Civil Engineering Reference
In-Depth Information
9.1 Introduction: the mechanism of carbonation-induced
corrosion of steel in concrete
Steel is normally passive when embedded in concrete, due to the alkalinity
of the pore water. This passivity can be destroyed by carbonation of the
concrete, that is, the gradual ingress of carbon dioxide into pores in the
concrete, which, when dissolved in the pore water, forms mildly acidic
carbonic acid.
This acid reacts with alkalis in the pore water to form a neutral salt and
water, so the carbonic acid reacts with the calcium hydroxide to form calcium
carbonate:
CO
2 +
H
2
O
→
H
2
CO
3
(9.1)
H
2
CO
3
+ Ca(OH)
2
→
CaCO
3
+ 2H
2
O
(9.2)
Once the calcium hydroxide is consumed the pH drops from 13 to 9 and
the passive layer decays. The steel then corrodes in the presence of the oxygen
and water available in the concrete pores.
Corrosion processes
When steel in concrete corrodes, it dissolves in the pore water and gives up
electrons:
Fe
→
Fe
2
+
+ 2e
-
(9.3)
This is the anodic reaction.
The two electrons must be consumed elsewhere on the steel surface to
preserve electrical neutrality:
2e
-
+ 2H
2
O + O
2
→
4OH
-
(9.4)
