Corrosion and formation of anodized foils for aluminum electrolytic capacitors

The chemical symbol of aluminum is Al, its atomic weight is 27.0, and its atomic number is 13, which means it has 13 electrons. The number of electrons in its outer shell is 3, so it can lose 3 electrons to become a trivalent cation during ionization. Metal aluminum is a white light metal with light-specific gravity, easy processing, good electrical and thermal conductivity, and strong corrosion resistance.

Corrosion of aluminum foil

The biggest advantage of aluminum electrolytic capacitors is the large capacitance per unit volume. This is due to the increased surface area of the anode foils used in aluminum electrolytic capacitors that have undergone chemical and electrochemical etching treatments. Due to the inclusion of metal elements such as Cu, Fe, and Pb in the aluminum foil with higher electrode potential than Al, its corrosion behavior in electrolyte solutions such as hydrochloric acid;

Anode area: Al-3e=A13+

A13++nH2O=A13+nH2O

Metal/Solution Interface A13++3CI-=A1C13

Cathode: 2H++2e=H2

There are various methods to expand the surface area of aluminum foil, such as chemical etching, DC etching, AC-DC etching, AC etching, displacement reaction, etc.

1. Chemical corrosion method

Due to the amphoteric nature of aluminum, it can be corroded in both acids and lyes. Acid has a strong erosive effect, especially hydrochloric acid and its salts, which can erode the defects on the surface of aluminum foil and form the starting point of corrosion. In terms of its corrosion mechanism, it mainly includes solubility corrosion and local micro-battery corrosion; aluminum foil In the lye, its corrosion form is mainly planar corrosion.

2. DC corrosion method

The DC etching method is to use the aluminum foil as the positive electrode of the power supply, and the other conductor as the negative electrode of the power supply, and applies a DC current in an electrolyte such as hydrochloric acid. Select the appropriate corrosion parameters such as current density, solution concentration, solution type, and liquid temperature. These parameters are determined according to the different requirements of the corrosion foil. Therefore, the etching method is still widely used so far. Most of the anode aluminum foil for high voltage and flash lamps use the DC etching method.

DC corrosion is tunnel corrosion, and the factors affecting tunnel corrosion are the composition and temperature of the etching solution, the time of electro-corrosion, the polarization potential or current, and the crystal structure of the surface state of the aluminum foil. Among them, the biggest influence is the polarization voltage or current, the CL content of the corrosion solution, and the corrosion temperature. The level of polarization potential has a significant impact on the pit density, pit growth direction, growth rate, and tunnel length. As the liquid temperature increases, the tunnel formation speed increases, while the tunnel width becomes narrower.

3. AC corrosion method.

The principle of AC corrosion is basically the same as the DC corrosion method, except that when the aluminum foil is in the positive half cycle of AC, the aluminum foil is corroded and does not corrode when it is in the negative half cycle. AC corrosion can produce high-density and uniform micro-etching pits on the surface of aluminum foil to obtain the largest specific surface area of the corroded foil, so it is widely used in low-pressure foil corrosion.

The frequency of the corrosion current also has a great influence. The experiment shows that the increase of the frequency, the decrease of the pitting holes, the increase of the density of the corrosion pits, and the improvement of the uniformity of the distribution of the corrosion holes are beneficial to the increase of the specific volume of the aluminum foil. When the frequency increases to a certain critical value, the pitting hole is easy to merge into a large hole because the pitting hole is too small. As a result, the specific surface area of the aluminum foil decreases, and the specific volume C decreases. As the current density increases, the specific volume increases, but when the current is too large, the formation of corrosion pits is rapid, which easily causes the corrosion film to fall off, resulting in a decrease in the specific volume C.

As the corrosion time prolongs, the etched holes grow fully, but if the corrosion time is too long, the aluminum foil will be thinned and the purpose of enlarging the surface will not be achieved. Therefore, there are many factors affecting the corrosion quality of the aluminum foil, and the formula of the etching solution and the etching solution must be comprehensively considered. Temperature, corrosion time, frequency of power supply and current density, etc. influence the corrosion process parameters. Corrosion is essentially an electrochemical process, and the corrosion of aluminum foil is affected by internal factors related to the aluminum metal itself, such as the properties, composition, structure, and surface state of aluminum, as well as the existence of mechanical deformation and internal stress; the second is related to corrosion. The external factors and the composition, concentration, liquid temperature of the corrosive liquid, the flow state of the corrosive liquid, the magnitude, and frequency of the applied current, etc.

The following factors are closely related to aluminum foil corrosion:

(1) Influence of the purity of aluminum foil

(2) The influence of mechanical deformation and internal stress.

(3) The influence of heat treatment.

(4) The influence of the pH value of the electrolyte solution.

(5) The influence of the composition and concentration of the corrosive liquid.

(6) The influence of temperature and current density of the corrosive solution.

(7) The influence of the length of corrosion time