Galvanic Corrosion
Situations often arise where two or more different metals are electrically connected under conditions permitting the formation of a corrosion battery. A situation then exists where one metal will be corroded preferentially in relation to the other metal to which it is physically connected. This is termed galvanic corrosion. Three ingredients are required— an electrolyte, a material to act as an anode, and another to act as a cathode—in addition to the metals. The electrolyte is the medium in which ionization occurs. The electrons flow from the anode to the cathode through a metal path. The loss of metal is always at the anode. Figure 19.10 illustrates the electrical and chemical interplay of electrochemical corrosion of two different metals. While this is similar to the corrosion battery discussed previously, it is usually faster acting and more severe.
Anodic reactions are always oxidation reactions which tend to destroy the anode metal by causing it to go into solution as tons or to revert to a combined state as an oxide. Cathodic reactions are always reduction reactions and usually do not affect the cathode material. The electrons that are produced by the anodic reaction flow through the metal and are used in the cathodic reaction. The disposition of the reaction products is often decisive in controlling the rate of corrosion. Sometimes they form insoluble compounds that may cover the metal surface and effectively reduce the rate of further corrosion. At other times the reaction products may go into solution or be evolved as a gas, and do not inhibit further reaction. Galvanic corrosion is an extremely important corrosion process and one that is frequently encountered. An understanding of it will be helpful in rounding out our knowledge of corrosion processes. The principles of galvanic corrosion may actually be utilized to advantage in the cathodic protection of surfaces by using sacrificial metal anodes or inorganic protective coatings.
The Galvanic Scale
Corrosion occurs at the rate which an electrical current, the corrosion driving current, can get through the corroding system. The driving current level is determined by the existing electrode potentials. Electrode potential is the tendency of a metal to give up electrons. This can be determined for any metal by measuring the potential between the specimen (metal) half-cell and the standard (hydrogen) half-cell. Tabulating the potential differences between the standard (hydrogen) half-cell and other elements opens an extremely important window to view one part of the corrosion spectrum. Such a tabulation, known as the electromotive series, is illustrated in Figure 19.11.
Utilizing the electromotive series, an engineer can determine the electrical potential between any two elements. This electrical potential is the algebraic difference between the single electrode potentials of the two metals. For example:
Zinc and copper:
+0.76 - 0.34 = +1.10 V potential Iron and copper:
+0.44 - 0.34 = +0.78 V potential Silver and copper:
-0.8 - 0.34 = -0.46 V potential
The electromotive series forms the basis of several possibilities for controlling and decreasing corrosion rates. It provides the data required to calculate the magnitude of the electrical driving force in a galvanic couple. The electrical driving force of an iron and copper couple may be thought of as promoting the following activities:
Oxidation of the iron (the anodic reaction)
Flow of electrons through the solid iron and copper couple
Cathodic reaction on the copper (reduction), where the electrons are used
Current (ionic) of Fe2+ and (OH)~ in the electrolyte
It is possible to arrange many metals and alloys in a series, known as the galvanic scale, which describes their relative tendency to corrode. Figure 19.12 is a listing of some of the industrially important metals and alloys, including the ones which are most frequently encountered. Bearing in mind that a metal located higher on the scale will corrode preferentially and thereby protect a metal lower on the scale from corrosion attack. the example shown in Figure 19.12 may be set up.

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