One practical use of the electrochemical impedance technique is rapid screening of either alloy corrosion resistance or process corrosivity as a function of process conditions. Curve-fitting of the impedance spectra using simple circuit analogue models can allow polarization or charge transfer resistances to be extracted even when there is limited understanding or characterization of the system under study. Practical corrosion predictions can be made from these results. Some successes and shortcomings in the technology are explored using corrosion of steel and 304SS in several environments. The results show that corrosion rates between 2.5 × 10^-4 and 250 mm/year can be estimated and predictions made even when only limited information on the solution chemistry and corrosion mechanism is available. Some discussion is presented on how one can gain confidence that the model circuit elements are providing reliable information.