Understanding multi-element alloy passivation in acidic solutions using operando methods

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.