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CORROSION OF METALS
➢ Corrosion
• To one degree or another, most materials
experience some types of interaction with a large
number of diverse environments
...

➢ Corrosion in materials
• Deteriorative mechanisms are different for the
three material types
• In metals, there is actual material loss either by
the solution which we call corrosion or by
deformation of nonmetallic scale or film or what
we call oxidation
...

The process is frequently also called corrosion
...

Polymers may dissolve when exposed to liquid
solvent or they may absorb the solvent as well
...

➢ What is Corrosion?
• “Process of surface degradation in metals due to
an Electrochemical reaction”
• Why is Corrosion a Concern to Metallurgist?
- Corrosion leads to a loss of:
• Cosmetic/ aesthetic value (face)
- Corrosion is the destructive and
unintentional attack on a metal, it is
electrochemical and ordinarily
begins at the surface
...
If the
reinforcement which is the rebar, is
corroded it will no longer perform
its function to give support to the
bridge
...
It has been estimated
that approximately 5% of an
industrialized nation’s income is
spent on corrosion prevention and
the maintenance or replacement of
products lost or contaminated as a
result of corrosion reaction
...

• Corrosion Kills

-

Undetected corrosion caused the
collapse of the Point Pleasant Bridge
over the Ohio River in 1967 and the
Mianus River Bridge in Connecticut
in 1983
- Forty-nine fatalities resulted from
these bridge failures
➢ The Elements of Corrosion
• What is an Electrochemical Reaction?
- Chemical reaction in which there is a transfer
of electrons from one chemical species to
another
- Reactions which are both chemical and
electrical in nature
- Also known as Redox Reactions
- Redox Recalled
• Metal atoms characteristically lose or
give up electrons in what we call
oxidation reaction
• Oxidation is sometimes called an anodic
reaction

•

-

Redox- A Deadly Combination
- This is made possible when we have
a conductive bridge connecting the
sources of these two reactions
- Example: Oxidation or Rusting of
Iron in Water which contains
dissolved oxygen
...
1st step: Iron is
oxidized to Fe2+
...
So, iron rust faster in
salt water than in fresh water
What Happens to Corrosion at High
Temperature?
• Usually, a temperature or pressure
increase directly leads to a higher
corrosion rate because electrochemical
reactions generally occur faster at higher
temperatures
...

• Corrosion occurs faster at high
temperature

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Corrosion of Metals
• All metals tend to oxidize
• Tendency differs from one metal to the
next
How does one know of a metal’s tendency to
oxidize?
• “Check the Electromotive Series for a
metal’s half cell potential”

•

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•

•

•

Half reactions as reduction reactions
with the electrons on the left-hand side
of the chemical equation
...

Corrosion rate
- Rate of material removal as a consequence of
the chemical action is an important corrosion
parameter
- This may be expressed as the corrosion
penetration rate (CPR): Thickness of metal
loss per unit of time
...
Familiar examples include
general rusting of steel and iron and the
tarnishing of silverware
• Probably the most common form of
corrosion
- Galvanic corrosion
• Occurs when two metals or alloys having
different compositions are electrically
coupled while exposed to an electrolyte
• The less noble or more reactive metal in
the
particular
environment
will
experience corrosion
...
If copper and steel tubings
are joined in a domestic water heater, the
steel will corrode in the vicinity of the
junction
Crevice corrosion
• Localized attack on a metal surface at, or
immediately adjacent to, the gap or
crevice between two joining surfaces
...
Usually, the width is
several thousands of an inch
Pitting corrosion
• Another form of very localized corrosion
attack in which small pits or holes form
...
Often going undetected and
with very little material loss until failure
occurs
Intergranular corrosion
• Occurs preferentially along grain
boundaries for some alloys and in
specific environments
• The net result is that a macroscopic
specimen disintegrates along its grain
boundaries
• Especially prevalent in some stainless
steels when heated to temperatures 500
C and 800 C for sufficiently long period
of time these alloys become sensitized to
intergranular attack
Selective leaching
• Found in solid solution alloys and occurs
when one element or constituent is
preferentially removed as a consequence
of corrosion processes
• Most common example is the
dezincification of brass in which zinc is
selective leached from a copper-zinc
brass alloy
...
In addition, the material
changes from a yellow to a red or copper
color
• Selective leaching may also occur with
other alloy systems in which aluminum,
iron, cobalt, chromium, and other
elements are vulnerable to preferential
removal
Erosion corrosion
• Arises from the combined action of
chemical attack and mechanical abrasion
or wear as a consequence of fluid motion

•

Virtually, all metal alloys to one degree
or another are susceptible to erosioncorrosion
...
The abrasive
action may erode away the film leaving
exposed a bare metal surface
...

Relatively soft metals such as copper and
lead are also sensitive to this form of
attack
...

- Stress corrosion
• Aka stress corrosion cracking
• Results from the combined action of an
applied tensile stress and a corrosive
environment
• In fact, some metals that are virtually
inert in a particular corrosive medium
becomes susceptible to this form of
corrosion when a stress is applied
...
While immerses in sea
water, stress corrosion cracks formed
along the bent region where the tensile
stresses are the greatest
...
This phenomenon is aptly referred
to as hydrogen embrittlement
...

Brittle
fracture
occurs
catastrophically as crack grow and
rapidly propagate
• Hydrogen in its atomic form diffuses
interstitially through the crystal lattice,
and concentrations as low as several
parts per million can lead to cracking
• Similar to stress corrosion in that a
normally ductile metal experiences
brittle fracture when exposed to both
tensile stress and a corrosive atmosphere
• High strength steels are susceptible to
hydrogen embrittlement and increasing
strength tends to enhance the material’s
susceptibility
➢ Corrosion Control

•

•

•

•

In most of the situations, metal corrosion can be
overseen, moderated or even halted by utilizing
the best possible systems
...
) Cathodic and Anodic Protection
• Cathodic Prevention
- Galvanic corrosion – typical
issue for metals submerged
together in seawater
...
Therefore,
galvanic corrosion prevention
regularly assaults sand bodies,
seawater apparatuses and oil and
gas pipelines
- Works by changing over
undesirable anodic sites on a
metal surface to cathodic
destinations
through
the
utilization of a restricting current
...
Example: tin is known for
anodic protection
...

- Involves coating of metal to
shield another metal (tin on steel)
- Called anodic protection due to
the fact that it makes the steel the
anode of an electrochemical cell
2
...
g
...
g
...
) Corrosion Inhibitors
• Chemicals that respond with the
metals surface or ecological gasses
causing corrosion protection
• The inhibitors procedure of abating
corrosion relies of 3 things:
1
...
) By decreasing the dispersion of
particles on the metal’s surface
3
...
) Design Modifications
• Can play a vital role when it comes to
corrosion prevention
• Process
can
assist
corrosion
prevention and increase the strength
of any existing protective anticorrosive coatings
• For best results: Metals should avoid
trapping dust and water, boost the
flow of air and avoid open holes
• Metal is easily accessible for routine
maintenance will also improve
longevity
...
) Metal Selection and Surface Conditions
• Materials Selection
- Most popular and critical method
of managing corrosion which
involves the selection of the right
and perfect materials for
particular corrosive environment
- Corrosion prevention behavior of
each metal and alloy is different
and also subjective
...
One simple way to
prevent corrosion is the use of

anti-corrosion metal such as
aluminum or stainless steel
...

The alteration of the corrosive
environment can be taken down
by decreasing the temperature,
diminishing velocity, separating
oxygen or oxidizers and
improving
concentration
...

6
...
The formation of patina
on copper is similar to the formation
of rust on iron
...
) Plating
• Process of covering one metal onto
another by hydrolysis
• A thin layer of metal frequently
nickel, tin or chromium, is kept onto
the substrate metal
• Chromium plating, copper plating,
silver plating



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