corrosive-damage-in-metals-and-its-prevention-1201432551681857-2

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CORROSIVE DAMAGE IN CORROSIVE DAMAGE IN 
MATERIALS & ITS PREVENTIONMATERIALS & ITS PREVENTION
Dr. T. K. G. NAMBOODHIRIDr. T. K. G. NAMBOODHIRI
Professor of Metallurgy (Retired)Professor of Metallurgy (Retired)
 
INTRODUCTIONINTRODUCTION
• Definition: Corrosion Definition: Corrosion 
is the degeneration of is the degeneration of 
 materials by reaction materials by reaction 
with environment. with environment. 
Examples: Rusting of Examples: Rusting of 
automobiles, buildings automobiles, buildings 
and bridges, Fogging and bridges, Fogging 
of silverware, Patina of silverware, Patina 
formation on copper.formation on copper.
 
UNIVERSALITY OF CORROSIONUNIVERSALITY OF CORROSION
• Not only metals, but non-metals like Not only metals, but non-metals like 
plastics, rubber, ceramics are also subject plastics, rubber, ceramics are also subject 
to environmental degradationto environmental degradation
• Even living tissues in the human body are Even living tissues in the human body are 
prone to environmental damage by free prone to environmental damage by free 
radicals-Oxidative stress- leading to radicals-Oxidative stress- leading to 
degenerative diseases like cancer, cardio-degenerative diseases like cancer, cardio-
vascular disease and diabetes.vascular disease and diabetes.
 
CORROSION DAMAGECORROSION DAMAGE
• Disfiguration or loss of appearanceDisfiguration or loss of appearance
• Loss of materialLoss of material
• Maintenance costMaintenance cost
• Extractive metallurgy in reverse- Loss of Extractive metallurgy in reverse- Loss of 
precious minerals, power, water and man-precious minerals, power, water and man-
powerpower
• Loss in reliability & safetyLoss in reliability & safety
• Plant shutdown, contamination of product Plant shutdown, contamination of product 
etcetc
 
COST OF CORROSIONCOST OF CORROSION
• Annual loss due to corrosion is estimated to be 3 Annual loss due to corrosion is estimated to be 3 
to 5 % of GNP, about Rs.700000 croresto 5 % of GNP, about Rs.700000 crores
• Direct & Indirect lossesDirect & Indirect losses
• Direct loss: Material cost, maintenance cost, Direct loss: Material cost, maintenance cost, 
over-design, use of costly materialover-design, use of costly material
• Indirect losses: Plant shutdown & loss of Indirect losses: Plant shutdown & loss of 
production, contamination of products, loss of production, contamination of products, loss of 
valuable products due to leakage etc, liability in valuable products due to leakage etc, liability in 
accidentsaccidents
 
WHY DO METALS CORRODE?WHY DO METALS CORRODE?
• Any spontaneous reaction in the universe Any spontaneous reaction in the universe 
is associated with a lowering in the free is associated with a lowering in the free 
energy of the system. i.e. a negative free energy of the system. i.e. a negative free 
energy changeenergy change
• All metals except the noble metals have All metals except the noble metals have 
free energies greater than their free energies greater than their 
compounds. So they tend to become their compounds. So they tend to become their 
compounds through the process of compounds through the process of 
corrosioncorrosion
 
ELECTROCHEMICAL NATUREELECTROCHEMICAL NATURE
• All metallic corrosion are electrochemical All metallic corrosion are electrochemical 
reactions i.e. metal is converted to its reactions i.e. metal is converted to its 
compound with a transfer of electronscompound with a transfer of electrons
• The overall reaction may be split into The overall reaction may be split into 
oxidation (anodic) and reduction oxidation (anodic) and reduction 
(cathodic) partial reactions(cathodic) partial reactions
• Next slide shows the electrochemical Next slide shows the electrochemical 
reactions in the corrosion of Zn in reactions in the corrosion of Zn in 
hydrochloric acidhydrochloric acid
 
ELECTROCHEMICAL REACTIONS IN ELECTROCHEMICAL REACTIONS IN 
CORROSIONCORROSION
DISSOLUTION OF ZN METAL IN HYDROCHLORIC ACID, 
 
222 HZnClHClZn  -------------------- -(1) 
Written in ionic form as, 
2
2 222 HClZnClHZn   ----------------------(2) 
The net reaction being, 
2
22 HZnHZn   ------------------------- (3) 
 
Equation (3) is the summation of two partial reactions, 
eZnZn 2*2  -----------------------------------------(4) and 
222 HeH 
 ------------------------------------------(5) 
Equation (4) is the oxidation / anodic reaction and 
Equation (5) is the reduction / cathodic reaction 
 
ELECTROCHEMICAL THEORYELECTROCHEMICAL THEORY
• The anodic & cathodic The anodic & cathodic 
reactions occur reactions occur 
simultaneously at simultaneously at 
different parts of the different parts of the 
metal.metal.
• The electrode The electrode 
potentials of the two potentials of the two 
reactions converge to reactions converge to 
the corrosion the corrosion 
potential by potential by 
polarizationpolarization
 
PASSIVATIONPASSIVATION
• Many metals like Cr, Ti, Many metals like Cr, Ti, 
Al, Ni and Fe exhibit a Al, Ni and Fe exhibit a 
reduction in their reduction in their 
corrosion rate above corrosion rate above 
certain critical potential. certain critical potential. 
Formation of a protective, Formation of a protective, 
thin oxide film.thin oxide film.
• Passivation is the reason Passivation is the reason 
for the excellent corrosion for the excellent corrosion 
resistance of Al and S.S. resistance of Al and S.S. 
 
FORMS OF CORROSIONFORMS OF CORROSION
• Corrosion may be Corrosion may be 
classified in classified in 
different waysdifferent ways
• Wet / Aqueous Wet / Aqueous 
corrosion & Dry corrosion & Dry 
CorrosionCorrosion
• Room Room 
Temperature/ Temperature/ 
High Temperature High Temperature 
CorrosionCorrosion
CORROSION
WET CORROSION DRY CORROSION
CORROSION
ROOM TEMPERATURE
CORROSION
HIGH TEMPERATURE
CORROSION
 
WET & DRY CORROSIONWET & DRY CORROSION
• Wet / aqueous corrosionWet / aqueous corrosion is the major is the major 
form of corrosion which occurs at or near form of corrosion which occurs at or near 
room temperature and in the presence of room temperature and in the presence of 
waterwater
• Dry / gaseous corrosionDry / gaseous corrosion is significant is significant 
mainly at high temperaturesmainly at high temperatures
 
WET / AQUEOUS CORROSIONWET / AQUEOUS CORROSION
 Based on the appearance of the corroded metal, Based on the appearance of the corroded metal, 
wet corrosion may be classified aswet corrosion may be classified as
• Uniform or GeneralUniform or General
• Galvanic or Two-metalGalvanic or Two-metal
• CreviceCrevice
• PittingPitting
• DealloyingDealloying
• IntergranularIntergranular
• Velocity-assistedVelocity-assisted
• Environment-assisted crackingEnvironment-assisted cracking
 
UNIFORM CORROSIONUNIFORM CORROSION
• Corrosion over the Corrosion over the 
entire exposed entire exposed 
surface at a uniform surface at a uniform 
rate. e.g.. rate. e.g.. 
Atmospheric Atmospheric 
corrosion.corrosion.
• Maximum metal loss Maximum metal loss 
by this form.by this form.
• Not dangerous, rate Not dangerous, rate 
can be measured in can be measured in 
the laboratory.the laboratory.
 
GALVANIC CORROSIONGALVANIC CORROSION
• When two dissimilar When two dissimilar 
metals are joined together metals are joined together 
and exposed, the more and exposed, the more 
active of the two metals active of the two metals 
corrode faster and the corrode faster and the 
nobler metal is protected. nobler metal is protected. 
This excess corrosion is This excess corrosion is 
due to the galvanic due to the galvanic 
current generated at the current generated at the 
junctionjunction
• Fig. Al sheets covering Fig. Al sheets covering 
underground Cu cablesunderground Cu cables
 
CREVICE CORROSIONCREVICE CORROSION
• Intensive localized Intensive localized 
corrosion within corrosion within 
crevices& shielded crevices & shielded 
areas on metal areas on metal 
surfacessurfaces
• Small volumes of Small volumes of 
stagnant corrosive stagnant corrosive 
caused by holes, caused by holes, 
gaskets, surface gaskets, surface 
deposits, lap jointsdeposits, lap joints
 
PITTINGPITTING
• A form of extremely A form of extremely 
localized attack localized attack 
causing holes in the causing holes in the 
metalmetal
• Most destructive formMost destructive form
• Autocatalytic natureAutocatalytic nature
• Difficult to detect and Difficult to detect and 
measuremeasure
• MechanismMechanism
 
DEALLOYINGDEALLOYING
• Alloys exposed to Alloys exposed to 
corrosives experience corrosives experience 
selective leaching out selective leaching out 
of the more active of the more active 
constituent. e.g. constituent. e.g. 
Dezincification of Dezincification of 
brass.brass.
• Loss of structural Loss of structural 
stability and stability and 
mechanical strengthmechanical strength
 
INTERGRANULAR CORROSIONINTERGRANULAR CORROSION
• The grain boundaries in The grain boundaries in 
metals are more active metals are more active 
than the grains because of than the grains because of 
segregation of impurities segregation of impurities 
and depletion of and depletion of 
protective elements. So protective elements. So 
preferential attack along preferential attack along 
grain boundaries occurs. grain boundaries occurs. 
e.g. weld decay in e.g. weld decay in 
stainless steelsstainless steels
 
VELOCITY ASSISTED CORROSIONVELOCITY ASSISTED CORROSION
• Fast moving Fast moving 
corrosives cause corrosives cause 
• a) Erosion-Corrosion, a) Erosion-Corrosion, 
• b) Impingement b) Impingement 
attack , and attack , and 
• c) Cavitation damage c) Cavitation damage 
in metalsin metals
 
CAVITATION DAMAGECAVITATION DAMAGE
• Cavitation is a special case Cavitation is a special case 
of Erosion-corrosion.of Erosion-corrosion.
• In high velocity systems, In high velocity systems, 
local pressure reductions local pressure reductions 
create water vapour create water vapour 
bubbles which get bubbles which get 
attached to the metal attached to the metal 
surface and burst at surface and burst at 
increased pressure, increased pressure, 
causing metal damagecausing metal damage
 
ENVIRONMENT ASSISTED ENVIRONMENT ASSISTED 
CRACKINGCRACKING
• When a metal is subjected to a tensile When a metal is subjected to a tensile 
stress and a corrosive medium, it may stress and a corrosive medium, it may 
experience Environment Assisted Cracking. experience Environment Assisted Cracking. 
Four types:Four types:
• Stress Corrosion CrackingStress Corrosion Cracking
• Hydrogen EmbrittlementHydrogen Embrittlement
• Liquid Metal EmbrittlementLiquid Metal Embrittlement
• Corrosion FatigueCorrosion Fatigue
 
STRESS CORROSION CRACKINGSTRESS CORROSION CRACKING
• Static tensile stress Static tensile stress 
and specific and specific 
environments produce environments produce 
crackingcracking
• Examples:Examples:
• 1) Stainless steels in 1) Stainless steels in 
hot chloridehot chloride
• 2) Ti alloys in 2) Ti alloys in 
nitrogen tetroxidenitrogen tetroxide
• 3) Brass in ammonia3) Brass in ammonia
 
HYDROGEN EMBRITTLEMENTHYDROGEN EMBRITTLEMENT
• High strength High strength 
materials stressed in materials stressed in 
presence of hydrogen presence of hydrogen 
crack at reduced crack at reduced 
stress levels.stress levels.
• Hydrogen may be Hydrogen may be 
dissolved in the metal dissolved in the metal 
or present as a gas or present as a gas 
outside.outside.
• Only ppm levels of H Only ppm levels of H 
neededneeded
 
LIQUID METAL EMBRITTLEMENTLIQUID METAL EMBRITTLEMENT
• Certain metals like Al Certain metals like Al 
and stainless steels and stainless steels 
undergo brittle failure undergo brittle failure 
when stressed in when stressed in 
contact with liquid contact with liquid 
metals like Hg, Zn, Sn, metals like Hg, Zn, Sn, 
Pb Cd etc.Pb Cd etc.
• Molten metal atoms Molten metal atoms 
penetrate the grain penetrate the grain 
boundaries and fracture boundaries and fracture 
the metalthe metal
• Fig. Shows brittle IG Fig. Shows brittle IG 
fracture in Al alloy by fracture in Al alloy by 
PbPb
 
CORROSION FATIGUE CORROSION FATIGUE 
 S-N S-N DIAGRAMDIAGRAM
 Synergistic action Synergistic action 
of corrosion & of corrosion & 
cyclic stress. Both cyclic stress. Both 
crack nucleation crack nucleation 
and propagation and propagation 
are accelerated by are accelerated by 
corrodent and the corrodent and the 
S-N diagram is S-N diagram is 
shifted to the leftshifted to the left
AirAir
CorrosionCorrosion
log (cycles to failure, Nf)
St
re
ss
 A
m
pl
it
ud
e
 
CORROSION FATIGUE, CORROSION FATIGUE, 
CRACK PROPAGATIONCRACK PROPAGATION
 Crack propagation Crack propagation 
rate is increased by rate is increased by 
the corrosive actionthe corrosive action
Log (Stress Intensity Factor Range,  K
lo
g 
(C
ra
ck
 G
ro
w
th
 R
at
e,
 d
a/
dN
)
 
PREVENTION OF CORROSIONPREVENTION OF CORROSION
• The huge annual loss due to corrosion is a The huge annual loss due to corrosion is a 
national waste and should be minimizednational waste and should be minimized
• Materials already exist which, if properly Materials already exist which, if properly 
used, can eliminate 80 % of corrosion used, can eliminate 80 % of corrosion 
lossloss
• Proper understanding of the basics of Proper understanding of the basics of 
corrosion and incorporation in the initial corrosion and incorporation in the initial 
design of metallic structures is essentialdesign of metallic structures is essential
 
METHODSMETHODS
• Material selectionMaterial selection
• Improvements in materialImprovements in material
• Design of structuresDesign of structures
• Alteration of environmentAlteration of environment
• Cathodic & Anodic protectionCathodic & Anodic protection
• CoatingsCoatings
 
MATERIAL SELECTIONMATERIAL SELECTION
• Most important method – select the Most important method – select the 
appropriate metal or alloy .appropriate metal or alloy .
• ““Natural” metal-corrosive combinations Natural” metal-corrosive combinations 
likelike
• S. S.- Nitric acid, Ni & Ni alloys- CausticS. S.- Nitric acid, Ni & Ni alloys- Caustic
• Monel- HF, Hastelloys- Hot HClMonel- HF, Hastelloys- Hot HCl
• Pb- Dil. Sulphuric acid, Sn- Distilled waterPb- Dil. Sulphuric acid, Sn- Distilled water
• Al- Atmosphere, Ti- hot oxidizersAl- Atmosphere, Ti- hot oxidizers
• Ta- Ultimate resistanceTa- Ultimate resistance
 
IMPROVEMENTS OF MATERIALSIMPROVEMENTS OF MATERIALS
• Purification of metals- Al , ZrPurification of metals- Al , Zr
• Alloying with metals for:Alloying with metals for:
• Making more noble, e.g. Pt in TiMaking more noble, e.g. Pt in Ti
• Passivating, e.g. Cr in steelPassivating, e.g. Cr in steel
• Inhibiting, e.g. As & Sb in brassInhibiting, e.g. As & Sb in brass
• Scavenging, e.g. Ti & Nb in S.SScavenging, e.g. Ti & Nb in S.S
• Improving other propertiesImproving other properties
 
DESIGN OF STRUCTURESDESIGN OF STRUCTURES
• Avoid sharp cornersAvoid sharp corners
• Complete draining of vesselsComplete draining of vessels
• No water retentionNo water retention
• Avoid sudden changes in sectionAvoid sudden changes in section
• Avoid contact between dissimilar metalsAvoid contact between dissimilar metals
• Weld rather than rivetWeld rather than rivet
• Easy replacement of vulnerable partsEasy replacement of vulnerable parts
• Avoid excessive mechanical stressAvoid excessive mechanical stress
 
ALTERATION OF ENVIRONMENTALTERATION OF ENVIRONMENT
• Lower temperature and velocityLower temperature and velocity
• Remove oxygen/oxidizersRemove oxygen/oxidizers
• Change concentrationChange concentration
• Add InhibitorsAdd Inhibitors
– Adsorption type, e.g. Organicamines, azolesAdsorption type, e.g. Organic amines, azoles
– H evolution poisons, e.g. As & SbH evolution poisons, e.g. As & Sb
– Scavengers, e.g. Sodium sulfite & hydrazineScavengers, e.g. Sodium sulfite & hydrazine
– Oxidizers, e.g. Chromates, nitrates, ferric saltsOxidizers, e.g. Chromates, nitrates, ferric salts
 
CATHODIC & ANODIC CATHODIC & ANODIC 
PROTECTIONPROTECTION
• Cathodic protection: Make the structure more Cathodic protection: Make the structure more 
cathodic bycathodic by
– Use of sacrificial anodesUse of sacrificial anodes
– Impressed currentsImpressed currents
 Used extensively to protect marine structures, Used extensively to protect marine structures, 
underground pipelines, water heaters and underground pipelines, water heaters and 
reinforcement bars in concretereinforcement bars in concrete
• Anodic protection: Make passivating metal Anodic protection: Make passivating metal 
structures more anodic by impressed potential. structures more anodic by impressed potential. 
e.g. 316 s.s. pipe in sulfuric acid plantse.g. 316 s.s. pipe in sulfuric acid plants
 
COATINGSCOATINGS
• Most popular method of corrosion protectionMost popular method of corrosion protection
• Coatings are of various types:Coatings are of various types:
– MetallicMetallic
– Inorganic like glass, porcelain and concreteInorganic like glass, porcelain and concrete
– Organic, paints, varnishes and lacquersOrganic, paints, varnishes and lacquers
• Many methods of coating:Many methods of coating:
– ElectrodepositionElectrodeposition
– Flame sprayingFlame spraying
– CladdingCladding
– Hot dippingHot dipping
– DiffusionDiffusion
– Vapour depositionVapour deposition
– Ion implantationIon implantation
– Laser glazingLaser glazing
 
CONCLUSIONCONCLUSION
• Corrosion is a natural degenerative Corrosion is a natural degenerative 
process affecting metals, nonmetals and process affecting metals, nonmetals and 
even biological systems like the human even biological systems like the human 
bodybody
• Corrosion of engineering materials lead to Corrosion of engineering materials lead to 
significant lossessignificant losses
• An understanding of the basic principles of An understanding of the basic principles of 
corrosion and their application in the corrosion and their application in the 
design and maintenance of engineering design and maintenance of engineering 
systems result in reducing losses systems result in reducing losses 
considerably considerably