|
Fig.
2 Installation of Zinc-Hydrogel Anode (Illustration copyrighted
by 3M)
HOW
IT WORKS
As
illustrated in Fig. 2, the zinc foil is applied to the surface of
structurally sound concrete structure. The pressure sensitive adhensive
(conductive hydrogel) bridges the gap between the zinc anode (foil
in this case) and the concrete surface – making possible the electrolyte
continuity across the anode and cathode (rebars).
The
zinc foil must be electrically connected to the rebar network by
wires so that electrons can flow through the wire from zinc (anode)
to rebars (cathode). Conventional current flows in the opposite
direction of electrons, i.e., from cathode to anode. Ionic current
is carried by charged species such as Cl-, Na+, OH-, Ca2+ etc. across
the electrolyte (concrete + hydrogel). The potential of rebar and
the current flowing between the zinc foil and the rebar can give
some indication about the degree of cathodic protection.
WHY
IT WORKS
Corrosion
of rebar is the process involving the following oxidation reaction:
Fe ==> Fe2+
+ 2e
A
neutral iron atom lost 2 electrons and became a positively charged
ion. The tendency of a metal to lose electrons can be considered
as the tendency of metal to corrode.
When
zinc foil is applied to the concrete surface but not connected
by wire to the rebar network, the steel rebar continues to corrode
at a normal rate. When zinc foil is electrically connected to
the rebar network, ELECTRONS flow from zinc foil into steel rebars,
thus reducing the steel’s tendency to lose electrons and hence
reducing the rate of corrosion (oxidation). This is because zinc
is a much more reactive metal than steel, it has a much greater
tendency to lose electrons than that of steel. Since electrons
carry negative charges, the accumulation of electrons within steel
rebar makes steel more negatively charged, leading to cathodic
polarisation, - the shift of rebar potential in the negative direction.
In general, the greater the tendency for the anode to donate electrons
to the cathode, the greater the degree of protection rendered
to the “cathode” (the steel rebar network). However, one must
remember that when an anode donates electrons to the cathode,
the anode material “sacrifices” itself. What this means is that
the useful life of the anode is both limited by the chemical/electrochemical
surface reactivity and the amount of physical material available.
POTENTIAL
APPLICATIONS
The
nature of the zinc hydrogel anode makes it a perfect rehabilitation
option for structures or certain sections of structures exposed
to atmospheres. The conductive hydrogel ensures the continuity of
electrolyte across the rebar-concrete-hydrogel-anode, and hence
cathodically protects the rebar network.
Tips:
It is a good idea to paint the zinc surface to preserve the anode
material by reducing corrosion of zinc from external atmosphere.
| 
