On the existence of our metals-based civilization I. Phase-space analysis

The stability of the barrier 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 using phase-space analysis, 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 0against the barrier layer thickness. A point of intersection of dL(-)/dt with dL(+)/dt indicates the existence of a metastable barrier layer having a steady-state thickness greater than zero. If (dL(-)/dt)(L= Lss) > (dL(+)/dt)(L= 0), the barrier layer cannot exist, even as a metastable phase, as the resulting thickness would be negative. Under these conditions, the surface is depassivated and the metal may corrode at a rapid rate. Depassivation due to potential-enhanced dissolution (transition to the transpassive state) and to acid attack on the barrier layer (acid depassivation) are explored. The boundaries for depassivation may be plotted in potential-pH space to develop kinetic stability diagrams as alternatives to the classical Pourbaix diagrams for describing the conditions under which metals or alloys exist in the passive state. (c) 2006 The Electrochemical Society.