Page 42 - Reliability and Maintainability of In service Pipelines

Introduction 31

corrosion of the reinforcing steel. The formation of rust can cause swelling in the
reinforcement and result in the cracking of the concrete cover.
However, the effect on concrete caused by the process of carbonation
is observed to be slow, and is determined by the rate of carbon dioxide diffusing
into the concrete. Although the rate of diffusivity depends highly on the
water cement ratio, level of hydration, humidity, temperature, permeability and
porosity of the concrete, sufficient thickness of concrete cover over the steel rein-
forcement can prolong the life span of concrete pipes as it requires more time to
penetrate through a thicker concrete, compared to a thinner concrete. With a
thicker concrete layer, the reinforcement is further protected against corrosion, for
longer periods of time.

Chloride Attack
In concrete pipes, chloride attack is widely recognized as the ability of chloride
ions to enter into the concrete and induce corrosion of the steel reinforcement.
It is by this action that, concrete pipes that are exposed to seawater often
undergo corrosion faster than concrete pipes that do not have a simultaneous
salt and water exposure. This creates a continuous corrosion favoring environ-
ment and with the oxide film lost in the previous carbonation process, due to a
reduced pH, the rate of corrosion in the reinforcing steel is further heightened.
An increased likelihood of chloride attack, however, is not only subject to
seawater exposure. In fact corrosion of the reinforcing steel in concrete pipes is
initiated when chloride concentrations exceed 0.6% of the binder mass of the
concrete (Hajkova, 2015). A threshold concentration of 0.026% (by weight of
concrete) is adequate enough to disrupt the oxide film on the surface of the steel
reinforcement, allowing corrosion to take place (Daily, 2017).

Galvanic Corrosion
The process of galvanic corrosion in concrete pipes occurs once the oxide film on
the surface of the steel reinforcement is disrupted. As the chloride ions do not
react with the concrete surface in a uniform distribution, it creates a gradient
effect, with some areas of the reinforcing steel exposed to a higher concentration
of chlorides and some areas not exposed at all. The areas exposed to higher con-
centrations have an increased likelihood of corrosion initiation, causing a reduc-
tion in the oxide film, while the oxide film on reinforcing steel that is not exposed
to chlorides remains intact.
Variances in the structural composition of the reinforcing steel, as well as
variances in residual stress, can also initiate the process of galvanic corrosion.
In this case, chlorides, reacting with the reinforcing steel in a uniform distribu-
tion, result in micro-cell corrosion, which is often localized and dominates the
corrosion process. Under this type of action, the anodic and cathodic sites on