Actions during service 165
the surrounding concrete, finally resulting in cracking and spalling of the
concrete cover.
A general distribution of anodes and cathodes on the steel surface combined with a change of their position during the corrosion process, will lead
to a quite uniform corrosion attack on the steel reinforcement. However, if
a small number of anodes is located at fixed points, localised corrosion can
occur (Rosenberg et al. 1989).
Considering the given information, the layman may question why steel
reinforcement is added to concrete containing oxygen and water, which
potentially leads to corrosion. The reason reinforcing steel in concrete normally does not corrode is because the pore solution in the concrete is totally
different from a typical aqueous environment (water, rain) on the earth’s
surface. The pore solution in a cementitious material typically has a high
pH value because of the high content of calcium hydroxide resulting from
cement hydration (see Chapter 1). The influence of the pore solution, or any
electrolyte in general, on the steel corrosion process is illustrated by a socalled Pourbaix diagram, as shown in Figure 5.39.
Based on thermodynamic information, a Pourbaix diagram shows the
conditions of electrochemical potential and pH value of an electrolyte
where corrosion is expected or where immunity or passivation is reached
(Pourbaix 1976). The electrochemical potential (expressed in Volts, V) is
the potential difference between the anode and a reference electrode, and
is arbitrarily defined as zero for the hydrogen electrode which is used as a
reference for all metal potentials (Rosenberg 1989). In the given Pourbaix
diagram, which is a simplified diagram for iron in an aqueous environment,
a typical natural environmental condition illustrates that iron (steel) can
14
Corrosion
Passivity
Natural environment
Concrete pore solution
Anodic
protection
Cathodic
protection
Immunity
Corrosion
Potential (V)
pH
7
0
–1
0
1
Figure 5.39 (Simplified) Pourbaix equilibrium diagram for iron in aqueous environment.
the surrounding concrete, finally resulting in cracking and spalling of the
concrete cover.
A general distribution of anodes and cathodes on the steel surface combined with a change of their position during the corrosion process, will lead
to a quite uniform corrosion attack on the steel reinforcement. However, if
a small number of anodes is located at fixed points, localised corrosion can
occur (Rosenberg et al. 1989).
Considering the given information, the layman may question why steel
reinforcement is added to concrete containing oxygen and water, which
potentially leads to corrosion. The reason reinforcing steel in concrete normally does not corrode is because the pore solution in the concrete is totally
different from a typical aqueous environment (water, rain) on the earth’s
surface. The pore solution in a cementitious material typically has a high
pH value because of the high content of calcium hydroxide resulting from
cement hydration (see Chapter 1). The influence of the pore solution, or any
electrolyte in general, on the steel corrosion process is illustrated by a socalled Pourbaix diagram, as shown in Figure 5.39.
Based on thermodynamic information, a Pourbaix diagram shows the
conditions of electrochemical potential and pH value of an electrolyte
where corrosion is expected or where immunity or passivation is reached
(Pourbaix 1976). The electrochemical potential (expressed in Volts, V) is
the potential difference between the anode and a reference electrode, and
is arbitrarily defined as zero for the hydrogen electrode which is used as a
reference for all metal potentials (Rosenberg 1989). In the given Pourbaix
diagram, which is a simplified diagram for iron in an aqueous environment,
a typical natural environmental condition illustrates that iron (steel) can
14
Corrosion
Passivity
Natural environment
Concrete pore solution
Anodic
protection
Cathodic
protection
Immunity
Corrosion
Potential (V)
pH
7
0
–1
0
1
Figure 5.39 (Simplified) Pourbaix equilibrium diagram for iron in aqueous environment.