Journal
JOURNAL OF PHYSICAL CHEMISTRY C
Volume 114, Issue 44, Pages 18853-18859Publisher
AMER CHEMICAL SOC
DOI: 10.1021/jp1078136
Keywords
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Funding
- Japan Society for the Promotion of Science [19206077, 21656180]
- Ministry of Education, Culture, Sports, Science and Technology, Japan [B01]
- Grants-in-Aid for Scientific Research [21656180, 19206077] Funding Source: KAKEN
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The growth of porous anodic films on iron has been examined at a constant current density of 50 A m(-2) in 0.1 mol L-1 NH4F-elhylene glycol electrolytes containing 0.1-1.5 mol L-1 water. Nanoporous films are formed in all the electrolytes, with the growth rate increasing with the decrease in the water content of the electrolyte. A barrier layer, in which a high electric field is applied during anodizing, thickens in proportion to the formation voltage at a ratio of 1.9 nm V-1, regardless of the water content of the electrolyte. However. there is a transition water content between 0.3 and 0.5 mol L-1. at which growth behavior changes. Above the transition level, the formation voltage is constant after an initial voltage rise, with the constant voltage slightly rising with a decrease in water content. In contrast, the formation voltage increases continuously to more than 150 V when the water contents are below the transition level. The anodic films are poorly crystalline and contain a significant amount of fluoride species. A high enrichment of fluoride species occurs near the metal/film interface when the water content in the electrolyte is below the transition level. Such enrichment is not as significant, or possibly absent, in electrolytes of increased water content.
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