July 2011 Archives

(1) Basic process of Anodic Oxidation

Basic process of Anodic Oxidation is as follows.

(2) Anodize process steps for various applications

The following are anodizing steps based on various applications. (Water cleaning omitted)

Construction Material (sash, spandrel, partition, door, fence, etc)

Industrial Products (Home appliance parts, heat exchangers, machine parts, etc.)

Cooking utensils (pots, kettles, warmers, etc.)

Ornamental products (automotive decoration parts, general decorative items, mirrors, name plates, camera parts, etc.)

The color of anodize coatings vary depending on the electrolytic process conditions such as aluminum alloy type, electrolyte bath, and power supply waveform. The [Table 1] and [Table 2] show electrolytic colorization under a DC power supply, and these colors will vary further by the coating thickness. Thicker coatings will have darker colors.

[Table 1] Natural Colorization of Anodize Coatings (Rolled material)

[Table 2] Natural Colorization of Anodize Coatings (Cast / Die-cast)

Therefore, the following must be considered when designing products with anodizing.

1. When selecting an aluminum alloy material, confirm that a desired color can be obtained by anodizing process.

2. When useable aluminum alloy is limited due to manufacturing method and strength for usage, and such alloy has its distinctive anodized color, use dying or electrolytic coloring to obtain the desired color.

3. When dark colorization is unavoidable due to specific condition such as applying a hard anodizing on some specific alloys, have the clients understand that the colors cannot be changed in order to preserve the wear resistant property of the coating.

Fig.1 shows a cross section schematic illustration of an anodizing layer.

(1) Barrier Layer

An electro-conductive coating film that forms during the early stage of the electrolytic process. This layer supports the growth of a porous layer that forms on the top. This is where a porous film is formed by a reaction of electrolytically dissolved aluminum and oxygen at the bottom of the barrier layer. As the porous layer film grows, the bottom portion of the barrier layer migrates lower while the thickness remaining the same. The thickness is approximately 20nm.

(2) Porous Layer

The anodizing layer is an aggregation of this porous layer. It is a collection of hexagonal layer cells with a centrally located micropore, viewed from the top. The diameter of the cells is 100~300nm and the diameter of the micropores is 10~20nm, depending on the condition of the electrolytic process.
Dye Colored Anodizing is accomplished by applying the coloring dye at the top portion of the micropores, and Electrolytically Colored Anodizing is accomplished by depositing metal or metal oxides at the bottom portion of the micropores.

Anodizing is also called Alumite processing for the name was a registered trade name for the process by Riken Chemical Labs a long time ago. Anodizing process and layer properties will be discussed below.

(1) Process

Refer to [Fig.1] below. The process uses the subject aluminum object as the anode (+ pole) and a carbon electrode as the opposing pole, and performs an electrolysis (applies a DC current) submerged in electrolyte baths. The surface aluminum of the subject dissolves and binds with oxygen to create a aluminum oxide surface layer. This layer is called anodic oxide layer (anodized layer).

(2) Layer Structure

When a acid solution is electrolysis processed, the anode normally emits oxygen but not in a case of aluminum anodes. The oxygen from the aluminum anode immediately binds before gasifying with dissolved surface layer metal and forms an aluminum oxide. The layer forming process is shown in [Fig.2] below.

A barrier layer forms during the initial phase of the electrolysis advancing to a formation of porous layer. As the process progresses, this porous layer grows to a certain thickness. As can be seen on the diagram, an anodized layer is made up of a electrically conductive but non-growing barrier layer, and isolating but growing porous layer.

Aluminum alloy materials can be classified into wrought alloys such as plates, bars, rods, and wire, and cast/die-cast alloys for sand/mold casting. They all have various characteristics and are designated by four digit JIS numbers*. The alloys are assigned 1000 class numbers and the lower two digits denote the associated information. For example, 99.7% aluminum is A1070 and AL-Mg alloy is A5052.

(1) Wrought Material

[Table 1] Wrought Aluminum Alloys and Characteristics

(2) Cast Aluminum Alloys

[Table 2] Types of Cast Aluminum Alloys

(3) Die-casting Aluminum Alloys

[Table 3] Die-casting Aluminum Alloys