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Corrosion mechanisms in reinforced concrete


Corrosion of reinforcement steel in concrete structure is one of the significant threats that can weaken and reduce the durability of a concrete structure. Concrete structures located in an aggressive environment such as coastal environment are more vulnerable to corrosion. Corrosion will adversely affect the reinforcement steel. Corrosion will occupy a bigger space which will exert stresses on concrete. These stresses will cause cracks and spalling of concrete, as shown in figure 1. Also, The steel cross-section will be reduced due to corrosion. The reduction of steel cross-section will result in the reduction of steel tensile strength.   


Figure 1





 The corrosion of reinforcement can be reduced and delayed by providing enough concrete cover, using epoxy-coated reinforcement, reducing the permeability of concrete by adding pozzolans such as micro silica, reducing the W/C, using corrosion inhibitor and using of stainless steel reinforcement. 

Corrosion is a chemical or electrochemical process which results in deterioration of materials. The corrosion occurs due to the difference in the dissolved ions in concrete, which create corrosion cells. The ions and electrons will flow between cathode and anodes, as shown in figure 2.  corrosion will occur in the anode in which the anode region losing section during the corrosion process. The iron turned into ferrous ion and electrons. These will flow from the anode to cathode. A cathode is a region with positive electrochemical potential. Consuming the electrons flowing from the anodic area, reducing the oxygen with the presence of water will create hydroxyl ion. The corrosion or electrochemical cell consists of a steel bar which acts as an electrical conductor and the concrete fluid which acts as an electrolytic environment. the below equations represent the corrosion process 


Anodic reaction:Fe→Fe2++2e−(oxidation reaction)Anodic reaction:Fe→Fe2++2e−oxidation reaction



Cathodic reaction:O2+2H2O+4e−→4OH−(redox reaction)Cathodic reaction:O2+2H2O+4e−→4OH−redox reaction


2Fe+O2+2H2O+4e−→2Fe(OH)22Fe+O2+2H2O+4e−→2Fe(OH)




Figure 2

The reason behind the presence of electrochemical potentials is the heterogeneities of the steel surface. The heterogeneities formed as a result of the different concentration of ions around steel surfaces such as chloride, alkalis, and oxygen. The difference in concentration of ions around the steel surface will create anode and cathode on the metal surface. Figure 3 illustrates the increase in metal volume due to the corrosion process. 




Figure 3


Despite the adverse effects of corrosion on a concrete structure, Fe3O4 is a useful product of the corrosion process. Fe3O4 can adhere to the rebar surface and generate a passive layer that protects the reinforcement from corrosive environment. The formation of this compound occurs only at high pH values when the concrete is alkaline. This is the passive state, while the concrete is alkaline and free from aggressive ions such as chloride. However, the passive protective film can be destroyed in a phenomenon called depassivation, by decreasing the pH of the concrete to values between 8 and 9 (carbonation) and/or by the existence of chloride ions dissolved in the pore solution.






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