© 2006 by Taylor & Francis Group, LLC
© 2006 by Taylor & Francis Group, LLC
© 2006 by Taylor & Francis Group, LLC
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Corrosion Testing — Practice 143<br />
SCI<br />
−333 mV<br />
−878 mV<br />
200 mV<br />
−333<br />
−515<br />
−696<br />
−878<br />
(mV)<br />
22.00 mm i 5 p/mm<br />
FIGURE 8.5 Volta distribution (mV) of coated steel before (top) and after (bottom) 5 weeks’<br />
weakly accelerated field exposure.<br />
Source: Forsgren, A. and Thierry, D., SCI Rapport 2001:4E. Swedish Corrosion Institute<br />
(SCI), Stockholm, 2001. Photo courtesy of SCI.<br />
figure, a large zone at low potential (–850 to –750 mV/NHE) can be clearly seen.<br />
Delamination, corrosion, or both is occurring at the transition area between the<br />
“intact” metal/polymer interface (zones at higher potential values, –350 to –200<br />
mV/NHE) and more negative electrode potentials. The corrosion that is starting here<br />
after 5 weeks will not be visible as blisters for nearly 2 years at the Bohus-Malmön<br />
coastal station in Sweden [25].<br />
8.2.5 SCANNING VIBRATING ELECTRODE TECHNIQUE<br />
R61C<br />
8.00 mm<br />
2 p/mm<br />
The scanning vibrating electrode technique (SVET) is used to quantify and map<br />
localized corrosion. The instrument moves a vibrating probe just above (100 µm or<br />
less) the sample surface, measuring and mapping the electric fields that are generated<br />
in the adjacent electrolyte as a result of localized electrochemical or corrosion<br />
activity. It is a well-established tool in researching localized events, such as pitting<br />
corrosion, intergranular corrosion, and coating defects. The SVET, which gives a<br />
two-dimensional distribution of current, is similar in many respects to the SKP; in<br />
fact, some instrument manufacturers offer a combined SVET/SKP system.<br />
<strong>©</strong> <strong>2006</strong> <strong>by</strong> <strong>Taylor</strong> & <strong>Francis</strong> <strong>Group</strong>, <strong>LLC</strong>