1. Corrosion
By :- Ratnadeepsinh Jadeja

2. INTRODUCTION
 Deterioration of materials by
chemical, electrochemical or
metallurgical interaction with their
environment
 Sometimes also applied to the
degradation of plastics, concrete
and wood, but generally refers to
metals

3. INTRODUCTION
• Almost all corrosion involves
electrochemical action of some kind
• Corrosion involves oxidation and reduction
• Loss of electron by metal is oxidation and
gain of electron by metal is reduction
• Oxidation involves reaction with oxygen at
high temperature, usually in absence of
moisture

4. CONSEQUENCES OF CORROSION
• More serious than the simple loss of a mass of metal
• Need for expensive replacements may occur even though the amount
of metal destroyed is quite small
The major harmful effects of corrosion can be summarized as follows:
• Reduction of metal thickness leading to loss of mechanical strength
and structural failure or breakdown
• Hazards or injuries to people arising from structural failure or
breakdown (e.g. bridges, cars, aircraft)
• Loss of time in availability of profile-making industrial equipment

5. CONSEQUENCES OF CORROSION
• Reduced value of goods due to deterioration of appearance
• Perforation of vessels and pipes allowing escape of their contents and
possible harm to the surroundings
• Loss of technically important surface properties of a metallic
component
• Mechanical damage to valves, pumps, etc, or blockage of pipes by
solid corrosion products

6. FACTORS INFLUENCING CORROSION
• Residual stress
• Voids, inclusions and dissolved gases
• Concentration and temperature of corrodant
• Existence of stray electric currents
• Surface films
• Presence of other ions in the solution
• Applied stresses
• Presence of impurities
• Gas absorb on the metal surface

7. CHEMISTRY OF CORROSION
• Metals are obtained from their ores
• These metals can therefore be regarded as being in a metastable
• Tend to lose their energy by reverting to compounds more or less
similar to their original states
• corrosion reactions are electrochemical in nature, at anodic sites on
the surface
• At a cathodic site the electrons react with some reducible component
of the electrolyte and are themselves removed from the metal
• The rates of the anodic and cathodic reactions must be equivalent
according to Faraday’s Laws

9. CHEMISTRY OF CORROSION
• The most common and important electrochemical reactions in the corrosion of
iron are thus
Anodic reaction (corrosion)
Fe→ Fe2+ + 2e
Cathodic reactions (simplified)
2H+ +2e → H2 …………………………………… (a)
H2O + 1/2 O2 + 2e → 2OH− ................(b)
• Reaction (a) is most common in acids and in the pH range 6.5 – 8.5 the most
important reaction is oxygen reduction (b).
• In (b) formation of solid corrosion debris from the reaction between the anodic
and cathodic products.

11. CHEMISTRY OF CORROSION
The rate at which attack is of prime importance is usually expressed
in one of two ways:
(1) Weight loss per unit area per unit time, usually mdd (milligrams per
square decimeter per day)
(2) A rate of penetration, i.e. the thickness of metal lost. This may be
expressed in American units, mpy (mils per year, a mil being a
thousandth of an inch) or in metric units, mmpy (millimeters per year).

12. TYPES OF CORROSION
• Direct corrosion
• Electrochemical and galvanic corrosion
• Liquid-Metal corrosion
• Corrosion of metal by gas
• Special corrosion types
 Uniform corrosion
 Atmospheric corrosion
 Pitting corrosion
 Intergranular corrosion
 Stress corrosion
 Corrosion fatigue
 Fretting corrosion
 Erosion corrosion

13. Direct corrosion
• This type of corrosion occurs mainly through the direct chemical
action of atmospheric gases such as O2, helogens, H2S, CO2, SO2, N2,
H2 or liquid metals on metal surface on the absence of moisture.

14. Electrochemical and galvanic corrosion
Electrochemical type of corrosion occurs when :
a. A metal is in contact with a conducting liquid
b. Two dissimilar metals or alloys are immersed partially in a conducting solution.
Electrochemical corrosion is due to the existence of separate anodic and cathodic
areas between which current flows through the conducting solution.

15. Electrochemical and Galvanic corrosion
Galvanic corrosion or Bimetallic corrosion:
When two dissimilar metals or alloys are electrically connected and
exposed to an electrolyte, the metal higher in electrochemical series
under goes corrosion.

16. Liquid-Metal corrosion
• Liquid Metal corrosion refers to environmental corrosion caused by
contact with a liquid metal. It is also known or as Liquid Metal
Cracking (LMC).
• There is a specific combination of liquid metals and stressed metals or
alloys that can lead to catastrophic intergranular cracking. For
example, carbon steels and stainless steels are susceptible to liquid
metal embrittlement by zinc and lithium; aluminum and aluminum
alloys are susceptible to liquid metal embrittlement by mercury and
zinc; copper and copper alloys are susceptible to liquid metal cracking
by mercury and lithium. Cracking is frequently observed to be a single
intergranular crack that propagates rapidly, at a rate of 25 cm/s.

17. Liquid-Metal corrosion

18. Corrosion of metal by gas
• Gaseous corrosion is a type of high-
temperature corrosion that occurs in
diesel engines, furnaces, gas turbines
and other machinery that interacts
with hot gas surrounded by
contaminants.
• Almost all alloys, materials and metals
of technological importance will
undergo oxidation and corrosion at
high temperatures, resulting in gaseous
corrosion. The rate, mechanism and
corrosion nature differ broadly
according to the existing temperatures
and environments.

19. Uniform corrosion
• Uniform corrosion also refers to
the corrosion that proceeds at
approximately the same rate
over the exposed metal
surface. Cast irons and steels
corrode uniformly when
exposed to open atmospheres,
soils and natural waters,
leading to the rusty
appearance.

20. Atmospheric corrosion
• Atmospheric corrosion is an electrochemical process, requiring the presence of
an electrolyte. Thin film "invisible" electrolytes tend to form on metallic surfaces
under atmospheric corrosion conditions, when a certain critical humidity level is
reached. For iron, this level is around 60%, in unpolluted atmospheres. The
critical humidity level is not a constant - it depends on the corroding material.

21. Pitting corrosion
Pitting corrosion is a localized form of corrosion by which cavities or
"holes" are produced in the material. Pitting is considered to be more
dangerous than uniform corrosion damage because it is more difficult
to detect, predict and design against. Corrosion products often cover
the pits.

22. Intergranular corrosion
• Intergranular corrosion is sometimes also called "intercrystalline
corrosion" or "interdendritic corrosion“.
• Intergranular corrosion (IGC), also known asintergranular
attack (IGA), is a form of corrosionwhere the boundaries of
crystallites of the material are more susceptible to corrosion than
their insides.

23. Stress corrosion
• Stress corrosion is another form of
corrosion that is important to many fields
including civil structures.
• Stress-corrosion occurs when a material
exists in a relatively inert environment but
corrodes due to an applied stress. The
stress may be externally applied or
residual.
• This form of corrosion is particularly
dangerous because it may not occur under
a particular set of conditions until there is
an applied stress. The corrosion is not
clearly visible prior to fracture and can
result in catastrophic failure.

24. Corrosion fatigue
Corrosion fatigue is fatigue in a
corrosive environment. It is the
mechanical degradation of a material
under the joint action of corrosion
and cyclic loading. Nearly all
engineering structures experience
some form of alternating stress, and
are exposed to harmful
environments during their service
life.

25. Fretting corrosion
Fretting corrosion refers to corrosion
damage at the asperities of contact
surfaces. This damage is induced under
load and in the presence of repeated
relative surface motion, as induced for
example by vibration. Pits or grooves
and oxide debris characterize this
damage, typically found in machinery,
bolted assemblies and ball or roller
bearings. Contact surfaces exposed to
vibration during transportation are
exposed to the risk of fretting
corrosion.

26. Erosion corrosion
Erosion corrosion is a degradation of material surface due to
mechanical action, often by impinging liquid, abrasion by a slurry,
particles suspended in fast flowing liquid or gas, bubbles or droplets,
cavitation, etc.

27. Corrosion testing
Corrosion testing method:
• Laboratory corrosion test
• Field corrosion test
Laboratory corrosion test:
1. Salt spray test:
• Standardized test method used to check corrosion resistance of
coated samples
• Accelerated corrosion test that produces a corrosive attack to the
coated samples in order to predict its suitability in use as a protective
finish

28. • The appearance of corrosion
products (oxides) is evaluated
after a period of time
• Cheap, quick, well standardized
and reasonably repeatable
• The apparatus for testing
consists of a closed testing
chamber, where a salted
solution (mainly, a solution of
5% sodium chloride) is atomized
by means of a nozzle

29. 2. Immersion in corroding media:
• Partial, alternate or total immersion in various corroding media
• Usually determine by the time required for appearance of initial
corrosion
• In other words, weight loss per time
• Stainless steel or non-ferrous are tested
3. Weathering test:
• The metal samples may be subjected to alternate wetting in
corrodants and drying

30. Field corrosion test:
• Test are carried out under those environmental condition which they
are expected to work in actual service
• Conducted in marine atmosphere, industrial, various soil condition
etc.
• Samples may be inspected periodically for loss of weight, pitting,
crack etc.

31. CORROSION PREVENTION
Three required ingredients for rust formation:
1. Water (solvent and reactant)
2. Oxygen (oxidizer reactant)
3. Electrolyte (ion transport in water)
Paint or polymer coating protect the metal and prevent corrosion but
even pinholes can lead to breakdown. By retarding either the anodic or
cathodic reactions the rate of corrosion can be reduced

32. CORROSION PREVENTION
(a) Coating the metal, in order to interpose a corrosion resistant
coating between metal and environment. The coating may consist of:
(i) another metal, e.g. zinc or tin coatings on steel,
(ii) a protective coating derived from the metal itself, e.g. aluminium
oxide on “anodised” aluminium,
(iii) organic coatings, such as resins, plastics, paints, enamel, oils and
greases.

33. CORROSION PREVENTION
(b) Alloying the metal to produce a more corrosion resistant alloy, e.g.
stainless steel
• Steel is alloyed with chromium and nickel.
• Stainless steel is protected by an invisibly thin, naturally formed film
of chromium sesquioxide Cr2O3.
(c) Cathodic Protection :
• Control the corrosion of a metal surface by making it the cathode of
an electrochemical cell
• protected metal to a more easily corroded "sacrificial metal" to act as
the anode
• The sacrificial metal then corrodes instead of the protected metal

34. Thank You