Crystallization of anodic titania on titanium and its alloys

Crystallization of amorphous anodic films grown at constant current density on sputtering-deposited titanium, and Ti-Si and Ti-Al alloys, in ammonium pentaborate electrolyte, has been examined directly by transmission electron microscopy. In the case of titanium, anatase develops at relatively low voltage in the inner film region, formed by inward migration of oxygen species. In contrast, the outer film region, formed at the film/electrolyte interface, is composed of amorphous oxide only. Oxide crystals are particularly found near the plane, separating the two regions, which is located at a depth of 35-38% of the film thickness. Oxide zones, of size ∼ 1 nm, with a relatively ordered structure, developed at the metal/film interface, are considered to lead to transformation of the inner region structure. The incorporation into the film of either aluminium or silicon species suppresses the formation of crystalline oxide to much increased voltages. However, eventually nanocrystals form at ∼40% of the film thickness, probably originating from pre-cursor nuclei in the air-formed on the as-deposited alloy.     

Ionic transport in amorphous anodic titania stabilised by incorporation of silicon species

Incorporation of silicon species from an alloy substrate into anodic titania is shown to stabilise the structure of the film, facilitating investigation of the ionic transport processes in amorphous titania grown at high efficiency. Thus, an amorphous anodic film developed on a sputtering-deposited Ti-6 at.%Si alloy formed to 100 V in phosphoric acid electrolyte in contrast to a partially crystalline film developed on relatively pure titanium at <20 V. Silicon species, which are immobile and act as marker species in the growing film, are present in the inner 58% of the film thickness. Evidently, the film material forms simultaneously at the film/electrolyte and alloy/film interfaces by co-operative transport of cations and anions, as is usual in amorphous anodic oxides. The phosphate anions incorporated from the electrolyte migrate inward at 0.34 times the rate of O²⁻ ions and hence are present in the outer 62% of the film thickness.     

Spark anodizing behaviour of titanium and its alloys in alkaline aluminate electrolyte

Spark anodizing of titanium, Ti-6Al-4V and Ti-15V-3Al-3Cr-3Sn in alkaline aluminate electrolyte produces highly crystalline anodic films consisting mainly of Al₂TiO₅ with α- and γ-Al₂O₃ as minor oxide phases, irrespective of substrate composition. However, the apparent efficiency for film formation decreases in the following order: Ti-6Al-4V, titanium and Ti-15V-3Al-3Cr-3Sn. A large amount of aluminium species are incorporated from the electrolyte, probably by plasma-chemical reaction, and become distributed throughout the film thickness. This distribution indicates that the electrolyte penetrates near to the film/substrate interface through the discharge channels. Thus, the outwardly migrating aluminium ions under a high electric field can be present even in the inner part of the anodic films. Voids are developed at the film/substrate interface, particularly on the vanadium-containing alloys, reducing the adhesion of the anodic film to the substrate.     

Anodic oxidation and dielectric behaviour of aluminium-niobium alloys

The anodizing behaviour of sputtering-deposited Al-Nb alloys, containing 21, 31 and 44 at.% niobium, has been examined in 0.1 M ammonium pentaborate electrolyte with interest in the composition and the dielectric properties of the anodic oxides. RBS and TEM revealed amorphous oxides, containing units of Nb₂O₅ and Al₂O₃ in proportion to the alloy composition. Xenon marker experiments indicated their growth through migration of the Nb⁵⁺, Al³⁺ and O²⁻ species, with cation transport numbers, in the range 0.31-0.35, and formation ratios, in the range 1.35-1.64 nm V⁻¹, intermediate between those of anodic alumina and anodic niobia. Al³⁺ ions migrate slightly faster than Nb⁵⁺ ions, promoting a thin alumina layer at the film surface, although this layer is penetrated by fingers of the underlying niobium-containing oxide of relatively reduced ionic resistivity. The incorporation of units of Nb₂O₅ into anodic alumina increases the dielectric constant from about 9 to the range 11-22 for the investigated alloys.     

Formation behavior of anodic TiO2 nanombes in fluoride containing electrolytes

TiO₂ nanotube layers can be formed with titanium in the electrolytes containing fluoride by electrochemical method. The role of fluoride ion, the crystallinity of the anodic oxide, and the chemical state were investigated. The results show the anodic film is composed of oxide and a little amount of hydroxide. The presence of F⁻ ions leads to chemical dissolution of Ti oxide layer and prevents hydroxide precipitation. Consequently, chemical dissolution rate increases with increasing the fluoride content in the range of 0–2% (in mass fraction) because F⁻ ions in electrolyte attack the interface and allow the ions of the electrolyte to easily penetrate into the interface. The as-anodized TiO₂ nanotubes exhibit an amorphous structure. Thermally treated nanotubes are composed of mixtures of the anatase and rutile phases.     

Incorporation of transition metal ions and oxygen generation during anodizing of aluminium alloys

Enrichment of nickel at the alloy/film interface and incorporation of nickel species into the anodic film have been examined for a sputtering-deposited Al-1.2at.%Ni alloy in order to assist understanding of oxygen generation in barrier anodic alumina films. Anodizing of the alloy proceeds in two stages similarly to other dilute aluminium alloys, for example Al-Cr and Al-Cu alloys, where the Gibbs free energies per equivalent for formation of alloying element oxide exceeds the value for alumina. In the first stage, a nickel-free alumina film is formed, with nickel enriching in an alloy layer, 2 nm thick, immediately beneath the anodic oxide film. In the second stage, nickel atoms are oxidized together with aluminium, with oxygen generation forming gas bubbles within the anodic oxide film. This stage commences after accumulation of about 5.4 × 10¹⁵ nickel atoms cm⁻² in the enriched alloy layer. Oxygen generation also occurs when a thin layer of the alloy, containing about 2.0 × 10¹⁹ nickel atoms m⁻², on electropolished aluminium, is completely anodized, contrasting with thin Al-Cr and Al-Cu alloy layers on electropolished aluminium, for which oxygen generation is essentially absent. A mechanism of oxygen generation, based on electron impurity levels of amorphous alumina and local oxide compositions, is discussed in order to explain the observations.     

Field crystallization of anodic niobia

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.     

TC4钛合金阳极氧化着色膜显色规律探讨

目的在不同电解液、电压等工艺参数下对TC4钛合金进行阳极氧化,获得彩色膜,分析探讨着色膜颜色随不同工艺参数变化的显色规律,并通过该显色规律分析着色膜显色机理。方法分别选用NaOH电解液、H3PO3电解液、Na2SiO3盐溶液对Ti6A14V钛合金进行氧化着色。通过金相显微镜、SEM、XRD、AFM和3nh色差仪等测试方法,分析氧化膜层显微组织、形貌特征、物相成分、膜层厚度与颜色变化。结果3nh色差仪测得膜层颜色值(L^*、a^*、b^*)随电压具有周期变化规律。在电压参数为120 V左右的起弧电压之前,三种电解液阳极氧化着色膜均是由非晶态的钛氧化物组成,显色规律一致,氧化膜层致密均匀,只是生长速率稍有不同。膜层显色是干涉加强光色与干涉减弱光色的互补光色的共同作用。通过钛合金氧化膜干涉光程差公式修正,推导出了薄膜厚度的理论计算公式,且AFM测试结果与理论计算得出的膜厚基本一致。随着电压继续升高,电解反应剧烈,宏观表面观察到微弧放电现象,电解过程过渡到微弧氧化阶段。结论在低电压阳极氧化阶段,TC4钛合金着色膜层是由致密均匀的非晶态钛氧化物组成,膜层生长方式是随电压均匀层状生长,显色原理主要是薄膜干涉原理。通过控制电压参数,可控制膜层厚度,继而得到理想的颜色。在Na2SiO3盐溶液中的膜层生长速率为1~1.7 nm/V。     

Baking treatment effect on materials characteristics and electrochemical behavior of anodic film formed on AZ91D magnesium alloy

The composition and microstructure of the anodic films formed on AZ91D Mg alloy, with or without baking, were investigated. The associated corrosion behavior of the anodized alloy in 3.5 wt% NaCl solution was also examined using electrochemical impedance spectroscopy (EIS). The results show that MgO was the main component in the anodic film which also contained some Mg(OH)₂, Al₂O₃, Al(OH)₃, and MgAl₂O₄. Both the amorphous and crystalline forms of anodic film were identified. The degree of crystallinity depended on baking temperature, which increased with increasing temperature in the range of 50-250 °C. The amounts of MgO and Al₂O₃ increased as a result of a dehydration reaction. The polarization resistance of anodized Mg alloy was improved significantly by increasing the oxide content in the anodic film. An optimum value of polarization resistance of anodic film was obtained for the alloy baked at 150 °C for 2 h followed by air cooling.     

Effects of electrolyte pH and temperature on dielectric properties of anodic oxide films formed on niobium

The effects of electrolyte pH and temperature on the structure and properties of anodic oxide films formed on niobium in phosphoric acid solution with the addition of NH₄OH for pH adjustment have been investigated. The film thickness formed at the same voltage slightly increased with increasing pH and significantly increased with increasing electrolyte temperature. The capacitance of the film was independent of electrolyte pH in an acid region, while it notably increased with increasing pH in an alkaline region. The relative permittivity of the film changed 43.7-80.5 when the electrolyte pH was increased from 1.6 to 10. The incorporation depth and content of phosphorus in the film were markedly suppressed at pH 10, and nitrogen was found to penetrate into a depth of 70%. Furthermore, the apparent transport number of Nb⁵⁺ ion decreased from 0.26 to 0.02 by a pH increase from 1.6 to 10. The notable changes in structure and dielectric properties of the anodic niobia film formed in the alkaline region would primarily be caused by the different incorporation behavior of electrolyte species such as phosphorous and nitrogen.     

The valence state of copper in anodic films formed on Al-1at.% Cu alloy

During anodising of Al-Cu alloys, copper species are incorporated into the anodic alumina film, where they migrate outward faster than Al³⁺ ions. In the present study of an Al-1at.% Cu alloy, the valence state of the incorporated copper species was investigated by X-ray photoelectron spectroscopy, revealing the presence of Cu²⁺ ions within the amorphous alumina film. However, extended X-ray irradiation led to reduction of units of CuO to Cu₂O, probably due mainly to interactions with electrons from the X-ray window of the instrument and photoelectrons from the specimen. The XPS analysis employed films formed on thin sputtering-deposited alloy/electropolished aluminium specimens. Such an approach enables sufficient concentrations of copper species to be developed in the anodic film for their ready detection.     

Influences of ion migration and electric field on the layered anodic films on Al–Mg alloys

Anodizing of sputtering-deposited Al–Mg alloys containing 27 and 32 at.% magnesium in sodium hydroxide electrolyte is shown to develop two-layered anodic oxide films. The outer layer contains aluminium and magnesium species, and is enriched in the latter species relative to the alloy, particularly towards the film surface. The inner layer also contains the two alloy species but is depleted in magnesium, due to Mg²⁺ ions migrating to the outer layer faster than Al³⁺ ions. The ratio of the thickness of the outer layer to that of the film increases with increase of magnesium content of the alloy. The presence of aluminium species in the outer layer is attributed to the penetration of the outer layer by oxide of the inner layer with lower ionic resistance. This mechanism of film growth appears to be sustainable to alloy concentrations to 40 at.%Mg, when the inner layer may no longer form. Enrichment of alloying elements can accompany film growth on Al–Mg alloys, as shown by enrichment of tungsten to 2–3 × 10¹⁵ atoms cm⁻² in an Al–26 at.%Mg–1 at.%W alloy.     

Formation of barrier-type anodic films on sputtering-deposited Al-Ti alloys

The formation of amorphous anodic films at constant current is investigated for sputtering-deposited Al-Ti alloys containing from 3-30 at.% Ti. The films were grown at high efficiency in a borate electrolyte and comprised a main region containing units of Al₂O₃ and TiO₂, with a thin surface region enriched in titanium species. The formation ratios of the films increased with increase of titanium content of the alloys. The presence of the outer region is explained by the faster migration of Ti⁴⁺ ions relative to that of Al³⁺ ions through the films.     

铝合金低硫酸浓度硬质阳极氧化膜生长及特性

目的探究铝合金在低浓度硫酸电解液中,阳极氧化膜的生长及特性。方法在3%H2SO4和18%H2SO4(均为质量分数)电解液中对6063铝合金进行硬质阳极氧化,通过对膜层生长过程中的电压-时间曲线及微观形貌进行分析,研究膜层的生长特性。结果在低浓度硫酸电解液中,氧化膜初期生长为"缺陷择优生长"方式,即在高表面能缺陷处不断形成氧化膜核心并铺展,直到相遇形成界面为止;后期生长为"交界面择优生长"方式,即在较薄氧化膜交界面不断溶蚀并产生Al3+和O2-反向传输,使氧化膜增厚。结论低浓度硫酸中阳极氧化膜的生长方式与传统阳极氧化膜显著不同,膜层更加致密,厚度、硬度和粗糙度较大。     

High resistivity magnesium-rich layers and current instability in anodizing a Mg/Ta alloy

Strikingly different morphologies of amorphous anodic films on a Mg/40 at.%Ta alloy are shown to result from single-stage and sequential anodizing procedures. The alloy, prepared by magnetron sputtering, was anodized galvanostatically in ammonium pentaborate (pH 8.3) and sodium silicate (pH 12.6) electrolytes at 293 K and studied by transmission electron microscopy, Rutherford backscattering spectroscopy, glow discharge optical emission spectroscopy and X-ray photoelectron spectroscopy. For one-step anodizing in the pentaborate electrolyte, a single-layered film, of approximate composition Ta₂O₅ · MgO, forms at a ratio of ∼1.8 nm V⁻¹. In the silicate electrolyte, an outer, magnesium-rich layer, containing silicon species, also forms, with a ratio of 0.8 nm V⁻¹. The outer layer can develop due to relatively fast migration of magnesium ions in the inner layer and the stabilization of the pH at the film surface, probably linked to generation of a silica gel that also limits loss of magnesium species to the electrolyte. Further thickening of the anodic film, in ammonium pentaborate electrolyte, produces fingers of low resistivity, inner oxide that penetrate the pre-existing, high resistivity outer layer, with a bi-modal distribution of finger sizes. When fingers reach the film surface, magnesium ions are ejected to the electrolyte. The absence of fingers in films grown in sodium silicate electrolyte is possibly due to prevention, by the silica gel, of their initiation.     

Formation, structure and composition of anodic films on AM60 magnesium alloy obtained by DC plasma anodising

Anodising of AM60 magnesium alloy (6% Al + 0.27% Mn) was studied in a solution containing 1.5 M KOH + 0.5 M KF + 0.25 M Na₂HPO₄ · 12H₂O with addition of various NaAlO₂ concentrations. The experiments were carried out in DC current galvanostatic mode. Observations of phenomena occurring at the sample surface plus voltage monitoring revealed three stages: traditional anodising, followed by microarc anodising and finally arcing. The film was porous and cracked, with poor bonding to the substrate. It was composed of magnesium and aluminium oxide, and contained all the elements present in the electrolyte. The aluminium concentration in the film was dependent on the concentration of aluminate ions in the electrolyte. The transition from microarc to arcing stage took place when the alloy surface was completely covered by the anodic film.     

Discontinuities in the porous anodic film formed on AA2099-T8 aluminium alloy

The anodizing behaviour of constituent particles (Al–Fe–Mn–Cu) and dispersoids (Al–Cu–Mn–Li and β′(Al₃Zr)) in AA2099-T8 has been investigated. Low-copper-containing Al–Fe–Mn–Cu particles anodized more slowly than the alloy matrix, forming a highly porous anodic oxide film. Medium- and high-copper-containing Al–Fe–Mn–Cu particles were rapidly dissolved, resulting in defects in the anodic film. The anodizing of Al–Cu–Mn–Li dispersoids is slightly slower than the alloy matrix, forming a less regular anodic oxide film. β′(Al₃Zr) dispersoids anodized at a similar rate to the alloy matrix. Further, the potential impact of the discontinuities in the resultant anodic films on the performance of the filmed alloy is discussed.     

Stress generated porosity in anodic alumina formed in sulphuric acid electrolyte

The generation of pores is investigated in anodic films formed at 5 mA cm⁻² on aluminium in 0.4 M sulphuric acid electrolyte at 293 K. The study follows the behaviour of a fine tungsten tracer layer, initially located in the aluminium, during anodizing. Significantly, the tungsten is incorporated into the anodic film with negligible loss of the tracer to the electrolyte. The findings indicate that pores develop primarily due to flow of film material in the barrier layer under the influences of the stresses of film growth. The flow of material from beneath pores toward the cell walls is accommodated by the increased thickness of the anodic film relative to that of the oxidized metal by a factor of about 1.35.     

The behaviour of chromium during anodizing of Al–Cr alloys

The anodic oxidation of dilute Al–Cr alloys, containing 0.8 and 1.7 at% Cr, has been investigated in order to understand the oxidation behaviour of the alloying element and its influences on the film composition and morphology. The alloys reveal two stages of oxidation: an initial stage, in which only aluminium atoms are oxidized to form a chromium-free anodic alumina film, and a subsequent stage, in which both aluminium and chromium are oxidized, in their approximate alloy proportions, with generation of a chromium-contaminated anodic alumina film. In the first stage, chromium is enriched in a thin layer of alloy, immediately beneath the anodic film, to an amount corresponding to a layer of average thickness 1.5 nm and of average composition, Al–20 at% Cr. Following the oxidation of chromium, oxygen is produced electrochemically within the film at or near the alloy/film interface, probably associated with the development of chromium-rich clusters in the enriched alloy layer and, subsequently, formation of semiconducting chromium-rich oxide. Thus, the film material formed at the alloy/film interface by inward migration of O^2- ions contains many oxygen-filled bubbles with associated high pressures. The chromium species present in the film migrate outward more slowly than Al³⁺ ions. Hence, a layer of chromium-free anodic alumina, which thickens as the film grows, is maintained adjacent to the film/electrolyte interface.     

Fe-30Ni-5NiO alloy as inert anode for low-temperature aluminum electrolysis

Fe-30Ni-5NiO alloy anodes were prepared by a spark plasma sintering process for aluminum electrolysis. NiO nano-particles with the size of ∼20 nm were dispersed in the anodes. The oxidation behaviors of the anodes were investigated at 800°C and 850°C, respectively. The electrolysis corrosion behaviors were tested in a cryolite-alumina electrolyte at a low temperature of 800°C with anodic current densities of ∼0.5 A/cm². The results indicated that the oxidation kinetic of the anodes followed a parabolic law. A continuous Fe₂O₃ film selectively formed on the surface of the anode during the electrolysis process. A semi-continuous Al₂O₃ layer was observed at oxide film/alloy interface, probably caused by an in-situ chemical dissolution process.