Civil Engineering Reference
In-Depth Information
6.2.2 Limit state function for degradation of carbon-based
conductive coatings
Our proposed model for oxidation of carbon in CP anodes takes into
account the real contact anode/concrete surface area. An 'efficiency factor'
is introduced to account for parallel reactions (6.1) and (6.2). This factor is
quantified, from laboratory studies and practical evidence, to be about 0.3
for a carbon anode in alkaline concrete at normal CP current densities. As
carbon oxidation proceeds the anode will degrade and eventually fail. The
precise mechanism is not identified at the moment. The options include, for
example, the decrease of the active surface area for oxidation according to
reactions (6.1) to (6.3). Also carbon oxidation could occur in a small layer
which becomes carbon depleted and whose thickness increases in time. As a
consequence, the conduction between anode and concrete decreases in time.
We opt for the latter mechanism as the most likely, based on microscopic
observations. For carbon particle filled conductive coating anodes, a limit state
function for oxidation induced degradation is introduced, Z
ox
. Its resistance
term R is the amount of carbon in a critical coating thickness in which carbon
particles can be oxidised before the conduction becomes too low. Its load term
S is the time and current dependent amount of oxidised carbon given by (6.5):
Z
ox
=
R - S
(6.6)
with load S =
F
(C) * I(A) * t from equation (6.5), and resistance
R
= mass of
carbon in a critical thickness for conduction loss due to oxidation.
With
F
(C) = 0.3 and
I
(A) known,
S
can be calculated.
R
must be estimated.
In the Berlin CP system, after 15 years, a layer of 0.5 - 1.0 mm (mean 0.8 mm)
of the anode had become devoid of carbon (Mietz et al., 2001). After 7 years,
the system stopped working properly. As conditions become increasingly
aggressive (due to a lower pH and a higher anode potential), degradation
may be non-linear with an increasing rate, so the critical thickness is equal to
or less than 7/15 * 0.8 = 0.4 mm. In about 7 years the system failed, so Z
ox
= 0 and for F(C) = 0.3 and I(A) = 8 mA/m
2
, the amount of oxidized carbon
was about 17 g/m
2
. Assuming that the same amount of oxidised carbon is
critical for the AHEAD conductive coating, R is 17 g/m
2
and the critical
thickness would be about 20 μm. Filling in these numerical values, the limit
state function for oxidative degradation of a carbon-based coating anode is
expressed in g of carbon per square metre as:
Z
ox
= 17 - 0.3 *
I(A) * t
(6.7)
with
I
(A) the anodic current density [mA/m
2
] and
t
the time [year].
As an example, a conductive coating system at 1 mA/m
2
will have a service
life of
t
= 17/0.3 = 57 years. This seems quite long. It should be realised that
this is a deterministic calculation based on mean values; the probability
