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CHEMMAT 121

Course summary for chemmat 121
by

Joshua Wilkinson

on 2 November 2015

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Transcript of CHEMMAT 121

CHEMMAT 121
AIM: To understand basic materials structure and properties in order to maximise their effectiveness
Materials:
METALS
CERAMICS
POLYMERS
COMPOSITES
BIO-MATERIALS

METALS
STRUCTURE
ATOMIC STRUCTURE
Metals assume a crystal structure

BCC
a= 4R/root3
No close-packed planes
Therefore more brittle than BCC

FCC
a= 4R/root2
Has a close packed plane (111)
Therefore super ductile

HCP
All slip planes in same direction making slip difficult.
VERY BRITTLE
Hexagonal Close-Packed
Face Centered Cubic
Body Centered Cubic
There structure is the first basis of its properties
STRUCTURAL DEFECTS
PLANAR DEFECTS
3 types of defects but
planar defects
have largest effect
Massive effect on strength as only
one set of bonds is broken in order for slip to occur
SCREW DISLOCATION
EDGE DISLOCATION
SOLID SOLUTIONS
Although metals are solids their atoms still diffuse about the lattice via
VACANCY DIFFUSION
or
INTERSTITIAL DIFFUSION

ALLOYING
Alloys are a mix of two or more elements and by having multiple phases in a solid we generate more 'boundaries' thus dislocations movement is inhibited.

If substitutional or interstitial atoms distort the parent lattice
dislocations require more energy to move through the lattice
The
more boundaries the better the prevention of dislocation movement
thus a fine dispersion of phases is best.
We use Phase Diagrams to determine how we can produce each alloy (page 37)

Note: Video on left show how to use phase diagrams and also are a link to many other topics
EXAMPLE METAL
STEEL
Steel is an Iron-Carbon alloy and it's behaviors are mapped by the iron-carbon phase diagram
Gamma or Austenite
FCC Structure therefore Ductile
Exists above 723*C
Non Magnetic
Alpha or Ferrite
BCC Structure therefore less ductile than Austenite
Exists between
room temp and approximately 910*C
Magnetic

Cementite
3
No slip systems so is very brittle and hard
A molecular compound

(Fe C)
Follow video to the left to revise other areas such as the development of pearlite.

Development of pearlite structure
Forms when the iron carbon
mix is slowly cooled from above 723*C
3
However the slower it is cooled the thicker the layers will be thus the weaker the material will get.
Each layer acts as a boundary to the other to
prevent dislocation movement thus increase strength.
Transformation is very slow as carbon must diffuse out forming
thick layers of Ferrite and thin layers of Fe C
STRENGTHENING
COMPOSITES
STRUCTURE
MICRO-STRUCTURE
Composites consist of a
combination of
either a
METAL, POLYMER or CERAMIC.
2 TYPES OF COMPOSITES
There are 2 types of composite arrangements:
NANO TECHNOLOGY
STRENGTHENING
It consist of normally only 2 phases the
MATRIX and the FIBRE

POLYMERS
STRUCTURE
ATOMIC STRUCTURE
Polymer Chain Types
Also one other type of polymer called a copolymer
The type of chain the polymer forms has a big effect on its mechanical properties
Polymer Isomers
There are 2 types of isomers mentioned:

POLYMERIZATION
There is 3 main ways to create polymers:

ELASTOMERS
Altering Properties
With Additives
MANUFACTURING OF
POLYMERS
Polymers are solids consisting of long chain molecules
These chains can be linked via covalent or weak Van der waals forces
Linear polymers have mer groups attached end to end .
Pack more tightly than Branched molecules so experience greater van der waals forces (HIGH CRYSTALINITY)
Branched polymers have mer groups attached end to end and mer groups attached to its sides.
Pack less tightly than Branched molecules so experience less van der waals forces meaning they're not as strong (CRYSTALINITY DEPENDENT ON WHETHER ATACTIC OR ISOTACTIC)
Cross-linked polymers have mer groups attached end to end and mer groups attached to its sides, these branches form links between chains thus covalently bonding them.
This makes them very strong.
Polymer Chain Types
Copolymers have more than one mer group within their structure.
MICRO-STRUCTURE
GRAIN BOUNDARIES
Grain boundaries have a significant effect on properties as they stop dislocation movement.
The hall petch equation calculates the yield stress depending on grain size (d)
Yield stress increases with smaller grain size(mm)

Ductile-Brittle Transition Temperature
BCC metals experience a Ductile-Brittle transition temperature
T

(similar to that of Thermoplastic glass transition temperature)

Above this temperature metal experiences increasing ductility.
T
This is because the atoms have enough thermal energy to overcome the energy required to slide a plane of atoms over another .
Thermosets and Thermoplastics
Thermoset Plastics
form cross-linked polymers
when made.
When they are cooled they
cannnot be re-heated
to soften as these cross-links have an equal chance of breaking as all other bonds in the polymer.
They are
very strong and rigid
due to the covalent bonding of chains

Thermosets:
Thermosplastics are held together by weak van der waals forces or secondary bonding.
This allows them to be re-heated as the secondary bonds break first allowing the chains to slide freely
They are
very soft and Ductile.

Ideal for processing and recycling.

Thermoplastics:
Glass Transition Temperature
Amorphous component of thermoplastic has a glass transition temperature.
Below this temperature the polymer does not have enough thermal energy for chains to easily slide past each other and thus is brittle and hard.
Crystaline and Amorphous Structures
Crystaline polymers and polymer molecules are highly ordered and thus can fit closely together and thus experience greater secondary bonding and greater strength.
Amorphous solids are also generally transparent
as light can pass through them. Crystalline solids are not
Atactic
Isotactic
Random arragement of branches either side of linear chain.
Forms an amorphous solid
as it cannot pack tightly into an ordered structure.
All branches on one side of linear chain.
Forms a highly crystaline solid
as it can pack tightly into an ordered structure.
ADDITION
NETWORK
POLYMERIZATION
CROSS-LINKING
The addition reaction occurs when a catalyst species is added to a mer group (e.g ethylene).
This breaks the double bond freeing the 2 electrons.
The electrons then bond with another mer group with it's double bond broken.
Forming a very long polymer chain
Produces a
3D cross-linked polymer
provided the mer has 2 sites that can react with other mers.
In the example below the
double bonded oxygen breaks off from the methanal
and
removes 2 hydrogens from the Phenol group.
The remaining
CH group bonds with the electrons
released after the hydrogens
in Phenol
are removed
This
forms a link between mer groups
, this happens multiple times creating a 3d network.
2
Formadehyde
Phenol
Cross-linked Polymer
+
=
+
H O
2
A
process known as VULCANISATION
is the process used to convert polymers into more durable materials

With the
addition of SULFUR double bonds are broken and 2 sulfurs bind to it and the adjacent broken double bond of another chain
.

This
forms cross-links between chains making it difficult for permanent chain-sliding to occur

In this course we will only be talking about using this process to convert NATURAL RUBBER to ELASTOMERIC RUBBER

Are generally
thermoplastics above their T
that have sufficient cross-linking.

G
They can
exhibit large non-linear elastic behaviour
(500-1000% Strain) with full recovery
Structural Requirements for Elastomeric Behaviour
Long Linear Chains
Amorphous Solid
Sufficient density of crosslinks
Must be above their T
G
Allow material to stretch much further
Secondary bonds of a crystalline structure prevent stretching. So must be majority amorphous
Polymer chains must be in constant thermal vibration (HIGH ENTROPY)
More crosslinks = Less Stretch
We need cross-links to hold the structure together though

Thus we need a balance between too much and too little
Thermoplastic Behaviour
Above T
Thermoplastics above their T have both a viscous behavior and also a elastic behavior.
To take this into account we use a combination of 2 models.
G
G
Elastic Model
Viscous Model
Models stretch of material with full retraction
Models material being strained with permanent chain sliding
occurring
Modelling Thermoplastic
Behaviour
EXAMPLE METAL
STEEL
Our ideal steel alloy is often
TEMPERED MARTENSITE
as it has
good ductility and also optimum strength.
To get tempered martensite we must
quench it down to room temperature
so as to prevent the carbon diffusing out to
Hold at a temperature at below 723*C to allow the carbon to diffuse out and form cementite precipitates.
(Fe C)
3
MARTENSITE
NEEDLE LIKE STRUCTURE
BCT structure (no slip systems so hard and brittle)
Metastable
so does not appear on phase diagram
No diffusion required.
Formed by
displacive transformation

TEMPERED MARTENSITE
Ductile Ferrite Matrix
Cementite precipitates
dispersed within it.
PPT halts dislocation movement
thus leading to an overall stronger material
The
Maxwell
model depicts a thermoplastic that stretches out but once unloaded
it does not fully recover due to permanent chain sliding .
The Kelvin-Voigt model depicts a thermoplastic that when stetching out and when unloaded. Its
stretching out and unloading motion is damped by viscous behavior however it fully recovers

STRAIN RELAXATION
The MAXWELL model is used to describe strain relaxation in thermoplastics.
When material is instantaneously strained while above its Tg the initial strain will reduce as permanent chain sliding takes place thus 'strain relaxation' occurs
.
0
e
-t/
Formula to find the stress at time t
E
Viscosity
Young's Modulus
Relaxation Time
MOST COMMON
FILLERS
PLASTICIZERS
STABILIZERS
FLAME RETARDANTS
REINFORCING ADDITIVES
Used to
improve strength
or simply
increase volume
.
Low molecular weight organic liquids added to polymer to reduce its T and make processing easier.

Done by
reducing secondary bond interactions
thus
enabling them to slide past each other
.

This
effectively lowers the T of the thermoplastic.

G
G
For Example:
Carbon Black is added to rubber to increase it's tear and abrasion resistance.
Added to
prevent thermal degradation
due to heat and exposure to UV radiation.
For example
Carbon Black absorbs UV and re-emits it as harmless radiation.
Also Zinc Oxide can scatter and reflect UV
Most
polymers are flammable
hence we can add
substances such as Cl or metallic salts
to
reduce the risk of combustion.
Strength and stiffness
in polymers can be
greatly increased with the addition of fibers from other materials.

This relates to COMPOSITES
For Example:
The addition of
glass fibers, carbon fibers
can greatly increase stiffness and strength
Extrusion Molding
Mainly
used for molding THERMOPLASTICS
, can be used for THERMOSETS but is rare
Compression Molding
Mainly
used for molding THERMOPLASTICS
, can be used for THERMOSETS but is rare
Injection Molding
Very versatile molding process,
can process both THERMOPLASTICS and THERMOSETS

Mainly
used for molding THERMOPLASTICS.
Blow Molding
THE MATRIX
THE FIBRE
The matrix phase is
usually metal or polymer to provide the composite ductility
.
The matrix also
binds the fibers together
and provides a
medium by which applied stress is transmitted through the matrix.
THIS BOND BETWEEN FIBRE AND MATRIX IS VERY IMPORTANT FOR COMPOSITE STRENGTH!

If matrix is ceramic, fibers are added to provide fracture resistance.
+
The fiber phase is normally the
strongest and stiffest component
and thus ideally should
carry the maximum proportion of the load.

For maximum strength the fibers should be aligned in direction of stress.

NOTE: Composites can also exist as simple laminates such as plywood (mix of soft and hard wood)
LAMINATES
FIBER-REINFORCED
Consist of layers of different materials glued together.
Layers ideally arranged perpendicular to applied stresses.
Examples include plywood (layers of soft and hard-wood)
Consist of fibers embedded in a matrix phase (no distinct layering as such)
Fibers ideally arranged adjacent to applied stresses.
Laminated Fiber Reinforced Composite
Combining both of them we can get a material like that below that gives them the best of both designs.
Force Transmission
MATRIX MUST TRANSFER MOST OF THE STRESS TO FIBERS.
This
force is transferred
to fibers
by shear forces between the fiber and the matrix
. Therefore we
need to make the bond between this interface as strong as possible.
Force to pull fiber out = x Interface Area
= x π x (Diameter of Fiber) x(Length Of Fiber)
= x πdL

= Given value (mPa)

Strengthening with Bond Strength in mind
In Tension
the
bond between fiber and matrix
must be broken therefore in tension we
want to maximize the bond strength
thus maximizing the composites strength.

Thus we want to
maximize the interface area in order to achieve greatest bond strength.


This is
done by decreasing diameter of the fiber
(Interface area proportional to inverse of diameter).




The most practically thin fiber we can obtain to maximize interface area are NANO FIBERS.

Nano fibers are the perfect fiber for composite materials under tension
, this is because they
have a very high surface area relative to its size
and also the
fiber structure itself can be perfect/without defects thus the fiber can reach it's huge theoretical strength.
CERAMICS
STRUCTURE
ATOMIC-STRUCTURE
Ceramics are held together by a mixture of covalent or ionic bonds.
ATOMIC STRUCTURE
2 MAIN IONIC STRUCTURES :
FRACTURE TOUGHNESS
TOUGHENING CERAMICS
OTHER PRODUCTION
TECHNIQUES
STRENGTHENING
ATOMIC STRUCTURE
IONIC BONDING
COVALENT BONDING
Consist of
+ve metallic Cations
bonded by surrounding
-ve non-metallic anions
and vice versa.
VERY STRONG
NOT DIRECTIONAL
Ionic structure dictated by the
NEED TO BLANCE CHARGES
and also the
RELATIVE SIZES OF CATIONS AND ANIONS
Occurs when valence electrons are shared between atoms,
These bonds are
THE STRONGEST
and are also
HIGHLY DIRECTIONAL
.
Whether it forms permanent or temporary dipole-dipole attractions is determined by the molecule's shape primarily as this influences it's symmetry.
Covalent bonded structures are either molecular compounds bonded by either permanent or temporary dipole-dipole attractive forces.

ROCK SALT STRUCTURE
General Formula:
AB
Co-ordination Number:
6
General Formula:
ABO
PEROVSKITE STRUCTURE
3
The perovskite structure can create what is known as a PIEZOELECTRIC Material.
PIEZOELECTRIC MATERIALS
As shown in the video below the
compression of a perovskite material where the central atom is displaced
will
induce a voltage across the material
due the the movement of the central atom over many of these unit cells.
The rock salt structure is essentially
2 interlocking FCC structures
.
Eg: NaCl, MgO, FeO
REFRACTORIES:
Materials that retain their strength at high temperature.
Used for things involving high heat such as furnaces.
MAIN COVALENT STRUCTURE
SILICATE STRUCTURE
The silicate structure is based on the silicate molecule shown on the left.
If
only some of them are joined
you
get chains, rings and sheet structures
such as the ones in the left .
If
all the oxgens are joined
you can get big
3D networks
such as quartz .

SHEET STRUCTURE
The bonds
within the sheet
are
VERY STRONG COVALENT BONDS.
The bonds
between sheets
are
VERY WEAK van der waals forces .
This gives the material
inherently low abrasion resistance.
Examples include Clay and Talc
NOTE: Joined oxygen's form incredibly strong covalent bonds
CONCRETE
Concrete is an example of a very important engineering ceramic.

It is made from a combination of of ceramic materials, Lime (CaO), Silica (SiO ), Alumina (Al O ), Iron Oxide (Fe O )
2
2
3
2
3
In Compression Cement is VERY STRONG
In Tension concrete is WEAK

Thus we often use concrete as a composite by reinforcing it with ductile Rebar, as shown on the right.
WEAKNESSES
IONIC STRUCTURES ARE PARTICULARLY BRITTLE
As shown in the diagram above when a ionic structure is stressed great enough to cause stress the cations of one plane line up with the cations of another as do the anions, this causes a repulsion effect thus forcing layers apart
Elastic and primarily plastic deformation is SEVERELY LIMITED
COVALENT AND IONIC STRUCTURES ARE VERY STIFF
BOTH COVALENT AND IONIC BONDS ARE VERY STRONG AND THUS BOTH COVALENTLY AND IONICLY BONDED STRUCTURES ARE VERY STIFF
WHY THEORETICAL STRENGTH IS NEVER REACHED
Brittle Materials are tested by bending them and their strength is called 'Flexual Strength' measured in MPa.
Theoretically ceramics have a VERY HIGH STRENGTH
, however this strength is
not reached due to micro imperfections.
Cracks, Holes and Pores are all major defects that have major implications on the materials strength.

Calculating the Flexual Strength
BEND
=
O
e
-nP
Constant
Perosity
Constant
Why do Imperfections reduce Strength
Cracks, Holes and Pores are all major defects that have major implications on the materials strength.

=
F
__
A
Area is reduced due to pores, holes and cracks such that the material experiences a greater than necessary Stress.
Essentially imperfections act as catastrophic stress concentrators

Critical Fracture Stress
Also Stress Intensity Factor
Geometric Factor
Applied Stress
Length of Crack
If the stress intensity factor ever reaches Kc the crack will rapidly propagate through the material resulting in failure
3 Methods:
Densification
Reinforce
Transformation Toughening
The aim of
densification is to reduce the porosity
of the ceramic thus increasing it's toughness.
Densification
can only occur during production
using a process called powder pressing followed by sintering.
Powder Pressing
Sintering
Powder pressing takes a quantity of
powdered ceramic material and compresses it under large pressures to form a solid shape.


Ideally the
pressure should be applied evenly
thus we use what is known as
ISOSTATIC PRESSING.
Sintering is used to densify a green compact.
This
process occurs via diffusion
and the predominant
driving force is to reduce surface area
. Ideally
powders should be used as they have a very high surface area
Because sintering requires diffusion, it
occurs at high temperatures
and thus the
rate that sintering occurs can be modeled by an Arrhenius equation
Rate Equation For Sintering
dP = 1 . Ae
-Q
________
RT
______
____
G
n
dt
P = Density
t = Time
G = Particle Size
A,n = Constants
Qs = Activation Energy
R = Universal Gas Constant
T = Temperature Kelvin


s
We can
increase the ductility and toughness
of a ceramic by turning it into a composite by
introducing ductile fibers to the brittle ceramic matrix

This is done in concrete reinforcing.
Zirconia ZrO is a polymorphic ceramic
2
Addition of a stabiliser (Y O )

preserves
the
cubic phase
down to room temperature.
If
less than
12 %
is added a second phase will be preserved.
This
phase should be monoclinic
but some (
3% bigger
)
tetragonal grains are also preserved because they are constrained by the cubic matrix
. This is a
metastable
ceramic.

As
cracks form
the
metastable tetragonal grains swell up to monoclinic grains
thus
closing the crack
and thus
increasing the ceramics toughness
.
2 3
HYDROPLASTIC FORMING
SLIP CASTING
FIRING
HYDROPLASTIC FORMING
With the
addition of water to ceramic particles the particles become very plastic.

For example Clay, we add water to allow it to be easily molded into shape.
SLIP CASTING
With the
addition of more water to ceramic particles the mixture turns into a slurry.

This
slurry is poured into the mold.

The
water diffuses out.

A
layer of solidified ceramic forms on the outer edge.
The
excess is poured out leaving a near finished mold.
FIRING
Once the ceramic has solidified in the desired shape it must be fired.
This
drives off the water and joins the ceramic particles together.
One of the processes that occurs is VITRIFICATION.
During VITRIFICATION some of the
ceramics components melt
and thus turn into a liquid, this
liquid flows between the solid particles and thus fills the gaps.
This is mainly used for pottery and whiteware.

CERAMIC EXAMPLE
GLASS
A type of ceramic with no long range order, most organic glasses are based on silica
TYPES OF OXIDES:
GLASS FORMING
These oxides can form networks with no long range order.
For example Silica and Borax
Remember the example for sheet structures
INTERMEDIATE
Can't form oxides on their own but can join in existing networks
For Example:
Lead oxide PbO (Lead Glass)
NETWORK MODIFIERS
Disrupt networks
to
give the glass particular properties.
(E.g lower viscosity)
For Example:
NaO (SODA)
CaO (Lime)
CERAMIC EXAMPLE
GLASS
Forming Glass
Viscosity
Properties Of Glass
Manufacture
Forming Glass
Whether a structure ends up glassy or crystalline
depends a lot on the cooling rate.
Cooled rapidly it forms a glass structure
When molten SiO is
cooled slowly it forms a crystal structure

2
Why?
The longer the molten solution has to cool the more time the solution has to line up in a highly ordered/crystalline arrangement
This Glassy state is also called the Amorphous or Vitreous State
Strain Point:
Fracture will occur before plastic deformation (brittle)
Annealing Point:
Diffusion rapid enough to remove residual stresses
Softening Point:
Temp Glass can be handled without deformation
Working Point
Viscosity low enough to be molded into shapes
Viscosity Equation
= e
Q
RT
___
O
VISCOSITY ( Pa.s)
ACTIVATION ENERGY (JOULES)
UNIVERSAL GAS CONSTANT
TEMPERATURE (KELVIN)
CONSTANT
Properties Of Glass:
EXCELLENT OPTICAL PROPERTIES
HIGH CHEMICAL STABILITY
ELECTRICAL INSULATOR
EXTREMELY BRITTLE

THERMAL INSULATOR:
NOTE:
THERMAL EXPANSION OF GLASS CAN BE ALTERED BY VARYING THE COMPOSITION, THUS CAN BE DESIGNED TO EXPAND WITH STEEL.
If we do not want tempered glass we use production techniques that finish with the glass being annealed to eliminate residual stresses.
Production Processes:
Float Process:
Blow Molding:
Usually used to produce flat glass
Usually used to produce bottle type items
Improving Fracture Toughness of Glass
This process
relies on the idea of putting the surface layer into compression to close any cracks
and thus
stop their propagation

Prince Rupert Drops
Critical Fiber Length
Due to fibers being expensive we must use them efficiently
Thus we have the concept of critical fiber length Lc:
This is the point at which increases in fiber length give you no gains in strength and thus you have a proportion of that fiber being redundant.
Calculating Stress Distribution
C
=
Stress Applied
/ sometimes the critical stress
V =
Volumetric Fraction of Matrix
(e.g 0.75)
=
Stress the particular material (Fiber or Matrix) = experiences
/ critical stress for that material
C
m
V =
Volumetric Fraction of Fiber
(e.g 0.25)
f
=
V
m
m
+
V
f
f
Calculating Isostrain Stiffness
C
=
Stress Applied
/ sometimes the critical stress
V =
Volumetric Fraction of Matrix
(e.g 0.75)
=
Stress the particular material (Fiber or Matrix) = experiences
/ critical stress for that material
C
m
V =
Volumetric Fraction of Fiber
(e.g 0.25)
f
=
V
m
m
+
V
f
f
E = Young's Modulus / Stiffness
= Strain experienced
E
C
E
m
E
f
Calculating Specific Modulus
The Specific Modulus represents the
strength to weight ratio
for a given material
E = Stiffness of material (calculated previously)
P = Density of material
Specific Modulus =
E
C
C
__
P
Failures in Composites
Tensile Failure
Compression Failure
Tensile Failure
Compression Failure
When a composite undergoes tensile failure it must propagate a crack across the composite.
To do this the
adhesive bonds between fiber and matrix must break
, this
requires a huge amount of energy and thus makes composites very tough.
The Crack will
either de-bond all the way up to one end of the fiber and back down or if the stress is great enough the fiber will break.
Under compression a typical composite (ductile matrix + stiff fibers) will simply collapse under compression
Once reaching this point a co-ordinated collapse of all fibers occurs creating 45* shear angles (as shown in the diagram below)
CORROSION
Concept of Corrosion
Corrosion Process
The corrosion process occurs when a metal atom leaves it's lattice forming an ion that then gets surrounded by water molecules.

This Process creates what is known as the HDL layer.

This layer allows an equilibrium to be reached when the HDL opposes further movement of metal ions. When this occurs the number of metal ions leaving the the lattice and the number of ions being pulled back onto the the lattice
Corrosion Driving Force
Using the hydrogen cell as a reference, we get a Electrode Potential for each type of electrode system.
The higher the the electrode potential the less reactive /(more inert) it becomes, the lower the electrode potential the more reactive /(more base) it becomes.

Rules:
The one with the
highest electrode potential will more easily oxidise
The one with the
lowest electrode potential will more easily be reduced

The
greater the difference between the 2 electrode potentials the greater the driving force for corrosion (HAS NO EFFECT ON RATE OF CORROSION)
Anodic and Cathodic Reactions
REMEMBER: O.I.L. R.I.G. AND C.C.R. A.A.O.
(OXIDATION IS LOSS ETC FROM SCHOOL)
Thus the
anodic reaction is oxidation
and the
cathodic reaction is reduction
.
Hence using a metal as an example:
M
ANODIC
M + e
+
-
M + e
+
-
M
CATHODIC
The Polarization Diagram
NET CURRENT
Voltage
E
m/m
+
OXIDATION
REDUCTION
When the electrode is changed from it's equilibrium potential it is said to be polarized and thus will be an anode or a cathode depending on it's polarization .
Note: The shallower the curve the less resistance to corrosion
The Polarization Diagram
Combining 2 cells (i.e Oxygen and Iron cells.) means one will polarize the other and vice-versa.
This gives us a corrosion volage which in turn gives us a exchange current density i
0
Differential Aeration Corrosion
The most important corrosion system is that where a metal is oxidized by Oxygen
When we have this system the
anodic reaction occurs in area of POOREST OXYGEN ACCESS
UNIFORM CORROSION
Cathodic and Anodic reactions occur uniformly along surface.
Therefore:
i = i
a c
TYPES OF CORROSION
PITTING CORROSION
Metal is coated in an Oxide layer
which prevents passage of Fe ions but can conduct electrons.
If this layer has
any breaks
along it
we will get pitting corrosion.
Leaves us with essentially a surface crack
Under Tensile Stress this could be a significant Problem
(DISCUSSED IN CERAMICS & FRACTURE MECHANICS).
This could create put the metal into a situation in which it could mean that 'a' is large enough that the stress intensity factor reaches it's critical value

Also note that if under tensile load this crack will suffer more intense corrosion due to it being a high energy area due to high stress.
(DISCUSSED SOON).
Also note that the
more conductive the electrolyte
the
faster the corrosion will take place
, this is because the
voltage lost across the resistive solution
means their is
less potential difference to drive the corrosion process.
Dissimilar Metals In Contact
Using this principle we can get 2 different results
Cathodic Protection
Corrosion Prevention
Anodic Protection
Anodic protection can be achieved by polarising a metal above it's Em.
Leaves us with essentially a surface crack
Under Tensile Stress this could be a significant Problem
(DISCUSSED IN CERAMICS & FRACTURE MECHANICS).
This could create put the metal into a situation in which it could mean that 'a' is large enough that the stress intensity factor reaches it's critical value

Also note that if under tensile load this crack will suffer more intense corrosion due to it being a high energy area due to high stress.
(DISCUSSED SOON).
Sacrificial Protection
Galvanic Attack
Sacrificial Protection
PREVENTS CORROSION OF IMPORTANT METAL
ENHANCES CORROSION OF IMPORTANT METAL
Galvanic Attack
Occurs when a
smaller area of the more reactive metal
is in contact with a
greater area of more inert metal
As the less reactive metal corrodes it
positively polarizes the more reactive metal.
This causes it to corrode faster than when it was alone
Big problem in instances where the main metal has been fastened down by bolts of the more reactive metal.
Occurs when a
larger area of the more reactive metal
is in contact with a
smaller area of more inert metal.
As the less reactive metal corrodes it negatively
polarizes the more inert metal below it's equilibrium potential
thus preventing it from corroding.
The most reactive metal will corrode first and then the more inert metal will corrode
FARADAY'S EQUATION
Gives rate of metal loss in a corrosion process or rate metal is deposited
m = mass in grams of metal lost/deposited
I = current in Amps
t = time in seconds
M = Atomic mass of metal (g/mol)
n = number of electrons given away or accepted
F = Faraday's Number (96,500 C/mol)
m =
I t M
nF
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Sacrificial Anode Method
Impressed Current Method
The aim of this is to
polarise the protected metal negatively so as to prevent the anodic reaction.

Sacrificial Anode Method
By
connecting a more reactive metal to a protected metal
we can polarize the protected metal below it's 'Em' and thus it cannot undergo it's anodic reaction.
The electrons given off by the attached sacrificial metal are sent to the protected metal and thus polarize it negatively.
Impressed Current Method
Another more long long-lasting method is the impressed current method.
This method polarises the protected metal negatively by simply
attaching it to a DC voltage source
and is
often used where the sacrificial anode method can't provide enough current.

Ideally the anode should not corrode at all (i.e be made of platinum)
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