Scanning Kelvin Probe

As described in Section 1.6.2, the scanning Kelvin probe (SKP) is practically unique as an electrochemical technique in that it does not require the presence of a bulk electrolyte and is capable of measuring localised free corrosion potentials (Ecorr) beneath humidity films and/or intact polymer layers. By moving the sample around beneath the probe a surface map of Ecorr may be obtained. Continuous scanning allows the surface maps to be put sequentially in an animation allowing the dynamic evolution of Ecoir to be studied. The SKP is used in this way to study FFC kinetics and also the effectiveness of different inhibitor coatings. The following section describes the apparatus used and the experimental procedures involved.

2.5.1 SKP Apparatus

A schematic diagram of the SKP apparatus is shown in Fig. 2.10. The reference probe (shown in Fig. 2.11) consists of a gold wire housed within a tapered glass capillary tube. Gold wire of diameter of 125 pm and 99.99% purity was use^

in this work unless otherwise stated. Wire was used as this produces the ideal tip geometry as if a truncated cone with a big vertex angle is used lateral resolution is reduced due to stray capacitance13. A 5 mm length of gold wire protrudes from the

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Figure 2.11: Schem atic diagram o f the SK P referen ce probe head.

Kelvin probe

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Figure 2.12: Schem atic diagram o f th e SKP calib ratio n w ith a PV B film present.

tapered end and is sealed with epoxy resin and then housed within a copper shroud.

The other end of the tube is attached to a loudspeaker and connected to a vibrator drive, which consists of a magnetically driven loudspeaker vibrated sinusoidally, at a frequency of 280 Hz, and normal to the sample surface. The vibrations are generated from the oscillator function on a Lock-In Amplifier (LI A), EG & G 7265. The LI A also filters out any background interference frequencies. The vibrations are transmitted mechanically to the gold wire (via the loudspeaker) which is mounted parallel with the surface of the specimen and about 100 pm above it. A peak-to-peak vibration amplitude of 40 pm was generated from this setup.

The Kelvin probe is fixed to a computer controlled tri-axial (x,y,z) stage and the sample is fixed in its local position. As described in Section 1.6.2 it is important to maintain a constant tip to sample distance and this is maintained at its optimum value using the micrometer levelling stage. Finally the probe and sample are mounted inside a stainless steel environment chamber.

2.5.2 SKP Operation

If a Volta potential exists between the probe and the surface then a change in the distance between the probe and the surface will cause a change the magnitude of the charge between the two. If the change is periodic, i.e. by vibrating the probe sinusoidally with respect to the test sample, then dC/dt and any non-zero Volta potential difference between the two produces an a.c. current as shown in Equation

1.26. Thus the vibration of the gold wire may cause an a.c. current generated in the external circuit connecting the sample and the reference probe and is initially amplified and converted to a.c. voltage. A corresponding d.c. output is sent from the amplifier to an integrator which adjusts the d.c. bias applied to the sample by the Trans-conductance amplifier so as to automatically null it (Ej=Q) and thus a “loop” is created.

The integrator makes the finite changes to equalise the Galvani potentials of the probe and the specimen in order to nullify the a.c. current. When the integrator alters the bias d.c. voltage, the system determines the effect it has on the a.c. current, gradually with smaller and smaller changes the system reaches null conditions. Once the stable condition is reached the integrator sends the magnitude of the reverse bias required (-£*/>), converted to a digital signal by the lock-in amplifier, to a computer.

2.5.3 SKP Calibration

The recorded A ^ f value is a measurement of the difference in localised potential existing between the surface and the probe. However, in terms of corrosion it is important to be able to get a measurement of the electrode potential, or the free corrosion potential, existing at point on the surface during corrosion experiments.

Therefore it is necessary to calibrate the SKP.

The probe electrode used in the SKP is made of gold wire, thus to ensure consistent results a calibration with a standard redox couple is required. In these experiments a series of redox couples have been used and also the effect of the PVB coating on A ^ f determined. The SKP was calibrated in terms of electrode potential using Ag/Ag+, Cu/Cu2+, Fe/Fe2+ and Zn/Zn2+ couples. A schematic diagram of a calibration cell is shown in Fig. 2.12. Calibration cells were prepared by machining wells (⁸mm diameter, 2mm deep) in discs of the relevant metal (15 mm diameter, 5 mm thick). These wells were then filled with a 0.5 mol dm' aqueous solution of the respective metal chloride salt (0.5 mol dm' nitrate salt in the case of Ag) and Volta potential difference values A ^ [ obtained with the SKP

The metal X base plate with a reservoir (shown in Fig. 2.12) is attached to the SKP in place of a sample. The reservoir is then filled with a 0.5M Xn+ aqueous and allowed to equilibrate after lowering the probe 1 0 0 pm above the reservoir surface and closing the SKP environment chamber. The Ay£®,f value is recorded at one point above the surface.

The corrosion experiments are carried out with the presence of an organic coating which is primarily a poly-vinyl-butyral (PVB) coating. Therefore it is also

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