CORROSION AND DEGRADATION OF MATERIALS

Transcription

1 LVIV POLYTECHNIC NATIONAL UNIVERSITY 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)

2 2 ABOUT LVIV Lviv one of the most beautiful cities in Ukraine. Lviv is the European pearl and the capital of Galicia. Our city have successfully remained intact throughout seven ages, preserving its historical atmosphere and old architecture.

3 3 ABOUT LVIV Life of the historical central part of Lviv goes slowly. You will not find noisy crowds or clatter of autos there. Taking a walk along central part of old city during all the day you will find many interesting рlaces. If you ever have been in Lviv, you will come here again and again.

4 4 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 corrosion takes place and the different ways unwanted corrosion can be controlled. Мanufacturing some metal products can be very costly. Corrosion deterioration of manufactured products, damaging their structure and ultimately rendering the product useless. Allowing corrosion is not cost efficient and can inhibit productivity. Understanding and preventing corrosion is important for maintaining infrastructures and machinery that face corrosion.

5 5 With knowledge of the types and an understanding of the mechanisms and causes of corrosion and degradation, it is possible to take measures to prevent them from occurring. For example, we may : change the nature of the environment, select a material that is relatively nonreactive, and/or protect the material from degradation.

6 6 Corrosion is defined as the destructive attack on a metal - it is electrochemical and ordinarily begins at the surface. The problem of metallic corrosion is significant. It has been estimated that approximately 5% of an industrialized nation s income is spent on corrosion prevention and replacement of products lost as a result of corrosion reactions.

7 Loss of mass in metals and alloys is called corrosion or the formation of nonmetallic film оn the surface of metals is called oxidation. Ceramic materials are relatively resistant to deterioration, which usually occurs at elevated temperatures or in rather extreme environments. This process is frequently also called corrosion. 7 DETERIORATIVE MECHANISMS ARE DIFFERENT FOR THE THREE MATERIAL TYPES

8 For polymers, mechanisms differ from those for metals and ceramics, and the term degradation is most frequently used. Polymers may dissolve when exposed to a liquid solvent, or they may absorb the solvent. Еlectromagnetic ultraviolet radiation and heat may cause alterations in their molecular structures. 8 DETERIORATIVE MECHANISMS ARE DIFFERENT FOR THE THREE MATERIAL TYPES

9 9 ELECTROCHEMICAL CONSIDERATIONS For metallic materials, the corrosion process is normally electrochemical. А chemical reaction in which there is transfer of electrons from one chemical species to another. Metal atoms characteristically lose or give up electrons in what is called an oxidation reaction. For example, a hypothetical metal M that has a valence of n (or n valence electrons) may experience oxidation according to the reaction: M M n+ + n e

10 10 ELECTROCHEMICAL CONSIDERATIONS THE TYPES OF CORROSION AND ORROSION DAMAGE in which M becomes an n positively charged ion and in the process loses its n valence electrons; e is used to symbolize an electron. Examples in which metals oxidize are: Fe Fe e Al Al e

11 11 ELECTROCHEMICAL CONSIDERATIONS Other reduction reactions are possible, depending on the nature of the solution to which the metal is exposed. For an acid solution having dissolved oxygen, reduction according to: O 2 + 4H + + 4e 2H 2 O For a neutral or basic aqueous solution in which oxygen is also dissolved: O 2 + H 2 O + 4e 4(OH )

12 12 ELECTROCHEMICAL CONSIDERATIONS 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 M n+ + e M n 1 + in which the metal ion decreases its valence state by accepting an electron. A metal may be totally reduced from an ionic to a neutral metallic state according to: M n+ + ne M

13 13 ELECTROCHEMICAL CONSIDERATIONS The location at which reduction occurs is called the cathode. It is possible for two or more of the preceding reduction reactions to occur simultaneously. An overall electrochemical reaction must consist of at least one oxidation and one reduction reaction and will be their sum.

14 14 ELECTRODE POTENTIALS Not all metallic materials oxidize to form ions with the same degree of ease. Consider the electrochemical cell shown in Figure 2. On the left-hand side is a piece of pure iron immersed in a solution containing Fe 2+ ions of 1 M concentration. The other side of the cell consists of a pure copper electrode in a 1 M solution of Cu 2+ ions. The cell halves are separated by a membrane, which limits the mixing of the two solutions.

15 Figure 1. The electrochemical reactions associated with the corrosion of zinc in an acid solution. (From M. G. Fontana, Corrosion Engineering, 3rd edition) 15 ELECTRODE POTENTIALS Figure 2. An electro-chemical cell consisting of iron and copper electrodes, each of which is immersed in a 1 M solution of its ion. Iron corrodes while copper electrodeposits

16 16 ELECTRODE POTENTIALS Now consider another galvanic couple consisting of the same iron half-cell connected to a metal zinc electrode that is immersed in a 1 M solution of Zn 2+ ions (Figure 3). In this case, the zinc is the anode and corrodes, whereas the Fe becomes the cathode. The electrochemical reaction is thus: Fe 2+ + Zn Fe + Zn 2+

17 17 ELECTRODE POTENTIALS Consequently, metallic materials may be rated as to their tendency to experience oxidation when coupled to other metals in solutions of their respective ions. A half-cell similar to those just described [i.e., a pure metal electrode immersed in a 1 M solution of its ions and at 25C (77F)] is termed a standard halfcell. Figure 3. An electrochemical cell consisting of iron and zinc electrodes, each of which is immersed in a 1 M solution of its ion. The iron electrodeposits while the zinc corrodes.

18 The corrosion rate, or the rate of material removal as a consequence of the chemical action, is an important corrosion parameter. This may be expressed as the corrosion penetration rate (CPR), or the thickness loss of material per unit of time. The formula for this calculation is where W is the weight loss after exposure time t; r and A represent the density and exposed specimen area and K is a constant, its magnitude depending on the system of units used. 18 CPR = KW pat CORROSION RATES Corrosion penetration rate as a function of specimen weight loss, density, area, and time of exposure

19 19 PASSIVITY Under particular environmental conditions, some normally active metals and alloys, lose their chemical reactivity and become extremely inert. This phenomenon, termed passivity, is displayed by chromium, iron, nickel, titanium, and many of their alloys. It is believed that this passive behavior results from the formation of a highly adherent and very thin oxide film on the metal surface, which serves as a protective barrier to further corrosion.

20 Inasmuch as there is an electric current associated with electrochemical corrosion reactions, we can also express corrosion rate in terms of this current, or current density the current per unit surface area of material corroding which is designated i. The rate r, in units of mol m 2 s r = i n F Expression relating corrosion rate and current density 20 is determined using the expression. PASSIVITY

21 21 PASSIVITY Stainless steels are highly resistant to corrosion in a rather wide variety of atmospheres as a result of passivation. They contain at least 11% chromium, which as a solidsolution alloying element in iron, minimizes the formation of rust; instead, a protective surface film forms in oxidizing atmospheres. Stainless steels are admitting to corrosion in some environments and therefore are not always stainless.

22 22 PASSIVITY Aluminum is highly corrosion resistant in many environments because it also passivates. If damaged, the protective film normally re-forms very rapidly. However, a change in the character of the environment may cause a passivated material to revert to an active state. Subsequent damage to a preexisting passive film could result in a substantial increase in corrosion rate, by as much as 100,000 times.

23 23 PASSIVITY This passivation phenomenon may be explained in terms of polarization potentiallog current density curves discussed in the preceding section. The polarization curve for a metal that passivates has the general shape shown in Figure 4.

24 24 PASSIVITY Figure 4. Schematic polarization curve for a metal that displays an active passive transition.

25 25 PASSIVITY At relatively low potential values, within the active region the behavior is linear, as it is for normal metals. With increasing potential, the current density suddenly decreases to a very low value that remains independent of potential; this is termed the passive region. Finally, at even higher potential values, the current density again increases with potential in the transpassive region.

26 26 FORMS OF CORROSION It is convenient to classify corrosion according to the manner in which it is manifest. Metallic corrosion is sometimes classified into eight forms: uniform, galvanic, crevice, pitting, intergranular, selective leaching, erosion corrosion, stress corrosion.

27 27 FORMS OF CORROSION The causes and means of prevention of each of these forms are discussed briefly. In addition, we have elected to discuss the topic of hydrogen embrittlement. Hydrogen embrittlement is a type of failure rather than a form of corrosion; however, it is often produced by hydrogen that is generated from corrosion reactions.

28 28 1.Uniform attack is a form of electrochemical corrosion that occurs with equivalent intensity over the entire exposed surface and often leaves behind a scale or deposit. In a microscopic sense, the oxidation and reduction reactions occur randomly over the surface. Familiar examples include general rusting of steel and iron and the tarnishing of silverware. UNIFORM ATTACK

29 29 UNIFORM ATTACK This is probably the most common form of corrosion. It is also the least objectionable because it can be predicted and designed for with relative ease.

30 30 GALVANIC CORROSION 2. Galvanic corrosion occurs when two metals or alloys having different compositions are electrically coupled while exposed to an electrolyte. The less noble or more reactive metal in the particular environment experiences corrosion (the more inert metal, the cathode, is protected from corrosion). As examples, steel screws corrode when in contact with brass in a marine environment. Аnd if copper and steel tubing are joined in a domestic water heater, the steel corrodes.

31 31 GALVANIC CORROSION Figure 5. Photograph showing galvanic corrosion around the inlet of a single-cycle bilge pump that is found on fishing vessels. Corrosion occurred between a magnesium shell that was cast around a steel core.

32 The rate of galvanic attack depends on the relative anodeto-cathode surface areas that are exposed to the electrolyte. 32 A number of measures may be taken to reduce the effects of galvanic 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. GALVANIC CORROSION

33 33 CREVICE CORROSION Electrochemical corrosion may also occur as a consequence of concentration differences of ions or dissolved gases in the electrolyte solution and between two regions of the same metal piece. For such a concentration cell, corrosion occurs in the locale that has the lower concentration. A good example of this type of corrosion occurs in crevices or under deposits of dirt or corrosion products where the solution becomes stagnant and there is localized depletion of dissolved oxygen.

34 34 CREVICE CORROSION 3. Corrosion preferentially occurring at these positions is called crevice corrosion. The crevice must be wide enough for the solution to penetrate yet narrow enough for stagnancy. Crevice corrosion may be prevented by using welded instead of riveted or bolted joints, using nonabsorbing gaskets when possible and removing accumulated deposits frequently.

35 35 CREVICE CORROSION Electrons from this electrochemical reaction are conducted through the metal to adjacent external regions. In many aqueous environments, the solution within the crevice has been found to develop high concentrations of H and Cl ions, which are especially corrosive. Many alloys that passivate are susceptible to crevice corrosion because protective films are often destroyed by the H and Cl ions.

36 36 Figure 6. Schematic illustration of the mechanism of crevice corrosion between two riveted sheets. (From M. G. Fontana, Corrosion Engineering, 3rd edition. CREVICE CORROSION

37 37 PITTING CORROSION 4. Pitting is another form of very localized corrosion attack in which small pits or holes form. They ordinarily penetrate from the top of a horizontal surface downward in a nearly vertical direction. It is an extremely type of corrosion, often going undetected and with very little material loss until failure occurs. An example of pitting corrosion is shown in Figure.

38 38 PITTING CORROSION The mechanism for pitting is probably the same as for crevice corrosion, in that oxidation occurs within the pit itself, with complementary reduction at the surface. It is supposed that gravity causes the pits to grow downward, the solution at the pit tip becoming more concentrated and dense as pit growth progresses.

39 39 PITTING CORROSION A pit may be initiated by a localized surface defect such as a scratch or a slight variation in composition. In fact, it has been observed that specimens having polished surfaces display a greater resistance to pitting corrosion. Stainless steels are somewhat susceptible to this form of corrosion; however, alloying with about 2% molybdenum enhances their resistance significantly.

40 40 INTERGRANULAR CORROSION 5. As the name suggests, intergranular corrosion occurs preferentially along grain boundaries for some alloys and in specific environments. The net result is that a macroscopic specimen disintegrates along its grain boundaries. This type of corrosion is especially prevalent in some stainless steels. It is believed that this heat treatment permits the formation of small precipitate particles of chromium carbide ( Cr 23 C 6 ) by reaction between the chromium and carbon in the stainless steel. These particles form along the grain boundaries, as illustrated in Figure 7.

41 41 INTERGRANULAR CORROSION Figure 7. Schematic illustration of chromium carbide particles that have precipitated along grain boundaries in stainless steel, and the attendant zones of chromium depletion

42 42 INTERGRANULAR CORROSION Both the chromium and the carbon must diffuse to the grain boundaries to form the precipitates, which leaves a chromium-depleted zone adjacent to the grain boundary. Consequently, this grain boundary region is now highly sensitive to corrosion.

43 43 INTERGRANULAR CORROSION Intergranular corrosion is an especially problem in the welding of stainless steels, when it is often termed weld decay. Figure 8 shows this type of intergranular corrosion. Figure 8. Weld decay in a stainless steel. The regions along which the grooves have formed were sensitized as the weld cooled. (From H. H. Uhlig and R. W. Revie, Corrosion and Corrosion Control, 3rd edition)

44 44 INTERGRANULAR CORROSION Stainless steels may be protected from intergranular corrosion by the following process: (1) subjecting the material to a high-temperature heat treatment in which all the chromium carbide particles are redissolved, (2) lowering the carbon content below 0.03 wt% C so that carbide formation is minimal, and (3) alloying the stainless steel with another metal such as niobium or titanium, which has a greater tendency to form carbides than does chromium so that the Cr remains in solid solution.

45 45 SELECTIVE LEACHING 6. Selective leaching is found in solid solution alloys and occurs when one element is preferentially removed as a consequence of corrosion processes. The most common example is the dezincification of brass, in which zinc is selectively leached from a copper zinc brass alloy. The mechanical properties of the alloy are significantly impaired because only a porous mass of copper remains in the region that has been dezincified. In addition, the material changes from yellow to a red or copper color.

46 46 SELECTIVE LEACHING Selective leaching may also occur with other alloy systems in which aluminum, iron, cobalt, chromium, and other elements are vulnerable to preferential removal.

47 47 EROSION CORROSION 7. Erosion corrosion arises from the combined action of chemical attack and mechanical abrasion or wear as a consequence of fluid motion.

48 48 EROSION CORROSION Virtually all metal alloys, to one degree or another, are susceptible to erosion corrosion. It is especially harmful to alloys that passivate by forming a protective surface film. The abrasive action may erode away the film, leaving exposed a metal surface. If the coating is not capable and rapidly re-forming as a protective barrier, corrosion may be severe. Relatively soft metals such as copper and lead are also sensitive to this form of attack. Usually it can be identified by surface grooves and waves having contours that are characteristic of the flow of the fluid.

49 49 EROSION CORROSION Erosion corrosion is commonly found in piping, especially at bends and changes in pipe diameter positions where the fluid changes direction or flow suddenly becomes turbulent. Propellers, turbine blades and pumps are also susceptible to this form of corrosion. Figure 9 illustrates the failure of an elbow fitting. One of the best ways to reduce erosion corrosion is to change the design to eliminate fluid turbulence effects. Other materials may also be used that resist erosion.

50 50 EROSION CORROSION Figure 9. Impingement failure of an elbow that was part of a steam condensate line. (Photograph courtesy of Mars G. Fontana. From M. G. Fontana, Corrosion Engineering, 3rd edition. Copyright 1986 by McGraw- Hill Book Company. Reproduced with permission)

51 51 STRESS CORROSION 8. Stress corrosion, sometimes termed stress corrosion cracking, results from the combined action of an applied tensile stress and a corrosive environment; both influences are necessary. In fact, some materials that are virtually inert in a particular corrosive medium become susceptible to this form of corrosion when a stress is applied. Small cracks form and then propagate in 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.

52 52 STRESS CORROSION Figure 10. A bar of steel bent into a shape using a nutand-bolt. While immersed in seawater, stress corrosion cracks formed along the bend at the regions where the tensile stresses are the greatest. (Photograph courtesy of F. L. LaQue. From F. L. LaQue, Marine Corrosion, Causes and Prevention)

53 53 STRESS CORROSION Most alloys are sensitive to stress corrosion in specific environments. For example, most stainless steels stress corrode in solutions containing chloride ions. Figure 11 is a photomicrograph showing an example of intergranular stress corrosion cracking in brass. The stress that produces stress corrosion cracking need not be externally applied; it may be a residual one that results from rapid temperature changes or occur for two-phase alloys in which each phase has a different coefficient of expansion. Also, gaseous and solid corrosion products can give rise to internal stresses.

54 54 STRESS CORROSION Figure 11. Photomicrograph showing intergranular stress corrosion cracking in brass. (From H. H. Uhlig and R. W. Revie, Corrosion and Corrosion Control)

55 55 STRESS CORROSION

56 56 HYDROGEN EMBRITTLEMENT 9. Various metal alloys, specifically some steels, experience a significant reduction in ductility and tensile strength when atomic hydrogen (H) penetrates into the material. This phenomenon is aptly referred to as hydrogen embrittlement. The terms hydrogen cracking and hydrogen stress cracking are sometimes also used. Strictly speaking, hydrogen embrittlement is a type of failure; in response to applied or residual tensile stresses, brittle fracture occurs catastrophically.

57 57 Conceptualized migration of hydrogen to the crack tip causing further cracking HYDROGEN EMBRITTLEMENT

58 58 HYDROGEN EMBRITTLEMENT Hydrogen in its atomic form (H as opposed to the molecular form, H) 2 diffuses through the crystal lattice, and concentrations as low as several parts per million can lead to cracking. Furthermore, hydrogen-induced cracks are most often transgranular, although intergranular fracture is observed for some alloy systems.

59 59 HYDROGEN EMBRITTLEMENT A number of mechanisms have been proposed to explain hydrogen embrittlement; most are based on the interference of dislocation motion by the dissolved hydrogen. Hydrogen embrittlement is similar to stress corrosion, in that a normally ductile metal experiences brittle fracture when exposed to both a tensile stress and a corrosive atmosphere.

60 60 HYDROGEN EMBRITTLEMENT Hydrogen sulfide, probably the most aggressive toxic, is found in petroleum fluids, natural gas, oil-well brines, and geothermal fluids. High-strength steels are sensitive to hydrogen embrittlement, and increasing strength tends to enhance the material s susceptibility. Martensitic steels are especially vulnerable to this type of failure; bainitic and ferritic steels are more resilient. Furthermore, FCC alloys (austenitic stainless steels and alloys of copper, aluminum, and nickel) are relatively resistant to hydrogen embrittlement.

61 61 CORROSION ENVIRONMENTS CAUSES OF DESTRUCTION AND DEGRADATION OF VARIOUS MATERIALS OF DIFFERENT CLASSES Corrosion environments include the atmosphere, aqueous solutions, soils, acids, bases, inorganic solvents, molten salts, liquid metals, and, last but not least, the human body. On a tonnage basis, atmospheric corrosion accounts for the greatest losses. Moisture containing dissolved oxygen is the primary corrosive agent, but other substances, including sulfur compounds and sodium chloride, may also contribute.

62 62 CORROSION ENVIRONMENTS CAUSES OF DESTRUCTION AND DEGRADATION OF VARIOUS MATERIALS OF DIFFERENT CLASSES This is especially true of sea atmospheres, which are highly corrosive because of the presence of sodium chloride. Sulfuric acid solutions (acid rain) in industrial environments can also cause corrosion problems. Metals commonly used for atmospheric applications include alloys of aluminum and copper, and galvanized steel.

63 63 CORROSION ENVIRONMENTS CAUSES OF DESTRUCTION AND DEGRADATION OF VARIOUS MATERIALS OF DIFFERENT CLASSES Seawater is generally more corrosive than freshwater, frequently producing pitting and crevice corrosion. Cast iron, steel, aluminum, copper, brass, and some stainless steels are generally suitable for freshwater use, whereas titanium, brass, some bronzes, copper nickel alloys, and nickel chromium molybdenum alloys are highly corrosion resistant in seawater.

64 64 CORROSION PREVENTION CAUSES OF DESTRUCTION AND DEGRADATION OF VARIOUS MATERIALS OF DIFFERENT CLASSES Some corrosion prevention methods were treated relative to the eight forms of corrosion; however, only the measures specific to each of the various corrosion types were discussed. Now, some more general techniques are presented; these include: material selection, environmental alteration, design, coatings, cathodic protection.

65 65 CORROSION PREVENTION CAUSES OF DESTRUCTION AND DEGRADATION OF VARIOUS MATERIALS OF DIFFERENT CLASSES Perhaps the most common and easiest way of preventing corrosion is through the selection of materials once the corrosion environment has been characterized. Standard corrosion references are helpful in this respect. Here, cost may be a significant factor. It is not always economically feasible to employ the material that provides the optimum corrosion resistance; sometimes, either another alloy and/or some other measure must be used.

66 66 CORROSION PREVENTION CAUSES OF DESTRUCTION AND DEGRADATION OF VARIOUS MATERIALS OF DIFFERENT CLASSES Inhibitors are substances that, when added in relatively low concentrations to the environment, decrease its corrosiveness. The specific inhibitor depends on both the alloy and the corrosive environment. Several mechanisms may account for the effectiveness of inhibitors. Other inhibitor attach themselves to the corroding surface and interfere with either the oxidation or the reduction reaction or form a very thin protective coating. Inhibitors are normally used in closed systems such as automobile radiators and steam boilers.

67 67 CORROSION PREVENTION CAUSES OF DESTRUCTION AND DEGRADATION OF VARIOUS MATERIALS OF DIFFERENT CLASSES Physical barriers to corrosion are applied on surfaces in the form of films and coatings. A large diversity of metallic and nonmetallic coating materials is available. It is essential that the coating maintain a high degree of surface adhesion, which requires some surface treatment. In most cases, the coating must be virtually nonreactive in the corrosive environment and resistant to mechanical damage that exposes the metal to the corrosive environment. All three material types metals, ceramics, and polymers are used as coatings for metals.

68 68 OXIDATION CAUSES OF DESTRUCTION AND DEGRADATION OF VARIOUS MATERIALS OF DIFFERENT CLASSES In addition, oxidation of metal alloys is also possible in gaseous atmospheres, normally air, in which an oxide layer forms on the surface of the metal. This phenomenon is frequently termed scaling, tarnishing, or dry corrosion. In this section, we discuss possible mechanisms for this type of corrosion, the types of oxide layers that can form, and the kinetics of oxide formation.

69 69 OXIDATION CAUSES OF DESTRUCTION AND DEGRADATION OF VARIOUS MATERIALS OF DIFFERENT CLASSES As with aqueous corrosion, the process of oxide layer formation is an electrochemical one, which may be expressed, for divalent metal M, by the following reaction: M O 2 MO

70 CAUSES OF DESTRUCTION AND DEGRADATION OF VARIOUS MATERIALS OF DIFFERENT CLASSES Thus, the oxide scale serves both as an electrolyte through which ions diffuse and as an electrical circuit for the passage of electrons. Furthermore, the scale may protect the metal from rapid oxidation when it acts as a barrier to ionic diffusion and/or electrical conduction; most metal oxides are highly electrically insulative. 70 OXIDATION

71 CAUSES OF DESTRUCTION AND DEGRADATION OF VARIOUS MATERIALS OF DIFFERENT CLASSES Several techniques are available for improving the oxidation resistance of a metal. Аpplication of a protective surface coating of another material that adheres well to the metal also is resistant to oxidation. In some instances, the addition of alloying elements will form a more adherent and protective oxide scale. 71 OXIDATION

72 CAUSES OF DESTRUCTION AND DEGRADATION OF VARIOUS MATERIALS OF DIFFERENT CLASSES Polymeric materials also experience deterioration by means of environmental interactions. However, an undesirable interaction is specified as degradation rather than corrosion because the processes are basically dissimilar. Мost metallic corrosion reactions are electrochemical, polymeric degradation is physiochemical phenomena. Furthermore, a wide variety of reactions and adverse consequences are possible for polymer degradation. Polymers may deteriorate by swelling and dissolution. 72 DEGRADATION OF POLYMERS

73 CAUSES OF DESTRUCTION AND DEGRADATION OF VARIOUS MATERIALS OF DIFFERENT CLASSES 73 DEGRADATION OF POLYMERS

74 CAUSES OF DESTRUCTION AND DEGRADATION OF VARIOUS MATERIALS OF DIFFERENT CLASSES Covalent bond rupture as a result of heat energy, chemical reactions and radiation is also possible. Because of the chemical complexity of polymers, their degradation mechanisms are not well understood. To cite a couple of examples of polymer degradation, polyethylene, if exposed to high temperatures in an oxygen atmosphere, suffers an impairment of its mechanical properties by becoming brittle. 74 DEGRADATION OF POLYMERS

75 CAUSES OF DESTRUCTION AND DEGRADATION OF VARIOUS MATERIALS OF DIFFERENT CLASSES When polymers are exposed to liquids, the main forms of degradation are swelling and dissolution. With swelling, the liquid or solute diffuses into and is absorbed within the polymer; the small solute molecules fit into and occupy positions among the polymer molecules. Thus the macromolecules are forced apart such that the specimen expands or swells. Swelling may be considered a partial dissolution process in which there is only limited solubility of the polymer in the solvent. 75 SWELLING AND DISSOLUTION

76 CAUSES OF DESTRUCTION AND DEGRADATION OF VARIOUS MATERIALS OF DIFFERENT CLASSES Dissolution, which occurs when the polymer is completely soluble, may be thought of as a continuation of swelling. Swelling and dissolution traits also are affected by temperature, as well as by the characteristics of the molecular structure. In general, increasing molecular weight, increasing degree of crosslinking and crystallinity, and decreasing temperature result in a reduction of these deteriorative processes. 76 SWELLING AND DISSOLUTION

77 CAUSES OF DESTRUCTION AND DEGRADATION OF VARIOUS MATERIALS OF DIFFERENT CLASSES Certain types of radiation [electron beams, x-rays, b- and g-rays, and ultraviolet (UV) radiation] possess sufficient energy to penetrate a polymer specimen and interact with the constituent atoms or their electrons. One such reaction is ionization, in which the radiation removes an orbital electron from a specific atom, converting that atom into a positively charged ion. 77 RADIATION EFFECTS

78 CAUSES OF DESTRUCTION AND DEGRADATION OF VARIOUS MATERIALS OF DIFFERENT CLASSES As a consequence, one of the covalent bonds associated with the specific atom is broken and there is a rearrangement of atoms or groups of atoms at that point. This bond breaking leads to either scission or crosslinking at the ionization site, depending on the chemical structure of the polymer and also on the dose of radiation. 78 RADIATION EFFECTS

79 CAUSES OF DESTRUCTION AND DEGRADATION OF VARIOUS MATERIALS OF DIFFERENT CLASSES In day-to-day use, the greatest radiation damage to polymers is caused by UV irradiation. After prolonged exposure, most polymer films become brittle, discolor, crack, and fail. For example, camping tents begin to tear, dashboards develop cracks, and plastic windows become cloudy. Radiation problems are more severe for some applications. Polymers on space vehicles must resist degradation after prolonged exposures to cosmic radiation. 79 RADIATION EFFECTS

80 CAUSES OF DESTRUCTION AND DEGRADATION OF VARIOUS MATERIALS OF DIFFERENT CLASSES Oxygen, ozone, and other substances can cause as a result of chemical reaction. This effect is especially prevalent in vulcanized rubbers that have doubly bonded carbon atoms along the backbone molecular chains and that are exposed to ozone, an atmospheric pollutant. 80 CHEMICAL REACTION EFFECTS

81 CAUSES OF DESTRUCTION AND DEGRADATION OF VARIOUS MATERIALS OF DIFFERENT CLASSES However, when these materials are subjected to tensile stresses, cracks and grow in a direction perpendicular to the stress; eventually, rupture of the material may occur. This is why the sidewalls on rubber bicycle tires develop cracks as they age. Apparently these cracks result from large numbers of ozone-induced scissions. Chemical degradation is a particular problem for polymers used in areas with high levels of air pollutants such as smog and ozone. 81 CHEMICAL REACTION EFFECTS

82 CAUSES OF DESTRUCTION AND DEGRADATION OF VARIOUS MATERIALS OF DIFFERENT CLASSES Thermal degradation corresponds to the scission of molecular chains at elevated temperatures; as a consequence, some polymers undergo chemical reactions in which gaseous species are produced. These reactions are evidenced by a weight loss of material; a polymer s thermal stability is a measure of its resilience to this decomposition. Thermal stability is related primarily to the magnitude of the bonding energies between the various atomic constituents of the polymer. 82 THERMAL EFFECTS

83 CAUSES OF DESTRUCTION AND DEGRADATION OF VARIOUS MATERIALS OF DIFFERENT CLASSES Many polymeric materials serve in applications that require exposure to outdoor conditions. Any resultant degradation is termed weathering, which may be a combination of several different processes. Under these conditions, deterioration is primarily a result of oxidation, which is initiated by ultraviolet radiation from the sun. 83 WEATHERING

84 CAUSES OF DESTRUCTION AND DEGRADATION OF VARIOUS MATERIALS OF DIFFERENT CLASSES Some polymers, such as nylon and cellulose, are also sensitive to water absorption, which produces a reduction in their hardness and stiffness. Resistance to weathering among the various polymers is quite diverse. The fluorocarbons are virtually inert under these conditions; some materials, including poly(vinyl chloride) and polystyrene, are sensitive to weathering. 84 WEATHERING

85 METHODS OF PREVENTING CORROSION One of the most effective means of corrosion prevention is cathodic protection; it can be used for all eight different forms of corrosion. Cathodic protection simply involves supplying, from an external source, electrons to the metal to be protected, making it a cathode. One cathodic protection technique employs a galvanic couple: the metal to be protected is electrically connected to another metal that is more reactive in the particular environment. 85 CATHODIC PROTECTION

86 METHODS OF PREVENTING CORROSION The latter experiences oxidation and, upon giving up electrons, protects the first metal from corrosion. The oxidized metal is often called a sacrificial anode, and magnesium and zinc are commonly used because they lie at the anodic end of the galvanic series. This form of galvanic protection for structures buried in the ground is illustrated in Figure CATHODIC PROTECTION

87 METHODS OF PREVENTING CORROSION Figure 12. Cathodic protection of (a) an underground pipeline using a magnesium sacrificial anode and (b) an underground tank using an impressed current. (From M. G. Fontana, Corrosion Engineering 87 CATHODIC PROTECTION

88 METHODS OFPREVENTING CORROSION 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 88 ANODIC PROTECTION

89 METHODS OF PREVENTING CORROSION ANOTHER EXAMPLE Zinc is attached to the steel hull of the vessel. 89 ANODIC PROTECTION

90 METHODS OF PREVENTING CORROSION SACRIFICIAL ANODE FOR A PIPELINE 90 ANODIC PROTECTION

91 METHODS OF PREVENTING CORROSION 91 Aluminium anodes mounted on a steel jacket structure using galvanic corrosion for corrosion control! Called cathodic protection ANODIC PROTECTION

92 METHODS OF PREVENTING CORROSION 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. 92 ANODIC PROTECTION

93 METHODS OF PREVENTING CORROSION The most widely used metallic coating method for corrosion protection is galvanizing, which involves the application of metallic zinc to carbon steel for corrosion control purposes. Hot-dip galvanizing is the most common process, and as the name implies, it consists of dipping the steel member into a bath of molten zinc. 93 CORROSION COSTS AND PREVENTIVE STRATEGIES STUDY

94 METHODS OF PREVENTING CORROSION Metallizing is defined as the application of very thin metallic coatings for either active corrosion protection (zinc or aluminum anodes) or as a protective layer (stainless steels and alloys). Application can be by flame spraying or electroplating. Other advanced processes such as plasma arc spraying can be used for exotic refractory metals for very demanding applications, but most of the advanced processes are not used for corrosion control. 94 CORROSION COSTS AND PREVENTIVE STRATEGIES STUDY

95 IDENTIFICATION OF CORROSION DAMAGES Test methods MECHANICAL CHEMICAL Metallographi c examination and X-RAY test methods Nondestructive methods 95

96 IDENTIFICATION OF CORROSION DAMAGES 96 METHODS OF METALLOGRAPHY

97 IDENTIFICATION OF CORROSION DAMAGES EN/ISO 17635:2010 (EN 12062) Acoustic - АТ Capillary - CТ Magnetic -МТx Optical - OТ Radiation - RT Heat - HV Electromagnetic - EM 97 METHODS OF METALLOGRAPHY

98 IDENTIFICATION OF CORROSION DAMAGES Methods of radiation control radiographic radioskopic Methods of radiation control based on the detection and analysis of penetrating ionizing radiation upon interaction with the controlled objects. radiometric 98 METHODS OF METALLOGRAPHY

99 IDENTIFICATION OF CORROSION DAMAGES The mechanism operates noise allows the judge maintenance or failure mechanism, and even the type of failure. 99 VIBRATION CONTROL METHOD

100 IDENTIFICATION OF CORROSION DAMAGES 100 VIBRATION CONTROL METHOD 1 - piezoceramic plates; 2 dampers; 3 - protector; 4 - liquid contact; 5 - the subject of control; 6 - transmitter housing; 7 - conductor; 8 - prism

101 IDENTIFICATION OF CORROSION DAMAGES Controls are carried out without any contact with the product 101 TYPES OF TRANSDUCERS EDDY

102 IDENTIFICATION OF CORROSION DAMAGES 102 METHOD MAGNETIZATION 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