Structure of the iron rust layer formed in air-solution alternating condition

Otani, Kyohei   ; Tsukada, Takashi ; Ueno, Fumiyoshi  

Previous study reported that corrosion rate of steel under gas/aqueous alternating condition is three times faster than under solution condition. In this study, the mechanism of corrosion acceleration of steel under gas/aqueous alternating condition was clarified by cross-sectional observation and analysis. The cross-sectional image of the rust layer shows that the layer was composed of multi-layer structure and the chemical analysis of Raman spectroscopy clarified that outer layer is FeOOH and inner layer is FeO. These results suggest that FeOOH is formed on the rust initially, and then FeOOH changes to FeO by cathodic reduction reaction. On the other hand, Cl ions concentrated at the rust/metal interface, therefore, anodic Fe dissolution would be accelerated by the concentration of Cl ions. From these reasons, the corrosion of steel under gas/aqueous alternating condition would be accelerated by the cathodic reduction reaction and the anodic Fe dissolution.