Civil Engineering Reference
In-Depth Information
'activating' noble metal oxides on the titanium surface cannot be oxidised any
further. Oxidation of chloride is relatively small and can be neglected.
6.2.1 Oxidation of carbon-based materials
Carbon-based anode materials, such as carbon-filled conductive coatings,
may suffer oxidising anodic attack (Brown & Tinnea; 1991). A tentative
model can be based on Faraday's law, taking the current density at the anode/
concrete interface into account, which may differ from the current density
per unit surface area of concrete. For a surface covering conductive coating,
the anode/concrete surface ratio is 1. A carbon oxidation efficiency factor
F(C)
with a value between 0 and 1 is introduced, that takes into account that
not all current oxidises carbon:
Δ
m
(C) =
F
(C) *
i
(A) *
A
(carbon) *
z
-1
*
F
-1
(6.4)
m(C)
amount of carbon oxidised per unit of time
[g/m
2
/s]
F(C)
the carbon oxidation efficiency factor [-], 0 <
F
(C) < 1
i
(A) the anode/concrete surface area current density [A/m
2
]
A
(carbon) the atomic mass of carbon, 12 [g/mole]
z
the number of electrons involved in reaction (6.2), 4 [-]
F
Faraday's constant, 96,500 [A s/mole].
with
Δ
For a surface covering carbon-based conductive coating CP system, a
typical current density of 1 mA/m
2
of concrete surface area equals an anode
current density of 1 mA/m
2
. If the total current is oxidising carbon to carbon
dioxide, so for F(C)
=
1, this current density would oxidise 1.0 g of carbon
per m
2
of anode surface area per year.
In more general terms, the amount of oxidation can be described with
only one parameter, the oxidation efficiency factor
F(C)
and two input
variables: the current density (assumed constant) and more conveniently
written as I(A) in mA/m
2
and time. Filling in numerical values for constants
in (6.4) and multiplying by time in years, we obtain:
Δ
M
(C) = 1.0 *
F
(C) *
I
(A) *
t
(6.5)
with
M
(C) the total amount of carbon oxidised per anode surface [g/m
2
]
F
(C) the carbon oxidation efficiency factor [-]
I
(A) the anode/concrete surface area current density [mA/m
2
]
t
the time [years].
Δ
For all the parameters listed above, time-averaged values should be
used. Evidence from the literature to test this model is scarce. Observations
on coating CP systems suggest a low level of attack (Polder et al., 2007).
Samples from structures with a conductive coating (AHEAD from Protector)
