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aluminum anodic oxidation process in mechanical design

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Author : jett NG
Update time : 2021-05-13 10:07:02

In-depth introduction of aluminum anodic oxidation process in mechanical design|


Aluminum is the most abundant metal element in the earth's crust and one of the most commonly used metal materials in mechanical design. It plays an important role in the development of aviation, construction, automobile and other industries with its excellent comprehensive performance.

In the mechanical design, in order to make the aluminum material give full play to its performance, it is often needed to treat the material, anodic oxidation treatment is one of them.
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Anodizing is a simple electrochemical process developed in the 20th century to form a protective coating of aluminum oxide on the surface of aluminum.

Purpose of anodizing Although aluminum has sufficient corrosion resistance to maintain its structural integrity when exposed to the atmosphere, surface corrosion can occur and damage its appearance.

The goal of anodizing is to produce a thick alumina surface layer that protects the aluminum from any corrosion and provides an inert surface that will maintain its initial appearance for many years.

Aluminium alloys are anodized to increase their resistance to corrosion and wear (wear and erosion) and to allow coloring, improved lubrication or improved adhesion. The anode layer does not conduct electricity.

The pretreated anodized coating is transparent so that the appearance of the underlying metal can be seen.

Thus, anodizing is compatible with the natural appearance of aluminum. Therefore, the pretreatment prior to anodizing is very important because it determines the visual appeal of the final anodized product. Aluminum surfaces can be treated by any mechanical or chemical method including shot blasting, polishing, brightening and etching.
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Etch a metal finish usually used to produce a satin finish in a solution based on sodium hydroxide.

Anodic oxidation process The process is called anodic oxidation process because the parts to be treated form the anode of the circuit. An alumina coating can be produced from aluminum by running an electric current through an acidic electrolyte bath immersed in the aluminum.

1. The oxide anodic film itself grows at the aluminum/alumina interface by continuously forming and dissolving very thin oxide layers; This is known as the barrier layer and its thickness is a function of the process voltage. A thick porous layer is formed on top of the barrier layer to make up the rest of the coating. Its thickness depends on the current flow through the circuit. The coating is an integral part of the metal and cannot flake or peel off. The structure of the coating consists of many very small holes that are filled with inert material during the sealing process.

Practical experience and weatherability tests show that the service life of anodic alumina exposed to the atmosphere depends on the thickness of the coating.

In contrast, the effects of the alloys used in such applications are negligible.


Films typically used for outdoor applications range in thickness from 10 to 30 microns. In engineering applications, especially for hard anodizing, the usual range is 30 to 250 um.


2. Colored anodic films can be used for a variety of decorative effects, from thick porous coatings that absorb colorants to thin transparent coatings that increase the interference effect of reflected light.

The most common anodic oxidation processes, such as sulfuric acid on aluminum, produce porous surfaces that are receptive to dyes. The resulting color will vary depending on the base metal alloy, but on some alloys it may be difficult to produce a lighter color. The dye has an acceptable UV resistance linearity when absorbed by the anode film.


Alternatively, metal (usually tin) can be electrolytically deposited at the bottom of the hole in the anode coating to provide a completely opaque color. These colours are created by the scattering and absorption of light and range from light champagne to black. Bronze is usually used for architectural purposes. You can also use this technique to produce grey and grey/blue.


Alternatively, the color can be formed as a whole on a film. This is made by mixing organic acids with sulphuric acid electrolytes and usually special alloys in an anodic oxidation process.


3. Sealing acidic anodizing solutions create holes in the anodized coating that retain the colorant and are usually sealed to prevent cosmetic deterioration of the coating during weathering and to prevent the loss of any colorant. An extended immersion in boiling deionized water or steam is the simplest sealing process in which the holes are filled with hydrated alumina.

Due to energy savings, a cold seal is commonly used, sealing the hole in a bath at room temperature by impregnation with a sealant (usually nickel hydroxide).

Advantages of anodizing Many metals are structurally weakened by oxidation processes, including corrosion, but do not corrode aluminum. Aluminum can actually be made more durable by a process called anodizing. The oxidized aluminum forms a coating because it replaces the original aluminum on the surface.


The result is an extremely hard and wear-resistant material on the surface of aluminum.


The correct anodizing process can make the anodizing film on aluminum almost as hard as diamond. Anodized alumina is often used in many modern buildings where metal frames are exposed to the environment. The anodizing process provides a natural protective appearance.


Application of anodized alumina Due to its durability, anodized alumina is used not only in construction applications, but also in many other applications. The car industry relies on anodized aluminum for trim, hubcaps, roof beams and protective covers on exposed parts. Furniture designers often use anodized aluminum as a frame for outdoor furniture, as well as a base metal for lighting fixtures and other decorative items. Modern appliances and computer systems may use anodized aluminum as a shield.


Although the anodic film is translucent, it does reflect some light from its outer surface. The dual reflection of the metal surface and the membrane surface gives the material a "flexible" appearance, which is particularly attractive to designers.


Anodic alumina may not be suitable for all applications due to its non-conductive properties.

History of anodizing and various processes Anodizing, first used on an industrial scale in 1923, is a chromic acid based process. Today, chromate anodizing is still used in special applications (for example, high-strength aluminum alloys used in the aerospace industry). Continuously evolving, the first sulfuric acid anodic oxidation process was patented in 1927. Sulfuric acid soon became and remains the most common anodized electrolyte. It includes everything from the heavy black dye coating used in high-tech equipment to cheap items, and mainly includes the anodizing of buildings to protect elements such as aluminum window frames from natural elements. Their natural color is light gray; Other colours can be achieved by introducing colourants into the film. Anodized alumina was first used in construction in the 1930s; Some of the buildings are still there and have been properly cleaned and the anodized aluminum looks like new.

Hard anodizing is a branch of anodizing of sulfuric acid, and the process conditions have been reached in Loamiatilliom O2X.MILII is pushed to achieve significantly harder, thicker, denser films. Its applications involve wear resistance, corrosion resistance, temperature effect, etc. A lubricant film can be introduced into the hole of the lubricant to improve the performance of the moving parts.


Oxalic acid anodizing was first patented in Japan in 1923 and later became widely used in Germany, especially in construction applications.


Phosphoric acid processes are a major recent development and have so far been used only as pretreatments for adhesives or organic coatings. Various proprietary and increasingly complex variations of all these anodizing processes have been developed industrially.


Anodizing can also be done in borate or tartrate baths that are insoluble in alumina. During these processes, the coating stops growing when the parts are completely covered, and the thickness is linearly related to the applied voltage. Widely used in the manufacture of electrolytic capacitors Environmental impact anodizing is a more environmentally friendly metal coating process. Usually, the by-products contain only small amounts of heavy metals. The most common anodized wastewater, aluminum hydroxide and aluminum sulfate can be recycled or used in industrial wastewater treatment systems.

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