A Correlated Potentiostatic Microscopic Study of Iron Passivation in Sulfuric Acid^★ Available to Purchase

Microscopic observations of surface and liquid phase processes are correlated with changes in the electrical output of an iron half cell undergoing cyclic activation and passivation in 1N sulfuric acid. When observed with crossed polarizers, no optical changes are seen during the activation step, but during the current decay associated with the onset of passivation, a roughly circular zone of greatly increased reflectance is seen to grow from the circumference of the specimen towards its center. The light emanating from this zone appears to be completely depolarized, and by ultra-microscopy, it is seen to contain colloidal particles. It is also seen that while the electrical and optical characteristics of the surface during the activation step are insensitive to flow conditions in the electrolyte adjacent to the surface, those during the passivation stage are very sensitive. It is suggested that prior to the establishment of a coherent passive oxide film on the iron surface, there is a precursory process which occurs in the liquid along the interface. This process is probably the precipitation of colloidally dispersed ferrous hydroxide.