Civil Engineering Reference
In-Depth Information
models for degradation of the anode and overlay adhesion have been set up
and validated from experimental evidence.
As part of a study into CP service life, an inventory was made of systems
operating in The Netherlands in 2004. The companies who installed them
were asked about maintenance, performance and failure of components.
Information was obtained from a total of 70 CP systems installed between
1987 and 2002. Some were not very well documented, however. Data on 52
of them provided sufficient information to be used. In particular the evaluation
of anode-copper connection and reference electrode performance was based
on the field data obtained.
6.2 Degradation of anode systems
CP current flow from the anode to the concrete involves oxidising and
acidifying electrochemical reactions at the anode/concrete interface. Their
type and relative proportion depend on the availability of reactants, the
electrical potential and the anode material. Their rate depends mainly on the
current density. At the anode/concrete (or anode/mortar) interface hydroxyl
ions are oxidised to oxygen gas by:
4 OH
-
(aq)
→
O
2
(g) + 2 H
2
O(I) + 4 e
(6.1)
This reaction tends to lower the pH and is equivalent to acid production.
Hydroxyl ions from the hardened cement paste tend to buffer a high pH. Acid
production may eventually dissolve the cement paste.
When an oxidisable material such as carbon is present, it may be oxidised
by:
C(s) + 2 H
2
O(I)
→
CO
2
(g) + 4 H
+
(aq) + 4 e
(6.2)
or
C(s) + H
2
O(I)
→
CO (g) + 2 H
+
(aq) + 2 e
(6.3)
In addition to consuming carbon, these reactions also produce acid and
tend to lower the pH. Reaction (6.2) occurs at a lower potential than reactions
(6.1) or (6.3), so thermodynamically it should be favoured. Experiments in
solution at high pH have shown that reaction (6.2) represents only about 20%
of the current and reaction (6.1) consumes about 80% (Eastwood et al. 1999),
so kinetics apparently dominate the course of the reactions. The slow kinetics
of reactions (6.2) and (6.3) under many different conditions are well known
from energy related research in which electrochemical oxidation of carbon
(coal) was studied.
Metals and chloride ions are potentially oxidisable. From the usual anode
materials, titanium is strongly passivated (at normal potentials and pH) and the
