Hard anodizing is a thick film anodizing process, which is a special anodizing surface treatment process for aluminum profiles
. In this process, the thickness of the anodic oxide film can reach about 250 microns, and the microhardness oxide film of 1500kg/mm2 can be obtained on pure aluminum, and the microhardness oxide film of 400-600kg /mm2 can be obtained on aluminum alloy.
Oxide film layer thermal conductivity is very poor, its melting point is 2050 ℃, the resistance coefficient is larger, the closed processing (dipping insulation or paraffin) breakdown voltage is 2000 v, in the atmosphere of high corrosion resistance, high wear resistance, is also a kind of ideal insulating layer, also have good insulation, and combined with the matrix metal very strong, and a series of advantages. Therefore, it is widely used in national defense industry and machine parts manufacturing industry. It is mainly used in aluminum profiles with high wear resistance, heat resistance and good insulation properties. Such as a variety of cylinder as the inner wall, piston, steam plug, cylinder, bearing, aircraft cargo floor, rolling rod and guide rail, hydraulic equipment, steam impeller, leveling machine, gear and cushion parts.
The hard oxidation process is used to replace the traditional hard chromium plating coating. Compared with the hard chromium process, it has the advantages of low cost, solid film binding, plating solution, cleaning waste liquid treatment and so on. However, the disadvantage of this process is that the mechanical fatigue strength of aluminum profiles is affected when the thickness of the film is large.
There are many hard anodic oxidation electrolytic methods, such as sulfuric acid, oxalic acid, propylene glycol, sulfosalicylic acid and other inorganic salts and organic acids. The power supply can be divided into DC, AC and AC-DC superimposed power supply, etc. At present, the following two kinds of hard anodic oxidation are widely used.
1. Direct current method for hard anodic oxidation of sulfuric acid;
2. Ac-dc gravity separation method for oxalic acid hard anodic oxidation.