The Methods for Anodizing of Aluminum
Surface Preparation
    No matter what method chosen to process aluminum and products, there must be dirt and defects to varying degrees on the surface. Such as dust, metal oxide (natural or aluminum oxide film which formed under high temperature), residual oil, bitumen, manual handing fingerprints (the major ingredient are fatty acid and nitrogen components), welding flux and salt corrosion, metal burrs, slight abrade, etc. So it is necessary to clean the surface by the method of chemical and physical before the oxidation treatment, so that exposed pure metal substrate and oxidized successfully, thus the artificial membrane that combined firmly with matrix and met the demands of color and thickness of super performance which has the best corrosion resistance and abrasion resistance can be gotten.
First: Degreasing
The methods of surface degreasing over aluminum and aluminum included organic solvent d
egreasing, surfactants skim, alkaline solution degreasing, acid solution degreasing, electrolytic degreasing, and emulsifying degreasing. The most effective is alkaline solution degreasing, especially thermal degreasing of sodium hydroxide solution among these methods.
Second: Alkali Etching agent
Alkali Etching agent is a process that aluminum products’ surface is cleaned in sodium hydroxide solution additional other material or not, it usually known as alkali corrosion or alkali wash, with the effect of additional treatment after surface degreasing, in order to further clean up grease and native oxide layer and slight scratch on the surface, thereby the pure metallic matrix is exposed and gets a higher quality of anodic film. In addition, the erosion surface of smoothness, satin, or different states of luster can be obtained by changing operation conditions such as the solution, temperature, handing time and others.
Third: Neutralization and aqueous cleaning
reaction in the shaft
The aluminum products that have been eroded in hot alkali solution have to acid leach with the purpose of dissolving the ash that has adhered to the surface after erosion is insoluble in the cold or hot water and residual alkali liquor, so that the pure metallic matrix is exposed. This process is called neutralization, luster or a bright dipping treatment.
Anodizing
Natural aluminum oxide because of soft and thin and poor corrosion resistance on the surface of products cannot be effective protective layer, let alone suit for dyes. The artificial oxidation film is mainly applied to chemical oxidation and anodic oxidation. Chemical oxidation is a process that parts of aluminum matrix metal reacts in weak alkaline or weak acid solution, and thickens natural oxide film or reacts other passivation membrane. The commonly used are chromate membrane and phosphoric acid membrane. They are both thin and good adsorption, dye and hole sealing can be performed.
1.The general principle of anodic oxidation film
Put aluminum or aluminum products as anode in electrolyte solution where the aluminum oxide film is generated on the surface using electrolytic action, this process is called as anodic oxidation film of aluminum and aluminum alloy. The material of high chemical stability like lead, stainless steel, aluminum and so on is as cathode in the electrolyte solution. The principle of aluminum anodic oxidation is essentially the principle of water electrolysis. When the electric current passes through, it releases hydrogen at the cathode; not only the molecular oxygen, also the atomic oxygen and ion oxygen (usually in the reaction with molecular oxygen) at the anode. The aluminum at the anode will be oxidized by generated oxygen and form anhydrous alumina film, but the generated oxygen does not all react with aluminum; part of it gives off in gaseous.
2.Choice of anodic oxidation electrolytic solution
The precondition of anodic oxidation film generated is that can dissolve easily in electrolyte. But this is not to say that in the presence of the dissolution of the electrolyte in the anodic oxidation can generate the same as the nature of the oxide film.
3.The types of anodic oxidation
As for the electric current, anodic oxidation is divided into direct current, alternating current, anodic oxidation, and pulse current; as for the electrolyte, it can be divided into sulfuric acid, oxalic acid, chromic acid, mixed acid and organic sulfonic acid mainly natural solution of colored anodic oxidation. According to the film's temper is divided into ordinary film, hard film (film), ceramic membrane, bright modified barrier layer of anodic oxidation layer, a semiconductor effect. Among them, the application of the method of the direct current electric sulfuric acid is the most common.

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