Instantaneous corrosion rates are typically calculated as a function of the exposure time from the slope of linear polarization resistance (LPR) scans using the well-known Stern-Geary equation and assuming a constant B value that does not vary during the test. However, this assumption is not valid when the corrosion mechanism changes during the test due to, for example, injection of a corrosion inhibitor, formation of protective films or passivation. A methodology was implemented to detect significant changes in the B values during a test, by evaluating the Current vs Potential data of LPR scans instead of only the polarization resistance (Rp) calculated from the slope. In addition, the Stern-Geary equation is a valid approximation only if one anodic and one cathodic reaction are present at the corrosion potential. An exact modified Stern-Geary equation was derived for cases where up to two cathodic and two anodic reactions are prominent at the corrosion potential. This procedure was used to interpret Rp vs time data from seawater corrosion at relatively low O2 concentrations. Under these conditions, not only the iron oxidation to Fe2+ and oxygen reduction reaction will occur, but the Fe2+/Fe3+ redox reaction may also be prominent at the corrosion potential. The application of this method was also illustrated using experimental data from a typical corrosion inhibitor qualification test, where chemical dosages are injected after a pre-corrosion period.