1. Corrosion is an electrochemical process involving oxidation and reduction reactions. It requires an anode, cathode, electrolyte, and an electrically conducting path. 2. At the anode, iron oxidizes to ferrous ions which then react with hydroxyl ions from the cathode to form iron hydroxide and iron oxide. The products occupy more volume than the original steel causing stresses in the concrete. 3. Chlorides from deicing salts or seawater can destroy the protective oxide layer and accelerate corrosion. Carbonation reduces concrete's alkalinity allowing the protective layer to break down.

1. CORROSION
 OXIDATION
 CORROSION
 PREVENTION AGAINST CORROSION

2. Corrosion Process
According to Newman, ―Corrosion of reinforcement embedded
in concrete is an electrochemical reaction, involving both
chemical processes and the flow of electricity between various
areas of steel and concrete
To complete a corrosion cell, an anode, a cathode, a metallic
connection between the anode and cathode, an ionic path,
moisture, and oxygen are required.

3. • At the anode, corrosion occurs through the
process of oxidation, a chemical reaction
where an electron is lost. The metallic
connection is provided by the reinforcing steel
and the ionic path is provided by the concrete
matrix (electrolyte)
Corrosion Process

4. • The driving force of corrosion is the difference in potential
between the anode and the cathode.
This potential may be created by
• 1. Differences in the surface of the steel bars. Since steel is an
alloy created
• from various elements (most notably iron and carbon), its
surface area has sites of differing electrochemical potentials.
• 2. Differences in electrolytes. These include differences in the
concentration of chlorides, oxygen, moisture, hydroxides, etc.
• 3. Presence of cracks. Cracks allow the more rapid ingress of
deteriorating
• chemicals and moisture.
Corrosion Process

5. • As corrosion occurs, the cross section of the steel is reduced
and the bond between the steel and concrete is damaged.
This loss of section and bond loss could reduces the strength
of the R/C member.
• As the cross section of the steel bar is reduced, the corrosion
by-products occupy a greater volume than the original steel.
• This increase can be up to 7 times the original volume of the
steel.
• The expansion causes tensile stresses to be exerted on the
surrounding of the concrete. As concrete is weak in tension,
the tensile forces cause local delamination ( See figures)
Corrosion Process

6. Corrosion Process

7. 1. At the anode, iron is oxidized to a ferrous state and electrons
are released.
2Fe → 2Fe+2 + 4e-
2. At the cathode, the lost electrons travel through the steel and
combine with oxygen and moisture to form hydroxyl ions O2 +
2H2O + 4e- → 4OH-
3. The ferrous ions from the anode then combine with the
hydroxyl ions from the cathode to produce ferrous hydroxide
2Fe+ + 4OH- → Fe(OH)2
4. Further oxidation, with the presence of oxygen and moisture,
produces ferric oxide
4Fe(OH)2 + 2H2O + O2 → 4Fe(OH)3
2Fe(OH)3 → Fe2O3 + 3H2O
Corrosion Process

8. Chloride Ingress and Carbonation
1. Chloride Ingress :
The presence of chlorides not only destroys the protective oxide
layer, but also fuels the corrosion process.)
Chlorides can be introduced to concrete during mixing or
service.
Calcium chloride (CaCl2) has been used as an accelerant at the
time of mixing. This facilitates the casting of concrete in cold
conditions and provides higher early strength concrete.
Chlorides may also be found in the aggregates and mixing water.
Service chloride contamination occurs because of deicing salts,
proximity to sea water, and ground water salts.

10. 2. Carbonation
The other process that causes the corrosion of reinforcing steel
is carbonation. While carbonation initially increases concrete‘s
compressive strength, modulus of elasticity, surface hardness,
and resistance to frost and sulphate attack, it has the
detrimental effect of reducing the alkalinity of the concrete.
Carbonation occurs when carbon dioxide and other gases from
the atmosphere penetrate through the surface pores and
capillaries of concrete.
When these gases react with water, carbonic acid is formed ,the
carbonic acid then reacts with the calcium hydroxide of the
hydrated cement paste to produce calcium carbonate
Corrosion Process

11. • CO2 + H2O → H2CO3
• H2CO3 + Ca(OH)2 → CaCO3 + 2H2O
• A reduction in pH occurs as the calcium carbonate does not
have a high alkalinity.
• Over time, carbonation will drop the pH levels to 8 – 9, and
the passive film will start to break down as the lower alkaline
concrete is not able to support the protective oxide layer
• Carbonation progresses inwards from the outer surface of the
concrete. Initially, the outer zone of concrete is affected and
over time, the depth of carbonation increases.
• While the rate of carbonation depends on the permeability of
the concrete, it also depends on the relative humidity (RH)
Corrosion Process

12.  A protective Cr2O3 layer forms on the surface of Fe
σ(Cr2O3) = 0.001 σ(Fe2O3)
 Upto 10 % Cr alloyed steel is used in oil refinery components
 Cr > 12% → stainless steels → oxidation resistance upto 1000o
C
→ turbine blades, furnace parts, valves for IC engines
 Cr > 17% → oxidation resistance above 1000o
C
 18-8 stainless steel (18%Cr, 8%Ni) → excellent corrosion resistance
 Kanthal (24% Cr, 5.5%Al, 2%Co) → furnace windings (1300o
C)
Alloying of Fe with Cr
Other oxidation resistant alloys
 Nichrome (80%Ni, 20%Cr) → excellent oxidation resistance
 Inconel (76%Ni, 16%Cr, 7%Fe)

13. Corrosion
THE ELECTRODE POTENTIAL
 When an electrode (e.g. Fe) is immersed in a solvent (e.g. H2O) some metal ions
leave the electrode and –ve charge builds up in the electrode
 The solvent becomes +ve and the opposing electrical layers lead to a dynamic
equilibrium wherein there is no further (net) dissolution of the electrode
 The potential developed by the electrode in equilibrium is a property of the
metal of electrode → the electrode potential
 The electrode potential is measured with the electrode in contact with a solution
containing an unit concentration of the ions of the same metal with the standard
hydrogen electrode as the counter electrode (whose potential is taken to be zero)
Metal
ions-ve
+ve

14. System Potential in V
Noble end Au / Au3+
+1.5
Ag / Ag+
+0.80
Cu / Cu2+
+0.34
H2 / H+
0.0
Pb / Pb2+
−0.13
Ni / Ni2+
−0.25
Fe / Fe2+
−0.44
Cr / Cr3+
−0.74
Zn / Zn2+
−0.76
Al / Al3+
− 1.66
Active end Li / Li+
−3.05
Standard electrode potential of metals
Standard potential at 25o
C
Increasingprope

15. Galvanic series
 Alloys used in service are complex and so are the electrolytes (difficult to
define in terms of M+
) (the environment provides the electrolyte
 Metals and alloys are arranged in a qualitative scale which gives a measure
of the tendency to corrode → The Galvanic Series
Environment Corrosion rate of mild steel (mm / year)
Dry 0.001
Marine 0.02
Humid with other agents 0.2
Galvanic series in marine water
Noble end Active end
18-8 SS
Passive
Ni Cu Sn Brass 18-8
SS
Active
MS Al Zn Mg
More reactive

16. Galvanic Cell
Anode
Zn
(−0.76)
Cathode
Cu
(+0.34)
e−
flow
Zn → Zn2+
+ 2e−
oxidation
Cu2+
+ 2e−
→ Cu
Reduction
or
2H+
+ 2e−
→ H2
or
O2 + 2H2O + 4e−
→ 4OH−
Zn will corrode at the expense of Cu

17. How can galvanic cells form?
Anodic/cathodic phases at the
microstructural level
Differences in the concentration of the
Metal ion
Anodic/cathodic electrodes
Differences in the concentration of
oxygen
Difference in the residual stress levels

18.  Different phases (even of the same metal) can form a galvanic couple at the
microstructural level (In steel Cementite is noble as compared to Ferrite)
 Galvanic cell may be set up due to concentration differences of the metal ion in the
electrolyte → A concentration cell
Metal ion deficient → anodic
Metal ion excess → cathodic
 A concentration cell can form due to differences in oxygen concentration
Oxygen deficient region → anodic
Oxygen rich region → cathodic
 A galvanic cell can form due to different residual stresses in the same metal
Stressed region more active → anodic
Stress free region → cathodic
O2 + 2H2O + 4e−
→ 4OH−

19. Polarization
 Anodic and Cathodic reactions lead to concentration differences near the
electrodes
 This leads to variation in cathode and anode potentials (towards each other)
→ Polarization
Current (I) →
Potential(V)→
Vcathode
Vcathode Steady state current
IR drop through the electrolyte

20. Passivation
 Iron dissolves in dilute nitric acid, but not in concentrated nitric acid
 The concentrated acid oxidizes the surface of iron and produces a thin protective
oxide layer (dilute acid is not able to do so)
 ↑ potential of a metal electrode → ↑ in current density (I/A)
 On current density reaching a critical value → fall in current density
(then remains constant) → Passivation

21. Prevention of Corrosion
Basic goal → • protect the metal • avoid localized corrosion
 When possible chose a nobler metal
 Avoid electrical / physical contact between metals with very different electrode
potentials (avoid formation of a galvanic couple)
 If dissimilar metals are in contact make sure that the anodic metal has a larger
surface area / volume
 In case of microstructural level galvanic couple, try to use a course
microstructure (where possible) to reduce number of galvanic cells formed
 Modify the base metal by alloying
 Protect the surface by various means
 Modify the fluid in contact with the metal
• Remove a cathodic reactant (e.g. water)
• Add inhibitors which from a protective layer
 Cathodic protection
• Use a sacrificial anode (as a coating or in electrical contact)
• Use an external DC source in connection with a inert/expendable electrode

22. UNIVERSALITY OF CORROSION
• Not only metals, but non-metals like plastics,
rubber, ceramics are also subject to
environmental degradation
• Even living tissues in the human body are
prone to environmental damage by free
radicals-Oxidative stress- leading to
degenerative diseases like cancer, cardio-
vascular disease and diabetes.

23. CORROSION DAMAGE
• Disfiguration or loss of appearance
• Loss of material
• Maintenance cost
• Extractive metallurgy in reverse- Loss of
precious minerals, power, water and man-
power
• Loss in reliability & safety
• Plant shutdown, contamination of product etc

24. COST OF CORROSION
• Annual loss due to corrosion is estimated to be 3 to 5
% of GNP, about Rs.700000 crores
• Direct & Indirect losses
• Direct loss: Material cost, maintenance cost, over-
design, use of costly material
• Indirect losses: Plant shutdown & loss of production,
contamination of products, loss of valuable products
due to leakage etc, liability in accidents

25. WHY DO METALS CORRODE?
• Any spontaneous reaction in the universe is
associated with a lowering in the free energy
of the system. i.e. a negative free energy
change
• All metals except the noble metals have free
energies greater than their compounds. So
they tend to become their compounds
through the process of corrosion

26. ELECTROCHEMICAL NATURE
• All metallic corrosion are electrochemical
reactions i.e. metal is converted to its
compound with a transfer of electrons
• The overall reaction may be split into
oxidation (anodic) and reduction (cathodic)
partial reactions
• Next slide shows the electrochemical
reactions in the corrosion of Zn in
hydrochloric acid

27. ELECTROCHEMICAL REACTIONS IN CORROSION
DISSOLUTION OF ZN METAL IN HYDROCHLORIC ACID,
222 HZnClHClZn +=+ -------------------- -(1)
Written in ionic form as,
2
2
222 HClZnClHZn ++=++ −+−+
----------------------(2)
The net reaction being,
2
2
2 HZnHZn +=+ ++
------------------------- (3)
Equation (3) is the summation of two partial reactions,
eZnZn 2*2
+→ -----------------------------------------(4) and
222 HeH →++
------------------------------------------(5)
Equation (4) is the oxidation / anodic reaction and
Equation (5) is the reduction / cathodic reaction

28. ELECTROCHEMICAL THEORY
• The anodic & cathodic
reactions occur
simultaneously at
different parts of the
metal.
• The electrode
potentials of the two
reactions converge to
the corrosion potential
by polarization

29. PASSIVATION
• Many metals like Cr, Ti, Al,
Ni and Fe exhibit a
reduction in their corrosion
rate above certain critical
potential. Formation of a
protective, thin oxide film.
• Passivation is the reason
for the excellent corrosion
resistance of Al and S.S.

30. FORMS OF CORROSION
• Corrosion may be
classified in
different ways
• Wet / Aqueous
corrosion & Dry
Corrosion
• Room Temperature/
High Temperature
Corrosion
CORROSION
WET CORROSION DRY CORROSION
CORROSION
ROOM TEMPERATURE
CORROSION
HIGH TEMPERATURE
CORROSION

31. WET & DRY CORROSION
• Wet / aqueous corrosion is the major form of
corrosion which occurs at or near room
temperature and in the presence of water
• Dry / gaseous corrosion is significant mainly
at high temperatures

32. WET / AQUEOUS CORROSION
Based on the appearance of the corroded metal, wet
corrosion may be classified as
• Uniform or General
• Galvanic or Two-metal
• Crevice
• Pitting
• Dealloying
• Intergranular
• Velocity-assisted
• Environment-assisted cracking

33. UNIFORM CORROSION
• Corrosion over the
entire exposed surface
at a uniform rate. e.g..
Atmospheric corrosion.
• Maximum metal loss by
this form.
• Not dangerous, rate can
be measured in the
laboratory.

34. GALVANIC CORROSION
• When two dissimilar metals
are joined together and
exposed, the more active of
the two metals corrode
faster and the nobler metal
is protected. This excess
corrosion is due to the
galvanic current generated
at the junction
• Fig. Al sheets covering
underground Cu cables

35. CREVICE CORROSION
• Intensive localized
corrosion within
crevices & shielded
areas on metal surfaces
• Small volumes of
stagnant corrosive
caused by holes,
gaskets, surface
deposits, lap joints

36. PITTING
• A form of extremely
localized attack causing
holes in the metal
• Most destructive form
• Autocatalytic nature
• Difficult to detect and
measure
• Mechanism

37. DEALLOYING
• Alloys exposed to
corrosives experience
selective leaching out of
the more active
constituent. e.g.
Dezincification of brass.
• Loss of structural
stability and mechanical
strength

38. INTERGRANULAR CORROSION
• The grain boundaries in
metals are more active than
the grains because of
segregation of impurities
and depletion of protective
elements. So preferential
attack along grain
boundaries occurs. e.g.
weld decay in stainless
steels

39. VELOCITY ASSISTED CORROSION
• Fast moving corrosives
cause
• a) Erosion-Corrosion,
• b) Impingement attack ,
and
• c) Cavitation damage in
metals

40. CAVITATION DAMAGE
• Cavitation is a special case
of Erosion-corrosion.
• In high velocity systems,
local pressure reductions
create water vapour
bubbles which get attached
to the metal surface and
burst at increased pressure,
causing metal damage

41. ENVIRONMENT ASSISTED CRACKING
• When a metal is subjected to a tensile stress
and a corrosive medium, it may experience
Environment Assisted Cracking. Four types:
• Stress Corrosion Cracking
• Hydrogen Embrittlement
• Liquid Metal Embrittlement
• Corrosion Fatigue

42. STRESS CORROSION CRACKING
• Static tensile stress and
specific environments
produce cracking
• Examples:
• 1) Stainless steels in hot
chloride
• 2) Ti alloys in nitrogen
tetroxide
• 3) Brass in ammonia

43. HYDROGEN EMBRITTLEMENT
• High strength materials
stressed in presence of
hydrogen crack at
reduced stress levels.
• Hydrogen may be
dissolved in the metal
or present as a gas
outside.
• Only ppm levels of H
needed

44. LIQUID METAL EMBRITTLEMENT
• Certain metals like Al and
stainless steels undergo
brittle failure when
stressed in contact with
liquid metals like Hg, Zn,
Sn, Pb Cd etc.
• Molten metal atoms
penetrate the grain
boundaries and fracture
the metal
• Fig. Shows brittle IG
fracture in Al alloy by Pb

45. CORROSION FATIGUE
• Synergistic action of
corrosion & cyclic
stress. Both crack
nucleation and
propagation are
accelerated by
corrodent
• Effect on S-N diagram
• Increased crack
propagation
AirAir
CorrosionCorrosion
log (cycles to failure, Nf)
StressAmplitude
Log (Stress Intensity Factor Range,  K
log(CrackGrowthRate,da/dN)

46. PREVENTION OF CORROSION
• The huge annual loss due to corrosion is a
national waste and should be minimized
• Materials already exist which, if properly
used, can eliminate 80 % of corrosion loss
• Proper understanding of the basics of
corrosion and incorporation in the initial
design of metallic structures is essential

47. METHODS
• Material selection
• Improvements in material
• Design of structures
• Alteration of environment
• Cathodic & Anodic protection
• Coatings

48. MATERIAL SELECTION
• Most important method – select the
appropriate metal or alloy .
• “Natural” metal-corrosive combinations like
• S. S.- Nitric acid, Ni & Ni alloys- Caustic
• Monel- HF, Hastelloys- Hot HCl
• Pb- Dil. Sulphuric acid, Sn- Distilled water
• Al- Atmosphere, Ti- hot oxidizers
• Ta- Ultimate resistance

49. IMPROVEMENTS OF MATERIALS
• Purification of metals- Al , Zr
• Alloying with metals for:
• Making more noble, e.g. Pt in Ti
• Passivating, e.g. Cr in steel
• Inhibiting, e.g. As & Sb in brass
• Scavenging, e.g. Ti & Nb in S.S
• Improving other properties

50. DESIGN OF STRUCTURES
• Avoid sharp corners
• Complete draining of vessels
• No water retention
• Avoid sudden changes in section
• Avoid contact between dissimilar metals
• Weld rather than rivet
• Easy replacement of vulnerable parts
• Avoid excessive mechanical stress

51. ALTERATION OF ENVIRONMENT
• Lower temperature and velocity
• Remove oxygen/oxidizers
• Change concentration
• Add Inhibitors
– Adsorption type, e.g. Organic amines, azoles
– H evolution poisons, e.g. As & Sb
– Scavengers, e.g. Sodium sulfite & hydrazine
– Oxidizers, e.g. Chromates, nitrates, ferric salts

52. CATHODIC & ANODIC PROTECTION
• Cathodic protection: Make the structure more
cathodic by
– Use of sacrificial anodes
– Impressed currents
Used extensively to protect marine structures,
underground pipelines, water heaters and
reinforcement bars in concrete
• Anodic protection: Make passivating metal
structures more anodic by impressed potential. e.g.
316 s.s. pipe in sulfuric acid plants

53. COATINGS
• Most popular method of corrosion protection
• Coatings are of various types:
– Metallic
– Inorganic like glass, porcelain and concrete
– Organic, paints, varnishes and lacquers
• Many methods of coating:
– Electrodeposition
– Flame spraying
– Cladding
– Hot dipping
– Diffusion
– Vapour deposition
– Ion implantation
– Laser glazing

54. Corrosion Management Strategies
• Factors governing Corrosion prevention methods
1. the level of chloride contamination
2. carbonation,
3. amount of concrete damage,
4. location of corrosion activity (localized or widespread),
5. the cost and design life of the corrosion protection
system,
6. the expected service life of the structure

55. Corrosion Management Strategies

56. 1. Sealers and Coatings
• The application of protective sealers and coatings helps to
prevent the initiation of corrosion. Properly applied
sealers and coatings do offer a significant increase in life
expectancy when installed before contamination of the
concrete.
• Sealers work by chemically reacting with the components
of concrete to fill the pores; thus, making it difficult for
water to penetrate the concrete surface. However, this
also inhibits water vapor from exiting the concrete
• Barrier protection by creating a physical barrier between
the concrete and the environment.
Corrosion Management Strategies

57. 2. Admixed Corrosion Inhibitors
• Admixed corrosion inhibitors, which are added to the
concrete at the time of mixing, are used to prevent the onset
of corrosion in R/C.
Corrosion Management Strategies
Corrosion Inhibitors
Cathodic
Inhibitors,
Mixed inhibitorsAnodic Inhibitors

58. • Anodic inhibitors work by stabilizing the protective film of the
concrete.
• It does so by interfering with the conversion of the ferrous
oxide to ferric oxide.
• The most commonly used anodic inhibitor is calcium nitrate.
By reacting with chlorides, higher concentrations of chlorides
are necessary for the initiation of corrosion.
• When using anodic inhibitors, using too low of a
concentration in aqueous environments has a possibility of
producing pitting corrosion
Corrosion Management Strategies

59. • Cathodic inhibitors work by reducing the amount of oxygen in
the concrete. However, cathodic inhibitors require a large
amount of material and are therefore impractical for use in
concrete. Furthermore, some cathodic inhibitors slow the
setting time of concrete
Corrosion Management Strategies

60. Surface Treatment (Coatings)
• Organic paints
• Chromating and phosphating:
– The Process - chromating and phosphating are surface-coating processes that enhance
the corrosion resistance of metals. Both involve soaking the component in a heated bath
based on chromic or phosphoric acids. The acid reacts with the surface, dissolving some
of the surface metal and depositing a thin protective layer of complex chromium or
phosphorous compounds
• Anodizing (aluminum, titanium)
– The Process - Aluminum is a reactive metal, yet in everyday objects it does not corrode or
discolor. That is because of a thin oxide film - Al2O3 - that forms spontaneously on its
surface, and this film, though invisible, is highly protective. The film can be thickened
and its structure controlled by the process of anodizing. The process is electrolytic; the
electrolyte, typically, is dilute (15%) sulfuric acid. Anodizing is most generally applied to
aluminum, but magnesium, titanium, zirconium and zinc can all be treated in this way.
The oxide formed by anodizing is hard, abrasion resistant and resists corrosion well. The
film-surface is micro-porous, allowing it to absorb dyes, giving metallic reflectivity with
an attractive gold, viridian, azure or rose-colored sheen; and it can be patterned. The
process is cheap, an imparts both corrosion and wear resistance to the surface.

61. • Electro-plating
– The Process -Metal coating process wherein a thin metallic coat is
deposited on the workpiece by means of an ionized electrolytic
solution. The workpiece (cathode) and the metalizing source material
(anode) are submerged in the solution where a direct electrical
current causes the metallic ions to migrate from the source material
to the workpiece. The workpiece and source metal are suspended in
the ionized electrolytic solution by insulated rods. Thorough surface
cleaning precedes the plating operation. Plating is carried out for
many reasons: corrosion resistance, improved appearance, wear
resistance, higher electrical conductivity, better electrical contact,
greater surface smoothness and better light reflectance.
Surface Treatment (Coatings)

62. Bluing:
Bluing is a passivation process in which steel is
partially protected against rust, and is named
after the blue-black appearance of the resulting
protective finish. True gun bluing is an
electrochemical conversion coating resulting from
an oxidizing chemical reaction with iron on the
surface selectively forming magnetite (Fe3O4), the
black oxide of iron, which occupies the same
volume as normal iron. Done for bolts called
“blackening”
Surface Treatment (Coatings)

63. Hot-dip Coating (i.e. galvanizing)
– Hot dipping is a process for coating a metal, mainly ferrous metals,
with low melting point metals usually zinc and its alloys. The
component is first degreased in a caustic bath, then pickled (to
remove rust and scale) in a sulfuric acid bath, immersed (dipped) in
the liquid metal and, after lifting out, it is cooled in a cold air stream.
The molten metal alloys with the surface of the component, forming a
continuous thin coating. When the coating is zinc and the component
is steel, the process is known as galvanizing.
– The process is very versatile and can be applied to components of any
shape, and sizes up to 30 m x 2 m x 4 m. The cost is comparable with
that of painting, but the protection offered by galvanizing is much
greater, because if the coating is scratched it is the zinc not the
underlying steel that corrodes ("galvanic protection"). Properly
galvanized steel will survive outdoors for 30-40 years without further
treatment.

64. CONCLUSION
• Corrosion is a natural degenerative process
affecting metals, nonmetals and even
biological systems like the human body
• Corrosion of engineering materials lead to
significant losses
• An understanding of the basic principles of
corrosion and their application in the design
and maintenance of engineering systems
result in reducing losses considerably