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LIFE
EXPECTANCY
As
in any other sacrificial anode CP system, the life expectancy
can be calculated if current output of the anode is known. It
is mainly determined by the amount of zinc anode material and
the rate (current output) at which zinc “sacrifices” itself (in
order to protect the cathode):
Manufacturer’s
data sheet suggests that the zinc mesh embedded in the snap-on fibreglass
jacket has a projected life of 46 years.
MAJOR
ATTRACTIONS AND APPLICATIONS
Compared
with conventional CP system, the snap-on zinc mesh anode jacket
has the following attractions:
- All-in-one,
self-contained sacrificial anode CP system
- Snap-on
type quick, easy installation
- Low
cost effective repairs
- Maintenance-free
- No
need for external power
- Long
life protection
- Jackets
are available in a wide range of sizes and can accommodate both
square and round pilings (Fig.10)
Major
applications include splash and tidal zones of steel reinforced
concrete structures. Optional Cast bulk zinc anode can be used on
fully submerged structures.
 
Fig.10
Jackets for square and round pilings
(Photo
copyrighted by FOSROC International and Alltrista Zinc products
Company)
ELECTROCHEMICAL
TREATMENT
Principles
In
all electrochemical restoration techniques a direct current
is applied between the reinforcement (cathode) and an external
anode in electrolytic contact with the concrete. Cathodic
protection (CP) is a permanent installation with design currents
below 10 mA/m2,
electrochemical chloride extraction (or removal, - EC or ECR)
and electrochemical realkalisation (ER) are applied only on
a temporary basis and use currents up to 1 A/m2.
In all three cases the electrochemical reactions at the cathode
(the
rebars) produce hydroxyl ions leading to an increase of the
pH near the rebar. This facilitates passivation of the steel.
Reaction (1b) is possible at very high current densities and
produces hydrogen and especially high tensile steels under
load could suffer hydrogen embrittlement.
2
H2O
+ O2
+ 4e ==> 4OH- (1a)
(at low current density)
2
H2O
+ 2e-
==> 2OH-
+ H2 (1b)
(at high current density)
At
the anode the possible oxidation reactions are oxygen evolution,
chlorine evolution or water decomposition:
2
H2O
==>O2
+ 4H+
+ 4e-
(2a) (if tap water is used)
4OH-
==> O2
+ 2H2O
+ 4e- (2a’) (if
alkaline solution is used)
2
Cl-
==>C12
+ 2e-
(2b)
H2O
+ C12
==>HCl + HClO (2b’)
These
reactions lead to an acidification of the electrolyte around
the anode (OH-
ions are converted into O2;
H2O
is converted into H+).
The decrease in pH value in the electrolyte around the anode
depends on the current density applied.
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