Civil Engineering Reference
In-Depth Information
Ionic
Current
1
/2O
2
+ H
2
O + 2e
-
→ 2OH
-
Fe → Fe
2+
+ 2e
-
Electronic Current
Anode
Cathode
Fe
2+
+ 2OH
-
→ Fe (OH)
2
Ferrous
Hydroxide
4Fe(OH)
2
+ O
2
+ 2H
2
O → 4Fe(OH)
3
Ferric Hydroxide
2Fe(OH)
3
→ Fe
2
O
3
.H
2
O + 2H
2
O Hydrated Ferric Oxide (rust)
electrical current: injecting electrons into the steel that forces the cathodic
reaction to predominate.
There are two cathodic reactions that can occur on the steel surface.
As well as hydroxyl production (equation 4.2) there is also a hydrogen
evolution reaction which occurs if the electrochemical potential of the steel
is too negative:
H
2
O + e
-
→
H + OH
-
(4.3)
This hydrogen evolution reaction can lead to hydrogen embrittlement
and severe precautions must be taken when applying impressed current
cathodic protection to prestressing or to structures that contain prestressing.
Advice on such applications can be found in Broomfield (2007) and NACE
(2001).
The chloride ion itself is negative and will be repelled by the negatively
charged cathode (reinforcing steel). It will move towards the (new external)
anode. With some anodes (particularly the carbon-based anodes) it may then
combine to form chlorine gas at the anode:
2Cl
-
→
Cl
2(gas)
+ 2 e
-
(4.4)
The other major reaction at all major anodes is the formation of oxygen:
2OH
-
→
H
2
O + ½O
2
+ 2e
-
(4.5)
and
H
2
O
→
½O
2
+ 2H
+
+ 2e
-
(4.6)
