September 2011 Archives

There are other types of functional alumite in addition to the ones explained earlier:

(1) Heat-absorbing alumite

The internal and external surfaces of the inner pot of an electric rice cooker are coated with fluororesin and treated with alumite, respectively. The inner pot of a rice cooker with the external surface blackened by secondary electrolysis of tin or nickel heats 10-percent faster and hence is more energy-efficient than rice cookers coated with silver-white alumite.
This is because an electrolytically blackened alumite film absorbs heat faster than a silver-white alumite film. The same principle applies to kettles consisting of a body fastened to a bottom plate electrolytically blackened for improvement of thermal efficiency.

(2) Thermal radiating

Aluminum heat sinks dissipate heat from electronic parts, such as power transistors. Conventionally, aluminum heat sinks are manufactured using aluminum extrusions with the sulfuric acid anodic oxide film dyeing blackend. For sufficient blackening, the sulfuric acid anodic oxide film needs to be 10 μm thick or thicker, but at the expense of heat radiation.
Therefore, secondary electrolytic coloring is used to blacken sulfuric acid anodic oxide films that are several micrometers thick. Metals such as nickel and tin are used for the blackening.

(3) Selective heat-absorbing alumite

Solar collectors and water heaters use inexhaustible solar energy to heat water and must be able to absorb solar heat efficiently while minimizing heat loss from the heated water.
This kind of thermal function is called selective heat absorption. Electrolytically blackened alumite films meet the requirement; they efficiently absorb solar thermal radiation with a wavelength of 0.2 to 1 μm while emitting minimal thermal radiation with a wavelength of 1 μm or more from the heated water.

(4) Insulating alumite

An alumite film is a good electric insulating material with a resistivity of 1012 to 1014 Ω. As such, alumite films are widely used in electronic parts.
The electrical properties of an alumite film, however, vary significantly depending on the type of electrolytic bath used, the electrolytic conditions applied, the type of the alumite film formed, and the finishing treatments such as sealing. Therefore, extreme care is necessary in the use of insulating alumite.

(5) Conductive alumite

A sulfuric acid anodic oxide film shows a significant decrease in electrical resistance when the pores are filled from the bottom to the top with highly conductive metal such as silver by secondary electrolysis. While a conventional sulfuric acid anodic oxide film has an electrical resistance of 1 × 108 Ωcm, a silver deposited film has an electrical resistance as low as 1 × 103 Ωcm.
Conductive alumite films are useful in preventing static electricity or electromagnetic interference shielding.

(6) Other

Alumite is also used for antivacterial, deodorant, catalytic, and other effects.

Non-magnetic Aluminum products can be made magnetic by filling the pores of the anodizing layer created on the surfaces with magnetic materials such as iron, nickel, cobalt or their alloys.

As the magnetic retention power is influenced by the size of the magnetic material particles as clearly shown in [Fig.2], it is said that the data storage capacity can be increased by enlarging the surface pore diameter by an enlarging process.

[Fig.3] shows the magnetic property of a coating with added magnetism by secondary electrolysis.

The mechanical parts used in the sliders are usually applied with lubricating oils. Under the circumstances where lubricating oils cannot be used, solid lubricants are required instead. Self-lubricating alumite has a film with pores in which one of the traditional solid lubricants, like graphite, molybdenum disulfide, or Teflon resin, is directly produced.

(1) Secondary electrolytic tin as lubricant

An alumite film formed with a sulfuric acid bath and then subjected to a secondary electrolytic tin bath will have an extremely low wear coefficient. Table 1 shows the results of such a test. As is clear from the table, the films with only their pores filled with tin showed increases in the wear coefficient proportional to the number of times the pores experienced friction. Meanwhile, the film with pores over deposited with tin showed a constant wear coefficient. Thus, an anodic oxide film formed on the aluminum, a relatively soft metal, and further coated with a thin film of soft metal such as tin or cadmium provides wear resistance under low loads.

[Table 1] Wear coefficients of tin-deposited films *

*Load of 99 g, steel ball diameter of 5 mm, stroke of 10 mm, and speed of 3.9 mm/sec

(2) Secondary electrolytic deposition of MoS₂

An anodic oxide film subjected to a secondary electrolytic bath consisting mainly of ammonium molybdate is provided with pores filled with directly produced highly lubricating molybdenum disulfide (MoS₂).

(3) Electrophoretic Teflon particles

When immersed in a solution containing dispersed particles (0.5 to 5 μm) of Teflon (tetrafluoroethylene) recognized for excellent lubricity and non-adhesiveness (resistance to adhesion), an anodic oxide film-covered work piece will become an anode for electrophoresis so that the pores in the alumite will be filled with negatively charged Teflon particles. This technique is well known as the Tufram process.

(4) Wear coefficients of anodic oxide films

Fig. 1 is a summary of the wear coefficients of the anodic oxide films described above:

Now, I will describe the characteristics of hard alumite.

(1) Bath temperature and film hardness

The typical hardness of the films produced at different temperatures of various electrolytic baths is shown in Fig. 1.

The graph clearly reveals that the film hardness varies significantly depending on temperature and types of electrolytic baths. As a result, the method in which the electrolytes of sulfuric acid and organic acid are industrially used (mixed acid method) and electrolysis is performed at low temperatures close to 0°C is in the mainstream.

In addition, the properties of the anodic oxide film are significantly affected by the composition, heat treatment, working history of the raw material, and the aluminum alloy. Therefore, even if electrolysis is performed using the same electrolyte and under the same conditions, a different raw material cannot provide a film with the same properties. The relationship between the material and film hardness is shown in Fig. 2. This graph is based on periodic reverse electrolyzing developed by Keigo Okubo.

(2) Hardness of film cross-section

Fig. 3 shows the hardness distribution of the hard film cross-sections. As the film thickness increases, the initially produced film surface in contact with the electrolyte is provided with the porous property due to dissolution by electrolytes, resulting in lower hardness.

(3) Wear resistance

The hard alumite film has high hardness and excellent wear resistance. Table 1 shows the results of tests conducted using a Taber abrasion tester. It was found that the hard alumite film has wear resistance more than 32 times that of raw aluminum and is better than hard chrome plating.

[Table 1] Comparison of wear resistance of various materials

(Amount of wear after 20,000 revolutions, with wear wheel CS-17 subjected to loads of 1 kg)