The Oxidation Process for Anodizing Aluminium Plate of Aluminium Profile

1.the principle of oxidation film generated from anodizing aluminium plate

　　The process that places the aluminium plate as an anode in electrolyte solution and electrolytically makes an alumina thin film on its surface,is called the anodizing treatment of aluminium plate.The cathode in the device is a material with high chemical stability in electrolytic solution,such as lead,stainless steel and aluminum etc.In essence,the principle of aluminium anodizing is the theory of water electrolysis.When the current passes,the hydrogen is released on the cathode,while on the anode,the precipitated oxygen is not only the molecular oxygen,but also the atomic oxygen(O)and the ionic oxygen,usually expressed as molecular oxygen in the reaction.Anhydrous alumina films is generated from aluminum oxidized by the oxygen precipitated on the anode, which does not act entirely with aluminum and some of them are precipitated as a gaseous form.  

２.The selection for oxidation electrolytic solution on anodizing aluminium plate

　　A precondition for the generation of anodizing films is that the oxidation film can be dissolved by electrolyte solution.However,not all the anodic oxidation in the dissolved electrolyte solution can produce oxidation film or the generated oxidation film with same properties.

３.Oxidation types of anodizing aluminium plate 

　　Anodizing are divided into DC anodizing,AC anodizing and pulse current anodizing according to the currents.Partition based on electrolytes are natural coloring anodizing of sulfuric acid,oxalic acid,chromic acid,mixed acid and sulfonic organic acid as the main solution.Division on the basis of properties of film layer are ordinary film,hard film(thick film),porcelain-like film,bright decorative layer,semi-conductive barrier layer and other anodic oxidation.Among these,the method of DC sulfuric acid anodizing is the most common application.

４.structures and properties for oxidation film of anodizing aluminium plate

　　Anodizing film consists of two layers,the porous thick outer layer  comes from the dense inner layer with dielectric property,the latter is called a barrier layer(also an active layer).Observation through an electronic microscope,the vertical and horizontal surface of the film layer almost has tubular holes perpendicular to the metal surface,which penetrate the outer layer of the film and arrive directly at a barrier layer of oxide film and metal interface.A honeycomb hexagonal prism formed by dense alumina with pores is called unit cell,and the entire film layer is composed of countless such unit cells.The thin and dense barrier layer with high hardness consists of an anhydrous alumina and can block current.The barrier layer’s thickness is about 0.03~0.05μm, which is 0.5% ~2.0% of the total film’s thickness.The porous outer layer of the oxidation film is mainly made up of amorphous alumina and a small amount of hydrated alumina, and also contains cation of electrolytes.When the electrolyte is sulfuric acid,normally,the sulfate content is 13%~17% in the film layer.Most of excellent features of the oxidation film are determined by the thickness and porosity of the porous outer layer,which are closely related to the anodizing conditions.

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