Corrosion Monitoring Using Scanning Kelvin Probe Analysis: A NonDestructive Evaluation of Coated Alu

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Corrosion Monitoring Using Scanning Kelvin Probe
AnalysisA Non-Destructive Evaluation of Coated
Aluminum Alloys in Air.

• Douglas C. Hansen and Gary E. Grecsek
• Princeton Applied Research
• Oak Ridge, TN 37831

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Introduction

• History of the Kelvin Probe
• Definition of the work function
• Theory of the Kelvin Probe
• Measurement technique
• Work function and Ecorr

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A Short History of The Kelvin Probe

• First described by Lord Kelvin in 1897 for the
  measurement of Volta potentials, contact
  potentials and /or surface potentials (for
  non-metals)
• Modified by Zisman in 1932 as a vibrating
  capacitor
• Improved and developed over the years by many
  others

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Work function ?

• Definition
• - the minimum work required to extract an
  electron in vacuum from the Fermi level ?F of a
  conducting phase through a surface.

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Relationship of ? to Surface Characterization

• Since the electron has to move through the
  surface region, its energy is influenced by the
  optical, electrical and mechanical
  characteristics of the region
• Therefore, work function (wf) is extremely
  sensitive to surface conditioning and is affected
  by adsorbed or evaporated layers, surface
  charging, oxide layer imperfections and
  surface/bulk contamination.

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Theory of the Kelvin Probe

• The diagram to the right illustrates two
• metals having constant separation, work
• functions and Fermi levels of E1, ?1 and
  E2,
• ?2, respectively.
• At first, the metals have no electrical
• contact and differing Fermi levels

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• When electrical contact is made, the flow of
  charge allows the
• Fermi levels to equalize, giving rise to a
  surface charge and
• potential difference, Vc (or contact
  potential).

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• The application of a backing potential, Vb
  allows the nulling of the
• surface charge where Vb -Vc which is the
  work function difference
• between the two materials

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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
• Less negative values for ?wf correlate to more
  active potentials for metals, and lower contact
  potentials for non-metals

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E2 Hawkeye Access Panel
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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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Al2024-T3 Test Panels

• Samples were prepared by applying approximately
  0.8 mils of MIL-P-23377 Type 1, Class C primer
  followed by 1.2 mils of MIL-C-85282 topcoat
• Clad with either a conversion coating or a
  non-chrome surface preparation.
• Non-exposed 2024-T3 clad with conversion
  coating, primer and polyurethane topcoat

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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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• Clad with a non-chrome surface preparation,
  primer and polyurethane topcoat.
• A scribe (X) was made in the coupon and was
  subjected to 12 N HCl for a period of one hour
  followed by 1000-hour humidity exposure to
  promote the growth of filiform corrosion.
• Work function analysis indicates a less negative
  value (-768 to -659 meV) within the scribe mark,
  which equates to a more active corrosion
  potential, as compared to the surrounding area.

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• Upon removal from the humidity chamber, the
  sample exhibited varying degrees of filiform
  corrosion emanating from the scribe mark.
• Filaments on the scribed test panel yielded ?wf
  values ranging from 0.795 to 0.830 meV
• These values correspond to more negative
  corrosion potentials, indicating more active
  sites than the surrounding area.

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Work Function Analysis of Test Panels 1 and 2
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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