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Scanning Vibrating Electrode Technique

Scanning Kelvin Probe

Localised Electrochemical Impedance Spectroscopy

1. Scanning Vibrating Electrode Technique

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Localised aqueous corrosion is an electrochemical process, whereby oxidation of anodic sites on a metal surface in an electrolyte causes electrons to flow through the metal substrate to adjacent cathodic areas. This flow of electrons through the metal is balanced by a flow of ionic current in the electrolyte, which in turn causes potential gradients to exist in solution close to sites of localised corrosion. The Scanning Vibrating Electrode Technique (SVET) uses a scanning microelectrode to measure these gradients in-situ, thereby locating and quantifying the corrosion activity at specific points on the sample surface. Measurement is made by vibrating a fine tipped microelectrode a few hundred microns above the sample, in a plane perpendicular to the surface. The electrochemical potential of the microelectrode is recorded at the extremes of the vibration amplitude, resulting in the generation of a sinusoidal AC signal. This signal is then measured using a lock-in amplifier, which is tuned to the frequency of probe vibration. Furthermore, the resulting signal, which is in effect a measure of the DC potential gradients in solution, can be converted to current density by a simple calibration procedure[1]. The technique is therefore able to make in-situ measurements of the localised corrosion activity occurring at the sample surface.

Figure 1: Schematic representation of SVP measurement.

The SVET method was originally devised by biologists for the measurement of extracellular currents near to living cells[2, 3]. Isaacs later developed the technique to study various localised corrosion processes, including stress corrosion cracking of 304 stainless steel[4] and corrosion inhibition by cerium salts[5]. More recently, other workers have successfully applied the SVET to the study of corrosion on coil-coated steel[6, 7, 8].

References

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2. Scanning Kelvin Probe

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The Scanning Kelvin Probe (SKP) is a non-contact, non-destructive device for spatially resolved determination of work function on conducting, semi-conducting and biological materials. Exploiting well-established principles of direct correlation between work function and surface condition, it is a tool with wide applications for both surface science and industrial uses. In particular, the technique has been employed extensively in the study of corrosion and coatings[9, 10, 11, 12, 13, 14, 15, 16, 17]

The series of schematic diagrams below illustrate the principles of local work function measurement using a Scanning Kelvin Probe. In Figure 2, the Kelvin probe tip (typically a metal disk electrode) has a Fermi level (Eprobe) which differs from that of the sample (Esample).

Figure 2: Kelvin probe and sample of differing Fermi levels (E) and Work Functions (Ø).

The difference in Fermi levels also implies a difference in the work function (Ø), which is defined as the minimum work required to extract an electron in vacuum from the Fermi level of a conducting phase through a surface. If electrical contact is made between the probe and sample, the Fermi levels (and therefore also the work functions) equalise, resulting in the generation of an electrical charge on the respective surfaces (Figure 3).

Figure 3: Electrical contact between Kelvin probe and sample causes Fermi levels and Work Functions to equalise, resulting in electrical charging at respective surfaces and potential difference (Vc).

This charge gives rise to a potential difference VC, which relates to the difference in work function such that;

-eVC = Øprobe - Øsample Equation 1

where 'e' is the electron charge.

If a backing potential (Vb) is introduced to the system, as shown below in Figure 4, then the surface charge will become zero at the unique point where;

Vb= - Vc Equation 2

The SKP100 measures the point of zero charge by vibrating the probe tip in a vertical plane perpendicular to the sample surface and measuring the AC current flow (IAC) that results if a surface charge exists. A lock-in amplifier is then used to measure IAC at the specific vibration frequency. The backing potential (Vb) is varied automatically to maintain zero current, and the work function difference is simply equal and opposite to Vb (as described in Equation 2).

Figure 4: Kelvin probe vibrated perpendicular to surface, and resulting current measured by a lock-in amplifier. Backing potential Vb applied to sample and at point where Vb = -Vc the surface charge will be zero and hence current (IAC) = 0.

The probe used for Kelvin measurements in the system described here is a 0.5 mm diameter Pt electrode, which is scanned above the sample surface using a computer controlled x,y,z scanning system. This then allows the measurement of work function to be spatially resolved across the surface.

In the field of corrosion science, measurements of work function can be taken one step further to allow the corrosion potential (Ecorr), at a specific point, to be determine, using the relationship[9]:

Ecorr = constant + (Øsample - Øprobe) Equation 3

where (Øsample - Øprobe) is the measured work function difference between the sample and probe. If Ecorr is measured, using a conventional reference electrode, then the constant term in the above equation can be calculated and Ecorr deduced.

References

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3. Localised Electrochemical Impedance Spectroscopy

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Electrochemical Impedance Spectroscopy (EIS) is an in-situ, non-destructive technique, which has been employed extensively in the study of electrochemical processes[18] and in particular is now widely accepted as a standard technique for the investigation of corrosion and coatings[19, 20]. The method involves applying a small, sinusoidal AC potential perturbation to a sample over a range of frequencies and measuring the current response. The impedance is calculated at each frequency by the ratio of voltage to current, and the experimental data may then be compared to model data from established AC circuit theory. The characterisation of individual processes by these electrical analogues can facilitate understanding and lead to more accurate predictions of corrosion rates and overall corrosion behaviour[21].

Difficulties arise in EIS, however, when attempting to study localised electrochemical processes, as the impedance is calculated from bulk current and voltage data, and is therefore averaged across the entire surface area of the sample. In an effort to make impedance measurements on a local scale, work by a number of authors has been carried out to combine established DC scanning probe methods with AC impedance techniques[22, 23, 24, 25],, resulting in the evolution of LEIS. The principles of LEIS are similar to those employed in traditional bulk EIS, in that a small sinusoidal voltage perturbation is applied to a working electrode sample and the resulting current is measured to allow calculation of impedance. However, rather than measure the bulk current from the whole of the active surface area, an electrochemical probe is scanned close to the surface in order to measure the local current distributions in the electrolyte. This probe incorporates two platinum electrodes, one that constitutes the tip of the probe and is electrochemically sharpened to approximately 5 m m diameter and a second that is in the form of a ring and is positioned at a distance (d) typically 3 mm away from the tip. Each of these electrodes is platinised in order to increase active surface area and therefore reduce the interfacial impedance between the electrode and the electrolyte. The potential difference between the two platinum electrodes (Vprobe) is measured via an electrometer and, for a known solution conductivity (k ), the local AC current (Ilocal) is calculated from Equation 4.

Equation 4

The ratio of AC voltage perturbation applied at the sample (Vapplied) to Ilocal then gives the value of local impedance (Zlocal) according to Equation 5.

Equation 5

This approach then brings spatial resolution to the measurement of electrochemical impedance, thus overcoming the limitations of "surface averaging" encountered in bulk EIS experiments. Furthermore, the use of a scanning electrochemical probe to measure localised current means that LEIS has two main modes of operation:

Applying a small voltage perturbation to the sample surface at a fixed frequency, whilst simultaneously scanning the probe (by means of a precision x, y, z scanning stage) above the surface of a specimen to measure the local AC current response.

Applying a small voltage perturbation to the sample surface over a range of frequencies, with the probe held fixed at a location of particular interest, in order to obtain the local impedance spectrum at that location.

Combination of these techniques permits both the location and quantification of areas of local impedance variations on the surface of samples. A two dimensional area scan at fixed frequency will identify areas of impedance variation, e.g. a defect in an otherwise intact organic coating would be relatively low in impedance compared to the surrounding areas. Once a particular area of interest has been identified by the area scan, the probe can then be moved to this location and a full frequency spectrum obtained. The data from the local impedance spectrum can then be modelled using the standard packages employed in bulk EIS.

A schematic representation of the typical LEIS set-up is given in Figure 5, below.

Figure 5: Schematic representation of LEIS set-up.

The system incorporates a precision x, y, z scanning stage based around stepper motors with a maximum step resolution of 1 m m and optical encoders on each axis for absolute positional readout. This stage is used to scan the probe above the stationary sample, which is held in an electrochemical cell. The cell set-up is similar to a standard three-electrode experiment, in that the sample acts as the working electrode, a Saturated Calomel Electrode (SCE) is a reference and platinum is used as a counter electrode. However, the counter electrode for the LEIS set-up given above is in the form of a ring, which is actually positioned on the scanning probe. It is important to note that this large electrode merely serves to supply the current from the electrochemical interface; the actual measurement of current for the purposes of the impedance calculations is facilitated through the two smaller platinised platinum electrodes on the probe, as described previously.

Though still a relatively new technique, LEIS has already proven to be a powerful tool in several areas of corrosion research, including the determination of surface inhomogeneities on stainless steel[22], organic coatings assessment[23, 24, 26, 27] and pitting corrosion[28].

References

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References

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1 SVP100 Manual, Version 1.23, Uniscan Instruments Ltd., Sigma House, Buxton, UK, SK17 9JB.

2 P. W. Davies, Fed. Proc., 25, p. 332 (1966).

3 L. F. Jaffe, R. Nucitelli, J. Cell. Biol., 63, p. 614 (1974).

4 H. S. Isaacs, J. Electrochem. Soc., 135(9), p. 2180 (1988).

5 H. S. Isaacs, A. J. Davenport and A. Shipley, J. Electrochem. Soc., 138(2), p. 390 (1991).

6 F. Zou, C. Barreau, R. Hellouin, D. Quantin and D. Thierry, Galvatech’95, p. 837 (1995).

7 D. A. Worsley, H. N. McMurray and A. Belghazi, Chem. Commun., p. 2369 (1997).

8 S. J. Badger, S. B. Lyon and I. M. Zin, "A Comparison of Various Chromate-Free Corrosion Inhibitors for use in Coil-Coated Galvanized Steel", Paper presented at 14th Int. Corr. Congress, Capetown, South Africa (1999).

9 M. Stratmann and H. Streckel, ‘On the atmospheric corrosion of metals which are covered with thin electrolyte layers - I. Verification of the Experimental technique’, Corro. Sci., 30(6/7), 681 (1990).

10 M. Stratmann and H. Streckel, ‘On the atmospheric corrosion of metals which are covered with thin electrolyte layers - II. Experimental results’, Corro. Sci., 30(6/7), 697 (1990).

11. M. Stratmann and H. Streckel, ‘On the atmospheric corrosion of metals which are covered with thin electrolyte layers - III.’, Corros.Sci., 30(6/7), 715 (1990).

12. M. Stratmann, H. Streckel et. al., Short Communication - ‘A new technique able to measure directly the delamination of organic polymer films’, Corro. Sci., 32(4), 467 (1991).

13. L. T. Han and F. Mansfield, ‘Scanning Kelvin probe analysis of welded stainless steel’, Corros.Sci., 39(1), 199 (1997).

14. A. Nazarov and D. Thierry, 'Analysis of surface carbon contamination on phosphated zinc surfaces by scanning Kelvin probe measurements', J. Electrochem. Soc., 145, L39-L42 (1998).

15 C. Chen, C. B. Breslin and F. Mansfeld, 'Scanning Kelvin probe analysis of the potential distribution under small drops of electrolyte', Werkstoffe und Korrosion, 49, 569 (1998).

16 P. Schmutz and G. S. Frankel, 'Characterization of AA2024-T3 by scanning Kelvin probe force microscopy', J. Electrochem. Soc., 145, 2285 (1998).

17. P. Schmutz and G. S. Frankel, 'Corrosion study of AA2024-T3 by scanning Kelvin probe force microscopy and in situ atomic force microscopy scratching', J. Electrochem. Soc., 145, 2295 (1998).

18 "Impedance Spectroscopy", J. R. Macdonald, Editor, Wiley, New York (1987).

19. F. Mansfeld and M. W. Kendig, Werkst. Korros., 36, 473 (1985).

20. S. Haruyama, M. Asari and T. Tsuru, in Corrosion Protection by Organic Coatings, M. W. Kendig and H. Leidheiser, Jr., Editors, PV87-2, p. 197, The Electrochemical Society Proceedings Series, Pennington, NJ (1997).

21. D. C. Silverman, "Primer on the AC Impedance Technique", in Electrochemical Techniques for Corrosion Engineering, R. Baboin, Editor, NACE, Houston, Texas (1986).

22. H. S. Isaacs and M. W. Kendig, Corrosion, 36, 269 (1980).

23 J. V. Standish and H. Leidheiser, Corrosion, 36, 390 (1980).

24. M. C. Hughes and J. M. Parks, "An AC Impedance Probe as an indicator of corrosion and defects in polymer/metal substrate system", in Corrosion Control by Organic Coatings, Henry Leidheiser Jr., Editor, p. 45-50, NACE, Houston, Texas (1981).

25. R. S. Lillard, P. J. Moran and H. S. Isaacs, J. Electrochem. Soc., 139, 1007 (1992).

26. M. W. Wittman, R. B. Leggat and S. R. Taylor, 'The Detection and Mapping of Defects in Organic Coatings Using Local Electrochemical Impedance Methods', J. Electrochem. Soc., 146, 4071 (1999).

27. A. M. Mierisch, J. Yuan, R. G. Kelly and S. R. Taylor, 'Probing Coating Degradation on AA2024-T3 Using Local Electrochemical and Chemical Techniques', J. Electrochem. Soc., 146, 4449 (1999).

28. I. Annergren, D. Thierry and F. Zou, Localized Electrochemical Impedance Spectroscopy for Studying Piting Corrosion on Stainless Steels" J. Electrochem. Soc., 144, 1208 (1997).