Aluminum galvanic coupling

Aluminum and Aluminum Alloys. Aluminum can be employed in sea water as a resistant material of construction. Experiments at Fort Bel voir, Virginia, and elsewhere, indicate that by proper corrosion-control practices, aluminum can be used for an entire plant which processes sea water. The sea water entering the plant should be free of all metallic ions, especially copper or nickel. It is essential, in such a plant, that no copper-base alloys be used at all and that galvanic couples to most other metals be avoided. 

Note the potentials of the graphite and the aluminum alloy that you determined. If these two are connected with an electrical contact, their potentials should move toward each other. Further, since the solution is relatively conductive, and assuming that the electrical lead connecting them was highly conductive, they would come to the same potential. Therefore connect the leads of the two electrodes together and connect them both to the positive (or V) lead of the voltmeter. Measure the potential of this galvanic couple relative to one of the reference electrodes and confirm that the couple potential does indeed rest somewhere in between the corrosion potentials of the two materials. 

Corrosion resistance of metallic coatings is dependent on the composition and nature of the electrolyte, oxygen concentration, polarization characteristics, ratio of cathodic to anodic area and the surface contaminants. If the corrosion potentials of two metals such as iron and aluminum are close to each other in a particular environment there may be reversal of the galvanic couple. 

As mentioned above, the environment has a significant effect on whether or not galvanic corrosion will be a problem. For example, carbon steel will corrode rapidly if equal or larger areas of Monel 400 are coupled with it in seawater. Conversely, carbon steel is compatible with Monel 400 in concentrated caustic solutions. Even freshwater can be sufficiently different from seawater couples incompatible in seawater work well in freshwater. For example, copper-steel and aluminum-steet couples are satisfactory for handling... 

Liao, C.-M., and Wei, R. P., Galvanic Coupling of Model Alloys to Aluminum - A Foundation for Understanding Particle-Induced Pitting in Aluminum Alloys, Electrochimica Acta, 45 (1999), 881-888. 

Aluminum is a good material for sea water service, if no copper and nickel ions are present and if all galvanic couples with most other metals can be avoided. 

In water, corrosion limits the use of aluminum to temperatures near 100°C, unless special precautions are taken. In air, corrosion limits its use to temperatures slightly over 300°C. Failure is caused by pitting of the otherwise protective Al(OH)3 film. The presence of chloride salts and of some other metals that form strong galvanic couples (for example, copper) can promote pitting. 

Galvanized steel is a common example of galvanic coupling where steel (Fe), with a standard electrode potential of —0.440 V vs. SHE, is cathodicaUy protected by zinc, which has a more active standard electrode potential of —0.763 V. Obviously, zinc is not a corrosion-resistant metal and cannot be classified as a barrier coating. It protects steel from corrosion through its sacrificial properties. Because zinc is less noble than iron in terms of the standard electrode potentials, it acts as an anode. The sacrificial anode (zinc) is continuously consumed by anodic dissolution reaction and protects the underlying metal (iron in steel) from corrosion. In practice, sacrificial anodes are comprised of zinc, magnesium alloys, or aluminum. 

Figure 7.8 shows a capacitive electrode system. One half-cell is an ordinary indifferent electrolyte-skin electrode. In the other half-cell, the electrolyte has been replaced by a dielectric so that there is no galvanic coupling between the electrode metal plate and the skin. The dielectric is usually a thin layer of a chloride or oxide of the metal in the electrode plate. The metal plate functions as a substrate for the thin dielectric that can be made from anodizing or oxidizing silver, aluminum, silicon, or titan. The dielectric must be robust and endure humidity and sweat arriving from the skin. The capacitance between the metal plate and the skin is dependent on contact area, dielectric thickness, and... 

As a result of extended storage times in the Disassembly Basins, corrosion of the aluminum-clad fuel has become evident Pitting corrosion is the dominant mode. The primary causes of pitting corrosion is the attack of aluminum from chloride and heavy metal ions and the galvanic couples that result from dissimilar metals. 

Galvanic couples betweoi stainless sted and alununtim-clad and couples between difre t aluminum alloys... 

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