CORROSIONCORROSIONEngineering Material ClassEngineering Material Class

Monday, 25 September 2006Monday, 25 September 2006

REFERENCESREFERENCES

Chandler, H., Chandler, H., Metallurgy for the Non MetallurgistMetallurgy for the Non Metallurgist, ASM, , ASM, Ohio, 1998. (1 copy in Student Design Facility)Ohio, 1998. (1 copy in Student Design Facility)Fontana, M. G., Fontana, M. G., Corrosion EngineeringCorrosion Engineering, 3, 3rdrd ed., McGraw- ed., McGraw-Hill, Singapore, 1987. (3 copies in Petra library)Hill, Singapore, 1987. (3 copies in Petra library)ASM Handbook volume 13: Corrosion, ASM, Ohio, 1987. ASM Handbook volume 13: Corrosion, ASM, Ohio, 1987. (reference book in Petra library)(reference book in Petra library)Roberge, P. R., Roberge, P. R., Handbook of Corrosion EngineeringHandbook of Corrosion Engineering, , McGraw-Hill, New York, 2000. (1 copy in Student Design McGraw-Hill, New York, 2000. (1 copy in Student Design Facility)Facility)Jones, D. A., Jones, D. A., Principles and Prevention of CorrosionPrinciples and Prevention of Corrosion, 2, 2ndnd ed., Prentice Hall, New Jersey, 1996. ed., Prentice Hall, New Jersey, 1996.

THE COST OF CORROSIONTHE COST OF CORROSION

EconomicEconomicCorrosion represents a constant Corrosion represents a constant

charge to a nation’s GNP in charge to a nation’s GNP in the order of 3-5%.the order of 3-5%.

Human Life and Human Life and SafetySafety

CountryCountry YearYear CostCost## % GNP% GNP

IndiaIndia

GermanyGermany

United United KingdomKingdom

AustraliaAustralia

JapanJapan

United United StatesStates

IndonesiaIndonesia

19611961

19691969

19701970

19731973

19741974

19751975

19851985

19901990

20022002

19861986

1.54*101.54*1099

19*1019*1099

1.36*101.36*1099

470*10470*1099

2.55*102.55*1099

82*1082*1099

150*10150*1099

180*10180*1099

300*10300*1099

101099

--

3.53.5

3.53.5

1.51.5

3.53.5

4.94.9

--

--

11

COST OF CORROSION - COST OF CORROSION - EconomicEconomic

Corrosion of bridges – they age and require Corrosion of bridges – they age and require replacementreplacement

A report by the New York department of transport: by 2010, 95% of all A report by the New York department of transport: by 2010, 95% of all New York bridges would be deficient if maintenance remained at the New York bridges would be deficient if maintenance remained at the same level as it was in 1981.same level as it was in 1981.

Corrosion in chemical companyCorrosion in chemical company

Corrosion in paper industryCorrosion in paper industry

Corrosion in oil industryCorrosion in oil industry

Corrosion of automobiles - fuel systems, Corrosion of automobiles - fuel systems, radiators, exhaust systems, and bodiesradiators, exhaust systems, and bodies

Indirect CostsIndirect Costs

Some of the more important sources of indirect Some of the more important sources of indirect costscosts : :

Plant DowntimePlant Downtime

Loss of ProductLoss of Product

Loss of efficiencyLoss of efficiency

ContaminationContamination

OverdesignOverdesign

DEFINITION OF CORROSIONDEFINITION OF CORROSION

The destruction or deterioration of a material because of reaction The destruction or deterioration of a material because of reaction with its environment. (Fontana)with its environment. (Fontana)The destructive result of chemical reaction between a metal or metal The destructive result of chemical reaction between a metal or metal alloy and its environment. (Jones)alloy and its environment. (Jones)

CORROSION SCIENCE is the study of the chemical and metallurgical CORROSION SCIENCE is the study of the chemical and metallurgical processes that occur during corrosion.processes that occur during corrosion.

CORROSION ENGINEERING is the design and application of methods CORROSION ENGINEERING is the design and application of methods to prevent corrosion.to prevent corrosion.

Ideally SCIENCE + ENGINEERING = to invent new and better Ideally SCIENCE + ENGINEERING = to invent new and better methods of prevention and apply existing methods more intelligently methods of prevention and apply existing methods more intelligently and effectively.and effectively.

ENVIRONMENTENVIRONMENT

Practically all environments are corrosive to some Practically all environments are corrosive to some degree.degree.Air and moistureAir and moistureFresh,distilled, salt and mined waterFresh,distilled, salt and mined waterRural, urban and industrial atmospheresRural, urban and industrial atmospheresSteam and other gases such as chlorine, Steam and other gases such as chlorine, ammonia,hydrogen sulfide, sulfur dioxide, and ammonia,hydrogen sulfide, sulfur dioxide, and fuel gases.fuel gases.

In general, the ‘inorganic materials are more In general, the ‘inorganic materials are more corrosive than the ‘organic.’corrosive than the ‘organic.’

Corrosion as Extractive Metallurgy Corrosion as Extractive Metallurgy in Reversein Reverse

Factor affecting choice of an Factor affecting choice of an engineering materialengineering material

Corrosion resistanceCorrosion resistance

CostCost

AppearanceAppearance

FabricabilityFabricability

Mechanical behaviourMechanical behaviour

AvailabilityAvailability

Factors affecting corrosion Factors affecting corrosion resistance of a metal resistance of a metal

Thermodynamics and electrochemistry : Thermodynamics and electrochemistry : understanding and controlling corrosion.understanding and controlling corrosion.

Metallurgical factorsMetallurgical factors

Physical chemistry: for studying Physical chemistry: for studying mechanism of corrosion reactions, surface mechanism of corrosion reactions, surface condition of metals, and other basic condition of metals, and other basic properties.properties.

Electrochemical AspectsElectrochemical Aspects

Figure 1 Schematic diagram of metal M dissolution, liberating into solution a metal ion M2+ and into the metal electrons, e-, which are consumed by reduction of H+ to H2.

FORMS OF CORROSIONFORMS OF CORROSION

Classification based on the appearance of Classification based on the appearance of the corroded metal.the corroded metal.

Can be identified by naked eye or Can be identified by naked eye or magnification.magnification.

Examination before cleaning is particularly Examination before cleaning is particularly desirable.desirable.

See various forms of corrosion pictured shematically in the acetate filmSee various forms of corrosion pictured shematically in the acetate film

Forms of CorrosionForms of Corrosion

Characteristics – Mechanisms – Preventive MeasuresCharacteristics – Mechanisms – Preventive Measures

Some of the 8 forms of corrosion are unique, but Some of the 8 forms of corrosion are unique, but all of them are more or less interrelated.all of them are more or less interrelated.Uniform corrosionUniform corrosionGalvanic, or two-metal corrosionGalvanic, or two-metal corrosionCrevice corrosionCrevice corrosionPittingPittingIntergranular corrosionIntergranular corrosionSelective leaching or parting Selective leaching or parting Erosion corrosionErosion corrosionStress corrosionStress corrosion

UNIFORM CORROSIONUNIFORM CORROSION

The most common form of corrosion.The most common form of corrosion.A uniform, regular removal of metal from the A uniform, regular removal of metal from the entire exposed surface. entire exposed surface.

- Metal ? -Metal ? -- Environment ? - Environment ? -

The metal becomes thinner and eventually fails.The metal becomes thinner and eventually fails.Represents the greatest destruction of metal on Represents the greatest destruction of metal on a tonnage basis.a tonnage basis.The life of equipment can be accurately The life of equipment can be accurately estimated on the basis of comparatively simple estimated on the basis of comparatively simple tests.tests.

Examples of Uniform CorrosionExamples of Uniform Corrosion

Steel or zinc in dilute HSteel or zinc in dilute H22SOSO44

Atmospheric corrosion : a sheet iron roofAtmospheric corrosion : a sheet iron roof

Carbon steel storage tank for sour (HCarbon steel storage tank for sour (H22S-S-

containing) crude oil after only two years containing) crude oil after only two years of service.of service.

Uniform attack can be prevented or Uniform attack can be prevented or reduced byreduced by

Proper materials including coatingsProper materials including coatings

InhibitorsInhibitors

Cathodic ProtectionCathodic Protection

Galvanic CorrosionGalvanic Corrosion

Two dissimilar metals are coupled in the Two dissimilar metals are coupled in the presence of corrosive solution.presence of corrosive solution.

There is a potential difference – the less There is a potential difference – the less resistant metal becomes resistant metal becomes anodicanodic and the more and the more resistant metal resistant metal cathodiccathodic..

The driving force for current and corrosion is the The driving force for current and corrosion is the potential developed between the two metals.potential developed between the two metals.

Dry-cell battery is an example of this type of Dry-cell battery is an example of this type of corrosion.corrosion.

Prevention of Galvanic CorrosionPrevention of Galvanic Corrosion

Select combinations of metals as close together as Select combinations of metals as close together as possible in the galvanic series.possible in the galvanic series.Avoid the unfavourable area effect of a small anode and Avoid the unfavourable area effect of a small anode and large cathode.large cathode.Insulate dissimilar metals wherever practicable.Insulate dissimilar metals wherever practicable.Apply coatings with caution.Apply coatings with caution.Add inhibitors.Add inhibitors.Avoid threaded joints for materials far apart in the series.Avoid threaded joints for materials far apart in the series.Design for the use of readily replaceable anodic parts or Design for the use of readily replaceable anodic parts or make them thicker for longer life.make them thicker for longer life.Install a third metal that is anodic to both metals in the Install a third metal that is anodic to both metals in the galvanic contact.galvanic contact.

CREVICE CORROSIONCREVICE CORROSION

Localised forms of attack that Localised forms of attack that result in relatively rapid result in relatively rapid penetration at small discrete penetration at small discrete areas.areas.Location : within crevices and Location : within crevices and other shielded areas on metal other shielded areas on metal surfaces exposed to surfaces exposed to corrosives (small volumes of corrosives (small volumes of stagnant solution) caused by :stagnant solution) caused by :

- HolesHoles- Gasket surfacesGasket surfaces- Lap jointsLap joints- Surface depositsSurface deposits- Crevices under bolt and rivet Crevices under bolt and rivet

headsheads- Wet packing materialsWet packing materials

Stainless steel bolt (bottom) inappropriately used Stainless steel bolt (bottom) inappropriately used in seawater after a five year exposure. (photo in seawater after a five year exposure. (photo copyright 2000 by George Dinwiddie, usedcopyright 2000 by George Dinwiddie, used by permission of www.alberg30.org)by permission of www.alberg30.org)

Cases of CREVICE CORROSIONCases of CREVICE CORROSION

Environmental Factors – Crevice Environmental Factors – Crevice CorrosionCorrosion

Deposits that may Deposits that may produce crevice produce crevice corrosion:corrosion:

- sand, dirt, corrosion sand, dirt, corrosion products (permeable), products (permeable), and other solids.and other solids.

To function as a To function as a corrosion site, a corrosion site, a crevice must be wide crevice must be wide enough to permit enough to permit liquid entry but liquid entry but sufficiently narrow to sufficiently narrow to maintain a stagnant maintain a stagnant zone.zone.

PITTINGPITTING

A form of extremely localised A form of extremely localised attack that results in holes in attack that results in holes in the metal.the metal.Holes – small or large in Holes – small or large in diameter, in most cases are diameter, in most cases are relatively small.relatively small.Pitting is unpredictable, Pitting is unpredictable, especially in conditions especially in conditions forming deep pits.forming deep pits.Difficult to detect pits because Difficult to detect pits because of their small size and because of their small size and because of the pits often covered with of the pits often covered with corrosion products.corrosion products.

The rate is variable, depending The rate is variable, depending on uncertain migration of on uncertain migration of corrodents into and out of the corrodents into and out of the pit.pit.Difficult to predict quantitatively Difficult to predict quantitatively and by laboratory tests :and by laboratory tests :

- Varying depths and number of Varying depths and number of pits.pits.

- Sometimes the pits require a Sometimes the pits require a long time – several months or long time – several months or a year to show up in actual a year to show up in actual service.service.

PreventionPrevention

Similar methods as suggested for Similar methods as suggested for combating crevice corrosion.combating crevice corrosion.Addition of 2% Mo to 18-8S (type 304) to Addition of 2% Mo to 18-8S (type 304) to produce 18-8SMo (type 316) results in a produce 18-8SMo (type 316) results in a very large increase in resistance to pitting.very large increase in resistance to pitting.To use materials that are known not to pit To use materials that are known not to pit in the environment under consideration: in the environment under consideration: hastelloy and titanium.hastelloy and titanium.Adding inhibitors.Adding inhibitors.

Effects of Metallurgical Structure on Effects of Metallurgical Structure on CorrosionCorrosion

Intergranular corrosion of Intergranular corrosion of austenitic stainless steels:austenitic stainless steels:

When heat treatments deplete When heat treatments deplete the g.b. of Cr by the g.b. of Cr by metallurgical reaction with metallurgical reaction with C (in the temp. range 425-C (in the temp. range 425-815815ooC). In that temp. C). In that temp. range, chromium carbides range, chromium carbides (Cr(Cr2323CC66) are insoluble and ) are insoluble and precipitates at g.b. precipitates at g.b.

What is the influence of Ni What is the influence of Ni and Mo ? and Mo ?

Control for Austenitic Stainless SteelsControl for Austenitic Stainless Steels

Employing high-temperature solution heat Employing high-temperature solution heat treatment (quench-annealing or solution-treatment (quench-annealing or solution-quenching).quenching).

Adding elements (stabilisers) that are strong Adding elements (stabilisers) that are strong carbide formers.carbide formers.

Lowering the carbon content to below 0.03%.Lowering the carbon content to below 0.03%.

SELECTIVE LEACHINGSELECTIVE LEACHING

The removal of one The removal of one element from a solid element from a solid alloy by corrosion alloy by corrosion processes.processes.

That element is more That element is more susceptible to corrosion susceptible to corrosion than the rest, more than the rest, more active electrochemically active electrochemically and are anodically and are anodically dissolved in galvanic dissolved in galvanic contact with the more contact with the more noble elements.noble elements.

Uniform dezincification – layer type (favour the high brasses/high Zn content)

Localised dezincification – plug type (favour the low brasses/low Zn content)

PreventionPrevention

Reducing the aggressiveness of the Reducing the aggressiveness of the environment (i.e. oxygen removal)environment (i.e. oxygen removal)Cathodic protectionCathodic protectionUse less susceptible material: Use less susceptible material:

- 15% Zn (red brass) 15% Zn (red brass) - Addition 1% Sn to a 70-30 brass (admiralty Addition 1% Sn to a 70-30 brass (admiralty

metal)metal)- Addition of As, Sb, P as ‘inhibitor’ (70%Cu, 29% Addition of As, Sb, P as ‘inhibitor’ (70%Cu, 29%

Zn, 1% Sn, 0.04% As)Zn, 1% Sn, 0.04% As)- Cupronickel (70-90%Cu, 30-10%Ni) for severely Cupronickel (70-90%Cu, 30-10%Ni) for severely

corrosive environments.corrosive environments.

EROSION CORROSIONEROSION CORROSIONThinning or removal of surface Thinning or removal of surface films by erosion from the flowing films by erosion from the flowing stream results in accelerated stream results in accelerated corrosion, called errosion corrosion, called errosion corrosion or impingement attack.corrosion or impingement attack.The attack is accelerated at The attack is accelerated at elbows, turbines, pumps, tube elbows, turbines, pumps, tube constrictions, and other constrictions, and other structural features that alter flow structural features that alter flow direction or velocity and increase direction or velocity and increase turbulence (agitator, blowers, turbulence (agitator, blowers, propellers, turbine blades, etc.)propellers, turbine blades, etc.)Corrosive mediums: gases, Corrosive mediums: gases, aqueous solutions, organic aqueous solutions, organic systems, and liquid metals. systems, and liquid metals. Appears as grooves, gullies, Appears as grooves, gullies, waves, rounded holes, and waves, rounded holes, and valleys.valleys.

CASES OF IMPINGEMENTCASES OF IMPINGEMENT

Stress Corrosion Cracking (SCC)Stress Corrosion Cracking (SCC)Conditions to produce SCCConditions to produce SCC

Critical environmentCritical environment

A susceptible alloyA susceptible alloy

Some component of Some component of tensile stresstensile stress