The growth kinetics and properties of potentiodynamically formed thin oxide films on aluminium in citric acid solutions (original) (raw)
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Anodic film growth on Al layers and Ta–Al metal bilayers in citric acid electrolytes
Electrochimica Acta, 2005
A systematic investigation has been undertaken of anodising behaviour of aluminium in citric acid solutions under changing the current density, electrolyte temperature, concentration and stirring conditions. Porous films of steady-state, controlled and extraordinary large morphologies have been obtained at the formation voltages of 260-450 V. The barrier layer thickness, cell and pore diameters appeared to be directly proportional to the formation voltage. For the first time, a porous anodic film with cell diameter of 1.1 m, barrier layer thickness of 0.5 m and pore diameter of 0.23 m was naturally grown on aluminium in 0.125 mol dm −3 citric acid electrolyte. The selected electrical and electrolytic conditions have been applied to sputter-deposited Ta-Al bilayers. The tantalum underlayer was anodised through the initially grown at 450 V porous alumina film with the formation of tantalum oxide 'columns' penetrating into the pores and mixing with the outer part of the alumina cell walls. During subsequent constant current reanodising of the Ta-Al bilayers in 0.5 mol dm −3 H 3 BO 3 , the voltage rose up to 1100 V, until voltage fluctuations terminated normal oxide growth. The columns thickened uniformly up to 1.42 m with the nm V −1 ratio of 1.3, exhibiting nearly ideal dielectric behaviour up to almost 1000 V of the formation voltage. The outer column material is composed mainly of stoichiometric Ta 2 O 5 mixed with small amount of Al 2 O 3 , while the innermost column material is tantalum monoxide with little content of boron species. Implications of these findings for the formation of composite dielectrics for high-voltage electrolytic capacitors are discussed.
Electrochimica Acta, 2001
The electrochemical behaviour of pure aluminium in citric acid solution was investigated with a rotating disk electrode (RDE) at the open circuit potential (OCP), in the presence and in the absence of chloride, using electrochemical impedance spectroscopy and Raman spectroscopy. The impedance diagrams are composed of four frequency domains: at very high frequency (VHF), a Warburg-like behaviour was attributed to the porosity effect of the external part of the oxide layer. A slightly depressed capacitive loop at high frequency (HF) was explained as the combination of two capacitive loops, one corresponding to the same phenomenon as in the VHF domain but insensitive to the porosity (barrier layer resistance in parallel to the oxide film capacity), the other one to the charge transfer resistance in parallel to the double layer capacity. An inductive loop at low frequencies (LF) appeared as due to the relaxation of oxygen species adsorbed at the film solution interface, while the very low frequencies (VLF) capacitive loop was ascribed to the coverage relaxation of adsorbed citrate species. Possible modulation of the passive layer thickness was also envisaged as a phenomenon at the origin of this latter loop. The diagrams shape is also insensitive to the addition of 104 ppm chloride ions to the solution, even though a diameter decrease of the HF capacitive loop is observed together with an increase of the frequency response. Raman Spectroscopy showed that citrate ions adsorb strongly to the electrode surface and that the time scale of this process is consistent with the frequency domain observed for the VLF capacitive loop. The interaction of citrate with the oxide layer is hindered by the addition of chloride ions to the electrolytic solution.
Anodic oxide cellular structure formation on aluminum films in tartaric acid electrolyte
Materials Letters, 1993
The kinetics of nucleation and growth of oxide cells during formation of anodic alumina films in tartaric acid electrolyte at a constant current density has been studied. The maximum number of oxide cells appears during the barrier oxide film growth and reaches the value of 9.2 x 1 O8 cm-z. At steady-state anodization, the anodic alumina film consists of 6.0 x 10s cm-' oxide cells with a dimension of 400 nm.
ChemInform, 1988
turrent efficiencies, dielectric constants and ion mobilities of variously formed anodic oxide films on tantalm in the presence of various electrolytes such as citric, tartaric, acetic and hydrocWoric acids have been evaluated. The cUIT~nt efficiencies of such films are unity upon an electrolyte concentration of 100 mol m-3• However, at higher elec*,olyte concentration, the experimental mass-gain of the tantalum sample during anodization is greater than that expe!::ted for the anodic formation of TazOs on tantalum, implying that mass-gain is probably due to the incorporation of anions into the films. The capacitances of such films depend on the concentration as well as nature of the aquebus electrolyte. The addition of cWoride ions has been found to' inhibit film growth, the effect being pro-nou~ced with increase in concentration of cWoride ions. Only tantalum ions are mobile during the growth of tanta-lum~xide films.
1988
turrent efficiencies, dielectric constants and ion mobilities of variously formed anodic oxide films on tantalm in the presence of various electrolytes such as citric, tartaric, acetic and hydrocWoric acids have been evaluated. The cUIT~nt efficiencies of such films are unity upon an electrolyte concentration of 100 mol m-3• However, at higher elec*,olyte concentration, the experimental mass-gain of the tantalum sample during anodization is greater than that expe!::ted for the anodic formation of TazOs on tantalum, implying that mass-gain is probably due to the incorporation of anions into the films. The capacitances of such films depend on the concentration as well as nature of the aquebus electrolyte. The addition of cWoride ions has been found to' inhibit film growth, the effect being pro-nou~ced with increase in concentration of cWoride ions. Only tantalum ions are mobile during the growth of tanta-lum~xide films.
Kinetics of Growth of Anodic Oxide Film on Tantalum in Aqueous Citric Acid
1987
Steady state kinetic data for the growth of oxide film on tantalum in aqueous citric acid have been obtained by eliminating the difference of film growth caused by different surface. conditions. The rate of increase of field with logarithm of ionic current density (Tafel slope) is independent of temperature. Various parameters of anodic oxidation have been evaluated using Dewald's theory. The entrance barrier energy is greater than the corresponding diffusion barrier energy and this suggests that the rate-controlling step in the kinetics of growth of anodic film is the ionic movement across the film and not at the metaV oxide interface. The value of net activation energy calculated using Dewald's theory agrees well with the value calculated using Dignam's equation. The data suggest that Dewald's theory which takes into account the space charge effect is better than other theories on film growth kinetics. '&19
2006
The polarization behaviour of pure aluminium was studied by potentiostatic, potentiodynamic and potential decay techniques in KClO 3 aqueous solutions, with and without different aggressive anions. In aqueous solution of KClO 3 without aggressive anions, the potential sweep from the cathodic to the anodic range led to the formation of an insoluble layer of aluminium oxide on the electrode surface, which passivates the working electrode. The presence of aggressive anions like as Cl", SCN - , prevents the formation of the passive film and accelerates the process of anodic dissolution, the more the greater is the concentration of the aggressive anion. Localized pitting is best described by a characteristic parameter called the pitting potential value (En). Ep is the protection potential and the pitting can be formed only in the E, - Ep polarization range. Electrochemical behaviour of anodised aluminium surface was also studied by (EIS) in KClO 3 solution. Samples were observed by ...
Corrosion Science, 2010
The characteristics of oxide films on Al and AlA1R alloys (R = rare earth metal = Ce, Y) galvanostatically formed (at a current density of 100 lA cm À2 ) in borate buffer solution (0.5 M H 3 BO 3 + 0.05 M Na 2 B 4 O 7 Á10H 2 O; pH = 7.8) were investigated by means of electrochemical impedance spectroscopy. EIS spectra were interpreted in terms of an ''equivalent circuit" that completely illustrate the Al(AlA1R alloy)/oxide film/electrolyte systems examined. The resistance of the oxide films was found to increase on passing from Al to AlA1R alloys while the capacitance showed an opposite trend. The stability of the anodic oxide films grown in the borate buffer solution on Al and AlA1R alloys was investigated by simultaneously measuring the electrode capacitance and resistance at a working frequency of 1 kHz as a function of exposure over a period of time to naturally aerated 0.01 M NaOH solution. Analyses of the electrode capacitance and resistance values indicated a decrease in chemical dissolution rate of the oxide films on passing from Al to AlA1R alloys.
The accurate determination of the impedance of oxide films in metal/oxide/electrolyte systems is shown to be possible through a modified procedure. The film impedance thus determined is found to be quite different from the measured cell impedance for anodic aluminium oxide formed on pure aluminium. The computed values of tan 6 and conductivity are found to be in agreement with the values reported earlier for anodic and thermal oxides.