Post on 06-Jul-2018
Basics of Corrosion and Its ProtectionBasics of Corrosion and Its Protection
U. Kamachi MudaliCorrosion Science and Technology Division
Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102Email: kamachi@igcar.gov.in
Cost of Corrosion
Reasons to Loose the Performance of ComponentsReasons to Loose the Performance of Components
CorrosionUniform, Localised
WearAdhesive Wear, AbrasiveWear, Corrosive Wear,
Fretting Wear
FractureDuctile, Brittle
FatigueLow Cycle, High Cycle,
Fretting
Surface ScaleOxidatation, Carburisation,
Sulphidation
2H+ + 2e- H2O2 + 4H+ + 4e- 2H2O (Acidic)O2 + 2H2O + 4e- 4H+ (Basic)
(Cathodic)
(Anodic) Corrosion is the “destructive interactionbetween a material and its environment”
“Rusting” is the most common type ofcorrosion attack observed on iron basematerials in all types of environments
Electrochemistry ofElectrochemistry of Aqueous CorrosionCorrosion
Corrosion occurs by metal dissolution (oxidation) at anode;electron released thereby is consumed by reduction of ions at ca thode;
During corrosion anodic current cathodic current
nenMM
e
e
3AlAl
2FeFe3
2
Uniform corrosion can be described as corrosion reaction thattakes place uniformly over the surface of the material, therebycausing a general thinning of the component and an eventualfailure of the material.
Corrosion rate measured in mils per year or mm per year.
Factors affectinggeneral corrosion
- Moisture- Temperature
- Contaminants- Corrosion Control
Uniform Corrosion on CarbonSteel Manhole
General / Uniform Corrosion
Breakdown of passivity
Aggressive ions, chemical and mechanicalbreak down of passive film, presence ofsusceptible sites at metal surface
Uniform corrosion Localized corrosion
Passivity of Metals and AlloysPassivity of Metals and AlloysState of metal surface characterized by low corrosion rate in a
potential region that is strongly oxidizing for the metal
Type 1: low corrosion rate on anodicpolarisation
Cr, Ti, Stainless steels, nickel
Type 2: low corrosion rates in activepotentials
Lead in sulfuric Acid
a) Slow dissolution of passive film forming Fe 3+
b) accelerated dissolution at a soluble salt island forming Fe 2+
c) direct anodic dissolution at a pit initiation site forming Fe 2+
Iron
Passive filmFeOOH
Salt islandFeOCl
Electrolyte
Fe Fe2+
OH-
OH-
OH- 2OH-
Fe3+
Fe3+
Fe3+
Fe3+
Fe3+
O=
O=
O=
O=
O=
O=
O=
O=
2OH-
H2O
H2O
Fe3+
Fe3+
Fe3+
Fe3+
O=
O=
O= 2OH-
2OH-
2OH-
Cl-
Cl-
H2O
H2O
H2O
(a) (b) (c)
Corrosion and Passive Film Formation in Metallic MaterialsCorrosion and Passive Film Formation in Metallic Materials
Naturally formed surface filmsprotect the surface until anaggressive ion penetrate anddamage the protective nature
Corrosion resistant surfaces were produced over iron and bronzes for retainingthe beauty and structural integrity over many centuries
Body, Chassis, Engine assembly,Brake controls, Coolant pumps,Crank case emissions, Cylinder
block, Carburetor, Exhaust systemand Cooling systems
Pitting corrosion, galvanic corrosion andcrevice corrosion occurred in the motorvehicle on both external and internal parts
Most of the corrosion problems are dueto wrong materials, poor design, impropermaintenance and abuse of vehicle
Corrosion - integral and unavoidable part of petroleum refining operation.
Refinery operations involve inflammable hydrocarbons, highly tox ic orexplosive gases, strong acids, caustic operating at high tempera tures.
Low temperature refinery corrosion : < 260 ºC, water
High temperature refinery corrosion : > 260 ºC, No water
Corrosive Agents in RefineriesInorganic compounds – salts – MgCl2, CaCl2, Hydrogen sulphide, HCl,
H2SO4, NaOH, crude oil contaminants, process chemicals
Refinery Corrosion Problems
Corrosion failure of boiler tube
Fish-mouth opening in FeedSuper heater Tube in
Hydrogen Unit
Foulingon
ManmadeSubstrates
Marine Corrosion Problems
Failedlocation
Heat Exchanger Failurein Fertiliser Industry
Corrosion failure of a carbomate heat exchanger was due to design failure in providingexcessive gap between the tubes and tube sheet leading to crevic e attack
Manifestations of Corrosion
Uniform CorrosionPitting CorrosionCrevice Corrosion
Intergranular CorrosionStress Corrosion Cracking
Fretting CorrosionHydrogen Embrittlement
Corrosion FatigueErosion CorrosionGalvanic Corrosion
High Temperature CorrosionMicrobially Influenced Corrosion
Localised Corrosion Happens at Heterogeneities present at the surface
Pitting Corrosion
Reasons for failureUse of Carbon Steel in Stainless Steel Unit
Ingress of ordinary water Into DM watersystem
Pitting Attack in Type 304 SS Heat ExchangerFailure occurred within six months of service as comparedto the guaranteed life of 5 to 10 years.Coil is filled with 70% methanol brine with 30% water, andthe outside of the coil was exposed to acetone vapourcontaining nitrogen.
(a)
(a) Crevice corrosion of type 316 stainless steel in acid conden sateunder a PTFE spacer; (b) typical schematic morphology with att ackgreatest at the mouth of the crevice.
Crevice corrosion is localized corrosionwhich might occur in small areas of stagnantsolution in crevices, joints and undercorrosion deposits .
Crevice Corrosion
(b)
Stress Corrosion Cracking
Unfortunate Facts of SCC
Cracking takes place inotherwise best alloys
Passive alloys resistantto general corrosion
High strength steelsprone to cracking
Cracking at RTin pure water
Material-Environment for SCC
Red fuming HNO3, hotmolten salts, methanol,salt water
Ti alloys
Water, moist air, Cl- &Cl- - H2S
High StrengthLow Alloy Steel
OH-, NO3-, CO3
- & HCO3-
solutions, anhydrousNH3
Carbon & LowAlloy Steel
HTHP Water, dilute Cl-,OH-, F-, polythionic acid
Austenitic SS(sensitised)
Cl-, OH-, HTHP WaterAustenitic SS(annealed)
EnvironmentMaterial
SCC in HeliumStorage Tank
frequencewave shape
mean stress
min
R-ratio
Kmax
KR = min
I
K
K
Kmax
mean
Corrosion Fatigue• Synergistic interaction of cyclicplastic deformation and environment
• Both crack initiation andpropagation are influenced
• Unlike SCC no specific corrosiveenvironment / materials combinationis necessary
Fatigue life data, S-N curves, for a high-strength steel under different
environmental conditions
Corrosion Fatigue of A Pump Shaft
Principal modes of high temperature corrosion in industrial envi ronments, as wellas the interaction between oxygen activity and principal corrodent activity
Applications above 600˚C- More than one mechanism acts to degrade the material
Modes of High Temperature Corrosion
O2, H2, H2OCO, CO2,H2S, SO2
NH3, N2, NO2Cl
Sulphur Halogen
Carbon
Nitrogen
Molten SaltsMolten Metal
Eutectics
Ash/SaltDeposits
Sulphidation Halogenation
Carburization
Nitridation
Molten Salt
CorrosionMolten MetalCorrosion
Low MeltingLiquification
Flue Ash/HotCorrosion
High temperature corrosion "drycorrosion" or "scaling“ is a form ofcorrosion that happens at temperatureswell above room temperature and doesnot require a liquid electrolyte.
“Oxidation” “Sulphidation” “Carburisation”“Metal Dusting” “Chloridation” “HotCorrosion".
High temperature corrosion occurs invarious industries such as:
power generation (nuclear and fossilfuel), aerospace and gas turbine, heattreating mineral and metallurgicalprocessing, chemical processing, refiningand petrochemical, automotive, pulp andpaper, waste incineration
Surface Scales by High Temperature Corrosion / Oxidation
Serious problem – Temperature and pressure, Hydrocarbon – danger offire – if rupture occurs.
Sulfidic corrosion
>2600C – 5400C - no liquid water - common problem in refineries
Sulphur compounds from crude oil, polysulfide, H 2S, mercaptans,aliphatic sulfides, disulfides
Sulfur compounds react with metal surface and form sulphide products
High Temperature Refinery Corrosion
Corrosion of Economizer Coil in Catalytic Reforming Unit
Hydrogen can degrade materials by:
hydrogen blistering hydrogen embrittlement decarburization hydrogen attack
Blistering
Hydrogen enters the lattice of a metal,diffuses to voids, creates high internalstresses blisters are created
Blistering may occur during exposure to:• hydrocarbons;• electroplating and pickling of metals;• chemical process streams;• H-containing atm. during welding;• general corrosive environments.
Cross section of a carbon steel plateremoved from a petroleum processstream showing a large hydrogenblister. Exposure time: 2 years.
Hydrogen Effects on Materials
Prevention of Blistering–use steels with few or no voids;use coatings; remove impuritiesthat can promote hydrogenevolution . . . S2- (particularly bad),As, CN-, etc.; use differentmaterials (Ni-base alloys have lowdiffusion rates for hydrogen).
Microbiologically Influenced Corrosion
Depositionof cells in
flow
Biofilm on 304SS
Crevice attack MOB
MIC refers to corrosionand ensuing loss of
metal caused bybiological organisms.
MIC is a commonproblem in industrialprocesses due to the
presence of microbes,adequate nutrients andcorrosive byproducts.
Good Engineering PracticeGood Engineering Practice
• Select material/environmental combination based on literature & experience• Reduce service and residual Stress
• Follow procedure/code/practice in Design/Fabrication/Operation
Improve EnvironmentImprove Environment
Control of Impurities
Improve MaterialImprove Material
Alloying element to reduce scaling Forming protective layers on surface
Adopt Protective MeasuresAdopt Protective Measures
Painting Coating Plating
New materials
Protection Measures Against Corrosion
Various Surface Treatment / Modification Methods
SurfaceModification /
Coatings
MECHANICALShot PeeningLaser PeeningRoll bonding
THERMALFlame HardeningLaser Treatment
Induction HardeningElectron Beam Heating
Diffusion (Hot Dip)Cladding
Spraying (hot)
THERMOCHEMICALBoridingNitriding
CarburizingMetallizing
Plasma, Ion TreatmentLaser Alloying
CHEMICALElectroplating
AnodisingElectroless plating
Self AssemblyElectrophoritic depositionChemical vapor deposition
(CVD)
PHYSICALSputtering
Laser TreatmentPhysical vapor deposition
(PVD)Evaporation
Ion implantationSpraying (cold)
Thickness
Proc
ess
0.1 μm
1 μm
10 μm
100 μm
1000 μm
Reasons to Choose Surface Treatment / Modification MethodsConservation of materials; desired surface morphology/finish; Fu nctional;Treatment efficiency; Thickness; Integrity; Simplicity; Cost & E conomics
Dependence of thermal processes taking during heating of metals byCO2 laser radiation on density of absorbed power and on time -of-
exposure (duration of action beam on material)
Laser Surface Modification
Laser Treatment in Industry
Information related to loss of performance through measurement o f thickness,Microstructure, strength, density through application of NDT met hods
Advanced Techniques
Laser Optic Tubing Inspection System(LOTIS)
Storage Tank Integrity Detection(FLOORSCANNER)
Low Frequency Eddy Current Testing(LOFEC)
Quantitative Ultrasonic Analysis Systemand Recorder (QUASAR)
AE Leak Detection System(TANKSCAN)
On-line Multiarray UltrasonicCorrosion Monitoring (FLEXIMAT)Pipeline Inspection Gauges (PIG)
Magnetic Flux Leakage (MFL)
Conventional Techniques
Eddy Current TestingUltrasonic Testing
Radiographic TechniquesAcoustic Emission testing
Laser TechniquesInsitu MetallographyReplica Techniques
In-Service Inspection for Corrosion Monitoring
Cathodic ProtectionAdvantages
- can be used for all metals- installation cost is low- corrosion rate can be reduced to zero
Limitations- can only be used in weak to
moderate environments- operating conditions must be
determined by empirical testing. Impressed Current
Sacrificial Anode
Corrosion Inhibitors A corrosion inhibitor is a chemical compound that, when added in
small concentration, stops or slows down corrosion of metals and alloys.
Inhibitors slow corrosion processes by either:
► Increasing the anodic or cathodic polarization behavior ( Tafel slopes)► Reducing the movement or diffusion of ions to the metallic surfa ce► Increasing the electrical resistance of the metallic surface.
Classification of inhibitors- passivating inhibitors- cathodic inhibitors- organic inhibitors- precipitation inhibitors- volatile corrosion Inhibitors
Examples of Corrosion inhibitors
Hexamine, phenylenediamine, dimethylethanolamine, sodium nitrite,cinnamaldehyde, condensation products of aldehydes and amines (imines),chromates, nitrites, phosphates, hydrazine, ascorbic acid, and others.
Summary
Corrosion of Soul