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Jul 30, 2018 - silicon nitride and from hafnium metal targets. Depositions were at ... silicon and hafnium nitrides produced by reactive rf sputtering was begun.
where W0 is the corrosion rate in the absence of inhibitor, and w is the corrosion rate in the same environment with the inhibitor added.
B. CLASSIFICATION OF INHIBITORS Inhibitor selection is based on the metal and the environment. A qualitative classification of inhibitors is presented in Figure 1. Inhibitors can be classified into environmental conditioners and interface inhibitors. 1. Environmental Conditioners (Scavengers) Corrosion can be controlled by removing the corrosive species in the medium. Inhibitors that decrease corrosivity of the medium by scavenging the aggressive substances are called environmental conditioners or scavengers. In near-neutral and alkaline solutions, oxygen reduction is a common cathodic reaction. In such situations, corrosion can be controlled by decreasing the oxygen content using scavengers (e.g., hydrazine ). © Minister of Natural Resources, Canada, 1999.
Uhlig's Corrosion Handbook, Second Edition, Edited by R. Winston Revie. ISBN 0-471-15777-5 © 2000 John Wiley & Sons, Inc.
*Form three-dimensional layers at the interface, so they are classified collectively as interphase inhibitors FIGURE 1. Classification of inhibitors.
2. Interface Inhibitors Interface inhibitors control corrosion by forming a film at the metal/environment interface. Interface inhibitors can be classified into liquid- and vapor-phase inhibitors. 2.1. Liquid-Phase Inhibitors Liquid-phase inhibitors are classified as anodic, cathodic, or mixed inhibitors, depending on whether they inhibit the anodic, cathodic, or both electrochemical reactions. 2.LL Anodic inhibitors. Anodic inhibitors are usually used in near-neutral solutions where sparingly soluble corrosion products, such as oxides, hydroxides, or salts, are formed. They form, or facilitate the formation of, passivating films that inhibit the anodic metal dissolution reaction. Anodic inhibitors are often called passivating inhibitors. When the concentration of an anodic inhibitor is not sufficient, corrosion may be accelerated, rather then inhibited. The critical concentration above which inhibitors are effective depends on the nature and concentration of the aggressive ions. 2.7.2. Cathodic inhibitors. Cathodic inhibitors control corrosion by either decreasing the reduction rate (cathodic poisons) or by precipitating selectively on the cathodic areas (cathodic precipitators). Cathodic poisons, such as sulfides and selenides, are adsorbed on the metal surface; whereas compounds of arsenic, bismuth, and antimony are reduced at the cathode and form a metallic layer. In near-neutral and alkaline solutions, inorganic anions, such as phosphates, silicates, and borates, form protective films that decrease the cathodic reaction rate by limiting the diffusion of oxygen to the metal surface. Cathodic poisons can cause hydrogen blisters and hydrogen embrittlement due to the absorption of hydrogen into steel. This problem may occur in acid solutions, where the reduction reaction is hydrogen evolution, and when the inhibitor poisons, or minimizes, the recombination of hydrogen atoms to gaseous hydrogen molecules. In this situation, the hydrogen, instead of leaving the surface as hydrogen gas, diffuses into steel causing hydrogen damage, such as hydrogen-induced cracking (HIC), hydrogen embrittlement or sulfide stress cracking. Cathodic precipitators increase the alkalinity at cathodic sites and precipitate insoluble compounds on the metal surface. The most widely used cathodic precipitators are the carbonates of calcium and magnesium. 2. L 3. Mixed inhibitors. About 80% of inhibitors are organic compounds that cannot be designated specifically as anodic or cathodic and are known as mixed inhibitors. The effectiveness of organic inhibitors is related to the extent to which they adsorb and cover the metal surface. Adsorption depends on the structure of the inhibitor, on the surface charge of the metal, and on the type of electrolyte. Mixed inhibitors protect the metal in three possible ways: physical adsorption, chemisorption and film formation. Physical (or electrostatic) adsorption is a result of electrostatic attraction between the inhibitor and the metal surface. When the metal surface is positively charged, adsorption of negatively charged (anionic) inhibitors is facilitated (Fig. 2). Positively charged molecules acting in combination with a negatively charged intermediate can inhibit a positively charged metal. Anions, such as halide ions, in solution adsorb on the positively charged metal surface, and organic cations subsequently adsorb on the dipole (Fig. 3). Corrosion of iron in sulphuric acid containing chloride ions is inhibited by quaternary ammonium cations through this synergistic effect . Physically adsorbed inhibitors interact rapidly, but they are also easily removed from the surface. Increase in temperature generally facilitates desorption of physically adsorbed inhibitor molecules. The most effective inhibitors are those that chemically adsorb (chemisorb), a process that involves charge sharing or charge transfer between the inhibitor molecules and the metal surface.
FIGURE 2. Adsorption of negatively charged inhibitor on a positively charged metal surface.
FIGURE 3(a). Positively charged inhibitor molecule does not interact with positively charged metal surface.
FIGURE 3(b). Synergistic adsorption of positively charged inhibitor and anion on a positively charged metal surface.
Chemisorption takes place more slowly than physical adsorption. As temperature increases, adsorption and inhibition also increase. Chemisorption is specific and is not completely reversible . Adsorbed inhibitor molecules may undergo surface reactions, producing polymeric films. Corrosion protection increases markedly as the films grow from nearly two-dimensional adsorbed layers to three-dimension films up to several hundred angstroms thick. Inhibition is effective only when the films are adherent, are not soluble, and prevent access of the solution to the metal. Protective films may be nonconducting (sometimes called ohmic inhibitors because they increase the resistance of the circuit, thereby inhibiting the corrosion process) or conducting (self-healing films).
Scavengers deplete the oxygen by chemical reaction; for example, hydrazine removes oxygen by the following reaction : 5O2 + 2(NH2-NH2) +± 2H2O + 4H+ + 4NO2"
FIGURE 4. Polarization diagram of an active-passive metal showing the dependence of the current on concentration of passivation-type inhibitor .
where n is the number of water molecules displaced by one inhibitor molecule. The ability of the inhibitor to replace water molecules depends on the electrostatic interaction between the metal and the inhibitor. On the other hand, the number of water molecules displaced depends on the size and orientation of the inhibitor molecule. Thus, the first interaction between inhibitor and metal surface is nonspecific and involves low activation energy. This process, called "physical adsorption," is rapid and, in many cases, reversible .
FIGURE 6. Homologous series of organic molecules (the molecules differ only in the hetero atom).
surface . Both in molecular and in dissociated forms VPIs adsorb either physically or chemically on the metal surface to inhibit corrosion.
D. EXAMPLES OF CORROSION INHIBITORS Inhibitors used in practice are seldom pure substances, but are usually mixtures that may be byproducts, for example, of some industrial chemical processes for which the active constituent is not known. Commercial inhibitor packages may contain, in addition to the active ingredients for inhibition, other chemicals, including surfactants, deemulsifiers, carriers (e.g., solvents) and biocides. The active ingredients of organic inhibitors invariably contain one or more functional groups containing one or more hetero atoms, N, O, S, P, or Se (selenium), through which the inhibitors anchor onto the metal surface. Some common anchoring groups are listed in Table 3. These groups are attached to a parent chain (backbone), which increases the ability of the inhibitor molecule to cover a large surface area. Common repeating units of the parent chain are methyl and phenyl groups. The backbone may contain additional molecules, or substituent groups, to enhance the electronic bonding strength of the anchoring group on the metal and/or to enhance the surface coverage. The outline of the constitution of an organic inhibitor is presented in Table 4. 1. Inhibitors Containing the Oxygen Atom Benzoic acid and substituted benzoic acids are widely used as corrosion inhibitors . Adsorption and inhibitor efficiencies of benzoic acids depend on the nature of the substituents. Electrondonating substituents increase inhibition by increasing the electron density of the anchoring group (-COOH group); on the other hand, electron-withdrawing, substituents decrease inhibition by decreasing the electron density. Percent inhibition as a function of substituents is presented in Figure 7.
a Anchoring and substituent groups are interchangeable, that is, the substituent group through which the inhibitor anchor onto the metal surface depends on the electron density, charge on the metal and the orientation of the molecule in a particular environment.
Hammett Constants FIGURE 7. Variation of inhibitor efficiency as a function of substituents (benzoic acid) (substituents with negative Hammett constants will attract electrons from the anchoring -COOH group, thereby decreasing the efficiency) . (Hammett constant is a measure of ability of the substituents to attract or repel electrons).
FIGURE 9. 8-Hydroxyquinoline surface chelation (Stable chelate complex is formed, but is soluble in aqueous medium—no corrosion inhibition) [2O]. Copyright NACE International. Reprinted with permission.
FIGURE 10. Pyrocatechols (forms insoluble chelate complex with the metal. Efficient corrosion inhibitor; also refer to Table 6) [2O].
(Fig. 10), forming adherent chelants on the metal surface, are effective inhibitors. Inhibition efficiency can be increased by decreasing the solubility through alkylation (increase in chain length) (Table 6) [2O].
See [2O]. Copyright NACE International. Reproduced with permission.
chromates (anodic inhibitors); amines, benzoates, mercaptons, and organic phosphates (mixed inhibitors); and polar or emulsifiable oils (film formers) . Atmospheres to which automobiles are exposed contain moist air, wet SC>2 gas (forming sulfuric acid in the presence of moist air) and deicing salt (NaCl and CaCl2). To control external corrosion, the rust-proofing formulations that are used contain grease, wax resin, and resin emulsion, along with metalloorganic and asphaltic compounds. Typical inhibitors used in rust-proofing applications are fatty acids, phosphonates, sulfonates and carboxylates. 6. Paints (Organic Coatings) Finely divided inhibiting pigments are frequently incorporated in primers. These polar compounds displace water and orient themselves in such a way that the hydrophobic ends face the environment, thereby augmenting the bonding of the coatings on the surface. Red lead (Pb3C>4) is commonly used in paints on iron. It deters formation of local cells and helps preserve the physical properties of the paints. Other inhibitors used in paints are lead azelate, calcium plumbate and lead suboxide . 7. Miscellaneous Inhibitors are used to control corrosion in boiler waters, fuel oil tanks, hot chloride dye baths, refrigeration brines, and reinforcing steel in concrete, and they are also used to protect artifacts.
F. OTHER FACTORS IN APPLYING INHIBITORS Some factors to be considered in applying inhibitors are discussed in the following paragraphs. 1. Application Techniques A reliable method should be applied for inhibitor application. A frequent cause of ineffective inhibition is loss of the inhibitor before it either contacts the metal surface or changes the environment to the extent required. Even the best inhibitor will fail if not applied properly. If the inhibitor is continuously applied in a multiphase system, it should partition into the corrosive phase, usually the aqueous phase. This partitioning is especially important when using water-soluble, oil-dispersible inhibitors. In batch treatment, the frequency of treatment depends on the film persistency. It is important that the corrosion rates are measured frequently to ensure that a safe level of inhibition is maintained. It is also important that the inhibitor contacts the entire metal surface and forms a continuous persistent film. When using volatile inhibitors, care must be taken in packaging to prevent the loss of inhibitor to the outside atmosphere. Inhibitors are added to the primers used in paint coatings. When moisture contacts the paint, some inhibitor is leached from the primer to protect the metal. The inhibitor should be incorporated in such a way that it protects the areas where potential corrosion can take place, and not leach completely from the primer during the service life. 2. Temperature Effects Organic molecules decompose at elevated temperatures. In general, film-forming inhibitors that depend on physical adsorption become less effective at elevated temperatures, so that larger treatment dosages may be required to maintain protective films. Chemisorption, on the other hand, increases with temperature due to the strengthening of chemical bonds. As a result, inhibitor efficiency increases with temperature up to the temperature at which decomposition of the inhibitor occurs.
FIGURE 11. Synergistic effect of mixing formaldehyde and furfuralimine .
Inhibitor Mixture, % FIGURE 12. Antagonistic effect of mixing narcotine and thiourea .
chemical per liter of fluid (or LD50, milligrams per kilogram) for exposure times of 24 and 48 h. The EC50 is the effective concentration of inhibitor to adversely affect 50% of the population. In general, ECso values are lower than LCso values because the former are the concentrations required to damage the species in some way without killing it. Some chemicals are excellent inhibitors, but are quite toxic and readily adsorbed through the skin. Toxicity of some inhibitors is presented in Table 7. There is a growing demand for corrosion inhibitors that are less toxic and biodegradable compared to current formulations. Green inhibitors displaying substantially improved environmental properties will be the inhibitors most widely used in the future.
G. REFERENCES 1. O. L. Riggs, Jr., in C. C. Nathan (Ed.), Corrosion Inhibitors, NACE, Houston, TX, 1973, p. 11. 2. M. G. Noack, Mater. Perform., 21(3), 26 (1982). 3. A. Frignani, G. Trabanelli, F. Zucchi, and M. Zucchini, Proceedings of 5th European Symposium of Corrosion Inhibitors, Ferrara, Italy, 1980, p. 1185. 4. V. S. Sastri, Corrosion Inhibitors: Principles and Applications, J. Wiley, New York, 1998, p. 39. 5. S. A. Levin, S. A. Gintzbergy, I. S. Dinner, and V. N. Kuchinsky, Proceedings of Second European Symposium on Corrosion Inhibitors, Ferrara, Italy, 1965, p. 765. 6. M. G. Fontana, Modern Theory—Principles, in Corrosion Engineering, McGraw-Hill, New York, 1986, pp. 445-481. 7. N. Hackerman and E. S. Snavely, in L. S. V. Delinder (Ed.), Corrosion Basics, NACE, Houston, TX, 1984, pp. 127-146. 8. S. Trasatti, J. Electroanal. Chem. Interf. Electrochem., 33, 351 (1971). 9. W. Lorenz, Z. Phys. Chem., 219, 421 (1962); 224, 145 (1963); and 244, 65 (1970). 10. Z. S. Smialowska and G. Wieczorek, Corros. Sci., 11, 843 (1971). 11. S. Trasatti, J. Electroanal. Chem., 53, 335 (1974). 12. J. O. M. Bockris and D. A. J. Swinkels, J. Electrochem. Soc., Ill, 736 (1964). 13. I. Langmuir, J. Am. Chem. Soc., 39, 1848 (1947). 14. R. G. Pearson, J. Am. Chem. Soc., 85, 3533 (1963); Science, 151, 172 (1966). 15. L L . Rosenfeld, V. P. Persiantseva, and P. B. Terentief, Corrosion, 20, 222t (1964). 16. A. Akiyama and K. Nobe, J. Electrochem. Soc., 117, 999 (1970). 17. P. G. Fox, G. Lewis, and P. J. Boden, Corros. Sci., 19, 457 (1979). 18. I. Singh, Corrosion, 49, 473 (1993). 19. T A. Skotheim (Ed.), Handbook of Conducting Polymers, Marcel Dekker, New York, VoIs. 1 and 2 (1986). 20. A. Weisstuch, D. A. Carter, and C. C. Nathan, Mater. Perform., 10(4), 11, (1971). 21. C. C. Nathan, in , 1973, p. 45. 22. S. Matsuda and H. H. Uhlig, J. Electrochem. Soc., Ill, 156 (1964). 23. Snow and ice control with chemical and abrasives, Highway Research Board, Washington, DC, Bulletin 152, 1960. 24. T. Rossel, Werkstoffe Korrosion, 20, 854 (1969). 25. H. H. Uhlig and R. W. Revie, Corrosion and Corrosion Control, 3rd ed., Wiley, New York, 1985, p. 274. 26. G. Trabenelli and F. Zucchi, Rev. Coat. Corros., 1, 97 (1972). 27. G. Trabanelli, F. Zucci, G. L. Zucchini, and V. Carassiti, Electrochim. Met., 2, 463 (1967). 28. I. N. Putilova, S. A. Balezin, and V. P. Barannik, Metal. Corros. Inhibitors, Pergamon, NewYork, 1960, pp. 17-24. 29. R. L. Martin, B. A. Alink, T. G. Braga, and A. J. McMahon, R. Weare, Environmentally acceptable water soluble corrosion inhibitors, CORROSION/95, Paper No. 36, NACE Houston, TX, 1995. 30. W. W. Frenier, Development and testing of a low-toxicity acid corrosion inhibitor for industrial cleaning applications, CORROSION-96, Paper No. 152, NACE Houston, TX, 1996.

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