Influence of pre-treatment on the surface composition of Al-Zn alloys

The influences of pre-treatments on the composition of Al-Zn alloys, containing 0.6-1.9 at.%Zn, are investigated using scanning and transmission electron microscopies, glow discharge optical emission spectroscopy and ion beam analysis. Alkaline etching, anodic alkaline etching and electropolishing result in zinc enrichments, just beneath the oxide/hydroxide films on the alloy surfaces. The enriched alloy, about 4-5 nm thick, contains zinc in solid-solution with aluminium, up to about 18 at.%. The highest and lowest enrichments are produced by alkaline etching and electropolishing respectively. For a given pre-treatment, the enrichment increases with increase in zinc content of the bulk alloy.     

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.     

Large-area self-ordered aluminium sub-micrometre dot arrays prepared by electropolishing on polycrystalline aluminium at constant current

The effects of electropolishing on the sub-micrometre structure pattern on the surface of polycrystalline aluminium were investigated. Under constant current, the electropolishing process results in large-area uniform sub-micrometre dot arrays despite the polycrystalline aluminium surface. Effects of temperature on the pattern of the surface structures were also investigated. Experimental temperature was shown to have a great influence on the structure pattern. When the electropolishing temperature increases, the structure of aluminium surface tends to transform from a dot pattern to stripe one and then to a corrosion structure.     

The electrochemical polishing behaviour of P/M high-speed steel (ASP 23) in perchloric-acetic mixed acids

The electropolishing behaviour of P/M high-speed tool steel (ASP 23) has been investigated in HClO₄-CH₃COOH mixed acids at a temperature range from −10 to 30 °C. The surface morphologies and roughness of polished samples were examined with field-emission scanning electron microscope (FESEM) and atomic force microscope (AFM). The results show that the steel can be electrochemically polished in the mixed acids with concentrations of HClO₄ from 20 to 80 vol%. Temper-induced carbides in both MC and M₆C forms can be leveled together with the steel matrix when the sample was polished at lower temperatures (<0 °C) in the mixed acids with higher HClO₄ contents (60-80 vol%), and the roughness (Ra) of the polished surface can be achieved to 30-50 nm. The electropolishing behaviour of the steel is under mass-transfer control and the anion ClO₄⁻, and/or its complexes, suggested to the governing species for the acceptor-limited mechanism during anodic dissolution in limiting-current plateau.     

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.     

Behaviour of copper during alkaline corrosion of Al–Cu alloys

Enrichment of copper beneath amorphous anodic films on relatively dilute, solid-solution Al–Cu alloys is necessary before copper can be oxidized and incorporated into the oxide layer. A similar enrichment arises during electropolishing, which also develops an amorphous oxide. In these cases, external polarization is applied, usually generating a relatively high oxidation rate. In contrast, enrichment behaviour at the corrosion potential has received less attention. The present study examines the corrosion of Al–Cu alloys, containing up to 6.7 at.% Cu, in 0.1 M sodium hydroxide solution at 293 K. Copper is again found to enrich in the alloy, similarly to behaviour with anodic polarization. However, following enrichment, discrete copper-rich particles appear to be generated in the corrosion product. These are suggested to be nanoparticles of copper, since the corrosion potentials of the alloys are low relative to that required for oxidation of copper. The corrosion rate increases with increase of both time and copper content of the alloy, probably associated with a greater cathodic activity due to an increasing number of nanoparticles. The corrosion proceeds with loss of aluminium species to the sodium hydroxide solution, but with retention of copper in the layer of hydrated alumina corrosion product.     

Effect of heat treatment on anodic activation of aluminium by trace element indium

The presence of trace elements in Group IIIA-VA is known to activate aluminium anodically in chloride environment. The purpose of this paper is to investigate the surface segregation of trace element In by heat treatment and resulting surface activation. Model binary AlIn alloys, containing 20 and 1000 ppm by weight of In, were characterized after heat treatment at various temperatures by use of glow discharge optical emission spectroscopy, electron microscopy and electrochemical polarization. Heat treatment for 1 h at 300 °C gave significant segregation of discrete In particles (thermal segregation), which activated the surface. Indium in solid solution with aluminium, obtained by 1 h heat treatment at 600 °C, also activated by surface segregation of In on alloy containing 1000 ppm In, resulting from the selective dissolution of the aluminium component during anodic oxidation (anodic segregation). The effect of anodic segregation was reduced by decreasing indium concentration in solid solution; it had negligible effect at the 20 ppm level. The segregated particles were thought to form a liquid phase alloy with aluminium during anodic polarization, which in turn, together with the chloride in the solution destabilized the oxide.     

Anodic oxides on InAlP formed in sodium tungstate electrolyte

Amorphous anodic oxide films on InAlP have been grown at high efficiency in sodium tungstate electrolyte. The films are shown to comprise an outer layer containing indium species, an intermediate layer containing indium and aluminium species and an inner layer containing indium, aluminium and phosphorus species_. The layering correlates with the influence on cation migration rates of the energies of In³⁺-O, Al³⁺-O and P⁵⁺-O bonds, which increase in this order. The film surface becomes increasingly rough with increase of the anodizing voltage as pores develop in the film, which appear to be associated with generation of oxygen gas.     

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.     

Pit growth behaviour of aluminium under galvanostatic control

The pit growth process on (1 0 0) aluminium under anodic pulse current in a mixed solution of 1 M HCl and 0.1 M H₂SO₄ at 30 °C has been evaluated using potential transient measurements and pit size distributions obtained by scanning electron microscopy. Sustained pit growth is observed for all pits during the initial anodic potential rise before reaching a steady-state etch potential, whereas a substantial fraction of the pits passivate at the steady-state etch potential. The pit growth rate during the initial potential rise is 3.4 μm s⁻¹, which is similar to that at the steady-state etch potential. The growth rates of active pits are potential-independent.     

Growth of anodic oxide films on AC2A alloy in sulphuric acid solution

Growth behaviour of anodic oxide films on AC2A Al cast alloy was investigated in sulphuric acid solution using SEM, optical microscope (OM) and confocal scanning laser microscope (CSLM) and energy dispersive spectroscopy (EDS). The AC2A alloy contains three different types of second-phase particles: Al–Cu, Al–Cu–Fe–Si and Al–Si particles. The growth of anodic oxide films was critically retarded by the presence of non-reactive particles of Al–Si, while little effect was observed by the presence of active particles of Al–Cu and Al–Cu–Fe–Si. The most severe retardation effect on the growth of anodic films on AC2A alloy resulted from agglomerated Al–Si particles.     

Electrochemical behaviour of microcrystalline aluminium in neutral fluoride containing solutions

The electrochemical behaviour of microcrystalline pure aluminium coating (mc-Al), fabricated by a magnetron sputtering technique, has been investigated in NaF as well as NaF + NaCl aqueous solutions. Results indicate that the anodic polarization characteristic changes with NaF concentration. F⁻ anions promote the formation of a passive film, whose semiconducting properties were investigated. When the F⁻ concentration is high ([F⁻] ? 0.03 mol/L) the passive film is mainly composed of aluminium fluoride, which is an n-type semiconductor on mc-Al and a p-type semiconductor on polycrystalline Al (pc-Al). In NaF + NaCl aqueous solutions, Cl⁻ and F⁻ ions compete in affecting the type of semiconducting passive films formed on mc-Al.     

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.     

Anodic reaction characteristics of tungsten in basic phosphate solutions

DC polarization and electrochemical impedance spectroscopy techniques were used to investigate the pH dependent anodic behaviour of tungsten in basic solutions. Anodic currents of W were pH independent between pH 8 and 11. Above pH 12 as pH increased the anodic currents of W also increased. The diffusion control in pH independent regime was observed to arise from slow diffusion of the solubilized species. At higher pH levels, however, OH⁻ ion diffusion from bulk of solution was observed to control the anodic process. Hydrate layer formation, which was observed with low frequency capacitive loop formation, accelerated with increase in pH.     

电解抛光法制备铝合金EBSD试样方法研究

目的 为在导电单相金属中获得高质量EBSD试样表面,研究电解抛光法制备铝合金试样的方法,并提供理论支持。方法 基于Jacquet黏膜模型和金属阳极原理,提出利用阳极极化曲线、电流-时间曲线和扫描电镜二次电子图像获得电解抛光工艺参数,批量制备铝合金EBSD试样的理论方法。采用恒电位法中的静态法记录稳定的电压-电流走势,以获得电流稳定的实验时间,在90 s内进行各电压下的电解抛光实验,获得电压与稳定电流的对应关系,并绘制阳极极化曲线。电流由持续稳定转至持续上升后的斜率与电压横坐标相交处为理论最低分解电压值。结合扫描电镜二次电子图像在最低分解电压以上观察抛光表面。结果 获得最优抛光电压值为31 V。利用电流随时间的变化曲线,结合黏膜模型分析,并通过扫描电镜二次电子图像验证,最优电压下的最佳抛光时间为12 s,该值是电流-时间曲线中的电流最低点。此工艺使制备的铝合金EBSD样品标定率为97%,是理想的电解抛光工艺。结论 采用阳极极化曲线获得的最优电压和最优电压下的最小电流规律由Jacquet黏膜模型支持,其所获得的电解抛光工艺能够制备出优质的样品表面,也能够为其他金属块体导电材料和其他需要电解抛光的实验类型提供获得最佳电解抛光工艺值的理论方法。     

The electrochemical polishing behavior of the Inconel 718 alloy in perchloric-acetic mixed acids

The electropolishing behavior of the Inconel 718 alloy was studied by using rotating disc electrode (RDE) in the HClO₄-CH₃COOH mixed acids with different HClO₄-concentrations. After electropolishing, surface morphologies of RDE specimens were examined with surface profiler, atomic force microscope and scanning electron microscope. According to the surface morphologies observed, three types of anodic dissolution behavior can be characterized in relation to the HClO₄-content in mixed acids; namely, leveling without brightening of the surface in the mixed acids with 10 and 20 vol% HClO₄, leveling and brightening of the surface in the mixed acids with 30 and 40 vol% HClO₄, and a matt and gray surface in the mixed acids with 50 vol% or more HClO₄. Anodic dissolution in the first and second dissolution types follows a mass-transfer controlled mechanism, in which a linear relationship between the reciprocal of limiting-current density and the reciprocal of square root of rotating speed of RDE specimen can be detected. Owing to precipitation of salt film on the polished surface of the Inconel 718 material, saturated dissolved metallic ions could be the chemical species for the mass-controlled mechanism. The salt film, in addition, could enhance the corrosion resistance of the Inconel 718 alloy.     

Effect of iron-containing intermetallic particles on the corrosion behaviour of aluminium

The effect of heat treatment on the corrosion behaviour of binary Al-Fe alloys containing iron at levels between 0.04 and 0.42 wt.% was investigated by electrochemical measurements in both acidic and alkaline chloride solutions. Comparing solution heat-treated and quenched materials with samples that had been subsequently annealed to promote precipitation of Al₃Fe intermetallic particles, it was found that annealing increases both the cathodic and anodic reactivity. The increased cathodic reactivity is believed to be directly related to the increased available surface area of the iron-containing intermetallic particles acting as preferential sites for oxygen reduction and hydrogen evolution. These particles also act as pit initiation sites. Heat treatment also causes depletion in the solute content of the matrix, increasing its anodic reactivity. When breakdown occurs, crystallographic pits are formed with {1 0 0} facets, and are observed to contain numerous intermetallic particles. Fine facetted filaments also radiate out from the periphery of pits. The results demonstrate that the corrosion of aluminium is thus influenced by the presence of low levels of iron, which is one of the main impurities, and its electrochemical behaviour can be controlled by heat treatment.     

Electrochemical behaviour of aluminium/steel rivet joints

The galvanic corrosion behaviour of a rivet joint of two sheets of the aluminium alloys EN AW-6014-T4 and EN AW-6082-T5 joined by an electrogalvanized steel blind rivet was investigated. The potentiodynamic polarization curves in a 5 wt.% NaCl solution show a potential reversion of the anodic and cathodic regions of the rivet joint. The surface potential was investigated with a capillary electrode before and after corrosion. From the measurements by the capillary electrode and SEM-observations an improved resistance against galvanic corrosion is expected after the dissolution of the zinc layer of the blind rivet.     

Electrochemical etching of AA5083 aluminium alloy in trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)amide ionic liquid

Trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)amide ([P_6,6,6,14][NTf₂]) ionic liquid is shown to react with AA5083 aluminium alloy under a two-step anodic polarisation, leading to partial passivation of the surface. Surface characterisation established that an electrochemical etching process had occurred, comparable to acid etching of aluminium. Energy dispersive X-ray spectroscopy (EDXS) and X-ray photoelectron spectroscopy (XPS) results indicated that magnesium de-alloyed from Mg₂Si intermetallic particles and metal fluorides were deposited onto the remaining Mg₂Si sites, leading to a decrease in the anodic corrosion kinetics (to one third of that of the control) as well as a 100 mV vs. SCE increase in the corrosion and pitting potentials.     

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.