The Scanning Kelvin Probe Technique: Applications for Corrosion, Coatings and Materials Characteriza

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The Scanning Kelvin Probe TechniqueApplications
for Corrosion, Coatings and Materials
Characterization
Princeton Applied Research
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Scanning Kelvin Probe

• SKP100
• The Kelvin Probe is a non-contact,
  non-destructive device for the determination of
  work function of conducting, semi-conducting, and
  coated materials. Exploiting well established
  principles of direct correlation between work
  function and surface condition, it is a tool with
  wide application for both surface science and
  industrial uses.

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Scanning Kelvin Probe

• Measurements of work function allows the
  corrosion potential (Ecorr), at a specific point,
  to be determined. Using the relationship
• ECORR Constant (Vb)
• where Vb is the measured work function
  difference between the probe and sample.
• The Scanning Kelvin Probe can be calibrated if
  Ecorr is measured, using a conventional reference
  electrode, then the constant term in the above
  equation can be calculated and Ecorr deduced from
  the data.

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Theory of the Kelvin Probe
?sample
? Work Function
?probe
E Fermi Level
Vc
Metal Probe
EProbe
EProbe
ESample
Metal Sample
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? Work Function
E Fermi Level
Metal Probe
EProbe
ESample

- - -

Metal Sample
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Vb
Metal Probe
EProbe
ESample
Vb -Vc
Metal Sample
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Vb
Iac 0 when Vb -Vc
Iac
Lock-in Amplifier
30 ?m
Iac
EProbe
ESample
Metal Sample
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• However, this produces a one-time measurement
  since the surface is now charged
• A time interval must occur before another
  measurement can be made
• In order to make a continuous measurement, a
  vibrating capacitance probe was developed.

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• This is defined by
• C Q ?0 A

V
d

• Where C is the capacitance
• Q is the charge
• V is the potential
• ?0 is the permittivity of the
  dielectric
• A is the surface area of the
  capacitor
• d is the separation between the
  probe and the sample

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• As separation d increases, the capacitance
• C decreases
• As the charge remains constant, the voltage, V,
  must increase
• As the probe oscillates above the sample, the
  voltage change is recorded

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• The peak-to-peak output voltage is defined as

Vptp (?V - Vb)RCo??sin(?t?)

• Where ?V voltage difference between the probe
  and
• the sample
• Vb backing potential
• R resistance of the converter
  feedback
• Co mean Kelvin probe capacitance
• ? angular frequency of vibration
• ? the phase angle
• ? modulation index (d1/d0)
• t time

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• When Vptp 0, the work function or contact
  potential of the surface is equal and opposite to
  Vb.

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Measurement Techniques

• Topographical measurement
• uses the measured difference in capacitance
  between the probe and the sample to determine the
  height of the probe above the surface being
  scanned (d is inversely proportional to C)
• Work function (?) measurement
• uses the backing potential, Vb, to measure the
  work function difference between the sample and
  the probe
• ?wf is equal and opposite to the work function of
  the sample

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Correlation of ? to Ecorr

• First demonstrated by M. Stratmann (1987)
• - Simultaneous measurement of wf and Ecorr

S. Yee, R.A. Oriani and M. Stratmann, JECS 138
(1991)
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Material and Coatings Analysis

• Zn-anodized steel
• Zn coating removed with conc. HCl
• Measurement made in air
• Sharp definition of exposed underlying steel vs
  intact Zn coating
• More negative values for ?wf correlate to more
  active potentials for metals, and lower contact
  potentials for non-metals

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• Work function measurement made in air at
• ambient temperature and humidity

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• Comparison of work function measurement
• and topographical measurement

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Treatment of AA2024-T3 Panels
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Test Panel 1

• Non-exposed 2024-T3 clad with conversion coating,
  primer and polyurethane topcoat
• Topographical analysis shows a grouping of
  holidays and raised areas
• Work function analysis shows an area of noble (or
  cathodic) potential surrounding an isolated
  island of active (or anodic) potential,
  possibly indicating a defect in the coating.

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Analysis of Test Panel 1

• ?wf values of -700 to -583 meV for areas of
  anodic potential

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Correlation of ? to Ecorr

• Work function measurements made in air
  immediately after corrosion potential measured
  while immersed in 0.598M NaCl (3.5).

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Test Panel 2
SKP100 Kelvin Probe
538.8m
459.3m
379.8m
300.2m
220.7m
Work Function (d ev)
141.1m
61.6m
-18.0m
-97.5m
-0.209
-177.1m
-256.6m
0
4K
8K
12K
16K
20K
24K
28K
32K
36K
40K
Displacement (µm)

• Chromate conversion coating, primer and topcoat
• Work function of exposed metal within scribe
  closely correlates with Ecorr of Cr3 (-0.744mV)
  using calibration curve

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SKP100 Kelvin Probe
455.6
436.4
417.3
398.1
378.9
Displacement (µm)
359.8
340.6
321.5
302.3
283.2
264.0
0
4K
8K
12K
16K
20K
24K
28K
32K
36K
40K
Displacement (µm)
Average depth of scribe 57.73?m
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Test Panel 4

• Work function for the exposed metal within the
  scribe is similar to that of previously published
  values in air
• Filaments directly below the center of the x
  exhibit more negative work functions, more active
  potentials.

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Filiform Corrosion of 2024-T3 Al Alloy
Alloy clad with non-chrome surface preparation,
primer and polyurethane topcoat.
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Individual Filament
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Comparison of Work Function Vs.Topography

• Test panel 4 scribe mark

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Rubber Sample Embedded with Steel Cords

• 19.8 x 4.6 x 0.35 cm, with two steel cords
  running the length of the sample embedded within
  the rubber at a depth of 0.4 mm
• The cords ran parallel to each other at a
  constant separation of 2.0 cm
• The conductivity of the rubber surrounding the
  embedded steel cords was reported as
• 1.5 x 105 ohms
• Measurement made at ambient temperature and
  humidity

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• Detection of the steel cord indicated by more
  negative work function, thicker rubber coating
  indicated by more positive work function

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Coated Steel Samples

• 41 mm diameter phosphated steel sample with
  organic coating (total thickness of sample 1 mm)
• 25 mm long scribe mark through coating to bare
  steel
• Measurement made at ambient temperature and
  humidity.

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• The area within the scratch (the exposed metal)
  were less active than the surrounding coating
• Evidence of delamination of the coating
  immediately adjacent to the scratch.

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Polymer Coated Galvanized Steel

• Galvanized steel sample with half of zinc coating
  removed
• Entire sample coated with a clear coat
• Measurement made at ambient temperature and
  humidity.

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• Clear difference between non-galvanized steel
  and galvanized portion
• Overcoating of polymer does not affect
  measurement of underlying surface.

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Preamplifier Circuit Board Analysis

• Base laminate, copper clad, 2 layer glass-based
  epoxy resin
• Topography measurement made to determine solder
  mask thickness over copper tracings
• Measurement made in ambient air and humidity.

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Summary and Conclusions

• The Scanning Kelvin Probe is capable of measuring
  differences in work function of coated samples in
  air at ambient temperature and humidity
• Work function measurements are able to
  distinguish between anodic areas of underlying
  metals with intact or physically compromised
  coatings
• Topographical measurements can reveal holidays in
  coatings with diameters as low as 70 microns
• Resolution of ?wf values is on the order of 1.0
  meV
• There is a good correlation between work function
  and corrosion potential for bare metals and
• Measurements in topography mode resulted in
  identification and quantification of holidays
  coatings as well as relative thickness of
  coatings.