The rare earth metal conversion coating process is recognized for its simple electrolyte composition, which is generally considered to be environmentally friendly. In this study, cerium conversion coatings were made in a cerium nitrate/hydrogen peroxide solution on AZ31 magnesium alloys after 20 s of pickling in hydrochloric acid (HCl) or hydrofluoric acid (HF) solutions. Results show that acid pickling in HF or HCl enhanced the adhesion and corrosion resistance of the cerium conversion coating. This enhancement is due to the inhibition of the blister formation, which presumably results from the rupture of the cerium oxide layer by hydrogen bubbles. Finally, the mechanism of HF and HCl acid pickling effect on the reaction rate and defect of the cerium conversion coating was discussed in detail.
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...
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