180
IBRACON Structures and Materials Journal • 2013 • vol. 6 • nº 2
Cathodic protection for concrete structures
pits repassivate. The
E
p
and
E
prot
values depend on several factors,
including chloride content, pH at the vicinity of corroding surface,
temperature and type of cement. Among these factors, the chloride
content and the pH of the concrete are the most important, namely:
n
the higher the chloride content, the higher
E
p
and
E
prot
values;
n
the lower the pH, the lower
E
p
and
E
prot
values.
According to Bertolini et al. [6],
E
p
values of the steel in concrete
vary from -259 mV (CCSE – copper/sulfate electrode) to -459 mV
(CCSE) and
E
prot
values from -559 mV (CCSE) to -809 mV
(CCSE). In general,
E
p
values are 300 mV more positive than
E
prot
values. Figure 2, drawn by Pedeferri [5], shows the behavior of steel
for different potentials and chloride contents at 20 
o
C. Different zones
are shown in Figure 2:
zone A
where pit can initiate and propagate;
zone B
where pit does not initiate but can propagate;
zone C
where
pit does not initiate or propagate;
zone D and E
where hydrogen
evolution occurs. Based on Figure 2 and on known protection crite-
ria (EN 12696 [7]; Pedeferri [5]), it can be concluded that:
n
new sound structures or chloride contaminated corrosion-
free structures:
the concrete is alkaline and the steel rein-
forcement is in a passive state without pits. Under these con-
ditions, the cathodic protection aims at keeping the potential
below the
E
p
potential (
zone B
in Figure 2). Thus, when the
chloride contamination reaches critical levels, the steel will re-
main passive without nucleation of pits. The goal in this case is
to prevent corrosion. The typical current density for this case is
between 0.2 mA/m
2
and 2 mA/m
2
(regarding the reinforcement
area). The variation of the current density is due to several fac-
tors, such as the concrete electrical resistivity and the chloride
contamination level;
n
structures presenting chloride induced corrosion:
the
concrete is alkaline and the steel reinforcement presents pit-
ting corrosion. Under these conditions, the cathodic protection
aims at decreasing the potential to values below the
E
prot
po-
tential. Thus, the formed pits will be repassivated and there
will be no formation of new pits. In prestressed concrete, the
steel should necessarily be maintained in
zone C
of Figure
2. In reinforced concrete, the potential can be maintained in
zones C or D
. When only the decrease in corrosion rate is de-
sired, the potential value can be kept at the lower part of
zone
B
. The typical protection current density for this case remains
between 2 mA/m
2
and 20 mA/m
2
(regarding the reinforcement
area). This variation is due to several factors, such as those
above mentioned and the degree of corrosion already present
on the steel reinforcement surface;
n
structures presenting carbonation induced corrosion:
the
pH of the concrete assumes lower values and the reinforce-
ment presents generalized corrosion. Under these conditions,
the cathodic protection is normally applied by decreasing the
potential to values between the reinforcement corrosion po-
tential and the iron equilibrium potential. As a result, a de-
crease in the corrosion rate is obtained.
In practice, for the application of the cathodic protection technique,
the knowledge of
E
p
and
E
prot
values is not necessary. Empirical
criteria are widely accepted, including the 100-mV depolarization
criterion (NACE RP0209 [2]).
In corroded atmospheric structures in which the reinforcement po-
tential value is more negative than -200 mV (CCSE), the adopted
protection current must be such that, at the most anodic location
in each 50-m
2
area, a minimum of 100 mV of “real” cathodic po-
From Figure 1, it is observed that, at high potential values, the steel/con-
crete interface potential remains in the passive domain at pH about 13
which is the usual pH of a sound concrete. The passive state is due to
the presence, on the steel surface, of a film composed of Fe
3
O
4
or Fe
2
O
3
oxides. Steel becomes immune to corrosion when the potential of steel/
concrete interface is brought to values ​below the equilibriumpotential val-
ue of the reaction Fe
3
O
4
+ 8H
+
+ 8e
-
ž
3Fe + 4 H
2
O (point 1 of Figure 1).
The immunity domain starts at the potential around -900 mV (EH, hydro-
gen electrode). At this potential, the reduction of hydrogen (2H
+
+ 2e
-
ž
H
2
; Point 2 in Figure 1) and the reduction of oxygen (O
2
+ 2H
2
O + 4e
-
ž
4OH
-
; point 3 in Figure 1) also take place. The reduction of oxygen also
occurs at a potential in the passivation domain, i.e., when steel is embed-
ded in a sound concrete (the range named
E
corr
shown in Figure 1).
When steel/concrete interface potential is maintained in the immu-
nity domain, both above mentioned reduction reactions determine
a progressive increase of the concrete alkalinity which may result
in concrete degradation if it contains reactive aggregates (alkali/ag-
gregate reactions). In addition, the increased volume of hydrogen
due to the 2H
+
+ 2e
-
ž
H
2
reaction may hinder the steel/concrete
adherence. In the case of prestressed concrete, this condition can
cause hydrogen induced fracture of high strength steels due to the
penetration of atomic hydrogen in the metal. Due to these possible
consequences, the traditional application of very negative poten-
tials in cathodic protection systems is not applied in atmospheric
concrete structures, especially in prestressed concrete.
The principle of cathodic protection applied in concrete structures
can be clarified by understanding the mechanism of occurrence of
pitting corrosion of steel reinforcement due to chloride ions. In this
case, the corrosion control through cathodic protection is done by
the pit repassivation principle or by the delay of pit nucleation.
Pitting corrosion occurs at potential values higher than a character-
istic potential known as pit potential (
E
p
). Once pit is initiated, it can
propagate at a potential more negative than
E
p
. To stop pit propa-
gation, it is necessary to reach a lower potential, known as repas-
sivation potential or protection potential (
E
pro
t
) below which active
Figure 2 – Schematic illustration of steel behaviour
in concrete for different potentials and chloride
contents at 20°C (Pedeferri [5])
1,2,3,4,5,6,7 9,10,11,12,13,14,15,16,17,18,...190