AC-IMPEDANCE STUDY ON THE ACTIVATION MECHANISM OF ALUMINUM BY INDIUM AND ZINC IN 3.5-PERCENT NACL MEDIUM

Activation of aluminium by In3+ and Zn2+ ions in chloride media has be en investigated by using potentiodynamic polarisation, potential vs. t ime measurements, electrochemical impedance spectroscopy and scanning electron microscopy studies. It is suggested that indium exhibits Inand In2+ intermediates during the dissolution and redeposition process es. Incorporation of any of these lower valent ions changes the defect structure of the passive film, leading to an increase in anionic vaca ncies and a decrease in the number of electrons, promoting active diss olution of aluminium. The role of zinc is to moderate localised attack by increasing the electron concentration and cation mobility, thereby lowering the corrosion rate at activated sites. (C) 1997 Elsevier Sci ence Ltd.