barcode generator vb.net source code Standard EMF Series of Metals in Software

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TABLE 35.1 Standard EMF Series of Metals
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metals in order of increasing activity, starting with gold (Au), which is the least active. If two of the metals listed were joined in a galvanic couple, the more active one would be attacked and plating or deposition of the less active one would occur. This is based on the fact that solutions contain only unit activity (concentration) of ions of each of the two metals. The standard EMF series is valid only for pure metals at 25 C and in equilibrium with a solution containing unit activity (concentration) of its own ions. If ion concentrations are greater than unit activity, the potentials are more positive; if less, the opposite is true.
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CORROSION 35.4
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PERFORMANCE OF ENGINEERING MATERIALS
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TABLE 35.2 Galvanic Series of Some Commercial Metals and Alloys in Seawater
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The galvanic series (Table 35.2) shows a similar relationship, except that impure metals such as alloys are also included and the medium is seawater. Other media, other concentrations, and other temperatures can further alter the order of the list. Therefore, care should be exercised in applying these data to a given galvanic corrosion situation except as a general, loose guide.
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35.3.3 Passivation Certain common engineering materials, such as iron, nickel, chromium, titanium, and silicon as well as their alloys (i.e., stainless steels), exhibit a characteristic of being able to behave both as a more active and as a less active (passive) material.
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CORROSION 35.5
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CORROSION
Note in the galvanic series (Table 35.2) that several stainless steels are listed twice, once as passive and once as active. Some common metals other than those mentioned also exhibit passivity, but to a lesser extent. A graphical representation of passivity is shown in Fig. 35.1. The three regions active, passive, and transpassive help to explain seemingly inconsistent behavior of active-passive materials under various degrees of attack severity.
FIGURE 35.1 Corrosion characteristics of an active-passive metal.
There are both advantages and disadvantages to be gained or suffered because of active-passive behavior. In very aggressive environments, a method called anodic protection can be used whereby a potentiostat is utilized to electrochemically maintain a passive condition and hence a low rate of corrosion. However, accelerated corrosion test results may be useless because increasing the corrosion power of the medium may cause a shift from a high active corrosion rate to a low passive condition, producing the invalid conclusion that corrosion is not a problem. Another example involves inhibitors which function by maintaining a passive condition. If the concentration of these inhibitors were allowed to decrease, high corrosion could result by passing from a passive to an active condition. Active-passive materials have a unique advantage in the area of corrosion testing and corrosion rate prediction. Potentiodynamic polarization curves can be generated in a matter of hours, which can provide good quantitative insights into corrosion behavior and prediction of corrosion rates in a particular environment. Most other corrosion testing involves months or years of testing to obtain useful results.
35.3.4 Crevice Corrosion and Pitting Crevice corrosion is related to active-passive materials which are configured such that crevices exist. Mated screw threads, gaskets, packings, and bolted or lapped joints
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CORROSION 35.6
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are common examples of crevices. Inside the crevice, oxygen or other corrosives required for passivation have restricted entrance, resulting in reduced concentration as they are consumed by corrosion in the crevice. When the concentration of these corrosives is low enough to fail to maintain passivity, the metal in the crevice becomes active. The large electrically connected, passivated surface outside the crevice completes a galvanic couple with a large adverse-area ratio, providing high attack rates within the crevice. Welding or forming can be used to avoid crevices. However, intergranular corrosion may occur in welded stainless steels (see Sec. 35.3.9). Pitting is a very localized attack that results in holes, or voids, on a metal surface. Although not restricted to active-passive metals, pitting is commonly related to these. With active-passive metals, pieces of dirt, scale, or other solid particles may rest on the bottom of a pipe or tank where velocities are not sufficient to move them. These particles form crevices, resulting in a localized attack similar to crevice corrosion.
35.3.5 Sacrificial Anodes Magnesium rods are placed in steel glass-lined hot-water tanks, and zinc is used to coat sheet steel (galvanized steel) to provide protection to the steel against corrosion. As the more active magnesium rod is attacked, the electrons generated are conveyed to the electrically connected steel tank, which needs protection only for regions where cracks or flaws exist in the glass lining. Similarly, for galvanized steel, protection is required only for regions of scratches or where steel edges are exposed.
35.3.6 Stress Corrosion Cracking In stress corrosion cracking (SCC), most of a metal s surface may show little attack, while fine intergranular or transgranular cracks may penetrate deeply into the surface. There may be a single continuous crack or a multibranched crack, or the entire surface may be covered with a lacy network of cracks. Usually dye penetrants and sectioning are needed to reveal the extent and depth of cracking. Certain classes of alloys and environments are susceptible to this phenomenon, and usually tensile stresses are involved, with crack penetration rates increasing with increasing tensile stress. The higher the strength condition of a given alloy, the greater seems to be the tendency to suffer SCC. Table 35.3 lists some materials and environments that have been known to produce SCC. Frequently, a difference in color or texture is noticeable between a stress corrosion crack and an adjacent region of overstress when the fracture is completed by mechanical means. Scanning electron micrographs are frequently useful in identifying SCC.
35.3.7 Selective Leaching Selective leaching refers to the chemical removal of one metal from an alloy, resulting in a weak, porous structure. Brass sink traps suffer this type of attack by zinc being leached out of the yellow brass, leaving behind a porous structure of reddish copper. Aluminum and silicon bronzes and other alloys are also subjected to selective leaching.
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