Page 141 - Corrosion Engineering Principles and Practice

116 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 117

The inhibitor efficiency of each solution can subsequently be
calculated using Eq. (5.24).
(CR − CR )
e
Inhibitor efficiency (%) = 100 uninhibited inhibited (5.24)
CR
uninhibited
where CR uninhibited is the corrosion rate of the uninhibited system
CR is the corrosion rate of the inhibited system
inhibited
The results obtained with LPR on these inhibiting solutions
are presented in Table 5.7 along with corrosion rates converted in
mm/year with the help of Table 3.2 in Chap. 3.
Electrochemical Impedance Spectroscopy
Electrochemical impedance spectroscopy (EIS) has been successfully
applied to the study of corrosion systems and proven to be a powerful
and accurate method for measuring corrosion rates for a few decades.
In this approach for determining the polarization resistance of a
metal, a measure of the electrochemical impedance is made at a series
of predetermined frequencies.
An important advantage of EIS over other electrochemical
techniques is the possibility of using very small amplitude signals
without significantly disturbing the properties being measured.
However, in order to estimate the polarization resistance (R ), that is
p
proportional to the corrosion rate at the monitored interface according
to Eq. (5.22), EIS results have to be interpreted with the help of a
model of the interface.
Amongst the numerous equivalent circuits that have been
proposed to model electrochemical interfaces only a few really apply
in the context of a freely corroding system. The first circuit is the
simplest equivalent circuit that can describe a metal/electrolyte
interface (Fig. 5.25). Its behavior is described by Eq. (5.25).

R
w
Z( ) = R + 1+ ( j R C )w p p dl b (5.25)
s

TCA R Corrosion Current Corrosion Rate Efficiency
p
–1
–2
(ppm) (W cm ) (mA cm ) (mm y ) (%)
2
0 14 1.55 18 0
250 25 0.87 10.1 44
1000 140 0.155 1.80 90
2000 1400 0.0155 0.18 99

TABLE 5.7 Inhibitor Efficiency of Trans-Cinnamaldehyde (TCA) to the Corrosion of
Carbon Steel Exposed to a 6 M HCl Solution