Problems in the Theoretical Treatment of Reactions Occurring at Easily Passivated Metal Anodes

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Abstract
Industrial anode materials should show small tendency to corrode in order to have a long life and, from energy considerations, should give low anodic overvoltages. The maximum corrosion in the active state in the presence and absence of complex formation is discussed of porous outer films of more difficulty soluble corrosion products. Corrosion in the passive state is treated as a property of the nonporous passive layer; several examples are presented, and the Flade potential for transitions from the active to the passive state and its properties are discussed. The influence of pH and complexing agents on passive corrosion is treated. Anode processes or, as treated here, redox processes such as the evolution of oxygen or chlorine can take place at passive electrodes only if the passive film has a sufficiently high electron conductivity. This is not the case with films on aluminum, titanium, and tantalum, for example. The overvoltage on passive anodes is a property of the passive film at the electrolyte side of the phase boundary. – Semiconducting materials and graphite as anode materials are discussed separately.