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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

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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