전기화학적 에칭에 의한 알루미늄 전해 커패시터 전극 제조

탁용석, 김행영, 최진섭, 이재광, 오재호

인하대학교 화학공학과

Preparation of the Aluminum Electrolytic Capacitor Electrode

by Electrochemical Etching

Yongsug Tak, Haengyoung Kim, Jinsub Choi, Jaekwang Lee, Jaeho Oh

Department of Chemical Engineering, Inha University

Introduction

Pitting is a common observation in aluminum corrosion. Aluminum pits show a unique geometry as a result of anodic etching in hot HCl solution. Cubic etch pits are initially formed in the early stage of etching and become tunnel-like structure after etch times of several seconds.(1) Tunnels grow along <100> crystallographic directions and growth velocities are several □per second with a 1-4 □ width. As temperature increases, tunnel growth rate increases.(2) Electrochemical tunnel etching of aluminum has been used for manufacturing an aluminum electrolytic capacitor to create a high surface area.  Surface area of aluminum is closely related with tunnel morphology, which depends on microstructure of the aluminum and etching conditions.

For high voltage applications, high-purity aluminum foil is dc etched in the sulfate-containing hot hydrochloric acid solution.(3) Sulfate ion is used as an oxide-forming agent at low temperature. Sulfate ions have a competitive adsorption with chloride ions and push the pitting potential towards positive potentials. This effect causes the morphological change of etched tunnels; increase of sulfate concentration increases tunnel density but decreases the maximum tunnel length. Tapering of tunnel is also observed in higher concentration of sulfuric acid. Pyun et al. suggested that sulfate ion has the aggressive nature for the pit growth above pitting potential at 60□C and 80□C.

In this work, the effect of sulfate ions on the aluminum etch pit formation was investigated with the morphology study by using a scanning electron microscopy and potential transient analysis.  Current interruption experiments were designed to induce the passivation during aluminum etch tunnel growth and the measured potential variations were interpreted to elucidate the role of sulfate ions during etch tunnel growth.

Experimental

The electrochemical cell body is consisted of a glass cylinder capped with a Teflon lid holding all three electrodes. The polycrystalline aluminum foil (capacitor grade 99.99%, Tokai, Japan) of 104 m thickness was used as the working electrode(WE). The geometric area of electrode during etching was 1 cm2. Prior to the etching experiment, the aluminum foil was chemically treated in 1M HCl at room temperature for 3min and then rinsed in deionized water. A platinum wire was used as counter electrode(CE) and Ag/AgCl/4M KCl was employed as the reference electrode(RE). RE was placed behind the WE holder. HCl and H2SO4(Aldrich, analytical grade) solutions used in this study were prepared with deionized water(Millipore, 18.2 M cm). Experiments were carried out at 65□C in stirred solution. Applied current waveform was modulated and supplied by a potentiostat/galvanostat(EG&G PAR 273A).  Accompanying potential transients were measured with a digital oscilloscope(TDS 350).

Results and Discussion

Effects of sulfate ion on the pit formation --- Figure 1 shows the potential transients when anodic current of 40 □/□ is applied with the variation of sulfuric acid concentration. The initial potential rapid increases fall on the same transient and it increases slowly to peak potential, Epp, then returns to the steady-state anodic potential, Ea. Both peak potential and the potential decay time, □D, from Epp to Ea increase with H2SO4 concentration.  Considering □D is a period of time needed for chloride ions to reach the metal/oxide interface, potential increases to Epp can be interpreted with the anodic oxide film formation process. After a sudden potential rise due to ohmic drop and double layer charging, potential increases at a rate of 3.8 V/s as the oxide film is thickened. It is commensurate with the film growth rate of 2.7 nm/s since potential increases by 1.4 V/nm. In the slow potential increase region of Fig. 1, etch pits are not found and average film growth rate is measured at 2.9 nm/s, similarly to 2.7 nm/s in H2SO4 only solution. The results suggested that potential increase to Epp can be attributed to the general oxidation of filmed surface and the thickness of films increases with increasing H2SO4 addition. Thickening of oxide films increases the potential decay time, □D, by requiring the longer penetration length of chloride ions through films. Pits are initiated during potential drop from Epp to Ea. The remaining surfaces are covered with sulfate-included oxide films and these are visible after etching of 10 s.

Inhibiting behavior of sulfate ions for the pit formation is investigated in hot HCl solution. H2SO4 addition moves pitting potential to the anodic direction by suppression of pit initiation. At more anodic potentials above pitting potential, anodic current increases with the increasing H2SO4 solution and it suggests the possible aggressive behaviors of sulfate ions for the bare aluminum dissolution. On the other hand, addition of Na2SO4 induces inhibiting behaviors over entire potential scan range. Above pitting potential, the effect of sulfate ions remains less clear because of their complex behaviors. However, since aluminum/aluminum oxide becomes unstable at low pH, it can be assumed that solution pH is more responsible than sulfate ions for the current increase above pitting potential.

Figure 2 shows the surface of aluminum specimen produced in which anodic current of 40 □/□ was applied during 10 s. Low magnification images indicate that, with increasing H2SO4 concentration, pit density increases and the average pit size diminishes, as observed in other works. In HCl only solution, number of pits are interconnected and have sharp edge lines of which reflects the etched surface. However, close examination of pits discloses the existence of oxide film over the pit mouth when sulfuric acid is added.  When 1M H2SO4 is added, edges of pit mouth are not clearly shown due to oxide debris formed by the mechanical rupture. Since current efficiency during tunnel etching is 110-115%, cathodic hydrogen evolution may be responsible for the oxide rupture. In case 2M H2SO4 addition, number of pits are found underneath oxide films and parts of pitted area are repassivated, that is, oxide formation effect becomes more pronouncing. AES depth profile indicates the inclusion of sulfate ions in oxide film. These results suggest that sulfuric acid have a significant influence on the oxide film formation during anodic etching in hot HCl solution.

References

1. R. S. Alwitt, H. Uchi, T. R. Beck, and R. C. Alkire, J. Electrochem. Soc., 131, 13 (1984).

2.  K. R. Hebert and R. C. Alkire, J. Electrochem. Soc., 135, 2146 (1988).

3.  A. Hibino, M. Tamaki, Y. Watanabe, and T. Oki, Sumitomo Light Metal

Technical Reports, 33, 26 (1992).

Figure 1  Initial potential transients for different concentrations of H2SO4. (a)

1M HCl, basis 1M HCl + (b) 1M H2SO4, (c) 2M H2SO4, (d) 3M

H2SO4, (e) 4M H2SO4. (anodic current=40 □/□, temperature=65□.)

Figure 2  SEM image of aluminum surface with different concentrations of

H2SO4.  Etching solutions; 1M HCl + 2M H2SO4,

etching time = 10 s, temperature = 65□.