Civil Engineering Reference
In-Depth Information
Acid degradation model
Equation (6.8) gives the tentative model for acid production at the anode,
which must be further quantified. Experimental evidence linking the
electrical charge passed to acid dissolution in the anode/concrete interface is
provided in (Polder et al., 2007, 2002). These papers describe microscopy
observations on samples from conductive coating and activated titanium CP-
systems, respectively, after several years of operation. In the coating study
(Polder et al., 2007), acid attack was not observed by the two techniques
used (PFM, SEM). Considering their resolution, this suggests that no
more than between 1% and 10% of the total electrical charge had actually
produced acid dissolution of concrete. Tentatively, it may be inferred that
NADF was between 0.01 and 0.1. A similar range can be inferred from
(Mussinelli et al., 1987). The titanium study (Polder et al., 2002) showed a
quantifiable amount of acid dissolution after 5 or 6 years at 17 to 80 mA/m
2
titanium surface (4 to 20 mA/m
2
by concrete surface). The observed amount
of dissolution was about one third of the amount expected from anodic
oxidation and ionic migration, neglecting diffusion. Apparently, diffusion
has compensated two-thirds of the acid formation. A fraction NADF of
about 0.07 of the total charge had actually produced acid. This estimate fits
well in the range of values derived from the coating study. Thus, the model
for net acid production for current densities between 1 and 20 mA/m
2
is
given by:
J
(acid,net) = 0.07 *
I
(A) /
F
(6.9)
with
J
(acid,net) in [mole/m
2
s].
With equation (6.9) we can calculate that for a CP current density of
I
(A)
mA/m
2
, a net amount of 0.023*
I
(A) mole of acid per m
2
per year will be
produced.
A recently developed numerical model was used to investigate acid
dissolution at the anode; details can be found in (Peelen et al., 2008).
Summarising, depletion of Ca(OH)
2
was found to progress with time,
depending on current density and on the diffusion coefficients for hydroxyl
and calcium ions. Diffusion coefficients were chosen on two levels that are
thought to represent wet and semi-dry concrete, with a value for hydroxyl
that is 10 times that of calcium. For high diffusion coefficients, the model
calculations predict an acid degradation rate corresponding to a NADF
of 0.063 for a current density of 1 mA/m
2
and 0.16 for 10 mA/m
2
. This
agrees quite well with our estimates from experimental evidence (0.07).
At low diffusion rates, NADF could be as high as 0.78. This seems quite
high, suggesting rapid acid attack that has not been observed in practice.
Apparently, further work is needed, in particular on diffusion coefficients of
ionic species used as input.
