Corrosion and its process in RC Structures

1. CORROSION OF
REINFORCEMENT

2. Corrosion:
• Corrosion can be defined as “Chemical or electrochemical
reaction between a material (usually a metal) and its
environment that produces deterioration of the material and
its properties”.
• It is the destruction of materials due to CHEMICAL REACTION with
environment and also LOSS OF STEEL due to rust formation.
• It is the conversion of IRON INTO OXIDES AND HYDROXIDES in the
presence of oxygen and water.
• Corrosion deteriorates concrete because the product of corrosion – FERRIC
OXIDE (brown color) occupies a greater volume than steel and exerts
BURSTING STRESS on surrounding concrete.

9. CORROSION MECHANISM:

10. CORROSION MECHANISM
Chemical
- Alkalinity of the concrete gets reduced to less than 10.0 by the
ingress of CO2 or the passive layer gets destroyed by the ingress of
chloride.
Electrochemical
- Cathodic and Anodic sites are formed resulting in the flow of
current with moist concrete serving as the electrolyte. In this process
the rate of corrosion is influenced by the oxygen supply.
Physical
-Rust (Corrosion product) experiences a volume growth as high as six
to seven times the original corroding metal. This volume growth
exerts physical expansive forces to the concrete surrounding the steel.

15. CORROSION PROCESS
• The corrosion process of reinforcement embedded in
concrete has two distinct periods namely, initiation period
and propagation period.
FINAL STATE
INITIATION PERIOD
PROPAGATION
OF CORROSION
CHANGE OF
CONCRETE PROPERTIES
CORROSION PERIOD
DEPASSIVATION
SERVICE LIFE
TIME
QUALITY
OF
CORROSION
PRODUCTS
Service life model for corrosion affected structures

16. Propagation period
•After the initiation of corrosion, the propagation
begins and this period has two distinct processes.
•One is that the corrosion follows an electrochemical
process and the other is the physical process due to
which damage to concrete occurs.
•During the propagation period, the corrosion
progresses at a rate depending on the availability of
oxygen and moisture.

33. Influencing factors
•The environment to which the structure is exposed
•The cover thickness
•Quality of cover concrete in terms of its alkalinity,
permeability and diffusion characteristics
•The type of steel
•Critical chloride level in concrete
•Presence of cracks
•pH value in concrete

36. DAMAGES OR EFFECTS DUE TO CORROSION:
• White patches
• Brown patches
• Occurrence of cracks
• Formation of multiple
cracks
• Spalling of cover of concrete
• Snapping of bars
• Buckling / Bulging of bars

38. CORROSION PROTECTION TECHNIQUES
 Coating to reinforcement
 Galvanized reinforcement
 Improving metallurgically by addition of certain elements
 Using stainless steel
 Using non-ferrous reinforcement
 Using corrosion inhibitors
 Coating to concrete
 Cathodic protection, either by means of impressed unit or
by sacrificial anodes.
 Electrochemical chloride removal
 Improving the cover concrete

39. METHODS OF CORROSION PROTECTION:
• Coating to reinforcement
• Galvanized reinforcement
• Improving metallurgically by addition of certain elements.
• Using Non-ferrous reinforcement
• Corrosion Inhibitors.
• Coating to concrete
• Cathodic protection
• Improving the concrete.

40. Coating to Reinforcement
• Coating- Steel bar- durable barrier to aggressive materials such as
chlorides.
• Initially the mill-scale is removed
• It provides adequate bond between epoxy and the steel.
• The bar is then heated to a controlled temperature.
• Electrostatically charged epoxy powder are deposited evenly on
the surface of the bar.
• The covering thickness 130 micron to 300 micron.

42. CATHODIC PROTECTION:
• Control corrosion of surface immersed in water or soil
• Cannot be used in surface exposed to atmosphere.
• Galvanising is a form of cathodic protection
• TYPES
1. Protection with galvanic anodes.
2. Current cathodic protection.

43. Protection with galvanic anodes:
• Purpose is to make the metal surface act as cathode.
• This is achieved by placing this cathode metal with another
corrodible anodic metal.
• Sacrifice method – since anodic material corrodes and sacrifices
to protect cathode material.

44. IMPRESSED CURRENT CATHODIC PROTECTION:
• Its an alternative source of direct electrical current usually a
rectifier that converts alternating current to direct current.
• Circuit is completed with an inert anodic material that is not
consumed in the process.
• Graphite and high silicon cast iron.

45. CORROSION RESISTING STEEL:
• Cement slurry coating – cement + silica
• Epoxy coating – Epoxy resin thro’ fusion.
• Plastic coating .
• Galvanising – zinc coating.

46. CORROSION INHIBITORS

47. Corrosion Inhibitors
•A substance which reduces the corrosion
•Types
- Anodic inhibitors
- Cathodic inhibitors
- Vapour phase inhibitors

48. ANODIC INHIBITORS
• Prevents corrosion reaction occuring at the anode.
• Forms an insoluble compound with the newly produced metal ions.
• Adsorbed on the surface- protective coating- reduces corrosion rate.
• Eg: Chromates, nitrates, phosphates, tungsten
• ADVANTAGE
* Effective
• DISADVANTAGE
* Dangerous – local attack

49. CATHODIC INHIBITORS
• Cathodic reaction are of two types:
- In an acidic solution
- In an neutral solution
• Acidic Solution
1) Evolution of hydrogen
2H+ + 2e- H2
2) Corrosion can be reduced in two ways:
- Slowing down diffusion of H+ ions to the cathode - adding
organic inhibitors
- Increasing the over voltage of hydrogen evolution – adding
antimony and arsenic oxides.

50. CATHODIC INHIBITORS
• Neutral Solution
H2O + ½ O2 + 2e- 2OH-
- eliminating the oxygen from neutral solution
thereby formation of OH- ions are inhibited.
(adding Na2SO3, N2H4)
- eliminating OH- ions from neutral solution
(adding Mg, Zn)

51. VAPOUR PHASE INHIBITORS
• Organic inhibitors- readily vaporize and form a protective layer on the
metal surface.
• Used in the protection of storage containers, packing materials,
sophisticated equipments etc.

52. Sodium benzoate as inhibitor
• It is a reinforcement coating method.
• In the patented process 2 % sodium benzoate is used in the
mixing water or a 10% benzoate cement slurry is used to paint
the reinforcement or both.
• Sodium benzoate is also act as an accelerator for compressive
strength.

53. Calcium nitrite as inhibitor
• Commonly adopted inhibitor
• Added to the concrete during mixing
• Typical dosage is of the order of 10-30 litres per m3 of concrete
depending on chloride levels in concrete.
• Calcium nitrite – liquid form containing about
30 % calcium nitrite solids by weight.
• Since most structures in a chloride environment reach a level of
about 7 kg of chloride ion per m3 , use of less than 18lit/m3 of
calcium nitrite is not recommended.

54. Mechanism
• In high PH of concrete , the steel is protected by a passive layer of
ferric oxide.
• Passive layer also contain ferrous oxide which initiate corrosion
when it react with chloride ions
• Calcium nitrate act as an inhibitor
• In which nitrite ions oxidize the ferrous oxide to ferric oxide thus
stabilizing the passive layer even in the presence of chloride