Sodium- and chloride-selective microelectrodes optimized for corrosion studies

Journal of Electroanalytical Chemistry 706 (2013) 13–24

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Sodium- and chloride-selective microelectrodes optimized for corrosion studies Valentine A. Nazarov a, Maryna G. Taryba b, Elena A. Zdrachek c, Kseniya A. Andronchyk c, Vladimir V. Egorov c,⇑, Svetlana V. Lamaka b a b c

Research Institute for Physical Chemical Problems, Leningradskaya Str., 14, 220030 Minsk, Belarus Instituto Superior Técnico, ICEMS, Technical University of Lisbon, Av. Rovisco Pais, 1049-001 Lisbon, Portugal Belarusian State University, Leningradskaya Str., 14, 220030 Minsk, Belarus

a r t i c l e

i n f o

Article history: Received 6 June 2013 Received in revised form 17 July 2013 Accepted 19 July 2013 Available online 3 August 2013 Keywords: Micro-potentiometry Ion-selective microelectrode SIET Corrosion SVET Localized electrochemical technique

a b s t r a c t New sodium and chloride ion-selective membrane cocktails for glass-capillary microelectrodes were developed satisfying the demands for corrosion studies. Membrane cocktails were optimized to provide a wide pH working range, good long-term stability and reproducibility of potential. The proposed sodium-selective cocktail contains sodium ionophore VI, potassium tetrakis(4-chlorophenyl) borate and 2-nitrophenyl octyl ether. The chloride-selective cocktail consists of tridodecylmethylammonium chloride and 2-nitrophenyl octyl ether. The proposed microelectrodes were successfully applied to measure the concentration of Cl and Na+ over corroding cut-edge samples of metallic coated steel and can be recommended for measuring the distribution of Na+ and Cl in corrosion applications. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Ions in the supporting electrolyte, or corrosion medium, Na+ and especially Cl play an important role in several types of corrosion. It is commonly thought that, to maintain electroneutrality, Cl fluxes to the anodic reaction zones where formation of Men+ is accompanied by local acidification of the medium due to the hydrolysis. Na+ fluxes to the zone of supporting cathodic reaction where formation of OH ions takes place, promoting the corrosion process. Hence, monitoring the evolution of local concentrations of Cl and Na+ together with pH and pMn+ measurements is of crucial importance for interpreting and modeling corrosion processes. Scanning ion-selective electrode technique using glass-capillary microelectrodes with liquid ion-selective membranes allows for obtaining direct information on local activities of the corresponding ions in a corrosion site. It therefore seems to be the most simple, convenient and informative method for this purpose. Potentiometric measurements with glass-capillary microelectrodes, in their present form, have been widely used by physiologists for extra- and intracellular measurements since the early 1970s [1]. Such electrodes are advantageous, essentially because of their low cost and uniform manufacturing technology. The electrode consists ⇑ Corresponding author. E-mail address: [email protected] (V.V. Egorov). 1572-6657/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jelechem.2013.07.034

of a microcapillary with a tip diameter of 0.1–5 lm filled with an ion-selective membrane cocktail in its lower part. An inner reference solution (usually aqueous KCl solution) is placed above the membrane with a silver wire, coated by electrodeposited silver chloride, immersed into it. The analytical properties of such electrodes are determined by the composition of the liquid membrane. This usually consists of an organic solvent immiscible with water (e.g. 2-nitrophenyl octyl ether), a liquid anion- or cation-exchanger providing so-called perm-selectivity of the membrane to anions or cations respectively (e.g. tridodecylmethylammonium chloride and potassium tetrakis(4-chlorophenyl) borate), and an ionophore (e.g. valinomycin) providing the selectivity of the membrane to the ion of interest. While there is no problem to prepare any membrane cocktail from individual components in the laboratory, those more in demand are also commercially available. The successful examples of application of glass-capillary microelectrodes in corrosion studies for pH [2–14], Zn2+ [15,16] and Mg2+ [6,8,14] activity mapping have been described. However, very few experimental results describe the Cl distribution in sites of pitting, crevice or cut-edge corrosion. Nguyen and Lin [17] used a double barrel chloride ion-selective microelectrode to follow the changes in Cl activity at the artificially delaminated area on the coating/metal interface and in a blister. They found that in both monitored sites of cold rolled steel, coated with an alkyd-based paint layer, Cl activity increased with immersion time. Luo et al.

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Table 1 Cocktail compositions for sodium and chloride ISMEs. # n/n

1* 2 3 4 5 6a 7 8 a b

Ionophore

Ion exchanger

Solvent, up to 100 w/w %

Name

w/w, %

Name

w/w, %

Na+-ionophore II Na+-ionophore VI Na+-ionophore VIII Na+-ionophore X Na+-ionophore VI Cl-ionophore I Cl-ionophore II –

10.0 6.1 2.8 0.7 6.0 5.0 1.0 –

Sodium tetraphenylborate KTClPhB KTClPhB KTClPhB KTClPhB – TDDMACl TDDMACl

0.5 2.3 0.5 0.2 2.1 – 0.1 10.0

NPhOE NPhOE NPhOE NPhOE DMNB NPhOEb NPhOE NPhOE

Commercially available membrane cocktails from Fluka [24]. Cocktail also contains 4.0 w/w % 1-Decanol and 1.0 w/w % tetradodecylammonium tetrakis(4-chlorophenyl) borate.

[18] measured Cl profiles over corroding pits. Lin et al. [19] measured accumulation of Cl over the crevice and pitting corrosion sites of stainless steel 304. Yan et al. [20] reported on the construction of combined Ag/AgCl – Ir/IrOx microelectrode that was used to monitor the increase in pH and depletion of Cl activity on X70 pipeline steel under cathodic protection. Du et al. [21] applied chloride selective microelectrodes to monitor Cl concentration at steel/concrete interfaces. Although accumulation of Na+ under a delaminated coating is reported in multiple sources no experimental results, reporting measured Na+ concentration could be found in the literature until recently. All the studies mentioned above present fragmentary information. No systematic studies were undertaken in the direction of measuring Cl and Na+ distribution in various sites of corroding materials due to a lack of reliable experimental protocols. Meanwhile, information concerning the distribution of ions from the corrosive environment around active sites may be vital for modeling corrosion processes and corrosion prediction. For example, while modeling galvanic coupling in aluminum alloys, Murer et al. [22] concluded that the obstacle 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. Recently Lamaka et al. [14] reported on simultaneous micro-potentiometric measurements of Na+ and Cl activities over an anodically polarized Pt wire. For these experiments a number of commercially available membrane cocktails, namely sodium ionophore II – Cocktail A (Fluka Ref. 71178) and slightly modified (to decrease the electric resistance of the membrane and response time) chloride ionophore I – Cocktail A (Fluka Ref. 99408) have been utilized. After [14] was published, our attention was drawn to a possible pH dependence of these commercially available membrane cocktails. Said cocktails were developed for biological applications, where pH hardly varies for more than one tenth of a pH unit. Therefore their pH sensitivity was never an issue. It should be mentioned here that, to obtain reliable results, a number of special requirements to analytical characteristics of Na+ and Cl-selective microelectrodes should be fulfilled. Firstly, as far as pH in the corrosion zone can vary greatly from about 2

in the anodic zone to about 12 in the cathodic zone, there are high demands on the working pH range of both Na+- and Cl-selective microelectrodes. Secondly, to register the alteration in Na+ and Cl activity which may be quite small (about 0.1–0.3 pa units of Cl and even less for Na+) high measurement precision is required. So the electrode should therefore provide small long-term potential drift and high potential reproducibility. Thirdly, high selectivity of the Na+-selective microelectrode against Zn2+, Al3+ and Mg2+ cations is necessary to prevent misinterpretation of Na+ activity measurements in anodic sites while working with metal coated steels and alloys. Lastly, to obtain scans including about one thousand or more experimental points within an appropriate timescale low response time of the used electrode is needed as well. However, it turned out that Na+ and Cl- selective microelectrodes based on the commercially available membrane cocktails do not fully satisfy the requirements mentioned above. So the aim of this work was to optimize the Na+ and Cl- SME membrane composition by varying ionophore and solvent nature in order to achieve the required analytical characteristics for the microelectrodes. Both newly developed glass-capillary microelectrodes were applied to trace the changes of Na+ and Cl concentration over a cut-edge of Al-Zn coated steel.

2. Experimental 2.1. Equipment Commercial Scanning Ion-selective Electrode Technique (SIET) equipment manufactured by Applicable Electronics (USA) and controlled using ASET software (ScienceWares, USA) with homemade glass capillary ion-selective electrodes (ISMEs) was used for performing the majority of the potentiometric measurements. ISMEs were made of single-barrel, standard-wall (330 lm), borosilicate glass capillaries with an outer diameter of 1.5 mm. A P-97 Flaming/Brawn Micropipette Puller (Sutter Instruments Company) was used to shape the cone tip. The diameter of the orifice of the tip was 1.5 ± 0.3 lm. The capillaries were silanized by injecting 200 ll of N, N-dimethyltrimethylsilylamine in a glass preparation

Table 2 The membrane compositions for sodium and chloride macro electrodes. # n/n

Ionophore

Ion exchanger

Name 1 2 3 4 5 6

Na+-ionophore Na+-ionophore Na+-ionophore Na+-ionophore Na+-ionophore Na+-ionophore

VI VI VI VI VI VI

w/w, %

Name

w/w, %

1.0 1.0 1.0 1.0 1.0 1.0

KTClPhB KTClPhB KTClPhB KTClPhB KTClPhB KTClPhB

0.4 0.4 0.4 0.4 0.4 0.4

PVC, w/w %

Plasticizer, up to 100 w/w %

33 33 33 33 33 33

NPhOE NPhPE NPhPhE DMNB FPhNPhE BNPhE

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Controller in a frequency range from 1  105 down to 1  102 Hz. All the spectra were recorded at open circuit potential while applying a 5 mV sinusoidal perturbation. A conventional three-electrode cell was used, consisting of a saturated calomel reference electrode, a platinum coiled wire as a counter electrode and the ion-selective microelectrode as working electrode. The cell was placed in a Faraday cage to avoid interferences from external electromagnetic fields and stray currents. The electrolyte in the cell (total volume 150 ml) was quiescent and equilibrated with air 0.05 M NaCl.

chamber at 220 °C. The silanized glass micropipettes were backfilled with an inner reference solution (0.05 M NaCl) and then tip-filled with a selective, ionophore-based, oil-like, membrane cocktail (the compositions of used membrane cocktails are presented in Table 1). The column length of the membrane was 50– 60 lm in the case of Na+-selective and 80–100 lm in the case of Cl-selective microelectrodes. An Ag/AgCl wire was inserted into the electrolyte from the back end of the capillary to provide an inner reference electrode electrical contact. A homemade Ag/AgCl/1 M KCl mini-electrode with an inner solution stabilized by 3% agar-agar was used as external reference electrode. The size of the orifice of this electrode was 2 mm. An IPA2 amplifier (input resistance >1015 X) was used to record the potential values. ‘‘Pseudo double-barrel’’ micro-electrode was used for verifying the absence of the IR drop effect on the measurements performed by developed Cl and Na+-SME. To this end, reference micro-electrode was fabricated by filling a glass-capillary with 0.05 M NaCl solution and insetting there a Ag/AgCl wire. The diameter of the orifice of the tip of this reference electrode was 10 ± 2 lm. This reference micro-electrode was positioned 10 lm away from Cl or Na+-SME by using a dual head stage manipulator (described in [8]). Both micro-electrodes, reference and ISME, constituted the ‘‘pseudo double-barrel’’ micro-electrode. Two micro-electrodes moved together during rastering. The distance between them, 10 lm, was constant. Ion-selective macro electrodes with plasticized membranes of ca.1 cm in diameter were used for a number of preliminary tests. The data obtained with macro electrodes is more reproducible as there is never a problem with leaking membrane cocktails or small air bubbles, notorious for distorting the response of the microelectrodes. Macro electrodes, fabricated according to [23] were used to optimize the composition of the membrane cocktails. The membrane compositions of these electrodes are presented in Table 2. In this case the measurements were performed using an 8-channel pH-meter-ionmeter Ecotest-120 (Econix, Russia) and saturated Ag/ AgCl reference electrodes EVL-1M3.1 (Izmeritel, Belarus). A multimeter Eutech PC 700 with combined pH electrode (ECFC7252101B) was used to control pH. The impedance of the microelectrodes was measured using Electrochemical Impedance Spectroscopy (EIS). The measurements were performed using an Autolab (The Netherlands) with a PCI4

O N

N O

O

O

O

O

Na -ionophore II H3C H3C

C12H25

H25C12

O

O

O

O +

Na -ionophore VIII

CH3 O

H3C H3C CH3

OR OR OR OR

N Cl

H3C CH3 N CH3

Mn N N

O

H3C

CH3 CH3

R= *

H17C8

O

H 3C H 3C H17C8

O

O Hg Hg O

CH3

CH3

O

CH3 -

Cl -ionophore I

O O

O

O

Na+-ionophore VI

+

O

O

O

O

O

O

O

CH3

H25C12

O

O

Sodium ionophore cocktail II A and chloride ionophore cocktail I A were purchased from Sigma–Aldrich. Custom cocktails were prepared using the following reagents: solvents – 2-nitrophenyl octyl ether (NPhOE), 2-nitrophenyl pentyl ether (NPhPE), 2-nitrophenyl phenyl ether (NPhPhE), 1,2-dimethyl-3-nitrobenzene (DMNB), 2fluorophenyl 2-nitrophenyl ether (FPhNPhE), benzyl 2-nitrophenyl ether (BNPhE); ionophores – N,N0 -Dibenzyl-N,N0 -diphenyl-1,2phenylenedioxydiacetamide (Na+-ionophore II), Bis[(12-crown4)methyl] dodecylmethylmalonate (sodium ionophore VI), Bis[(12-crow-4)methyl] 2,2-didodecylmalonate (Na+-ionophore VIII), 4-tert-Butylcalix[4]arene-tetraacetic acid tetraethylester (Na+-ionophore X), meso-Tetraphenylporphyrin manganese(III)chloride complex (Cl-ionophore I), 4,5-dimethyl-3,6-dioctyloxyo-phenylene-bis(mercurytrifluoroacetate) (Cl-ionophore II); ion exchangers – potassium tetrakis(4-chlorophenyl) borate (KTClPhB), tridodecylmethylammonium chloride (TDDMACl). All the cocktail reagents were Selectophore™ grade products from Fluka. The structural formulas of the used ionophores are presented in Fig. 1. Membrane cocktails were prepared according to Table 1 by weighing about 1 mg of ionophore and adding the appropriate amount of ion-exchanger and solvent into a polypropylene vial. Then the components were thoroughly mixed using an ultrasonic bath. Commercially available buffer solutions with pH 1.679 (at 25 °C), 4.008 (at 25 °C), 6.865 (at 25 °C), 9.180 (at 25 °C), 10.012 (at 25 °C), 11.0 (at 20 °C) and 12.00 (at 20 °C) purchased from Fluka were utilized for calibration of glass pH electrodes (ECFC7252101B) using a Eutech PC 700 Meter. It should be mentioned here that the concentrations of Na+- and Cl in commercially available pH buffer O

O

O

O

2.2. Reagents

-

Cl -ionophore II

+

Na -ionophore X Fig. 1. The structural formulas of sodium and chloride-selective ionophores used in this work.

O

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V.A. Nazarov et al. / Journal of Electroanalytical Chemistry 706 (2013) 13–24

solutions is not constant, so these buffer solutions produce unpredictable responses with Na+- and Cl – SMEs and thus cannot be used to determine the working pH range of these electrodes. Hence homemade buffer solutions were prepared with constant (0.05 M) Na+ and Cl concentrations using the scheme of universal buffer preparation described in [25] and modified for our purposes. A mixture containing 0.4 M boric, acetic and phosphoric acids was combined with specific portions of 1 M sodium hydroxide and 1 M sodium chloride solutions and then diluted to 50 ml with MilliPore water (q > 18 MX cm). The exact pH values of the prepared solutions were determined using calibrated pH selective glass macro electrodes and were close to those expected. This set of homemade buffers is called hereafter pH buffers, type I. The composition and measured pH values of the prepared buffer solutions (type I) with a constant Na+ concentration are given in Table 3. The composition of buffer solutions (type I) with a constant Cl concentration was analogous to the one presented in Table 3 with the exception of the sodium chloride aliquot volume: aliquots of 1 M sodium chloride solution were 2.50 ml for each buffer solution. Also, a series of solutions with specific pH was prepared in the presence of constant chloride background and containing no other anions except for chloride and hydroxide (pH buffers, type II). The composition and the measured pH values of these buffer solutions are given in Table 4. These pH buffer solutions were used in cases when chloride-selective electrodes, due to low selectivity, showed responses to the anions containing in the universal buffer mixture (acetate, borate, phosphate).

2.3. Determination of analytical parameters of microelectrodes To determine the working pH-range of Na+- and Cl-selective electrodes, their potentials were measured against external reference electrode in pH buffers of type I or II (for Cl-selective electrodes). The range was defined as a pH-region within which potential deviation does not exceed ±2 millivolts from the average value in 0.05 M Na+ or Cl solutions. To define the potential drift, the slope of potential by time dependence obtained by monitoring the electrode response in 0.05 M NaCl solution for 2 h was used. Potential reproducibility was determined as the mean value of 10 measurements of the potential after a microelectrode was exposed in 0.05 M NaCl solution for 4 min. Response time (slim, and s95) (see IUPAC recommendations [26,27] and the review from Maccà [28]) was measured in a ‘‘dual drop cell’’, specifically designed for such measurements [29]. The selectivity of Na-SME over Zn2+, Al3+ and Mg2+ was determined using a separate solution method [26,30].

Table 3 The composition and measured pH values of prepared pH buffer solutions (type I) with constant Na+ concentration (the prescribed aliquots are used to prepare 50 mL of buffer solution). Measured pH

Vacid’s

2.1 3.0 4.1 5.0 6.3 7.1 8.3 9.4 10.2 11.0 12.0 12.6

1.25 1.25 1.25 1.25 1.25 1.25 1.25 1.25 1.25 1.25 1.25 0.63

mixture (0.4M each),

ml

VNaOH – 0.44 0.63 0.88 1.06 1.31 1.50 1.75 1.94 2.06 2.50 2.50

(1M),

ml

VNaCl 2.50 2.06 1.88 1.63 1.44 1.19 1.00 0.75 0.56 0.44 – –

(1M),

ml

Table 4 The composition and measured pH values of prepared buffer solutions (type II) with constant Cl concentration (the prescribed aliquots are used to prepare 50 ml of buffer solution). Measured pH

(0.1M),

VHCl

2.1 3.1 5.2 8.3 9.3 10.9 11.9

5.00 0.50 – – – – –

VNaCl ml

(0.1M),

20.00 24.50 – – – 25.00 25.00

ml

VNH4Cl (0.1M), ml

(0.1M),

VNH3

– – 25.00 25.00 25.00 – –

– – – 2.50 25.00 – –

VNaOH ml

(0.1M),

ml

– – – – – 0.50 5.00

2.4. SVET and micro-potentiometric measurements (SVET–SIET) The developed microelectrodes were applied to map Cl and Na+ distribution over the cut-edge of metallic coated steel. The thickness of the steel substrate was 0.6 mm. The steel was dip coated with an Al–Zn alloy (55 wt% Al, 43.4 wt% Zn and 1.6 wt% Si). The thickness of the metallic coating were was 20–25 lm. The steel samples were cut into small pieces (0.5–3 mm) and embedded in epoxy resin. The cathodic and anodic activity above the cut-edge sample was monitored using Scanning Vibrating Electrode Technique (SVET) that recorded local current density and Scanning Ion-selective Electrode Technique (SIET) providing information about Na+ and Cl distribution. The local activities of Na+ and Cl were mapped 50 lm above the surface on a 31  31 grid. The time of acquisition for each data point was 2.5 s, resulting in a total scan time of about 1 h. This period also includes the time for the electrode to move from point to point. Quasi-simultaneous measurements of local current density and local ion concentration (SVET–SIET) in and around the corrosion site were performed as described elsewhere [11]. The vibration probe of SVET and glass capillary were positioned at a distance of 50 lm in order to avoid cross-talk effects, breaking of the glass electrode in collisions with the vibrating probe and excessive solution stirring. A dual head stage manipulator allowed for precise positioning of the two measuring tools: vibrating probes of SVET plus Cl or Na+ selective microelectrodes to monitor local current density and ion distribution. The dual head stage was mounted on 3D step motors which moved with a lateral resolution of 0.8 lm. A time lag between acquiring each current density and micro-potentiometric data-point was less than 2.6 s. Thus, one SVET– SIET scan yielded two independent, simultaneously acquired maps showing ionic current density and ion distribution. Experiments were performed during continuous immersion in either 0.005 M or 0.171 M (1 w/w%) NaCl. All experiments were performed in a Faraday cage at room temperature (21 ± 3 °C). Insulated Pt–Ir probes (MicroProbes for Life Science, USA), with platinum black deposited on a spherical tip of 15 lm diameter were used as vibrating electrodes for the SVET measurements. The SVET probe was positioned 100 ± 3 lm above the surface, vibrating in the planes perpendicular (Z) and parallel (X) to the sample’s surface. The vibration amplitude was 32 lm (peak to peak), the vibration frequencies of the probe were 124 Hz (Z) and 325 Hz (X). Only the data collected from the vibration perpendicular (Z) to the sample was considered and presented in this work.

3. Results and discussion First of all, proceeding from actual measuring conditions taking place in corrosion studies, the desirable parameters for Na+- and Cl-selective microelectrodes were established. The following parameters were considered: linear range of response, value of

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V.A. Nazarov et al. / Journal of Electroanalytical Chemistry 706 (2013) 13–24

E, mV

NaII NaVI NaVIII NaX

380

360

340

320

300

280 2

4

6

8

10

12

14

pH Fig. 2. Response of the Na-SMEs based on NPhOE solvent and different ionophores to pH in buffer solutions of type I (see Table 1 for cocktail composition and Table 3 for the composition of used pH buffer solutions).

E, mV

+

Na -ionophore VI

NPhOE DMNB NPhPE NPhPhE FPhNPhE BNPhE

120 110 100 90 80 70 60 1

2

3

4

5

6

7

8

9

10 11 12 13 14

pH Fig. 3. Response of the Na+-macroelectodes based on Na+-ionophore VI and different plasticizers to pH (see Table 2 for membrane composition) To obtain these dependences macroelectrodes were immersed in a 0.015 M HCl solution, then pH was in situ adjusted by addition of small amounts of NaOH solutions of different concentration and controlled by pH glass electrode.

3.1. Sodium-selective electrode As a wide working pH-range is one of the most crucial parameters for successful employment of the electrode in corrosion studies, first of all we have investigated pH sensitivity of both the Na+- and Cl-selective microelectrodes. The first step for the membrane optimization consisted of studying the influence of the ionophore nature on the working pH-range of Na+-selective microelectrodes based on NPhOE plasticizer. The data are presented in Fig. 2. The response of the electrode based on Na+-ionophore X is affected by pH at the top. Besides, that microelectrode displayed a very low (about half of Nernstian) function slope, so it was not used in further studies. The variation of potential in the pH range from 2 to 12 was 6.9 mV for ionophore VIII, 6.3 mV for ionophore II and 5.2 for ionophore VI. Thus, Na+-ionophore VI was chosen for further studies. At the next step we studied the influence of the solvent nature on potential as a function of pH dependence of the electrodes based on the Na+-ionophore VI, Fig. 3. In order to obtain the most reliable results and taking into account the simplicity of measurements technique this part of study was performed via macro electrodes with plasticized PVC membranes. At the first glance, four out of six electrodes (except for those plasticized with BNPhE and FPhNPhE) possess a rather wide working pH-range. Potential deviations from the average values do not exceed ±2 mV in the pH range 2–11. In order to quantify the pH sensitivity of the studied electrodes, the slope (mV/pH unit) of linear regressions for E vs. pH dependences was calculated. The resulting values were: 0.13 for NPhOE, +0.12 for NPhPE, +0.27 for DMNB and +0.26 for NPhPhE. Other analytical characteristics of the studied electrodes are presented in Table 5. Evidently, all the electrodes possess very similar selectivity, linearity range and electrode function slope. However, taking into account their pHsensitivity, the electrodes with the membranes plasticized with NPhOE and NPhPE can be regarded as the most appropriate. More detailed studies were performed with NPhOE-based membrane placed in glass-capillary microelectrode. The analytical parameters of this electrode are presented in Table 6. Response of the developed electrode to sodium ions is shown in Fig. 7(a). It is clear that this electrode is superior to that based on commercial membrane cocktail and its parameters almost entirely correspond to those desired. So, this electrode can be proposed for corrosion studies using micro-potentiometry and SIET in particular. 3.2. Chloride-selective electrode

Nernstian slope, working pH range, potential drift, potential reproducibility, selectivity and response time. Data presented in Table 6 shows that neither the Na+- nor Cl-selective microelectrodes filled with Fluka membrane cocktails A77178 (based on Na+ ionophore II) and A99408 (Cl ionophore I) respectively, thoroughly satisfy the desirable parameters.

First of all the potential vs. pH dependence of Cl-selective microelectrodes based on commercially available cocktail (ionophore I cocktail A) was obtained. The results are presented in Fig. 4. The actual working pH-range of this electrode lies in between 3 and 9 (Fig. 4(a)), i.e. it does not satisfy the requirements for corrosion studies as stated above. The working pH-range of membrane cocktail, based on Cl-ionophore II, was found to be

Table 5 Analytical characteristics of Na-selective macroelectrodes based on Na+-ionophore VI and different plasticizers (see Table 2 for membrane composition). Analytical characteristic

Plasticizer NPhOE

DMNB

NPhPE

NPhPhE

FPhNPhE

BNPhE

5  101–2  104 50.4 2–11

5  101–2  104 52.7 2–11

5  101–2  104 53.3 2–11

5  101  2  104 51.5 2–11

5  101–2  104 53.0 2–11

5  101–2  104 51.8 2–11

1.4 3.7 3.8 3.7

1.4 3.8 3.9 3.6

1.4 3.8 3.7 3.6

1.2 3.7 3.7 3.4

1.4 3.8 3.9 3.6

Linearity range, mol L1 Slope, mV/decade Working pH-range Selectivity coefficients

LogK Pot Naþ ;Menþ

K+ Al3+ Zn2+ Mg2+

1.3 3.7 3.8 3.6

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Table 6 Analytical characteristics of Na+- and Cl-SME based on developed and commercial membrane cocktails. Analytical characteristic

1

Linearity range, mol L Slope, mV/decade Working pH-range Potential drift, mV h1 Potential reproducibility, mV

10 –10 >53 2–12 <1 ±1.0 <3

log K Pot Na;Al3þ

b

3

b

Na-II cocktail A 1

10 –10 54.9 4–10 1.4 ±3.2 2.2

a

Na-VI, NPhOE

4

1

10 –10 57.4 3–12 0.3 ±0.9 3.9

4

Cl-I cocktail A 1

3

b

TDDMA Cl, NPhOE

10 –10 51.6 3–9 2.2 ±1.2 –

101–104 58.7 2–12 0.9 ±0.2 – –

log K Pot Na;Zn2þ

<3

2.8

3.4

–

log K Pot Na;Mg2þ

<3

3.5

4.3

–

–

slim, s s95, s

<3 <1 <1  1011

 3 [31] 1  1011

1.7 0.65 5  1010

– – 7  1010

2.3 0.71 1.4  1011

Impedance, X a

a

Desirable value

1

Commercially available. Developed in this work.

E, mV 0

Cl- - ionophore I, Cocktail A (Fluka)

E, mV

Cl- - ionophore II, TDDMA Cl, NPhOE

90

-20

80 70

-40

60 -60

50 40

-80

30

(a) -100

(b)

20 2

4

6

8

10

12

14

2

4

6

pH

8

10

12

14

pH

E, mV

E, mV

20

TDDMA Cl, NPhOE

50

Ag/AgCl - electrode

18 16

0

14 -50 12 10

(c) 2

-100 4

6

8

10

12

14

pH

(d) 2

4

6

8

10

12

14

pH

Fig. 4. Response of the studied Cl-SMEs to pH in buffer solutions of type I (see Table 1 for cocktail composition and Table 3 for the composition of used pH buffer solutions).

even narrower: between 5 and 12, as can be seen in Fig. 4(b). Besides, that electrode possessed a rather long response time (see Fig. 5). It should be mentioned here that the potential of anionselective electrodes based on metal containing ionophores often suffer from a strong pH influence due to metal hydrolysis [32]. This high pH sensitivity of commercially available Cl-SME is not a problem for biological applications, where pH hardly varies for more than one tenth of a pH unit. However, in corrosion studies, where pH varies in a wide range, high sensitivity to pH could lead to an underestimation of the actual Cl concentration at acidic pH and an overestimation at alkaline pH. These results motivated us to discard the idea of using ionophore-based chloride electrode at all, and to search in other directions. In particular, an attempt was undertaken to use an Ag/AgCl electrode of the second kind as an indicator microelectrode for Cl determination. The electrode displayed a rather wide working pH-range (from 2 to 12, see Fig. 4(c)) but the function slope was significantly lower than Nernstian (about 46.7 mV/decade). The

most crucial fact, however, was the experimental difficulties of fabricating an electrode of such a type with tip diameter less than 10 lm. Hence it was not small enough for high resolution micropotentiometric measurements performed using SIET or SECM (Scanning Electrochemical Microscopy in potentiometric mode). The adjacent points on the map are separated from each other by 10–100 lm and ion concentration is mapped 10–50 lm over the active surface. Besides, we aimed at designing the glass-capillary micro-electrodes in this work. This allows for all the microelectrodes used in micro-potentiometric measurements in corrosion laboratory to be fabricated using the same equipment and procedures. It is well know that the selectivity of anion-selective electrodes based on quaternary ammonium salts obey the Hofmeister’s extraction series [33]. Thus, the selectivity of the Cl selective electrode based on ‘‘pure anion exchanger’’, in absence of ionophore, is very low. However, taking into account that under actual measurement conditions the corrosion medium does not contain any

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of such a type of electrode to Cl against OH should be about 102–103, i.e. low enough to neglect the OH influence on the electrode potential in 0.05 M Cl solutions. On the other hand, quaternary ammonium salts have no inclination to hydrolysis, so one could expect a rather wide working pH-range. It turned out surprisingly that the TDDMACl-based microelectrode also showed a strong pH-dependence (Fig. 4(d)) when it was tested in pH buffer solutions of type I (see Table 3 for the composition). However, the reason for this behavior is a very low selectivity of the studied electrode to chloride against other anions in the absence of any ionophore. So, in spite of the constant chloride background in the universal buffer solutions, the electrode may respond to the change of acetate, phosphate and borate activities as well as to the activities of their partially neutralized anion forms with pH. This assumption is confirmed by the results of pH response measurements in the solutions which contain no other anions except for chloride and hydroxide, Fig. 6. The composition of these type II pH buffer solutions is presented in Table 4. One can see that the electrode displays a wide working pH-range between 2 and 12. These results once again highlight the high susceptibility of ionselective microelectrodes to minor details, such as composition of buffer solution, and illustrate the importance of careful experiment planning when working with ion-selective microelectrodes. Other analytical parameters of the exclusively TDDMACl-based electrode, such as linearity range, Nernstian slope, potential drift, potential reproducibility and the response time are quite good as well (see Table 6) and fully satisfy the required parameters. Response of the developed electrode to chloride ions is shown in Fig. 7(b). Therefore, this electrode can be recommended for corrosion studies. Note, that if ions more lipophilic than Cl (such as  NO 3 , SNC , and salicylate) present in the corrosive medium, this Cl-selective microelectrode must not be used as measured Cl values would be highly overestimated.

pH 1.99

4.18

3.08

5.25

6.15

7.14

8.08

9.32

100

E, mV

80

60

40

20 0

5

10

15

20

25

30

35

40

t, min

Fig. 5. Time graph of the response of Cl-SME based on Cl-ionophore II to pH in buffer solutions of type I. See Table 3 for the composition of pH buffer solutions.

E, mV 280

260

240

220

200

3.3. Response time and the impedance of Na+- and Cl-selective microelectrodes

180 2

4

6

8

10

The response time is one of the most important characteristic of microelectrodes used for scanning measurements since it predetermines the acquisition time of a scan composed of multiple points. The response time was measured according to IUPAC recommendations [26] using a ‘‘dual drop cell’’ as described in [29]. Two different approaches were used for quantification of the response time, slim and s95. The response time of the microelectrode, slim, is the time elapsed between the instant when the microelectrode was brought into contact with the solution and the instant at which the potential/time slope (DE/Dt) reaches a limiting value (1 mV/min by IUPAC recommendation [26]). The time needed to obtain 95% of the total potential change, s95, was also determined. Note that s95 is not recommended by IUPAC as it provides highly

12

pH Fig. 6. Response of TDDMACl-based Cl-SME to pH in buffer solutions of type II. See Table 4 for the composition of pH buffer solutions.

anions, except for Cl and OH, high selectivity is not necessary. Therefore we tested an electrode with a ‘‘pure anion exchange membrane’’ based on TDDMACl without any ionophore. According  to [34] the chloride-hydroxide anion exchange constant (log K OH Cl ) in an extraction system water/solution of trinonyloctadecylammonium chloride in toluene is 2.8 ± 0.2, so the selectivity coefficient

E, mV

E, mV

(a)

400

Θ = 54,7 mV/decade

350

350

R = 0,999

300

300

250

250 +

200

pa (Na ) 1

2

3

(b)

400

4

Θ = -58,7 mV/decade

R = 0,999

-

200

pa (Cl ) 1

5 +



2

3

4

5

Fig. 7. Responses of the optimized Na - and Cl -SMEs in the corresponding ions solutions.

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Fig. 8. Graphical representation of response time slim and s95 as potential of the electrode changes from initial value, E0, to stationary value, Estat. (a) Activity of the main ion, Na+, was decreased from 0.01 to 0.001 M. (b) Activity of Cl was increased from 0.001 to 0.01 M.

Fig. 9. Bode plots of glass-capillary without a membrane, newly developed and commercially available Cl- and Na+-selective microelectrodes.

underestimated potential values, but as this value is often found in the literature, we also present it here for comparison. The response time calculated from the curves presented in Fig. 8 is s95 = 0.64 s and slim = 0.85 s for Na+-SME and s95 = 0.71 s and slim = 0.84 s for Cl-SME. The impedance of the developed microelectrodes was measured using EIS. The Bode plots for Cl- and Na+- SMEs along with glasscapillary filled only with internal solution are presented in Fig. 9. Given the very small diameter of the orifice of glass-capillaries, the impedance of the developed microelectrodes is rather high, ca. 1011 ohm. The impedance values for the newly developed and commercially available microelectrodes are presented in Table 6. The impedance of the capillary filled only with internal reference solution was ca. 6  107 X. 3.4. Application of Na+- and Cl-selective microelectrodes for corrosion related studies Cut-edge samples of steel coated with Al–Zn alloy (55 wt% Al, 43.4 wt% Zn and 1.6 wt% Si) were tested by simultaneous SVET– SIET. Distribution of Na+ was mapped in conjunction with recording ionic current density over the same site. Then SVET maps were taken simultaneously with rastering Cl concentration above another sample of metallic coated steel. Typical distributions of Na+ and Cl concentrations and corresponding local current density distribution together with the optical micrograph are presented in Fig. 10. The black contours on the cut-edge samples indicated

in the SVET and SIET maps identify the areas corresponding to the metallic coating and the steel substrate. The X and Y coordinates in the data maps represent the size of the mapped area in lm. The measured concentrations of Na+ and Cl are presented as –log10 (ion concentration), also known as pCl and pNa, by analogy with pH. Cathodic activity – oxygen reduction accompanied by alkalinization – occurs along the steel substrate whereas anodic activity – Zn and Al dissolution – is localized in the metallic coating. This is in line with the previous reports [3,4,13]. The lower pCl values (higher chloride concentration) are found in the middle of the steel substrate where cathodic activity takes place. Depletion of Cl is observed in the metallic coating correlating with the anodic activity identified there by SVET. The opposite is revealed for Na+ distributions. The lower pNa values (higher sodium concentration) correlate with the areas showing anodic activity in the current density distribution map. In our recent article [35] we reported the results of a systematic study of local Na+ and Cl distribution measured over the cut-edge samples of metallic coated steels. Just as it is reported here, an unexpected distribution of Na+ and Cl was found in it for zinc, aluminum and zinc/aluminum metallic coated steel. Depletion of Cl in anodic sites (metallic coating) and its accumulation in cathodic sites (steel) was accompanied by an increase of Na+ concentration in anodic sites and decrease of Na+ content in cathodic sites. Such a distribution was explained by the formation of Cl and OH containing corrosion products and charge compensation effects. The described distribution of Na+ and Cl was partially confirmed by ex situ analysis of corrosion products performed by SEM-EDS and Raman spectroscopy. Interested readers may refer to [35] for the detailed explanation. Nevertheless, we realize that described distribution of Na+ and Cl is counter intuitive. Indeed, one would expect an increase of Cl content in the anodic sites compensating for high H+ and Men+ concentration and a corresponding distribution of Na+ [14,19,35]. Potentiometry in general and measurements with ion-selective micro-electrodes in particular, is a field where erroneous results can be obtained very easily and artifacts are not easy to recognize. Potentiometric measurements with microelectrodes can be affected by many factors thus yielding biased data. Among the factors that could severely affect the measurements under given conditions, the IR drop effect seems to be the most dangerous. Several experiments were performed in order to verify the absence of the IR drop effect in the collected data. The use of a double barrel microelectrode, with one barrel being a reference electrode and the other one serving as ion-selective electrode, should reveal the IR drop effect if any. Here we performed SIET measurements with a ‘‘pseudo double barrel’’ microelectrode. This means that the reference micro-electrode and working Cl- or Na+-SME electrode were positioned on the dual

V.A. Nazarov et al. / Journal of Electroanalytical Chemistry 706 (2013) 13–24

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Fig. 10. Optical micrographs (a) and (c) taken after 12 h of immersing the cut-edge samples in 0.005 M NaCl. Metallic coating is located on the left and right side of the cutedge steel sample, (a) and (c). The black contours of the sample on (d)–(g) facilitate correlating active zones with the metallic coating and steel. SVET (f) and (g); and corresponding SIET pNa (d) and pCl (e) distribution were measured after 12 h of immersion. Optical micrograph (b) shows two microprobes: platinum black SVET probe and ion-selective glass-capillary microelectrode positioned together to perform simultaneous SVET–SIET measurements results of which are presented in (d)–(f) and (e)–(g).

head stage ca. 10 lm apart. In this setup the IR drop is negligible as both micro-electrodes move together while scanning the sample. The results are presented in Fig. 11. The Cl and Na+ distribution measured in this way does not differ significantly from the one recorded with a static mini-reference electrode, Fig. 10. Depletion of Cl is observed in the metallic coating and its increase is evident in cathodic site over steel. Increase of Na+ concentration occurs above the anodic sites and its depletion accompanies cathodic activity. Another way of revealing the IR drop effect is conducting the measurements in a high concentration of supporting electrolyte. Measurements of Cl and Na+ distribution were repeated in 1 wt% (0.171 M) NaCl instead of 0.005 M NaCl. The results are presented in Fig. 12. The Cl and Na+ distribution is very similar to that observed in dilute NaCl. A slight decrease of Cl in anodic sites and increase in cathodic sites, was observed. In Fig. 11(c) note rather peculiar distribution of precipitated corrosion products: across the cut-edge sample instead of along the metallic coating. Important that the Cl distribution corresponds to this unusually shaped

precipitates rather than SVET map that reflects the IR drop. The results of both types of the experiments indicate an absence of influence of the IR drop effect on micro-potentiometric measurements performed with the developed Cl- and Na+-selective microelectrodes.

4. Conclusions Commercially available membrane cocktails, tested in our conditions, namely sodium ionophore II – Cocktail A (Fluka Ref. 71178) and chloride ionophore I – Cocktail A (Fluka Ref. 99408), showed rather significant pH dependence that made them unsuitable for corrosion related measurement where pH changes within a wide range. New membrane compositions for Na+ and Cl-selective microelectrodes were formulated and optimized to be used for corrosion related studies. The pH dependence of electrode function is improved so that it allows for an unbiased application of said

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Fig. 11. (a) pCl, (c) current density and (e) pNa contour line image overlapped with corresponding optical micrograph of the sample. Optical and contour line images exactly correspond to the (b) pCl (d) current density and (f) pNa mappings. The metallic coating is located on the top and on the bottom of the cut-edge steep sample. The black contours of the sample are repeated in (b), (d) and (f). The measurements were performed after (a) and (b) 1 h; (c) and (d) 3 h; (e) and (f) 5 h of immersion in pH neutral 0.005 M NaCl solution. A pseudo double-barrel micro-electrode with reference electrode moving together with ISME was used to obtain pCl and pNa maps. Both electrodes are visible in the optical image (a). The vibrating probe of SVET is presented in the optical micrograph (c).

V.A. Nazarov et al. / Journal of Electroanalytical Chemistry 706 (2013) 13–24

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Fig. 12. (a) and (b) current density distributions corresponding to (c) pCl and (d) pNa mappings. SVET and SIET measurements were recorded over cut-edge sample of hot dip Al–Zn coated steel after 8 h of exposure to 0.171 M NaCl. Corresponding optical micrographs (e) and (f) of two samples are also presented. Black rectangle indicates scanned area. Metallic coating is located on the top and the bottom of the cut-edge steel sample. The circles on (e) and (f) are the gas bubbles (probably H2) formed along dissolution of Zn and Al of metallic coating.

microelectrodes for monitoring activities of Na+ and Cl over alkaline, cathodic and acidic anodic corrosion sites. The importance of the use of ‘‘pure’’ pH buffer solutions when working with liquid membrane cocktails was highlighted. Said buffer solutions must not influence the potential readings in any other way apart from pH. The new microelectrodes were fully characterized. The electrodes possess an improved response time along with long- and short-term stability of potential. The proposed microelectrodes were successfully applied to measure concentrations of Cl and Na+ over corroding cut-edge samples of metallic coated steel and can be recommended for measuring distribution of Na+ and Clin corrosion applications. Acknowledgements Yegor Morozov from IST is gratefully acknowledged for the help with EIS measurements. FP7 Marie Curie IRSES Project ‘‘SISET: Enhancing Scanning Ion-Selective Electrode Technique’’ Project FP7-PEOPLE-IRSES-GA-2010-269282, BRFFR ‘‘Science IC – 2013’’

Project C13IC-023, RFCS Project AtCorAS, GA 2011-CT-201100015. FCT Projects, SFRH/BD/72602/2010, PTDC/CTM/108446/ 2008, PTDC/CTM-NAN/113570/2009, PTDC/CTM-MET/113645/ 2009, and PTDC/CTM-MET/112831/2009 for the financial support. References [1] J.L. Walker Jr, Ion specific liquid ion exchanger microelectrodes, Anal. Chem. 43 (1971) 89–93A. [2] J.O. Park, C.H. Paik, Y.H. Huang, R.C. Alkire, Influence of Fe-rich intermetallic inclusions on pit initiation on aluminum alloys in aerated NaCl, J. Electrochem. Soc. 146 (1999) 517–523. [3] K. Ogle, V. Baudu, L. Garrigues, X. Phillip, J. Electrochem. Soc. 147 (2000) 3654– 3660. [4] K. Ogle, S. Morel, D. Jacquet, Observation of self-healing functions on the cut edge of galvanized steel using SVET and pH microscopy, J. Electrochem. Soc. 153 (2006) B1–B5. [5] H. Ding, L.H. Hihara, Localized corrosion currents and pH profiles over B4C, SiC, and Al2O3 reinforced 6092 aluminum composites I in 0.5M Na2SO4 solution, J. Electrochem. Soc. 152 (2005) B161–B167. [6] S.V. Lamaka, O.V. Karavai, A.A. Bastos, M.L. Zheludkevich, M.G.S. Ferreira, Monitoring local spatial distribution of Mg2+, pH and ionic currents, Electrochem. Commun. 10 (2008) 259–262.

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