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S canning R eference E lectrode T echnique :   
         Examples of of colour maps obtained.
Reference on Scanning Reference Electrode Technique by CMC staff:

P.S. Sidky: Proceedings of Corrosion & Environment Conference (1998), Bath, UK, pages 185-201.
These scans are of the field
(i.e. potential gradient dV/dz, proportional to current ) over the same specimen whose potential scan is shown above.
Note the amount of detail; scan is rotated to reveal detail.

The field ranges from -0.03 to 0.129 mV/micron
on Z-axis (red).

Same field scan,
rotated slightly

These scans show the vertical field but the
horizontal fields
dV/dx and dV/dy
are also plottable from
the scan data collected.

So horizontal currents can also be obtained.
Potential scan across 1 mm  scratch on
painted steel
in 0.035% NaCl,
after 26 hours

1 mV plot resolution.
Potential scan across 1 mm wide
scratch on coaltar
epoxy painted
steel in 3% NaCl.

0.7 mV span

1 microvolt plot
Same specimen
3 hours later

2 mV span on

1 microvolt
plot resolution.
Same specimen
after 4 hours

2 mV span on

1 microvolt
plot resolution.
Potential scan
over pit in an
stainless steel

60 mV span on

1 mV plot resolution
Potential scan
over pits in
another unpainted
stainless steel

6 mV span on

20 microvolt
plot resolution

A "Point-In-Space" calibration graph is required to obtain current values.
A linear scan is made over a 70 micron dia. vertically-mounted wire with 4 microamps passing from it to a large Cu anode.
Maximum potential with probe directly over wire tip is 3 mV and peak width at half-maximum is 0.6 mm.
A plot of potential vs current is linear over this point.
ADVANTAGES over SVET scanners:-
Gives vertical and horizontal currents (SVET only gives vertical current component).
Also gives potentials (SVET only gives currents).

Absence of vibratory stirring allows sharp resolution of defects (SVET has a vibrating electrode).

Unique feature: Automatic probe contouring allows scanning of non-flat surfaces such as weld beads and
paint blisters (SVET requires a flat surface and its probe is damaged if it contacts a surface).

No need to fix specimens accurately horizontal , so results are independent of operator skill.

Your own probe can be used, allowing experimentation with extra-narrow probe tips for the experimenter.
potential scan
over a pitted
stainless steel

Potential vs  distance (X & Y) map.

The paint defect, seen as a peak, is shown in an SEM photograph below.

The paint defect shown as a peak in the colour scan above, is shown here in this SEM photograph.





This scan is of the electrochemical
potential over a scratched painted
steel surface in 3% NaCl, with the
1 mm scratch parallel to the y-axis
and located at x = 1.3 mm.

The scan shows that the scratch
region nearest to y = 0 is more anodic
than the rest.

The corresponding field scan,
proportional to the current from the
scratch is shown next below this one,
rotated to different angles. All scans
can be rotated and tipped, in steps.

Plot resolution is 1 mV

NOTE: This picture is
much sharper (higher
resolution) than shown
here in this small copy.