Corrosion and Cathodic Protection

protection of tanks and underground structures it is necessary to understand the nature and mechanism of the corrosion that makes such protection necessary. F. N. Speller, in his book on corrosion (1), offers the following statements on the subject: "Corrosion of metals may be defined broadly as the chemical action of their environment, often resulting in their deterioration or destruction. It occurs because in many environments, most metals are not inherently stable and tend to revert to some more stable combination, of which the metallic ores, as found in nature, are familiar examples. "Recent work on the fundamentals of corrosion has shown that the essential phenomena are the same for all metals and alloys, differing only in degree but not in kind. "It may now be regarded as established that in substantially all cases of corrosion at ordinary temperatures and these comprise the most important practical cases the driving force of the corrosion reaction between metal and environment is electrochemical. The magnitude of this electrochemical potential, which varies with the environment and the metal, determines the tendency of the reaction to proceed, whereas the rate of corrosion is determined mainly by the resistance to the continued progress of the reaction set up by certain of the corrosion by-products." McKay and Worthington, in their book on the corrosion resistance of metals and alloys (2, p. 33) indicate a series of metals and alloys showing the tendency to corrode, as follows: