Civil Engineering Reference
In-Depth Information
The first process is carbonation. It might be considered as concrete's
equivalent to the oxidation of steel. Steel is formed by the removal of oxygen
and other oxidising elements and compounds from iron ores. Concrete
(or more accurately cement) is formed by driving off carbon dioxide from
the constituents of cement clinker. Just as corrosion is the process of the
oxygen recombining with steel or iron to revert back to the lowest chemical
energy state, so the reaction of concrete with carbon dioxide is a comparable
reaction. The difference is that iron oxide has drastically different (inferior)
properties compared to steel while carbonated concrete has very similar
properties to uncarbonated concrete except that it loses its alkalinity. This
loss of alkalinity means that embedded steel becomes vulnerable to corrosion
as the protective passive oxide layer breaks down.
The other process that will lead to reinforcement corrosion is chloride
ingress. Chloride ions from sea salt, de-icing salt or from contaminated
concrete mix constituents will break down the passive layer without
damaging the concrete once there is a sufficient concentration at the steel
surface along with sufficient oxygen and water.
Regardless of the cause of depassivation, the corrosion process proceeds
in the same manner for carbonated and chloride-contaminated structures.
Iron dissolves at the anode, forming Fe
2
+ irons and liberating electrons:
The anodic reaction: Fe
→
Fe
2+
+ 2e
-
(4.1)
The two electrons (2e
-
) created in the anodic reaction must be consumed
elsewhere on the steel surface to preserve electrical neutrality. In other
words we cannot have large amounts of electrical charge building up at one
place on the steel. There must be another chemical reaction to consume the
electrons. This is a reaction that uses water and oxygen to create alkaline
hydroxyl ions:
The cathodic reaction: 2e
-
+ H
2
O + ½O
2
→
2OH
-
(4.2)
This is illustrated in
Figure 4.1.
The hydroxyl ions increase the local
alkalinity and therefore will strengthen the passive layer, warding off the
effects of carbonation and chloride ions at the cathode. Note that water and
oxygen are needed at the cathode for corrosion to occur, but not at the anode.
The ferrous ions go on to react with hydroxyl ions, water and oxygen, to
create rust which has approximately seven times the volume of the original
steel.
We can see from the reactions and the figure that there is a benign
cathodic reaction producing hydroxyl ions, i.e. alkalinity. If we can force
this to occur across the reinforcing steel network and displace the anodic
reaction elsewhere, we will protect the steel from corrosion. An impressed
current cathodic protection system does this by installing an anode system
on the concrete surface or embedded in the concrete and applying a small
