The Corrosion of Metals---I. Mechanism of Corrosion Processes

LL metals are corrodible under the appropriate circumstances. The most important metal industrially, iron, is probably the most corrodible under ordinary conditions. Many estimates have been made of the value of iron and steel products destroyed by corrosion.1 While much depends upon the basis of calculation it seems reasonable to conclude that the annual cost of corrosion in this country is of the same order as the interest on the public debt or nearly one third of the cost of the federal government in normal times. The common nonferrous metals--zinc, lead, copper, aluminum, nickel and tin--are more resistant to corrosion largely because of their tendencies to form protective surface films. In the atmosphere under favorable circumstances tests have indicated, for example, that in the form of sheet 0.03 inch in thickness and exposed on one side as in the case of roofings, zinc, copper and lead if mechanically undisturbed would resist corrosion for more than one, two and three centuries respectively.2 Once a protective film is formed it may preserve the metal indefinitely. Under other circumstances these metals may readily corrode. Contact with large inert soil particles may result in the perforation of cable sheathing 0.10 inch in thickness in about 8 years. 3 Tin, although resistant to corrosion in air and pure water, is severely corroded by alkalies, and aluminum is attacked by both alkalies and acids. The 20 The Corrosion of Metals--I. Mechanism of Corrosion Processes By R.