PREVENTION OF CONCRETE STRUCTURES AGAINST CORROSION




Any process of deterioration and consequent loss of a solid metallic material, through an unwanted chemical or electrochemical attack by its environment, starting at its surface, is called corrosion.


PASSIVITY

         Passivity is a phenomenon by which a metal exhibits corrosion resistance

         It is the result of formation  of a highly protective, but very  thin and invisible film on the surface of the metal.

Causes of corrosion in RC structures
Ø  Carbonation

Ø  Chloride attack


Ø  Carbonation

         Reduction of pH in cover concrete which causes loss of passive oxide layer

         Low pH caused by reaction of Calcium Hydroxide (Ca(OH))2 in concrete with atmospheric Carbon Dioxide

Ø  CHLORIDE ATTACK

         Caused by chlorides breaking up passive oxide layer

         The moisture in concrete pores will act as electrolyte, and the area adjacent to concentration of chloride ions on the reinforcement steel will become cathode initiates corrosion.

Principle of  corrosion
         when there  is difference in electrical potential along the steel reinforcement, an electrochemical cell is set up.

         One part of the steel becomes anode and other becomes cathode and the pore water acts as an electrolyte.

         these positively charged ferrous ions at the anode passes into solution
         the negatively charged free electron passes through the steel into cathode 
         They are absorbed by the constituent of electrolyte  and combine with water and oxygen to form hydroxyl ions
         These hydroxyl ions combine with ferrous ions  to form ferrous hydroxide which is further oxidized  to ferric hydroxide


Prevention in new structure
         Corrosion can be controlled during construction by enhancing the quality of the material used and adjusting to the local environment.
  
The following approaches are commonly used:
     Corrosion inhabiting admixtures
     Coating to reinforcement
     Coating to concrete surface



Corrosion  inhabiting admixtures
         Use of corrosion inhabiting admixtures such as Nitrates, Phosphates, and Benzoates can prevent or delay corrosion.
    typical dosage :   10 – 30 lts/m3 of concrete



Coating to reinforcement


      Steel can be made more corrosion resistant by altering its structure through metallurgical process.

Ø  Material  used for coating Epoxy
Ø  zinc
Ø  Epoxy coating:

         The bar in the plant heated to a controlled temperature, epoxy powder  are deposited evenly on the surface of the bar.


Galvanized  coating:

         Dipping the steel bars in molten zinc, these results in a coating of zinc bonded to the surface of steel.



Coating to concrete:
      The protective coatings are based on acrylics Such as Silicones, chlorinated rubber, epoxy phenol which retains the breathing property of concrete.
      Coating used in the concrete serve the dual purpose of protection and decoration, increases clean ability by reducing dust.

Rehabilitation of corroded structure
Electrochemical treatment of concrete is becoming more popular as a repair and restoration option to stop corrosion and extend the service life of existing RC structures.

Ø  Electrochemical methods:

     realkalisation
     chloride extraction
Realkalisation

            used in carbonation induced corrosion




Chloride extraction

              used in treatment of chloride induced corrosion      


Electrochemical chloride extraction:
         Removes chlorides from concrete by ion migration

         Regenerates the protective passive layer around the reinforcing steel


Mechanism:
         An anode is embedded in an electrolyte media is applied to the surface of the concrete.

         The cathode is connected to reinforcing steel in the concrete

         The two terminals are connected to a DC power supply
         chloride ions being negative migrate towards the positive electrode, the anode.
         Since this is external to the concrete, the chloride content of the concrete is reduced  particularly on and around the negatively charged reinforcing steel.
         Simultaneously  the electrolytic production of hydroxyl ions at the reinforcing steel surface results in a high pH being generated around the steel.
         Thus concrete becomes chloride free and high alkaline concrete














Advantages:
         Non-destructive repair method
         Removes the cause of corrosion
         Architectural and exposed aggregate finishes can be maintained
         No need of expensive structural support
         Lower cost
         Extended service life
         Fast installation and removal
         No permanent maintenance required
         More environmentally friendly



Conclusion:
      Corrosion being one of the major problem for deterioration of concrete  the solution for this is also based on the principles of chemical reaction and electricity. 


      The ultimate aim is to solve the problem of corrosion to increase the durability of concrete structures.


      Selection of any system of prevention , protection  and remedial measures will depend on the case and exposure conditions of concrete during its service life




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