The Mechanism of Passivating-Type Inhibitors

All passivating‐type inhibitors are oxidizing agents. However, not all oxidizing agents are inhibitors. A passivating ‐type inhibitor functions by producing local‐action current which anodically polarizes a metal into the passive potential region and thereby provides the means for obtaining a noble mixed potential. This mechanism is independent of whether passivity is caused primarily by oxide or adsorbed films. The major factors which determine whether a particular system will exhibit passivity are: ( a ) reversible potential of the redox system created by the oxidizing inhibitor; ( b ) electrochemical parameters for reduction of the inhibitor on the metal surface (exchange current, Tafel slope, and limiting diffusion current), and ( c ) critical anodic current and Flade potential of the metal. If oxygen is present in the system, some chemicals may function by changing its reduction kinetics. It is further shown that the amount of inhibitor found associated with the surface is not necessarily related to adsorption. Data are presented for passivation of stainless steel and titanium. The mechanism is discussed in terms of various oxidizing agents including oxygen, chromate, molybdate, and pertechnetate.