Chapter 16 - Corrosion of Metals

Abstract

This chapter focuses on corrosion of metals in aqueous systems, as this is a major engineering and economic problem. Aqueous corrosion of metals is much more important than dry oxidation of metals at near-ambient temperatures, because, if liquid water is present, diffusion of the ions and molecules involved in the electrochemical corrosion process is greatly facilitated. Most corrosion problems encountered in practice involve only a single metal. In bimetallic corrosion, the anodic and cathodic surfaces are well defined as being different metals, and are established instantly on placing the metals in electrical contact. The corrosion that results can be very vigorous, but one can usually arrange either to avoid using dissimilar metals together or else to place an electrically insulating gasket between them. Even single metals, however, are subject to aqueous corrosion by essentially the same electrochemical process as for bimetallic corrosion. The metal surface is virtually never completely uniform; even if there is no preexisting oxide film, there will be lattice defects, local concentrations of impurities, and, often, stress-induced imperfections or cracks, any of which could create a local region of abnormally high free energy that could serve as an anodic spot. This electrochemical differentiation of the surface means that local galvanic corrosion cells will develop when the metal is immersed in water, especially aerated water. A more effective source of anodic and cathodic regions on a single metal is variation in the thickness, or even local absence of a protective oxide film.