Corrosion of steel rebars in concrete
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Steel corrodes in the presence of air and water. Although there are porosity and moisture in a concrete, steels usually does not corrode inside it. It happens because the concrete alkalinity, which forms a “passive” layer on the steel surface protect the metal. This passivating layer can be destroyed by two different processes: carbonation and chloride attack (Broomfield, 1996).
According to Shreir, Jarman and Burstein (1994), steel rebars corrosion in concrete is an interesting type of corrosion to be study. This process is influenced by the quantity of water because how much more water present in concrete, more permeable it is and then, the corrosion process ocurrs easily (Hover, 2011). Carbonation is a process, which the concrete reacts with carbon dioxide reducing the pH to a rate that makes steel does not be passive anymore. This process starts in the concrete surface, spreads to steel and may cause corrosion in the metal. The level of carbonation depend on the level of porosity, permeability and cement content and the concrete thicker determinates how much the concrete embedded will be protected (Shreir; Jarman; Burstein, 1994).
Chloride is an element that causes corrosion in steel rebars because it overcomes the concrete protection. According to Montemor, Simoes and Ferreira (2003), chloride ions from contaminated mix constituents or from surrounding environment destroy the passive film of steel surface. Both process of chloride ingress in the concrete and then the rebar corrosion are complex and depend on different aspects, as temperature cycles and wet-dry cycles (Shi; Xie; Fortune; Gong, 2012).
The chemical reactions that occurs in these corrosion processes are the same for both processes and they generate rust in the steel surface . The steel dissolves in the pore water and loss electrons, while water and oxygen are consumed (Shreir; Jarman; Burstein, 1994).
The equations below represent the anodic and cathodic reactions:
ANODIC REACTION:
Fe → Fe2+ + 2e-
CATHODIC REACTION:
H2O + 1/2 O2 + 2e– → 2OH–
Another stage represented by the equations bellow occur for “rust” to form. There is the formation of ferrous hydroxide, which becomes ferric hydroxide and then is transformed in hydrated ferric or rust. According to PCA (2014), the formation of hydrated ferric oxide generates a tensile stress, which results in cracking and spalling in concrete because occurs an expansion. The volume of rust in bigger than steel volume.
Fe2+ + 2OH– → Fe(OH)2
(Ferrous hidroxyde)
4Fe(OH)2 + O2 + 2H2O → 4Fe(OH)3
(Ferric hydroxide)
2Fe(OH)3 → Fe2O3.H2O + 2H2O
(Hydrated ferric oxide or rust)
===========================
Steel corrodes in the presence of air and water. Although there are porosity and moisture in a concrete, steels usually does not corrode inside it. It happens because the concrete alkalinity, which forms a “passive” layer on the steel surface protect the metal. This passivating layer can be destroyed by two different processes: carbonation and chloride attack (Broomfield, 1996).
According to Shreir, Jarman and Burstein (1994), steel rebars corrosion in concrete is an interesting type of corrosion to be study. This process is influenced by the quantity of water because how much more water present in concrete, more permeable it is and then, the corrosion process ocurrs easily (Hover, 2011). Carbonation is a process, which the concrete reacts with carbon dioxide reducing the pH to a rate that makes steel does not be passive anymore. This process starts in the concrete surface, spreads to steel and may cause corrosion in the metal. The level of carbonation depend on the level of porosity, permeability and cement content and the concrete thicker determinates how much the concrete embedded will be protected (Shreir; Jarman; Burstein, 1994).
Chloride is an element that causes corrosion in steel rebars because it overcomes the concrete protection. According to Montemor, Simoes and Ferreira (2003), chloride ions from contaminated mix constituents or from surrounding environment destroy the passive film of steel surface. Both process of chloride ingress in the concrete and then the rebar corrosion are complex and depend on different aspects, as temperature cycles and wet-dry cycles (Shi; Xie; Fortune; Gong, 2012).
The chemical reactions that occurs in these corrosion processes are the same for both processes and they generate rust in the steel surface . The steel dissolves in the pore water and loss electrons, while water and oxygen are consumed (Shreir; Jarman; Burstein, 1994).
The equations below represent the anodic and cathodic reactions:
ANODIC REACTION:
Fe → Fe2+ + 2e-
CATHODIC REACTION:
H2O + 1/2 O2 + 2e– → 2OH–
Another stage represented by the equations bellow occur for “rust” to form. There is the formation of ferrous hydroxide, which becomes ferric hydroxide and then is transformed in hydrated ferric or rust. According to PCA (2014), the formation of hydrated ferric oxide generates a tensile stress, which results in cracking and spalling in concrete because occurs an expansion. The volume of rust in bigger than steel volume.
Fe2+ + 2OH– → Fe(OH)2
(Ferrous hidroxyde)
4Fe(OH)2 + O2 + 2H2O → 4Fe(OH)3
(Ferric hydroxide)
2Fe(OH)3 → Fe2O3.H2O + 2H2O
(Hydrated ferric oxide or rust)
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