion environment or under low anodic polarisation voltage. The WBE method appeared to be more sensitive and could provide more detailed information of corrosion processes than the SRET. This work has revealed two different mechanisms of localised corrosion initiation: (i) The initiation of localised corrosion due to the disappearance of minor anodes; and (ii) The initiation of localised corrosion due to the initiation of new localised anodic sites.
References [1] Q. Meng, T. Ramgopal, G.S. Frankel, Electrochem. Solid State Lett. 5 (2002) B1–B4. [2] Y.J. Tan, S. Bailey, B. Kinsella, A. Lowe, J. Electrochem. Soc. 147 (2000) 530. [3] Y.J. Tan, Corrosion Science: A Retrospective and Current Status, in: G.S. Frankel, H.S. Isaacs, J.R. Scully, Sinclair, (Eds.), PV2002-13, The Electrochemical Society Proceedings Series, Pennington, NJ, (2002). [4] Y.J. Tan, Prog. Org. Coat. 19 (1991) 89. [5] Y.J. Tan, S. Bailey, B. Kinsella, Corros. Sci. 43 (2001) 1905–1929. [6] Y.J. Tan, T. Liu, N.N. Aung, Novel experiments using the wire beam electrode method: (III) measuring electrochemical corrosion parameters from both the metallic and electrolytic phases, Corros. Sci. (in press), doi:10.1016/j.corsci.2004.11.021. [7] H.S. Isaacs, B. Vyas, ASTM-STP 727 (1981) 3–33. [8] H.S. Isaacs, M.P. Ryan, L.J. Oblonsky, in: Proceedings of Research Topic Symposium, Corrosion 97, NACE (1997) 65–79. [9] H.N. McMurry, Corrosion, NACE 57 (2001) 313. [10] N. Missert, R.G. Copeland, F.D. Wall, C.M. Johnson, J.C. Barboiir, P. Kolula, Corrosion Science: A Retrospective and Current Status, in: G.S. Frankel, H.S.Isaacs, J.R. Scully, Sinclair, (Eds.), PV200213, The Electrochemical Society Proceedings Series, Pennington, NJ (2002).
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[11] Y.J. Tan, N.N. Aung, T. Liu, Novel experiments using the wire beam electrode method: (I) studying electrochemical noise signatures from localised corrosion processes, Corros. Sci. (in press), doi:10.1016/j.corsci.2004.11.019. [12] M.G. Fontana, Corrosion Engineering, Third Ed., McGraw-Hill Book Company, Singapore, 1987, p. 69.
Novel corrosion experiments using the wire beam electrode. (IV) Studying localised anodic dissolution of aluminium Tie Liu, Yong-Jun Tan *, Bernice Zee Mei Lin, Naing Naing Aung School of Materials Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore Received 12 January 2004; accepted 6 November 2004 Available online 19 February 2005
Abstract An electrochemically integrated multi-electrode array namely the wire beam electrode (WBE) has been applied in novel experiments to study the anodic dissolution behaviour of aluminium (AA1100), which was exposed to corrosive media with and without the presence of corrosion inhibitor potassium dichromate. The objective of this work is to demonstrate the applicability of the WBE for investigating corrosion processes under anodic polarisation. Anodic current measurements and mapping have been made, for the first time, with the WBE surface being anodically polarised. Pitting potential as well as anodic dissolution profile has been successfully determined by mapping anodic dissolution currents over the anodically polarised WBE surface. The pitting potential determined using the WBE method was found to correlate well with that determined using the conventional pitting scan method; and the anodic dissolution profile determined using the WBE method was found to correlate with maps obtained using the scanning reference electrode technique (SRET). Potassium dichromate was found to significantly affect the pitting potential, anodic dissolution profile and pitting initiation characteristics of aluminium. Two mechanisms of localised corrosion initiation have been identified. For WBE surface under free corrosion or low anodic polarisation conditions, the initiation of localised corrosion was found to be due to the disappearance of minor anodes, *
Corresponding author. Tel.: +65 67905175; fax: +65 67909081. E-mail address: [email protected] (Y.-J. Tan).
0010-938X/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2004.11.022
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which lead to accelerated dissolution of a few major anodes. For WBE surface under large anodic polarisation, the initiation of localised corrosion was found to be due to the formation of active new anodic sites. This work suggests that the WBE method is useful for understanding the electrochemical behaviour of localised anodic processes, and their dependence on externally controllable variables. 2005 Elsevier Ltd. All rights reserved. Keywords: Localised corrosion; Wire beam electrode; Scanning reference electrode technique; Aluminium; Corrosion inhibitor
1. Introduction The corrosion resistance of many important engineering materials such as aluminium relies upon a passive film that protects its surface. In order to prevent this passive film from breakdown in aggressive media, corrosion inhibitors are widely added to the environment. Corrosion inhibitors such as chromate or dichromate are believed to enhance the passive film, prevent the initiation of localised anodes, and repair any damage in the passive film. However some important issues regarding aluminium localised corrosion and its inhibition have not been fully understood, for instance, the mechanisms of localised passive film breakdown, and the means by which chromates interact with the passive film are still controversial. Frankel et al. [1] recently questioned if chromate is an anodic inhibitor that prevents corrosion by significantly inhibiting anodic dissolution process. Their results suggested that the mechanism of localised corrosion inhibition of aluminium alloys by chromate must be something other than inhibition of anodic dissolution in an active pit or crevice [1]. These issues have received increasingly more attention during recent years, primarily due to the search for environmentally friendly replacements of traditional toxic corrosion inhibitors by mimicking the inhibition mechanism of the effective hexavalent chromium-containing inhibitors. Obviously the success of this research approach depends heavily upon the understanding of these issues. This present work aims to contribute to this research effort by introducing a new experimental method that could be used to achieve better understanding of the mechanisms of passive film breakdown and anodic dissolution processes, and the interaction between passive film and corrosion inhibitors. The key strategy of this work is to apply an electrochemically integrated multi-piece electrode, namely the wire beam electrode (WBE) [2–6], in a novel experimental set-up. The WBE is an array of mini-electrodes, which constitutes a unique integrated multi-electrode system that effectively could simulate a conventional one-piece electrode surface in electrochemical corrosion behaviour. Its addressable multi-electrode structure is a special feature that could allow the measurement of local electrochemical changes such as local currents flowing between corrosion anodes and cathodes located over its working surface. The WBE was originally developed for the study of the electrochemical heterogeneity of organic coatings, and later its application has been extended to the study of localised corrosion and various other nonuniform electrode processes. In this work, the WBE has been utilised, for the first time, under external anodic
T. Liu et al. / Corrosion Science 48 (2006) 67–78
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polarisation to enable the monitoring of the initiation of localised anodes, and also the distribution of anodic dissolution currents. The scanning reference electrode technique (SRET) [7–9] and the conventional cyclic potentiodynamic polarisation method have also been applied in this work, in conjunction with the WBE method. The objective of this preliminary work is only to demonstrate the applicability of the WBE method in studying the fundamental processes that underlie the localised corrosion of aluminium and its interaction with corrosion inhibitors. This work will be followed by more detailed investigation on aluminium corrosion processes and mechanism.
2. Experimental Fig. 1 shows an experimental set-up used in this work, which incorporates a threeelectrode electrochemical cell with aluminium WBE as working electrode, a saturated calomel electrode (SCE) as reference electrode and a platinum electrode as
Fig. 1. The schematic set-up of experiment using the WBE in combination with SRET, and photos showing the WBE and its working surface.
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auxiliary electrode. The WBE used in this work was made of one hundred aluminium (AA1100) wires that were pre-coated with epoxy coating and bound together using epoxy. Each aluminium wire had a surface area of about 1.17 mm2 and the WBE array had a dimension of 15 mm · 15 mm. The working surface of the WBE was polished with 320, 800 and 1000 grit silicon carbide paper and cleaned with deionised water and ethanol before being installed in the electrolyte tank of a SRET instrument (SP100 SRET system, EG&Company, USA), which functioned as an electrochemical cell. A solution containing 0.5 M NaCl was used in all experiment as the base corrosive media. Sodium dichromate (K2Cr2O7) was added into the base solution for studying inhibition effects. In order to study anodic dissolution processes, the WBE surface was polarised anodically under potentiostatic control, which was realised by connecting the WBE to the ÔWEÕ terminal of an AutoAC (ACM Instruments, UK) functioning as a potentiostat. The impressed anodic polarisation current for each wire (e.g. Ik for wire k) was measured by connecting another GillAC, which functioned as a zero resistance ammeter, in sequence between the chosen wire terminal and all other terminals (shorted together) using a computer controlled automatic switch (custom made). This was repeated for all 100 wires so that an anodic polarisation current distribution map that represents the current distribution across the WBE surface was generated. More details regarding the data analysis have been described previously [2,3,5,6]. SRET measurements were also carried out, alternately with WBE measurements, over the anodically polarised WBE surface, to map current distribution in the electrolytic phase. To avoid the probe tip contacting the specimen surface during the SRET measurements, the WBE surface was adjusted with a spirit level to ensure that its surface was flat and vertical to the probe tip. The distance between the probe tip and the WBE surface was carefully adjusted to approximately 100 lm, by moving the probe tip away from the specimen surface under computer control with the aid of a video camera. The ÔgainÕ of the SRET system was set at 2000 and the scan speed was set at 20 mm/s. The scanning ranges for X-direction and Y-direction were set as 15,000 lm. Each SRET measurement produces a map containing 384 scan lines with 512 data points per line. WBE and SRET measurements were repeated regularly, at certain desired intervals, during the duration of electrode exposure under anodic polarisation. During no measurement periods, all the wire terminals of the WBE were connected together to allow electrons to move freely between wires, in a similar way as would be the case with a single-piece mild steel electrodes with a larger surface area. All the experiments were conducted at air-conditioned room temperature (approximately 20 C).
3. Results and discussions Localised aluminium corrosion is believed to initiate due to the polarisation of aluminium above its pitting potential (Epit) by external or internal factors, such as the galvanic effect of a noble particle in the passive film or the presence of local
T. Liu et al. / Corrosion Science 48 (2006) 67–78
71
chemistry that causes a higher local electrode potential [10]. In order to simulate and study such processes, in this work, external anodic polarisation was applied upon aluminium WBE surfaces that were exposed to corrosive media. WBE and SRET measurements were carried out to study the anodic dissolution by mapping current distributions in the metallic and electrolytic phases. It is expected that when the electrode is polarised to a potential above its Epit, according to conventional understanding on localised corrosion, WBE and SRET measurements should detect phenomena that are related to the initiation of localised anodic sites. The presence of corrosion inhibitor potassium dichromate should prevent the formation of such anodic sites. 3.1. Aluminium corrosion in 0.5 M NaCl solution Fig. 2 shows maps measured from a WBE surface before and after applying external anodic polarisation. Under this corrosion environment, as shown in
Fig. 2. WBE and SRET maps measured from aluminium WBE exposed to 0.5 M NaCl solution before and after applying external anodic polarisation.
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Fig. 2 (continued )
Fig. 3, aluminium electrode had an open circuit potential of around 900 mV (vs. SCE) and a pitting potential of approximately 760 mV. Before external polarisation was applied, as shown in WBE map of Fig. 2a, aluminium was under localised corrosion with anodic sites distributing over the WBE surface in a random manner. The anodic current values were low with the maximum anodic current density of only 0.048 mA/cm2. SRET were not able to detect such currents, and thus the map obtained was ÔemptyÕ. This result indicates that the SRET had a limitation in detecting this type of slow aluminium corrosion that produced only small currents flowing between local anodes and cathodes. It is well know that the sensitivity of SRET measurement is dependent heavily upon the ionic current value, the conductivity of the electrolyte, the distance between the probe tip and the specimen and also the distribution of anodes and cathodes on the specimen surface [6].
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Fig. 3. Cyclic potentiodynamic polarisation measurement (pitting scan) of aluminium WBE exposed to 0.5 M NaCl with and without inhibitor.
When the WBE surface was anodically polarised to 767 mV, which is above the open circuit potential but 7 mV below the pitting potential, the WBE map in Fig. 2b indicates that anodic dissolution was accelerated with the maximum anodic dissolution current density increased from 0.048 to 0.28 mA/cm2. An interesting observation is that anodic dissolution sites remained almost unchanged after the application of anodic polarisation. The SRET was still unable to detect any ionic current in the electrolytic phase.
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When anodic polarisation of the WBE surface was further increased to 747 mV, which is 13 mV above the pitting potential, as shown in the WBE map of Fig. 2c, anodic dissolution currents were further increased to a maximum value of 0.718 mA/cm2. SRET measurements were able to detect anodic dissolution processes, as shown in Fig. 2c, revealing similar anodic dissolution behaviour to that in the corresponding WBE map. Again the anodic dissolution sites were found to remain almost unchanged and that the anodic dissolution current density increased after the application of higher anodic polarisation. An unexpected phenomenon observable in Fig. 2 is that the number of main anodic sites over the WBE surface decreased with the increase in polarisation voltage. For instance, the numbers of main anodes shown in Fig. 2a and c are 9 and 5, respectively. This result is surprising, since conventional understanding on localised corrosion initiation expects that new localised anodic sites should initiate when the electrode is polarised to a potential above its Epit. This result suggests a mechanism of localised corrosion initiation—a process that minor anodes disappear, and anodic dissolution currents concentrate on a few main anodes. Similar pitting initiation mechanism has been observed in other experiments of studying localised corrosion related electrochemical noise [11]. The SRET maps in Fig. 2 also reveal another phenomenon that SRET is able to effectively detect ionic currents and their distribution when a WBE surface is polarised above its Epit. This result suggests that the SRET could be used to detect Epit, and more importantly the profile of pitting. Similar phenomena have been observed repeatedly in other experiments using WBE or conventional one-piece electrodes. When anodic polarisation was removed and thus aluminium WBE corrosion returned to free corrosion status, as shown in Fig. 2d, the WBE patterns became similar to that in Fig. 2a and the SRET map became ÔemptyÕ again. A difference between Fig. 2d and c is that some new major cathodes formed during the anodic polarisation processes. The mechanism behind this change is not clear and requires further investigation. 3.2. Aluminium corrosion in 0.5 M NaCl solution containing 0.5 M K2Cr2O7 When corrosion inhibitor potassium dichromate was added to the corrosive solution, as shown in Fig. 3b, pitting potential of aluminium became much less negative (approximately 650 mV vs. SCE). When the WBE specimen was exposed to such a corrosion condition without any external polarisation, as shown in Fig. 4a, localised corrosion occurred with anodic sites distributing over the WBE surface in a random manner. The corrosion behaviour was similar to that in Fig. 2a, except that the anodic current density values (maximum 0.025 mA/cm2) were lower than that in Fig. 2a (maximum 0.048 mA/cm2). These lower anodic dissolution currents suggest that potassium dichromate inhibited anodic dissolution. Again, SRET were not able to detect such currents flowing over the WBE surface, and thus the map obtained was ÔemptyÕ. The SERT measurements were not able to detect ionic current in the solution even when the WBE was polarised to 611 mV, 39 mV above its pitting potential. This
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Fig. 4. WBE and SRET maps measured from aluminium WBE exposed to 0.5 M NaCl solution with 0.5 M K2Cr2O7 added, before and after applying external anodic polarisation.
result clearly demonstrates that potassium dichromate effectively inhibited anodic dissolution. The WBE map in Fig. 4b indicates that anodic dissolution was
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Fig. 4 (continued )
accelerated with the maximum anodic dissolution current density increased from 0.025 to 0.164 mA/cm2. Similar to the observations made in the previous experiment, the anodic dissolution sites remained almost unchanged. However, when polarisation was extended to 30 min and 1 h, as shown in Fig. 4c and d, major changes occurred to WBE and SRET maps. Major anodic dissolution anodes, which are different from the major anodic sites in Fig. 4a and b, are observable form both the WBE and SRET maps. Anodic dissolution currents increased significantly to a maximum value of 4.733 and later 5.346 mA/cm2. SRET measurements were able to detect anodic dissolution processes, as shown in Fig. 4c and d, revealing similar anodic dissolution behaviour to that in the corresponding WBE map. This result suggests another anodic dissolution mechanism: The initiation of localised corrosion due to the initiation of new localised anodic sites. This process requires a higher anodic polarisation voltage and also an initiation period. This
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mechanism appears to agree with conventional understanding on localised corrosion initiation [10,12]. When polarisation voltage was removed and thus the aluminium WBE returned to free corrosion status, as shown in Fig. 4e, the WBE map became similar to that in Fig. 4a, and SRET map became ÔemptyÕ again. The newly formed anodic dissolution sites in Fig. 4c and 4d ceased to exist. The mechanism behind this phenomenon is not clear and requires further investigation.
4. Conclusions The wire beam electrode and the scanning reference electrode technique have been used in combination to study the anodic dissolution of aluminium in 0.5 M NaCl solution, and the inhibiting effects of potassium dichromate. Under polarisation conditions, SRET and WBE techniques were used successfully in determining corrosion behaviour and profile below and above the pitting potential (Epit), and the effects of inhibitors. It was found that with inhibitors, larger polarisation were required for pitting to initiate, in particular, long duration was required for pit to grow. It was also found that the SRET was not sensitive enough to monitor free corrosion process occurring on the surface of aluminium exposed to relatively mild corrosion environment or under low anodic polarisation voltage. The WBE method appeared to be more sensitive and could provide more detailed information of corrosion processes than the SRET. This work has revealed two different mechanisms of localised corrosion initiation: (i) The initiation of localised corrosion due to the disappearance of minor anodes; and (ii) The initiation of localised corrosion due to the initiation of new localised anodic sites.
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