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8/8/2019 Corrosion Bird's Eye View
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Leaking Underground Storage Tank
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CloseClose--Up of Leaking Underground StorageUp of Leaking Underground StorageTankTank
This is a classic example of normal galvanicThis is a classic example of normal galvanic
corrosion experienced on USTs.corrosion experienced on USTs.
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3030
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Water Water
for for Life Life
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88
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Microorganisms such as bacteria,
fungi, moulds and mildews
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Above ground storage tanks ( A ST) where the ambient
summer temperatures are ideal for bacterial colony growth
Microbialogical Corrosion Attack
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Microbially Microbially--influenced influenced corrosioncorrosion of of metalsmetals atat seasea isis aabigbig safetysafety and and financial financial problem problem caused caused byby thethe production production of of damagingdamaging substancessubstances suchsuch asas hydrogenhydrogensulphidesulphide byby sulphatesulphate--reducingreducing micromicro--organismsorganisms within within
biofilmsbiofilms onon thethe surfacessurfaces..
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ThereThere areare manifoldmanifold reasonsreasons toto avoidavoid corrosioncorrosion inin maritimemaritime applicationsapplicationsaboveabove--seasea oror subsub--seasea.. SuitableSuitable standardsstandards toto protectprotect partsparts likelike boltingsboltings andandfastenersfasteners forfor many many years years fromfrom corrosioncorrosion areare thethe cathodiccathodic corrosioncorrosion
protectionprotection oror thethe useuse of of costcost intensiveintensive duplexduplex andand supersuper duplexduplex steelssteels..
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Steel Life C ycle
R ebar
Scrap
SteelBillet
ContinuousCasting
La
le(refining)
Electric ArcFurnace
SteelMaking By-pro
ucts
FencingPro
ucts
HotR ollers
Scrap Yar
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Why Metals Corrode
Refining-Corrosion Cycle
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Why Metals CorrodeWhy Metals Corrode
Corrosio Cy le ExampleCorrosio Cy le Example
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Cr ystalline Structure of Cr ystalline Structure of Perfect SolidsPerfect Solids
Unit Cells for BodyUnit Cells for Body--Centered Cubic, FaceCentered Cubic, Face--CenteredCenteredCubic, and He agonal CloseCubic, and He agonal Close--Packed StructuresPacked Structures
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Cr ystalline Structure of I erfect Solids
Point Defects in a Cr ystalline StructurePoint Defects in a Cr ystalline Structure
Self-interstitiali urity ato Matri ato
Substitutionali urity ato
Interstitiali urity ato
Vacancy(Void)
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Half Half--Cell Potential Measure ent andCell Potential Measure ent andCu/CuSOCu/CuSO44 Reference lectrodeReference lectrode
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Electro otive For ce Series
Half Half--Cell Potential Measure entCell Potential Measure ent
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Electro otive For ce Series
Electro otive For ce Series for MetalsElectro otive For ce Series for Metals
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Measure ent of Pi eline PotentialsMeasure ent of Pi eline Potentials
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Corrosion Cell Between Dissi ilar MetalsCorrosion Cell Between Dissi ilar Metals
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2020
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Corrosion CellCorrosion Cell
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http://www.corrosionclinic.com/corrosion_online_lectures/dl.gif
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Original surfacebefore corrosion
Surface after
corrosion
General Corrosion
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General Corrosion
Methods to Control General CorrosionMethods to Control General Corrosion
11. Use corrosion. Use corrosion--resistant materials.resistant materials.22. Apply coatings.. Apply coatings.33. Use corrosion inhibitors.. Use corrosion inhibitors.44. R emove oxygen from the system.. R emove oxygen from the system.
55. Use cathodic protection.. Use cathodic protection.66. Add corrosion allowance.. Add corrosion allowance.
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Galvanic Corrosion
Originalsurface
Surface aftercorrosion
Anodic metalCathodic
metal
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Galvanic CorrosionGalvanic Corrosion
Dissimilar Metal Couple Mechanism
Old Pipe Old PipeNew Pipe
Current FlowCurrent Flow
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Galvanic Seriesof SomeCommercial Alloys in
Seawater
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Galvanic Corrosion in a Highly Conductive Solution
Highly conductive electrolyteHighly conductive electrolyte
Widespread corrosion Widespread corrosion
Anodic metal
Cathodicmetal
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Galvanic Corrosion
Galvanic Corrosion in a Mildly Conductive Solution
Mildly conductive electrolyteMildly conductive electrolyte
ConcentratedConcentratedcorrosioncorrosion
Anodic metal
Cathodic
metal
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A rea Princip
le for
A rea Princip
le for Ga
l vanic
CorrosionGa
l vanic
Corrosion
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Galvanic CorrosionGalvanic Corrosion
Methods to Control Galvanic CorrosionMethods to Control Galvanic Corrosion
1.1. Select metals that are close together in theSelect metals that are close together in thegalvanic series.galvanic series.
2.2. Do not use small anodes with large cathodes.Do not use small anodes with large cathodes.
3.3. Insulate dissimilar metals completely.Insulate dissimilar metals completely.
4.4. Apply and maintain coatings. Apply and maintain coatings.
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Galvanic Corrosion
(contd)
5.5. Use corrosion inhibitors.Use corrosion inhibitors.
6.6. Avoid Avoid threaded joints for materialsthreaded joints for materialsthatthat areare farfar apartapart in the galvanic series.in the galvanic series.
7.7. Use cathodic protection.Use cathodic protection.
8.8. UseUse thicker anodicthicker anodic parts or designparts or designthemthem forfor easy replacement.easy replacement.
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A R epresentation of Intergranular
Corrosion
Grainboundary
Intergranularcorrosion
Grain
Intergranular CorrosionIntergranular Corrosion
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Intergranular Corrosion
R epresentation of a Sensitized Grain Boundary in Type 304 Stainless Steel
Grain
Grain
Grain
Chromium-depleted
area
Grainboundary
Chromiumcarbide
precipitate
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Intergranular Corrosion
Methods to Control Intergranular CorrosionMethods to Control Intergranular CorrosionFor Austenitic Stainless Steels:For Austenitic Stainless Steels:
1.1. Avoid Avoid temperatures that causetemperatures that causesensitization (400sensitization (400 900900°°C).C).
22.. Add alloying elements that form carbides Add alloying elements that form carbideseasier than chromium (e.g., titanium oreasier than chromium (e.g., titanium or
niobium).niobium).
3. ower the carbon content below 0.02%.3. ower the carbon content below 0.02%.
4. Follow special welding procedures.4. Follow special welding procedures.
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Pitting
R epresentation of Pitting Mechanism
Original surface
Pit Pit
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Pitting
Growth of a Pit in an Aerated Sodium
Chloride Solution
Sodium ChlorideSodium Chloride
SolutionSolution
O2O2
O2
O2
Na+ Na+ Na+
O2 O2
OH- OH- OH- OH-
O2 O2CI-CI-
CI-
CI-
CI-CI-
CI-
CI-
CI-
M+M+
M+
M+
M+
M+M+
H+ H+
e
e e
e
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Pitting
Methods to Control Pitting CorrosionMethods to Control Pitting Corrosion
1. Use cathodic protection.1. Use cathodic protection.
2. Coat vessels and pipe.2. Coat vessels and pipe.
3. Keep3. Keep equipment cleanequipment clean..
4. Use4. Use corrosion inhibitorscorrosion inhibitors or oxygen scavengersor oxygen scavengers
5. Select5. Select the proper alloysthe proper alloys..
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Crevice Corrosion
Concentration Cell at the Air-Water Interfaceof an Electrolyte Droplet
Aerated Aeratedareaarea
OO22
e
e
Metal oxideMetal oxidedepositsdeposits
Active corrosion in Active corrosion inunaerated areaunaerated area
Drop of Drop of electrolyteelectrolyte
Fe++Fe++
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Crevice Corrosion
Concentration Cell in the Crevices of a Bolted Flange
A nodic Areasin Crevices
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CreviceCorrosion
ConcentrationCell in t e
Crevices of a
olte lan e
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Crevice Corrosion
e Later ta es of Crevice Corrosion
et een o olte lates
Na +
O2
O2e
O2
e
e
ee
OH -
e
O2 Na +
M +
CI -
H+O2 O2 O2
CI -
CI -
CI -
CI -
CI -
CI -
CI -
H+
H+
M +M +
M +
M +
M +
M +
OH -
OH -
OH -
OH -OH -
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Filiform Corrosion
Parts of a Filament
Tail Head
Motion
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FiliformFiliform CorrosionCorrosion
The R eactions in a Corrosion Filament on aSteel Surface
Protectivecoating
Direction of growth
Filament
headO2
O2H2O H2OO2
high H
Steel Fe2+
low HFe (OH)3
Corroded track
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Filiform CorrosionMethod to ControlMethod to Control FiliformFiliform CorrosionCorrosion
1.1. R educeR educe thethe humidity humidity of theof the environment,environment,
if if possiblepossible..
2.2. UseUse coatingscoatings that are lessthat are less permeablepermeable toto water vapor. water vapor.
3.3. Cleaner metalCleaner metal surfaces may reducesurfaces may reduce filiformfiliformcorrosion.corrosion.
Erosion Corrosion
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Erosion-Corrosion
R epresentation of Erosion-CorrosionSmooth groves or wavesTurbulent fluid flow
Tubing inlets for condensers
Original surface
FlowSurface aftercorrosion
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Sulfide Stress Cracking
Sulfide Stress Cracking
Tensile
H
Advancingcrack
H
H
H
H
H
H
H H
H
Forces
Tensile
Forces
Metal
Hydrogenatoms
H
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Stress Corrosion Cracking
Stress Corrosion
Cracking
Advancingcrack
Metal
Corrosionproductfilm
Tensile
Forces
Tensile
Forces
M2+M2+
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Stress Corrosion Cracking
Met o s to Control Stress Corrosion CrackingMet o s to Control Stress Corrosion Cracking11.. C ange t eC ange t e esignesign or re uce t e loaor re uce t e loa to lowerto lower
t e stress.t e stress.
22.R
emove.R
emove corrosive agentscorrosive agents..33. Use. Use cat o iccat o ic protection only protection only w en SCC w en SCC isis
known to cause failures.known to cause failures.
44. Apply . Apply coatingscoatings..55. Use. Use corrosion in ibitorscorrosion in ibitors..
66.. SelectSelect materialsmaterials t at are resistantt at are resistant to SCC.to SCC.
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Corrosion Fatigue
Alternating Tensile and CompressiveStresses in a rotating Member
oad
Bearing
Support
Bearing
Support
Compression
Tension
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Corrosion Fatigue
Appearance of Corrosion Fatigue Failureof a R otating Member
Initiation site( corrosion pit ormechanical damage )
Smooth surface
( with beach marks )
R ough fracturesurface
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Corrosion Fatigue
Methods to Control Corrosion FatigueMethods to Control Corrosion Fatigue1.1. Use more corrosion resistant metal.Use more corrosion resistant metal.
2.2. R emove water from the system.R emove water from the system.
3.3. R educe stresses on the structure as follows:R educe stresses on the structure as follows:
change the design.change the design. heat treat the material to relieveheat treat the material to relieve
internal stressesinternal stresses shotshot--peen the surface to createpeen the surface to create
compressive stresses.compressive stresses.4.4. Use corrosion inhibitors to prevent theUse corrosion inhibitors to prevent the
start of pits.start of pits.
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De-Alloying(Selective eaching)
Plug TypeDezincification
Dezincifiedplugs
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De-Alloying (Selective eaching)
MethodsMethods toto ControlControl SelectiveSelective eachingeaching
11. Use an alloy that is less susceptible to. Use an alloy that is less susceptible to
selective leaching.selective leaching.
22. Use corrosion inhibitors to make the. Use corrosion inhibitors to make theenvironment less corrosive.environment less corrosive.
33. R emove oxygen to make the environment. R emove oxygen to make the environmentless corrosive.less corrosive.
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A n Electrochemical CorrosionCell on A n Electrochemical CorrosionCell on thetheSurface of a MetalSurface of a Metal
HowCathodic Protection Works
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HowCathodic Protection Works
Electrons Introduced from an External SourceElectrons Introduced from an External Source
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How C t odic rot ction or s
C t odic olarization Caus d b y Dir ct Curr nt
H C h di P i W k
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HowCathodic Protection Works
A fter Cathodic Protection A fter Cathodic Protection Current isCurrent is A pplied A pplied
O ti f I d S t
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Operation of Impressed Systems
A Typical Impressed Current System with a
Surface A
node Bed
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R emove the Oxygen
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