Abstract
Influences of electrolyte, pre-thermal treatment and substrate composition have been examined to elucidate the mechanism of field crystallization of anodic niobia formed on magnetron-sputtered niobium. The field crystallization occurs during anodizing at 100 V in 0.1 mol dm-3 ammonium pentaborate electrolyte at 333 K, with the crystalline oxide growing more rapidly than the amorphous oxide, resulting in petal-like defects. The nucleation of crystalline oxide is accelerated by pre-thermal treatment of the niobium at 523 K in air, while vacuum treatment hinders nucleation. Notably field-crystallization is also absent in 0.1 mol dm-3 phosphoric acid electrolyte or when anodizing Nb-10at.%N and Nb-29at.%W alloys in the ammonium pentaborate electrolyte. The behaviour is explained by the role of the air-formed oxide in providing nucleation sites for field crystallization at about 25% of the thickness of the subsequently formed anodic film, the location being due to the growth mechanism of the anodic oxide and the nature of crystal nuclei. Incorporation of tungsten, nitrogen and phosphorus species to this depth suppresses the field crystallization. However, boron species occupy a relatively shallow layer and are unable to affect the nucleation sites.
Original language | English |
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Pages (from-to) | 580-593 |
Number of pages | 14 |
Journal | Corrosion Science |
Volume | 49 |
Issue number | 2 |
DOIs | |
Publication status | Published - 2007 Feb |
Externally published | Yes |
Keywords
- A. Sputtered films
- B. SEM, GDOES, RBS
- C. Anodic films
ASJC Scopus subject areas
- Chemistry(all)
- Chemical Engineering(all)
- Materials Science(all)