Iron, galvanizing

Oxygen corrosion only occurs on metal surfaces exposed to oxygenated waters. Many commonly used industrial alloys react with dissolved oxygen in water, forming a variety of oxides and hydroxides. However, alloys most seriously affected are cast irons, galvanized steel, and non-stainless steels. Attack occurs in locations where tuberculation also occurs (see Chap. 3). Often, oxygen corrosion is a precursor to tubercle development. 

C19-0030. In Section Exercise, an aluminum-iron galvanic cell was proposed. Using information presented in this section, explain why this cell is unlikely to make a good batteiy. 

Fittings and unions malleable iron, galvanized A338 and A197 Class 150 malleable iron galvanized to A153 1706 ... 

Figure 7. Increased corrosion rate of iron galvanically coupled with platinum in...

E6.9. Construct the Evans diagram for a nickel and iron galvanic couple immersed in 1 M acidic solution. Using the electrochemical parameters Hsted in Exercise E6.8, estimate graphically the following parameters and tabulate the results ... 

Steel corrodes by electrochemical reactions. In the presence of oxygen, at anodic areas ferric ions and at cathodic areas hydroxyl ions are formed. Aluminum generally corrodes more slowly than steel because of a dense, coherent layer of aluminum oxide. However, aluminum corrodes more rapidly than iron under either highly acidic or basic conditions. Also, salt affects the corrosion of aluminum more than it affects the corrosion of iron. Galvanized steel is protected since zinc acts as a sacrificial anode and a barrier preventing water and oxygen from reaching the steel surface. 

Iron may be protected by coating with tin (tin cans) or with zinc (galvanized iron). Galvanized iron does not corrode as long as zinc is present. By contrast, when a tin can is scratched, the exposed iron underneath corrodes rapidly. Explain the difference between zinc and tin as protective coatings against iron corrosion. 

Fig. 10.6 The Fe/Zn system. A break in the zinc coating on iron (galvanized iron) will, to a limited extent, continue to protect the iron (cathodic) as the zinc (anodic) dissolves...

The analogy of the galvanic cell described above, with a copper and iron galvanic couple, illustrates why iron corrodes and copper does not corrode. Naturally, copper does not corrode because it acts as a cathode whereas iron corrodes and generates a more negative potential and it acts as an anode. 

Rnst can also be prevented by placing a sacrificial electrode in electrical contact with the iron. The sacrificial electrode must be composed of a metal that oxidizes more easily than iron (that is, it must be below iron in Table 18.1). The sacrificial electrode oxidizes in place of the iron (just as the more easily oxidizable species in a mixture is the one to oxidize), protecting the iron from oxidation. A related way to protect iron from rusting is to coat it with a metal that oxidizes more easily than iron. Galvanized nails, for example, are coated with a thin layer of zinc. Since zinc has a more negative electrode potential than iron, it... 

An electric potential or voltage exists between the two cell halves, and its magnitude can be determined if a voltmeter is connected in the external circuit. A potential of 0.780 V results for a copper-iron galvanic cell when the temperature is 25°C (77°F). 

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