A physical model describing corrosion processes on a 3‐D inhomogeneous surface is developed. The derived theoretical transfer function is discussed in terms of transport and charge‐transfer controlled corrosion steps. Simulations show “depressed capacitive semicircles” depending on the hydrodynamic conditions and the metal corrosion rate. In principle, the differential corrosion rate can be determined by an exact analysis of the complete transfer function of the system under test. The use of the polarization resistance and the application of the well‐known Stern‐Geary relation lead to exact information on the corrosion rate under certain limiting conditions only. The model is tested by experimental impedance results obtained for the system , aerated, , in the absence and in the presence of hexan (1,6)‐biphosphonic acid as inhibitor. The system‐specific parameters obtained by a nonlinear fit procedure are discussed in terms of the model used. The differential corrosion rate is checked by nonelectrochemical solution analysis. The porous oxidic 3‐D surface layer acts as an interphase inhibitor.