In a circulating alkaline electrolyte, the current-potential curves and impedance plots reveal that a flow-through electrode made of zinc particles is quasi-blocked by a surface layer near the equilibrium potential and sharply activated at either anodic or cathodic polarizations. For slighlty clogged pores, the results of Electrode Impedance Spectroscopy (EIS) are shown to agree with the De Levie model of cylindrical pores. A least square fitting fives the potential dependencies of both geometrical parameters of pores and the penetration depth of current. It is concluded that even with a forced convection, the penetration depths of anodic and cathodic currents remain very small.