On the Tenuous Nature of Passivity

The use of reactive metals and their alloys (e.g., Ni-Cr-Mo-W-Fe and Fe-Cr-Ni alloys) for constructing machines that are exposed to ambient environments relies upon a continuing state of kinetic passivity of the metal surface.  Without this state, which is due to the formation and continued existence of a “passivating” oxide film, the alloy would react rapidly with components of the ambient environment (oxygen, water) and the structural integrity of the system would be compromised.  The stability of the barrier oxide layers of bilayer passive films that form on metal and alloy surfaces, when in contact with oxidizing aqueous environments, is explored within the framework of the Point Defect Model (PDM) using phase-space analysis (PSA), in which the rate of growth of the barrier layer into the metal, (dL⁺/dt), and the barrier layer dissolution rate, (dL^-/dt), are plotted simultaneously against the barrier layer thickness.  A point of intersection of dL^-/dt with dL⁺/dt indicates the existence of a metastable barrier layer, with a steady state thickness that is greater than zero, and hence specifies the conditions that must be met for “passivity”.