Spatially controlled microstructural variation using a free-surface flow driven by a rotating cylinder electrode - Growth of anodic oxide films on A1 6061

Growth and characterization of oxide films with spatially controlled microstructure on Al 6061 using a coaxial rotating, axially translating electrochemical reactor (CRATER) are reported. This electrode/flow combination allows growth of oxide films with spatially controlled microstructure on a single working electrode (WE) under well-defined conditions of mass transfer, electrode potential, and residence time. This is achieved by restricting electro-oxidation to a "window'' of narrow axial extent just below the free surface of an electrolyte whose coaxial inner and outer boundaries are a rotating counter electrode and an axially translating WE, respectively. Axial translation of the WE maps temporal variation of a controlled parameter (e.g., applied potential, mass-transfer rate) to axial variation of oxide microstructure on the WE. In the present work the applied potential is scanned linearly in time, leading to each element of the WE undergoing electro-oxidation over a narrow range of potential. Each element of the WE has the same exposure time in the reactive window and is exposed to the same mass-transfer conditions. This produces an oxide film in which the cell size and pore size vary systematically with potential along the length of the WE. The film displays a high degree of azimuthal uniformity. Continuous scanning of process variables in a single experiment by this approach allows identification of desirable microstructures in narrow ranges of operating conditions. Application of CRATER methodology to electrodissolution and electrodeposition is discussed. (C) 2002 The Electrochemical Society.