Civil Engineering Reference
In-Depth Information
6.2.4 Acid production
The CP current density must be converted into a rate of acid production.
Possible electrode reactions and ionic migration and diffusion processes have
been schematically represented in
Figure 6.2
(Bertolini et al., 2004). They
will be briefly discussed.
Electrolysis
The total amount of oxidation at the anode is the sum of all possible reactions:
oxidation of hydroxyl, chloride and/or carbon. Only chloride oxidation does
not form acid. Its contribution is probably small and can be neglected. Here
it is simply assumed that all anodic processes consume hydroxyl ions. The
'flux' of hydroxyl consumption at the anode becomes equal to the total CP
current divided by Faraday's number.
Migration
The current also causes mass transport through the concrete by ionic
migration. Each ion carries a fraction of the current equal to its transport
number
t
(
i
). In aqueous solution comparable to concrete pore liquid (0.1
to 0.3 molar NaOH and/or KOH with a small amount of Ca(OH)
2
),
t
(OH)
is 0.8 and
t
(Na) is 0.2 (neglecting K and Ca). The migration fluxes of
hydroxyl and sodium are given by the current multiplied by their transport
number. Of the hydroxyl produced at the cathode, a fraction
t
(OH) leaves
the cathode area, that is 0.8 *
I
(CP)/
F
. The excess negative charge at the
cathode is compensated by inflow of an equivalent amount of positively
charged sodium ions. The migration of hydroxyl ions will reduce the acid
production at the anode. Some net consumption of hydroxyl will result
anyway: an amount of sodium ions of 0.2 *
I
(CP)/
F
migrates from the
MIGRATION
Cathode to anode OH
Anode to cathode Na
-
-
+
+
Anode
-
+
Steel
CATHODIC REACTION
O
2
+ 2 H
2
O + 4 e → 4 OH
-
ANODIC REACTIONS
4 OH
-
O
2
+ 2 H
2
O + 4 e → 4 OH
-
4 OH
-
→ O
2
+ 2 H
2
O + 4 e
→ O
2
+ 2 H
2
O + 4 e
2 Cl
-
2 Cl
-
→ Cl
2
↑ + 2 e
C + 2 H
2
O → CO
2
+ 4 H
+
+ 4 e
→ Cl
2
↑ + 2 e
C + 2 H
2
O → CO
2
+ 4 H
+
+ 4 e
Concrete
DIFFUSION
To anode OH
-
and Na
+
Overlay (mortar)
-
+
