the simple in corrmetals

8/14/2019 The Simple in CorrMetals

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CORROSION

INTRODUCTION:

Corrosion is the destruction or deterioration ofmaterial because of reaction with its environment,or destruction of materials by chemical or

electrochemical reaction with environment ratherthan straight mechanical. The importance of corrosion studies alwaysconcentrated on three main things:

The first area of significance is economic; including theobjective of reducing losses resulting from thecorrosion of material.

The second is improved safety of operatingequipment, because corrosion may fail withcatastrophic consequences. Examples are pressurevessels, radioactive material, boilers, etc.The third is conservation of metal resources becausethe worlds supply of these resources is limited andthe wastage of them includes losses of waterreserves associated with the production and

fabrication of metal structures.

Environment plays a big role in corrosion of metals.Practically all environments are corrosive to somedegree; some examples are air and moisture;fresh,distilled, salt and mine waters; rural, urban andindustrial atmospheres; steam and other gases

such as chlorine gas, ammonia, H2S, SO2 and fuel

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gases; mineral acids such as HCL, H2SO4 andHNO3; organic acids such as acetic and formic;solvents, vegetable and petroleum oils; and foodproducts .On the other hand, corrosion damagecause economic loss to the country as well as theenvironmental problems, either by direct losses orindirect losses.

1) By Direct losses: and this represents the costs ofreplacing corroded structures, such ascondenser tubes, pipelines, metal roofing etc.,

including necessary labor, also include theextra cost of using corrosion-resistant metalsand alloys or chemicals, such as corrosioninhibitors .The first studies indicate that thecost of corrosion in Jamahiriya industrial andoil fields up to 50 million Dinars per year.

2) Indirect losses: Corrosion failures can sometimes

cause many unexpected problems that needmoney and control to avoid them. Examplesof indirect losses are as follows :

I) Shut down: Unexpected corrosion failures in anyplant need shut down or stopping of the plant torepair the failure. In this case the total cost willbe replacement of corroded part plus the cost of

the lost production.ii) Lost of product: losses of oil, gas, or water occurthrough a corroded pipe system until repairs aremade. Gas leaking from a corroded pipe mayenter the basement of a building and cause anexplosion.

iii) Contamination of product: In many cases the

market value of the product is directly related toits purity and quality. A small amount of

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corrosion, which introduces certain metal ionsinto the solution, may cause catalyticdecomposition and contamination of a product.v) Effects on safety: The handling of hazardousmaterials, such as toxic gases, strong acids andradioactive substances demands the use ofmaterials of construction which minimizecorrosion failures.Corrosion failure of a small component mayresult in failure of the entire structure. Corrodingequipment can cause some fairly harmlesscompounds to become explosion.vi) Appearance: Badly corroded and rustedequipment in plant or any used metal surfacewould leave a poor impression on the observer.Much money is spent on painting of cars andchoosing expensive material like stainless steelor aluminum for use in out side surfaces or trimof buildings.

Sheet

Ironore

(Ironoxide)

Pipe

Fig. (1)

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Mine

Steel milReductionRefining

CastingRollingShaping

Auto body(Atmosphere)

UndergroundPipeline

(Soil and water)

Rust

Hydratediron oxide


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Fig. (3)

Thermodynamics and electrochemistry are ofgreat importance for understanding corrosion.

For example: if we ask a question:

Will corrosion occur for any system?

The answer will be:

Depends on thermodynamics of the system

(studying electrochemical equilibrium, electrode

potential, chemical equilibrium).

The second question will be if the system has a tendency forcorrosion then how fast will be?

The answer will be:

Depends on Kinetics. (Studying the kinetic of the

rate of corrosion reaction is controlled by the system)

rate of electron transfer.

ELECTROCHMICAL CONCEPT OF CORROSION

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Electrochemical

Corrosionresistance

Physicalchemical

Metallurgical

Thermodynamic


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Corrosion of metal in an electrolyte (in contact withmoist soil or water )is basically an electrochemicalreaction. The corrosion reaction usually involvesanodic areas (anodes) at which metal solution(corrosion) takes place, and cathodic areas(cathodes) at which a reducing action, not usuallyaffecting the metal occurs. Electrical current flowsfrom the anode to the cathode in the electrolyteand from cathode to anode in the metal. An areawill be anodic or cathodic as determined by itstendency to go into solution.

The tendency for any chemical reaction to goincluding the reaction of a metal with itsenvironment is measured by freeenergy change G (delta). The more negative the value of G, thegreater is the tendency for the reaction to go. So ifwe consider the following reaction at 25 C.

i) Mg + H2O(1)+1/2O2(g) Mg(OH)2(S)

G=-142,600 cal.

ii) Cu+H2O(1)+1/2O2(g) Cu(OH)2(S)

G= -28,600 cal.

iii) Au+3/2H2O(1)+3/2O2(g) Au2(OH)3 (S)

G=+15,700 cal.

The values of G shows that Mg has high tendencyto corrode in aerated water environment thancopper. Gold in equation (iii) has no tendency atall.Correspondingly does not corrode in aqueousmedia.

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The tendency for a metal to corrode can also beexpressed in terms of electromotive force (e.m.f)of the corrosion cells. Since electrical energy isexpressed as volts x Coulombs (joules), thereactions between G in joules and e.m.f in volts,E, is G= - n F E

Where n= No. Electrons involved in reaction

F= Faraday (96,500 Coulombs)

Therefore the greater the value of E for any cell,the greater is the tendency for the overall reactionfor the cell to go.

Corrosion of Iron: Since we are primarilyconcerned with the corrosion of iron as metal usedin many plants exposed to environment (moist,soil, or water) the electrochemical corrosionreaction of iron will be as follows:

If piece of iron is in contact with solution containing(H+). The iron Fe will tend to go to solution and H2gas will deposit on the surface of iron.

The over all reaction may be written as:

Fe + 2H+ Fe++ + H2This reaction may be broken down into twoseparate reactions as follows:

At the anode Fe Fe++ + 2e

At the cathode 2H+ + 2e H2

Electrons are transferred in the metal from theanode to the cathode, Electrical current flowing inthe opposite direction from the cathode to anode.

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Therefore the pre-requirements for corrosion are:

I) Anodic process: dissolution of metal (electronproduction)

ii) Cathodic process: electron consumption

iii) Electron-movement in the metal i.e. (conductivity)

iv) Ion-movement in the solution i.e. (electrolyticconductivity)

Types or (forms)of corrosion :

1.Uniform Attack: It is the most common form ofcorrosion. It is normally characterized by achemical or electrochemical reaction, whichproceeds uniformly over the entire exposedsurface, or covers a large area.

Uniform attack can be prevented or reduced by :

a) Proper materials.

b) Inhibitor.

c) Cathodic protection.d) Protective coating

2. Galvanic corrosion or two-metal corrosion :

When two dissimilar metals with differentpotentials are immersed in a corrosive or

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conductive solution, this potential differencesproduce electron flow between them. Themore active metal (less resistant) becomesanodic and less active metal (moreresistant)cathodic.

3. Pitting Corrosion :is localized accelerated attack resulting in

the formation of pits or holes in the metal.

4. Erosion Corrosion :is the acceleration or increase in rate of

attack on a metal because of relativemovement between a erosive & corrosivefluid and the metal surface.

5. Stress Corrosion : Stress Corrosion or stress cracking is the

combined effect of static tensile strength andthe corrosive environment on a metal.

For stress corrosion to occur, both thepresence of tensile stress and specificcorrosive environment are necessary.

6. Microbial Corrosion :Is the deterioration of a metal by corrosion

processes, which occur directly, or indirectlyas a result of the activity of microorganismswhich are present in a variety of environmentssuch as soil, natural water, seawater and innatural petroleum products. A good example isthe sulfur-oxidizing bacteria, which arecapable of oxidizing elemental sulfur, or sulfurbearing compounds to sulfuric acid, creating

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extremely corrosive conditions: (aerobicsulfur-oxidizing bacteria)

2S + 3O2 + 2H2O 2H2SO4

(In the presence of Thiobacillus, Ferrooxidans)

7.Underground or Soil Corrosion:Corrosion conditions for any metal structure

buried underground vary over a wide rangeaccording to the soil medium.

The factors affecting corrosiveness of a soil are:

a) its acidityb) degree of aerationc) its moisture and salts contentd) electrical conductivitye) presence of bacteria and micro-organismsf) soil texture (particle size )

Soil corrosion is purely electrochemical in character

CORROSION CONTROL:Corrosion damage can be completely avoided

only under ideal conditions.Although such ideal conditions are impossible toapply, it is possible to minimize corrosionconsiderably.Since the types of corrosion are so numerous theconditions under which corrosion occurs are so

extremely varied, it is not surprising thatdifferent methods are used to deal withcorrosion problems.The most common ways for controllingcorrosion are as follows:

1. Protection by proper design:A proper selection of the material for anyparticular corrosive environment and consulting

a corrosion engineer during the design time are

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the best means of controlling and evenpreventing corrosion.

For Example:Avoidance of dissimilar metal contact in thepresence of a corroding solution. Like in watersupplying system galvanized iron tanks areunder rapid corrosion in the presence of copperpipes, because galvanic cell will form betweenCu and Zn.

2. Improving the characteristics of the metal:

Impurities in metal causes heterogeneity, whichdecreases corrosion resistance of metal .Anincrease in both strength and corrosionresistance of most commercial metals is bestimproved by alloying them with suitable alloyingelements.Example of alloying iron base metal, which ismost significant because of the wide range use

of stainless steel.Many alloying elements (like P, Cu, Si, Al) areuseful in giving iron a limited corrosion resistantonly. However, alloying effect of Cr. with ironincreases corrosion resistance in almost allenvironments.

3. Cathodic Protection:

Is a method of preventing or reducing metalcorrosion in an electrolytic medium by supplyingelectrons to the metal structure to be protected.In all corrosion cells, corrosion occurs only at theanode, while various reactions may take place atthe cathode, the metal does not go into solutionand is protected from corrosion .If a metalstructure in the soil or immersed in water is

made the cathode so that the surface picks up

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current from the electrolyte ,the structure willnot corrode. Cathodic protection can beachieved in either of two ways; by the use ofgalvanic (sacrificial) anodes, or by impressedcurrent.

Galvanic anode systems employ reactive metals asauxiliary anodes electrically connected to thestructure to be protected. The difference inpotentials between the anode and structure, asindicated by their relative positions in theElectro-chemical series, causes a positivecurrent to flow in the electrolyte from the anodeto metal structure, so that the whole surface ofstructure be comes cathode.Impressed-current systems employ inert or non-galvanic anodes with an external source of D.C.power to impress a current from anode tocathode.

The following figures show the principles of both systems:The principle of cathodic

Protection by sacrificial Anodes.

Fig. (5)

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Wires joining sacrificial anodes to pipe line

Current Sacrificial anodes near to pipe

Pipeline buried in earth

The principle of cathodic protection

By impressed current.

Transformer Rectifier

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Tank Current

Anode

Fig. (6)

4. Use of Inhibitors:

A corrosion inhibitor may be defined as asubstance which when added in small quantities

to the aqueous corrosive environment,effectively decreases the corrosion rate of a

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metal. Inhibitors are organic or inorganicsubstances that dissolve in the corrodingmedium, but are capable of forming a protectivelayer of some kind at either the anodic orcathodic areas.

Anodic inhibitors: (such as chromate,phosphate, or other ions of transition elementswith a high oxygen content). These compoundsare adsorbed on the metal surface forming aprotective film or barrier thereby reducing thecorrosion rate.

Cathodic inhibitors: (such as amines. hetrocyclicnitrogen compounds are capable to adsorb atthe metal surface and decrease cathodicreaction e.g.

2H + 2e- H2 in this case it decrease thehydrogen diffusion in the medium.

5.Protective coatings: It is probably the oldest ofthe common procedures for corrosion prevention.

A coated surface isolated the underlying metalfrom the corroding environment. There areorganic coatings like paints or inorganic-like

metallic coatings.

CORROSION MONITORING :

The following figure summarizes the techniques available for

monitoring corrosion in an operating plant.

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Fig. (7)

References

1. FONTANA, M.,Corrosion Engineering, international

edition 1987, Mc Graw Hill Book Co. (third edition )

2. Corrosion for Science and Engineering, second edition

published 1995, produced by Long man Singapore publishers

(Pte) Ltd.

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CORROSION

MONITIRING

ThicknessMeasurement

VisualInspection

Weight loss coupon

And other test pieces

Electrical resistanceProbe

Measurement of plant

Corrosion potential

Linear polarizationmeasurement

Sentinal

Holes

Hydrogenprobes


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3. D.L.Piron, The Electrochemistry of Corrosion, second

printing 1991, published by NACE International.

4. ASM Hand book, formerly Ninth Edition, Metals Handbook,

volume 13 Corrosion, page 118 / forms of corrosion,

prepared under the direction of the ASM INTERNATIONAL

Handbook Committee.

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