Elektrolytische Tunnelätzung von Aluminiumfolie

Electrolytic tunnel etching of aluminium foil The most effective method of etching aluminum foil for high voltage electrolytic capacitors is the electrolytic tunnel etching with direct current in an aqueous solution of chloride salts. Surprisingly the classical electrochemical method for determining the potentiostatic current-time curves of annealed high purity aluminum foil delivers a pure tunnel etching in a narrow range around the pitting potential. The potentiostatic current-time curves show the dynamic of the very fast and on an already etched aluminum foil not repeatable process of forming the tunnels (tunnel diameter ca. 0.2 μm; tunnel length 10–100 μm) and immediate after tunnel formation the about 1000 times slower and controllable process of widening these tunnels in the region of passivation current density.     

Electrochemical etching model in aluminum foil for capacitor

A novel tunnel growth model is proposed to reveal how hydrogen is transported out of tunnels and explain the phenomena during etching process of aluminum foil for capacitor. Experimental results indicated that tunnel density increased and tunnel width decreased with temperature increasing at the temperature range of 70–80 °C. The pressure in electrolyte had an effect on the tunnel density. The tunnel density after etching at 0.2 atm was larger than that at 1 atm. A pulse electrochemical etching process is described according to the novel tunnel growth model. It considers that hydrogen bubble of nanometer dimension is absorbed on the tunnel’s walls surface during the pulse electrochemical etching process, and the saturated hydrogen at the end of the tunnel is accumulated to be a large bubble before hydrogen is transported out of the tunnel. The large bubble will emanate from the end of tunnel when the pressure in the bubble is equal to that outside. The wall surface passivation phenomena is explained by this model; the naturally corrugated texture with ripples of about 0.1 μm in tunnel’s walls surface is regarded to be produced by the potential periodical changes, which are caused by the large hydrogen bubble at the end of the tunnel. At the same time, the effect of temperature and pressure on the morphology of the tunnel is also investigated by use of the model. The pressure of the large hydrogen bubble in the tunnel is calculated according to the date in other references when the period of the pulse electrochemical etching is inferred to be 3 ms at their experimental conditions. The proposed process of pulse electrochemical etching could explain well the calculation results and the SEM images of etched tunnels under the same experimental conditions.     

Tunnel morphology of aluminum foil etched by a two-step DC etching method

A two-step DC etching of aluminum foil for capacitor is proposed. The first step DC etching was processed in a solution of HCl + H₂SO₄, the second was finished in a solution containing NaCl with some addition or no addition. Experiments indicated that the tunnel morphology was variable with the composition of electrolyte during the second step DC etching. When the solution of NaCl was used as electrolyte, the density of tunnels was increased. However, when some organic compound was added in the electrolyte of NaCl, the tunnels’ length was increased, and many branch tunnels were fabricated.     

Effect of sulfuric acid on pit propagation behaviour of aluminium under AC etch process

Pit propagation on high purity aluminium electrode in 2 M HCl solutions with and without H₂SO₄ under an alternating current (AC) has been examined. Pit development and potential transients were dependent on the H₂SO₄ concentration. In the sulfate-free etchant, most pits developed from the pretreated surface, with little tendency to form clusters of pits. With increasing H₂SO₄ concentration the size of the pit clusters increased. There is an optimal H₂SO₄ concentration, which is 0.01 M H₂SO₄ in this study, to form a deep etched layer of uniform thickness with high surface area. At H₂SO₄ concentrations higher than 0.01 M, the pit propagation proceeded on limited foil surface sites and deep etched regions were formed locally, since sulfate ions assisted passivation and reduced the number of pit nucleation sites on foil surface. Analysis of potential transients during the anodic half-cycle supports the hypothesis that sulfate ions retarded the pit nucleation.     

咪唑啉缓蚀剂对电解电容器用铝箔直流扩面增容的作用

合成了油酸咪唑啉和丙烯酸咪唑啉化合物,通过红外谱对其进行了结构表征;用Tafle曲线和交流阻抗等方法研究了其对铝箔的缓蚀作用,并研究了其对铝箔直流扩面增容的影响。结果表明,在2.0 mol/L的HCl溶液中两种化合物对铝箔均有缓蚀作用,为阴极型缓蚀剂,腐蚀液中添加两种化合物后,铝箔直流扩面的蚀孔密度增加、分布均匀、使铝箔的比电容提高约25%。     

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.     

Effects of pretreatment on the aluminium etch pit formation

The effect of chemical pretreatments on the electrochemical etching behavior of aluminium was investigated with the topographic studies of surface and the analysis of initial potential transients. Two-step pretreatments with H₃PO₄ and H₂SiF₆ result in a high density of pre-etch pits on aluminium surface by the incorporation of phosphate ion inside the oxide film and the removal of surface layer by aggressive fluorosilicic acid solution. It generates a high density of etch pits during electrochemical etching and results in the capacitance increase of etched Al electrode by expanding the surface area, up to 61.3 μF/cm² with the pretreatment solution of 0.5 M H₃PO₄ at 65 °C and 10 mM H₂SiF₆ at 45 °C.     

高压电子铝箔阳极电解扩孔行为

研究发孔铝箔在盐酸和硝酸溶液中的阳极极化行为与扩孔特性的关系。阳极扩孔的基本条件是将发孔箔的内、外表面都控制在钝化状态下。在盐酸溶液中,阳极极化时存在点蚀电位和一个较小的钝化电位区,当扩孔施加的电流密度大于临界点蚀电流密度时,铝箔表面发生二次发孔,导致形成孔蚀族与并孔,腐蚀箔厚度减薄,比电容显著降低。在硝酸溶液中,阳极极化时存在一个宽阔的钝化电位区,因此硝酸扩孔比盐酸扩孔容易控制,不会发生二次发孔。提高盐酸或硝酸浓度与温度均可以增大最大维钝电流密度,即增大扩孔的最大电流密度,提高扩孔的生产效率。     

Effect of microcrystallization on pitting corrosion of pure aluminium

A microcrystalline aluminium film with grain size of about 400 nm was prepared by magnetron sputtering technique. Its corrosion behaviour was investigated in NaCl containing acidic solution by means of potentiodynamic polarization curves and electrochemical noise (EN). The polarization results indicated that the corrosion potential of the sample shifted towards more positive direction, while its corrosion current density decreased compared with that of pure coarse-grain Al. The EN analysis based on stochastic model demonstrated that there existed two kinds of effect of microcrystallization on the pitting behaviour of pure aluminium: (1) the rate of pit initiation is accelerated, (2) the pit growth process was impeded. This leads to the enhancement of pitting resistance for the microcrystallized aluminium.     

Growth and passivation of aluminum etch tunnels at on-off controlling direct current in 6 wt. % HCI solution

Growth and passivation of tunnels within Al foil by on-off controlling DC etching in 6 wt.% HC1 solution has been investigated.It was found that,in a given etchant solution at a special temperature,the longest tunnel length was only a function of the turn-on interval of DC.The potential of AI foil broke at on-off controlling DC by the result from anode polarization curves and potential-time (E-t) responding curves.When DC was switched on,the potential increased abruptly over pitting potential,leading to nucleation of pits at the surface and the growth of tunnels at special length.When DC was switched off,the potential decreased rapidly to a passive value,leading to stoppage of nucleation and death of tunnels.By this way,the longest tunnel length can be controlled and a non-piercing layer can be obtained.Hence,etching of Al foil at this current is beneficial for maintaining a good mechanical strength.     

Growth and passivation of aluminum etch tunnels at on-off controlling direct current in 6 wt.% HCl solution

Growth and passivation of tunnels within Al foil by on-off controlling DC etching in 6 wt.% HCI solution has been investigated. It was found that, in a given etchant solution at a special temperature, the longest tunnel length was only a function of the turn-on interval of DC. The potential of Al foil broke at on-off controlling DC by the result from anode polarization curves and potential-time （E-t） responding curves. When DC was switched on, the potential increased abruptly over pitting potential, leading to nucleation of pits at the surface and the growth of tunnels at special length. When DC was switched off, the potential decreased rapidly to a passive value, leading to stoppage of nucleation and death of tunnels. By this way, the longest tunnel length can be controlled and a non-piercing layer can be obtained. Hence, etching of Al foil at this current is beneficial for maintaining a good mechanical strength.     

FIB/SEM study of AA2024 corrosion under a seawater drop: Part I

The role of metallic microstructure in 0.5 μl seawater droplet corrosion of aluminium alloy 2024 (AA2024) has been investigated. Focussed ion beam/scanning electron microscopy (FIB/SEM) was used to determine the relationships between the corrosion products formed at specific sites at the surface and the underlying attack mechanisms. Dealloying of S-phase particles, matrix/particle interfacial attack and grain boundary attack were the predominant attack modes. Cooperative behaviour between IM particles was made possible by networks of etched grain boundaries, which provided a connecting path.     

Combined anodic/cathodic transient currents within nucleating pits on Al–Fe alloy surfaces

Pit nucleation and metastable growth on microelectrodes of aluminium and some Al alloys in chloride solution below the pitting potential are described by recurring current transients. Pit nucleation is a sharp, violent event and is observed repeatedly on all materials tested: pure aluminium, titanium and iron binary aluminium alloys, and alloy 1050A. At the imposed electrode potential, current transients due to pit nucleation recorded for pure aluminium and a solid solution titanium binary alloy are exclusively anodic. Those from alloys containing iron intermetallics sometimes exhibit both anodic and cathodic wings. The cathodic wing is due to transient hydrogen evolution at sites which border iron-rich intermetallic particles: these are preferred pit nucleation sites.     

Corrosion of aluminium in copper-aluminium couples under a marine environment: Influence of polyaniline deposited onto copper

In this study, we examined how aluminium corrosion in Al-Cu/PANI galvanic couples in a marine environment is influenced by deposition of polyaniline (PANI) on copper. Polarization curves and immersion assays in 0.1 M NaCl were performed. The morphologies of etched Al and corrosion products were observed by SEM, and the Al ions in solution were quantified by atomic absorption spectroscopy. A reduction in aluminium damage due to galvanic corrosion was observed as a result of decreased effective area for the oxygen reduction reaction on Cu/PANI electrode. Furthermore, an electrochemical reduction of PANI from leucoemeraldine to emeraldine base is proposed.     

Preparation and characterisation of aluminium pigments coated with silica for corrosion protection

Aluminium pigments with a layer of silica were prepared by a sol–gel method using tetraethoxysilane as precursor and ethylenediamine as catalyst. Under the optimum conditions, the corrosion protection factor can reach 99.3% and average grain almost remains the same size after coating, indicating that the coated aluminium pigments have excellent chemical stability and good dispersibility. FTIR and EDS analyses demonstrate that a layer of silica coating has been formed on the flaky aluminium particle. SEM, AFM and BET analyses show that a smooth and dense silica coating layer has been formed.     

Dielectric breakdown and healing of anodic oxide films on aluminium under single pulse anodizing

Single pulse anodizing of aluminium micro-electrode has been employed to study the behaviour of dielectric breakdown and subsequent oxide formation on aluminium in alkaline silicate and pentaborate electrolytes. Current transients during applying pulse voltage have been measured, and surface has been observed by scanning electron microscopy. Two types of current transients are observed, depending on the electrolyte and applied voltage. There is a good correlation between the current transient behaviour and the shape of discharge channels. In alkaline silicate electrolyte, circular open pores are healed by increasing the pulse width, but such healing is not obvious in pentaborate electrolyte.     

Diphenolic Schiff bases as corrosion inhibitors for aluminium in 0.1 M HCl: Potentiodynamic polarisation and EQCM investigations

The effects of novel synthesised two Schiff bases on the corrosion of aluminium in 0.1 M HCl were investigated using potentiodynamic polarisation and electrochemical quartz crystal microbalance measurements. Results show that inhibition efficiencies increase with increase in inhibitor concentration. This reveals that the inhibition occurs through adsorption of the inhibitor molecules on the metal surface. Adsorption of these inhibitors follows Temkin adsorption isotherm. The correlation between the inhibitor performances and their molecular structures has been investigated using quantum chemical parameters obtained by MNDO (modified neglect of diatomic overlap) semi-empirical method. Calculated quantum chemical parameters indicate that Schiff bases adsorbed on aluminium surface by chemical mechanism.     

Surfactant inhibition of aluminium pigments for waterborne printing inks

Micrometre-sized aluminium particles are used as pigments in silver inks and for environmental reasons it is desirable to develop water-based formulations of such pigments. In waterborne coatings, however, aluminium is prone to react with water, with subsequent hydrogen gas evolution and loss of the silvery lustre. The protection against water of aluminium pigments by adsorption of either a nonionic or an anionic surfactant has been evaluated. Phosphate esters with different lengths of the hydrocarbon tail have been synthesised and tested, and were found to provide very effective protection of the pigments. Sodium dodecyl sulphate (SDS), on the other hand, was completely ineffective as inhibitor, even though the adsorption isotherm was about the same as for the structurally similar sodium dodecyl phosphate (SDP). This difference may be explained by formation of different types of complexes with the aluminium oxide surface. Outer-sphere complexes are suggested for SDS, whilst SDP can form more stable inner-sphere complexes. Tests with the non-surface active methyl phosphate as inhibiting agent gave insufficient protection, indicating that surface activity is crucial for the inhibition, and that the use of the phosphates as such is not enough to give proper protection. The work shows that amphiphilic compounds having phosphate as anchoring group are efficient in providing water resistance to aluminium pigments.     

Effects of polymer corrosion inhibitor on widening etch tunnels of aluminum foil for capacitor

We investigated the effects of polymeric corrosion inhibitor polystyrene sulfonic acid (PSSA) additive to 3% HNO₃ solution on widening tunnels of pre-etched aluminum foil by electrochemical DC etching for aluminum electrolytic capacitors, using scanning electron microscopy and polarization curves. With trace PSSA, the dissolution of exterior surface of etch tunnels of Al foil is suppressed and the dissolution of interior surface of etch tunnels of Al foil is facilitated, respectively. The tunnels transform from circular cone to circular column in shape and pits-merging on the surface is weakened, leading to significant increase in the surface area and specific capacitance of the Al foil. The amounts of reduced thickness and weight of Al foil during the widening process of etch tunnels can be decreased if PSSA is employed.     

A morphological study of filiform corrosive attack on cerated AA2024-T351 aluminium alloy

SEM and EDS studies were carried out to characterise filiform attack on a cerated AA2024-T351 aluminium alloy with a polyurethane topcoat. The filiforms developed on AA2024-T351 were sectioned, stripped of corrosion product and etched to reveal the grain structure. Examination of sections through the filaments and the filaments themselves, revealed severe local attack in the form of pitting resulting in grain etch out, grain boundary attack and subsurface etch out. Chloride ions were detected deep within pits and the subsurface etch out. The observations were similar to those found with filiform corrosion on chromated and coated surfaces. The observations led to development of a filiform corrosion model naming the volume expansion of the corrosion product as the principal cause for delamination.