Aluminum----Anodise Aluminum sheets
I. Introduction to anodised aluminum sheets
Anodised aluminum sheet is to place the aluminum sheet in corresponding electrolyte (such as sulfuric acid, chromic acid, oxalic acid, etc.) as the anode for electrolysis under specific conditions with externally applied current. The aluminum sheet of the anode is oxidized and has an aluminum oxide film formed on the surface; the film is 5-20μm thick, and a hard anodic oxidation film can be up to 60-200μm thick. After anodization, the aluminum sheet achieves greater hardness, better abrasive resistance (up to 250-500 kg/mm2), and good resistance to heat; the hard anodic oxidation film also features a melting point of up to 2320 K, excellent insulativity, high breakdown voltage (up to 2000V), and enhanced resistance to corrosion (it may stand salt spray under ω=0.03NaCl for thousands of hours without being corroded). Additionally, the oxide film is distributed with numerous pores that can absorb various lubricants, thus making the anodised aluminum sheet suitable for manufacturing engine cylinder or other wear-resistant parts; the strong absorbability of the film can also be well utilized to form various bright colors. Still ferrous metals or their alloys (such as aluminum, magnesium their alloys and so on) can also be anodised.
II.Applicable scope of the anodised aluminum sheets
The anodised aluminum sheets are widely used in such fields as mechanical parts, airplane and automobile parts, precise instruments, wireless electrical appliances, building decorations, mechanical housing, illumination lamps, electronic consumables, art wares, household appliances, indoor decorations, sign boards, furniture automobile decorations, etc.
III. Advantages of the anodised aluminum sheet
(1) Good processability: the anodised aluminum sheet is strongly decorative, featuring moderate hardness and easy bending and shaping; it can be processed by continuous pressing at high speeds, and directly made into products without requiring complex surface treatment, thus having greatly reduced the production period and the production cost of products.
(2) Good weatherability: an anodised aluminum sheet with oxide film of standard thickness (3μm) stands colorfast for a long time when it is used indoors, with no corrosion, oxidation or rusting; a thickened anodised aluminum sheet (thickness of 10μm) can be used outdoors with colorfastness under long-time exposure to sunlight.
(3) Prominent metallic features: anodised aluminum sheets may have their hardness on a par with gems and are well resistant to scratches; they have no paint coating on the surface and thus have retained the metallic gloss of aluminum sheets, protruding the modern metallic features and promoting the class and added value of the products.
(4) Good resistance to fire: as the anodised aluminum sheet is purely made of metal without any paint or chemical substance, it neither combusts at a temperature as high as 600 degrees, nor generates any poisonous gas, and thus satisfies the requirements for environmental protection.
(5) Good resistance to dirt: the anodised aluminum sheet retains no finger print, and thus no stains will be left; it can also be easily cleaned, without producing any corroded spot.
(6) Wide applicability: the anodised aluminum sheet is widely used, and applicable to metal aluminum ceiling, curtain wall plate, aluminum composite panel, fireproof board, honeycomb aluminum panel, single aluminum sheet, electric appliance panel, cabinet panel, furniture panel, etc.
IV. Anodization process of aluminum sheets
1. General principle of aluminum sheet anodization generating aluminum oxide film
Anodization of aluminum sheet refers to the process of generating aluminum oxide film on the surface of an aluminum sheet through electrolysis when the aluminum sheet is put in the electrolyte as the anode. In the anodization device, a material with high chemical stability in the electrolyte, such as lead, stainless steel, aluminum, etc., serves as the cathode. The basic principle of aluminum anodization is the electrolysis of water. When the current circulates, hydrogen is released from the cathode, while on the anode are not only oxygen molecules, but also oxygen atoms (O) and oxygen ions deposited (those are often expressed as oxygen molecules in the reaction). The aluminum of the anode is then oxidized by the oxygen deposited on it, forming a water-free aluminum oxide film; meanwhile, not all the oxygen generated reacts with the aluminum, and some is released as gas.
2. Selection of electrolyte for anodization of aluminum sheets
The prerequisite of forming the oxide film on the anode is that the oxide film is soluble in the electrolyte. However, this does not mean that all anodization in an electrolyte with solubility may generate oxide films or oxide films with similar properties.
3. Types of anodization of aluminum sheet
According to the form of current, anodization can be classified into DC anodization, AC anodization and pulse current anodization; according to the types of electrolyte, it can be classified into sulfuric acid anodization, oxalic acid anodization, chromic acid anodization, mixed acid anodization and sulfo organic acid-based natural coloring anodization ; according to the properties of the films, it can also be classified into anodization for regular film, anodization for hard film (thick film), anodization for porcellaneous film, anodization for glossy finish and anodization for semi-conductive barrier layer. Among those, the AC sulfuric acid anodization is the most commonly used.
4. Structure and properties of the oxide film formed during aluminum sheet anodization
The oxide film obtained through anodization consists of two layers. The external layer is thick and porous, which grows on the dense internal layer with dielectric property; the internal layer is called the barrier layer (also known as active layer). When the film is observed under an electronic microscope, both its transverse and longitudinal surfaces appear to have vasiform pores perpendicular to the metal surface; the pores go through the external layer to the barrier layer between the oxide film and the metal surface. Taking a pore as the axle, a honeycomb-shaped six-edge prism is formed by the dense aluminum oxide around, which is called a unit cell. The entire film consists of numerous unit cells. The barrier layer is formed by water-free aluminum oxide that is thin, dense and hard with resistance to passage of current; it is around 0.03-0.05μm thick, accounting for 0.5%-2.0% of the general thickness of the film. The external porous layer of the oxide film is formed by amorphous aluminum oxide, small amount of hydrated alumina and still some cations from the electrolyte. When the electrolyte is sulfuric acid, the content of sulfate in the film is generally around 13%-17%. Most favorable properties of the oxide film are determined by thickness and porosity of the external porous layer, which are closely related to the anodization conditions.
VII. Basic processing procedures of surface treatment are as follows:
Procedures for anodization of aluminum sheets: racking – degreasing – rinsing – alkaline corrosion – rinsing – neutralizing – rinsing - anodizing – rinsing – pore sealing – rinsing – off rack – testing – packing.
Among the procedures, degreasing, alkaline corrosion, neutralizing, anodizing and pore sealing are key to ensure the oxidation quality.