Passivity: enabler of our metals based civilisation

Abstract

The origin of passivity, as viewed from the Point Defect Model, is presented in terms of the phase space analysis of the rate law for the growth of passive films. It is shown that passivity is due to the formation of a metastable oxide and hence, the occurrence of passivity is a kinetic phenomenon, in which the rate of film growth of the oxide at the metal/barrier layer interface into the metal at zero barrier layer thickness must exceed the rate of barrier layer dissolution at the barrier layer/solution (bl/s) interface, in order for the barrier layer to exist. If this relationship does not hold, the barrier layer disappears and the surface becomes depassivated. Depassivation is illustrated with respect to transpassive dissolution, acid depassivation, pitting corrosion, flow assisted corrosion, impingement attack, resistive depassivation and other phenomena. The theory also leads to the development of Kinetic Stability Diagrams (KSDs), in which the potential for depassivation is plotted against pH to define regions of depassivation and passivation, so that regions in potential versus pH space can be defined within which passive films may exist as protective, metastable entities. The author offers these diagrams as kinetically inspired alternatives to the classical Pourbaix diagrams. One form of depassivation that occurs at highly localised regions on a metal surface is pitting corrosion. The theory for pitting is now highly developed and certain aspects of the subject are reviewed here. It is argued that the theory of pitting is sufficiently well established that the deterministic prediction of pitting damage is practical, in many systems. The theory is illustrated here with respect to the pitting of Type 403 stainless steel (SS), which is used extensively for the manufacture of low pressure steam turbines in steam cycle systems for the generation of electrical power.