The kinetics of passivation and dissolution of many corrosion resistant engineering alloys remain incompletely characterized. Clarification of such kinetics and the unique role of alloying elements were investigated upon Ni-22% Cr and Ni-22% Cr-6% Mo, wt%, in an acidic chloride-free sulfate environment using simultaneous electrochemistry, on-line Inductively Coupled Plasma-Mass Spectroscopy (ICP-MS), and Single Frequency-Electrochemical Impedance Spectroscopy (SF-EIS). The combination of methods enabled in-operando tracking of the total current densities for (i) oxidation, (ii) cation ejection by multiple paths, and (iii) oxide film growth during non-steady state passivation. An improved and physically accurate interpretation of passivation processes is presented and clarifies and alters the frequently made assumption that Cr₂O₃ is the dominant oxide on Ni-Cr alloys throughout oxidation.