Bimetallic couples

Beyond the simple resistance of a material of construction to dissolution in a given chemical, many other properties enter into consideration when makiug an appropriate or optimum MOC selection for a given environmental exposure. These factors include the influence of velocity, impurities or contaminants, pH, stress, crevices, bimetallic couples, levels of nuclear, UV, or IB radiation, microorganisms, temperature heat flux, stray currents, properties associatea with original production of the material and its subsequent fabrication as an item of equipment, as well as other physical ana mechanical properties of the MOC, the Proverbial Siebert Changes in the Phase of the Moon, and so forth. 

Galvanic corrosion is location specific in the sense that it occurs at a bimetallic couple (Fig. 16.2). It is metal specific in the sense that, typically, corrosion affects the metal that has less resistance in the environment to which the couple is exposed. Hence, in principle, we would anticipate galvanic corrosion of relatively reactive metals wherever they are in physical contact with relatively noble metals in a sufficiently aggressive, common environment. Experience has shown, however, that all such couples do not necessarily result in unsatisfactory service. This is because of the interplay of various critical factors that influence galvanic corrosion. These critical factors are discussed in the next section. 

Finally, it is important to point out that although in localised corrosion the anodic and cathodic areas are physically distinguishable, it does not follow that the total geometrical areas available are actually involved in the charge transfer process. Thus in the corrosion of two dissimilar metals in contact (bimetallic corrosion) the metal of more positive potential (the predominantly cathodic area of the bimetallic couple) may have a very much larger area than that of the predominantly anodic metal, but only the area adjacent to the anode may be effective as a cathode. In fact in a solution of high resistivity the effective areas of both metals will not extend appreciably from the interface of contact. Thus the effective areas of the anodic and cathodic sites may be much smaller than their geometrical areas. 

Fig. 1.28 Evans diagram illustrating a corrosion process (e.g. a bimetallic couple) in which the area of the cathode is not equal to that of the anode, (o) so that and (b) > S(,...

Table 1.23 Weight loss of iron for Fe/M bimetallic couples in l o NaCl ...

Similar effects to those shown in Fig. 1.65 have been observed by Pryor and Keir in certain bimetallic couples having steel cathodes. Figure 1.66... 

Fig. 1.69 Effect of resistivity of solution on the distribution of corrosion on the more negative metal of a bimetallic couple, (a) Solution of very low resistivity and (b) solution of very high resistivity. Note that when the resisitivity is high the effective areas of the cathodic and anodic metals are confined to the interface between the two metals...

Deaeration has occasionally been used as a means of controlling bimetallic corrosion under conditions of total immersion, and this method of control can be used successfully, if physical conditions permit, provided that the less noble metal is not sufficiently electrochemically active to permit rapid evolution of hydrogen at the more noble metal, as is observed, for instance, in many bimetallic couples involving magnesium anodes. 

The use of soluble inhibitors as a means of controlling bimetallic corrosion presents many technical problems. Apart from the fact that this method is limited in applicability to recirculating systems, efficient anodic inhibitors, such as chromates, are frequently quite specific in their action and so certain bimetallic couples, such as the Al-Cu couple in chloride solutions, are... 

Disadvantages Possible interaction effects on other buried structures (Section 10.6) subject to the availability of a suitable a.c. supply source or other source of d.c. regular electrical maintenance checks and inspection required running costs for electrical supply (usually not very high except in the case of bare marine structures and in power stations where structures are often bare and include bimetallic couples) subject to power shutdowns and failures. 

Access of air and water will also affect the corrosion rate. Metal inserts in corrosive plastics are most actively attacked at the plastic/metal/air interfaces with certain metals, notably aluminium titaniumand stainless steel, crevice effects (oxygen shielding and entrapment of water) frequently accelerate attack. Acceleration of corrosion by bimetallic couples between carbon-fibre-reinforced plastics and metals presents a problem in the use of these composites. 

The reaction rate per unit area i. For a corroding metal the partial anodic and cathodic current densities cannot be determined directly by means of an ammeter unless the anodic and cathodic areas can be separated physically, e.g. as in a bimetallic couple. If the metal is polarised a net current 4 for cathodic polarisation, and for anodic polarisation, will be obtained and can be measured by means of an ammeter. 

Laboratory tests used in the development of inhibitors can be of various types and are often associated with a particular laboratory. Thus, in one case simple test specimens, either alone or as bimetallic couples, are immersed in inhibited solutions in a relatively simple apparatus, as illustrated in Fig. 19.34. Sometimes the test may involve heat transfer, and a simple test arrangement is shown in Fig. 19.35. Tests of these types have been described in the literatureHowever, national standards also exist for this type of test approach. BSl and ASTM documents describe laboratory test procedures and in some cases provide recommended pass or fail criteria (BS 5117 Part 2 Section 2.2 1985 BS 6580 1985 ASTM 01384 1987). Laboratory testing may involve a recirculating rig test in which the intention is to assess the performance of an inhibited coolant in the simulated flow conditions of an engine cooling system. Although test procedures have been developed (BS 5177 Part 2 Section 2.3 1985 ASTM 02570 1985), problems of reproducibility and repeatability exist, and it is difficult to quote numerical pass or fail criteria. 

In the corrosion of iron by neutral aerated water, in a bimetallic couple with, for example, copper or platinum as the cathode (as in Fig. 16.2), the initial product at the anode is Fe2+(aq) ... 

Poorly applied coatings to bimetallic couplings can permit serious galvanic and pitting corrosion to occur, especially where small anode areas are exposed. 

The general Hamiltonian is given by Eq. (6.3), where the sum is over all the j-k pairs. The treatment is in fact based on bimetallic coupling. The S levels are obtained and the hyperfine coupling for each S level is given by... 

All these variants are complementary to one another and, by using different experimental conditions, ligands, reductants, and modes of preparation, this approach can apparently be used to prepare any type of bimetallic couple. However, the catalytic properties of the final catalyst will depend strongly on the technique used. The selection of one variant over another depends on the desired structure of the bimetallic phase. 

Figure 1.27 A mixed potential plot for the bimetallic couple of iron and zinc. The figure also explains the higher corrosion rate of iron than zinc in hydrochloric acid solution. Despite the more positive reduction potential of iron, the evolution of hydrogen on iron has a high exchange current density (Reproduced from Corrosion for Science and Engineering, Tretheway and Chamberlain, Copyright Pearson Education Ltd)...

In a thermocouple, heating one junction of a bimetallic couple and cooling the other produces electromotive force in the circuit. This observation was originally was made by Seebeck in 1821. Besides the use of thermocouples, transistor electronics and semiconductors are important areas of interest for thermoelectric phenomena. Thermocouples made of semiconductors can develop relatively large electromotive potentials and are used to convert heat into electricity. 

Figure 7.5 shows a composed of a bimetallic couple metal wires a and b with one junction maintained at temperature T and the other maintained at T+ dT. An electromotive force E causes a current / to pass through the wires. A Peltier heat qpe(T + dT) per unit current will be absorbed at the warm junction and an amount of heat qpe(T) will be given off at the cool junction. To maintain a temperature gradient, Thomson heat (q l h i)(dT) must be supplied to the metal a, and an amount of heat (r/Th h)(c/7 j must be removed from b, since the current is in the opposite direction in metal wire b. In a closed work cycle, the electric energy is fully converted to heat. Therefore, the energy balance per unit current by the first law of thermodynamics is... 

Figure 7.5. A bimetallic couple of metals a and b, the two junctions (points 2 and 3) are held at different temperatures Tand T + dT. qPe and g,-h show the Peltier and Thomson heats respectively, while Eis the electromotive force.

BIMETALLIC COUPLE - A joint or union of two dissimilar metals. 

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