SRET and SVET Flashcards
Advantages of scanning techniques
-spatially resolve and quantify
conventional measure values associated with total area
and do not yield info on size shape and spatial distribution
-Surface quantities estimated without need to perturb the corroding surface, allows repeating and time resolved scans
Disadvantages of scanning techniques
-Since no direct contact with surface, frequently a non-trivial problem to calculate surface quantities with any degree of accuracy
Principles of SRET
- Two reference electrode, one is a microtip electrode and the other is a reference electrode (SCE)
- Microtip moving at constant height above surface
- Potential of microtip measures relative to the SCE
- In practise a point source will produce a bell shaped distribution of E, where maximum value directly above (X, Y = O)
- WHM of potential distribution peak is whm=3.46 x z
- Implies theoretical spatial resolution, difficult to resolve two point current sources if they are not separated by a distance greater than the value of the whm
Calibration of SRET
- Using a galvanostat to pass a known current through an electrolyte of known conductivity between the point electrode and a larger area remote electrode
- SRET potential as a function of different applied currents, slop of E vs i provides a calibration factor
Advantages of SVET over SRET
- Directly measure components of corrosion current density
- Improved sensitivity
- Improved spatial resolution
Principles of SVET
-Scanning microtip vibrated at constant amplitude
-Vibrating SVET measures the normal electrical field strength which is given by differentiating the electric potential with respect to the z component
whm = 1.552 x z
SVET apparatus
- Microtip
- Vibrator driver
- Vibrator drive amplifier
- Lock in amplifier
- triaxial motor driver
- Microcomputer
Experimental comparison of SRET to SVET
-Improvements can be shown using a point current source
-linear relationship between point source current and SVET signal over three orders of magnitude
-Detection limit for SRET is 100nA
-Detection limit of SVET is 1 nA (x100 more sensitive)
Signal data over surface visually shows SVET superior resolution
3 ways for calibrating the SVET
- Calculation
- The use of a point current source
- Use of a tube cell
Calibration (Calculation)
-Providing the electrolyte conductivity (k) and the peak to peak amplitude (app) of the vibration is known, by Ohms law, the peak to peak voltage signal (Vpp) is related to the current flux density along the axis of probe vibration (jz):
Vpp = jz (app/k)
In practise can be difficult to measure app with accuarcy
Calibration (Use of a point current source)
- Same concept as SRET, however calibration is made with respect to current density rather than current
- Current density is given by the applied current divided by 2πz^2 (where z is the height of the probe)
- Plot of Vpp vs i/2πz^2 provides a means of converting the SVET voltage signal to a value of current flux density along the axis of vibration
- Advantage of this technique is that you can confirm if the SVET is working properly
- Disadvantage is that control of the probe height is critical (needs to be exactly the same for calibration and experiments)
Calibration (Use of a tube cell)
- nanogalvanostat used to pass a known current between 2 Pt gauze electrodes
- One compartment consists of a nylon beaker, the other a large volume electrolyte filled perspex tank both containing an electrode
- compartments linked together by a vertical orientated electrolyte filled glass tube
- During calibration the SVET is inserted into the tube opening with a Ag reference electrolyte immersed in large tank electrolyte
- Current flux constant, cell current divided by internal area of cross section (minus the area of SVET)
- Calibration plot of Vpp vs applied current density
- Slope used for conversion factor for SVET signal to current density in the plane of the scan
Advantage and disadvantages of tube cell calibration
A-quick simple, no control of height required
D-Gives no info about the SVET probe response characteristics
SVET limitations
- Only able to resolve localised corrosion features which are separated by distances greater than the scan height (h)
- Anode-cathode distance > h, flux lines cross the plane of scan and detected by SVET
- when considerable less than h, the flux lines do not cross the plane of the scan and are undetected
- Not good for general corrosion
