corrosion and various types

1. Corrosion
Definition:
• As per IUPAC,
• “Corrosion is an irreversible
interfacial reaction of a material
(metal, ceramic, polymer) with its
environment which results in its
consumption or dissolution into the
material of a component of the
environment. Often, but not
necessarily, corrosion results in
effects detrimental to the usage of
the material considered.
Exclusively physical or mechanical
processes such as melting and
evaporation, abrasion or mechanical
fracture are not included in the term
corrosion”
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2. Corrosion
Types of corrosion
• Corrosion can be classified
• Chemical and
electrochemical
• High temperature and low
temperature
• Wet corrosion and dry
corrosion.
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3. Each form of corrosion has a specific arrangement of anodes and cathodes
and specific patterns and locations depending on the type can exist.
Types
• Uniform corrosion.
• Galvanic corrosion,
• concentration cells,
• water line attack
• Pitting.
• Dezincification
• Dealloying (selective leaching)
• Atmospheric corrosion
• Erosion corrosion
• Fretting Crevice corrosion
• cavitation
• Stress corrosion,
• intergranular and transgranular corrosion,
• hydrogen cracking and embrittlement
Corrosion fatigue.
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4. Corrosion
Crevice corrosion
• Crevice corrosion is a
localized attack on a metal
adjacent to the crevice between
two joining surfaces (two
metals or metal-nonmetal
crevices). The corrosion is
generally confined to one
localized area to one metal.
This type of corrosion can be
initiated by concentration
gradients (due to ions or
oxygen). Accumulation of
chlorides inside crevice will
aggravate damage
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5. Crevice corrosion
Factor affecting crevice corrosion
• Materials: alloy composition
• metallographic structure.
• Environmental conditions such as
pH, oxygen concentration
• Halide concentrations,
• temperature.
• Geometrical features of crevices
• surface roughness
• Metal to metal or metal to
nonmetal type.
• Filiform corrosion is a special
type of crevice corrosion
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6. Corrosion
Pittig corrosion
• Pitting corrosion is a localized
phenomenon confined to smaller areas.
Formation of micro-pits can be very
damaging. Pitting factor (ratio of deepest
pit to average penetration) can be used to
evaluate severity of pitting corrosion
which is usually observed in passive
metals and alloys.
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7. Uniform corrosion
Uniform corrosion
• Uniform corrosion is a very
common form found in
ferrous metals and alloys
that are not protected by
surface coating or inhibitors.
A uniform layer of „rust‟ on
the surface is formed when
exposed to corrosive
environments Atmospheric
corrosion is a typical
example of this type.
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8. Galvanic corrosion
• Galvanic corrosion often referred to
as dissimilar metal corrosion occurs
in galvanic couples where the active
one corrodes. EMF series
(thermodynamic) and galvanic series
(kinetic) could be used for prediction
of this type of corrosion. Galvanic
corrosion can occur in multiphase
alloys.
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9. Corrosion
Intergranular corrosion
• Localised attack at or nearer to grain
boundaries in a metal or alloy can be
termed as intergranular corrosion.
Generally the following factors contribute
to intergranular corrosion.
• Impurities and precipitation at grain
boundaries. Depletion of an alloying
element (added to resist corrosion) in the
grainboundary area.
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10. Corrosion
Erosion corrosion:
• Erosion corrosion is the
deterioration of metals and alloys
due to relative movement between
surfaces and corrosive fluids.
Depending on the rate of this
movement, abrasion takes place.
This type of corrosion is
characterized by grooves and
surface patterns having
directionality. Typical examples
are
• Stainless alloy pump impeller,
Condenser tube walls.
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11. Corrosion
Fretting corrosion
• Corrosion occurring at contact regions
between materials under load subjected
to slip and vibration can be termed
Fretting. Such friction oxidation can
occur in engine and automotive parts.
Fretting is known to occur at bolted tie
plates on rails.
• Parameters promoting fretting include:
• Relative motion between two surfaces.
Interface under load.
• Both the above produce slip and
deformation of surfaces. Wear-
oxidation and oxidation-wear theories
are proposed to explain fretting
corrosion.
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12. Selective leaching (Dealloying) refers to selective dissolution of active metal
phase from an alloy in a corrosive environment.
Examples
• Brass containing copper and zinc.
Since zinc is anodic to copper,
selective dezincification occurs in
a corrosive medium, enriching the
cathodic copper in the matrix
(colour of brass turns red from
yellow).
• Graphitization of grey cast iron-
graphite being cathodic enhances
dissolution of iron in the matrix,
leaving behind a graphite
network.
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13. Examples
• Tin Bronzes in hot brine or steam-
Destannification.
• Precious metal alloys such as
gold containing copper or silver –
strong acids, sulfide environment
- preferential dissolution of
copper or silver.
• Cupro-nickel alloys in condenser
tubes-denickelisation
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14. Erosion corrosion
• Erosion corrosion is the
deterioration of metals and
alloys due to relative
movement between surfaces
and corrosive fluids.
Depending on the rate of this
movement, abrasion takes
place. This type of corrosion
is characterized by grooves
and surface patterns having
directionality. Typical
examples are
• Stainless alloy pump impeller,
Condenser tube walls.
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15. • All equipment types
exposed to moving fluids
are prone to erosion
corrosion
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16. Prevention of corrosion
Approaches
• The choice of any one control
technique depends on
economics, safety aspects and
other technical considerations.
• Design
• Materials selection
• Protective coatings
• Inhibitors and environmental
alterations
• Corrosion allowances
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17. Proper design of equipment
• In the design of equipment,
fittings such as baffles, valves
and pumps to be considered
• Elimination of crevices
Complete drainage of liquids
• Easy to clean
• Facilitate easy access to
inspection and maintenance
• Avoid bimetal contacts –
Insulation of Joints.
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18. Coatings
• Barrier between corrosive
environment and metal.
• Coatings may serve as sacrificial
anodes (zinc on steels ) or release
substances that resist corrosion.
• Metal coatings - Silver, copper,
nickel, chromium, tin, lead on
steels (ensure pore - free,
uniform, adherent coating;
favorable anode / cathode ratio to
minimize galvanic attacks).
• coatings of – Zinc, aluminium,
cadmium on steels. (steel is
cathodic to plated metal).
• Coatings can be applied through hot
dipping, hot spraying, electroplating,
electroless plating, vapour deposition
and metal cladding.
• Aluminium, stainless steel, titanium,
platinum etc can be cladded on
various metallic substrate for
enhanced corrosion protection
(physical or chemical).
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19. Types of paint coatings
• Good adhesion, flexibility,
impact resistance and
protection from chemicals,
moisture, and atmospheric
conditions.
•
• Lacquer – synthetic resins (vinyl
chloride, acrylic, rubber).
• Latex (Acrylics and Vinyls)
• Oil-based and Epoxy coatings
(good bending, hard and flexible)
• Coal – tar – epoxy.
• Poly – urethanes, polyester and
vinyl ester (hard, brittle or
elastomeric).
• Organic zinc rich coatings (organic
barrier + galvanic Zn protection)
• Co-polymeric protective
coatings.(thermoplastic –
copolymer - aromatic coatings).
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20. Corrosion
Type
• Stress corrosion cracking)
• Corrosion fatigue
Prevention stratergies
• More resistant alloys.
Remove tensile stress,
control of environment
(elimination of chlorides
• Eliminate cyclic stress and
corrosive environment.
More rigid design to reduce
stresses due to
vibrations. Avoid stress
concentration in design.
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21. Type
• Hydrogen embrittlement
• Galvanic Corrosion
Prevention Stratergies
Choice of less – susceptible alloy
/ coatings. Avoid cathodic
protection (steels in acid
Environments)
Selection of metals / alloys closer
in galvanic series. Favorable
cathode to anode ratio.
Coating taking care not to
create smaller anodes with
larger cathodes, insulation of
dissimilar joints
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22. Type
• Crevice corrosion
• Erosion corrosion and
cavitation
Prevention strategies
• Proper design of junctions and joints
to minimize crevices. Welded joints
preferable to rivets and bolts. Pitting
and crevice corrosion are enhanced in
stagnant / slow flowing solution.
Provide drainages.
• Design to reduce velocity and
turbulence, avoid abrupt changes in
flow directions.
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23. Biocorrsion
• Biomaterials engineering deals
with scientific study of the
structure and properties of
biomaterials in relation to their
interaction with biological
systems. Biomaterials have to
interface with human
biological systems so as to
evaluate, augment or replace
any organ, tissue or function of
the body.
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24. • Body temp:37 C
• pH :7.4
• Chemical composition of
body fluids: contains water,
dissolved oxygen, sodium,
chloride, bicarbonate,
calcium, potassium,
magnesium, phosphate,
amino acids, proteins,
plasma, lymph, saliva and
various organic compounds
• Various biological molecules
influence the electrochemical
behavior of implant metals and
alloys. Anodic and cathodic
reactions are influenced by
adsorption and interaction of such
organic polymers. For example,
protein adsorption on implant
materials can interfere with
oxygen and hydrogen redox
reactions. Besides, bacteria
present in the human body can
also influence the corrosion
behavior of implant materials.
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25. Biocompatibility
A biocompatible (bio) material
should not disrupt normal body
function to any significant
level.
• Many factors such as size of implant,
material properties and surface-
chemical characteristics influence
biocompatibility.
• Long-term stability of metal implants
critical for patient health and survival.
• - Stents
• -Arthroplasty
• - Fracture fixation
• - Pacemakers
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26. Ideal implant materials should be
• Inert
• Nontoxic to body
• Corrosion resistant
• Inexpensive
• Strong
• Fatigue resistant
Biocorrosion can lead to
• Bone loss
• Structral deformity
• Osteolysis
• Metal accumulation in body
fluids (nickel, cobalt and
chromium) impart toxicity
(metal sensitization). Retrieval
of wear particle remnants from
the body fluids can help with
failure analysis and
development of newer implant
materials.
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