|
|
Hokkaido University Collection of Scholarly and Academic Papers >
Theses >
博士 （工学） >
Formation Behavior of Nanostructured Anodic Films on Metals in Fluoride Containing Organic Electrolytes
|
Files in This Item:
|
|
|
Related Items in HUSCAP:
|
| Title: | Formation Behavior of Nanostructured Anodic Films on Metals in Fluoride Containing Organic Electrolytes |
| Other Titles: | フッ化物含有有機電解液中における金属上へのナノ構造アノード酸化皮膜の生成挙動に関する研究 |
| Authors: | Shahzad, Khurram Browse this author |
| Issue Date: | 26-Dec-2016 |
| Publisher: | Hokkaido University |
| Abstract: | Porous anodic alumina films with self-ordered nanopore channels have been used for surfacetreatment of aluminum and its alloys to improve corrosion and wear resistance. The films are also of recentinterest as key materials for nanodevices and as template for fabrication of nanomaterials. Self-orderingelectrochemical anodizing has been recently extended to other metals including titanium, zirconium,niobium, iron and stainless steel, and these nanostructured anodic films have many promising applications.The extensions to many metals and alloys were mainly achieved by finding novel organic electrolytescontaining fluoride and small amounts of water. Although extensive studies on formation mechanism ofporous anodic alumina films formed in aqueous acid electrolytes have been conducted, little is known aboutthe growth mechanism of the anodic films in the organic electrolytes, particularly on iron that is the mostimportant practical metal. Understanding the growth mechanism is of significant importance to control themorphology and composition of the anodic films. Thus, this study focused on elucidating the growthmechanism of the nanostructured anodic films on iron. Further, in this study it was found that the fluoridecontainingorganic electrolytes with a wide range of water concentrations are suitable for the formation ofcompact, so-called barrier-type anodic films on magnesium, which is also a practically important lightmetal.This dissertation includes six chapters. Chapter 1 describes the general introduction of anodizingand objective of this dissertation. In chapter 2, the preparation of materials and experimental conditionsemployed for this study as well as major characterization techniques were described in detail.Chapter 3 describes the critical factors influencing the film morphology and growth mechanism onthe anodic film on iron. Water concentration in electrolyte influences largely the film structure andcomposition of the anodic film as well as the distribution of fluoride-rich layer within the anodic film. Forinstance, scalloped metal/film interface, typical of porous anodic alumina, was observed only when thewater concentration was ≥ 1.5 mol dm-3. At lower water concentrations the interface became flat. A thinnerbarrier layer beneath the porous layer was formed by an increase in water concentration. Carbon andfluoride incorporations were also suppressed at high water concentration. TEM observations clearlydisclosed that fluoride enrichment occurred at the cell boundaries as well as at the scalloped metal/filminterface when the water concentration was 1.5 ml dm-3. The preferential dissolution of fluoride-rich cellboundaries caused the development of self-ordered nanotubular anodic films at high water concentrations.In contrast, no fluoride enrichment was found at cell boundaries when the water concentration was low (0.1mol dm-3). Thus, no nanotubular film was formed at low water concentrations. Through the findings in thischapter, a transition of the growth mechanism from “field-assisted dissolution” to “field-assisted flow” withwater concentration was proposed.In Chapter 4, a barrier-type anodic film was formed on magnetron-sputtered iron by employingpotentiodynamic anodizing at a relatively fast sweep rate, and the influence of electric field on thedissolution of the barrier-type anodic film was examined. It was found from Rutherford backscatteringspectroscopy that the barrier-type anodic film formed in ethylene glycol electrolyte containing 0.1 mol dm-3 NH4F and 0.1 mol dm-3 water consists of two layers, comprising an outer iron hydroxy-fluoride layer andan inner iron fluoride layer, with the latter layer forming as a consequence of the faster migration of fluorideions in comparison with oxygen species. The barrier layer was formed even at a current efficiency as lowas 49%. The application of electric field to the barrier-type film on iron accelerated the chemical dissolutionof the anodic film at the film/electrolyte interface. Below the critical field of approximately 2.8 MeV cm-1,uniform thinning of the anodic film occurred due to field-assisted dissolution, while a nanoporous film wasdeveloped above the critical field. The critical field for anodic film on iron is much lower than that recentlyexamined on aluminum (~5.5 MeV cm-1).In chapter 5, the uniform growth of barrier-type anodic film on magnetron-sputtered magnesiumwas examined in fluoride and phosphate-containing ethylene glycol-H2O mixed and aqueous electrolytes.The addition of phosphate to fluoride-containing organic electrolytes induced amorphization of anodic filmand increases the volume expansion factor. The anodic films consisted of phosphate-incorporatedoxyfluoride and the concentration of incorporated phosphate decreased gradually with an increase in waterconcentration. The anodic films consist of two layers with an inner layer containing less amount ofphosphate. The outer layer is formed at the film/electrolyte interface by the migration of Mg2+ ionsoutwards, while the inner layer is formed at the metal/film interface. The efficiency of film growth reducedfrom 100 % at 0.1 mol dm-3 H2O to ∼ 52% in aqueous electrolyte. Despite the low efficiency in aqueouselectrolyte, a barrier-type anodic film with a uniform thickness was developed. The reason for the formationof the barrier-type anodic film, not the porous-type film, at such low efficiency was discussed.Chapter 6 is the general conclusion of this dissertation and future prospects of anodizing researchare described. |
| Conffering University: | 北海道大学 |
| Degree Report Number: | 甲第12503号 |
| Degree Level: | 博士 |
| Degree Discipline: | 工学 |
| Examination Committee Members: | (主査) 教授 安住 和久, 教授 村越 敬, 教授 長谷川 靖哉, 教授 幅﨑 浩樹, 准教授 青木 芳尚 |
| Degree Affiliation: | 総合化学院（総合化学専攻） |
| Type: | theses (doctoral) |
| URI: | http://hdl.handle.net/2115/64448 |
| Appears in Collections: | 課程博士 (Doctorate by way of Advanced Course) > 総合化学院(Graduate School of Chemical Sciences and Engineering)
学位論文 (Theses) > 博士 （工学）
|
|
