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Metal Corrosion and its Prevention
Mechanical Engg. Department
SRES, Sanjivani College of Engineering,
Kopargaon. (MS) 423603
• Corrosion is natural phenomenon which attacks
metal by chemical, electrochemical action converts
metal into metallic compound , such as an oxide ,
hydroxide or sulphate.
• Destruction or deterioration and consequent loss of
solid metallic materials , starting at its surface due to
chemical or electro chemical attack, by their
• Corrosion is major problem worldwide in all
• It impacts on safety , health and environmental
Causes of Corrosion
• Pure or noble metals such as gold, silver, platinum,
aluminum , copper do not corrode since they are
chemically uncombined in their natural state.
• Following conditions must exist before corrosion.
i) Presence of metal that will corrode (anode)
ii) Presence of cathode
iii) Presence of conductive liquid (electrolyte)
iv)Electrical contact between
anode and cathode
• Elimination of any one of these
condition will reduce or stop
Classification of Corrosion
• on the basis of working
i) Low temperature Corrosion
ii) High temperature Corrosion
• on the basis of working
i) Dry or Chemical Corrosion
ii) Wet or Electrochemical
i) Dry or Chemical Corrosion
• It occurs when oxygen in air reacts with metal ,
without presence of liquid.
• It is very sensitive to temperature
• Rate of dry corrosion varies from metal to metal ,
as a result of mechanisms involved.
• Oxide layer on steel & iron is known as rust.
• In aluminum , copper oxide layer formed due to
reaction with atmospheric oxygen ,stops further
• As oxide layer formed , prevents further contact of
oxygen as film is non porous.
• This stopping is known as Passivations.
• Active Corrosion- steel, CI , Mg.
ii) Wet or Electrochemical Corrosion
• It is electrochemical phenomenon , which occurs
in galvanic cell.
• Particularly when two metals are in contact with
a liquid , liquid containing salt and electric
potential is formed between the metals.
• in some cases in single metal , when comes in
contact with liquid or electrolyte , a anode and
cathode areas are created.
• When anode area starts corroding with respect
to cathode areas , is termed as wet corrosion.
Mechanism of Corrosion
• Electrochemical reaction – transfer of electrons
• Electrochemical corrosion cell- Electrical circuit and its
associated chemical reaction are called
electrochemical corrosion cell.
i) Anode- Oxidation occurs, metal dissolution takes place
& production of electrons
ii) Cathode – electrons are consumed & reduction
iii) Metal Path – movement of electron from
anode to cathode
iv) Electrolyte- ionic current flows
Anodic & cathodic reactions in corrosion cell
2Fe Fe e+ −
→ + 2 22 4 4O H O e OH− −
+ + →
At Anode-Oxidation At Cathode -reduction
• Type of Metal
• Heat treatment & Grain direction
• Presence of dissimilar metal
• Anode & cathode surface area
• presence of electrolyte
• Availability of oxygen
• time of exposure to corrosive environment
Factors Influencing Corrosion
Types of corrosion
1. Pitting Corrosion- Aluminum, Mg alloys, Cu alloy.
Mechanism of pitting Corrosion
• Factors influencing pitting Corrosion
1. Alloying elements
2. Surface finish
2.Stress corrosion & season cracking
• It is combined action of corrosive environment
and mechanical stresses which often due to poor
design or poor maintenance practice.
• Stress corrosion cracking (SCC) is intergranular
cracking of metal caused by stress& corrosion.
Mechanism of stress corrosion cracking
Transgranular cracking in SCC
Control or Prevention of SCC
1. Control of stress level and Hardness.
2. Avoid chemical corrosion that causes SSC
3. Control of temperature
4. Modification in design , that will reduce SCC
3. Cavitation Corrosion
• Protective films are removed from metal
surface during fluid flow.
• Low pressure development due to
discontinuity in flow path , causes vapour
bubbles to form in fluid steam.
• Due to collapsing of bubbles shock
waves produced and removes protective
film from metal surface
Cavitation damage in pump impeller
Control of cavitation Corrosion-
1. Careful material selection i.e. high alloy steels containing
Ni, Cr, W
2. Use of soft rubber coating , it absorbs cavitation energy
3. Smooth Surface finish
4. Hydrogen Embrittlement
• Hydrogen blisters results from hydrogen atoms ,
formed during sulfide corrosion process on steel.
• Hydrogen atom
combines to form
Hydrogen blisters on steel surface
Mechanism of hydrogen blistering
2 3 4 23 4 4Fe H O Fe O H+ → +
5. Inter Granular Corrosion
• Form of corrosion where boundaries of grain of
metal are more susceptible to corrosion than
• Difference in Corrosion
potential between grain
and grain boundary.
e.g. CuAl2 in Al alloys,
Cr23C6 in stainless steel.
Inter granular corrosion in Austenitic stainless steel
• Formation of chromium carbide precipitants
(Cr23C6) at grain boundary occurs when stainless
steel of type 300 are heated to a temperature of
400 to 9000 C in rolling mill or fabrication or
Non sensitized condition sensitized condition
Microstructure of 304 S-S
Prevention of Intergranular Corrosion
Knife Line Attack (KLA)
• Use of low grade stainless steel (304 L, 316L grades)
due to which less carbon will be available for
precipitation of chromium carbide.
• Use of stabilized grade of stainless steel (321, 322
• Use of post weld treatment.
6. Crevice Corrosion
• Localized attack that occurs in restricted areas ,
gaskets, flanges, fasteners, lap joints.
• Occurs between two metal plassets.
Prevention of crevice corrosion
• Avoid crevice during fabrication i.e. smooth
• avoid solution get into crevice i.e. apply
grease to nuts and bolts.
• routine cleaning
• Apply external coating.
7. Erosion corrosion
• It results from loss of film under high velocity or high
turbulent fluid flow.
• Corrosion reaction which is accelerated by relative
movement between corrosive fluid and metal surface.
• High Turbulence intensity
• impact velocity , angle, relative density between
particle and fluid,
erosion corrosion of
control of erosion corrosion
• selection of resistant materials- high wear
resistance , high hardness
• Decreasing flow velocity
• minimizing the turbulence
8. General or Uniform Corrosion
• It is attack over entire exposed surface or large area of
• Metal loss is distributed uniformly across exposed
9. Galvanic Corrosion
• It occurs when two metals , with different compositions
are connected in presence of electrolyte.
• Current will flow from anode to cathode.
• Rate of galvanic corrosion depends on difference in
control of Galvanic corrosion
• selection of metals that are close together in
• Insulate metal from each other
• Painting of both metals
1. Active corrosion protection
• To influence reactions which proceed during
• e.g. Addition of inhibitors to aggressive medium.
• By mechanically isolating package contents from
aggressive corrosive agents by using protective
layers , films, coatings.
• This method neither changes ability of package
contents to corrode nor aggressiveness of
2. Passive corrosion protection
3.Permanent corrosion protection
• To provide protection at the place of use.
• e.g. stable oxide layer (nonporous film)forming
• Stresses occurring during transport, handling,
storage are much greater than those occuring at
place of use.
• e.g. extreme variation in temperature which
results in condensation during transportation.
4. Temporary Corrosion protection
• Chemicals that react with metallic surface &
gives certain level of protection.
• inhibitors has chemical bonding with base
metal , it is more adherent to base metal , due
to which corrosion reaction stopped.