CORROSION AND DEGRADATION OF MATERIALS · CORROSION AND DEGRADATION OF MATERIALS Head of...

CORROSION AND DEGRADATION OF MATERIALS

Head of Department: Sc.D., Prof. Zoia Duriagina

Institute of Engineering Mechanics and Transport

Department of Applied Materials Science and Materials Engineering (AMSME)

LVIV POLYTECHNIC

NATIONAL UNIVERSITY

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LVIV POLYTECHNIC NATIONAL UNIVERSITY

ABOUT LVIV

Lviv – one of the mostbeautiful cities in Ukraine.Lviv is the European pearland the capital of Galicia.Our city have successfullyremained intactthroughout seven ages,preserving its historicalatmosphere and oldarchitecture.

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ABOUT LVIV

Life of the historical centralpart of Lviv goes slowly. Youwill not find noisy crowds orclatter of autos there.Taking a walk along centralpart of old city during all theday you will find manyinteresting рlaces.If you ever have been in Lviv,you will come here again andagain.

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CORROSION AND DEGRADATION OF MATERIALS

The main sections that we will consider:• The types of corrosion and corrosion damage, • Causes of destruction and degradation of various materials, • Methods of preventing corrosion.

In this lecture we will explore the process by which corrosiontakes place and the different ways unwanted corrosion can becontrolled. Мanufacturing some metal products can be verycostly. Corrosion deterioration of manufactured products,damaging their structure and ultimately rendering the productuseless. Allowing corrosion is not cost efficient and can inhibitproductivity. Understanding and preventing corrosion isimportant for maintaining infrastructures and machinery thatface corrosion.

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THE TYPES OF CORROSION AND CORROSION DAMAGE

With knowledge of the types and anunderstanding of the mechanismsand causes of corrosion anddegradation, it is possible to takemeasures to prevent them fromoccurring.For example, we may :• change the nature of the

environment,• select a material that is relatively

nonreactive,• and/or protect the material from

degradation.

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Corrosion is defined as thedestructive attack on a metal - it iselectrochemical and ordinarilybegins at the surface.The problem of metallic corrosionis significant. It has been estimatedthat approximately 5% of anindustrialized nation’s income isspent on corrosion prevention andreplacement of products lost as aresult of corrosion reactions.

DETERIORATIVE MECHANISMS ARE DIFFERENT FOR THE THREE MATERIAL TYPES7



Loss of mass in metals and alloys is called corrosion or theformation of nonmetallic film оn the surface of metals iscalled oxidation.Ceramic materials are relatively resistant to deterioration,which usually occurs at elevated temperatures or in ratherextreme environments. This process is frequently also calledcorrosion.

DETERIORATIVE MECHANISMS ARE DIFFERENT FOR THE THREE MATERIAL TYPES8



For polymers, mechanisms differ from those for metals andceramics, and the term degradation is most frequently used.Polymers may dissolve when exposed to a liquid solvent, orthey may absorb the solvent.Еlectromagnetic ultraviolet radiation and heat may causealterations in their molecular structures.

ELECTROCHEMICAL CONSIDERATIONS9



For metallic materials, the corrosion process is normallyelectrochemical. А chemical reaction in which there istransfer of electrons from one chemical species toanother. Metal atoms characteristically lose or give upelectrons in what is called an oxidation reaction.For example, a hypothetical metal M that has a valence ofn (or n valence electrons) may experience oxidationaccording to the reaction:

𝑀 → 𝑀𝑛+ + 𝑛𝑒−



THE TYPES OF CORROSION AND ORROSION DAMAGE

in which M becomes an n positively charged ion and inthe process loses its n valence electrons;e is used to symbolize an electron.Examples in which metals oxidize are:

Fe → 𝐹𝑒2+ + 2𝑒−

Al → 𝐴𝑙2+ + 3𝑒−




Other reduction reactions are possible, depending on thenature of the solution to which the metal is exposed. For anacid solution having dissolved oxygen, reduction according to:

For a neutral or basic aqueous solution in which oxygen is alsodissolved:

𝑂2 + 4𝐻+ + 4𝑒− → 2𝐻2𝑂

𝑂2 + 𝐻2𝑂 + 4𝑒− → 4(𝑂𝐻−)




Any metal ions present in the solution may also be reduced;for ions that can exist in more than one valence state(multivalent ions), reduction may occur by

in which the metal ion decreases its valence state by acceptingan electron. A metal may be totally reduced from an ionic to aneutral metallic state according to:

𝑀𝑛+ + 𝑒− → 𝑀 𝑛−1 +

𝑀𝑛+ + 𝑛𝑒− → 𝑀




The location at which reduction occurs is called thecathode. It is possible for two or more of the precedingreduction reactions to occur simultaneously. An overallelectrochemical reaction must consist of at least oneoxidation and one reduction reaction and will be theirsum.

ELECTRODE POTENTIALS14



Not all metallic materials oxidize to form ions with the samedegree of ease. Consider the electrochemical cell shown inFigure 2. On the left-hand side is a piece of pure ironimmersed in a solution containing ions of 1 Mconcentration. The other side of the cell consists of a purecopper electrode in a 1 M solution of ions. The cellhalves are separated by a membrane, which limits the mixingof the two solutions.

𝐹𝑒2+

𝐶𝑢2+




Figure 1. The electrochemical reactionsassociated with the corrosion of zinc inan acid solution. (From M. G. Fontana,Corrosion Engineering, 3rd edition)

Figure 2. An electro-chemical cellconsisting of iron and copper electrodes,each of which is immersed in a 1 Msolution of its ion. Iron corrodes whilecopper electrodeposits




Now consider another galvanic couple consisting of the sameiron half-cell connected to a metal zinc electrode that isimmersed in a 1 M solution of ions (Figure 3). In thiscase, the zinc is the anode and corrodes, whereas the Febecomes the cathode. The electrochemical reaction is thus:

𝑍𝑛2+

𝐹𝑒2+ + 𝑍𝑛 → 𝐹𝑒 + 𝑍𝑛2+




Consequently, metallic materialsmay be rated as to their tendencyto experience oxidation whencoupled to other metals insolutions of their respective ions.A half-cell similar to those justdescribed [i.e., a pure metalelectrode immersed in a 1 Msolution of its ions and at 25C(77F)] is termed a standard half-cell.

Figure 3. An electrochemical cellconsisting of iron and zinc electrodes,each of which is immersed in a 1 Msolution of its ion. The iron electro-deposits while the zinc corrodes.

CORROSION RATES18



The corrosion rate, or the rate of material removal as aconsequence of the chemical action, is an important corrosionparameter. This may be expressed as the corrosion penetrationrate (CPR), or the thickness loss of material per unit of time. Theformula for this calculation is

where W is the weight loss afterexposure time t; r and A representthe density and exposed specimenarea and K is a constant, itsmagnitude depending on thesystem of units used.

Corrosion penetration rate—as

a function of specimen weight loss,

density, area, and time of exposure

𝐶𝑃𝑅 =𝐾𝑊

𝑝𝐴𝑡

PASSIVITY19



Under particular environmentalconditions, some normally active metalsand alloys, lose their chemical reactivityand become extremely inert. Thisphenomenon, termed passivity, isdisplayed by chromium, iron, nickel,titanium, and many of their alloys. It isbelieved that this passive behavior resultsfrom the formation of a highly adherentand very thin oxide film on the metalsurface, which serves as a protectivebarrier to further corrosion.

PASSIVITY20



Inasmuch as there is an electriccurrent associated withelectrochemical corrosion reactions,we can also express corrosion ratein terms of this current, or currentdensity — the current per unitsurface area of material corroding— which is designated i.The rate r, in units of

is determined using the expression.

𝒓 =𝒊

𝒏 ∙ 𝑭

Expression relating

corrosion rate and current

density 𝑚𝑜𝑙𝑚2 ∙ 𝑠

PASSIVITY21



Stainless steels are highly resistant to corrosion in arather wide variety of atmospheres as a result ofpassivation.They contain at least 11% chromium, which as a solid-solution alloying element in iron, minimizes the formationof rust; instead, a protective surface film forms inoxidizing atmospheres.

Stainless steels are admitting to corrosion in someenvironments and therefore are not always “stainless.”

PASSIVITY22



Aluminum is highly corrosion resistant in manyenvironments because it also passivates. If damaged, theprotective film normally re-forms very rapidly. However, achange in the character of the environment may cause apassivated material to revert to an active state.

Subsequent damage to a preexisting passive film couldresult in a substantial increase in corrosion rate, by asmuch as 100,000 times.

PASSIVITY23



This passivation phenomenon may be explained interms of polarization potentiallog current densitycurves discussed in the preceding section. Thepolarization curve for a metal that passivates hasthe general shape shown in Figure 4.

PASSIVITY24



Figure 4. Schematicpolarization curvefor a metal thatdisplays an active–passive transition.

PASSIVITY25



At relatively low potential values, within the“active” region the behavior is linear, as it is fornormal metals. With increasing potential, thecurrent density suddenly decreases to a very lowvalue that remains independent of potential; thisis termed the “passive” region.Finally, at even higher potential values, the currentdensity again increases with potential in the“transpassive” region.

FORMS OF CORROSION26



It is convenient to classify corrosion according to the mannerin which it is manifest.Metallic corrosion is sometimes classified into eight forms:• uniform,• galvanic,• crevice,• pitting,• intergranular,• selective leaching,• erosion–corrosion,• stress corrosion.

FORMS OF CORROSION27



The causes and means of prevention of each of theseforms are discussed briefly. In addition, we have electedto discuss the topic of hydrogen embrittlement.

Hydrogen embrittlement is a type of failure rather than aform of corrosion; however, it is often produced byhydrogen that is generated from corrosion reactions.

UNIFORM ATTACK28



1.Uniform attack is a form of electrochemical corrosionthat occurs with equivalent intensity over the entireexposed surface and often leaves behind a scale or deposit.

In a microscopic sense, theoxidation and reductionreactions occur randomlyover the surface. Familiarexamples include generalrusting of steel and ironand the tarnishing ofsilverware.

UNIFORM ATTACK29



This is probably the most common form of corrosion. It isalso the least objectionable because it can be predictedand designed for with relative ease.

GALVANIC CORROSION 30



2. Galvanic corrosion occurs when two metals or alloyshaving different compositions are electrically coupledwhile exposed to an electrolyte. The less noble or morereactive metal in the particular environment experiencescorrosion (the more inert metal, the cathode, isprotected from corrosion).As examples, steel screws corrode when in contact withbrass in a marine environment. Аnd if copper and steeltubing are joined in a domestic water heater, the steelcorrodes.




Figure 5. Photograph showinggalvanic corrosion around theinlet of a single-cycle bilgepump that is found on fishingvessels. Corrosion occurredbetween a magnesium shellthat was cast around a steelcore.




The rate of galvanic attack depends on the relative anode-to-cathode surface areas that are exposed to the electrolyte.

A number of measures may be taken to reduce the effects ofgalvanic corrosion significantly including the following:1. If coupling of dissimilar metals is necessary, choose two

that are close together in the galvanic series.2. Avoid an unfavorable anode-to-cathode surface area

ratio; use an anode area as large as possible.3. Electrically insulate dissimilar metals from each other.4. Electrically connect a third, anodic metal to the other

two.

CREVICE CORROSION33



Electrochemical corrosion may also occur as aconsequence of concentration differences of ions ordissolved gases in the electrolyte solution and betweentwo regions of the same metal piece. For such aconcentration cell, corrosion occurs in the locale that hasthe lower concentration.

A good example of this type of corrosion occurs in crevicesor under deposits of dirt or corrosion products where thesolution becomes stagnant and there is localized depletionof dissolved oxygen.

CREVICE CORROSION34



3. Corrosion preferentially occurring at these positions iscalled crevice corrosion. The crevice must be wideenough for the solution to penetrate yet narrow enoughfor stagnancy.

Crevice corrosion may be prevented by using weldedinstead of riveted or bolted joints, using nonabsorbinggaskets when possible and removing accumulateddeposits frequently.

CREVICE CORROSION35



Electrons from this electrochemical reaction areconducted through the metal to adjacent externalregions. In many aqueous environments, the solutionwithin the crevice has been found to develop highconcentrations of H and Cl ions, which are especiallycorrosive.

Many alloys that passivate are susceptible to crevicecorrosion because protective films are often destroyedby the H and Cl ions.

CREVICE CORROSION36



Figure 6. Schematic illustration of the mechanism of crevice corrosion betweentwo riveted sheets. (From M. G. Fontana, Corrosion Engineering, 3rd edition.

PITTING CORROSION37



4. Pitting is another form of very localized corrosion attack inwhich small pits or holes form. They ordinarily penetrate fromthe top of a horizontal surface downward in a nearly verticaldirection. It is an extremely type of corrosion, often goingundetected and with very little material loss until failure occurs.An example of pitting corrosion is shown in Figure.

PITTING CORROSION38



The mechanism for pitting is probably the same asfor crevice corrosion, in that oxidation occurs withinthe pit itself, with complementary reduction at thesurface.It is supposed that gravity causes the pits to growdownward, the solution at the pit tip becoming moreconcentrated and dense as pit growth progresses.

PITTING CORROSION39



A pit may be initiated by a localized surface defectsuch as a scratch or a slight variation in composition.In fact, it has been observed that specimens havingpolished surfaces display a greater resistance topitting corrosion.Stainless steels are somewhat susceptible to thisform of corrosion; however, alloying with about 2%molybdenum enhances their resistance significantly.

INTERGRANULAR CORROSION40



5. As the name suggests, intergranular corrosion occurspreferentially along grain boundaries for some alloys and inspecific environments. The net result is that a macroscopicspecimen disintegrates along its grain boundaries. This typeof corrosion is especially prevalent in some stainless steels.

It is believed that this heat treatment permits the formationof small precipitate particles of chromium carbide ( )by reaction between the chromium and carbon in thestainless steel. These particles form along the grainboundaries, as illustrated in Figure 7.

𝐶𝑟23𝐶6




Figure 7. Schematic illustration of chromium carbide particles thathave precipitated along grain boundaries in stainless steel, and theattendant zones of chromium depletion




Both the chromium and the carbon must diffuse to thegrain boundaries to form the precipitates, which leaves achromium-depleted zone adjacent to the grainboundary. Consequently, this grain boundary region isnow highly sensitive to corrosion.




Intergranular corrosion is an especially problem in the weldingof stainless steels, when it is often termed weld decay.Figure 8 shows this type of intergranular corrosion.

Figure 8. Weld decay in astainless steel. The regions alongwhich the grooves have formedwere sensitized as the weldcooled. (From H. H. Uhlig and R.W. Revie, Corrosion andCorrosion Control, 3rd edition)




Stainless steels may be protected from intergranularcorrosion by the following process: (1) subjecting thematerial to a high-temperature heat treatment in whichall the chromium carbide particles are redissolved, (2)lowering the carbon content below 0.03 wt% C so thatcarbide formation is minimal, and (3) alloying thestainless steel with another metal such as niobium ortitanium, which has a greater tendency to form carbidesthan does chromium so that the Cr remains in solidsolution.

SELECTIVE LEACHING45



6. Selective leaching is found in solid solution alloys andoccurs when one element is preferentially removed as aconsequence of corrosion processes.The most common example is the dezincification of brass,in which zinc is selectively leached from a copper–zincbrass alloy. The mechanical properties of the alloy aresignificantly impaired because only a porous mass ofcopper remains in the region that has been dezincified. Inaddition, the material changes from yellow to a red orcopper color.

SELECTIVE LEACHING46



Selective leaching may also occur with other alloy systemsin which aluminum, iron, cobalt, chromium, and otherelements are vulnerable to preferential removal.

EROSION–CORROSION47



7. Erosion–corrosion arises from the combined action ofchemical attack and mechanical abrasion or wear as aconsequence of fluid motion.




Virtually all metal alloys, to one degree or another, aresusceptible to erosion–corrosion. It is especially harmful to alloysthat passivate by forming a protective surface film.

The abrasive action may erode away the film, leaving exposed ametal surface. If the coating is not capable and rapidly re-formingas a protective barrier, corrosion may be severe.

Relatively soft metals such as copper and lead are also sensitiveto this form of attack. Usually it can be identified by surfacegrooves and waves having contours that are characteristic of theflow of the fluid.




Erosion–corrosion is commonly found in piping, especially atbends and changes in pipe diameter — positions where thefluid changes direction or flow suddenly becomes turbulent.Propellers, turbine blades and pumps are also susceptible tothis form of corrosion. Figure 9 illustrates the failure of anelbow fitting.

One of the best ways to reduce erosion–corrosion is tochange the design to eliminate fluid turbulence effects.Other materials may also be used that resist erosion.




Figure 9. Impingementfailure of an elbow that waspart of a steam condensateline. (Photograph courtesyof Mars G. Fontana. FromM. G. Fontana, CorrosionEngineering, 3rd edition.Copyright © 1986 byMcGraw- Hill BookCompany. Reproduced withpermission)

STRESS CORROSION51



8. Stress corrosion, sometimes termed stress corrosioncracking, results from the combined action of an appliedtensile stress and a corrosive environment; both influences arenecessary.

In fact, some materials that are virtually inert in a particularcorrosive medium become susceptible to this form of corrosionwhen a stress is applied. Small cracks form and then propagatein a direction perpendicular to the stress (Figure 10).

Failure behavior is characteristic of that for a brittle material,even though the metal alloy is in fact ductile.

STRESS CORROSION52



Figure 10. A bar of steelbent into a shape using anutand-bolt. Whileimmersed in seawater,stress corrosion cracksformed along the bend atthe regions where thetensile stresses are thegreatest. (Photograph courtesy

of F. L. LaQue. From F. L. LaQue,Marine Corrosion, Causes andPrevention)

STRESS CORROSION53



Most alloys are sensitive to stress corrosion in specificenvironments. For example, most stainless steels stresscorrode in solutions containing chloride ions.Figure 11 is a photomicrograph showing an example ofintergranular stress corrosion cracking in brass.

The stress that produces stress corrosion cracking need notbe externally applied; it may be a residual one that resultsfrom rapid temperature changes or occur for two-phase alloysin which each phase has a different coefficient of expansion.Also, gaseous and solid corrosion products can give rise tointernal stresses.

STRESS CORROSION54



Figure 11. Photomicrographshowing intergranularstress corrosion cracking inbrass. (From H. H. Uhlig andR. W. Revie, Corrosion andCorrosion Control)

STRESS CORROSION55



HYDROGEN EMBRITTLEMENT56



9. Various metal alloys, specifically some steels,experience a significant reduction in ductility and tensilestrength when atomic hydrogen (H) penetrates into thematerial. This phenomenon is aptly referred to ashydrogen embrittlement.The terms hydrogen cracking and hydrogen stresscracking are sometimes also used. Strictly speaking,hydrogen embrittlement is a type of failure; in responseto applied or residual tensile stresses, brittle fractureoccurs catastrophically.




Conceptualized migration of hydrogen to the crack tip causing further cracking




Hydrogen in its atomic form (H as opposed to themolecular form, ) diffuses through the crystal lattice,and concentrations as low as several parts per millioncan lead to cracking.

Furthermore, hydrogen-induced cracks are most oftentransgranular, although intergranular fracture isobserved for some alloy systems.

𝐇𝟐




A number of mechanisms have been proposed to explainhydrogen embrittlement; most are based on the interferenceof dislocation motion by the dissolved hydrogen. Hydrogenembrittlement is similar to stress corrosion, in that a normallyductile metal experiences brittle fracture when exposed toboth a tensile stress and a corrosive atmosphere.




Hydrogen sulfide, probably the most aggressive toxic, isfound in petroleum fluids, natural gas, oil-well brines, andgeothermal fluids.High-strength steels are sensitive to hydrogenembrittlement, and increasing strength tends to enhancethe material’s susceptibility.Martensitic steels are especially vulnerable to this type offailure; bainitic and ferritic steels are more resilient.Furthermore, FCC alloys (austenitic stainless steels andalloys of copper, aluminum, and nickel) are relativelyresistant to hydrogen embrittlement.

CORROSION ENVIRONMENTS61


CAUSES OF DESTRUCTION AND DEGRADATION OF

VARIOUS MATERIALS OF DIFFERENT CLASSES

Corrosion environments include the atmosphere,aqueous solutions, soils, acids, bases, inorganicsolvents, molten salts, liquid metals, and, last but notleast, the human body.On a tonnage basis, atmospheric corrosion accounts

for the greatest losses.Moisture containing dissolved oxygen is the primarycorrosive agent, but other substances, including sulfurcompounds and sodium chloride, may also contribute.





This is especially true of sea atmospheres, which arehighly corrosive because of the presence of sodiumchloride.Sulfuric acid solutions (acid rain) in industrialenvironments can also cause corrosion problems.

Metals commonly used for atmospheric applicationsinclude alloys of aluminum and copper, and galvanizedsteel.





Seawater is generally more corrosive thanfreshwater, frequently producing pitting and crevicecorrosion. Cast iron, steel, aluminum, copper, brass,and some stainless steels are generally suitable forfreshwater use, whereas titanium, brass, somebronzes, copper–nickel alloys, and nickel–chromium–molybdenum alloys are highly corrosion resistant inseawater.

CORROSION PREVENTION64




Some corrosion prevention methods were treatedrelative to the eight forms of corrosion; however, onlythe measures specific to each of the various corrosiontypes were discussed. Now, some more generaltechniques are presented; these include:• material selection,• environmental alteration,• design,• coatings,• cathodic protection.





Perhaps the most common and easiest way of preventingcorrosion is through the selection of materials once thecorrosion environment has been characterized.

Standard corrosion references are helpful in this respect.Here, cost may be a significant factor. It is not alwayseconomically feasible to employ the material thatprovides the optimum corrosion resistance; sometimes,either another alloy and/or some other measure must beused.





Inhibitors are substances that, when added in relatively lowconcentrations to the environment, decrease itscorrosiveness. The specific inhibitor depends on both the alloyand the corrosive environment. Several mechanisms mayaccount for the effectiveness of inhibitors.

Other inhibitor attach themselves to the corroding surface andinterfere with either the oxidation or the reduction reaction orform a very thin protective coating.Inhibitors are normally used in closed systems such asautomobile radiators and steam boilers.





Physical barriers to corrosion are applied on surfaces in theform of films and coatings.

A large diversity of metallic and nonmetallic coating materialsis available. It is essential that the coating maintain a highdegree of surface adhesion, which requires some surfacetreatment. In most cases, the coating must be virtuallynonreactive in the corrosive environment and resistant tomechanical damage that exposes the metal to the corrosiveenvironment. All three material types—metals, ceramics, andpolymers—are used as coatings for metals.

OXIDATION68




In addition, oxidation of metal alloys is also possible ingaseous atmospheres, normally air, in which an oxidelayer forms on the surface of the metal.This phenomenon is frequently termed scaling,tarnishing, or dry corrosion.

In this section, we discuss possible mechanisms forthis type of corrosion, the types of oxide layers thatcan form, and the kinetics of oxide formation.

OXIDATION69




As with aqueous corrosion, the process of oxide layerformation is an electrochemical one, which may beexpressed, for divalent metal M, by the followingreaction:

𝑀 +1

2𝑂2 → 𝑀𝑂

OXIDATION70




Thus, the oxide scale serves both as an electrolytethrough which ions diffuse and as an electrical circuitfor the passage of electrons.

Furthermore, the scale may protect the metal fromrapid oxidation when it acts as a barrier to ionicdiffusion and/or electrical conduction; most metaloxides are highly electrically insulative.

OXIDATION71




Several techniques are available for improving theoxidation resistance of a metal. Аpplication of aprotective surface coating of another material thatadheres well to the metal also is resistant tooxidation.

In some instances, the addition of alloying elementswill form a more adherent and protective oxide scale.

DEGRADATION OF POLYMERS72




Polymeric materials also experience deterioration bymeans of environmental interactions. However, anundesirable interaction is specified as degradation ratherthan corrosion because the processes are basicallydissimilar.Мost metallic corrosion reactions are electrochemical,polymeric degradation is physiochemical phenomena.

Furthermore, a wide variety of reactions and adverseconsequences are possible for polymer degradation.Polymers may deteriorate by swelling and dissolution.





Covalent bond rupture as a result of heat energy,chemical reactions and radiation is also possible. Becauseof the chemical complexity of polymers, their degradationmechanisms are not well understood.

To cite a couple of examples of polymer degradation,polyethylene, if exposed to high temperatures in anoxygen atmosphere, suffers an impairment of itsmechanical properties by becoming brittle.

SWELLING AND DISSOLUTION75




When polymers are exposed to liquids, the main forms ofdegradation are swelling and dissolution. With swelling,the liquid or solute diffuses into and is absorbed withinthe polymer; the small solute molecules fit into andoccupy positions among the polymer molecules.

Thus the macromolecules are forced apart such that thespecimen expands or swells. Swelling may be considered apartial dissolution process in which there is only limitedsolubility of the polymer in the solvent.

SWELLING AND DISSOLUTION76




Dissolution, which occurs when the polymer iscompletely soluble, may be thought of as a continuation ofswelling. Swelling and dissolution traits also are affectedby temperature, as well as by the characteristics of themolecular structure.

In general, increasing molecular weight, increasing degreeof crosslinking and crystallinity, and decreasingtemperature result in a reduction of these deteriorativeprocesses.

RADIATION EFFECTS77




Certain types of radiation [electron beams, x-rays, b-and g-rays, and ultraviolet (UV) radiation] possesssufficient energy to penetrate a polymer specimen andinteract with the constituent atoms or their electrons.One such reaction is ionization, in which the radiationremoves an orbital electron from a specific atom,converting that atom into a positively charged ion.

RADIATION EFFECTS78




As a consequence, one of the covalent bondsassociated with the specific atom is broken and thereis a rearrangement of atoms or groups of atoms at thatpoint. This bond breaking leads to either scission orcrosslinking at the ionization site, depending on thechemical structure of the polymer and also on thedose of radiation.

RADIATION EFFECTS79




In day-to-day use, the greatest radiation damage topolymers is caused by UV irradiation. After prolongedexposure, most polymer films become brittle, discolor,crack, and fail. For example, camping tents begin to tear,dashboards develop cracks, and plastic windows becomecloudy.

Radiation problems are more severe for someapplications. Polymers on space vehicles must resistdegradation after prolonged exposures to cosmicradiation.

CHEMICAL REACTION EFFECTS80




Oxygen, ozone, and other substances can cause as a result ofchemical reaction. This effect is especially prevalent invulcanized rubbers that have doubly bonded carbon atomsalong the backbone molecular chains and that are exposed toozone, an atmospheric pollutant.

CHEMICAL REACTION EFFECTS81




However, when these materials are subjected totensile stresses, cracks and grow in a directionperpendicular to the stress; eventually, rupture of thematerial may occur. This is why the sidewalls onrubber bicycle tires develop cracks as they age.Apparently these cracks result from large numbers ofozone-induced scissions. Chemical degradation is aparticular problem for polymers used in areas withhigh levels of air pollutants such as smog and ozone.

THERMAL EFFECTS82




Thermal degradation corresponds to the scission of molecularchains at elevated temperatures; as a consequence, somepolymers undergo chemical reactions in which gaseous speciesare produced.

These reactions are evidenced by a weight loss of material; apolymer’s thermal stability is a measure of its resilience to thisdecomposition.

Thermal stability is related primarily to the magnitude of thebonding energies between the various atomic constituents ofthe polymer.

WEATHERING83




Many polymeric materials serve in applications thatrequire exposure to outdoor conditions.Any resultant degradation is termed weathering,which may be a combination of several differentprocesses. Under these conditions, deterioration isprimarily a result of oxidation, which is initiated byultraviolet radiation from the sun.

WEATHERING84




Some polymers, such as nylon and cellulose, are alsosensitive to water absorption, which produces areduction in their hardness and stiffness.

Resistance to weathering among the variouspolymers is quite diverse. The fluorocarbons arevirtually inert under these conditions; some materials,including poly(vinyl chloride) and polystyrene, aresensitive to weathering.

CATHODIC PROTECTION85


METHODS OF PREVENTING CORROSION

One of the most effective means of corrosion prevention iscathodic protection; it can be used for all eight differentforms of corrosion.Cathodic protection simply involves supplying, from anexternal source, electrons to the metal to be protected,making it a cathode.One cathodic protection technique employs a galvaniccouple: the metal to be protected is electrically connectedto another metal that is more reactive in the particularenvironment.




The latter experiences oxidation and, upon giving upelectrons, protects the first metal from corrosion. Theoxidized metal is often called a sacrificial anode, andmagnesium and zinc are commonly used becausethey lie at the anodic end of the galvanic series.

This form of galvanic protection for structures buriedin the ground is illustrated in Figure 12.




Figure 12. Cathodic protection of (a) an underground pipeline usinga magnesium sacrificial anode and (b) an underground tank using animpressed current. (From M. G. Fontana, Corrosion Engineering

ANODIC PROTECTION88



SACRIFICIAL ANODES

• Galvanization of Steel• Dip steel sheet in molten zinc. Get a pretty thin coating.• Zinc will be anode. Steel exposed by crack is the cathode.

Since we have a huge anode having to be served by a small cathode, corrosion rate will be slow.

Large area anode (zinc)

Tiny cathode (steel)

An example of a favorable area ratio. Bad deal: huge cathode, tiny anode

ANODIC PROTECTION89



ANOTHER EXAMPLE

• Zinc is attached to the steel hull of the vessel.

ANODIC PROTECTION90



SACRIFICIAL ANODE FOR A PIPELINE

ANODIC PROTECTION91



Aluminium anodes mounted on a steel jacket structure – using galvanic corrosion for corrosion control! Called cathodic protection

http://upload.wikimedia.org/wikipedia/en/f/f1/Anodes-on-jacket.jpg

http://upload.wikimedia.org/wikipedia/en/f/f1/Anodes-on-jacket.jpg

ANODIC PROTECTION92



PASSIVATION OF THE ANODE

• We have two examples already. Stainless and aluminum.• A thin oxide layer forms on the surface and isolates the

metal from the environment.• Zn, Mg, Cu and Ti are also capable of passivation under

normal conditions of operation.• Steel will also passivate in the presence of an alkaline

environment, such as rebar in concrete.• Corrosion inhibitors. Some of these, such as the

chromates, are capable of coating a steel and passivating it.• Coatings, paints, etc.

CORROSION COSTS AND PREVENTIVE STRATEGIES STUDY93



The most widely used metallic coating method forcorrosion protection is galvanizing, which involves theapplication of metallic zinc to carbon steel for corrosioncontrol purposes.

Hot-dip galvanizing is the most common process, andas the name implies, it consists of dipping the steelmember into a bath of molten zinc.

CORROSION COSTS AND PREVENTIVE STRATEGIES STUDY94



Metallizing is defined as the application of very thinmetallic coatings for either active corrosion protection (zincor aluminum anodes) or as a protective layer (stainlesssteels and alloys).

Application can be by flame spraying or electroplating.Other advanced processes such as plasma arc spraying canbe used for exotic refractory metals for very demandingapplications, but most of the advanced processes are notused for corrosion control.

95


IDENTIFICATION OF CORROSION DAMAGES

Test methods

MECHANICAL

CHEMICAL

Metallographic examination

and X-RAY

test methods

Non-destructive

methods

METHODS OF METALLOGRAPHY96






EN/ISO 17635:2010 (EN 12062)

• Acoustic - АТ• Capillary - CТ• Magnetic -МТx• Optical - OТ• Radiation - RT• Heat - HV

• Electromagnetic - EM




Methods of radiationcontrol based on the detection and

analysis of penetrating ionizing radiation upon interaction with the

controlled objects.

Methods of radiation control

radiographicradioskopic

radiometric

VIBRATION CONTROL METHOD99



The mechanism operates noise allows the judge maintenance or failure mechanism, and

even the type of failure.

VIBRATION CONTROL METHOD100



1 - piezoceramic plates; 2 – dampers; 3 - protector; 4 - liquid contact; 5 - the subject

of control; 6 - transmitter housing; 7 -conductor; 8 - prism

TYPES OF TRANSDUCERS EDDY101



Controls are carried out without any contact with the product

METHOD MAGNETIZATION102



Applying a magnetic field to the object of control near the surface of which there is

a defect that occurs spatial differentiation magnetic field and

created by field dipole

CORROSION AND DEGRADATION OF MATERIALS · CORROSION AND DEGRADATION OF MATERIALS Head of...

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