Evaluation of Alloy Anodes for Aluminum‐Air Batteries: III . Mechanisms of Activation, Passivation, and Hydrogen Evolution

The electrochemistry of pure aluminum and six alloys containing elements such as Ga, In, Tl, Mg, and Mn in at temperatures of 25°, 50°, and 80°C is reported. Alloy electrodissolution and hydrogen evolution were explored separately by delineating the current/voltage curves for the anodic and cathodic partial reactions using the technique described in Part II of this series (1). Aluminum is shown to be a passive metal in concentrated potassium hydroxide solution, at least at 25° and 50°C, with the active‐to‐passive transition being evident at the lowest temperature. At potentials more positive than the open‐circuit value, aluminum dissolves in the transpassive mode yielding a nearly linear current/voltage curve. The minor alloying elements (particularly Ga, In, Tl, Mn, and Mg) passivate aluminum by forming a protective layer on the base aluminum. Activation of the alloy occurs by the oxidation of this layer to form soluble products; in those alloys containing gallium the oxidation of Ga to appears to determine the critical activation potential . The alloying elements also inhibit hydrogen evolution, partly by decreasing the exchange current density but also by modifying the Tafel constant. The kinetic data for hydrogen evolution have been used to estimate surface coverages of the alloying elements as a function of potential and temperature.