Anomalously large differences between values of the polarization resistance R(p) measured in the time and frequency domains pose a serious problem in determination of the corrosion rate from the Stern-Geary equation. A theoretical model is used to derive expressions for a quantitative interpretation of the effect of a number of parameters on the apparent R(p) values obtained by de and ac measurements. Problems associated with the determination of the true R(p) value are discussed, and some relevant questions concerning the minimum frequency needed for impedance measurements are identified.