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Growth of porous anodic films on zirconium and zirconium alloys in glycerol/fluoride electrolytes

Muratore, Francesca

[Thesis]. Manchester, UK: The University of Manchester; 2011.

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Abstract

Anodic films have been produced on zirconium and zirconium alloys potentiostatically (at either 20 or 40 V) in 0.35 M ammonium fluoride in glycerol, with interest in the addition of small amounts of water (up to 5 vol.%) to the electrolyte on their growth, morphologies and compositions. Scanning and transmission electron microscopies have been employed to analyse morphologies of the films, which appeared to be porous under all the investigated conditions.Rutherford backscattering spectroscopy and nuclear reaction analysis, used as techniques to investigate film compositions, disclosed the presence of zirconium, oxygen, fluorine, carbon and nitrogen in the films. The contents of fluorine and oxygen in the films were found to increase and decrease respectively by decreasing the amount of water added to the electrolyte from 5 to 0 vol.%. Moreover, the content of fluorine increased by decreasing the applied formation voltage, from 40 to 20 V, for films formed in electrolytes containing similar amounts of added water.In order to get information on the distribution of the species in the films, cross-sections of selected specimens were produced by focused ion beam and analysed by analytical transmission electron microscopy. Oxide-rich nanotubes were revealed embedded in a fluoride-rich matrix, suggesting that the mechanism of growth of the anodic films is governed by different migration rates of the anionic species in the film base, with F- ions, being the fastest anions. The relative amounts of the oxide-rich and fluoride-rich materials were related to the composition of the electrolyte, with the fluoride regions being less extensive and the oxide-rich nanotubes being thicker-walled by adding small amounts of water. Moreover, nanotubes are constituted of two shells (an outer one surrounding the pores and an inner one located between the outer shell and the matrix), suggesting differences in the composition in these two regions, presumed to be due to the incorporation of carbon species, being the slowest migrating species, in the outer shell.The fluoride-rich matrix chemically dissolved following 1 h immersion of the specimens in the formation electrolytes, promoting the transition from porous to nanotubular morphologies. Ageing of the specimens in deionized water for similar times did not significantly influence the morphologies and compositions of the anodic films.