Metals and alloys under highly oxidizing conditions: in situ studies for elucidation of “transpassive” processes

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One of the least understood potential ranges in many fields of electrochemical processes is at highly anodic (oxidizing) potentials of so-called passive metals. In essence, three processes can take place in this potential regime: metal dissolution, oxide film formation, and oxygen evolution (from water oxidation). The project targets detailed studies of electrochemical behaviour and mechanisms in the transpassive region of selected technologically relevant passive metals and alloys. An understanding of these processes is essential for a sustainable performance of metallic materials in many applications where passive alloys may be exposed to highly oxidizing conditions, such as in their use as bipolar plates in electrolyzers. Even though the phenomenon of transpassivity has been long recognized, many open questions exist in fundamental understanding and the contribution of involved reaction pathways. This is due to the complex nature of possible electrode reactions taking place under transpassive conditions, as they often involve solid state oxide formation, dissolution (formation of solvatized metal cations) and gas formation. All these reactions depend on the material/electrolyte system, are potential- and time-dependent, and often occur in overlap of processes under dynamic equilibrium conditions; therefore in situ (analytical) techniques are most desired for elucidation of reaction mechanisms. Conventional investigations on reactivity rely mainly on monitoring the electrochemical behaviour which is not able to detect non-faradaic side reactions or to distinguish between different overlapping anodic oxidation reactions. In the current project a unique, in situ respirometry technique will be coupled with electrochemistry, combined with post-polarization characterization of the material surfaces. With this approach, we aim to achieve a more universal understanding on the critical factors in transpassive reactivity, and on the interplay of metal dissolution, oxide formation and oxygen evolution. In view of practical applications we will investigate technologically interesting metals and alloys, including model systems such as pure metals of the constituents of the alloys and binary model alloys. Three groups of materials will be of interest: i) metals and alloys with known transpassive dissolution originating from the oxidation of the passivating film into soluble higher-valent species; ii) other passive systems enabling water oxidation reaction to take place on the surface (electron conductive passive films); iii) valve metals. The data is brought together to bridge the nature of the oxide layer (its ionic and electronic properties, solubility) to the catalytic nature of the surface for water oxidation. This understanding should help to develop design strategies for specific applications of metallic materials in the high anodic potential regime in aqueous solutions.