Passive immunity

Although there are many differences between immunity systems in plants and animals, there are similarities. Both systems have two kinds of immunity passive and active. Alkaloids may take part in both systems. 

Immunization has been the dream of the traditional Chinese medicine. In modem tiuKs, in the lack of time for active immunization, passive immunization with immimoglobulins has solved many urgent problems. Active immunization has prevented infectious mass diseases, such as diphtheria, hepatitis, influenza, measles, meningococcal, mumps, pneumococcal, poho, rabies, mbella, tetanus, typhus, varicella, and yellow fever, as well as several diseases of domesticated animals. 

Fig. 14M The potentiallpH diagram for magnesium, (a) the detailed diagram. Lines correspond to different concentrations of Mg, as marked, (b) simplified form, defining regions of immunity, passivity and corrosion. Data from Pourbaix in "Atlas of Electrochemical Equilibria in Aqueous Solutions", Pergamon Press, 1966.

In spite of the foregoing limitations, the corrosion scientist and engineer can derive a wealth of information by consulting the relevant potential/pH diagrams. The regions of immunity, passivity, and pos.sible corrosion are demarcated, and the most common corrosion products are shown. Studying the relevant diagram is an excellent way to start a new corrosion study, but it. should never be the only tool used to solve the problem. 

Of very corrosion-resistant materials we have metals such as tantalum and zirconium (both resistant due to passivity), and platinum (immune, passive in strongly oxidizing environment) (see the Pourbaix diagrams for tantalum and platinum in Figure 3.11). Tantalum is attacked by some alkaline solutions, fluoric acid and hot, concentrated sulphuric acid. The metal is used, e.g. for repair of glass-lined equipment and for handling of pure chemicals. Zirconium is used in nuclear... 

These diagrams indicate the regions of immunity, passivation and corrosion of pure metallic elements in pure water at 25°C. Fig. 9.1 (a, b) gives the potential-pH diagrams of Cr and Ti. Cr is the element responsible for the passive behaviour of stainless steels and Co-Cr-Mo alloys. In Fig. 9.1a passivation by a Cr(OH)3 film is assumed. The film is thermodynamically stable over a wide range of pH and potential values. Below pH 4 the film... 

Resistance of ceramics to corrosion is due to one of three basic behaviors immunity, passivation, or kinetically limited corrosion. When a ceramic is thermodynamically incapable of spontaneous reaction with its environment it is referred to as having immunity. When the necessary thermodynamic data are available this type of corrosion resistance may be predicted by calculation. Metals, except for the precious metals such as gold, do not exhibit immunity. 

Figure 6.41. Pourbaix diagram for Fe where the areas of immunity, passivation, and corrosion are indicated. The diagram has been calculated for an ion concentration of 10 mol/e.

The potential-pH diagrams have been published for most metals. They provide valuable information on conditions for which a metal is immune, passive, or submitted to corrosion by the electrolyte. Figure 1.16 also shows the E-pH lines of the hydrogen (H+/H2) and the oxygen (O2/H2O) electrodes. These redox systems are important for the attack of metals in many environments. Since the Fe/Fe + line is below the H /H2 line, it can be predicted that iron is attacked in acidic electrolytes due to oxidation by hydrogen ions. A more detailed discussion is presented in Section 5.3. 

Simplified form, defining regions of Immunity, passivity and corrosion. Data from M. Pourbalx in Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon Press, 1966. 

In passive immunotherapy immune globulin (Ig) is an effective replacement in most forms of antibody deficiency (14). In the past, plasma was used instead of immune globulin, but plasma is rarely indicated in the 1990s because of the risk of disease, particularly AIDS, transmission. Because plasma contains many factors in addition to immunoglobulins (Igs), plasma is, however, of particular value in patients with protein-losing enteropathy, complement deficiencies, and refractory diarrhea. 

Eig. 2. The thermodynamic regions of corrosion, immunity, and passivation of iron in an iron—water system assuming passivation by a film of Ee202 (H)-... 

Metals that depend on a relatively thick protective coating of corrosion product for corrosion resistance are frequently subject to erosion-corrosion. This is due to the poor adherence of these coatings relative to the thin films formed by the classical passive metals, such as stainless steel and titanium. Both stainless steel and titanium are relatively immune to erosion-corrosion in most cooling water environments. 

Pourbaix has evaluated all possible equilibria between a metal M and HjO (see Table 1.7) and has consolidated the data into a single potential-pH diagram, which provides a pictorial summary of the anions and cations (nature and activity) and solid oxides (hydroxides, hydrated oxides and oxides) that are at equilibrium at any given pH and potential a similar approach has been adopted for certain M-H2O-X systems where A" is a non-metal, e.g. Cr, CN , CO, SOj , POj", etc. at a defined concentration. These diagrams give the activities of the metal cations and anions at any specified E and pH, and in order to define corrosion in terms of an equilibrium activity, Pourbaix has selected the arbitrary value of 10 ° g ion/1, i.e. corrosion of a metal is defined in terms of the pH and potential that give an equilibrium activity of metal cations or anions > 10 g ion/1 conversely, passivity and immunity are defined in terms of an equilibrium activity of < 10 g ion/1. (Note that g ion/1 is used here because this is the unit used by Pourbaix in the S.I, the relative activity is dimensionless.)... 

It should be noted that Fig. 1.15 (top) is based entirely on thermodynamic data and is therefore correctly described as an equilibrium diagram, since it shows the phases (nature and activity) that exist at equilibrium. However, the concepts implicit in the terms corrosion, immunity and passivity lie outside the realm of thermodynamics, and, for example, passivity involves both thermodynamic and kinetic concepts it follows that Fig. 1.15 (bottom) cannot be regarded as a true equilibrium diagram, although it is based on one that has been constructed entirely from thermodynamic data. 

Although the zones of corrosion, immunity and passivity are clearly of fundamental importance in corrosion science it must be emphasised again that they have serious limitations in the solution of practical problems, and can lead to unfortunate misconceptions unless they are interpreted with caution. Nevertheless, Pourbaix and his co-workers, and others, have shown that these diagrams used in conjunction with E-i curves for the systems under consideration can provide diagrams that are of direct practical use to the corrosion engineer. It is therefore relevant to consider the advantages and limitations of the equilibrium potential-pH diagrams. 

Fig. 1.38(Equilibrium potential-pH diagram for the Cr-H20 system and (< ) potential-pH diagram showing zones of corrosion, passivity and immunity (after Pourbaix )... 

Under these circumstances the metal s surface within the crevice became active and it corroded with the formation of a yellowish-white corrosion product that was identified as being mainly rutile TiOj. On the other hand, a Ti-0- 13Pd alloy was found to be immune from crevice corrosion, since the presence of the palladium facilitated passivation of the metal surfaces forming the crevice. 

Griess has observed crevice corrosion of titanium in hot concentrated solutions of Cl , SOj I ions, and considers that the formation of acid within the crevice is the major factor in the mechanism. He points out that at room temperature Ti(OH)3 precipitates at pH 3, and Ti(OH)4 at pH 0-7, and that at elevated temperatures and at the high concentrations of Cl ions that prevail within a crevice the activity of hydrogen ions could be even greater than that indicated by the equilibrium pH values at ambient temperatures. Alloys that remain passive in acid solutions of the same pH as that developed within a crevice should be more immune to crevice attack than pure titanium, and this appears to be the case with alloys containing 0-2% Pd, 2% Mo or 2[Pg.169]

In cases where passivity is impossible, corrosion can be prevented if the metal can reach equilibrium with the melt (case 1). The system usually undergoes some corrosion initially, when traces of oxidising impurities are reduced and the redox potential of the melt falls (Fig. 2.33). Finally, after a certain amount of corrosion has occurred, the metal becomes immune and corrosion ceases. In Fig. 2.33 complete equilibrium between metal and melt was still not quite reached even after several hundred hours exposure. 

Stress-corrosion cracking based on active-path corrosion of amorphous alloys has so far only been found when alloys of very low corrosion resistance are corroded under very high applied stresses . However, when the corrosion resistance is sufficiently high, plastic deformation does not affect the passive current density or the pitting potential , and hence amorphous alloys are immune from stress-corrosion cracking. 

Fig. 6.1 Theoretical domains of corrosion, immunity and passivation of silver, at 25°C...

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