In Situ Measurement of Al3+ Concentration Profile during Al Anodization using Digital Holographic Interferometric Microscope

Abstract

In this study, the ionic mass transfer phenomenon of Al3+ in a sulfuric acid (pH 1) accompanying the anodization of an Al electrode was investigated using a digital holographic interferometric microscope. The anodization of an Al electrode is very popular technique to functionalize Al plates however, the anodization mechanism in the initial stage has not yet been understood, especially for the ionic mass transfer phenomenon. The investigation revealed the refractive index profile corresponding to the Al3+ concentration profile in the electrolyte. The Al3+ concentration on the electrode surface increased because of the Al dissolution in the initial stage of the anodization, and the thickness of the diffusion layer was proportional to the square root of time. The Al3+ surface concentration started to decrease after approximately 20 s probably because the Al dissolution reaction was hindered, and the Al oxide layer formation progressed. Additionally, ex situ scanning electron microscopy observation showed pores on the electrode surface at 80 s, which indicated that the pore formation was initiated from approximately 40 s with the progress of the oxide film formation. This research must be a steppingstone in order to understand the mechanism of anodization of Al.