Anodizing is a surface treatment to metal such as aluminum and titanium to provide an oxide layer. Anodizing is carried out by means of an electrolytic treatment. The oxide layer is hard, porous and durable. Thereafter the corrosion resistance can be improved by closing off the pores (sealing) .
Method of treatment.
Various electrolytes can be used in order to carry out an anodising process. Depending on the material for anodizing, and the desired properties of the anodic layer an electrolyte and treatment type will be selected.The US military standard MIL-A-8625 is a global distinction of three types of processes created for anodizing of aluminum (alloys):
- Chromic acid anodizing
- Sulfuric acid anodizing
- Hard anodizing
All aluminum alloys can have a thin surface layer by means of anodizing (<25μm), but varies their suitability for hard coating.Hard Coating is anodising, but in a special way. The coating may have a thickness of 50μm or more and has black to gray-black color.
Method of treatment.
The anodising process consists of the following process steps;
Clean. At first the metal to be covered is cleaned to remove the dirt and the natural oxide layer.
Any reduction of the thickness by etching with an acid, in order eventually to be able to come out of the original size, the so-called “dimensionally stable anodizing ‘.
The actual anodising. To increase the layer of an oxide crystal by means of direct current, by placing the metal to be covered in a bath containing sulfuric acid or chromic acid wherein the metal as an anode is connected (a form of electrolysis), hence the designation anodizing. During or after a step of growing the porous crystals can be colored with different colorants (pigments).
Sealing (sealing) of the porous crystals by steam or boiling water.
At hard anodising, type III anodizing, coating thickness standard 25 to 50 microns. A hard-anodized layer is to bring naturally darker colored and more difficult in a specific color.
- In the construction industry, in order to protect aluminum window frames, against corrosion.
- As electrical insulating layer.
- As a black layer on cold plates, so the heat is increased.
- The bicycle and automobile construction to beautify the appearance, to protect it against corrosion, and in the case of front forks, wheels and dampers: reduce friction.
- In mechanical engineering for improved pressure load and wear properties. It also can be made in the anodic layer teflon-like materials in place of colorant. As a result, the friction decreases and creates a non-adhesive layer.
- In the printing technique for the wet-ability of offset printing plates and the application of a light-sensitive layer.
- The lacquer technique as adhesive for paints
- As decoration: to beautify the appearance, usually with colors. The aluminum oxide is colorless and transparent as glass. Lighter colors make additional use of this transparency resulting translucent metallic colors.