Study Of Expanding Surface Corrosion Process And Controlling Of Pit Initiation Sites On The Anodic Foils Used For Aluminium Electrolytic Capacitor

Aluminum electrolytic capacitors achieve extensive applications in the field of assembly ofelectronic products. At the same time, the demands of high performance aluminum electrolyticcapacitors with miniaturization, high specific capacitance and et al are growing drastically. Recentdevelopment of small electronic devices with increasing capacitance is to increase the specificsurface area of the anode aluminum foils by gaining a great amount of tunnels on the [001] direction.In this paper, the factors as quality of aluminum foils, pretreatment process, corrosion process,some auxiliary methods of adding ultrasound treatment and corrosion inhibitors so as to influencedthe expanding surface corrosion were discussed and analyzed. Also, the nanotechnology such asporous anodic alumina templates, the self-assembled structure of the polystyrene spheres （PS） andUV nano lithography techonology were introduced to the corrosion process to control the pitsinitiation. The main contents and innovative contentare as follows:（1） The level of electronic aluminum foils in trace elements produced by China had satisfied thestandards of JCC in Japan which was on behalf of the world standard, especially on the control of Feand Mg trace element. In terms of organizational structure, domestic aluminum foils had cubictexture, and the content was above95%. However, the domestic electronic aluminum foils had moredefects on the surface, and the grain size was too small, which was bad for uniform pit corrosion andtunnel growth.To deal with these defects, we studied pretreatment process, corrosion process, some auxiliarymethods and adopted the nanotechnology to improve tunnel corrosion.（2） The pretreatment methods as acid cleaning, alkaline cleaning, surface oxidation andactivation treatment were studied in this paper. By comparing surface morphology, distribution ofthe pits, size, and analyzing the Tafel and IV electrochemical curves, we found that pretreatmentprocess prior to pitting corrosion could improve the defects on the surface of the aluminum foils.Compared to acid cleaning, alkaline cleaning was easier to remove surface stains and uneven floor.The surface oxidation after cleaning the surface was in favor of forming the square holes andincreasing the number of pits. Etching pits were easily formed on the surface of the aluminum foilafter Cl^-activation treatment, and the pit number increased. However, the diameter size of theetching holes was too small, and lots of them were less than0.5μm because of the strong erosion ofCl^-ions. These small holes would be blocked during the forming process, which could not improvespecific surface area.（3） The parameters during the pitting corrosion process had a greater influence on themorphology of corrosion hole. The process parameters such as concentration of hydrochloricacid/sulfuric acid, current and temperature were discussed in this paper. By comparing surface morphology, distribution of the pits, the size, and the capacitance, it was showed that with theincrease of H₂SO₄concentration, the porosity on the aluminum foil was improved, but the smallerpore diameter and tunnel length was obtained. It was concluded that1M HCl+3M H₂SO₄solutionwas most beneficial for pitting corrosion. The effect of the temperature on the pitting corrosion wassimilar to that of sulfuric acid. As the temperature increased, the porosity was increased, butachieved smaller pore diameter and tunnel length. Furthermore, the corrosion on the surface ofaluminum foils was serious when the temperature reached85℃. It led to decrease of the materialmechanical performance. Therefore,78℃was selected as the corrosion temperature in this study.The current density also had a greater influence on the porosity, especially in the low current zone.However, the size of the etching tunnels changed rarely with the current density increasing. Thepitting corrosion process was phenomenal, and lots of etching tunnels combined, as the currentdensity reached0.4A/cm². Hence,0.3A/cm²was selected as the corrosion current density in thisstudy. By analyzing the Tafel curve, the ΔE, the difference between the Epit （pitting potential） andEcorr （self-corrosion potential） reflected the difficulty of the pitting corrosion on the entire surfaceof aluminum foils.（4） With the assistance of the ultrasonic treatment, the pitting corrosion and the etching tunnelsgrowth were also improved. The exchange rate of the solution outside and inside the holes was fasterby utilizing the ultrasonic treatment, and the process of the generated AlCl₃and Al₂（SO₄）₃reachingto the saturated concentration was delayed. So the tunnels continued to grow, and the length of thetunnels became longer. Also, the density of the etching tunnels increased with the assistance ofadding corrosion inhibitors. The distribution of tunnels was improved with addition of the corrosioninhibitors, and the reason for this was similar to the effect of SO₄^2-ions.（5） To some extent, nano templates could control the pit initiation sites on aluminum foils. Thenovel template of the porous alumina proposed in this paper could obviously control the pits alongthe combined hole to form, and the fabrication of this novel template was easy. The monolayerself-assembly nano structures of polystyrene spheres could also control pits occurred on the surfaceof the aluminum foil. However, this control process just existed in the first few seconds duringcorrosion, because the PS easily falled off from the aluminum substrate. The preloading printingprocess by the nano template had no beneficial effect on the distribution of the etching pits. UV nanolithography was also introduced to control the distribution of pitting. To some extent, this technologycould control the pits formation on aluminum surface. However, there were a great amount ofnon-through holes due to the immature technology of the UV nano lithography on the aluminumfoils. Therefore, the experiment based on this template was hard to carry out. This study willcontinue to be carried out in our future work.