Abstract
Oxide layers on titanium foils were produced by galvanostatically controlled plasma electrolytic oxidation in 12.9 M sulfuric acid with small amounts of phosphoric acid added up to a 3% mole fraction. In pure sulfuric acid, the oxide layer is distinctly modified by plasma discharges. As the time of the process increases, rough surfaces with typical circular pores evolve. The predominant crystal phase of the titanium dioxide material is rutile. With the addition of phosphoric acid, discharge effects become less pronounced, and the predominant crystal phase changes to anatase. Furthermore, the oxide layer thickness and mass gain both increase. Already small amounts of phosphoric acid induce these effects. Our findings suggest that anions of phosphoric acid preferentially adsorb to the anodic area and suppress plasma discharges, and conventional anodization is promoted. The process was systematically investigated at different stages, and voltage and oxide formation efficiency were determined. Oxide surfaces and their cross-sections were studied by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The phase composition was determined by X-ray diffraction and confocal Raman microscopy.
Highlights
The oxide layer of metals such as titanium can be tailored for specific applications
One derived technique is plasma electrolytic oxidation (PEO), in which the applied voltage exceeds a critical point and causes the initial oxide layer to reform by characteristic breakdowns, which often induce plasma conditions
Since the ratio of the two compounds in the mixture is critical for obtaining specific properties, we explored the effect of small amounts of H3PO4 in concentrated H2SO4
Summary
The oxide layer of metals such as titanium can be tailored for specific applications. The most common technique used to artificially grow a passive layer is anodic oxidation, in which moderate voltages promote a denser and thicker oxide layer compared with the naturally formed oxide. With the increasing scope of applications, new demands on materials have evolved. To meet these needs, researchers have developed new techniques from classical anodic oxidation. One derived technique is plasma electrolytic oxidation (PEO), in which the applied voltage exceeds a critical point and causes the initial oxide layer to reform by characteristic breakdowns, which often induce plasma conditions. PEO-treated titanium can be used for a variety of applications, such as biomedical prostheses, automotive components, and photocatalytic devices [1]
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