Alumilite process
Aluminum or aluminum alloy products as the anode, placed in the electrolyte solution for electricity treatment, the use of electrolytic action to form alumina film on the surface of the process, known as aluminum and aluminum alloy anodizing treatment. After anodizing treatment, aluminum surface can form several microns - hundreds of microns of oxide film. Compared with the natural oxide film of aluminum alloy, its corrosion resistance, wear resistance and decoration have been improved significantly.
Chinese name
Aluminium anodizing
Foreign name
Aluminium natural anodised
Original reason
Principles of water electrolysis
Cathode reaction
H2:2H + + 2e → H2
Anodic reaction
4OH-4e → 2H2O + O2
directory
1 Basic Information
2-anode structure
3 Sealing process
4 sacrificial anode
Basic information editing
The principle of anodizing aluminum is essentially the principle of hydroelectrolysis. When an electric current passes through, the following reactions occur:
At the cathode, H2 is emitted in the following reaction: 2H + + 2e → H2
On the anode, 4OH-4e → 2H2O + O2, the oxygen precipitated is not only molecular oxygen (O2), but also includes atomic oxygen (O) and ionic oxygen (O-2), which is usually expressed as molecular oxygen in the reaction.
The aluminum as the anode is oxidized by the oxygen precipitated on it to form an anhydrous Al2O3 film: 2Al + 3[O] = Al2O3 + 1675.7KJ It should be noted that not all of the generated oxygen interacts with the aluminum, and some of it precipitates out in gaseous form.
Types of anodizing Anodizing has long been widely used in industry. There are many methods under different names, which can be summarized as follows:
According to the current type, it can be divided into direct current anodizing, alternating current anodizing, and pulse current anodizing, which can shorten the production time to reach the required thickness, and the film is thick, uniform and dense, and the corrosion resistance is significantly improved.
Divided by electrolyte: sulfuric acid, oxalic acid, chromic acid, mixed acid and organic sulfonic acid solution of natural coloring anodizing.
According to the properties of the film, there are: ordinary film, hard film (thick film), porcelain film, bright modification layer, semiconductor barrier layer and other anodic oxidation.
Direct current sulfuric acid anodizing process is the most common application, because it is suitable for aluminum and most aluminum alloy anodizing treatment; The film layer is thick, hard and wear-resistant, and better corrosion resistance can be obtained after sealing. The film layer is colorless and transparent, strong adsorption ability and easy to stain; Low processing voltage, less power consumption; The processing process does not need to change the voltage cycle, which is conducive to continuous production and practical operation automation; Sulfuric acid is less harmful to human body than chromic acid, wide supply, low price advantages.
Come nearly ten years, the construction industry of our country gradually USES ALUMINUM doors AND Windows and OTHER decorative ALUMINUM MATERIAL, their surface treatment production line IS this method.
Anode structure editing
1) Structure of anodic oxide film Anodic oxide film is composed of two layers, the outer layer is called porous layer, thick, loose porous, low resistance. The inner layer, called the barrier layer (also known as the active layer), is thin, dense and has high resistance. The porous outer layer grows on top of a dense inner layer with dielectric properties. In general, the anodized film is an array of hexagonal columns, each of which has a star-shaped hole filled with solution, like a honeycomb structure, with a wall twice as thick as the pore diameter.
(1) Barrier layer The barrier layer is composed of anhydrous AI2O3, thin and dense, with high hardness and the effect of preventing the passage of current. (2) The porous outer oxide film is mainly composed of amorphous AI2O3 and a small amount of r-AI2O3.H2O, as well as electrolyte anions.
The pore size of the oxide film is between 100nm~200nm, the thickness of the oxide film is about 10 microns, the porosity is about 20%, and the pore distance is between 300~500nm. The cross-section of the oxide film shows that the oxide film hole is basically tubular structure and the dissolution reaction of the oxide film is basically at the bottom of the hole. And the general sulfuric acid DC anodized film aperture is about 20nm, if it is 12 micron oxide film, it is how deep the fine tubular structure ah! Assuming this is a 1m diameter well, it will be 600m deep.
Most of the excellent characteristics of oxide film, such as corrosion resistance, wear resistance, adsorption, insulation and other properties are determined by the thickness and porosity of the porous outer layer, but the two are closely related to the anodizing conditions, so the anodizing conditions can be changed to meet the requirements of different use of the film. Film thickness is a very important performance indicator of anodized products, and its value directly affects the corrosion resistance, wear resistance, insulation and chemical coloring ability of the film. In the conventional anodizing process, the film thickens over time. After reaching the maximum thickness, they gradually become thinner with the extension of processing time. Some alloys, such as AI-Mg and AI-Mg-Zn alloys, are particularly obvious. Therefore, the oxidation time is generally controlled within the maximum film thickness time.
2) Properties and applications of anodic oxide film Anodic oxide film has higher hardness and wear resistance, strong adhesion capacity, strong adsorption capacity, good corrosion resistance and electrical insulation and high thermal insulation. Because of these special properties, it has been widely used in various aspects.
The main uses are: (1) improve the wear resistance, corrosion resistance, weather corrosion resistance of parts. (2) The transparent film generated by oxidation can be colored into a variety of color films. (3) as a capacitor dielectric film. (4) Improve the binding force with organic coating. For coating base. (5) for electroplating, enamel bottom. (6) Other applications being developed, such as solar absorption panels, ultra-high hardness films, dry lubrication films, catalyst films, nanowires, deposition of magnetic alloys in porous films as memory elements.
Closed process editing
Aluminum oxide film is porous film, no matter whether there is coloring treatment, before being put into use should be closed treatment, so as to improve its corrosion resistance and weather resistance. There are three types of treatment methods, namely high temperature hydration reaction sealing, inorganic salt sealing and organic sealing.
(1) High temperature water closure
This method uses the hydration reaction of aluminum oxide film with water to change amorphous plasma film into hydrated crystalline film:
Hydration reaction can be carried out at room temperature and high temperature, but at high temperature, especially at the boiling point, the hydration crystallization film generated is a very stable and irreversible crystallization film, therefore, the most commonly used aluminum oxide film closed treatment is boiling water method or steam method.
(2) inorganic salt seal
Inorganic salt method can improve the fastness of organic dyes, so it is commonly used in chemical coloring method.
① Acetate method
② silicate method
(3) organic closure method
This is to dip oil, paint or paint aluminum oxide film, due to the high cost and increase the process flow, so it is not used, more or the above two types of methods, and the first kind of high temperature water method as the mainstream.
Sacrificial anode editing
Aluminum alloy sacrificial anode is suitable for cathodic protection against metal corrosion of ships, machinery and equipment, ocean engineering and harbor facilities as well as pipelines and cables in sea mud.