Mechanisms Of Corrosion Processes
The analysis of corrosion processes comprises examining the special features in a given metal's anodic dissolution, establishing the nature of the cathodic reaction (which is coupled with metal dissolution), and defining in greater detail the loci of the anodic and cathodic partial reaction. In corrosion, the equilibrium potential of reduction of the oxidizing agent is always more positive than that of dissolution of the metal (at the given solution composition). The main cathodic reactions in metal corrosion are hydrogen evolution and the reduction of dissolved oxygen. It is only in special cases when the corresponding reactants are available that chlorine, nitric acid, or other oxidizing agents will be reduced. Hydrogen evolution occurs at much more negative potentials than oxygen reduction. Hence, corrosion coupled with hydrogen evolution can be observed only for metals having sufficiently negative equilibrium potentials—the alkali and alkaline-earth metals, aluminum, magnesium, zinc, iron, and so on—and is encountered predominantly in acidic and alkaline media. Oxygen-depolarized corrosion occurs in contact with air, most often in neutral solutions (atmospheric corrosion, waterline corrosion in seawater, etc.).
In Section 13.7 we noted that the rate of corrosion (spontaneous dissolution) of metals depends on the shape and position of both the anodic polarization curve for metal dissolution and the corresponding cathodic curve and is determined by the point of intersection of these curves. The anodic curves 1 and 2 in Fig. 22.2a are for metals with a more negative and more positive potential, respectively. For the former, the state of the system corresponds to point A; the corrosion current is high, owing to the high rate of hydrogen evolution. For the latter, the potential is in a region where no hydrogen is evolved. Oxygen reduction is the only possible cathodic reaction. Because of the limited solubility of oxygen in water, this reaction occurs with concentration polarization (limiting current density il), which imposes a limit on the overall rate of the process: diffusion-controlled corrosion (point B). Oxygen-depolarized corrosion occurs mainly when the liquid film on the metal is thin, so that oxygen access to the electrode is sufficiently fast. In solutions, the rate of oxygen-depolarized corrosion depends on stirring intensity.
- FIGURE 22.2 Schematic polarization curves for spontaneous dissolution: (a) of active metals; (b) of passivated metals. (1,2) Anodic curves for active metals; (3) cathodic curve for hydrogen evolution; (4) cathodic curve for air-oxygen reduction; (5) anodic curve of the passivated metal.
Passivation of the metal and the associated sharp decline of its anodic dissolution rate have a strong effect on corrosion rates (curve 5 and the point of intersection C in Fig. 22.2b). Passivation is encountered more often under the effect of oxidizing agents (e.g., in the presence of oxygen).
When the metal surface is homogeneous, the anodic and cathodic partial reaction will be distributed uniformly over all surface segments; at different times both the anodic and the cathodic reaction will occur at each segment. The surface of a metal's liquid amalgam can be cited as an example of an ideally homogeneous surface. Rather good homogeneity is found as well on annealed surfaces of highly pure solid metals.
A nonuniform distribution of the reactions may arise when the metal's surface is inhomogeneous, particularly when it contains inclusions of other metals. In many cases (e.g., zinc with iron inclusions), the polarization of hydrogen evolution is much lower at the inclusions than at the base metal; hence, hydrogen evolution at the inclusions will be faster (Fig. 22.3). Accordingly, the rate of the coupled anodic reaction (dissolution of the base metal) will also be faster. The electrode's OCP will become more positive under these conditions. At such surfaces, the cathodic reaction is concentrated at the inclusions, while the anodic reaction occurs at the base metal. This mechanism is reminiscent of the operation of shorted galvanic couples with spatially separated reactions: Metal dissolves from one electrode; hydrogen evolves at the other. Hence, such inclusions have been named local cells or microcells.
The idea that metal corrosion could be due to local-cell action was put forward in 1830 by Auguste Arthur de la Rive, and became very popular. An extreme view derived from this idea is the assertion that perfectly pure metals lacking all foreign inclusions will not corrode. However, it does not correspond to reality. It was established long ago
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