corrosion lecture manchester
TRANSCRIPT
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CorrosionPSLP Short course : November 2012
Prof. R Akid
Corrosion & Protection Centre
University of Manchester
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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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Lessons to be learnt
! Incompatibility
! Cu alloy end plate/ steel
bolts
1962
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Lessons to be learnt
! 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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+
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--
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+++++
+ +
++
++ +
+
1 2 3
1 Helmoltz layer
2 Gouy-Chapman layer
3 Bulk solution
The electrode-electrolyte interface
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-+
+
+
+
A more active than C
--
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+
+
+
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+
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--
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+
+
+
+
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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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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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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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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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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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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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