Aluminum and its alloys it will be oxidized in air is known , the aluminum surface is naturally formed oxide film of non- crystalline , it will make the surface of the aluminum metal lose their luster , though this layer of the natural oxide film causes the aluminum metal surface slightly passivated , but very thin , about 4 ~ 5nm, and the porosity , the mechanical properties are poor, it can not effectively prevent the air of various media further corrosion of the aluminum metal .
After anodic oxidation treatment , the aluminum and its alloys can be obtained on the surface layer is much more than the natural oxide film thickness of the dense layer ( from tens to hundreds of micrometers or microns). This layer and then through the artificial oxide film sealing treatment , non- crystalline form of the oxide film into a crystalline oxide film , pores can be closed, so the surface luster of the metal over the long term , corrosion resistance , mechanical strength are improved , dyed also receive decorative appearance. Since aluminum and its alloy products after anodizing has many features , so aluminum anodizing surface treatment of aluminum products in a wider application . In industrial applications can be divided into the following categories .
( 1) to prevent corrosion products : the anodic oxidation film obtained after proper sealing treatment in the atmosphere has good stability . Whether from the sulfuric acid solution , oxalic acid or chromic acid solution obtained in the normal process of the oxide film , which is a very good corrosion resistance , such as household aluminum pan , pot, washing interior and so on. Chromic acid oxidation method is particularly suitable for aluminum welding parts and riveting pieces .
( 2 ) protection - decoration : high transparency in the obtained oxide film , the oxide film can be adsorbed with a variety of organic dyes or inorganic pigments characteristics , the oxide film obtained bright light of various colors and patterns , and in recent years not the emergence of new technology , such as less oxidation times a colored fireworks pattern , wood grain pattern , oxidation offset transfer printing , ceramic oxide , so that the aluminum look more beautiful looking, this layer of color film is both a decorative layer , but also resists , such as lighters , fountain pens and crafts.
( 3 ) as the hard wear layer : in sulfuric acid or oxalic acid solution can be adjusted by the anodization process conditions , to obtain thick and hard layer , the use of film holes and to store the selective absorption properties of the fuel , used effectively to work under conditions of friction state , both lubrication and wear characteristics , such as cars and tractors engine cylinder , piston .
( 4 ) as an electrical insulating layer : Anodized Aluminum and its alloys oxide film having a large resistance characteristics, and the resistance is proportional to the thickness of the film , this feature as an electrical insulating property has a certain practical significance , can be used as capacitors the dielectric can also be used for alumina leather outer cable sheath , its surface layer for insulation instead of rubber and plastic foreskin foreskin is more popular in foreign countries , when the thickness of 27.5μm , the breakdown voltage of 441V. The use of phenolic resin as pore filling , which can increase wear voltage 2 times in the oxalic acid solution , when the film thickness increases , the availability of resistance 200Ω, the breakdown voltage of 980V for quality insulating layer , of course, to use this feature except for the conductor , but also can be used for other electrical appliances and so on.
( 5 ) for painting , plating the bottom : the oxide layer having a porous adsorption capacity and good , and the film and the organic film with good adhesion , the underlying paint can be used . Of phosphoric acid anodized aluminum plating film can be used as the bottom .
( 6 ) used in modern architecture : Since the introduction of aluminum electrolytic coloring process , in recent years, the domestic aspects of architectural aluminum electrolytic coloring applied to the increasing , it is not only suitable for architectural bronze shades , black, red , with its excellent abrasion resistance conventional oxidation in general , particularly worth mentioning is very good fastness , more than 20 years in the atmosphere Needless sun dew color , which is incomparable with oxidative coloring . Oxidized aluminum electrolytic coloring method can be used in building windows and doors not only to do with it shop counters , shelves, etc. are also particularly suitable.
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