Corrosion Bird's Eye View

8/8/2019 Corrosion Bird's Eye View

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Leaking Underground Storage Tank



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.



3030



Water Water

for for Life Life



88



Microorganisms such as bacteria,

fungi, moulds and mildews



Above ground storage tanks ( A ST) where the ambient

summer temperatures are ideal for bacterial colony growth

Microbialogical Corrosion Attack



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..



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..



Steel Life C ycle

R ebar

Scrap

SteelBillet

ContinuousCasting

La

le(refining)

Electric ArcFurnace

SteelMaking By-pro

ucts

FencingPro

ucts

HotR ollers

Scrap Yar



Why Metals Corrode

Refining-Corrosion Cycle



Why Metals CorrodeWhy Metals Corrode

Corrosio Cy le ExampleCorrosio Cy le Example



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



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)



Half Half--Cell Potential Measure ent andCell Potential Measure ent andCu/CuSOCu/CuSO44 Reference lectrodeReference lectrode



Electro otive For ce Series

Half Half--Cell Potential Measure entCell Potential Measure ent



Electro otive For ce Series

Electro otive For ce Series for MetalsElectro otive For ce Series for Metals



Measure ent of Pi eline PotentialsMeasure ent of Pi eline Potentials



Corrosion Cell Between Dissi ilar MetalsCorrosion Cell Between Dissi ilar Metals



2020



Corrosion CellCorrosion Cell



http://www.corrosionclinic.com/corrosion_online_lectures/dl.gif



Original surfacebefore corrosion

Surface after

corrosion

General Corrosion



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.



Galvanic Corrosion

Originalsurface

Surface aftercorrosion

Anodic metalCathodic

metal



Galvanic CorrosionGalvanic Corrosion

Dissimilar Metal Couple Mechanism

Old Pipe Old PipeNew Pipe

Current FlowCurrent Flow



Galvanic Seriesof SomeCommercial Alloys in

Seawater



Galvanic Corrosion in a Highly Conductive Solution

Highly conductive electrolyteHighly conductive electrolyte

Widespread corrosion Widespread corrosion

Anodic metal

Cathodicmetal



Galvanic Corrosion

Galvanic Corrosion in a Mildly Conductive Solution

Mildly conductive electrolyteMildly conductive electrolyte

ConcentratedConcentratedcorrosioncorrosion

Anodic metal

Cathodic

metal



A rea Princip

le for

A rea Princip

le for Ga

l vanic

CorrosionGa

l vanic

Corrosion



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.



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.



A R epresentation of Intergranular

Corrosion

Grainboundary

Intergranularcorrosion

Grain

Intergranular CorrosionIntergranular Corrosion



Intergranular Corrosion

R epresentation of a Sensitized Grain Boundary in Type 304 Stainless Steel

Grain

Grain

Grain

Chromium-depleted

area

Grainboundary

Chromiumcarbide

precipitate



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.



Pitting

R epresentation of Pitting Mechanism

Original surface

Pit Pit



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



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..



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++



Crevice Corrosion

Concentration Cell in the Crevices of a Bolted Flange

A nodic Areasin Crevices



CreviceCorrosion

ConcentrationCell in t e

Crevices of a

olte lan e



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 -



Filiform Corrosion

Parts of a Filament

Tail Head

Motion



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



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



Erosion-Corrosion

R epresentation of Erosion-CorrosionSmooth groves or wavesTurbulent fluid flow

Tubing inlets for condensers

Original surface

FlowSurface aftercorrosion



Sulfide Stress Cracking

Sulfide Stress Cracking

Tensile

H

Advancingcrack

H

H

H

H

H

H

H H

H

Forces

Tensile

Forces

Metal

Hydrogenatoms

H



Stress Corrosion Cracking

Stress Corrosion

Cracking

Advancingcrack

Metal

Corrosionproductfilm

Tensile

Forces

Tensile

Forces

M2+M2+



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.



Corrosion Fatigue

Alternating Tensile and CompressiveStresses in a rotating Member

oad

Bearing

Support

Bearing

Support

Compression

Tension



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



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.



De-Alloying(Selective eaching)

Plug TypeDezincification

Dezincifiedplugs



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.



A n Electrochemical CorrosionCell on A n Electrochemical CorrosionCell on thetheSurface of a MetalSurface of a Metal

HowCathodic Protection Works




Electrons Introduced from an External SourceElectrons Introduced from an External Source



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




A fter Cathodic Protection A fter Cathodic Protection Current isCurrent is A pplied A pplied

O ti f I d S t



Operation of Impressed Systems

A Typical Impressed Current System with a

Surface A

node Bed



R emove the Oxygen

Corrosion Bird's Eye View

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