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Anodizing of aluminum

Anodizing of aluminum

To protect aluminum products from corrosion and strengthen the structure of its surface, so-called "oxidation" is used, which creates a thick film on the surface of the product. It can be either chemical oxidation in a solution of chromium or anodizing using anodic polarization of the product in the electrolyte. In other words, anodizing is the process of creating an oxide film on the surface of metals and alloys. The main purpose of this procedure is to reduce the metal's tendency to corrosion, as well as to improve the appearance of the metal product.

The most common technology for anodizing aluminum is so-called sulfuric acid anodizing based on the chemical composition of the anode solution (electrolyte). As a result of the anodizing procedure, a thick anodic coating with pores of different sizes mounts on the aluminum surface. The coating thickness and pore size depend on the concentration of sulfuric acid in the anode electrolyte, the temperature of the anode solution, and the current density that flows through the electrolyte to the aluminum surface.

 By its structure, the anode coating consists of a porous layer and a barrier layer located under it. The thickness of the barrier layer depends on the composition of the electrolyte and technological parameters. When anodizing, the barrier layer is formed first, and its thickness directly depends on the value of the anodizing density. 

After the barrier layer is formed, a porous crystal structure is formed on its outer side. During its formation, the barrier layer first dissolves, and then, as the current value and the temperature increases, the surface layer dissolves to form a porous one.

Pure aluminum of the highest quality is better anodized than alloys with other metals. The appearance of the anode coating and its properties (wear resistance, corrosion resistance, etc.) depend on both the type of aluminum alloy and compliance with the technology in its production. The size, shape, and distribution of intermetallide (consisting of two or more metals) particles also affect the quality of anodizing. The chemical composition of the aluminum alloy is essential in products that require brilliant anodizing, in which case the level of insoluble particles must be possibly low.

The anodizing process consists of three stages:

1. Preparatory stage, during which the aluminum product is mechanically and electrochemically treated. The surface is cleaned, sanded, and degreased. Then the product is placed in an alkaline solution for its etching. The last stage of preparation is the immersion in an acidic solution, where it is clarified, after which the product is thoroughly washed from the acid.

2. The chemical anodizing of the aluminum. To do this, the product is suspended on special brackets and placed in a bath with an electrolyte between two cathodes. Solutions of sulfuric, oxalic, chromic, and sulfosalicylic acids can act as electrolytes, sometimes with the addition of organic acid or salt. Sulfuric acid is the most common electrolyte, but it is not possible to properly process products with small holes or gaps. Chromic acids are better suited for this purpose. Oxalic acid, in turn, creates the best insulation coatings in different colors. Different acid concentrations and current densities give different end product results. Increasing the temperature and lowering the current density gives a soft and porous film. As the temperature decreases and the current density increases, the hardness of the coating rises. The temperature range in the sulfuric acid electrolyte ranges from 0 to 50 degrees Celsius, and the density range -- from 1 to 3 A/dm2. The electrolyte concentration can vary between 10-20% of the volume, depending on the need.

During the anodizing process, the anode cells, including the pores, form a hexagonal structure, which, according to experts, fulfills the principle of minimum energy and does not depend on the type of electrolyte used. The hexagon shape has an energy origin.

The thickness of the anodic coating increases with the duration of anodizing. However, the degree of thickness growth depends on several factors, such as the type of electrolyte, current density, processing time, etc. Initially, there is a rapid and constant increase in the actual thickness, and then a decrease in the rate of thickness growth begins until a stage occurs at which the thickness remains almost constant, despite the continued supply of electric current. This is because during anodizing there is a continuous increase in the thickness of the coating and its dissolution under the influence of the electrolyte (sulfuric acid solution). 

The size of the anode cells directly depends on the anodizing parameters. As the voltage scales up, the size of the anode cell increases and the number of pores decreases accordingly. The ratio between cell size and voltage is approximately linear, meaning the higher the voltage, the larger the cell size.

3. The third and most vital stage is the stage of consolidation.  Since the surface of the product becomes porous and soft after anodizing, it is necessary to close the pores. This procedure is performed by immersing the product in heated freshwater, or by steam treatment, or with a specialized solution.  However, if the product is planned to be stained later, then fixing is not performed, since the paint itself fills the space in the pores.

Four methods are used for color anodizing:

1. Impregnation of the porous layer with special stain (the adsorption method). After the bath with the electrolyte, the product is immersed in a solution with a stain heated to a certain temperature (55-75 degrees Celsius), for some time (from 5 to 30 minutes), and then further compacted to build a thicker colored layer.

2. Electrochemical deposition into the pores of various metals (the method of electrolytic dyeing, also known as black anodizing of aluminum) is to obtain a colorless anode film first, and then continue the process in an acidic solution of salts of certain metals (copper, manganese, tin, etc.). The color of the finished product is obtained from bronze to black.

3. Special alloying due to the precipitation of particles in the volume of the porous layer, but not in the pores themselves – the method of integral dyeing. With this method, organic salts are added to the electrolyte solution for anodizing, thanks to which the product is stained.

4. Electrolytic staining with the use of special alloying due to additional expansion of the pores near their bottom (the interference dyeingmethod). Technologically similar to the method of integral staining, but allows you to get more shades, thanks to the formation of a special reflective layer.

In TSPROF K03 knife sharpeners the pivot-arm of the rotary mechanism is necessarily subjected to anodizing. This part is constantly subjected to load during sharpening and friction from the clamps moving along it. Anodizing is performed to protect against excessively rapid wear of the pivot-arm surface, so its wear resistance increases.

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