Early stage of iron anodic oxidation measured by 25ms-resolution X-ray reflectometry

Anodic oxidation at metal/water interface is a general phenomenon that commonly takes place on many metals. Time evolution of the oxide thickness is proportional to log(t)[1]. However, early stage clearly deviates from log(t) and the mechanism has not been understood yet in spite of its fundamental importance[2]. To reveal the early-stage mechanism, precise structural basis is needed. Here we report 25ms-resolution X-ray reflectivity experiments[3] to investigate the early stage of anodic oxidation at Fe (100), (110) and (111) in pH 8.4 borate buffer solution as model materials. Experiments were performed at the BL13XU of SPring-8, Japan with a 25keV monochromatic X-ray. The film density in the early stage is lower than in the later stage by 13 %, which suggests that the film structure is a highly defective spinel oxide. Moreover, the growth rate in the early stage is found to be proportional to about t^-1.5 but in later stage is t^-1. The presumed mechanism based on the Point Defect Model[1] will be discussed in the presentation.
[1] C. Y. Chao et al., Journal of The Electrochemical Society, 128, 1187 (1981).
[2] J. Kruger et al., Journal of The Electrochemical Society, 114, 43 (1967).
[3] H. Fujii et al., Journal of The Electrochemical Society, 166, E212 (2019).