Civil Engineering Reference
In-Depth Information
oxides and contained relatively smaller amounts of Fe
3+
oxides. The outer
layer contained relatively larger amounts of Fe
+3
oxides than the inner layer.
However, this fi lm is also likely heterogeneous and discontinuous. Jones
(1982) reported that corrosion initiation is likely a result of the electrostatic
attraction between the positively charged metal surface (without an oxide
layer) and the negatively charged chlorides. Hime and Erlin (1987) reported
that the chlorides react with the unprotected ferric and ferrous ions to fi rst
produce ferrous chloride. The ferrous chloride then reacts with hydroxyl
ions to produce iron hydroxide (Fe(OH)
2
), freeing the chlorides to further
react with more ferric and ferrous ions. Figure 19.1 shows a general sche-
matic of the corrosion process. Note that water is converted to hydroxyl
ions and the chlorides are released to further react with the base steel. This
indicates that once chlorides are transported to the steel reinforcement
surface, they are always available for further corrosion reactions. It also
shows that the availability of water is necessary for the reaction to continue.
Note that the rate at which the chlorides are transported to the steel surface
is a signifi cant parameter for determining the time to corrosion initiation
and continued propagation.
In addition to chloride-induced corrosion, corrosion of the steel rein-
forcement can initiate as a result of carbonation of the concrete. Carbon-
ation of the concrete occurs when the alkalis in concrete react with acidic
components of the atmosphere, particularly carbon dioxide (CO
2
). Carbon-
ation results in progressively reduced alkalinity of the concrete. Because
the stability of the passive fi lm is dependent on high alkalinity, the protec-
tion from the passive layer is reduced with decreasing concrete alkalinity,
resulting in increased corrosion activity of the reinforcement. Unlike chlo-
ride-induced corrosion where localized attack is common, carbonation-
induced corrosion is more associated with general or uniform corrosion.
Cl
-
H
2
O
Chloride-contaminated
concrete
Cl
-
O
2
H
2
O
H
2
O
Cl
-
O
2
FeCl
-z
+ OH
-
Fe(OH)
2
+ Cl
-
x
Cl
-
Mill-scale
Cl
-
Cl
-
O
2
H
2
O
O
2
H
2
O
Cl
-
Cl
-
O
2
H
2
O
Cl
-
O
2
Fe
2+
Passive film
2e
-
Not to scale
Fe
Fe
Fe
Fe
Fe
Anode
Cathode
Steel reinforcement
19.1
Simplifi ed schematic of chloride-induced corrosion.
Search WWH ::
Custom Search