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8/6/2019 Mse3 Notes
1/17
8/6/2019 Mse3 Notes
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ALUMINIUMPROPERTIES
+ low density (2.7 g/cc) < steel (7.9 g/cc)
+ good electrical conductivity
+ good thermal conduction
+ Corrosion resistant
+ High coefficient of thermal expansion
+ soft and ductile
+ Relatively low molting point (660C).
- Iow strength
-price
APPLICATIONS
BIG 4:
BUILDING (roofing,cladding,heat exchange components)
PACKAGING (foods,beverages)
TRANSPORT (high performance cars, aerounautics)
ELECTRICAL (conductors, cables)
TYPES OF ALLOYS ( 4-DIGIT SERIES)
WROUGHT ALLOYS CAST ALLOYS
NON-HEAT TREATABLE HEAT TREATABLE
Work hardening to strengthen
Reduces ductility and formability
[ e.g. 5000 series Al-Mg
(Mg2Al3 non coherent precipitate) ]
PROPERTIES:
+ high strength
+readily welded
+high corrosion resistance
-poor ductility
-reduced properties if Mg>4%
APPS:
Ship building
automotive
SOLID SOLUBILITY function of T
[ e.g. 2000 series Al-Cu-Mg (3.5% Cu,0.5% Mg) ]
PROPERTIES:
+ high strength
+good ductility
APPS:
aircraft
Car bodies
No universally accepted nomenclature
PROPERTIES:
+ low melting point
+ good surface finishing
+ no gas solubility
+ light weight
- High shrinkage
APPS:
automotive
aerospace castings
With Silicon:
xThermal exp coeff
Weldability
Corrosion R
Special alloy:12%Si-15Cu-1%Mg-2%Ni
For piston-->dispersion hardening
IMPORTANT ALLOYS: T4: solution + natural ageing
T6: solution + artificial ageing Heat treated ( stable )
Other properties:
AGE HARDENED Al VS(5-6% Zn, 2.5% Mg)
PURE ANNEALED Al
Tensile strength (MPa)
Yield strength (Mpa)
572
503
45
17
MAGNESIUM
PROPERTIES
+ low density (1.74 g/cc) < Al (2.7 g/cc)
+ easily machinable
+ low melting point (650C)
- Pure Mg is weak
- difficult to cast
- price ( for aerospace industry)
SOLID SOLUTION with Al,Zn,Zr,Th
In WWI Mg-Al-Zn-->0.2% Mn added for corrosion R --> still principal casting alloy
Zr to refine grains ( higher mech properties)
In Al alloys, oxide film protects from oxidation
In Mg alloys, over 850C, surface burst into flame
T6 artificially aged
Most used alloys: T4 naturally agedSand or die casting
CASTING (90% european production) WROUGHT
SAND CASTING (largest use of Mg alloys)
Faster rate of solidification( high mech prop, less grain size
Mg has low volume heat c apacity-->less heat to remove casting
Zn:Al:Mg production rates = 1.0:1.6:1.9
PERMANENT MOULD CASTING
Processes:
Mg casting have similar properties of Al castings, but higher strength-weight ratio
Automotive crankcase
Formula 1 racing car gearbox
Helicopter gearcase
Casting production:
Mg difficult to deform < 250C
Hot working (extrusion, forging, rolling), 300C
8/6/2019 Mse3 Notes
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TITANIUM
PROPERTIES
+ low density (4.5 g/cc)
+ strength up to 1400 Mpa
PURE METAL:
+ low corrosion resistance
- low mech properties
HCP()structure 882C
BBC() structure 882C
Applications:
Heat exchangers (pipes - tubes)
valves for petrolchemical industry
ALLOYS
Solid-solution strengthening without altering transformation T (Tin, Zr)
-stabilising elements - transformation T (Al, O2,H)
-stabilising elements - transformation T (v,ta,Mo,Nb)
Eutectoid reaction - transformation-phase structure @ T room
4 groups:
General properties
+corrosion resistance
+high T properties(up to 535C)
[above 535C oxyde film breaks down]
-alloys
(5%Al-2.5% Sn)
-alloys - alloys [ MOST USED ]
( TI-6%Al-4%V also Ti-6/4)
+ corrosion R
+ oxydation R
+ high T strength
+ weldable
+ can be aged to
produce higher strength
Apps:
Beams, fitting for aerospace ind.
High-strength fasteners
+ superplastic (up to 1000%)
Apps:
For aircraft parts
Some for bioengineering
(joints,plates,valves)
TYPES OF PROCESSES
CASTING FORMING MACHINING
TMP=1678C
High affinity with O,N,H Casting undervacuum
Ti-6Al-4V
Ti-5Al-2.5 Sn
Most used alloys:
But only 1% of Ti consumpion
(low thermal conductivity-->larger thermal gradient)
Chemical industry
Few precision casting for aerospace
Application:
high power
High T forming
Dies heating control (low th gradient)--
-->Localised chilling and cracking --
--> Expensive tooling(die faces in Ni or Co for high T)
Require:
High resistance to deformation ---
--> Ti alloys less forgeable than others--
Or
High strain rate () to reduce chilling
Isothermal forging (in vacuum)
[reduce pressure - but expensive]
(e.g. high pressure compressor disc)
-->require:
Low thermal conductivity --
-->high cutting T --
-->chips stick to tool cutting edge --
-->Reducing tool life
FORCES IN FORMING
In cold working: f=Kn
In hot working: f=cm
k=strength coefficient
n=work hardening exp
=strain rate [s-1]
m=strain rate sensivitity exp
RECRYSTALLISATION
WORK HARDENING
(high strain rates,
Recrystallisation can't take place)
LIGHT ALLOYS 2venerd 15 aprile 2011
19:47
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HIGH T MATERIALS
Retention of strength at high T
Creep R
Oxidation R
Corrosion R
Wear/spalling R
Properties:
Depends on
Applications and T
Engines
(reciprocating, gas turbines...)
Burners in boilers
Furnace components
Tooling for hot-working
High T environments
Ti alloys
Steels
Super-alloys (Ni-based)
Ceramics
Materials
TI ALLOYS
-
Alloys
More =
mech prop
Near-
Alloys
[heat
treatable
To improve
mech prop]
Ti-6Al-4V
Higher strength and creep R
HEAT TREATED for max creep R
WELDABLE [for light weight high p compressor drum
Assembly development for Rolls-Royce
aerospace engines
HEAT TREATABLE for good balance of properties
[ fatigue R, toughness, creep R]
Process:
solution treatment
(1028 C for 2h)
Oil quench
Age 700 C for 2h
Air cooling
Apps:
High p compressors
On aircraft engines
General properties
Al stabilizes -phase;
Mo stabilzes -phase
Used up to 600 c, above 600 C OXYDATION
Above 600 C-->INTERMETALLICS (compounds of Ti) [generally TiAl (up to 870 c) , Ti3Al up to 700 C ]
Reduce density, improve strength and R at high T
Nb,W added to improve Troom properties
STEELS FOR HIGH TMax T for different steels:
PLAIN carbon 315 C
LOW-ALLOY steel
[Cr,Mo,V,W]
425 C
MEDIUM alloys
Hot-work TOOL steel
550 C
AUSTENITIC ss 750 C
AUSTENITIC STAINLESS STEELS
[ Fe - Cr -Ni, also smaller amounts of Ti and Al]
Bcc Cr tends to stabilize (bcc ferrite)
Fcc Ni tends to stabilize (fcc austenite)
which resist at high T
Ti-Al-->precipitation hardening of intermetallic phases Ni3Ti and Ni3Al
Famous austenitic SS : 18-8 Cr-Ni
Also very Creep resistant
Apps: exhaust parts in IC engines
Up to 11%
Corrosion R
High T stability
HIGH T ALLOYS - COATINGS01 April 2011
13:51
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NDT
checks for defects without destroying products (vs tensile test, creep test, impact/toughtness test)
do not determine mechanical properties
NDT = general QUALITY CONTROL PROCEDURE [on 100% products or statistical analysis(sampling)]
1st test: always VISUAL INSPECTION
x-rays-rays
Radiography
Ultrasonic testing
Magnetic particle inspection
Eddy current testing
Dye-penetrant
Other tests:
RADIOGRAPHY
For detection ofINTERNAL DEFECTS
x-rays or -rays --> sources of penetrating radiations
The flaw, or discontinuity, must have different
absorption caractheristics than the material itself
X-RAYS (William Wanken, 1895)
Sensitivity in radiography:
%sensitivity=x/x 100%
Where
x=thickness
x=smallest thickness available
I.Q.I. image quality indicator (penetrometer) -->
to find sensitivity
Type: STEP-HOLE I.Q.I.
The smaller the %sensitivity, the better
Each step has hole
with =h step
x
x
DETECTOR
VISIBILITY reduces
-RAYS
Intensive radiation of a single wavelength
from radiactive source
Cobalt 60: best isotope
(higher energy than x-rays)
Used for thick,absorbitive materials
More portable than x-ray equipment
e.g used ON-SITE for weldments in p ipeline
Intensity of -ray source decreases with time,
such that
I(t)=I0exp(-t)
Where
I0=original intensity
=decay constant [s-1]
When exp(-t)=0.5 -->
t is half-life of isotope
For Co60--> t(1/2)=5.3 years-->
no longer useful after 2 half lives(I=25%I0)
ULTRASONIC TESTING
Used to detect VERY SMALL INTERNAL FLAWS
All that is needed is an internal surface caused by a
discontinuity.
A piezoelectric transducer introduces pulses of sound
into a test piece at high frequencies ( >100 KHz).
Velocity of sound through various media is know:
Air @ 330 m/s
Al @ 6250 m/s
Stainless steel @ 5740 m/s
A defect when a component of known material results
in the ultrasound wave being reflected
(partially reflected rather than transmitter-->
recorded by oscilloscope)
NDT non destructive testing01 April 2011
13:51
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MAGNETIC PARTICLES INSPECTION
For defection offlaws (discontinuities) near the surface
of ferromagnetic materials (Fe,Ni,Co)
A magnetic field is induced in the material to be tested, producing line of flux.
The discontinuity must be perpendicular to the line of flux and close to the surface.
Place part in magnetic coil
Induce current in part
Place part close to magnet
Ways to induce field:
Flaw creates N-S poles opposite to part's N-S poles.
Parts can be dry or diluted (to help magnetic flow)
Quench cracks
Fatigue cracks
Cracks induced by machining processes (e.g. grinding)
Used for:
AC-surface flaws only
DC-surface or deeper (sub-surfaces) flaws
EDDY CURRENT TESTING
For all electricity conducting materials (all metals)
AC current flowing in a conductive coil produces electromagnetic fiel, which
in turn induces Eddy current in the sample.
These Eddy currents induce additional electromagnetic fields in the sample,
which interact with the applied field.
By determining the effect of the sample on the field, information can be
deduced concerning the structure and properties of the sample.Changes in
the electrical conductivity or magnetic permeability of the sample can be
detected and these changes can be due to changes in composition, micro-
structure or flaws close to the surface.
Go/no go test (accept/reject) - Wheatstone bridge type apparatus
DYE PENETRANT INSPECTION
For surface breaking defects RELATIONSHIP BETWEEN NDT AND FRACTURE MECHANICS
Fracture mechanics = effects of cracks on strength of material
If size of flaw is known after NDT, prediction can be made wheter the flaw
will cause fracture for a given applied stress (F.M.)
NDT supplies INFO needed to make the decision if the component can be in
service until next scheduled test
R1=R2
R3 : standard component in coil
R4: production(test) component in coil
e.g. mictrostructure variation :depth of case in a case-hardened steel
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Expensive
Difficult to machine
For materials:
Less waste and use of energy
Processes:
Metal forming
Metal casting
METAL FORMING
Plastic deformation of metals
Classifications:
Uniaxial tensile
Uniaxial compressive
Biaxial tensile
By applied system of stress:
DIRECT PROCESSES INDIRECT PROCESSES
Compressive stress -->induces 2 compressive stresses
on mutally perpendicular planes
Tensile stress --> induces 2 compressive stresses on mutually perpendicular planes
[only COLD WORKING -->because rely on STRAIN HARDENING - improve also mechanical properties]
e.g ROLLING
e.g. EXTRUSION
e..g WIRE DRAWING
e.g. DEEP DRAWINGe.g TUBE DRAWING
1
3
2
1
3
2
Multi-drawing
DIE SECTION
a b
C d
L
Die parts
Casing: protect the nib
a- bell (lubricant sprayed as part enters
b-approach core (1st contact part-nib-->die angle )
+ length = + tool life
- length = - energy loss
c-bearing or parallel:
d-relief (elastic spring.back)
Nib (usually WC hard and resistant):
Load vs die angle Die angle affects load force to pull the wire:
Ideal load : F= A2 lnR (R=Ai/Af)
Friction
Redundant work load
Real load affected by:
Large = - friction, + redundant
Al 24
Cu 12
Steel 6
Every material has optimum :
TYPES:
Sinkinga)
Fixed plugb)
Floating plugc)
Moving madreld)
Require: hollow cylinder [from hot-forming or boring)
+ closer dimensions
+mechanical properties
It is COLD FINISHING:
Direct compression (e.g. rolling, forging, extrusion)
Indirect compressin (e.g. wire drawing, deep drawing)
Biaxial tensile (e.g. stretch forming of sheet metal)
By stress system induced in workpiece:
Process: deep drawing + pressing [bending + stretching]
Applications: from circular sheet to cup
Areas:
a-b: IRONING( pure radial drawing die-blank holder)
b-c: STRETCH FORMING over die radius - SLIDING (possible THINNING)
c-d: STRETCHING die-punch ; sliding along surface
d-e: STRETCH FORMING punch radius ; metal doesn't slide over punch -->low hardnesse-f: BASE OF CUP - no deformations (original metallurgical conditions )-->higher hardness
(constant p and clearance)
Problems:wrinkling (b) -->require outer pressure ringor blank holder
INHOMOGENEOUS PROPERTIES
[blanks differentially annealed -center is in cold-worked conditions
Ideal load(to deform plastically metal) - function of drawing ratio R1/R2
Friction load
Ironing load
LOAD VARIATION DURING CYCLE -3 components:
CASE STUDY: MANUFACTURE OF CANS (beverages)
3000 Al-Ma
5000 Al-Mg
-->non-heat treatable work hardening series:
Materials: Al alloys -->strengthen by WORK HARDENING
Also Tin as light,stable and non-reactive with food
PROCESSES
DRAWING &IRONING DRAWING & REDRAWING
NB: before 3-piece can process
For beverages
Cans thickness:
Al from 0.42 to 0.15 mm
Tin from 0.32 to 0.10 mm
For food
drawing and redrawing
Stampin,calibrating,trimming
2 machines :
Cans thickness:
From 0.18 to 0.20 mm
NEAR NET SHAPE PROCESSES
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INVESTMENT CASTING
Material used: Al alloys
[e.g. turbochargers ]
Applications: aeroengine and high performance car engines
Wax injection and cluster assembly
Mould invested and then wax removed
[wax melts at 65C ]
Added fire refractory
Metal pouring and removal-->finished part
Steps:
High T require higher performance
High T materials used
APPLICATION: TURBINE BLADES CASTING
COOLING
SHELL MOULD
CERAMIC CORE
WAX
Removed chemically
[caustic soda -remove
ceramics, not metals]No machining required
ALTERNATIVE
Turbine blade in operation = creep test [tensile strength at high T]
At high T, grain boundaries softer than grains --> grain boundary sliding
SINGLE CRYSTALEQUIAXIAL [ ISOTROPIC COOLING] COLUMNAR [DIRECTIONAL COOLING
CASTINGvenerd 15 aprile 2011
20:49
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GAS TURBINE AEROENGINE
TYPES
TURBOPROP (propeller)
TURBOFAN high bypass ratio
TURBOFAN low bypass ratio
TURBOJET
SUBSONIC SPEED
(civil aircrafts)
SUPERSONIC SPEED
(military aircrafts - apart Concorde)
EFFICIENCY
MATERIAL REQUIREMENT
high specific strength (spec strength=strength/weight) at high T
Fan blades
Compressor disks
Compressor blades
Combustion chamber
Turbine disks
Turbine blades
CRITICAL COMPONENTS
COOL END
HOT END
COOL END
FAN BLADES
for high bypass ratio TurboFan engines (1 st developed by Rolls Royce - RB211)
2m diameter
4000 rpm rotation
Dimensions:
Denser material = higher stress --->try lower density material first
1st selection: CRP (carbonfiber reinforced plastics) -->very high specific strength (3 times steel)
2nd selection: Ti alloy blades -->toughness, but also density
Problem: require machining-->expensive process, expensive material, difficult to machine
Main alloy: Ti 6-4 [4% V] also called IMI 318
PRECISION FORGING (larger presses and better process control)
Material important characteristics: from RESONANT FREQUENCY
E=young's modulus
I=moment of inertia
L=ideal length
Preferred high natural frequency f>resonant f -->avoid vibrations
Require high EI and low
Propulsivep Thermal t
Higher blades number:
aerodynamicsfuel consumptions
Panels : creep formed (aka superplastic forming)
[require very fine equiaxed grain size (
Materials:
Forged Al alloys (Al-Cu) after WW2-->up to 200 C [as compression ratio , Texit ]
Steels (stainless steels) --> heavy, but good at high T
Ti (from 1960 to today)-->300 to 600 C, lighter than steels [ISOTHERMAL FORGING]
NB: if Texit too high, used alloys for turbine blades
Same materials as discs, but no fatigue
resistance
Materials:
Forged Al alloys (Al-Cu)
Steels (stainless steels)
Ti -->IMI834 up to 600 C
HOT ENDTURBINE
DISCS BLADES
Require:
Same as compressor discs
Materials:
Ni-based superalloys
' precipitates (Ni3Al or Ni3Ti)-->persists at high T
Strengthen mechanism: precipitation hardening
Most demanding service conditions
Require:
Creep resistance
High T resistance
Materials:
Steels (stainless steels) in the beginning
Ni-based superalloys
Process: investment casting
Selection steps:
Pick best material (Ni-based superalloys)
If not enough, cooling systemIf not enough, coatings
If not enough, switch to CERAMICS
(problems with toughness)
Failed Test with chicken-->material not tough enough
[impact damage on bird strikes - failure along fibers]
CASE STUDIES01 April 2011
13:52
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CRYSTALLOGRAPHY
DEFECTS
POINT LINEAR(DISLOCATIONS) INTERFACIAL(BOUNDARIES) IMPURITIES
Vacancy(up with T)
Interstitial(small concentrations)
Edge
Screw
Mixed
[ BURGER VECTORS ]
External surfaces
Grain boundaries
Twin boundaries
Stacking faults
If desired, in alloys:
Interstitial
substitutional
plastic deformation SLIP SYSTEMS
(related to dislocation density) (depend on the structure)
STRENGTH ALTERATION
Grain size reduction Solid solution alloying Strain hardening
Finer material stronger than coarse grains
(GREATER GRAIN BOUNDARY AREA=less motion of
dislocations)
Hall Petch equation:
Alloying with impurity atoms
INTERSTITIAL or SUBSTITUTIONAL solid solution
(up Yield and tensile strength)
LATTICE STRAIN on surrounding host atoms
Strength up as metal plastically deformed
COLD WORKING-WORK HARDENING
Common crystal systems:
FCC
BCC
TETRAGONAL
HEXAGONAL
CREEP
elevated T
static mechanical stresses
Many materials are have to bear:
[e.g., turbine rotors in jet engines ;
steam generators under centrifugal stresses]
3 areas:
Primary = transient creep.
[decreasing creep rate |
increasing creep resistance or strain hardening]
Secondary = steady-state creep
[constant creep rate |
balance between strain hardening and recovery]
Tertiary =
[rate acceleration |ultimate failure ( rupture)
from microstructural or metallurgical changes]
CREEP= time-dependent and permanent deformation
of materials when subjected to a constant load or stress.
(undesirable phenomenon |limits lifetime of part |
in metals important above 0.4 Tm ]
CREEP TEST = constant load test
Always until rupture (creep rupture tests)
CREEP CURVE = SUPERPOSITION OF CURVES IMPORTANT PARAMETER:
Steady state creep rate
VARIABLES
Stress
T
GENERAL FORMULA
CREEP
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CREEP MECHANISMS
Slip
Sub-grain formation
Grain-boundary sliding
Principal deformation processes at high T:
Dislocation glide
Dislocation creep
Diffusion creep
Grain boundary sliding
principal creep mechanisms: More than 1 mech per time:
In series:
Dominates fastest mech
In parallel:
Dominates slower mech
DISLOCATION CREEP DIFFUSION CREEPGRAIN BOUNDARY
SLIDINGdislocation glide + vacancy diffusion
steady-state creep rate : balance between
rate of strain hardening (h)and
rate of thermal recovery (r)
Diffusion vs dislocation climb
General theory:
Power-law relation (intermediate )
Harper-Dorn Creep:
[at low stresses
Linear relation n=1)
At High stresses ( )
In high T, low stress
NABARRO-HERRING
(stress-directed atomic diffusion)
COBLE CREEP
(at lower T -
grain boundary diffusion)
Not significant for steady-state creep rate.
important for:
initiation of intergranular fracture
maintaining grain contiguity during
diffusional flow mechanisms
DEFORMATION MECHANISMS MAP (stress-T diagram)
The regions of the map = dominant deformation mechanism @given stress-temperature condition.
The boundaries= combinations of stress and T where respective strain rates are equal.
Steady state creep > 0.5 T/Tm
Assume creep as single activated process (Arrhenius eq):
To find activation energy Q(T differential creep test:
DATA EXTRAPOLATION METHOD (LARSON-MILLER)
L-M PARAMETER:
T in K
tr in hours
CREEP 2
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CORROSION
Materials experience interaction with diverse environments
some interactions impair a materials usefulness
(mechanical properties ,physical properties, appearance)
Deteriorative mechanisms different for 3 material types:
by dissolution (corrosion)
By formation of nonmetallic scale or film (oxidation)
In metals actual material loss:
In ceramics:corrosion only at high T or extreme environments
In Polymers:degradation
Corrosion =destructive and unintentional attack of a metal
it is electrochemicaland ordinarily begins at the surface.
Huge problem: 5% nation's income spent on corrosion prevention and
the maintenance or replacement of products lost or contaminated
[occasionally used to advantage e.g. architecture]
Product appearance
Maintenance and operating costs
Plant shutdowns
Contamination of product
Loss of valuable product
Effects on safety and reliability
Product liability
Adverse Effects:
Corrosion Engineering= design and application of methods to prevent corrosion
Corrosion Management=process of reviewing applied Corrosion Engineering considerations.
[Corrosion is efficiently and adequately controlled only with both CE and CM]
CORROSION RATE EXPRESSION
Percentage weight loss
Milligrams per square centimeter per day
Grams per square inch per hour
Corrosion rates may be expressed in different ways:
But do not indicate corrosion resistance in terms of penetration:
Better indicator: The corrosion penetration rate (CPR)
W = weight loss [mg]
= density [g/cm3]
A=area [ cm2]
t= exposure time [hours]
K is 87.6 for mm/y.
CORROSION OF METALS
ELECTROCHEMICAL CONSIDERATIONS
For metallic materials, corrosion process is electrochemical
(chemical reaction with transfer of electrons from one chemical species to another)
Metal atoms: oxidation reaction (takes place on ANODE)
Reduction:transfer of electrons to other species
Reduction in metal ions (occurs in CATHODE)
or
Oxidation + reduction= electrochemical reaction
e.g:
Zinc in acid
Iron in water
Zinc in Cu sulphate
ELECTRODE POTENTIALS
Not all metals experience oxidation with the same degree of ease
E.g.:
Fe-Cu
Associated V: 0.78 V
Fe-Zn
Associated V: 0.323 V
GALVANIC COUPLE
one metal becomes an anode and corrodes
other metal becomes cathode
Two metals electrically connected in a liquid electrolyte:
2 series:
Galvanic series:represents relative reactivities ofmetals
and commercial alloys in seawater
CorrosionThe Thermodynamic Driving Force
Most metals and alloys subject corrosion in different environments
(more stable in an ionic state than as metals)
In thermodynamic terms:
net decrease in free energy in going from metallic to oxidized states.
[all metals occur in nature as compounds]
Exceptions:gold and platinum.
PASSIVITYPASSIVITY :Some active metals and alloys, under
particular environmental conditions, lose their
chemical reactivity and become extremely inert.
Chromium
Iron
Nickel
Titanium
And many of their alloys.
Happens to:
Passive behaviour from :
formation of adherent and very thin oxide film on
metal surface, which serves as a protective barrier
to further corrosion.
Stainless steels: at least 11%Cr-->highly resistant
to corrosion
Aluminum: is highly corrosion resistant alsobecause passivates
(If damaged, the protective film normally reforms
very rapidly)
But subsequent damage to a pre-existing passive
film could result in a substantial increase in
corrosion rate (by as much as 100,000 times)
Standard emf series :
standard half cell (hydrogen) coupled with
other metal half cells and ranked by Voltage
CORROSION01 April 2011
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CORROSIVE ENVIRONMENTS
- The atmosphere (greater losses)-->Al,Cu,galvanized steel
- Aqueous solutions-->cast iron, steel, Al,Cu,brass,sstainless steel
- Soils-->cast iron, plain carbon steel
- Acids
- Bases
- Inorganic solvents
- Molten salts
- Liquid metals
- The human body
oxide layer formation for divalent metal (oxidation and reduction half reaction)
Pilling Bedworth ratio
Where:
AO is the molecular weight of the oxide
Am is the atomic weight of the metal
po and pm are the oxide and metal densities,
Other factors influencing corrosion resistance:
- A high degree of adherence between film and metal.
- Comparable coefficients of thermal expansion for metal and oxide.
- The oxide should have a relatively high melting point and good high-temperature plasticity.
Several techniques for improving the oxidation resistance of a metal:
- Application of a protective surface coating (PAINTING)- addition of alloying elements will form a more adherent and protective oxide scale -->
more favorable Pilling-Bedworth ratio and/or improving other scale characteristics.
< 1 -->the oxide film porous and unprotective because insufficient to fully cover the metal surface.
= 1 --> ideal
> 1 -->If the ratio is greater than unity, compressive stresses result in the film as it forms.
> 2-3 the oxide coating may crack and flake off, continually exposing a fresh and unprotected metal surface.
for no t divalent metals:
oxydation occurs at metal-scale interface
reduction half reaction occurs at scale-gas interface
SCALING KINETICS
oxide scale reaction [normally on the surface]-->rate of reaction: measuring weight gain per unit area(W) as f(time)
porous
flakes off [P-B ratios 2]
Oxide layers:
[oxygen available for reaction as
oxide does not act as a reaction
barrier]
W-t linear relationship
Sodium
Potassium
Tantalum
e.g. Oxidation of
Oxide layers:
very thin less than 100 nm
form at low T
W-t logarithmic relationship
Aluminum
Iron
copper
e.g oxidation of (@Troom)
nonporous
adheres to metal surface
Oxide layers:
rate of layer growth controlled by
ionic diffusion.
W- t parabolic relationship
Iron
Copper
Cobalt
e.g. oxidation of
CORROSION 2
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Uniform
Galvanic
Crevice
Pitting
Intergranular
Selective leaching
Erosion-corrosion
Stress-corrosion
Metallic corrosion classified into eight forms:
FORMS OF CORROSION
UNIFORM ATTACK
electrochemical corrosion with equivalent
intensity over the entire exposed surface and
often leaves behind a scale or deposit.
In a microscopic sense, the oxidation arc:
reduction reactions occur randomly over the
surface.
Some familiar examples include:
rusting of steel and iron
Tarnishing of silverware
Most common corrosion
Easy to predict and design
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
environment will corrode;
the more inert metal, the cathode, will be protected
from corrosion.
For example:
Steel screws in contact with brass in a m arine
environment
- copper and steel tubing in a domestic water heater
DEFENSES
include the following:
choose two dissimilar metals close together in the
galvanic series.
Avoid small anode-to-cathode surface area ratio; use
an anode area as large as possible.
Electrically insulate dissimilar metals from each other.
lectrically connect a third, anodic metal to the other
two; this is a form of cathodic protection.
CREVICE CORROSION
Electrochemical corrosion as a consequence ofconcentration
differencesof ions or dissolved gases in the electrolyte solution, and
between two regions of the same metal piece.
For such a concentration cell, corrosion occurs in the locale that has the
lowerconcentration.
[ in crevices and recesses or under deposits of dirt or corrosion products]
The crevice:
wide enough for the solution to penetrate
narrow enough for stagnancy
MECHANISM
After oxygen has been depleted within the crevice, oxidation of the
metal occurs at this position.
Electrons from this electrochemical reaction are conducted through the
metal to adjacent external regions, consumed by reduction .
Many alloys that passivate are susceptible to crevice corrosion because
protective flms are often destroyed by the H - and Cl- ions.
Precautions:
welded instead of riveted or bolted joints.
Removing accumulated deposits frequently
Designing containment vessels to avoid stagnant areas and ensure
complete drainage.
PITTING
localized corrosion attack in which small pits
or holes form.
extremely insidious : undetected and with
very little material loss until failure occurs.
MECHANISM
same as for crevice corrosion in that
oxidation occurs within the pit itself, with
complementary reduction at the surface.
It is supposed that gravity causes the pits to
grow downward.
May be initiated by localized surface defect
such as a scratch or a slight variation in
composition.
Polished surfaces display a greater r esistance
to pitting corrosion.Stainless steels suffer it;
alloying with about 2% Mo enhances their
resistance significantly.
INTERGRANULAR CORROSION
preferentially along grain boundaries for some alloys and in
specific environments.
Prevalent in some stainless steels when heated to
temperatures between 500 and 800C for suff iciently long
time periods formation of small precipitate particles of
chromium carbide (Cr23C6).
These particles form along the grain boundaries which
leaves an adjacent chromium-depleted zone so this grain
boundary region is now highly susceptible to corrosion.
Protective measures:
1.Subjecting the sensitized material to a high-temperature
heat treatment in which all the chromium carbide particles
are redissolved.
2.Lowering the carbon content below 0.03 wt.% C so that
carbide formation is minimal.
3. Alloying the stainless steel with another metal such as
niobium or titanium, which has a greater tendency to form
carbides than does chromium so that the Cr remains in solid
solution.
In solid solution alloys and occurs when one element or
constituent is preferentially removed as a consequence of
corrosion processes.
Most common example:
dezincification of brass
[ zinc selectively leached from a copper-zinc brass alloy]
mechanical properties of the alloy significantly impaired
material changes from yellow to a red or copper color
May also occur with other alloy systems in which aluminum,
iron, cobalt, chromium, and other elements are vulnerable
to preferential removal.
In each case, initial corrosion dissolves both components of
the alloy but the more noble metal. (E.g. copper in the case
of brass)is then precipitated from solution at the surface.
This leads to increased solution of the parent alloy due to
galvanic effects and hence further deposition of copper.
In other cases, a given phase in a multiphase material may
be more prone to attack in a process known as selective
attack.
EROSION - CORROSION
From combined action ofchemical attack and
mechanical abrasion or wear as a
consequence of fluid motion and all metal
alloys are susceptible.
Especially harmful to alloys that passivate
forming a protective surface film:
The abrasive action erode the filmIf coating not capable of continuously
reforming, corrosion may be severe.
Relatively soft metals such as c opper and lead
are also sensitive to this form of attack.
Identified by surface grooves and waves having
contours that are characteristic of the flow of
the fluid.
Fluid properties:
Increasing fluid velocity enhances rate of
corrosion
More erosive solution when particulate solids
are present
Commonly found in piping, especially at bends,
elbows, and abrupt changes in pipe diameter
positions where the fluid changes direction or
flow suddenly becomes turbulent.
Propellers, turbine blades, valves, and pumps
are also susceptible to this form of corrosion.
Prevention
Change design to eliminate fl uid turbulence
and impingement
Use materials that resist erosion
Hard ceramic linings in steel pipes.
Removal of particulates and bubbles from
solution
STRESS CORROSION
From combined action of tensile stress and corrosive
environment
Small cracks form and propagate in direction
perpendicular to the stress with the r esult that failure
may eventually occur.
Failure behavior is of brittle material, even though the
metal alloy is intrinsically ductile.
Cracks may form at low stress levels(below the tensile
strength)
Most alloys are susceptible to stress corrosion in specific
environments, especially at moderate stress levels:
Most stainless steels stress corrode in solutions
containing chloride ions
brasses are especially vulnerable when exposed to
ammonia
The stress that produces stress corrosion cracking need
not be externally applied, may be residual one from
rapid temperature changes and uneven contraction, or
for two-phase alloys in which each phase has a different
coefficient of expansion.
Gaseous and solid corrosion products that are entrapped
internally can give rise to internal stresses.
PreventionProbably the best measure to take in reducing or totally
eliminating stress corrosion is to lower the magnitude of
the stress.
SELECTIVE LEACHING
HYDROGEN EMBRITTLEMENT
metal alloys, specifically steels, experience reduction in ductility and
tensile strength when H enters the material.
Hydrogen embrittlement is a type of failure:
- In response to applied or residual tensile stresses, brittle fracture
occurs as cracks grow and 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.
Hydrogen induced cracks are most often transgranular, although
intergranular fracture observed for some alloys.
Hydrogen embrittlement is similar to stress corr osion.
High-strength steels are susceptible to hydrogen embrittlement, and
increasing strength tends to enhance the material's susceptibility:
- Martensitic steels are especially vulnerable.
- Bainitic, ferritic, and spheroiditic steels are more resilient.
- FCC alloys are relatively resistant to hydrogen embrittlement,
mainly because of inherently high ductilities.
[strain hardening alloys will enhance embrittlement]
Prevention
Reducing tensile strength of the alloy via a heat treatment.
Removal of the source of hydrogen.
"Baking" the alloy at an elevated temperature to drive out any
dissolved hydrogen.
Substitution of a more embrittlement-resistant alloy.
CORROSION 3
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CORROSION PREVENTION
General techniques:
- Materials selection
- Environmental alteration
- Design
- Coatings
- Cathodic Protection
- Corrosion Inhibitors
MATERIAL SELECTION
most common and easiest way:
selection of materials once the corrosion
environment has been characterized.
[Cost may be a significant factor]
INHIBITORS
Substances that, when added in relatively low concentrations
to the environment, decrease its corrosiveness.
depends both on the alloy and on the corrosive environment.
Several mechanisms for the effec tiveness of inhibitors:
- Some react with and eliminate chemically active species
(such as dissolved oxygen).
- Other inhibitor molecules attach to the corroding surface
and interfere with either the oxidation or the reduction
reaction.
- Others form a very thin protective coating.
Inhibitors are used in closed systems such as automobile
radiators and steam boilers.
DESIGNShould allow for complete drainage in the case of
a shutdown, and easy washing.
Since dissolved oxygen may enhance the
corrosivity of many solutions, the design should, if
possible, include provision for the exclusion of air.
Other examples of intelligent design:
- Weld rather than rivet tanks (Crevice corrosion)
- Avoid excessive mechanical or thermal stresses
on components exposed to corrosive media.
(Stress-corrosion)
- Avoid sharp bends in piping with high velocities
and/or solids in suspension (Erosion-corrosion)
COATINGSPhysical barriers to corrosion as films and coatings.
Essential:
high degree of surface adhesion
Coating nonreactive in corrosive environment
resistant to mechanical damage
All three material typesmetals, ceramics, and
polymersare used as coatings for metals.
CATHODIC PROTECTIONUsed for all eight different forms of corrosion and
may completely stop corrosion.
Oxidation occurs by the generalized reaction
Cathodic protection simply involves supplying, from an
external source, electrons to the metal to be protected,
making it a cathode [reverse reaction]
layer of zinc applied to surface of steel
by hot dipping.
In the atmosphere and most aqueous
environments, zinc is anodic and will protect steel.
extremely slow rate of corrosion of zinc coating
as quite large ratio of anode-to-cathode surface A
IMPRESSED CURRENT
Source of electrons is an impressed current from
an external dc power source.
Terminal (-) connected to the structure
Terminal (+) to inert anode (often graphite)
high-conductivity backfill material provides good
electrical contact between the anode and
surrounding soil.
A current path exists between the cathode and
anode through the intervening soil, completing the
electrical circuit.
ENVIRONMENTAL ALTERATION
Lowering fluid T &/or v reduces corrosion rate;
increasing or decreasing the concentration of some
species in the solution will have a positive effect
[e.g.the metal may experience passivation]
CATHODIC PROTECTION METHODS
GALVANIC COUPLE
One technique employs a galvanic couple: the
metal to be protected electrically connected to a
more reactive metal. The latter experiences
oxidation, and protects the first metal (sacrificial
anode) [Mg,Zn]
GALVANIZATION
CORROSION 4