Kinetic Stability Diagrams and the Prediction of Passivity

摘要

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, corrosive 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~+/di), 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". The condition for passivity is stated simply as dL~-/dt > (dL~+/dt)_(L=0). This formalism leads to the derivation of "Kinetic Stability Diagrams" (KSDs) for describing passivity in potential-pH space as alternatives to the classical Pourbaix diagrams. The KSDs describe the region in potential-pH space where the barrier layer of the passive film may exist as a metastable entity, noting that the layer can never exist as a thermodynamically stable entity, as proposed by Pourbaix. Outside of this region, the surface is depassivated by the dissolution rate exceeding the barrier layer growth rate at zero layer thickness, either due to the dissolution rate becoming highly potential dependent(oxidative depassivation, such as transpassive dissolution) or due to enhanced barrier layer dissolution due to the increase on concentration of some aggressive species (acid depassivation upon acidification). The derivation and interpretation of a KSD for a chromium-containing nickel-base alloy is used to illustrate the application of these new, kinetically-based diagrams and a comparison is made with the classical Pourbaix diagram to illustrate the differences between the two approaches.