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181 IBRACON Structures and Materials Journal • 2013 • vol. 6 • nº 2
A. ARAUJO | Z. PANOSSIAN | Z. LOURENÇO
crease up to the value named instant off potential (CP off in Figure
3) also known as “real” IR-free potential because the sudden in-
crease is a consequence of the potential decay due to soil resistiv-
ity. Then, an asymptotic increase of the steel potential is observed
reaching a steady state value named Enatural (Figure 3).
The absolute value of the difference between the natural potential
and the instant off potential (Enatural – ECP off) is the “real” depolariza-
tion value of the steel/soil interface which is the value that should
be equal or above 100 mV to achieve the above mentioned 100-
mV depolarization criterion. If in a given cathodic protection sys-
tem, the “real” depolarization value is lower than 100 mV, higher
cathodic protection current should be applied.
According to Bennet and Turk [9] and Pedeferri [5], a period of 4 h is
required for the Enatural stabilization. However, in some cases, this time
may be insufficient to achieve the stabilization. In these cases, larger
periods may be adopted which may reach 24 h (Pedeferri [5]). It is
worth mentioning that the potential stabilization depends on the oxygen
availability and not of the cathodic protection efficiency. In water saturat-
ed structures, where the availability of oxygen is lower, the stabilization
time is greater (Glass, Hassanein, Buenfeld [10]; NACE RP0209 [2]).
The cathodic protection effectiveness is ensured by continuously mea-
suring the steel potential in concrete with respect to reference electrodes.
This may be done by burying sensors (permanent electrodes) in the
concrete in most critical areas aiming at representing different corrosion
conditions of the entire structure to be protected or in places where the
potential control is of the most importance. Mostly used reference elec-
trodes are: Ag/AgCl/KCl-0.5 mol/L; Mn/MnO2/NaOH-0.5 mol/L and ac-
tivated titanium with or without mortar backfill. For monitoring buried or
submerged concrete structures, external (immersed or buried) zinc elec-
trodes or copper/copper sulfate electrodes may be used (Lourenço [11]).
3. Impressed current cathodic protection
In the impressed current cathodic protection method, the electric
current is provided by applying an electrical potential using an ex-
ternal electric power source. Typically, an alternating current rec-
tifier is used with its positive pole connected to the anodes and
larization should be achieved. In a sound concrete or in a slightly
chloride-contaminated concrete in which the reinforcement poten-
tial value is more positive than -200 mV (CCSE), which indicates
that the steel is passivated, there is no need for adoption of this
minimum threshold of cathodic polarization (NACE RP0209 [2]).
Figure 3, proposed by Raupach and Bruns [8], illustrates the ap-
plication of the “real” 100-mV polarization criterion. In Figure 3, it is
observed that, when the reinforcement is not protected, it assumes
its natural corrosion potential (Ecorr in Figure 3). When cathodic
protection is applied, the steel potential is shifted into cathodic di-
rection, assuming the value CP on of Figure 3. After switching off
the protection current or, in the case of sacrificial protection, after
disconnecting the anode, the steel potential presents a sudden in-
Figure 3 – Typical potential decay measurement
after disconnecting the cathodic protection
system (Raupach and Bruns [8])
A B
Figure 4 – Activated titanium mesh application. Mesh fixation on the concrete surface (a).
Cover mortar applied by projection (b)