corrosion lecture manchester

8/10/2019 Corrosion Lecture Manchester

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CorrosionPSLP Short course : November 2012

Prof. R Akid

Corrosion & Protection Centre

University of Manchester

[email protected]


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References

Corrosion for Scientists & EngineersChamberlain & Trethewey

Corrosion

Shreir (Vol 1&2)

Corrosion Engineering

Fontana & Greene

Corrosion: Fundamentals, Testing, andProtection

ASM Handbook Volume 13A:


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Lessons to be learnt

! Incompatibility

! Cu hull / Fe nails

1763


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! Incompatibility

! Cu alloy end plate/ steel

bolts

1962


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! Incompatibility

! SS bearing and Mg alloy

wheel

1982


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Cost of Corrosion

! Direct replacement

! Indirect costs

!

product loss! environment

! production loss

! safety

!

increased labour


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The cost of Corrosion is currently around 3-4% GDP.Typical values for a recent US survey are given belowTotal costs $137.9 billion

See http://www.corrosioncost.com/downloads/pdf/index.htm

Bridges, railroads Gas, Electricity distribution

Road, air , sea

Oil & gas,chemicals

DefenceNuclear waste

1 Hurricane Katrina every year!


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Corrosion is bad for business !

1.4% drop in share price= $2Bn


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Why metals & alloys corrode! Metal - unstable

! Oxide - stable

!

Reverse of metalextraction


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Formation of Corrosion Cells

Microstructurematrix/grain boundary

Inclusions

AerationOxygen rich -Cathode

Oxygen starved - Anode

Heat Treatment- Welding

- sensitisation

Mechanical Workingstrained area - anodeunstrained - cathode

Differentials


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Uniform corrosion

M = Mn++ ne-

O2+ 2H2O + 4e-= 4OH-pH !7

2H++ 2e-= H2 pH "7

Apply Faradays law to predict corrosion rate

Corrosion product


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Localised corrosion

M = Mn++ ne-

O2+ 2H2O + 4e-= 4OH-pH !7

2H++ 2e-= H2 pH "7

Faradays law not applicableto predict corrosion rate


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Pourbaix Diagram : Fe/H2O


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What is Corrosion Potential ?


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--

-

-

-

-

+

+

+

+

+

+

+

--

-

-

-

+++++

+ +

++

++ +

+

1 2 3

1 Helmoltz layer

2 Gouy-Chapman layer

3 Bulk solution

The electrode-electrolyte interface

-

-

-

-+

+

+

+

A more active than C

--

-

-

-

-

+

+

+

+

+

+

+

--

-

-

-

-

+

+

+

+

e-

A C

E

i

EC - uncoupled

EA- uncoupled

A-CCoupled

A C

Separation of charge

Metal [ve]

Solution [+ve] (cations)


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Standard Hydrogen Electrode


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STANDARD ELECTRODE POTENTIALS(Volts)

Na++ e- = Na -2.71Mg2++ 2e-= Mg -2.37Al3++ 3e-= Al -1.66Mn2++ 2e-= Mn -1.18Zn2++ 2e-= Zn -0.76Fe2++ 2e-= Fe -0.44

Cr3+

+ 3e-

= Cr -0.41Co2++ 2e-= Co -0.28Ni2++ 2e-= Ni -0.25Sn2++ 2e-= Sn -0.14Pb2++ 2e-= Pb -0.132H++ 2e- = H2(g) 0.00 Sn4++ 2e-= Sn2+ 0.15

Cu2+

+ 2e-

= Cu 0.34Ag++ e-= Ag 0.80O2 (g) + 4H

++ 4e-= 2H2O 1.23Au3++ 3e-= Au 1.50


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Comparison of standard electrode potentials

ElectrodePotential(V)

Au/Au3+= 1.5

Cu/Cu2+= 0.34

Hg2Cl2/Cl-= 0.242

H2/H+= 0.0

Fe/Fe2+= - 0.44

Zn/Zn2+= - 0.76

Ecell for Cu + Fe = Ecathode Eanode = 0.34 (-0.44) = 0.78V

SCE


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Measurement of Corrosion Potential

! Ref to ECorr


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Mechanisms

differential aeration; metal dissolution/hydrolysis;

lack of passivation


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Factors affecting Corrosion Rate

! Rate of electron transfer at metal surface

! Supply/transport of species to/from surface

!

Conductivity of solution! Passivity of metal

! Anode/Cathode surface area ratio


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Corrosion Rate Measurement

Laboratory-based methodsElectrochemical techniques

(icorr!B/Rp)where B= ba.bc/2.3 (ba + bc) = Stern-Geary Equation

Note Rp= Polarisation Resistance (R=V/I : Ohms Law)


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Polarisation Curves (Evans diagrams)

"a = 2.303 RT/#nFand C = log io

and $a = "a log ia C

$c = "c log ia C

"a"a

"c


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Determination of Rp

Rp = 1/Slope

Current(mA)

Voltage (mV)

+

-

- +

Note Ohms Law: R=V/I

But, slope of a graph = y/x = I/V

Therefore Rp = 1/Slope

Icorr= (ba.bc)/2.3 Rp. (ba+bc)

Potential change 20 mV


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Q1 - Anodic/Cathodic polarisation curvesfor C steel in 3.5% NaCl

Current (!)

0.1 1 10 100 1000

-1300

-1200

-1100

-1000

-900

-800

-700

-600

Icorr = 3!Bc = 170 mV

Ba = 70 mV

Voltage

(mVvsRef)


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Current (!)

0.1 1 10 100 1000

-1300

-1200

-1100

-1000

-900

-800

-700

-600


Ba = 70 mV

Voltage

(mVvsRef)


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Current (!)

0.1 1 10 100 1000

-1300

-1200

-1100

-1000

-900

-800

-700

-600


Ba = 70 mV

Voltage

(mVvsRef)


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Current (!)

0.1 1 10 100 1000

-1300

-1200

-1100

-1000

-900

-800

-700

-600


Ba = 70 mV

Voltage

(mVvsRef)


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Current (!)

0.1 1 10 100 1000

-1300

-1200

-1100

-1000

-900

-800

-700

-600


Ba = 70 mV

Voltage

(mVvsRef)


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Current (!)

0.1 1 10 100 1000

-1300

-1200

-1100

-1000

-900

-800

-700

-600


Ba = 70 mV

Voltage

(mVvsRef)


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Corrosion Calculations

! Rate of metal loss can be calculated using Faradays

Law.!

Given that a metal has a molecular wt M, valency z, density ,g cm3, and is

corroding uniformly over its surface with a current i given in A, thefollowing expression may be used to determine the total weight loss m,

centimetres per unit area (s) per unit time, t, is:

m/s = Mit/zF

where m =%sd and F = Faraday constant (96487 As)


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Example

! A storage tank containing sulphuric acid

corrodes at a rate of 50 A/cm2. Calculate the

minimum metal thickness required for a life of10 years assuming that a safety factor of 2mm

thickness is required.

where z=2, M = 55.85, %= 7.68 g/cm3


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Answer

Corr rate = 50A/cm2Lifetime = 10 years (3.1536 x 108 s)

Final thickness d = 2 mm

m/s = Mit/zF and m =%sd

d = Mit/%zF

= 55.85g . 50x10-6A/cm2 . 3.1536 x 108 s

7.86 g/cm3. 2 . 96487 As

= 0.58 cm ( 5.8 mm) in 10 years

Initial thickness = 2 mm + loss = 7.8 mm

Note: For Fe corrosion 1 A/cm2= 0.0116 mm/y


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Corrosion rate calculations

1. A mild steel tank corrodes at a rate of 120 A/cm2. Calculate theminimum metal thickness required for a life of 10 years assuming a safetyfactor of 2mm. z=2, M=55.85, 7.86g.cm-3. F=96487 A.s

2a. A cylindrical tank, 2m high by 0.5m diameter contains aerated sea water toa depth of 0.6m. Inspection after 8 weeks revealed a loss of 500g of metal.

Calculate the corrosion current, current density and corrosion rate. Assumingan initial wall thickness of 3mm, calculate the expected life of the tank. z=2,M=55.85, %=7.86g.cm-3. F=96487 A.s. Note 1A/cm2= 0.0116mm/y loss

2b. After 1 year a cathodic protection system was installed. Calculate the

maximum corrosion current density that can be tolerated to ensure a tank lifeof a further 20 years.


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Forms of Corrosion!

Uniform

! Pitting

! Galvanic

!

Crevice

! Selective (de-alloying, IG)

! Stress-assisted (SCC. CF, HE)

Localised


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Influence of material composition on

uniform corrosion! corrosion behaviour

related to formation of

oxide films


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Example of Weathering Steel

Angel of the North, Newcastle


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Pit mechanism

Specific reactions in stainless steel Fe = Fe2++ 2e-Fe2++ H2O = Fe(OH)

++ H+CrCl3+ 3H2O = Cr(OH)3+ 3HCl (drop in pH)MnS + 2H+= H2S + Mn

2+

Stages of Pitting corrosionStage I Breakdown of passive film (High chloride ion concentration in solution)Stage II Metal dissolution (localised corrosion)Stage III Repassivation (giving metastable pitting) or pit propagation due to stable pitting.


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Examples

Cu piping/hotwater system

C film(cathode)oninner surface


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Examples

316 Stainless steel inleisure pool environment

Screw locationStainless steel piping, water treatment plant


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Pitting

Potenti

alV

vs,SCE

Log corrosion current

(A/unit area)

1

2

3

45

Eb

Erp 4. Passive film breakdown (pitting) (Eb)

1. Active dissolution

2. Passive film formation

3. Passivity

5. Repassivation Erp


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Critical Pitting Temperature

Cr, Ni, Mo, N Eb

Si, Ti, S, C Eb


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Galvanic

! Galvanic series

! conductivity of soln

! anode/cathode area

! distance between

couples


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Galvanic Series


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Crevice

! differential aeration

! separation of anode/

cathode sites

!

local hydrolysis and

acidification


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Crevice mechanism

General reaction: M++ Cl-+H2O = MOH + HClSpecific reactions in the case of stainless steels we have:

CrCl3+ 3H2O = Cr(OH)3+ 3HCl (local solution pH drops)and

FeCl2+2H2O = Fe(OH)2+ 2HCl

Stages of Crevice corriosionStage 1 Depletion of oxygen in the crevice solutionStage II Increase in acidity and chloride content of the crevice solutionStage III Permanent breakdown of passive film and onset of localised corrosionStage IV Propagation of crevice corrosion


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Selective dissolution

! Intergranular corrosion

! enrichment/depletion at

grain boundaries

!

De-metallification! Zn/Cu in brasses

! Fe/C in graphitic cast iron


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Sensitisation (weld decay)


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Microbiologically Influenced Corrosion (MIC)

(Bacteria & Biofilms)

}

ColonisationofSulphateReducing

Bacteria(SRB)

H2S formation

LocalisedCorrosion(pitting)

Microorganisms,

especially bacteria,colonise surfaces toform Biofilms

Biofilm formation;up to 48hrs dependingupon temperature


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Consequences of MIC


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Bacterial-activesol-gel coatings

Anti-'microbial induced corrosion' (MIC) coating

Combination of anti-corrosion sol-gel coating and protective bacteria.

Uniform distribution of protective bacteria fixed on the surface

Substrate

'Biocoat'

Viable bacterial cells

immobilised in coating


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Bio-active coating - field trials

Patents: GB 2425976 (15.11.06) Akid/WangGB 2425975 (18.04.07) Akid/Wang


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Application of a biological active

anticorrosion coatings for Microbial Induced Corrosion

The sol-gel biological active coatings on Al

samples after 6 months field trials in an estuarine

environment (brackish)

Bare Al alloyR/T curedAbiotic sol-gel

Biocoat

0.5 mm


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Comparison of sol-gel coatings with non-viable(dead) and viable (live) endospores

27 weeks Whitby Harbour

J. Gittens, H. Wang, TJ. Smith, R. Akidand D. Greenfield (2010)Biotic sol-gel coating for the inhibition of corrosion in seawater ECS Transactions, 24, 211-229


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Stress Corrosion Cracking

Material

Stress

Environment

SCC


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SCC SusceptibilityMetal/alloy Environment

Al Alloy NaCl and H2O solutions, H2O vapour

Cu Alloys Ammonia solutions and vapours, Water, Amines

Mg Alloys NaCl, NaCl/K 2Cr2O7solutions

Steels NaOH, NaCl and NO3 solutions, Mixed acids,

H2S gas/solutions

Stainless steels Acidic NaCl solutions at elevated temperatures, MgCl2, H2S


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Metallographic features

of SCC

Intergranular

Transgranular

Brass Spheroidal Cast Fe


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Example

Aloha Airlines (1988)

E l Fli b h 1 t J 1974


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Example: Flixborough, 1stJune 1974

Manufacture of Caprolactam (for Nylon 6:6)

Leak of cyclohexane from reactor No. 5 led to by 20bypass system being installed this leaked leading to explosion

28 Killed

36 Injured


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Prevention of SCC

! Remove or reduce working/residual stresses

! Induce compressive residual stresses

! Control the operating environment,composition,

pH, temperature or potential! Apply anodic or cathodic protection

! Change alloy composition or structure


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Corrosion Fatigue

! combination of cyclic

stress and envment

! all combinations of

material/envmentsusceptible

! CF strength not related to

UTS


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Corrosion Fatigue (CF)

S

tress

Time

Stress

Time

SCC CF

Load History


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CF development from pit sites!

Structural Steel

! Seawater

! frequency, 0.2Hz

!

R = 0.1


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CP -No Failure

103 104 105 106 107200

400

600

800

1000

In-Air

Fatigue Limit

-1250 mV (vs SCE) (1 Hz)

Air -No Failure

StressAm

plitude(MPa)

Number of Cycles to Failure

0.6M NaCl (1 Hz)

Air (5 Hz)

Assessment of CF (SN curves)

StressAmplitude(MPa


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Prevention of CF

! Increase corrosion resistance of material

! Reduce aggressiveness of environment (dilution

or inhibitors)

!

Lower applied stresses/induce compressivestresses

! Prevent metal/environment contact - metallic or

non-metallic coatings

! Apply cathodic protection


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Corrosion Fatigue Case Study


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Hydrogen Embrittlement

! Premature failure of high

strenth steels

! Stress corrosion cracking

!

Pick up from pickling and

electroplating processes

( H atom)

+

H2(gas)

Adsorption of H Metal/Metal bond failure


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Approaches to Corrosion Prevention

1. Design

2. Coatings

3. Modification of the environment (Inhibitors, O2. pH)

4.

Material Selection5. Anodic Protection

6. Cathodic Protection


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Material Selection and Design

Considerations

Material Selection

Material Availabilityand Cost

Maintainability

Mechanical

Properties

Physical Properties

Existence of PreviousKnowledge of Design

Corrosion Resistance

Suitability for CorrosionControl Measures Fabrication

RequirementsFire

Resistance


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Design


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C steel versus stainless steel


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Design Aspects water traps

weld orsealant

poor improved

poor improved

sealant


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Design Aspects water traps

poor improved

Poor improved

water trapfree

drainage

welds


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Design Aspects coating aspects

poor improved

poor improved

lack ofcoverage


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Design Aspects galvanic couples

sealant

Al alloy

Cu Alloy

non-metallicgasket/sleeve


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Design Aspects flow effects

erosion-corrosionand poor drainage

turbulent flow non-turbulent flow

poor

improved


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Design Dangers!


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Protection Mechanisms - Coatings

Barrier Coatings non metallic, non conducting

environment/substrate contact

eliminated

Sacrificial (Base) Coatings coating dissolves in preference to

substrate

Noble Coatings higher corrosion resistance than

substrate, must be pore and crackfree


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Defects in coating systems


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Coatings

! Electrodeposition substrate must be conducting, i.e. metallic

coating thickness &1-50 microns

! Electroless deposit uses chemical reducing agents, no external

DC supply required, can coat non-conductingsubstrates

! Electrophoresis charged particles in solution are electrostatically

attracted to a substrate, can get rapid deposition

!

Hot dipping low melting point coating applied to substrate

thick coatings can be applied

! Spraying can spray paints, metals, ceramics

! Cladding provides a laminate coating to the substrate,

often in sandwich form, must ensure satisfactory

bond between cladding and substrate!

Vacuum/

Vapour depositionhi-tectype coatings, very expensive therefore very thin


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Corrosion Inhibitors

Anodic

Forms a passive film

! eg. orthophosphates

!

raises pH!

needs Ca ions in solution

! Silicates

!

as above

! Nitrites

!

oxidising agent

Cathodic

Blocks cathodic reactions

through precipitation

reaction! eg. As, Sb ions

! reduce H evolution reaction

(could increase risk of HE)

! Polyphosphate compounds

!

form on cathode sites


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1 icorr No inhibitor

2 icorr Anodic inhibitor

3 icorr Cathodic inhibitor

4 icorr Mixed inhibitor

Influence of Inhibitors on corrosion rates

No inhibitor

No inhibitor1

Anodic inhibitor

2

Cathodic inhibitor34

Potential

Current density


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Basis of cathodic/anodic protection

(Anodic)

Answers to Questions


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Answer No. 1;Require value of d for a 10 year life (3.1536 x 108seconds)But d = 2mm after 10 years (safety factor)

m/s = Mit/zF and m=%sdd=Mit/%zF = (55.85g x 120x10-6A.cm-2x 3.1536x10-8s)/(7.86g.cm-3x 2 x 96487 A.s)

= 1.426cm in 10 years (14.26 mm)Therefore minimum metal thickness = 14.26mm + 2mm = 16.26 mm.

Answer No. 2aSurface area = Base + sides = 'r2+ 2'rh = 11387cm2m = 500g after 8 weeks (4.8384 x 106s)

Current density (i) = mnF/sMt = (500g x 2 x 96487 A.s)/ (11387cm2x 55.85g x 4.8384 x 106s)=31.36 x 10-6A.cm-2

Corrosion rate = 31.36 x 10-6A.cm-2x 0.0116 mm/y (0.00116cm/y) = 0.364 mm/yLifetime = 3mm/0.364mm/y = 8.24 years

Answer No. 2b

After 1 year 0.364 mm lost, therefore 2.636mm wall thickness remains.2.636 must last 20 years which equates to 2.363/20 = 0.132 mm/y.Given 0.0116mm/y = 1A/cm2, then current density for 0.132 mm/y = 11.4A/cm2

Answers to Questions

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