THE CORROSION OF METALS 23 



tions. As a consequence, metals corrode somewhat less rapidly in- 

 doors but the character of the attack generally resembles that shown 

 out of doors. Underground exposures to soils and waters are often 

 severe and cause ferrous structures to fail unless protective non- 

 metallic coatings are employed. Considerable progress has been 

 made within the past ten years in determining the corrosivity of soils 

 and developing adequate preservative coatings." 



The best measure of the tendency of a metal to corrode is, in thermo- 

 dynamical terms, the decrease in free energy which accompanies the 

 chemical reaction involved in the process, i.e., the difference in energy 

 between the initial and final state of the system. This may be ob- 

 tained by simple calculation and is of value in showing whether or not 

 it is possible for corrosion to occur under the conditions defined. 

 There is no assurance however, that reactions which are possible will 

 actually take place within a reasonable time, if at all. Calculations ^ 

 show that, exposed to the atmosphere containing moisture, aluminum, 

 zinc, tin, iron, nickel, copper and silver may corrode to their respective 

 hydroxides. If, oxygen be excluded the last three metals listed cannot 

 corrode and iron only to the lower state of oxidation. These calcula- 

 tions, however, give no information as to the rate of corrosion or the 

 mechanism by which it takes place, matters of great practical im- 

 portance. 



The rate of corrosion or of any other chemical reaction bears no 

 direct relationship to the energy changes involved; it cannot be pre- 

 dicted but must be measured in some form.^" Obviously the rate of 

 corrosion depends upon the nature of the chemical reactions at the 

 surface of the metal. Generally, secondary reactions are involved and 

 the slowest step in the process controls the rate. The limiting factoi 

 is usually some sort of barrier,- — a film of gaseous or solid corrosion 

 products at the surface. 



The mechanism by which corrosion occurs may be one either of 

 direct combination of the metal and non-metal or the replacement by 

 the corroding metal of hydrogen or another metal in compounds. The 

 oxidation of metals, particularly at higher temperatures, halogenation 

 reactions, such as the chlorination of aluminum, and the reaction of 

 copper and sulfur, are examples of direct combination. In many of 

 the reactions which occur in the atmosphere, such as the formation of 

 tarnish films, the processes are somewhat obscure. When zinc corrodes 

 in the ordinary atmosphere an oxide film forms in the early stages 

 which is pseudomorphic with the metal ^^ but which is converted 

 eventually into the ordinary granular form of zinc oxide. The rate 

 of corrosion of zinc is determined by the rate of this conversion 



