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