Pitting Behavior of Aluminum Foil during Alternating Current Etching in Hydrochloric Acid Containing Sulfate Ions

Abstract

Sulfate ions in sodium chloride solutions effectively enhance the capacitance of aluminum foils etched with direct current. This study explored the effects of sulfate ions on pitting behavior of aluminum foils for low-voltage electrolytic capacitor during etching with alternating current. Experimental results indicate that the presence of sulfate ions in hydrochloric acid reduced the loss of aluminum due to undermining by the cubic pits. Furthermore, sulfate ions adsorbed on the aluminum surface increased the breakdown potential of the surface film and led to passivation of the existing pits. Consequently, the aluminum foil etched in the solution with the addition of sulfate ions displayed a more uniform pitted structure than that etched in the solution solely composed of hydrochloric acid.Aluminum foils for low-voltage electrolytic capacitors are generally etched in hydrochloric acid with alternating current, 1-6 in which the pits that form during the anodic half-cycle are passive during the subsequent cathodic half-cycle by the precipitation of the etch film. 1,2 This results in a cauliflower-like pitted structure on both sides of the foil, leaving the central core unetched. 3,4 The capacitance of the foil is closely related to its pitted structure, which is controlled by the nucleation, growth, coalescence, and propagation of the pits. Such pitting behavior is strongly influenced by the waveform of the alternating current 4,5 ͑ac͒, the composition of the electrolyte, and the additives such as aluminum ions, transition metal compounds, phosphates, sulfates, tartrates, and oxalates, 6-9 bath temperature and pH, 4,5 as well as factors related to the purity and microstructure of the foil including grain structure, defects and textures, and type and amount of the various precipitates. 10,11 Sulfate ions in sodium chloride solutions have been shown to enhance the capacitance of aluminum foils etched with the direct current ͑dc͒ via refinement of etch configuration and the increase in the population density of the tunnel pits. [12][13][14] The presence of sulfate ions also causes a shift of the breakdown potential in the noble direction 14-16 and an increase in the induction time for the onset of pitting of aluminum during anodic polarization in chloridecontaining solutions. 16,17 Competitive adsorption 14-16,18 of sulfate ions with chloride ions and the formation of aluminum sulfate 14 have been proposed to explain the effects of sulfate ions on the pitting behavior of aluminum. The competitive adsorption mechanism stems from the fact that sulfate ions are a potential corrosion inhibitor, 16,19,20 while chloride ions are an aggressive species inducing pitting on most of aluminum alloys. 16,21 A model has been well established to explain the pitting corrosion of aluminum in chloridecontaining solutions, and involves sequentially adsorption of chloride ions on the oxide surface, penetration of chloride ions through the oxide film by migration through oxygen vacancies or by oxidation film dissolution, and localized dis...

Corrosion behaviour of twin-roll cast 8006 aluminium alloy foil with different thicknesses for the food packaging industry

“…The surface of the TRC180 sample depicts relatively less degeneration due to the pitting corrosion and less accumulation of NaCl on the surface. Owing to the detrimental effect of the potentiodynamic polarization tests, a large scale of the surface filled with pits from the pitting corrosion (Liang et al, 2018;Lin and Li, 2006;Calabrese et al, 2018;Quartiermeister et al, 2019). The EDS result summarized in Table 1 indicates an increase in sodium and chlorine content after the test compared to the bare samples due to the NaCl saline solution.…”

Effect of Additives on Smut-Layer Formation and Pitting during Aluminum Etching in Hydrochloric Acid

“…NMR analysis.-Samples for the solid-state NMR were prepared by carefully removing the dried etch film produced during graining and placing the resulting powder in a 4 mm ZrO 2 rotor. Cross polarization ͑CP͒ magic angle spinning ͑MAS͒ solid-state NMR measurements were carried out on a Bruker Avance 250 spectrometer operating at 65.28, 63.00, and 101.42 MHz for 27 Al, 13 C, and 31 P, respectively. Chemical shifts were referenced to external aqueous Al͑NO 3 ͒ 3 , adamantane ͓38.3 ppm relative to tetramethylsilane ͑TMS͔͒, and 85% H 3 PO 4 , respectively.…”

“…Pitting can also be used for surface processing where the localized corrosion of the metal is used as an advantage to create uniform surface roughness on the substrate by varying parameters like total applied charge, frequency, current, or electrolyte concentration. Such processes include a number of electrochemical manufacturing techniques like electrochemical machining ͑ECM͒, 6,7 electrochemical micromachining ͑EMM͒, 8,9 production of high-surfacearea capacitor foils, [10][11][12][13] and ac electrograining for lithographic plates [14][15][16][17][18][19][20] with convoluted surfaces that increase surface area for enhanced water-retention properties and photosensitive coating adhesion.…”

Elucidation of the Anodization and Silver Incorporation Impact on the Surface Properties of AA1050 Aluminum Alloy

“…87,88 In addition, the high intensity of the S2p peak shows that it is surrounded by electronegative elements, revealing that this element is present in the form of sulfate anions. 89,90 Traces of nitrogen were detected on the sample surfaces after silver incorporation (group 3 -Fig. 9c).…”