© 2006 by Taylor & Francis Group, LLC

142 Corrosion Control Through Organic Coatings
that for a coating to provide corrosion protection to steel, it should have an initial
impedance of at least 10 8 /cm 2 , a value also suggested by others [15], and that
after aging, the impedance should have decreased by no more than three orders
of magnitude. Sekine has reported blistering when the coating resistance falls
below 10 6 /cm 2 , regardless of coating thickness [17-19].
For more in-depth reviews of the fundamental concepts and models used in EIS
to predict coating performance, the reader is directed to the research of Kendig and
Scully [20] and Walter [21-23].
8.2.4 SCANNING KELVIN PROBE
The scanning Kelvin probe (SKP) provides a measure of the Volta potential (work
function) that is related to the corrosion potential of the metal, without touching the
corroding surface [24]. The technique can give a corrosion potential distribution,
with a spatial resolution of 50 to 100 µm, below highly isolating polymer films. The
SKP is an excellent research tool to study the initiation of corrosion at the metal/polymer
interface.
Figures 8.4 and 8.5 show the Volta potential distribution for a coil-coated sample
before and after 5 weeks of weakly accelerated field testing [25]. In the “after”
SCI
−293 mV
−510 mV
100 mV
−293
−365
−438
−510
(mV)
28.00 mm i 5 p/mm
R612
10.00 mm
2 p/mm
FIGURE 8.4 Volta distribution (mV) of coated steel before exposure.
Source: Forsgren, A. and Thierry, D., SCI Rapport 2001:4E. Swedish Corrosion Institute
(SCI), Stockholm, 2001. Photo courtesy of SCI.
© 2006 by Taylor & Francis Group, LLC