References cited in this section 7. I. Epelboin and M. Keddam, J. Electrochem. Soc., Vol 117, 1970, p 1052 9. I. Epelboin, C. Gabrielli, M. Keddam, and H. Takenouti, Electrochemical Corrosion Testing, F. Mansfeld and U. Bertocci, Ed., STP 727, ASTM International, 1981, p 150–192 12. M. Keddam, Corrosion Mechanisms in Theory and Practice, P. Marcus, Ed., Marcel Dekker Inc, 2002, p 97 13. H. Takenouti, Electrochemistry, Vol 67, 1999, p 1063 14. K.E. Heusler, Z. Elektrochem., Vol 62, 1958, p 582 15. J.O'M. Bockris, D. Drazic, and A.R. Despic, Electrochim. Acta, Vol 4, 1961, p 325 16. M. Keddam, O. R. Mattos, and H. Takenouti, J. Electrochem. Soc., Vol 128, 1981, p 257, 266 17. N. Benzekri, M. Keddam, and H. Takenouti, Electrochim. Acta, Vol 34, 1989, p 1159 18. M. Itagaki, M. Tagaki, K. Watanabe, Denki Kagaku (Electrochemistry), Vol 63, 1995, p 425 19. O.E. Barcia, O.R. Mattos, and B. Tribollet, J. Electrochem. Soc. Vol 139, 1992, p 446 20. C. Gabrielli, M. Keddam, F. Minouflet-Laurent, and H. Perrot, Electrochem. Solid-State Letters, Vol 3, 2000, p 418 21. I. Annergren, M. Keddam, H. Takenouti, and D. Thierry, Electrochim. Acta, Vol 41, 1996, p 1121 22. R.P. Frankenthal, J. Electrochem. Soc., Vol 116, 1969, p 580 23. M. Keddam, O.R. Mattos, and H. Takenouti, Electrochim. Acta, Vol 31, 1993, p 1158 F. Huet, R.P. Nogueira, and H. Takenouti, Aqueous Corrosion Reaction Mechanisms, Corrosion: Fundamentals, Testing, and Protection, Vol 13A, ASM Handbook, ASM International, 2003, p 52–60 Aqueous Corrosion Reaction Mechanisms François Huet, Ricardo P. Nogueira, Bernard Normand, and Hisasi Takenouti, Université Pierre et Marie Curie and UPR 15 of CNRS, “Laboratoire Interfaces et Systèmes Electrochimiques,” Université Pierre et Marie Curie References 1. H. Kaesche, Metallic Corrosion, Principles of Physical Chemistry and Current Problems, NACE International, Houston, TX, 1985 2. F. Mansfeld and U. Bertocci, Ed., Electrochemical Corrosion Testing, STP 727, ASTM International, 1981 3. J.C. Scully, Ed., Treatise on Materials Science and Technology, Vol 23, Corrosion: Aqueous Processes and Passive Films, Academic Press, 1983 4. J.R. Scully, D.C. Silvermann, and M.W. Kendig, Ed., Electrochemical Impedance: Analysis and Interpretation, STP 1188, ASTM International, 1994 5. K.J. Vetter, Electrochemical Kinetics, Academic Press, 1967 6. A.J. Bard and L.R. Faulkner, Electrochemical Methods, Fundamentals and Applications, John Wiley & Sons, Inc., 1980 7. I. Epelboin and M. Keddam, J. Electrochem. Soc., Vol 117, 1970, p 1052 8. C. Wagner and W. Traud, Z. Elektrochem., Vol 44, 1938, p 391 9. I. Epelboin, C. Gabrielli, M. Keddam, and H. Takenouti, Electrochemical Corrosion Testing, F. Mansfeld and U. Bertocci, Ed., STP 727, ASTM International, 1981, p 150–192 10. M. Pourbaix, Lectures on Electrochemical Corrosion, Plenum Press, 1973 11. U.R. Evans, The Corrosion and Oxidation of Metals, Arnold Publications, Inc., 1960 12. M. Keddam, Corrosion Mechanisms in Theory and Practice, P. Marcus, Ed., Marcel Dekker Inc, 2002, p 97 13. H. Takenouti, Electrochemistry, Vol 67, 1999, p 1063 14. K.E. Heusler, Z. Elektrochem., Vol 62, 1958, p 582 15. J.O'M. Bockris, D. Drazic, and A.R. Despic, Electrochim. Acta, Vol 4, 1961, p 325 16. M. Keddam, O. R. Mattos, and H. Takenouti, J. Electrochem. Soc., Vol 128, 1981, p 257, 266 17. N. Benzekri, M. Keddam, and H. Takenouti, Electrochim. Acta, Vol 34, 1989, p 1159 18. M. Itagaki, M. Tagaki, K. Watanabe, Denki Kagaku (Electrochemistry), Vol 63, 1995, p 425 19. O.E. Barcia, O.R. Mattos, and B. Tribollet, J. Electrochem. Soc. Vol 139, 1992, p 446 20. C. Gabrielli, M. Keddam, F. Minouflet-Laurent, and H. Perrot, Electrochem. Solid-State Letters, Vol 3, 2000, p 418 21. I. Annergren, M. Keddam, H. Takenouti, and D. Thierry, Electrochim. Acta, Vol 41, 1996, p 1121 22. R.P. Frankenthal, J. Electrochem. Soc., Vol 116, 1969, p 580 23. M. Keddam, O.R. Mattos, and H. Takenouti, Electrochim. Acta, Vol 31, 1993, p 1158 F. Huet, R.P. Nogueira, and H. Takenouti, Aqueous Corrosion Reaction Mechanisms, Corrosion: Fundamentals, Testing, and Protection, Vol 13A, ASM Handbook, ASM International, 2003, p 52–60 Aqueous Corrosion Reaction Mechanisms François Huet, Ricardo P. Nogueira, Bernard Normand, and Hisasi Takenouti, Université Pierre et Marie Curie and UPR 15 of CNRS, “Laboratoire Interfaces et Systèmes Electrochimiques,” Université Pierre et Marie Curie Selected References • A.J. Bard and L.R. Faulkner, Electrochemical Methods, Fundamentals and Applications, John Wiley & Sons, Inc., 1980 • C. Gabrielli, “Identification of Electrochemical Processes by Frequency Response Analysis,” Solartron Instruments, Farnborough, U.K., 1980 • C. Gabrielli, “Use and Applications of Electrochemical Impedance Techniques”, Technical Report 24, Solartron Instruments, Farnborough, U.K., 1997 • H. Kaesche, Metallic Corrosion, Principles of Physical Chemistry and Current Problems, NACE International, 1985 • M. Keddam, Anodic Dissolution, Corrosion Mechanisms in Theory and Practice, P. Marcus, Ed., Marcel Dekker Inc., 2002, p 97 J. Kruger, Passivity, Corrosion: Fundamentals, Testing, and Protection, Vol 13A, ASM Handbook, ASM International, 2003, p 61–67 Passivity Jerome Kruger, Johns Hopkins University Introduction ALL METALS AND ALLOYS, with the exception of gold, have a thin protective corrosion product film present on their surface resulting from reaction with the environment. If such a film did not exist on metallic materials exposed to the environment, they would revert back to the thermodynamically stable condition of their origin—the ores used to produce them. Some of these films—the passive films—on some, but not all, metals and alloys have special characteristics that enable them to provide superior corrosion- resistant metal surfaces. These protective “passive” films are responsible for the phenomenon of passivity. The first metal found to exhibit the phenomenon of passivity was iron. Uhlig (Ref 1) has written a review of the history of passivity that lists three 18th century scientists—the Russian Lomonosov in 1738, the German Wenzel in 1782, and the Briton Keir in 1790—who observed that the highly reactive surface of iron became unexpectedly unreactive after immersion in concentrated nitric acid. This effect was first called passivity by Schönbein. This unexpected phenomenon of passivity occupies a central position in controlling corrosion processes, enabling the use of metallic materials in the many technologies of the 21st century. Moreover, it is the breakdown of the passive film that leads to the inability of metals and alloys to perform their assigned functions because of localized corrosion failure modes such as stress corrosion, pitting, crevice corrosion, and corrosion fatigue. Its importance to materials technology transcends, however, corrosion science and corrosion engineering. For example, one of the main reasons silicon replaced germanium in semiconductor device technology was that silicon forms effective passive films and germanium does not (Ref 2). Early work in the area of passivity that had an enormous impact on providing technology with improved engineering materials is, of course, the development of the stainless steels. This has promoted the continual development of a large number of alloys that exhibit corrosion resistance because of the protection provided by the passive film. An improved understanding of the role that alloying constituents play in determining the properties of this passive film will lead to guidelines that can be used to develop engineering alloys with improved corrosion resistance. The scope of this article limits the discussion of all of the details on the subject of passivity. Moreover, the passivation behavior of all of the various metals and semiconductors that exhibit passivity is not given. Instead, this article discusses the classic passive metal iron and its alloys as illustrative examples of metals exhibiting passivity. References 3, 4, 5, 6, 7 provide a more extensive treatment of the subject of passivity in general and passivity of other metals and semiconductors in addition to iron. References cited in this section 1. H.H. Uhlig, Passivity of Metals, R.P. Frankenthal and J. Kruger, Ed., Electrochemical Society, 1978, p 1–28 2. A.G. Revesz and J. Kruger, Passivity of Metals, R.P. Frankenthal and J. Kruger, Ed., Electrochemical Society, 1978, p 137 3. R.P. Frankenthal and J. Kruger, Ed., Passivity of Metals, Electrochemical Society, 1978 4. J. Kruger, Int. Mater. Rev., Vol 33, 1988, p 113–130 5. H. Hasegawa and T. Sugano, Ed., Passivation of Metals and Semiconductors, Part II, Passivity of Semiconductors, Pergamon Press, 1990 6. K.E. Heusler, Ed., Passivation of Metals and Semiconductors, Materials Science Forum, Vol 185–188, Trans Tech Publications, 1995 7. M.B. Ives, J.L. Luo, and J.R. Rodda, Ed., Passivity of Metals and Semiconductors, Proc. Vol 99–42, Electrochemical Society, 2001 J. Kruger, Passivity, Corrosion: Fundamentals, Testing, and Protection, Vol 13A, ASM Handbook, ASM International, 2003, p 61–67 Passivity Jerome Kruger, Johns Hopkins University General Aspects Importance of Passivity to Corrosion-Control Technology. If the passive film did not exist, most of the technologies that depend on the use of metals could not exist because the phenomenon of passivity is a critical element in controlling corrosion processes. Therefore, the destruction of passivity at local breakdowns leads to a large part of the corrosion failures of metal and alloy structures—localized attack such as pitting, crevice corrosion, stress corrosion, and corrosion fatigue. The development of the stainless steels in the 1920s is regarded as a major application of the phenomenon of passivity. This development has contributed significantly to modern technology by providing the design engineer with engineering materials such as the large number of iron and nickel-base alloys as well as many other alloy systems that exhibit superior corrosion resistance—this effort continues today. Types of Passivity. There are two types of passivity: • Type 1. A metal active in the electromotive force (emf) series is passive when its electrochemical behavior in a given environment becomes that of a metal noble in the emf series (low corrosion rate, noble potential). • Type 2. A metal is passive while, still from the standpoint of thermodynamics at an active potential in a given environment, it exhibits a low corrosion rate (low corrosion rate, active potential). This type of passivity can be termed “practical passivity.” Only type 1 passivity is considered here. Examples of metals or alloys exhibiting such passivity are nickel, chromium, titanium, iron in oxidizing environments, stainless steels, and many others. Examples of type 2 passivity are lead in sulfuric acid and iron in an inhibited pickling acid. A major characteristic of a type 1 passive system is the existence of a polarization curve (i, current density, or rate, versus E, potential, or driving force), of the sort shown in Fig. 1. It illustrates well a restatement of the definition of type 1 passivity as first proposed by Wagner (Ref 8). He suggested that a metal becomes passive when, upon increasing its potential in the positive or anodic (oxidizing) direction, a potential is reached where the current (rate of anodic dissolution) sharply decreases to a value less than that observed at a less anodic potential. This decrease in anodic dissolution rate, in spite of the fact that the driving force for dissolution is brought to a higher value, is the result of the formation of a passive film. Fig. 1 The idealized anodic polarization curve for an iron-water system exhibiting passivity. Three different potential regions are shown; the active, passive, and pitting or transpassive regions. E p is potential above which the system becomes passive and exhibits the passive current density i p . The critical current density for passivation is i c . Another more practical definition has been provided by an ASTM standard: “passive—the state of metal surface characterized by low corrosion rates in a potential region that is strongly oxidizing for the metal” (Ref 9). Employing Passivity to Control Corrosion. Passivity can be used to control corrosion by using methods that bring the potential of the surface to be protected to a value in the passive region. This can be accomplished by the following tactics: • Using a device called a potentiostat, a current can be applied to the metal to be protected that will set and control the potential at a value greater than the passivating potential, E p . This method of producing passivity is called anodic protection (Fig. 1). • For environments containing damaging species such as chloride ions that cause pitting, the potentiostat or other devices that control the potential can be used as in the item above to set the potential to a value in the passive region below the critical potential for pitting, E pit . • Alloys or metals that spontaneously form a passive film, for example, stainless steels, nickel, or titanium alloys, can be used in applications that require resistance to corrosion. Usually a pretreatment such as that described below is desirable. • A surface pretreatment can be carried out on an alloy capable of being passivated. The use of such a pretreatment has been a standard practice for stainless steels for many years. The passivating procedure involves immersion of thoroughly degreased stainless steel parts in a nitric acid solution followed by a thorough rinsing in clean, hot water. The most popular solution and conditions of operation for passivating stainless steel is a 30 min immersion in a 20 vol% nitric acid solution at 49 °C (120 °F). However, other solutions and treatments may be used, depending on the type of stainless steel being treated (Ref 10). • The environment can be modified to produce a passive surface. Oxidizing agents such as chromate and concentrated nitric acid are examples of passivating solutions that maintain a passive state on some metals and alloys. References cited in this section 8. C. Wagner, Corros. Sci., Vol 5, 1965, p 751–764 9. Definitions of Terms Relating to Corrosion and Corrosion Testing, G 13, Annual Book of ASTM Standards, ASTM, 1983 10. D. Peckner and I.M. Bernstein, Handbook of Stainless Steels, McGraw-Hill, 1977, Ch 24, p 24 J. Kruger, Passivity, Corrosion: Fundamentals, Testing, and Protection, Vol 13A, ASM Handbook, ASM International, 2003, p 61–67 Passivity Jerome Kruger, Johns Hopkins University Thermodynamics of Passivity Thermodynamics provide a guide to the conditions under which passivation becomes possible. A valuable guide to thermodynamics is the potential-pH diagram, the Pourbaix diagram. Pourbaix's Atlas of Electrochemical Equilibria in Aqueous Solutions (Ref 11) describes applications of these potential-pH equilibrium diagrams to corrosion science and engineering. One major application is the establishment of the theoretical domains or conditions for corrosion, immunity, and passivation. Figure 2 shows a simplified diagram for the iron-water system. The three theoretical domains show on a thermodynamic basis the potential- pH conditions where no corrosion is possible (immunity), where a corrosion-product film forms that may confer protection against corrosion (passivation), and where corrosion is expected (corrosion). (Pourbaix designates the immunity domain as that of “thermodynamic nobility” and the total of the passivation and immunity domains as that of “practical nobility.”) Whether the film is passive (protective) or not is a kinetic consideration and not a thermodynamic one (see the section “Nature of the Passive Film” in this article). Such Pourbaix diagrams can identify metals capable of forming films that, depending on their properties, may or may not be protective, and conditions can be determined where there is a transition from passivation to activation. One could call the equilibrium diagrams a “road map of the possible.” Fig. 2 Simplified potential-pH equilibrium diagram (Pourbaix diagram) for the iron-water system. Above equilibrium line A oxygen is evolved, and below equilibrium line B hydrogen is evolved. Source: Ref 11 The diagrams can, therefore, be used as a basis for identifying the active, passive, and transpassive regions of active-passive polarization curves (see Fig. 1). Thus, potentials above the oxygen-evolution line (the line marked A in Fig. 2) are in the transpassive region. Also, the diagrams can be used to interpret the reasons for loss of the protective nature of the passive film in the transpassive region. For example, the protective layer on stainless steels that contain chromium involves Cr(III); at higher potentials Cr(III) is oxidized to Cr(VI), and the protective Cr 2 O 3 becomes the soluble chromate ion, resulting in the loss of corrosion resistance. Usually, the passive regions of the polarization curves correspond to potentials in the equilibrium diagrams where protective solid compounds are stable. However, even though the active regions of the polarization curves usually lie in regions labeled as “corrosion” on the potential-pH diagrams, this is not always the case. For example, iron can passivate in sulfuric acid solutions under conditions where the diagrams would predict corrosion and, hence, an active condition, but the rate of passive-film dissolution is so extremely slow that the film is metastable and thereby prevents metal dissolution. Reference cited in this section 11. M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, 2nd ed., Pergamon Press, 1966 J. Kruger, Passivity, Corrosion: Fundamentals, Testing, and Protection, Vol 13A, ASM Handbook, ASM International, 2003, p 61–67 Passivity Jerome Kruger, Johns Hopkins University Kinetics of Passivity From the standpoint of the kinetics, passivity can be characterized as the conditions existing on a metal surface because of the presence of a protective film that markedly lowers the rate of corrosion, even though from thermodynamic (corrosion tendency) considerations one would expect active corrosion. Figure 1, which depicts an idealized anodic polarization curve for a metal surface, can serve as a basis for describing in a general way the kinetics of passivation. Anodic polarization curves obtained from a real system under practical conditions (Ref 12) are shown in Fig. 3. They still include the general features of the ideal curve (Fig. 3). Figure 3 shows a comparison of iron and 304L stainless steel in H 2 SO 4 . In Fig. 1, where the anodic polarization curve is that of a metal that exhibits an ability to become passive, the current initially increases with an increase in potential, but when the potential reaches the value of the passivating potential, E p , the critical current density for passivation, i c , is reached, and a marked drop in current density (corrosion rate) is observed. This is the onset of passivity, and the current density remains low at i p as the potential is increased to higher values. If the potential is increased to sufficiently high values, the current density begins to rise, and either pitting results or the transpassive region is entered. In the transpassive region, oxygen evolution and possibly increased corrosion takes place. Fig. 3 Comparison of anodic polarization curves for iron and 304L stainless steel in 1 N H 2 SO 4 . Adapted from Ref 12 The corrosion potential of a metal surface is controlled by the intersection of the anodic (potential increases in the positive direction) and cathodic (potential increases in the negative direction) polarization curves where the anodic and cathodic reaction rates are equal. Therefore, even though a metal may be capable of exhibiting passivity, its corrosion rate will depend on where the cathodic polarization curve intersects the passive metal anodic curve of the type shown in Fig. 1. Figure 4 shows three possible cases. If the cathodic reaction produces a polarization curve such as A, which is indicative of oxidizing conditions, the corrosion potential will be located in the passive region, and the system can exhibit a low corrosion rate. If the cathodic reaction produces curve C, which is indicative of reducing conditions, the corrosion potential will be in the active region, and the corrosion rate can be high. Curve B represents an intermediate case where passivity, if it exists at all, will be unstable, and the surface will oscillate between active and passive states. Fig. 4 Intersections of three possible cathodic polarization curves (straight lines A, B, C) with an anodic polarization curve for a system capable of exhibiting passivity. The corrosion rate depends on the current density at the intersection. Curve A produces a passive system, curve C an active system, and curve B an unstable system. Reference cited in this section 12. M.G. Fontana and N.D. Greene, Corrosion Engineering, McGraw-Hill, 1967, p 336–337 J. Kruger, Passivity, Corrosion: Fundamentals, Testing, and Protection, Vol 13A, ASM Handbook, ASM International, 2003, p 61–67 Passivity Jerome Kruger, Johns Hopkins University Nature of the Passive Film It is now widely believed that a film is responsible for the condition of passivity—one of the major accomplishments of past research. The understanding of the nature of passive films has been greatly enhanced in recent years, resulting in the development of many models of the passive film by the application of a whole array of in situ and ex situ techniques developed over the past 25 to 30 years. Two examples of collections of such models (Fig. 5) have been given by Sato (Ref 13) and Cohen (Ref 14). This section discusses five of the properties of passive films: the thickness, the composition, the structure, and the electronic and mechanical properties. These five aspects are discussed using, for the purpose of illustration, the films on iron and iron alloys. Since all of the properties that determine the nature of the passive film are interrelated, the discussion of each is artificially limited. [...]... and A.J Schrott, Corros Sci., Vol 35, 1993, p 19 25 84 L.J Oblonsky, Passivity of Metals and Semiconductors, M.B Ives, J.L Luo, and J.R Rodda, Ed., Proc Vol 9 9-4 2, Electrochemical Society, 20 01, p 25 3 25 7 85 J Kruger, Proc Corrosion and Corrosion Protection, R.P Frankenthal and F Mansfeld, Ed., Vol 8 1-8 , Electrochemical Society, 1981, p 66–76 J Kruger, Passivity, Corrosion: Fundamentals, Testing, and. .. 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