Immunity zone

Figure 16.16 is a representation of Fig. 16.1 a in terms of the type of reaction it shows, as a function of pH, the zones where corrosion by dissolution occurs, where initial corrosion forms insoluble oxides on the surface that impede further reaction (passive zone) and the region where the metal is stable (immune zone). 

Ideally, we would like to depress the potential sufficiently to reach the immune zone. However, that is very close to the hydrogen evolution potential (lower dotted line) at pH 12 which is where steel in concrete lies. For the reasons discussed earlier in this section, we want to avoid hydrogen evolution, so we aim for the area below the pitting potentials. This is discussed in Appendix 1 of BSEN12696 (2000) and also in the section on criteria 7.4. 

Another way of looking at the theory of cathodic protection is to look at the Pourbaix diagram (Pourbaix, 1973 Morgan, 1990) for iron in chloride solution. This shows that there are conditions where steel corrodes, and areas where protective oxides form and an area of immunity to corro.sion depending upon the pH and the potential of the steel. Ideally we would like to depress the potential sufficiently to reach the immune zone (Figure 6.2). In practice we do not do that for reasons described in the Section 6.5 on control criteria. 

Pourbaix Diagrams. Plots of equilibrium pH vs electrochemical potential E describe the effects of aqueous corrosion on borosilicate and silicate glasses. They are applicable to weathering studies and to ground water attack on nuclear waste glasses. The diagrams display any immune zone between active corrosion... 

Hastelloy C-4 is almost totally immune to selective intergranular corrosion in weld-heat-affected zones with high temperature stabihty in the 650-I040 C (I200-I900 F) range Hastelloy C-22 has better overall corrosion resistance and versatihty than either C-4 or C-276 (in most environments). 

This relay may be used only under unrestricted fault conditions, with three CTs as shown, tf the scheme is used under a restricted fault condition, with the fourth CT in the neutral, the directional relay will remain immune to any fault occurring outside the zone of the three CTs, as the fault current through the fourth CT will offset the residual current, detected by the three CTs (.Section 2l.6.,f), rendering the whole scheme non-functional. 

The running of telephone lines through power lines is long discontinued. They are now run on separate structures within a city and nearby areas at audio frequency (— 0.3-3.4 kHz), and maintain enough distance from HT power distribution lines. They are therefore almost unaffected from such disturbances. Nevertheless, interferences must be kept in mind when installing these lines so that they are out of the inductive interference zone of the power lines. The latest method in the field of communications to avoid disturbances is to use underground optical fibre cables, where possible, as discussed later. Optical fibre cables are totally immune to such disturbances. 

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. 

The region of immunity [Fig. 1.15 (bottom)] illustrates how corrosion may be controlled by lowering the potential of the metal, and this zone provides the thermodynamic explanation of the important practical method of cathodic protection (Section 11.1). In the case of iron in near-neutral solutions the potential E = —0-62 V for immunity corresponds approximately with the practical criterion adopted for cathodically protecting the metal in most environments, i.e. —0-52 to —0-62V (vs. S.H.E.). It should be observed, however, that the diagram provides no information on the rate of charge transfer (the current) required to depress the potential into the region of immunity, which is the same (< —0-62 V) at all values of pH below 9-8. Consideration of curve//for the Hj/HjO equilibrium shows that as the pH... 

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 )... 

As may be seen from the potential-pH diagram " (Fig. 6.3) platinum is immune from attack at almost all pH levels. Only in very concentrated acid solutions at high redox potentials (i.e. under oxidising conditions) is there a zone of corrosion. This accounts for the solubility of platinum in aqua regia. Platinum is also prone to complex-ion formation, and this can lead... 

Marine mammals have adapted to an aquatic or semi-aquatic lifestyle with such features as reduced or eliminated limb structures, a blubber layer for thermoregulatory, nutritional, buoyancy and locomotory roles, and reproductive strategies conducive to fitness in a given biogeographic zone (habitat). The environmental challenges faced by newborn marine mammals at birth may explain there largely precocious nature and the apparent maturity of the immune system of newborn marine mammals [1],... 

Selenium is a significant component of the enzymatic system of the glutafion for antioxidant protection. It is included in the composition of 200 enzymes engaged in different biochemical reactions, demonstrates the immune-tropic, antiteratogenic and anticancer properties, betters the functional state of muscles, especially myocarditis, and takes part in hormones synthesis of the thyroid gland. Selenium s deficit in soil is the cause of hearth deficiency in endemic zones. 

Targeted screening approaches specifically zone in upon plants more likely to contain bioactive molecules in the first place. For example, plants which seem to be immune from predation may well be producing substances toxic to, for example, insects. These in turn are also likely to have some effect on human cells. 

Flatworm parasites are well recognized for their ability to live for decades in environments where they are in contact with potentially damaging immune factors. This adaptability reflects the fact that these parasites have evolved mechanisms to evade immune effector mechanisms, and more remarkably, to sense and utilize components of the host immune system for their own development. Schistosomes, digenetic trematodes, are undoubtedly the most well-studied parasitic flatworms. These parasites infect over 200 million people in tropical and subtropical zones, and cause severe disease in approximately 5% of those infected. Of immunological interest is the fact that pathology in schistosome-infected individuals is caused largely by the immune response... 

Overall, the defects in lymphocyte homeostasis and immune response may also include defective responses to the lysolipids, LPA and SIP, which regulate Rho pathways in numerous cells, including lymphocytes (Lin and Boyce, 2006). Rubtsov et ah (2005) found a decrease in marginal zone B cells in Lsc / mice and these MZB cells had an enhanced migratory response to SIP. In contrast to the observations in neutrophils, the MZB cells also showed an increased adherence in the absence of Lsc that could prevent appropriate migration in response to SIP. This would correspond... 

Rubtsov, A., Strauch, P., Digiacomo, A., Hu, J., Pelanda, R., and Torres, R. M. (2005). Lsc regulates marginal-zone B cell migration and adhesion and is required for the IgM T-dependent antibody response. Immunity 23, 527-538. 

We now turn to an anatomical description of lymph nodes. The lymph node is surrounded by a thick, fibrous capsule and is subdivided into compartments by trabeculae. Inside the capsule is the subcapsular or marginal sinus, which forms the entry point of lymphatic fluid into the node, via the afferent vessel. The lymph node cortex, which lies beneath the subcapsular sinus, is the location of the primary and secondary lymphoid follicles. The primary follicles are comprised of B-lymphocytes. An immune response stimulates B-cells to replicate and differentiate, converting the primary follicle into a secondary follicle or germinal center, surrounded by a zone of small lymphocytes. The paracortex surrounds the germinal centers and primary follicles and contains mostly T-lymphocytes. The medulla is composed of medullary cords, consisting of macrophages and plasma cells, and medullary sinuses. The medullary vessels include the arteries and veins, and the afferent and efferent lymphatic vessels, respectively, deliver the lymphatic fluid into and out of the lymph node. 

The diverse contributions to what is now simply referred to as culture shock highlight the complexities of the concept. For the traveller, particularly the traveller moving across time zones into altered daylight and weather conditions, there are assaults on the body s immune system and functioning. A rich travel medicine literature has now developed and is replete with dramatic example of the difficulties faced by some tourists. 

Thien, M., T. G. Phan, S. Gardam, M. Amesbuiy, A. Basten, F. Mackay, and R. Brink. 2004. Excess BAFF rescues self-reactive B cells from peripheral deletion and allows them to enter forbidden follicular and marginal zone niches. Immunity 20 785-798. 

Wen, L., J. Brill-Dashoff, S. A. Shinton, M. Asano, R. R. Hardy, and K. Hayakawa. 2005. Evidence of marginal-zone B cell-positive selection in spleen. Immunity 23 297-308. 

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