Outer Hydrous Layer on the Passive Oxide Film

Outer Hydrous Layer on the Passive Oxide Film 

In neutral solution, Fe dissolved in the electrolyte can be anodi-cally deposited on the electrode to form a hydrous layer When iron electrode is passivated after the passage of active dissolution, the ferrous ions dissolved in the active potential region are deposited in the passive potential region to form the relatively thick hydrous layer. 

This effect of Fe concentration indicates that when iron is passivated by an anodic potential sweep from the active potential region, the thick hydrous oxide layer would be formed by the anodic deposition of Fe dissolved from the iron in the active potentials. Much thicker oxide films on iron have been reported by various authors in neutral solutions. The origin of the thick films may be Fe accumulated in the solutions. 

Since the anodic deposition from Fe does not take place in acidic solution, the thictoess is independent of the concentration of Fe ions in solution at pH lower than four. 

The composition of the anodic deposition layer was studied by using ex-situ reflection electron diffraction by Cohen et al. They found Y-FeOOH for the deposited film formed on platinum from perchlorate solution containing Fe. Ohtsuka et al. estimated the composition from Raman spectroscopy as well as in-situ EQCM and ellipsometiy to be amorphous Fe00H-H20 (i.e., FefOHls)/  

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