Anodic Dissolution of Metals—Anomalous Valence

Abstract

INTRODUCTIONWhen metals are dissolved anodically in aqueous and nonaqueous media, the weight of metal dissolved is sometimes greater than that calculated from Faraday's law assuming normal oxidation states. Among several metals of industrial importance, AI, Be, Cd, Fe, Pb, Mg, Ti, and Zn exhibit this phenomenon. The usual implication is that these metals dissolve as ions with an ionic valence or oxidation number less than normal, e.g., Mg+, Be+, etc.Actually, anomalous anodic dissolution, i.e., that which results in a weight loss of an anode larger than that based on coulometric data, was first reported for Al over a century ago by its discoverers, Wohler and Buff. 1 Yet interestingly enough, despite the considerable research which has focused on this problem since that time, the exact nature of anomalous dissolution of metals remains in dispute.One mechanism which has received considerable support is the concept of univalent (unipositive) ion formation, sometimes referred to as the transitory ion mechanism.The unipositive (uncommon) ion mechanism has been especially espoused in large part by two major groups: Epelboin and co-workers at the University of Paris and Davidson, Kleinberg, and co-workers at the University of Kansas, Lawrence. The mechanism was first employed by Epelboin to explain the apparent valence of 1.40 for Zn dissolving anodically in aqueous perchlorate solution. 2 This value was obtained both from Straumanis, James, and co-workers at the University of MissouriRolla have attributed anomalous anodic dissolution of metals in aqueous solution, as have many other electrochemists, to the competition of secondary processes with an electrochemical oxidation step excluding that of transitory ion formation. They have explained the anomalous dissolution of many metals in aqueous media as arising from related phenomena: local action, film-controlled polarization, and anodic disintegration. The reader should be made aware that these phenomena often involve, but are not necessarily confined to, secondary processes frequently encountered in the electrochemistry of metals dissolving in dilute aqueous systems. Indeed, the major complaint of Epelboin is that too many electrochemists choose to confine their studies of anomalous anodic dissolution within the framework of classical electrochemistry. Despite the conflicting philosophies of the different approaches, considerable credit must be accorded to Epelboin and his colleagues in their extensive and well-planned efforts to come up with a universal and quantitative mechanism of anodic dissolution based on sound kinetic and thermodynamic principles.There is no argument that experimental and theoretical considerations of anomalous dissolution from existing and new points of view are essential to research and development with regard to corrosion protection and inhibition, batteries and other energy devices, electrochemical synthesis, and other associated electronic properties of interfaces. Accordingly, the following review is presented with the hope that it will ...

Probabilistic cellular automata model of generalised corrosion, transition to localised corrosion

“…As the peninsula grows, the anodic, acidic region may close in onto the base of the peninsula causing the rupture of the metallic connection and leading to its detachment and formation of an island. The appearance of such metallic islands can be related with the chunk effect 1 [18,[26][27][28][29][30]. This island is progressively dissolved with probability P diss .…”