Page 7 - Riem-Vol6_nº2

1. Introduction
Traditionally, cathodic protection is used as a method of combating
corrosion of submerged or buried metal structures, being widely used
for protection for ships’ external bodies and for buried pipes. In recent
decades, this protection technique has been used for the rehabilita-
tion of deteriorated concrete structures (Chess [1]; NACE RP0209 [2])
and was adopted for the first time in 1973 for a bridge in California
(United States) which presented severe corrosion of the reinforcement
(Beamish and Belbol [3]). Besides the use of degraded structures, the
cathodic protection technique has been used in new structures as a
corrosion prevention technique. Such usages may occur in partially
or completely buried or submerged structures or in atmospheric struc-
tures, both for reinforced concrete or prestressed concrete.
According to Broomfield [4], until 1994, there were more than one mil-
lion square meters of cathodic protection applied in concrete structures
in the United States and Canada and, probably, another million or more
in the rest of the world. According to Pedeferri [5], until 1996, the ca-
thodic protection technique had already been applied in about 500,000
m2 of concrete structures presenting chloride-induced corrosion and
140,000 m2 of new structures, the latter mostly prestressed concrete
structures.
The general concept of cathodic protection in concrete structures
is defined according to the steel state (passive state or active state)
and according to the corrosion induced agent, namely:
n repassivation of nucleated pits: such as concrete structures
presenting chloride-induced corrosion;
n reduction of generalized corrosion rates: such as structures
presenting carbonation-induced generalized corrosion or chlo-
ride-induced advanced corrosion in which the pits have been
coalesced giving rise to generalized corrosion;
n cathodic prevention: such as new structures (passive state)
prone to chloride contamination or chloride-contaminated
structures at levels below the critical threshold.
Despite the application of cathodic protection being suited to struc-
tures exposed to different conditions and states of preservation, it is
more widely used in atmospheric structures subjected to chloride con-
tamination, such as those located in marine environments, industrial
environments with chloride sources and, in some countries, in envi-
ronments that use de-icing salts. The chloride contamination associ-
ated with high humidity determines the decrease of electrical resistiv-
ity of concrete. Thus, smaller electrical current densities to achieve
the desired level of protection are required which is not the case for
concrete structures without a chloride contamination. Carbonated-
concrete structures require higher current densities because the car-
bonation increases the concrete resistivity.
The application of cathodic protection in concrete structures can be
done by means of two methods: impressed current and sacrificial an-
odes. Normally, the impressed current cathodic protection is used in
most atmospheric structures. However, in high humidity environment,
such as splash or tidal zones, sacrificial anode cathodic protection is
also used successfully. The sacrificial anode technique is used for
submerged or buried structures as well.
Both the impressed current and the sacrificial anodes are dependent
mainly on the characteristics of the structure to be protected as well
as the characteristics of the environment to which the structure will be
exposed. In addition to these factors, the ease of anode installation,
the cost, the aesthetic, the required lifetime and the maintenance are
factors to be considered for an effective cathodic protection design.
In Brazil, the application of cathodic protection technique in concrete
structures exposed to high aggressive environments is unusual. Typi-
cally, stringent project criteria are adopted. These include a high cov-
ering thickness and a high quality concrete. In the case of deteriorated
structures, traditional techniques for recovery and concrete surface
treatments are usually adopted. In practice, these criteria are not al-
ways a guarantee for concrete structure durability. Additionally, the
effectiveness of the traditional techniques regarding the lifetime exten-
sion of structures exposed to very high environments is questionable.
Thus, the importance of the topic addressed in this work is justified.
It is expected that a better knowledge of the cathodic protection prin-
ciples will encourage Brazilian engineers to consider its application in
both new structures as well as for the recovery of deteriorated con-
crete structures.
2. Cathodic protection principle
The principle of cathodic protection technique is to reduce the
steel/concrete potential to more negative values ​than its natu-
ral corrosion potential (Ecorr). This is obtained by applying, to the
steel reinforcement, a direct current which may be provided by an
external electric power supply (impressed current protection) or
by an electrical connection of the steel reinforcement to a more
active metal (sacrificial anode protection). In either method, the
supply current must be suitable to negatively polarize the steel/
concrete interface.
Classically, the cathodic protection brings the steel/environment
potential to very close or into the immunity domain of the iron/water
Pourbaix diagram. In concrete structures, this concept can be ap-
plied only to submerged or buried structure. For atmospheric struc-
tures, the concept is different. To better understand this, consider
Figure 1 which shows the Pourbaix diagram.
179 IBRACON Structures and Materials Journal • 2013 • vol. 6 • nº 1
A. ARAUJO | Z. PANOSSIAN | Z. LOURENÇO
Figure 1 – Potential/pH equilibrium Pourbaix
diagram of iron in water (25°C, 1 atm). The main
reactions which take place in the passive/immune
transition region are shown for pH = 13