In-situ visualization of local corrosion by Scanning Ion-selective …gecea.ist.utl.pt/.../23_2010_Lamaka_BookChapter_SIET_sm.pdf · 2011-02-21 · of ion-selective electrodes [14],

In-situ visualization of local corrosion by Scanning Ion-selective Electrode

Technique (SIET)

SvetlanaV. Lamaka1, Ricardo M. Souto

2 and Mário G. S. Ferreira

1,3

1ICEMS, Instituto Superior Técnico, Av. Rovisco Pais, TU Lisbon, 1049-001, Lisboa, Portugal 2Department of Physical Chemistry, University of La Laguna, E-38200 La Laguna (Tenerife), Spain 3CICECO, Department of Ceramics and Glass Engineering, University of Aveiro, 3810-193, Aveiro, Portugal

Potentiometry with ion-selective microelectrodes measures activity of specific cations and anions at the solid/liquid interface from the solution side. Anodic and cathodic components of corrosion processes change local concentration of H+, OH-, metal cations and ions of the supporting electrolyte. These changes can be detected and quantified by SIET, making this technique an alternative, or complement, to methods based on potential and current changes across the interface. This mini-review describes SIET basics and demonstrates several examples of SIET applications relevant to corrosion studies.

Keywords corrosion; localized techniques; SIET; SVET; ion-selective micro-electrode; pH-microscopy

1. Introduction

Localized physical-chemical processes occur at active solid/liquid interfaces in nature as well as in many technical systems. Information on the spatial distribution of the ionic species consumed or released on the active sites is of primary importance for understanding many chemical and biological processes. Conventional electrochemical methods provide information about the overall activity on the entire surface. In-situ sensing of the processes in active sites, pores or defects can be achieved only by using localized techniques such as SECM (Scanning Electrochemical Microscopy), SVET (Scanning Vibrating Electrode Technique) and SIET (Scanning Ion-selective Electrode Technique). Addressing solid/liquid interfaces processes at micro-scale provides a key for clarifying mechanisms of the respective electrochemical reactions. Corrosion of different metallic materials is associated with anodic oxidation (metal dissolution followed by hydrolysis of formed Men+) and cathodic reduction (water and oxygen reduction accompanied by accumulation of OH-). Hence, corrosion processes not only involve electrochemical reactions but also acid-base interactions. Moreover, different anions present in the medium can either catalyze or inhibit the anodic dissolution of metals, accelerating the process or inducing passivation. Thus, identification and quantification of chemical species produced or consumed is crucial for understanding the corrosion phenomena. The local activity of different ionic species can be assessed by using ion-selective microelectrodes (ISMEs). Two NACE Whitney Award Lectures, by Norio Sato, 1989 [1], and Hugh Isaacs, 2000 [2], deal with the importance of acid-base equilibria for studying the intimate details of corrosion processes. In his pioneering research on mechanism of the pitting of metals José Galvele developed the idea that local acidification caused by hydrolysis of dissolving metal cations creates special conditions for pit growth [3]. This once again emphasizes the importance of acid-base equilibria in view of studying corrosion processes. Almost all features (apart from aeration) of corrosion pit propagation according to the Fontana-Greene mechanism [4] can be quantified by using ion-selective microelectrodes: pH at cathodic and anodic sites, concentration of metal cations and their hydrolysis, and Cl- migration to the areas of anodic activity. In spite of such impressive possibilities local potentiometry with ion-selective microelectrodes is not a widely-spread technique in corrosion laboratories. This review gathers several representative examples of SIET measurements, revealing the capability and the potential applications of local potentiometric measurements with ion-selective microelectrodes for corrosion related studies.

2. SIET basics for corrosionists

Scanning ion-selective electrode technique (SIET) works as a micro-potentiometric tool allowing measurements of specific ions at a quasi-constant micro-distance over an active surface in solution. Potentiometric measurements are conducted in a two electrode galvanic cell under zero current conditions. A potentiometric cell is composed of a reference electrode and an ion-selective microelectrode. A SIET device contains the following major parts: an ion-selective microelectrode mounted on a 3D computerized stepper-motors system, used to position and move the microelectrode over the sample. The sample in turn is placed on a movable holder where a reference electrode (e.g. Ag/AgCl mini-electrode) is also mounted. A video camera equipped with a long-distance lens providing magnification up to 400 times is located over the sample. The potential difference measured in the potentiometric cell is amplified and digitalized. This experimental setup is the same, for the most part, as used in Scanning Electrochemical Microscopy (SECM) in potentiometric mode [5]. In microbiology membrane-

Microscopy: Science, Technology, Applications and Education A. Méndez-Vilas and J. Díaz (Eds.)

2162 ©FORMATEX 2010

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transport processes are studied using similar techniques, Microelectrode Ion Flux Estimation (MIFE) [6-8] and Self-Referencing Ion-Selective probes (SERIS) [9, 10]. The difference is that MIFE and SERIS estimate ion fluxes by vibrating the ion-selective microelectrode with a frequency of 0.3 to 0.05 Hz. For a better understanding of the principle of modern potentiometry readers may refer to several excellent reviews [11, 12]. Recommendations on nomenclature of ISE [13], guidelines for experimentalists on how to assess of the limits of ion-selective electrodes [14], time response [13, 15, 16] and selectivity [17] are also available. The extensive reviews on performance characteristics, including selectivity, of ISE based on more than 500 ionophores selective to ca. 50 different ionic species [18 - 21] can be useful when choosing a membrane composition for a particular application. Different membrane cocktails for microelectrodes and Selectophore grade purity individual chemicals (ionophores, ionic sites and membrane solvents) are commercially available [e.g. 22]. Before ion-selective microelectrodes became interesting for corrosion scientists, the glass-capillary microelectrode with liquid membrane was introduced by life scientists [23]. Since then, microbiologists have kept developing cutting edge instrumentation and experimental approaches for micro-potentiometric research. Today, glass-capillary microelectrodes are well established analytical tools. The principles of experimental work with microelectrodes are summarized in [24, 25]. A detailed chronology of development and state of the art in potentiometric microelectrodes is reported in [26]. Recent advances in the application of potentiometric microsensors for microbiological studies are reviewed in [6, 10, 27, 28]. A glass-capillary microelectrode filled with a selective ionophore-based oil-like membrane is presented in Fig. 1a. The column length of the membrane, used for different applications, is about 10 to 100 µm. The diameter of the orifice of an ion-selective glass-capillary microelectrode varies from 0.1 to 5 µm. The microelectrode also comprises an inner reference electrolyte and an Ag/AgCl wire inserted into the electrolyte to provide a reference electrode. Although glass-capillary microelectrodes are widely used, experimental work with these tools is not an easy task: they are highly fragile, their life time is usually limited to one day and their detection limit is biased by the flux of primary ions from the ion-selective membrane and inner reference electrolyte. Several approaches to design robust solid-contact ion-selective microelectrodes which do not require an inner reference solution have been reported recently. Pt or Au wires were sealed in glass capillaries, conically bevelled around the measuring disk with subsequent deposition of conductive polymer and drop casting of a polyvinylchloride (PVC)-based ion-selective membrane [29]. A carbon fiber coated with conductive polymer was dipped inside the membrane cocktail placed in the tip of a glass capillary [30]. Another original idea is to etch a microwire inlaid in a glass capillary forming a microcavity that afterwards is filled with a conducting polymer and a liquid membrane [31]. A solid-contact ionophore-based ion-selective microelectrode recently designed by our group is shown in Fig. 1b [16]. The microelectrode is based on an insulated needle-shaped Pt-Ir wire with an exposed apex. The ion-to-electron transducer is made of conductive polymer poly(3-octylthiophene-2,5-diyl) and placed between an ion-selective membrane and the metallic tip. The ion-selective PVC-based membrane is deposited on the layer of conductive polymer. The length of the ion-sensitive tip of the electrode is about 10 µm.

Fig.1 a) glass-capillary ion-selective micro-electrode with liquid membrane, and b) solid-contact ion-selective microelectrode with plasticized PVC-based membrane.

Calibration of the ion-selective microelectrode has to be performed before and after measuring a sample to correlate the results to the Nernst equation:

Ei = i

i

i lnaFz

RTE +0 , (1)

where F is the Faraday constant, R is the universal gas constant, and T is the absolute temperature. The calibration of the electrodes is recorded as potential, Ei, vs. time dependence with sequential increase of ion activity, ai.. Apart from calibration values, such a method provides important characteristics of a microelectrode as potential stability, drift and rough estimation of time response. The Nernstian slope of electrode function is calculated taking into account activities rather than concentrations. The activity coefficient fi for i ion is calculated according to the extended Debye-Hückel equation:


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IB1

IAz logf

i

2i

iα+

=− , (2)

where iz is the charge number of ion species i; I is the ionic strength of the solution; A is the Debye-Hückel constant, A

= 0.5085 for aqueous solutions at 298K; B is a characteristic constant for the solvent, which depends on temperature and dielectric constant of solvent; B = 0.3281 for aqueous solutions at 298K; αi is the approximate effective ionic radius of i ion, α = 8Å for Mg2+, 4Å for Na+ and 3Å for Cl- [32]. The selectivity of a microelectrode can be determined using a modified separate solution method which involves calibration of the electrode in solutions of the main and then of the interfering ion [13, 17]. As an example, the selectivity of the Mg2+-SME towards Na+ which, along with Cl-, are the main components of a corrosion medium, can

be calculated as follows [33]. A standard cell potential E 0+Na

, in solutions of Na+ only, is calculated by the equation of

the corresponding ion function. E 02+Mg

is determined in the same way, Fig. 2. The selectivity coefficient pot

Na,Mg2logK ++

can

then be calculated by using the equation:

RTln10

Fz )E(ElogK

2

22

Mg0

Mg

0

Na

pot

Na,Mg

+

++++ −= (3)

Obviously, the better the selectivity for the main ion (Mg2+) over the interfering ion (Na+), the lower the value

ofpot

,NaMg2K ++ . Calculated selectivity of Mg2+-SME is pot

NaMg

K ++ ,2log = -3.1 [33], while pot

NaZnK ++ ,2

log = -1.4 for Zn2+-SME

[34]. This means that Mg2+activity down to 6·10-5 M could be determined in 0.05M NaCl solution, and only 3·10-4 M of Zn2+ can be quantified in 0.005M NaCl supporting electrolyte. The limiting value of selectivity coefficients over interfering Na+ cation can be calculated using the Nikolsky-Eisenman equation. Thus, if potentiometric measurements are carried out in a 0.05M NaCl supporting electrolyte, and minimal concentration of the main cation to be determined

is 1·10-4 M, the selectivity coefficient n to be at least pot

NaMeK ++ ,

log = -4.2, =++

pot

NaMeK

,2log -2.9, and pot

NaMeK ++ ,3

log = -

1.6 for mono-, double- and triple-charged cations respectively.

Fig. 2 Response of Mg2+-selective microelectrode in a series of Mg2+ and Na+ pure solutions. The calibration curves were used to calculate the selectivity coefficient of Mg2+-SME to Na+.

Special attention must be given to the dynamic characteristics of the microelectrodes, namely stability of potential and response time. One of the most important characteristics of microelectrodes used for scanning measurements is the response time. It predetermines the acquisition time of the entire scan that often requires more than 1000 consecutive points. Semi-isochronous acquisition of all points of the scan ensures that the system under study did not change significantly during the measurement. Response time is predetermined by the combined resistance-capacitance of the electrochemical cell. The higher the RC value the slower the response.

The response time can be measured in a “dual drop cell” specifically designed for such measurements [16], schematically shown in Fig. 3. Quantification of the response time can be done in two ways. Firstly, as defined by the IUPAC recommendations [13], the response time of the electrode, τlim, is the time elapsed between the instant when the electrode was brought into contact with the solution and the instant at which the potential/time slope (∆E/∆t) reaches a limiting value (e.g. 1 mV/min or 0.017 mV/s). Secondly, the time needed to obtain 95% of the total potential change, τ95, can also be determined for comparison, as this value (along with τ90) is often found in literature.

The response time of the glass-capillary microelectrodes with liquid membranes varies from 1 to 5 s: τlim = 4.6 and

τ95 = 2.6 s were found for the Zn2+-SME [34], the response times were τlim = 1.3 and τ95 = 0.30 s for the pH-SME, and τlim

= 2.9 and τ95 = 1.6 s for the Mg2+-SME [33]. The solid-contact microelectrodes recently developed by our group [16] exhibited shorter response time, τlim = 0.96 and τ95 = 0.33 s for the pH-SME, and τlim = 2.58 and τ95 = 1.1 s for the Mg2+-SME. Although the values of time response quantified as τ95 (or τ90) are often found in literature, the differential response time τlim specified by the current IUPAC recommendations [13] appears to be more practical for real scanning measurements. SIET mapping implies point by point measurements of ion activity. Decrease of time of acquisition of



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each data point to the value of τ95, usually results in blurring of ionic mapping, wheras time of acquisition set up at the value of τlim leads to sharper scanning images.

Fig. 3 Schematic drawing of “dual drop cell” for response time measurements of microelectrodes. The microelectrode is moved by PC-controlled motors from one drop with a lower concentration of the main ion (e.g. pH 9) to another drop with a higher concentration of the main ion (pH 8). Reprinted from [16], reproduced with permission from Electroanalysis, Wiley-VCH Verlag GmbH&Co.

Other important characteristics of microelectrodes, such as long and short-term potential stability, lower detection limit and pH region of functioning, have to be also verified while designing new microelectrodes for corrosion-related studies.

3. Anodic dissolution of pure Al, Mg, Zn and Fe in view of acid-base equilibria

The anodic dissolution of a metal leads to the formation of hydrated cations, Men+, that undergo hydrolysis forming hydroxyl complexes accompanied by acidification:

−+ +→ neMeMe (aq)n

(4)

(aq)(aq)1)-(nK

2(aq)n H(Me(OH))OHMe

1hyd +++ +→+ (5)

The rate of hydrolysis is determined by the stability of the (Me(OH))(n-1)+ complex, where 1stK is reference data1:

(aq)1)-(nK

(aq)n (Me(OH))OHMe

1st +−+ →+ (6)

1stwn

1)(n1hyd KK

][OH

][OH

][Me

]][H[(Me(OH))K ⋅=⋅=

−

−

+

++−

, (7)

where Kw is the dissociation constant of water, 1.0·10-14 at 25oC. If [Men+], which depends on intensity of anodic dissolution, is known, acidification caused by hydrolysis of Me2+ can be calculated from eq. (5):

][MeKlogpH n1hyd

+⋅−= (8)

Table 1 contains the stability constants of (Me(OH))(n-1)+ complexes for the metals most intensively studied in corrosion, the corresponding hydrolysis constants, and the pH values calculated by eq. (8) for [Men+] = 0.1 and 0.0001M.

Table 1 Stability and hydrolysis constants, and pH values for the hydrolysis of Al3+, Mg2+, Zn2+ and Fe3+ species

1 Stability constants quoted in different handbooks may vary within 1 to 2 orders of magnitude. For all calculations in this paper, including graphic

calculations with Hydra-Medusa, reference data taken from [35] were used.



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Metal Complex and stability

constant 1stK

Hydrolysis constant, 1hydpK

pH at [Men+]

0.1 and 0.0001 M

Al Al(OH)2+ 1.07·109 4.5 3.0 4.6 Mg Mg(OH)+ 3.63 ·102 11.4 6.2 7.0 Zn Zn(OH)+ 1.10·106 8.0 4.5 6.0 Fe Fe(OH)2+ 6.31·1011 2.2 1.6 3.1

Although additional hydrolysis of (Me(OH))(n-1)+ to (Me(OH)2)+− )2(n and further is possible, it is unlikely to occur in

the naturally-corroding environment due to the accumulation of H+ from the first stage of the hydrolysis reaction. On the other hand, formation of hydroxychloride complexes is a common process accompanying hydrolysis in corrosion media and it shifts the hydrolysis equilibrium to the right promoting further acidification. Figure 4 presents the dependencies of pH calculated taking into account all stability constants tabulated for metal hydroxides and hydroxychlorides, thus the pH values presented in Table 1 and Figure 4 vary slightly.

a)-5 -4 -3 -2 -1 00

2

4

6

8

pH

Log [Al3+]TOT b)-5 -4 -3 -2 -1 00

2

4

6

8

pH

Log [Mg2+]TOT

c)-5 -4 -3 -2 -1 00

2

4

6

8

pH

Log [Zn2+]TOT d)-5 -4 -3 -2 -1 00

2

4

6

8

pH

Log [Fe3+]TOT

Fig. 4 pH values caused by hydrolysis of Al3+, Mg2+, Zn2+ and Fe3+ species depending on the concentration of cation Men+. Initial pH for all graphs = 5.5, concentration of supporting electrolyte NaCl is 0.05M. The calculations were made using the Hydra-Medusa software [36]. Thereby, if anodic dissolution and cathodic reduction are separated, pH at anodic sites during iron dissolution can be strongly acidic, lower than 2. Oxidation of aluminum is also accompanied by acidification down to pH 3. Dissolution of sacrificial zinc anode causes a weakly acidic reaction, while Mg2+ is not capable of changing the pH of a neutral corrosion medium. Talking about localized corrosion, pH in confined micro-sites will strongly depend on the rate of corrosion reaction, conditions of ionic diffusion, proximity of cathodic site, temperature and additional ions of corrosion medium that may form complexes with Men+ more stable than corresponding hydroxides and hydroxychlorides. Obviously, localized pH of corroding alloys may differ from the pH of anodic dissolution of pure metals. For example, pH in anodic site of corroding AZ31 magnesium alloy was found to be slightly acidic, 5.3 [37], that is lower than given in Table 1 and Figure 4b. Such low pH was ascribed to the co-dissolution and hydrolysis of Al along with Mg, since the AZ31 contains ca. 2.5-3.5% aluminum.



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4. Examples of SIET measurements

4.1 pH-selective microelectrodes: local cathodic alkalinization and anodic acidification

Traditionally a tridodecylamine-based ion-selective membrane (H+-ionophore I cocktail B, Fluka, Ref. 95293) is used for localized pH-measurements [37 - 40]. The slope of the linear regression is close to the Nernstian behaviour, -54.7 ± 0.8 mV/pH (cf. Figure 5). The pH range of linear response of this membrane is from 5 to 12 [22, 33]. Such microelectrodes can be used for corrosion studies of Zn- and Mg-based substrates, where the pH in anodic and cathodic zones varies between 5 and 12 [37, 40, 41].

Fig. 5 Dynamic a) and concentration b) calibration curves of pH-selective glass-capillary microelectrode based on tridodecylamine. Given that pH of pitting corrosion of Al can drop as low as 3, the use of tridodecylamine-based pH-selective membrane is not suitable for unbiased pH microscopy of Al-based materials. Another ionophore, 4-nonadecylpyridine (Fluka, Ref. 25292), can be used when the expected range of pH is extended into the acidic region [42 - 44]. Typical dynamic and concentration potentiometric responses of a pH-selective microelectrode based on 4-nonadecylpyridine are presented in Figure 6. The microelectrodes exhibit stable and reproducible potential readings in the pH range 2-10, with a linear slope of -55.4 ± 0.4 mV/pH.

Fig.6 Dynamic a) and concentration b) calibration curves of pH-selective glass-capillary microelectrode based on 4-nonadecylpyridine. The membrane cocktail is composed of 6 wt% 4-nonadecylpyridine, 12 mol% potassium tetrakis(4-chlorophenyl)borate and membrane solvent 2-nitrophenyloctyl ether. The inner filling solution contains a buffer solution formed by 0.01M KH2PO4 and 0.1M KCl. Apart from electrochemical reactions, corrosion processes involve acid-base interaction, which in fact diversify the corrosion phenomena to such a great extend. Thus, the importance of localized pH measurements for corrosion studies is hard to overestimate. Mapping of pH over a sample of galvanized steel coated with the inhibitor-doped pre-treatment is a striking example [45]. Figure 7 shows that pH in anodic zone reached 3.5. As shown in Figure 4c, dissolution and hydrolysis of zinc alone could not cause such a low pH in the defect. Participation of iron in the anodic dissolution seems to be the only plausible explanation for this high local acidification. Indeed, the inhibitor introduced into the pre-treatment is likely to hinder dissolution of the sacrificial zinc layer and the exposed iron substrate undergoes anodic



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dissolution resulting in the major acidification of the defect area. This essential conclusion about the detrimental effect of the corrosion inhibitor in a particular coating formulation could not be attained by only using localized electrochemical techniques which measure net positive and negative currents such as SVET.

Fig. 7 a) pH mapping over the galvanized steel sample with the artificial defect. pH in anodic zone dropped to 3.5, surrounded by the whitish border of corrosion products visible in the optical micrograph of the scanned area, b). Ding et al. used pH-microscopy to study particulate-reinforced aluminum-matrix composites [39, 46, 47]. Double-barrel pH microsensors were used by Park et al. to measure the local pH near intermetallic inclusions of 6061 aluminum alloy [38, 48]. One chamber was used as a reference electrode and the other chamber contained a pH-sensitive cocktail. pH distribution over corroding cut edge of galvanized steel substrates was studied by Ogle et al. [40, 41, 49]. Information on pH of anodic and cathodic sites can be used not only for clarification of corrosion mechanisms and purposes of corrosion prediction and modelling. It is of paramount importance for choosing adequate corrosion inhibitors, as they usually operate in a specific pH range only [33, 44, 50]. The properties of self-healing systems, that often include pH-sensitive triggers of inhibitor release, can be tailored if the pH of corrosion onset is known [44, 51-52]. For example, in [51] we described a self-healing anticorrosion coating system for 2024 aluminum alloy based on inhibitor-containing reservoirs composed of stratified layers of oppositely charged polyelectrolytes. Branched polyelectrolyte reservoirs deliver inhibiting species “on demand” when starting corrosion process changes the local pH. This becomes possible as the configuration of polyelectrolyte molecules which form the reservoir shells changes to the “open” state if local pH is lower than 4 or higher than 8, corresponding to pH of anodic and cathodic reactions in the course of aluminum corrosion. When corrosion has not started yet, or is already terminated by the inhibitor, polyelectrolyte shells are in the “closed” state preventing spontaneous leakage of inhibitor.

4.2 Cation-selective microelectrodes: measuring dissolving metals during the course of anodic reaction

Although the values of local pH can be of a great help for revealing the cations participating in the reactions of anodic dissolution (see Figures 7 and 8), quantification of generated cations is required for accurate calculations of the rate of anodic reactions during numerical modelling of corrosion and self-healing processes. Modern potentiometry offers a wide range of synthesized ionophores suitable for designing microelectrodes selective to cations of interest for corrosion-related applications. Key reviews on the properties of hundreds of ionophore-based ion-selective electrodes are available [18 - 22]. One of the most successful cation-selective microelectrodes developed so far for corrosion applications is Mg2+-SME. Figure 8 presents an example of sequential pMg, pH and current density measurements performed over a magnesium alloy [37]. To passivate the alloy, highly susceptible to corrosion, the surface was coated with a thin protective sol-gel film and then five defects were punched with a needle exposing small separate areas for corrosion attack. After several hours of immersion, the current density, H+ and Mg2+ distributions were measured over the sample surface. All four images are complementary. Anodic dissolution in the first defect registered by SVET was accompanied by local increase of Mg2+ concentration and slight acidification due to co-dissolution of Al. Cathodic reduction in defects 2, 3 and 4 led to local alkalinization: 2H2O+2e- → 2OH-+H2↑ and decrease of Mg2+ activity relative to the bulk concentration. This local depletion of Mg2+ can be explained by the formation of insoluble complexes of Mg(OH)2 in the alkaline pH region. Mixed anodic and cathodic activity in the fifth defect was also resolved by pH and Mg2+-selective microelectrodes making it an interesting example demonstrating the sensitivity and resolving power of the technique. pH and pMg (-log10[Mg2+]) were mapped sequentially using glass-capillary microelectrodes with liquid membranes. pH-SME was based on tridodecylamine (H+-ionophore I cocktail B, Fluka, Ref. 95293), and Mg2+-SME was based on N,N′′-octamethylene-bis(N′-heptyl-N′-methyl-methylmalonamide), Fluka, Ref. 63083, and it also contained 3 wt% potassium tetrakis(4-chlorophenyl)borate, 25 wt% chloroparaffin, and 62 wt% 2-nitrophenyloctyl ether [37].



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Another example of successful adoption of cation-selective microelectrodes in corrosion practice is the Zn2+-selective microelectrode developed recently [34]. The membrane cocktail was composed of 7 wt% tetra-n-butyl thiuram disulfide [53], 22.8 wt% (150 mol% relative to ionophore) sodium-tetrakis[3,5-bis(trifluoro-methyl)phenyl]borate, 1.4 wt% tetrakis(4-chlorophenyl)borate tetradodecyl-ammonium (ETH 500), and 68.8 wt% 2-nitrophenyloctyl ether. The developed Zn2+-SME was successfully applied to relevant cases in corrosion research: zinc dissolution, zinc electroplating and corrosion in defects of painted galvanised steel.

Fig. 8 Sol-gel coated AZ31 magnesium alloy with five artificial defects immersed in 0.05M NaCl solution. Optical micrograph (a), distribution of local ionic current density measured by SVET (b), and ionic distribution: pH (c) and pMg (d), mapped by glass-capillary micro-electrodes.

4.3 Na+ and Cl- selective microelectrodes: the role of supporting electrolyte in charge compensation

It is apparent that the ions of corrosion medium, namely Cl- and Na+, participate in the ionic equilibriua around active sites, and therefore interpretation and modelling of electrochemical processes should take ionic interactions into consideration. Hayase et al. [54] calculated the decrease of Na+ and increase of Cl- concentration where the accumulation of H+ takes place. Luo et al. [55] measured Cl- profiles over the corrosion pits. Lin et al. [56] measured accumulation of Cl- over the crevice corrosion sites of stainless steel 304. However, due to the lack of reliable experimental protocols no systematic studies were undertaken in that direction. On the other hand, information concerning the distribution of ions from the corrosive environment around active sites may be vital for modelling corrosion processes and corrosion prediction. For example, while modelling galvanic coupling in aluminum alloys, Murer et al. [57] concluded that the blocking step for the application of numerical mass transport models was the lack of input data on the local dissolution rate of aluminum matrix as a function of the chemistry of the system, namely, pH and Cl- concentration. By using a dual head stage micromanipulator (Biomedizinische Geräte, Germany), a device that allows for precise positioning of two microelectrodes, the distributions of Cl- and Na+ ions were mapped simultaneously over the anodically-polarized 25 µm diameter Pt wire, see Figure 9. The applied current induced the anodic oxidation of water resulting in acidification of the solution around the platinum wire according to the reaction:

+− +→− 4HO4eO2H 22 , (9)

An increase of Cl- activity and decrease of Na+ activity were observed over the Pt current source. Taking into account that H+ are by far the fastest ions in solution (the diffusion coefficients +H

D =9.31·10-5 cm2/s, +NaD =1.33·10-5 cm2/s

and −ClD =2.03·10-5 cm2/s), H+ ions are likely to form a local positively-charged zone over the source of acidified

solution. Probably, Cl- migrate to the zones with high concentrations of extraneous H+ to compensate for the surplus of positive charge and maintain a charge balance. For the same reason Na+ cations are forced outwards from the same sites resulting in local depletion of Na+ content. An increase in Cl- concentration over the active sites, where cations are generated, has a far-reaching impact in corrosion. It contributes to the autocatalytic nature of anodic dissolution of metals and prevents repassivation: a higher generation rate of Men+ provokes a higher migration rate of Cl- which, in



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turn, accelerates the corrosion process. This effect is of greater relevance in the case of hydrolysing cations, like Al3+

and Fe3+, since their dissolution is accompanied by a higher rate of local acidification. Accumulation of Cl- in confined micro-sites, like pits or defects in a coating, plays a critical role in the propagation and spreading of corrosion processes.

Fig. 9 a) Optical micrograph of scanned area; b) simultaneously recorded pCl and c) pNa mapping over the anodically polarized Pt wire current source in 0.05MNaCl supporting electrolyte. pCl = -log10 [Cl-] and pNa = -log10 [Na+]. Sodium Ionophore II - Cocktail A (Fluka, 71178) was used as a membrane for the Na+- SME. The Nernstian slope of Na+-SME was 61.8±0.4 mV/dec. The commercially available membrane for Cl--SME (Chloride Ionophore-Cocktail A, Fluka, Ref. 24902) was modified to decrease the electric resistance of the liquid and decrease response time. The total amount of cationic-anionic site, tetradodecylamonium tetrakis (4-chlorophenyl)borate (ETH 500), was brought to 5 wt% against 1 wt% in the commercial sample. The Nernstian slope of Cl--SME was -54.6 ± 0.5 mV/dec.

4.4 Quasi-simultaneous SVET-SIET measurements

SVET measures the potential differences in solution due to the ionic fluxes produced above the sample surface during electrochemical corrosion as they flow between the anode and the cathode. The vibrating probe (VP) detects a potential difference between the extremes of their movements. The vibrations of the probe convert the DC potential gradients it detects along the vibration direction into an AC signal with a frequency of the vibrations. SVET is widely used as a powerful electrochemical technique for corrosion related studies [33, 34, 37, 39 - 41, 45 - 47, 51, 52, 58 - 69]. However, a platinised platinum vibrating probe used for SVET cannot differentiate the chemical nature of the species that generate the electrical field. A combination of SIET and SVET measurements provides important information about both components of the electrochemical process, thus characterising it in terms of potential/current as well as in terms of chemical equilibria. Both techniques, SVET and SIET, with pH- and Mg-SMEs were used sequentially to study the corrosion and passivation of coated magnesium alloys [33, 37, 68]. Chemical and electrochemical phenomena occurring on the cut edge of galvanized steel were analysed by SVET and pH-SIET along with calculating ionic equilibria [40, 41]. SVET and pH-SIET measurements allowed for a detailed analysis of chemical processes in cathodic and anodic sites of aluminum alloys [39, 46]. Supplemented by SEM/EDS and XRD analysis, SVET and SIET revealed distribution and chemical nature of the corroding products contributing to elucidation of the mechanisms of corrosion and passivation of aluminum alloys [39, 46, 47]. Despite the useful information extracted, there is always a considerable time-lag between SVET and SIET measurements that cannot be neglected, especially if the SVET and SIET data are used for calculations and modelling. In a recent publication [69], we disclosed the experimental setup for the time and space correlated SVET-SIET measurements. A dual head stage micromanipulator allowed for precise positioning (± 1 µm) of two microprobes, and ASET software (ScienceWares) enabled sequential reading of SVET and SIET data channels. Point-by-point scanning was made with a time-lag of 1.5 seconds between acquiring SVET and SIET readings. No cross-talk between the vibrating probe and the ion-selective microelectrode could be detected providing that VP and ISME are kept at least 30 µm apart. An optical photograph of experimental setup including positioned together VP and ISME, reference Ag/AgCl and auxiliary Pt ring electrodes is presented in Figure 10a. Figure 10b shows an optical micrograph of vibrating probe and ion-selective microelectrode ready for acquiring SVET and SIET signals.



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Fig. 10 a) Optical photograph of experimental setup for quasi-simultaneous recording of SVET and SIET signals. b) Optical micrograph of vibrating probe (VP) of SVET and glass-capillary ion-selective microelectrode of SIET brought together to measure current density and ion distribution quasi-simultaneously. To illustrate quasi-simultaneous SVET-SIET measuremetns the following model can be considered. The SVET-SIET system measured the distribution of pH and current density over a 25 µm diameter Pt wire embedded into epoxy resin. An anodic current of +100 nA was driven through the system, and resulted in a local acidification according to eq. (9). Measured values of simultaneously recorded pH and current density are plotted in Figure11.

Fig. 11 a) Optical micrograph of scanned area; b) pH changes over anodically polarized Pt wire current source; c) SVET current density distribution recorded simultaneously with SIET measurements.

The combined SVET-SIET measurements correlate electrochemical oxidation-reduction processes with acid-base chemical equilibria. The use of both techniques in one experiment provides a powerful tool for the investigation of mechanisms and kinetics of electrochemical corrosion processes in confined micro-sites such as defects in coatings or pits. Simultaneously rather than sequentially measured distribution of specific ions and current density also provides a higher quality of local data that can be used for modelling and prediction of corrosion.

Acknowledgements The support by Fundação para a Ciência e a Tecnologia (FCT, Portugal) through the project PTDC/CTM/108446/2008 and FP7 through “MUST” project, contract NMP3-LA-2008-214261is gratefully acknowledged. The authors are grateful to Ms. M. Taryba and Dr. F. Montemor from Instituto Superior Técnico and Dr. A. Bastos, Mrs. O. Karavai, Dr. M. Zheludkevich from University of Aveiro in Portugal for their collaboration. Alan M. Shipley from Applicable Electronics and Dr. H. S. Isaacs from Brookhaven National Laboratory, USA, are acknowledged for their support and interesting discussions.

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Documents

In-situ visualization of local corrosion by Scanning Ion-selective …gecea.ist.utl.pt/.../23_2010_Lamaka_BookChapter_SIET_sm.pdf · 2011-02-21 · of ion-selective electrodes [14],