Advantages of SKP
- Can measure the electrochemical potential of surfaces covered by extremely thin and resistive electrolyte layer
- Purely potentiometric, electrically least perturbing
- Probe tips very small, characterization of localised electrochemical phenomena
- Study local electrochemical processes under poorly conducting films
Disadvantages of SKP
- No improvement in lateral resolution, coupling between different surface sites result in kelvin probe detecting only mixed potential
- Only electrode potential and not current, no direct measure of rate of reactions
- Data not always straightforward, additional potential differences may interfere
Potential definition
- Work done in bringing a unit of positive charge to that point from infinity
- Always relative (differences)
Galvani potential difference
- ΔΦ
- Potential difference between two points in the bulk of two phases
- Measurable only when the two phases have identical chemical composition
Volta potential difference
- Δψ
- Contact potential between two metals is the potential difference between two points:
- point 1 close to surface of metal 1
- point 2 close to surface of metal 2
- Where M1 and M2 and uncharged and in contact
Surface potential
- χ
- Change in electric potential when moving from inside to just outside the surface of a bulk phase
The fermi level
- electrochemical potential of an electron inside a solid conducting or semi conducting bulk phase
- Determined by both the chemical (bonding) and electrical environment
The work function
- W
- Minimum energy to remove an electron from inside solid to a point immediately outside the solid (fermi to vacuum)
What causes contact potential?
- when metals with dissimilar work functions are brought into contact, a volta potential difference or contact potential is produced
- Initially there is no electrical contact between two metals
- Fermi levels are unconnected and free to float relative to one another
- When electrical contact is made, fermi levels equalise
- Inequality of Wa and Wb now produces a volta potential difference and corresponding energy difference for an electron
Principle components of SKP instrument
- The tip of the reference probe and the surface of the metal under study act as two plates of a parallel plate capacitor
- C = ε x ε0 x A / d
- The bias (U) is adjusted such that the current becomes zero (null current)
- It the quantity -U that is measured and reported as Δψ
Lateral resolution
- Best estimated by scanning across a sharp step in actual volta potential, so that any lateral broadening in the measured volta potential distribution is due to limitations in probe resolution
- Step broadening increases with probe height (d)
- SKP lateral response width ΔL50 from where the ΔE changes from 0.25 to 0.75 (normalised axis)
- ΔL50 = 0.884d + 0.4D (D=probe diameter)
- The slope arises from the electric field distribution spreading as d increases
SKP in cathodic disbondment experiments
- Draw the apparatus and the graphs example
- Scans are conducted immediately after addition of electrolyte and at hourly intervals after that for 24 hours
- Ecorr vs distance profiles are produced
Relationship between Volta potential and Electrode potential
-Equations
Regions of Ecorr in CD
4 characterised regions (draw diagram)
1) Region immediately next to defect where Ecorr is same as freely corroding metal
2) A region of approx slight linear Ecorr gradient due to ohmic potential drop from migrating ions
3) Sharp transition to high Ecorr, at the delamination front
4) A region of as yet undelaminated polymer (high Ecorr)
How contact potential gives rise to the storage of capacitance between two metals
If two metals with a contact potential (volta potential difference) when connected, are in close close proximity and separated by a dielectric medium, they will act as plates of a capacitor storing electrical charge Q = C x Δψ
- If a bias is added to the circuit a unique value of U may be found U = - Δψ
- The volta potential of the two metals will become the same and the charge stored between the plates will be zero
Two important variable limiting SKP lateral resolution
- Probe diameter
- probe sample distance
