Anodizing Process and Advantages of Aluminum Plate
1. General Principle of Formation of Oxidation Film on Anodic Alumina Plate:
The process of forming alumina film on the surface of aluminium sheet by electrolysis is called anodic oxidation treatment of aluminium sheet. The cathode in the device is a material with high chemical stability in electrolytic solution, such as lead, stainless steel, aluminum, etc. The principle of aluminum anodization is essentially the principle of water electrolysis. When the current passes through, hydrogen is emitted on the cathode; on the anode, the oxygen precipitated is not only molecular oxygen, but also atomic oxygen (O) and ionic oxygen, usually expressed as molecular oxygen in the reaction. Aluminum, as an anode, is oxidized by the oxygen precipitated on it to form an anhydrous alumina film. The oxygen generated does not all interact with aluminium, and part of it is precipitated in the form of gases.
2. Selection of electrolyte solution for anodic alumina plate oxidation:
A prerequisite for the growth of anodic oxidation film is that the electrolyte should dissolve the oxide film. But this does not mean that the anodic oxidation in all electrolytes with dissolution can produce oxide film or the oxide film formed has the same properties.
3. Types of oxidation of anodic alumina plates:
Anodic oxidation can be divided into direct current anodic oxidation, alternating current anodic oxidation and pulse current anodic oxidation. According to the electrolyte, there are sulfuric acid, oxalic acid, chromic acid, mixed acid and natural colored anodic oxidation with sulfonated organic acid as the main solution. According to the nature of the film, there are: ordinary film, hard film (thick film), ceramic film, bright modification layer, semiconductor barrier layer and other anodic oxidation. The commonly used anodizing methods and process conditions for aluminium and its alloys are shown in Table - 5. Among them, direct current sulfuric acid anodization is the most widely used method.
4. Structure and properties of anodic alumina oxide film:
Anodic oxide film consists of two layers. The porous thick outer layer grows on the dense inner layer with dielectric properties. The latter is called the barrier layer (also known as the active layer). Electron microscopic observation showed that almost all the longitudinal and transverse surfaces of the film presented tubular holes perpendicular to the metal surface, which penetrated through the outer layer of the film until the barrier layer between the oxide film and the metal interface. A honeycomb hexahedron consisting of dense alumina around the main axis of each pore is called crystal cell. The whole film is composed of numerous such cells. The barrier layer is composed of anhydrous alumina, which is thin and compact, has high hardness and prevents current from passing through. The thickness of the barrier layer is about 0.03-0.05 um, which is 0.5-2.0% of the total film. The porous outer layer of the oxide film is mainly composed of amorphous alumina and a small amount of hydrated alumina. In addition, it contains cations of electrolyte. When the electrolyte is sulfuric acid, the sulfate content in the membrane is 13% - 17% under normal conditions. Most of the excellent properties of the oxide film are determined by the thickness and porosity of the porous outer layer, which are closely related to the anodic oxidation conditions.
(1) Good processability: the positive alumina board has strong decorative, moderate hardness, easy bending and forming, continuous high-speed stamping, convenient for direct processing into products, no need for complex surface treatment, greatly shorten the production cycle of products and reduce production costs.
(2) Weather resistance is good: standard thickness of oxide film (3 micron) of anodic alumina plate indoor use for a long time does not change color, corrosion, oxidation, rust. Anodic alumina sheets with a thicker oxide film (10 micron) can be used outdoors, and can be exposed to the sun for a long time without discoloration.
(3) Metal sensitivity: Anodized aluminium sheet has high surface hardness, gem grade, good scratch resistance, no paint covering, retaining the metal color of aluminium sheet, highlighting modern metal sensitivity, improving product grade and added value.
(4) High fire resistance: pure metal products, no paint and any chemical substances on the surface, 600 degrees high temperature, no combustion, no toxic gases, in line with the requirements of fire protection and environmental protection.
(5) Strong anti-fouling: no fingerprints, no stains, easy to clean, no corrosion spots.
(6) Strong applicability: wide application, suitable for metal aluminium ceiling, curtain wall, aluminium-plastic panel, fire-proof panel, honeycomb aluminium panel, aluminium veneer, electrical panel, cabinet panel, furniture panel, etc.