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183
IBRACON Structures and Materials Journal • 2013 • vol. 6 • nº 2
A. ARAUJO | Z. PANOSSIAN
|
Z. LOURENÇO
of inert anodes such as titanium, platinum and nickel, the only pos-
sible anodic reaction is water oxidation (2H
2
O
4H
+
+ O
2
+ 4e
-
).
If the anode material oxidizes, which is the case of soluble
anodes such as zinc, the main reaction is anode oxidation
(Zn + 2H
2
O
Zn(OH)
2
+ 2H
+
+ 2e
-
). At the cathode surface, the
reduction of oxygen occurs (O
2
+ H
2
O + 4e
-
4OH
-
). When the
steel/concrete potential is sufficiently polarized up to the iron im-
munity domain, the reduction of iron oxides (Fe
3
O
4
+ 8H
+
+ 8e
-
3Fe + 4H
2
O) and the reduction of hydrogen (2H
+
+ 2e
-
H
2
) also
take place.
As can be seen in Figure 6, the cathodic protection current ap-
plied on the steel reinforcement may cause other chemical modi-
fication in the concrete. Negative ions (hydroxides, chlorides,
carbonates and sulfates) are repelled by the cathode and the
positive ions (calcium, sodium, potassium, zinc and hydrogen)
are attracted to the cathode. Thus, the cathodic protection effect
is not only the corrosion mitigation, but also the restoration of the
passive film on the steel surface due to the decrease of the chlo-
ride concentration in the vicinity of the protected reinforcement
(Lourenço and Costa [12]).
4. Sacrificial anode cathodic protection
(galvanic protection)
In the sacrificial anode galvanic protection method, the cathodic
electric current is provided by means of the natural potential dif-
ference between the two distinct metals, the one of them being
the steel prestressed (cathode) and the other a less noble metal
(anode). The mostly used metal and alloys in this cathodic protec-
tion method are zinc, aluminum or magnesium and their alloys (13
ACI 222.3R [13]).
Beyond an adequate potential difference, this method also de-
mands a conductive environment in order to guarantee the electric
current flowing between the steel reinforcement and the anode. In
concrete structures, such conditions are normally achieved when
concrete is exposed to a continuous humid atmosphere or in im-
mersion condition. When these conditions are not achieved, the
concrete resistivity will change and may assume very high values
causing a deep decrease of the electric current which can compro-
mise the protection of the prestressed. It is worth emphasizing that
this method is not applied for structures subjected to stray current.
In Brazil, the most known galvanic protection system is the use of
pure zinc tablets embedded with a conductive and alkaline mortar.
They are galvanically connected to the exposed reinforcement be-
fore applying the repair mortar. According to NACE 01105 [14], this
system has the objective to provide greater efficiency to the patch
repair since it can delay the corrosion onset and also can avoid the
appearance of incipient anodes adjacent to the repaired site.
Abroad, beside the tablets, jacketing systems are also used. This
system is composed of a zinc mesh fixed to a fiberglass plate.
The jacket system can be successfully used for the rehabilitation
of on-shore piles. This is done by using two-piece stay-in-place fi-
berglass forms lined with a zinc mesh anode electrically connected
to the steel reinforcement. Then, the jacket is filled with a suitable
cement mortar [15].
Another system, both for new and damaged structures, is the met-
allization of the concrete surface which is done by spraying a thin
layer of pure zinc or aluminum/indium alloy on the concrete sur-
face. According to NACE publication 01105 [14], in some cases, it
is also necessary to use a moisture retention promoter (hydrophilic
product) under the film. In marine structures, in which periodic con-
crete humidification occurs, the system can maintain a current of
10.8 mA/m
2
which satisfies the 100-mV depolarization criterion for
many years.
Figure 7 – Tapes installation (a). Concreting (b)
Electric power supply (c).
General view after repairing (d)
A
B
D
C
