We define a reflective-transmissive coefficient to characterize the boundary conditions of real electrochemical systems with a diffusion flux of charged species. For these systems we obtain from a generalized diffusion equation, for processes with a non-vanishing relaxation time, a generalized Warburg-like impedance for a finite length diffusion with general boundary conditions. The influence of the reflective-transmissive coefficient on the generalized Warburg impedance is studied and related to the reduction and corrosion nature of the interface. We also interpret this impedance in terms of a general transmission line.