Page 151 - Corrosion Engineering Principles and Practice

126 C h a p t e r 5 C o r r o s i o n K i n e t i c s a n d A p p l i c a t i o n s o f E l e c t r o c h e m i s t r y 127

1.E + 10

1.E + 09
Exposure Time
1.E + 08

Impedance (q) 1.E + 07

1.E + 06

1.E + 05

1.E + 04

1.E + 03
1.E – 01 1.E + 00 1.E + 01 1.E + 02 1.E + 03 1.E + 04 1.E + 05
Frequency (Hz)
FIGURE 5.30 Magnitude of impedance of a coating versus frequency. Low
frequency impedance values show good correlation with long-term exposure
behavior. (Courtesy of Guy D. Davis, DACCO SCI, Inc.)
electrochemical noise (EN) has been found to be uniquely appropriate
for monitoring the onset of events leading to localized corrosion
and understanding the chronology of the initial events typical of
this type of corrosion.
The EN technique differs in many ways from other electrochemical
techniques used in corrosion. One important difference is that ENA
does not require that the sensing element be polarized in order to
generate a signal. However, it is also possible to measure current
noise under an applied potential, or measure potential noise under
an applied current. The potential and current between freely corroding
electrodes (in many cases < 1 µV and < 1 nA) are measured with
sensitive instrumentation. A measurement frequency of 1 Hz is
usually appropriate to provide meaningful data. For simultaneous
measurement of electrochemical potential and current noise, a three-
electrode sensor is required. In field corrosion monitoring, the three-
sensor elements are usually made of the same material.
Whilst the measurement of electrochemical noise is relatively
straightforward, the data analysis can be complex and inconclusive.
Even if ENA was first applied in field corrosion monitoring in the late
1960s, an understanding of the method of analysis is still evolving,
partly because the technique has been used to look at several types of
corrosion. The relationships between potential and current noise are
inherently complex to analyze quantitatively because the naturally