Actions during service 177
As a result, the corrosion process of the steel can be very local, creating
‘pit corrosion’. Within a pit, a microclimate can be obtained, as shown in
Figure 5.46. The pH value in the pit decreases and more chloride ions are
attracted, making the micro-environment even more corrosive. Iron ions
Fe 2+ react with chloride ions Cl − and can be transported away from the
reinforcement. After further oxidation to ferri-oxides and ferri-hydroxides,
these corrosion products can become visible on the concrete surface, showing brownish colours. Furthermore, the expansive formation of corrosion
products can also lead to cracking and spalling of the concrete cover, as is
the case in carbonation-induced carbonation. However, due to pit corrosion, it is possible that the steel reinforcement fails due to an insufficient
remaining cross section. This is a special risk in case of prestressing strands,
which are more sensitive to chloride-induced corrosion than classical reinforcement. Prestressing strands or wires can fail due to pit corrosion, even
before corrosion products become visible on the concrete surface. Due
attention should be given to this risk when applying excessive amounts of
de-icing salts on prestressed road bridges.
In this context, stress-corrosion cracking and hydrogen embrittlement can also be mentioned, which can be considered a synergistic action
between mechanical and environmental loading. Prestressing steel is particularly susceptible to this kind of attack, although failure typically also
requires insufficient concrete quality, inadequate mortar injection of the
cable ducts, or corrosion prior to injection (Rosenberg 1989). In cathodic
regions, hydrogen is formed, which can penetrate the steel. As a result,
the steel lattice structure is distorted and the steel becomes more brittle.
Furthermore, atomic hydrogen (H) can be transformed to molecular
Concrete
HCl
H 2 O
OH
–
Cl
–
Fe
2+
pH 5
pH 13
Passivation layer
Steel
Figure 5.46 Chloride-induced pit corrosion of steel reinforcement in concrete.
As a result, the corrosion process of the steel can be very local, creating
‘pit corrosion’. Within a pit, a microclimate can be obtained, as shown in
Figure 5.46. The pH value in the pit decreases and more chloride ions are
attracted, making the micro-environment even more corrosive. Iron ions
Fe 2+ react with chloride ions Cl − and can be transported away from the
reinforcement. After further oxidation to ferri-oxides and ferri-hydroxides,
these corrosion products can become visible on the concrete surface, showing brownish colours. Furthermore, the expansive formation of corrosion
products can also lead to cracking and spalling of the concrete cover, as is
the case in carbonation-induced carbonation. However, due to pit corrosion, it is possible that the steel reinforcement fails due to an insufficient
remaining cross section. This is a special risk in case of prestressing strands,
which are more sensitive to chloride-induced corrosion than classical reinforcement. Prestressing strands or wires can fail due to pit corrosion, even
before corrosion products become visible on the concrete surface. Due
attention should be given to this risk when applying excessive amounts of
de-icing salts on prestressed road bridges.
In this context, stress-corrosion cracking and hydrogen embrittlement can also be mentioned, which can be considered a synergistic action
between mechanical and environmental loading. Prestressing steel is particularly susceptible to this kind of attack, although failure typically also
requires insufficient concrete quality, inadequate mortar injection of the
cable ducts, or corrosion prior to injection (Rosenberg 1989). In cathodic
regions, hydrogen is formed, which can penetrate the steel. As a result,
the steel lattice structure is distorted and the steel becomes more brittle.
Furthermore, atomic hydrogen (H) can be transformed to molecular
Concrete
HCl
H 2 O
OH
–
Cl
–
Fe
2+
pH 5
pH 13
Passivation layer
Steel
Figure 5.46 Chloride-induced pit corrosion of steel reinforcement in concrete.