Title: The Scanning Kelvin Probe Technique: Applications for Corrosion, Coatings and Materials Characteriza
1The Scanning Kelvin Probe TechniqueApplications
for Corrosion, Coatings and Materials
Characterization
Princeton Applied Research
2Scanning 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.
3Scanning 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.
4Theory of the Kelvin Probe
?sample
? Work Function
?probe
E Fermi Level
Vc
Metal Probe
EProbe
EProbe
ESample
Metal Sample
5? Work Function
E Fermi Level
Metal Probe
EProbe
ESample
- - -
Metal Sample
6Vb
Metal Probe
EProbe
ESample
Vb -Vc
Metal Sample
7Vb
Iac 0 when Vb -Vc
Iac
Lock-in Amplifier
30 ?m
Iac
EProbe
ESample
Metal Sample
8- 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.
9- 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
10- 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
11- 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
12- When Vptp 0, the work function or contact
potential of the surface is equal and opposite to
Vb.
13Measurement 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
14Correlation 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)
15Material 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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17- Work function measurement made in air at
- ambient temperature and humidity
18- Comparison of work function measurement
- and topographical measurement
19Treatment of AA2024-T3 Panels
20Test 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. -
21Analysis of Test Panel 1
- ?wf values of -700 to -583 meV for areas of
anodic potential
22Correlation of ? to Ecorr
- Work function measurements made in air
immediately after corrosion potential measured
while immersed in 0.598M NaCl (3.5). -
23Test 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 -
24SKP100 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
25Test 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.
26Filiform Corrosion of 2024-T3 Al Alloy
Alloy clad with non-chrome surface preparation,
primer and polyurethane topcoat.
27Individual Filament
28Comparison of Work Function Vs.Topography
29Rubber 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
30- Detection of the steel cord indicated by more
negative work function, thicker rubber coating
indicated by more positive work function -
31Coated 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.
32- 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. -
33Polymer 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.
34- Clear difference between non-galvanized steel
and galvanized portion - Overcoating of polymer does not affect
measurement of underlying surface.
35Preamplifier 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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39Summary 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.