Influence Of Trace Elements And Pre-treatment On Cube Texture And Pit Corrosion Of High Voltage Aluminum Foils

The electrolytic capacitor aluminum foil with high specific capacitance is a high-tech and high value-added product. It is the main material for capacitors which are the basic devices of the electronic industry. The domestic aluminum foil has high corrosion capacitance in the chromic acid system, but uneven pitting and few etching pits are produced in the environmental of H2SO4 corrosion system, which result in small surface area and low capacitance after forming and etching. In order to promote pitting ability and corrosion homogeneities of aluminum foils, the microstructure and recrystallization texture evolution of high voltage aluminum foils have been investigated using optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), secondary ion mass spectrometer (SIMS), electrochemical technique. Combined with first principle simulation, the influence of heat treatment, surface treatment and the amount and distribution of trace elements on the homogeneity of etching pit has been studied. The main conclusions are as following:(1)The distribution of trace elements in pure aluminum was investigated by first principle simulation. According to the vacancy transportation principle, a model was established to describe the kinetics process of segregation of trace elements on clean surface via atomic movement through vacancies. The effect of doped trace elements on the electrochemical properties of aluminum surface was investigated, and a straightforward equation was suggested to desbribe the surface chemical potentials of metals and the electrochemical dissolution potentials of alloy surfaces. The calculation results provide a reference for the future design of high voltage aluminum foils.(2) The effect of heat treatment on the volume fraction of cube texture, microstructure and the pitting ability was studied. The results indicated that properiety speed of heating-up and rapid cooling at the elevated temperatures led to homogeneous dispersion of trace elements on aluminum foil surface and more corrosion pits that ensure high specific capacitance.(3) The surface treatment was used to improve the uniformity of pitting on the surface of the aluminum foils. The sodium hydroxide pre-treatment before pitting can remove the oxide film on the foils, and the hydrochloric acid pre-treatment can introduce Cl-ion into the surface of foils. NaOH+HCl co-pre-treatment reduces the corrosion potential and increases the corrosion current density. The area and density of the pit etching on the surface were promoted after pre-treatment, and the uniformity of pitting was also improved significantly.(4) The effects of In trace element on the deformation microstructure, the evolution of cold rolling texture and recrystallization texture of the high purity aluminum foils were investigated. The results demonstrated that In element can refine the grains of the aluminum foils and ensure the percentage of recrystallization cube texture in the aluminum foils higher than 95%.(5) The influence of the content and distribution of trace In, the final annealing and surface treatment on the homogeneity of etching pit has been investigated, and the mechanism was explored. The enrichment and the distribution of trace element In on the foil surface can be adjusted by carefully controlling the heat-treatment parameters. The mechanism of pitting behavior of trace In on aluminum surface was similar to that of Pb. Most of the enriched In was found to be near the oxide film/aluminum matrix interface. Since the potential of In is much higher than that of aluminum, a local electrochemical cell was formed between In and Al substrate. Thus the efficiency of corrosion was improved and the etched area became larger. The corrosion pits of the etched aluminum foil became more uniform.(6) A new method without homogenizing treatment of ingot was established to fabricate high voltage aluminum foils with high content of cube texture and excellent pitting corrosion properties. Taking advantage of three-layer-electrolysis high-purity aluminum and partial high-purity aluminum, more than 95%volume fraction of cube texture and 98% corrosion region distributed well can be obtained through special designed process. Moreover, it introduced the gallium into aluminum foils directly. It has the advantages of lower cost, higher efficiency and more economy benefits.