A wall-jet electrode reactor and its application to the study of electrode reaction mechanisms Part III: Study of the mechanism of the a.c. electrolytic graining of aluminium in hydrochloric acid

The mechanism of the a.c. electrolytic graining of aluminium in hydrochloric acid is determined from the analysis of the potentiostatic transient behaviour of the system aluminium–electrolyte under anodic and cathodic polarization and comparison of experimentally determined transients with calculated values derived from a candidate mechanistic scheme. It has been established, that the oxidation of aluminium in the development of a distinct surface morphology occurs according to the Al3+ ions being dissolved from the surface and removed to the bulk of the solution, hence forming pits. Al(Cl)3 is a solid intermediate. The morphology developed, is determined by the excess of Cl– ions created at the electrode surface, with respect to the bulk concentration. The accumulation of Cl– ions is governed by the ratio between the rate constant for the formation of Al(Cl)3, set by the flux of charges forced across the electrode–solution interface per unit surface area taking part in the active dissolution of aluminium and the mass transport rate of the Cl– ions. The reduction of H+ ions in the cathodic half period of the applied alternating current is mass transport controlled. The concomitant rise in interfacial pH causes Al3+ ions formed in the preceding anodic half period, which are not yet removed from the electrode–solution interface, to precipitate as aluminium.     

Influence of magnesium and aluminium ions on the copper a.c. deposition into aluminium anodic oxide film nanotubes

The influence of Mg²⁺ and Al³⁺ ions on a.c. deposition of copper nanowires into aluminium anodic oxide film (AOF) nanotubes has been studied using cyclic voltammetry and d.c. plasma emission spectrometry. From the analysis of copper quantities deposited into the Al AOF nanotubes (m _Cu), 0.02 M MgSO₄ concentration was found to be optimal for Cu(II) solutions. Moreover, it was shown that Mg²⁺ and Al³⁺ ions not only prevent the breakdown of the barrier layer of AOF, but change the rate of copper deposition and modify the shape of the m _Cu against pH plots depending on the a.c. voltage applied. From the analysis of the quantities of magnesium (m _Mg) incorporated into the Al AOF nanotubes, presumably in the form of Mg(OH)₂, the m _Mg against pH dependences were determined in MgSO₄ and MgSO₄ + CuSO₄ solutions. An increase in m _Mg from 30 g dm^–2 to 1 mg dm^–2 at pH 1.5 and from 6 g dm^–2 to 16 g dm^–2 at pH 7.0 was found under the same a.c. treatment conditions from MgSO₄ solutions without and with Cu²⁺ ions, respectively, indicating the incorporation of Mg(OH)₂ into the Al AOF nanotubes to be lower up to about one hundred times in the case of Cu deposition. Based on the experimental results, it was suggested that incorporation of the Mg(OH)₂ particles into the Al AOF nanotubes occurred simultaneously with growing copper nanowires under a.c. bias is insignificant, if the pH of the CuSO₄ + MgSO₄ solution is 2.5.     

铝合金阳极氧化膜着金色的研究

研究了以银盐为主盐的电解着金色工艺,研究了着色液中各成分及操作条件的影响,分析了影响着色效果的各因素,该着色工艺稳定性好,使用寿命长,着色膜耐蚀,耐磨,耐晒性优良。     

Incorporation of zirconia nanoparticles into coatings formed on aluminium by AC plasma electrolytic oxidation

Composite ceramic coatings were formed on aluminium by AC plasma electrolytic oxidation (PEO) using Na₆P₆O₁₈ or Na₂SiO₃ · 5H₂O/KOH electrolytes with monoclinic zirconia nanoparticles in suspension. The coatings grown in Na₂SiO₃ · 5H₂O/KOH electrolyte revealed γ-Al₂O₃ and amorphous phase; α-Al₂O₃ and AlPO₄ were additionally produced with the Na₆P₆O₁₈ electrolyte. Higher temperature zirconia phases, possibly tetragonal and orthorhombic, in addition to the monoclinic phase, were indicative of elevated temperatures at sites of microdischarges. Further, local melting resulted in zirconium-rich dendrites in the coating formed in silicate electrolyte. Zirconium was mainly located in the relatively compact, outer layer of the coating, constituting ∼70–90% of the coating thickness. Nanoparticles appeared to be incorporated at the coating surface and following transport to the interface regions between the inner and outer layers along short-circuit paths through the outer coating.     

Effects of electrolytic composition on the electric double-layer capacitance at smooth-surface carbon electrodes in organic media

As a fundamental research on the optimization of electrolyte composition in practical electrochemical capacitor device, double-layer capacitance at Glassy Carbon (GC) and Boron-doped Diamond (BDD), as typical smooth-surface carbon electrodes, has been studied as a function of the electrolyte composition in organic media. Specific capacitance (differential capacitance: F cm⁻²) determined by an AC impedance method, in which no contribution of mass-transport effects is included, corresponded well to integrated capacitance evaluated by conventional cyclic voltammetry. The specific capacitance at the GC electrode varied with polarized potential and showed clear PZC (potential of zero charge), while the potential dependence of the capacitance at BDD was very small. The effects of the solvent and the electrolytic salt on the capacitance behavior were common for both electrodes. That is, the sizes of the solvent molecule and the electrolytic ion (cation) strongly affected the capacitance at these smooth-surface carbon electrodes.     

(Ba0.5Sr0.5)TiO3 modification on etched aluminum foil for electrolytic capacitor

A new method was proposed to form (Ba_0.5Sr_0.5)TiO₃–Al₂O₃ composite oxide film on etched aluminum foils. The specimens were covered with (Ba_0.5Sr_0.5)TiO₃ (BST) layer by dip-coating in citrate solution and subsequent heat-treatment under 400–650 °C, finally by anodizing in a hot boracic acid and borate solution. The BST powders heated under different temperatures were characterized by X-ray diffraction (XRD) and the specific capacitance of the coated specimens heat-treated under different temperatures and times was measured. It is found that the specific capacitance increases initially with enhancing the temperature and reaches to maximum at 550 °C, but slightly decreases with the heat-treatment time. The capacitance was increased by about 35% after BST coating.     

Investigation of the growth processes of coatings formed by AC plasma electrolytic oxidation of aluminium

The formation of alumina-based coatings on aluminium by AC plasma electrolytic oxidation (PEO) has been investigated using a silicate electrolyte with selective additions of fine zirconia particles. The coatings comprised an amorphous barrier layer, a relatively dense intermediate layer and a more porous outer layer that contained silicon species. Zirconia was incorporated non-uniformly into the outer layer and, to a limited extent, the intermediate layer, as both particles and a component of cellular microstructures. Following treatments firstly in zirconia-containing electrolyte and secondly in zirconia-free electrolyte, the zirconia did not extend beyond about the middle of the intermediate layer, indicating its limited inward mass transfer during microdischarges. The coating efficiency decreased at oxidation times in excess of 40 min due to dissolution of either the substrate or the coating, or physical loss of coating material. The oxidation of aluminium consumed on average ∼29% of the anodic charge; the remainder was used mainly in generation of oxygen gas.     

Anodic processes in plasma electrolytic oxidation of aluminium in alkaline solutions

In this paper, basic electrochemical processes (such as oxide film growth, anodic dissolution and oxygen liberation) on an aluminium anode in a model alkaline solution are considered under conditions of galvanostatic DC plasma electrolytic oxidation (PEO). The experiments performed include: (i) recording and analysis of the main electrical characteristics of the process; (ii) determination of the oxide layer thickness; (iii) anodic gas collection and composition analysis and (iv) electrolyte analysis to determine dissolved aluminium. Four different stages of the PEO process have been identified, characterised by various rate proportions of the partial anodic processes. Overall current efficiency of the oxide film formation has been estimated to be in the 10-30% range. The film growth rate decreases significantly with increasing electrolyte concentration from 0.5 to 2 g l⁻¹ KOH, since the rate of anodic dissolution increases. Oxygen evolution is shown to be the main electrochemical process at the potentials corresponding to the plasma stages of the electrolysis. The overall rate of oxygen liberation at the anode exceeds the Faraday yield, which is probably due to the radiolytic effect of the plasma discharge on the adjacent electrolyte volume.     

Zinc impregnation of the anodic oxidation layer of 1050 and 2024 aluminium alloys

The porous oxide layer obtained by phosphoric anodic oxidation (PAO) of 1050 and 2024T3 aluminium alloys is modified by impregnation with zinc under alternating voltage. The resulting current against applied voltage relationship shows that a threshold voltage is required to deposit the zinc. Beyond a low critical voltage, V _{c L}, zinc electrocrystallization starts near the barrier layer and grows with time through the porous oxide layer whatever the alloy used as substrate. For the 2024T3 alloy, beyond a high critical voltage V _{c H}, S.I.M.S. analysis shows that zinc is also present on top of the oxide layer. The distribution of zinc particles depends on the porous layer morphology: formation of zinc needles in a columnar form (1050 alloy) and dispersion of zinc particles in a disorganized structure (2024T3 alloy). The polarization curves obtained in a 3 wt % NaCl solution show a decrease in anodic and cathodic currents indicating a protective effect of zinc impregnation, confirmed by electrochemical impedance analysis.     

电化学溶液pH值对聚吡咯铝电容器性能的影响*

采用化学聚合和电化学聚合两步法制备聚吡咯铝电解电容器,研究了电化学溶液pH值对聚吡咯的微观形貌及聚吡咯铝电解电容器的电容量和等效串联电阻的影响。结果表明,随着电化学溶液pH值增加,聚吡咯的颗粒增大,分布越来越松散,电容器的电容量减小、等效串联电阻增大。在电化学聚合溶液pH=2条件下,制备的聚吡咯致密性高,颗粒小且均匀,且相应的电容器具有最高的电容量和最低的等效串联电阻,其值分别为18.3μF和20.4 mΩ。     

废旧锂离子电池正极材料与铝箔电解剥离浸出研究

提出了-废旧锂电池正极材料的电解浸出新工艺.以铅板为阳极,锂电池正极材料为阴极,利用外加电流的阴极保护铝箔,实现在浸出钴的同时剥离铝箔,使铝箔得以完整回收.研究了浸出过程各因素对钴铝浸出影响,最佳电解浸出条件为:电流密度为15.6 mA/cm2,硫酸浓度为0.4 mol/L,柠檬酸质量浓度为36 g/L,温度为25℃,电解时间为120 min,钴浸出率为90.85％,铝溶解率为5.8％.对后续工艺进行了讨论,通过对剥离的正极粉料综合回收处理,萃取除杂,草酸铵沉钴,得到钴质量分数为31.3％合格的草酸钴,钴的综合回收率大于99.8％.     

Characterization of porous aluminium oxide films from a.c. impedance measurements

An equivalent circuit (EC) that reproduces the a.c. impedance of porous aluminium oxide films in a highly approximate manner is proposed. The results reveal that electrochemical impedance spectroscopy (EIS) is a powerful tool for obtaining detailed information on the electrochemical properties of both the porous and barrier layer on which the corrosion resistance of aluminium depends. The impedance at a given frequency can be used for accurate calculation of the electrochemical parameter for the oxide film represented by each element of the EC. In this way, the effects of any factor on sealing and ageing of anodized aluminium oxide films can be precisely analysed. The EIS technique provides an effective, advantageous alternative to existing seal quality control tests.     

On the enhanced antibacterial activity of plasma electrolytic oxidation (PEO) coatings that incorporate particles: A review

Biomaterials are substances of artificial or natural origin that are used to improve, treat, heal or replace the tissue or bone of a human or animal body. Due to the ever-increasing progress and the widespread use of biomaterials for various biomedical purposes, different methods are used to modify their surface characteristics. One of the problems facing biomaterials such as implants, prostheses, and stents is the presence of various bacteria that can cause adverse side effects such as infection, swelling, and tenderness. This raises the issue of their resistance to bacterial infection, a subject that needs to be thoroughly investigated. So far, a variety of methods have been developed to treat or coat biomaterials and make them resistant to bacterial infections. One of the most promising approaches is the plasma electrolytic oxidation (PEO) process. This process is not only successful in the formation of porous, hard, corrosion-resistant, wear-resistant, and biocompatible coatings but also can be easily manipulated to introduce antibacterial agents to the coatings structure. The addition of nano- or micro-sized particles in the electrolytes has been proven to not only modify the composition and structure of the PEO coatings, but also bring about a strong antibacterial activity. In light of recent advances in this field, the following review aims at discussing different aspects of particles addition in PEO electrolytes when the antibacterial activity is the main concern.     

The effect of plasma electrolytic oxidation process parameters on the tribocorrosion properties of TiO2 coatings

Despite the fact that Titanium and its alloys are materials which have excellent corrosion-resistant properties, they have poor wear and friction performance under tribological conditions. The aim of this study is to find suitable parameters for the surface treatment of Cp-Ti substrates which are used under saline environment. In this study, TiO₂ coatings were grown on Cp-Ti substrates at different frequencies which are parameters of the coating process. Due to its low cost and ability to achieve high thicknesses, The Plasma Electrolytic Oxidation (PEO) method was applied to grow TiO₂ coatings. The microstructures, morphology, and crystallographic structure were analyzed using SEM and XRD. Tribocorrosion properties of the coatings were investigated using a combination of the pin-on-disk wear test and potentiodynamic polarization test units. The frequency is known to have a strong impact on the PEO process. The impacts of frequency changes on the PEO coating performance were examined under a constant voltage. As result, the increase of the frequency caused smaller pores and cracks in the surface morphology of the coating and at the same time this yielded an increment on the tribocorrosion behavior of the coating.     

Examination of the double-layer capacitance of an high specific-area C-cloth electrode as titrated from acidic to alkaline pHs

The titration of the interfacial capacitance of a C-cloth electrochemical capacitor electrode was undertaken for the purpose of evaluating the cyclic voltammetry (CV) responses as a function of the pH of an aqueous electrolyte over the range of 0–14. As the pH was increased from 0 (aqueous H₂SO₄) to a progressively more alkaline (NaOH) solution of pH 11, a 30% loss in capacitance is seen, partly attributable to the disappearance of the pseudocapacitive peaks based on the oxidation/reduction of the surface quinone groups. As the pH is increased above 11, the capacitance increases ca. 20%, as other surface functionalities (a pyrone derivative and another unknown species) become activated in alkaline solution. When the C-cloth is titrated from pHs of 14 to 2, the CV current profile shrinks (with a concomitant decrease in charge-storage capacity). However, at electrolyte pHs below ca. 2, the quinone functionalities are activated and the charge-storage capacity increases ca. 30% above the initial value. It was found that the CV current responses in pH 14 electrolytes are smaller when the titration is base-initiated, rather than acid-initiated, due to an activation step required to make the pyrone functionalities active in alkaline solution.     

Liquid phase oxidation of 3,4-dihydroxybenzoic acid and the effect of the products on trihydroxy aluminium precipitation yields

Organic compounds are known to interfere with the precipitation of trihydroxy aluminium (gibbsite) in the Bayer process. Studies have been carried out of the effect of 3,4-dihydroxybenzoic acid on precipitation and of the beneficial effect that can be achieved by oxidising the acid. Measurements of precipitate yields both in the presence of the acid and of products obtained by oxidising the acid were made. Oxidation in the presence of various metal ions was also examined. The yield of precipitate was found to decrease in the order Cu²⁺ (1·9 mM) > Fe³⁺ (2·2 mM) > blank (no added organics) > Mn⁴⁺ (44·3 and 2·3 mM) > Mn²⁺ (2·3 mM) > Mn²⁺ (44·3 mM) ≈ uncatalyset oxidation > Fe³⁺ (43·5 mM) > Cu²⁺ (38·2 mM) > untreated sample. This relative ordering was generally unaffected by reaction temperature or oxygen partial pressure. Soluble manganese salts were formed by partial dissolution of MnO₂ in the alkaline solutions. These salts were required for oxidation of the 3,4-dihydroxybenzoic acid to products which, collectively, did not poison the trihydroxy aluminium precipitation process.     

The synthesis of micro and nano WO3 powders under the sparks of plasma electrolytic oxidation of Al in a tungstate electrolyte

Plasma electrolytic oxidation (PEO) is commonly known as a coating technique. However, the present study shows that PEO of pure Al in 10?g?l^?1 Na₂WO₄·2H₂O leads to the synthesis of micro and nano sized powders of WO₃. The as-synthesized WO₃ powders have been characterized by SEM, TEM and photocatalytic tests. The plasma discharges during the PEO process have been investigated by real-time imaging and spectrographic method. The energetic features of the single discharge at different stages of PEO have been evaluated. It was suggested that the electrolyte species were directly decomposed into tungsten oxides within the discharge channels and then the oxides re-entered the electrolyte due to the lower melting and boiling points of WO₃, and more importantly, its tendency of sublimation.     

Influence of chloride ion concentration on the electrochemical corrosion behaviour of plasma electrolytic oxidation coated AM50 magnesium alloy

The electrochemical degradation of a silicate- and a phosphate-based plasma electrolytic oxidation (PEO) coated AM50 magnesium alloy obtained using a pulsed DC power supply was investigated using potentiodynamic polarisation and electrochemical impedance spectroscopy (EIS) in NaCl solutions of different chloride ion concentrations viz., 0.01 M, 0.1 M, 0.5 M and 1 M. The surface of the PEO coated specimens after 50 h of immersion/EIS testing was examined by optical microscopy and scanning electron microscopy. The results showed that the corrosion deterioration of PEO coated magnesium alloy in NaCl solutions was significantly influenced by chloride ion concentration. The silicate-based coating was found to offer a superior corrosion resistance to the magnesium substrate than the phosphate-based coatings in lower chloride ion concentration NaCl solutions (0.01 M and 0.1 M NaCl). On the other hand both these PEO coatings were found to be highly susceptible to localized damage, and could not provide an effective corrosion protection to Mg alloy substrate in solutions containing higher chloride concentrations (0.5 M and 1 M). The extent of localized damage was observed to be more with increase in chloride concentration in both the cases.     

Electrochemical behaviour of the antimony electrode in sulphuric acid solutions—III. identification of corrosion products after long-term polarization

During the oxidation of pure antimony (Sb) in sulphuric acid (H₂SO₄) solutions a passivating layer is formed on the electrode surface. In the early stage of oxidation the layer consists of amorphous Sb oxide and/or hydroxide, but with longer oxidation times aggregates of whisker-like crystals of a trivalent oxidation product, Sb₄O₄(OH)₂SO₄, are formed. The identification of this compound was done by ex situ X-ray powder diffraction, and the result was confirmed by electron microprobe and X-ray photoelectron spectroscopic analysis. By means of an extraction procedure succeeded by atomic absorption spectrometric analysis it was shown that the Sb ions dissolved during the oxidation exist in H₂SO₄ in the trivalent state even at potentials well above the equilibrium potential of the Sb(V)/Sb(III) couple.     

Anodic oxidation of azo dye C.I. Acid Red 73 by the yttrium-doped Ti/SnO2-Sb electrodes

This work was conducted to study the ability of anodic oxidation of azo dye C.I. Acid Red 73 (AR73) using the yttrium-doped Ti/SnO₂-Sb electrodes. The effects of Sb doping level, yttrium doping level, thermal decomposition temperature and cycle times of dip-coating thermal decomposition on the properties of the electrodes were investigated. The results showed that the excellent electrochemical activity of Ti/SnO₂-Sb-Y electrode can be achieved at a 7∶1 molar ratio of Sn∶Sb and thermal decomposition temperature of 550°C. Moreover when the cycle times of dip-coating and thermal decomposition were up to 10 times, the performance of the electrode tends to be stable. The Ti/SnO₂-Sb electrodes doped with yttrium (0.5 mol-%) showed the most excellent electrochemical activity. In addition, the influences of operating variables, including current density, initial pH, dye concentration and support electrolyte, on the colour removal, chemical oxygen demand (COD) removal and current efficiency were also investigated. Our results confirmed that the current efficiency increased with the concentrations of dye and sodium chloride. Moreover, increasing the current density and the initial pH would reduce the current efficiency.