Chapter 1 - Dependence of the Corrosion Behavior of Metals Attacked by an External Current on Their Initial State

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This chapter addresses the corrosion behavior of metals attacked by an external current. It is customary to say that a metal that is spontaneously dissolving (corroding) at an appreciable rate is in an active state. When the current of the anodic direction is impressed on an actively corroding metal, the stream of electrons is directed from the metal to an external circuit. During anodic polarization of the metals, the major anodic process on which the main share of the anodic current is spent consists of the oxidation of the solution components on their surface. In chloride-containing aqueous solutions, the products of the oxidation are, according to the solution composition, oxygen, chlorine, and compounds of oxygen and chlorine. The influence of the external anodic current on a metal in an electrolyte solution is usually considered adequate for the creation of an electric contact with another, more electropositive metal, and also for the introduction of an oxidizer into the solution. In both of these cases, shifting of the metal potential to the positive side takes place. When the same potential values are reached under the influence of either of these two kinds of action, a similar corrosion behavior of the metal can be expected as a result. The aggressiveness of the external current is aggravated by its character of nonuniform distribution on the surface of metallic structures and the ability of the current to concentrate on metal areas with a weakened passive film or on defects in the coatings.