Abstract
Pores in the porous-type anodic oxide film of aluminum have conventionally been thought to grow vertically in a straight tube-like manner. However, branching and nano-branching pores (referred to as feather-like) typically observed in films generated in chromic acid solutions are considered non-steady-state morphologies. However, are these formed only under specific conditions? These have also been observed in neutral borate electrolytes, highlighting the need to clarify the dominant factors governing pore morphology. In this study, we report the results of observations and discussions on the fine structure of pore morphology, including typical films grown in oxalic acid or sulfuric acid, using state-of-the-art analytical techniques to elucidate the dominant factors influencing pore formation [1].Observation of the initial pore formation in the three types of electrolytes, sulfuric acid, oxalic acid, and phosphoric acid, reveals that the initial pores are considerably smaller in diameter and more numerous than the main pores. This is presumed to be due to pore formation through dissolution under low electric field strength caused by the presence of a thick barrier layer. Observation of the growth process of pores generated in sulfuric acid and oxalic acid electrolytes using high-resolution TEM revealed that neither film grows in a tubular manner but rather undergoes repeated branching at a fine scale. The cause of nano-branching in the main pores is similarly attributed to thickening of the barrier layer for some reason in the electrolyte, resulting in a decrease in the electric field. Thus, it is considered that pores do not grow in a straight tube-like manner but rather universally involve branching, and the grown pores are subsequently smoothed out by chemical dissolution.These results thus indicate that the formation of radial nano-branched pores is not unique to chromic acid films. Instead, this is a universal phenomenon of anodic films that frequently occurs when the electric field strength decreases during anodizing for the same reason. When Thompson et al. first reported horizontal cross-section observations of various porous anodic films in 1978, they stated that “steady-state anodizing is not as regular as previously assumed”. This remains true, and elucidation of the actual structure of the porous film is important for more effective application of the film.[1] S. Ono, H. Asoh, Electrochem. Comm. (2021). Figure 1
Published Version
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