Corrosion potential additivity principle

Andersen et al. predicted that similar results would be expected for the corrosion of other multivalent metals oxidizing via lower oxidation states. They also pointed out that their interpretation was consistent with the kinetics of the corrosion of copper in oxygenated HCl solutions. Here the final product is Cu and thus there is no vulnerable intermediate. In consequence, the rate of copper dissolution from either Nj-saturated or 02-saturated HCl solutions was the same at a given potential in conformity with the additivity principle. 

The principle of electrochemical corrosion protection processes is illustrated in Figs. 2-2 and 2-5. The necessary requirement for the protection process is the existence of a potential range in which corrosion reactions either do not occur or occur only at negligibly low rates. Unfortunately, it cannot be assumed that such a range always exists in electrochemical corrosion, since potential ranges for different types of corrosion overlap and because in addition theoretical protection ranges cannot be attained due to simultaneous disrupting reactions. 

Electrochemistry finds wide application. In addition to industrial electrolytic processes, electroplating, and the manufacture and use of batteries already mentioned, the principles of electrochemistry are used in chemical analysis, e.g.. polarography, and electrometric or conductometric titrations in chemical synthesis, e.g., dyestuffs, fertilizers, plastics, insecticides in biolugy and medicine, e g., electrophoretic separation of proteins, membrane potentials in metallurgy, e.g.. corrosion prevention, eleclrorefining and in electricity, e.g., electrolytic rectifiers, electrolytic capacitors. 

Anodic protection was developed using the principles of electrode kinetics and is difficult to understand without introducing advanced concepts of electrochemical theory. Briefly, anodic protection is controlled by the formation of protective passive film on metals and alloys using an externally applied potential. Anodic protection is used to a lesser degree because of the limitations on metal-environment systems for which anodic protection is viable. In addition, it is possible to accelerate corrosion if proper controls are not implemented during anodic protection. 

The ( a — Ec) term can be reduced by applying a cathodic current to the corroding sample so that its potential at least reaches the equilibrium potential of the anode E, where the corroding electrode is cathodically protected. It is necessary to apply an additional current to ensure that the potential is a few millivolts more negative that the a value to achieve complete protection from corrosion. Cathodic protection can be achieved by applying the current externally or by coupUng the specimen of interest to sacrificial anodes of metals such as Al, Mg, Zn. These principles are illustrated in Fig. 14.9. 

The designing of cathodic protection systems is rather complex, however, it is based on simple electrochemical principles described earlier in Chapter 2. Corrosion current flows between the local action anodes and cathodes due to the existence of a potential difference between the two (Fig. 5.1). As shown in Fig. 5.2, electrons released in an anodic reaction are consumed in the cathodic reaction. If we supply additional electrons to a metallic structure, more electrons would be available for a cathodic reaction which would cause the rate of cathodic reaction to increase and that of anodic reaction to decrease, which would eventually minimize or eliminate corrosion. This is basically the objective of cathodic protection. The additional electrons are supplied by direct electric current. On application of direct current, the potential of the cathode shifts to the potential of the anodic area. If sufficient direct current is applied, the potential difference between the anode and cathode is eliminated and corrosion would eventually cease to occur. 

In order to construct mixed-potential diagrams to model a corrosion situation, one must first gather (1) the information concerning the activation overpotential for each process that is potentially involved and (2) any additional information for processes that could be affected by concentration overpotential. The following examples of increasing complexity will illustrate the principles underlying the construction of mixed-potential diagrams. 

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