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.     

Anodic films formed on magnesium in organic, silicate-containing electrolytes

Anodic films were prepared on magnesium in electrolyte solutions consisting of 3.0 M KOH and various concentrations of sodium silicate and aqueous ethylene-glycol solutions. The anti-corrosion properties of the films formed in the ethylene-glycol electrolyte solutions ranging from 10 to 40 wt% far exceeded that of the anodic film produced using the HAE method. The film formed under optimal conditions had excellent anti-corrosion properties, which were more than 10-fold greater than those of the HAE anodic films. The anodic film consisted of two layers—a heterogeneous porous layer and a barrier layer; moreover, both carbon and silicate were detected in the anodic film.     

铝阳极氧化膜的显微组织与性能研究

研究了ＬＹ１１硬铝合金硫酸法阳极氧化膜的组织结构及其性能,讨论了电解液组成和工艺条件对它们的影响。结果表明,电解液中Ｈ２ＳＯ４浓度增大,易得到较厚的多孔型氧化膜;而稀Ｈ２ＳＯ４（１０％体积比）电解液,可获得致密、无孔洞的相对较薄的氧化膜,其耐蚀性、电绝缘性和表面硬度均明显改善。工艺操作参量中,保持较低的电解液温度、合适的阳极电流密度及氧化时间,有利于膜层综合性能的提高。     

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.     

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.     

Porous anodic oxides on titanium and on a Ti-W alloy

The growth of a nanoporous anodic oxide on titanium and a Ti-20 at.% W alloy, both deposited by magnetron sputtering, in a glycerol/phosphate electrolyte at 453 K is reported. The oxide formed on titanium is a mixture of amorphous titania and anatase. However, that on the alloy is amorphous only and forms at increased efficiency, about 27%. The amorphous structure is considered to be stabilized by incorporated units of WO₃, which are distributed uniformly throughout the anodic film. The growth of the porous oxides is suggested to be associated with loss of film species at the film/electrolyte interface at the base of pores, with new oxide forming exclusively at the metal/film interface by inward migration of O²⁻ ions.     

Plasma electrolytic oxidation of pre-anodized aluminium

The influence of an anodizing pre-treatment in sulphuric acid is investigated on plasma electrolytic oxidation (PEO) of aluminium in silicate electrolyte under constant rms current. The presence of the anodic film is shown to promote the establishment of a micro-arc regime that is favourable for growth of the PEO coating. The incorporation of the pre-formed film into the coating appears to proceed by thermal transformation of the anodic alumina, accompanied by formation of oxide beneath the pre-formed layer. The final coatings contain α- and γ-Al₂O₃, with increased concentrations of silicon, sodium and potassium in an outer region of the coating.     

Influence of pH on the passivation behavior of 254SMO stainless steel in 3.5% NaCl solution

The potentiodynamic polarization measurement of 254SMO stainless steel (UNS 31254) was conducted in 3.5% NaCl solutions with pH ranging from 0.1 to 5. The results indicated that this stainless steel offered excellent pitting corrosion resistance in corrosive environments. Further, it also exhibited various features on the polarization curves in different pH solutions. The electrochemical constant-potential passivation treatment performed at different pH followed by XPS analysis revealed that the primary constituents of the outermost layer of the passive films formed in the weak (pH 5) and strong (pH 0.8) acid solutions are iron oxides and Cr₂O₃ and Cr(OH)₃, respectively. Molybdenum oxides, primarily in the six-valence state, existed in the outermost layer of the passive film. Only very weak signals corresponding to that of nickel oxides were detected in the film formed in the weak acid (pH 5) solution. The ICP-MS analyses indicated selective dissolution of a significant amount of Fe and a few Mo and Ni ions during the passivation treatment in the strong acid (pH 0.8) solution. No Cr dissolution was observed; this indicated that the Cr in the film is relatively stable. XPS depth profiling results showed that a similar bilayer-structured film was formed in both the solutions (pH 0.8 and 5); the outer layer of this film is primarily composed of Cr(OH)₃ and Mo(VI), and the inner layer, Cr₂O₃ and Mo(IV). The results of the examinations of passive film formations and dissolution by XPS and ICP-MS were consistent with the polarization curves.     

Anodizing of magnesium alloy AZ31 in alkaline solutions with silicate under continuous sparking

Anodization is a useful technique for forming protective films on magnesium alloys and improves its corrosion resistance. Based on the alkaline electrolyte solution with primary oxysalt developed previously, the optimum secondary oxysalt was selected by comparing the anti-corrosion property of anodic film. The structure, component and surface morphology of anodic film and cross-section were analyzed using energy dispersion spectrometer (EDS), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The corrosion process was detected by electrochemical impedance spectroscopy (EIS). The results showed that secondary oxysalt addition resulted in different anodizing processes, sparking or non-sparking. Sodium silicate was the most favorable additive of electrolyte, in which anodic film with the strongest corrosion resistance was obtained. The effects of process parameters, such as silicate concentration, applied current density and temperature, were also investigated. High temperature did not improve anti-property of anodic film, while applying high current density resulted in more porous surface of film.     

Breakdown of passive film on nickel in borate solutions containing halide anions

The effect of Cl⁻, Br⁻ and I⁻ anions as aggressive agents on the anodic behaviour of nickel electrode in deaerated Na₂B₄O₇ solutions have been investigated by galvanostatic polarization technique. Lower concentrations of the halide anions have no effect on the mechanism of nickel passivation. An increase in the halide anions concentration causes oscillation of the potential in the oxygen evolution region. This could be attributed to the destruction of the passivity by halide anions and repassivation of the film by anodic current and/or OH⁻ anions. Higher aggressive anion concentrations cause breakdown of the passive film and initiated pitting corrosion. As the temperature increases, the breakdown potential is shifted towards the more negative direction. On the other hand, as the pH of the solution increases, the breakdown potential is shifted toward more positive direction, indicating increased protection of the passive film. The activation energy, , of the oxide film formation in the presence of Cl⁻ anions was calculated and was found to be 21 kJ/mol.     

Generation mechanism of microdischarges during plasma electrolytic oxidation of Al in aqueous solutions

The generation mechanism of microdischarges during plasma electrolytic oxidation (PEO) of aluminium was investigated at constant current densities in aqueous alkaline solutions. Microdischarges were generated only in weakly alkaline solutions under high applied voltages. The breakdown voltage of the anodic oxide film was not dependent on the applied anodic current density while it was strongly dependent on the OH⁻ concentration in the solution. The size and density of microdischarges became larger and lower, respectively, with increasing PEO treatment time. The mechanism of microdischarge generation could be explained based on the movement of ions in the oxide film and resistive property of the oxide film.     

Behaviour of a fast migrating cation species in porous anodic alumina

The behaviour of Nd³⁺ ions is examined in porous anodic alumina films formed at 5 mA cm⁻² in 0.4 M phosphoric acid at 293 K on aluminium substrates that contain a buried 5 nm-thick tracer band of Al-Nd alloy. The Nd³⁺ ions migrate outward in the barrier region about twice as fast as Al³⁺ ions. The neodymium was located in the anodic film by transmission electron microscopy and scanning electron microscopy, and quantified by Rutherford backscattering spectroscopy. The Nd³⁺ ions migrated to the cell walls and to pore base, depending upon their location in the substrate relative to the alumina cells and pores. Nd³⁺ ions that reached the pore base were lost to the electrolyte. The outward transport of the Nd³⁺ ions was greatest beneath the pores and least at the cell boundaries, resulting in transformation of the planar tracer layer of the substrate to a roughly hemispherical shape in the film. The behaviour contrasts with that of a tracer band of slowly migrating W⁶⁺ ions, which reveals an approximately inverse distribution, while W⁶⁺ ions are retained within the film.     

Long-term anticorrosion behaviour of polyaniline on mild Steel

Anticorrosion performances of polyaniline emeraldine base/epoxy resin (EB/ER) coating on mild steel in 3.5% NaCl solutions of various pH values were investigated by electrochemical impedance spectroscopy (EIS) for 150 days. In neutral solution (pH 6.1), EB/ER coating offered very efficient corrosion protection with respect to pure ER coating, especially when EB content was 5-10%. The impedance at 0.1 Hz of the coating increased in the first 1-40 immersion days and then remained constant above 10⁹ Ω·cm² until 150 days, which in combination with the observation of a Fe₂O₃/Fe₃O₄ passive film formed on steel confirmed that the protection of EB was mainly anodic. In acidic or basic solution (pH 1 or 13), EB/ER coating also performed much better than pure ER coating. However, these media weakened the corrosion resistance due to breakdown of the passive film or deterioration of the ER binder.     

Passivity of polycrystalline NiMnGa alloys for magnetic shape memory applications

The corrosion and passivation behaviour of bulk polycrystalline martensite Ni₅₀Mn₃₀Ga₂₀ and austenite Ni₄₈Mn₃₀Ga₂₂ alloys was compared in electrolytes with different pH values. Linear anodic and cyclic potentiodynamic polarisation methods and anodic current transient measurements have been conducted for the alloys and their constituents to analyze free corrosion, anodic dissolution and passive layer formation processes. Electrochemically treated alloy surfaces were characterized with scanning electron microscopy (SEM) and angle-resolved x-ray photoelectron spectroscopy (XPS). The electrochemical response of both alloys is in principal similar and is dominated by the Ni oxidation. In acidic solutions (pH 0.5 and 5) a slightly higher reactivity is detectable for the martensitic alloy which is mainly attributed to enhanced dissolution processes at the multiple twin boundaries. In weakly acidic to strongly alkaline solutions (pH 5-11) both alloys exhibit a low corrosion rate and a stable anodic passivity. While air-formed films comprise NiOOH, Ga₂O₃ and MnO₂, passive films formed in near neutral media (pH 5-8.4) are composed of Ni(OH)₂, NiOOH and Ga₂O₃ in the outer region and of NiO, MnO₂ and MnO in the metal-near region.     

5-甲基苯并三氮唑作为电腐蚀抑制剂在铜电化学机械平坦化中的应用

根据电化学分析,5-甲基苯并三氮唑(m-BTA)的腐蚀抑制能力要高于苯并三唑(BTA)的。当羟基乙叉二膦酸(HEDP)电解液中同时含有m-BTA及氯离子时,其抑制解离能力比只含有m-BTA的更好,即使施加更高的阳极氧化电位依然能保持良好的抑制能力。由电化学阻抗谱法、纳米划痕实验以及能谱分析结果得知,m-BTA抑制能力的提升是因为整体钝化膜厚度的增加而引起的。由X射线光电子能谱分析得知,氯离子与m-BTA钝化层形成[Cu(I)Cl(m-BTA)]n高分子化合物,使得整体钝化层厚度增加。因此,在含有m-BTA的HEDP电解液中添加氯离子有助于m-BTA钝化层抑制能力的增强,进而更有效的电位操作区间得到扩展。     

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.     

Effects of hydrogen on the anodic behavior of Alloy 690 at 60 °C

Polarization and electrochemical impedance spectroscopy (EIS) measurements, Mott-Schottky (M-S) analysis and X-ray photoelectron spectroscopy (XPS) were used to investigate the effects of hydrogen on the anodic behavior of a one-dimensionally (1D) 25% cold worked (CW) Alloy 690 thermal treated (TT) in a boric acid and sodium sulphate solution at 60 °C. The pre-hydrogen-charged specimen exhibited a higher anodic current than that of the uncharged specimen below the transpassive potential. The charged hydrogen can be trapped in the metal. Electrochemical impedance spectroscopy (EIS) showed that the resistance capacitance loop of the hydrogen-charged specimen was significantly smaller than that of the uncharged specimen. Mott-Schottky analyses indicated that the passive film formed on Alloy 690 at −0.2 V_SCE was an n-type semiconductor, with a p-n hetero-junction at 0.2 V_SCE. Charged hydrogen increased the carrier density and the thickness of the passive film both at −0.2 V_SCE and 0.2 V_SCE. The Ni/Cr ratio in the surface film decreased after hydrogen charging, indicating that charged hydrogen could enhance the oxide film growth by increasing the OH⁻ (O²⁻) concentrations through its reaction with vacancies.     

Pore development in anodic alumina in sulphuric acid and borax electrolytes

The formation of porous anodic films on an Al-3.5 at.%W alloy is compared in sulphuric acid and borax electrolytes in order to investigate pore development processes. The findings disclose that for anodizing in sulphuric acid, the pores develop mainly due to the influences of field-induced plasticity of the film and growth stresses; in borax, field-assisted dissolution dominates. The films formed in sulphuric acid are consequently much thicker than the layer of oxidized alloy and tungsten species are retained in the film. In contrast, with borax, the films and oxidized alloy layers are of similar thickness and tungsten species are lost to the electrolyte. Efficiencies of film growth are also significantly different, about 65% in sulphuric acid and about 52% in borax. The retention of tungsten species during anodizing in sulphuric acid is due to the localization of tungsten in the inner regions of the barrier layer and cell walls, with a layer of anodic alumina separating the tungsten-containing regions from the electrolyte. For borax, the tungsten is distributed more uniformly through the film material, enabling loss of tungsten species to the electrolyte from the pore base.     

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.     

Enrichment behaviour of gallium in heat and surface treatments of Al–Ga foils

The behaviour of gallium is examined after heat and surface treatments of aluminium foils containing either 120 or 1300 ppm gallium, with low enrichments of gallium at the surfaces arising from foil manufacture. Vacuum heat treatment at 823 K for 20 ks caused negligible additional enrichment, probably associated with the high solubility of gallium in aluminium. Subsequent alkaline etching in 0.25 M sodium hydroxide for 60 s at 348 K increased significantly enrichments, to about 3.2×10¹⁴ and 3.6×10¹⁵ Ga atoms cm⁻² for the foils containing 120 and 1300 ppm gallium respectively. The enrichments were probably located in both the metal and the overlying etching products. In contrast, electropolishing in perchloric acid/ethanol eliminated pre-existing enrichments, probably due to activation of the foil, with no enrichment developing during the electropolishing procedure. Barrier-type anodic films formed on the alkaline-etched foils contained gallium species, with increased concentrations at the film surface. Gallium enriched the metal region immediately beneath the anodic film during anodizing and led eventually to detachment of the growing film from the etched foil, followed by growth of a second film beneath the detached film. The combined amounts of gallium in the anodic film and enriched metal layer were similar to the levels at the surface regions of the etched foils. The anodic films formed on the electropolished foils, with no pre-existing enrichment, were free of gallium species and remained attached to the foils, since insufficient gallium could enrich in the metal beneath the anodic film during the relatively short period of anodizing.