Page 156 - Corrosion Engineering Principles and Practice

130 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 131

0 0.09
0.08
–5
0.07
–10 0.06
Potential (mV) –15 0.05 Current (uA)

0.04
–20 0.03
Decrease in Current 0.02
–25 Band Width Indicates
Decrease in General 0.01
Corrosion Activity.
–30 0
01:24:57
02:05:19
01:53:12
01:37:03
00:40:20
00:52:26
01:04:32
01:16:38
00:16:08
00:28:14
02:13:23
00:04:02
00:00:00 00:08:04 00:12:06 00:20:10 00:24:12 00:32:16 00:36:18 00:44:22 00:48:24 00:56:28 01:00:30 01:08:34 01:12:36 01:00:00 00:20:40 01:28:59 01:33:01 01:41:05 01:49:10 01:57:15 02:01:17 02:09:21
Time
FIGURE 5.37 Electrochemical current noise (large band) and potential noise
(lower signal) in a debutanizer overhead piping obtained with the Concerto VT noise
system. (Courtesy of CAPCIS Ltd.)
The analysis of EN results obtained between two working
electrodes (WEs) with an imposed or naturally developing asymmetry
can be carried out by considering Eq. (5.27), which indicates that the
noise impedance of a cell (Z ) depends on the impedances of the two
n
WEs, as well as their noise levels, represented by the power density
spectra (Y i and Y i ) obtained by performing the analysis of noise
2
1
signals with either fast Fourier transform (FFT) or with the maximum
entropy method (MEM).
+
Y f ( ) Y f ( ) Y f ( )
Z f ) = Y V I f ( ) = | ( ) 2 | | ( )| Y i 1 i 1 ( ) | ( )| Y i 2 ( ) (5.27)
(
Z f Z f ( )|
i 2
+
n
1
f
2
2
Z
f
f
Z
f
1
2
For the simplest case of two WEs with the same impedance (Z = Z )
2
1
the noise impedance is equal to the modulus of the electrode impedance
|Z( f )|. This result is valid independently of the origin of the noise
signals (localized or uniform corrosion, bubble evolution due to
the cathodic reaction) and the shape of the impedance plot, even if the
noise levels of the two electrodes are different. In such case, noise
measurements are equivalent to impedance measurements for which
the external signal perturbation has been replaced by the internal noise
generated by the corrosion processes [24].
However, when the two WEs do not have the same impedance,
the noise impedance analysis requires a more cautious interpretation.
Depending on the source of the current noise the measured impedance
may either be that of a quiet cathode or that of a quiet anode while in