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Slide1
T207 – Tutorial block 4
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https://learn1.open.ac.uk/mod/oucontent/view.php?id=8322Slide2Slide3
T207 – Tutorial 05
Parts 1 - 3
Corrosion
Material PropertiesSlide4
Corrosion
List the different types of corrosion.
Suggest ways of preventing the types of corrosion.Slide5
Corrosion
What is corrosion?
What causes corrosion?
What is necessary for corrosion to occur?Slide6
Corrosion
Corrosion is an electro chemical process that removes electrons from one material and passes them to another.
Corrosion is caused by having two dis-similar metals in contact through an electrolyte and results in an electric current passing between them. Slide7Slide8
Corrosion
There are FOUR basic requirements in order for corrosion to occur
An ANODE
A CATHODE
An ELECTROLYTE (a solution that will allow electrons to flow
An ELECTRIC CURRENT (or flow of electrons)Slide9
Anode & Cathode
Anode
Loss of electron in oxidation
Oxidation always occurs at the anode
Cathode
Gain of electron in reduction
Reduction always occurs at the cathodeSlide10
Corrosion
The chemical process is dependent upon the Galvanic series and Electro-potential
These series show the electrical potentials (as measured against a datum, such as hydrogen) that are usually listed in order of their NOBILITY (or likelihood to corrode)Slide11Slide12
Corrosion
Other corrosion mechanisms
See section 4
of Block 5 Part 1
Pitting
corrosion (doesn’t require 2 dissimilar metals – usually a scratch in a protective surface)Slide13
Corrosion
Other corrosion mechanisms
See section 4
of Block 5 Part 1
Pitting
corrosion (doesn’t require 2 dissimilar metals – usually a scratch in a protective surface)Slide14
Corrosion
Other corrosion mechanisms
See section 4
of Block 5 Part 1
Stress-corrosion cracking (combination of a tensile stress, chemical environment and a material susceptible to chemical attackSlide15Slide16
Corrosion
Other corrosion mechanisms
Crevice corrosion (occurs where conditions suggest corrosion shouldn’t take place; a crevice where moisture can collect and stagnate)Slide17
Corrosion
Other corrosion mechanisms
Erosion
and cavitation
corrosion (fast flowing fluids where low pressures cause air bubbles to collapse)Slide18
Preventing corrosion
You need to remove at least one of the 4 requirements for corrosion to occur
This can be in the form of a layer of protective material, sacrificial metal to corrode etc.Slide19
Preventing corrosion
Use the same metals (not dis-similar)
Avoid crevices where moisture can collect and stagnate
Avoid water accumulation by sealing joints etc.
Joint design
Welding design (double V joints smoothed over)
Coatings, painting etc.
Sacrificial metalsSlide20
Polymers
Polymers can degrade due to
Elevated temperatures for prolonged periods
UV degradation
Chemical degradationSlide21
UV light and polymers
UV light can attack the long molecular chains, breaking them down.Slide22
Chemical degradation of polymers
Usually associated with elevated temperatures and an acidic or alkaline environmentSlide23
Material Properties
How does the atomic structure of metals differ from ceramics?
How does this affect their mechanical properties?Slide24
Metals
Metals are made of a crystalline structure that is fairly simple and repetitive.
They have been investigated through X ray diffraction techniques and the images analysed to ascertain the underlying crystalline structureSlide25
Basic crystalline structuresSlide26
Basic crystalline structures
Face centred cubic
Usually more ductile than other structuresSlide27
Basic crystalline structures
Body centred cubic
Less tightly packed (more free space)Slide28
Basic crystalline structures
Hexagonal close packing
Each atom is surrounded by 6 other atomsSlide29
Slip planes
Atoms can slip (move) in the crystalline structureSlide30
Slip planes
Atoms can slip (move) in the crystalline structureSlide31
Slip planes
The slip planes are associated with the easiest path for atoms to move past each other.
Close packed structures have several slip planes and so they are more malleable and ductile than other packing structuresSlide32
2D slip planesSlide33Slide34
Dislocations
Dislocations are imperfections in the metallic crystalline structure which make it easier for atoms to move and thus increase the ductility of metals.
This ductility is also associated with the extended plastic behaviour of metals (deformation after the elastic limit has been achieved).
Plasticity requires the presence of dislocations and the ability of them to move through the crystalline structureSlide35Slide36Slide37Slide38
Structure of ceramics
Ceramic materials are crystalline (as metals) but are usually compounds of two or more elements
The arrangement of the atoms is specific (not random) and this affects the packing structure.
Size of atoms affects the structure (large and small ions may not form a rectangular structure)Slide39
Sodium chloride (salt)Slide40
Barium Titanate
Ba
Ti
O4Slide41
Amorphous ceramics
Usually oxides, these have no close packed structure and hence have no crystalline structure as suchSlide42
Amorphous ceramicsSlide43Slide44
Differences between metals and ceramics
Metal bonding allows the atoms to move past each other (slip planes) which give
srise
to ductility, malleability and plasticity
Ceramic atoms are chemically bonded to each other, so there are NO slip planes for the atoms to move past each other
Force on a ceramic will lead to fracture of the bonds, and hence the material (but at high temperatures there may be limited slip)Slide45
Structure of polymers
Polymers are made up of long strings of atoms, which are built up of regular or irregular sub stringsSlide46
Bonding in polymers
Individual chains are strongly chemically bonded, but links with other chains is from weaker inter-atomic Van der Vaal's forcesSlide47
How polymer chains appear Slide48
Transition temperatures
From block 2, the ductile brittle temperature is the temperature where the material will suffer a ductile failure rather than brittleSlide49
Melting temperature
This is the temperature when the substance changes phase from solid to liquid (T
m
)Slide50
Melting temperature
This is the temperature when the substance changes phase from solid to liquid (T
m
)Slide51
Glass temperature
For an amorphous ceramic, it is the temperature when the material will become less brittle and less stiffSlide52
Stress strain relationships for polymersSlide53
Stress strain relationships for polymersSlide54
Stress strain relationships for polymersSlide55
Viscoelasticity
Where a polymer exhibits both viscous and elastic behaviour
Viscous is where polymer chains slip pass each other
Elastic is where the chains are subjected to tensile forces
Tine dependent (hysteresis behaviour on
laoding
/unloading)Slide56Slide57
Increasing strength of metals
Controlling the strength of metals is all about controlling how the atoms slip past each other and impeding the passage of dislocations.Slide58
Impeding dislocation movement
Plastically deforming the material will cause dislocations to interfere, entangle and impede the motion of other – WORK HARDENING
As crystal boundaries impede the motion of the dislocations, reducing the size of the crystal grain size also impedes the movement of dislocations – GRAIN SIZE STRENGTHENINGSlide59
Impeding dislocation movement
Changing the size of the atoms (alloying with other atoms) – this addition of alloying elements to make a solid solution is called SOLUTION HARDENING
Discrete particles of different composition of the second
phas
can present obstacles to dislocation movement – AGE HARDENING or PRECIPITION HARDENINGSlide60
Hall-Petch Equation
SAQ 3.1 Page 123 Block 5 Part 3
The 0.1% proof stress of an aluminium alloy was measured for different grain diameters, d, produced by annealing at various temperatures. At the largest grain diameter of 0.5 mm the 0.1% proof stress was 110 MN m
-2
and the value of k was found to be 0.45 MN m
-3/2
. Estimate the 0.1% proof stress with a grain diameter of 0.04 mm.
d = 0.5 mm = 0.0005 m
σ
t
= 110 MN m
-2
= 110x10
6
N m
-2
and k = 0.45 MN m
-3/2
= 0.45x10
6
N m
-3/2
Using the Hall-
Petch
equation
σ
t
= σ
0
+
kd
–0.5Slide61
Hall-Petch Equation
SAQ 3.1 Page 123 Block 5 Part 3
d = 0.5 mm = 0.0005 m σ
t
= 110 MN m
-2
= 110x10
6
N m
-2
and k = 0.45 MN m
-3/2
= 0.45x10
6
N m
-2
Using the Hall-Petch equation
σ
t
= σ
0
+
kd
–0.5
Know d,
σ
t
and k Need to find
σ
0
σ
0
= σ
t
-
kd
–0.5
σ
0
= 110x10
6
- 0.45x10
6
x 0.0005
-0.5
So
σ
0
= 89875388 N m
-2
= 90 MN m
-2Slide62
Hall-Petch Equation
SAQ 3.1 Page 123 Block 5 Part 3
d = 0.5 mm = 0.0005 m σ
t
= 110 MN m
-2
= 110x10
6
N m
-2
and k = 0.45 MN m
-3/2
= 0.45x10
6
N m
-2
Using the Hall-Petch equation
σ
t
= σ
0
+
kd
–0.5
Know d,
σ
t
and k Need to find
σ
0
σ
0
= σ
t
-
kd
–0.5
σ
0
= 110x10
6
- 0.45x10
6
x 0.0005
-0.5
So
σ
0
= 89875388 N m
-2
= 90 MN m
-2
σ
t
= 90 + 0.45
d
–0.5
With a grain diameter, d = 0.04 mm = 0.00004 m
σ
t
= 90
10
6
+ (0.45
10
6
0.00004
–0.5
)
σ
t
= 161151247 N m
-2
=
161 MN m
–2Slide63
Hall-Petch Equation
I was asked to explain the transposition of a formula
in Exercise 3.1 of Block 5 Part 3 to mke k the subject.
σ
t
= σ
0
+
kd
–0.5
Know
σ
t
and
σ
0
need to find k
σ
t
– σ
0
=
kd
–0.5
(σ
t
– σ
0
) / d
-0.5
=
k or k =
(σ
t
– σ
0
) / d
-0.5
A negative index means the inverse so 1 / d
-0.5
= d
0.5
So k = (σ
t
– σ
0
) d
0.5
σ
0
= 90x10
6
N m
-2
σ
t
= 113x10
6
N m
-2
d= 0.0004 m
k = (113x10
6
– 90x10
6
) x 0.0004
0.5
k = 460000 = 460x10
3
N m
-3/2Slide64
Strengthening polymers
Change the chain structure by copolymerisation or modification of the repeated monomer unit
Alignment of the chains in the direction of loading
Fabrication of composites – such as the Mosquito aircraft in WW2 – made by cabinet makers with wood and glue. Now the Boeing Dreamliner continues the work!