Chapter 1: Introduction
The purpose of anodizing aluminum alloy: improve the surface hardness of the workpiece, wear resistance, corrosion resistance and other properties, good insulation performance, can significantly change and improve the appearance and performance of aluminum alloy. Aluminum alloy through chemical pretreatment, can also be electroplating, electrophoresis, spraying, etc., the surface of aluminum alloy with metal coating or organic polymer coating, to further improve the decorative and protective effect of aluminum alloy.
1.2 Aluminum alloy surface treatment technology
Aluminum alloy surface treatment technology: surface mechanical pretreatment (mechanical polishing or sweeping, etc.), chemical pretreatment or chemical treatment (chemical conversion or chemical plating). Electrochemical treatment (anodizing or electroplating, etc.), physical treatment (spraying, enamel enameling and other physical surface modification techniques), etc.
1.3 Aluminum alloy and anodizing
Different components of aluminum alloys are suitable for different purposes of anodizing, such as the anodizing performance of aluminum-copper alloys (especially bright anodizing) is generally not good,
Chapter 2 Surface mechanical treatment of aluminum
The appearance and applicability of aluminum and its alloy products depend largely on the surface pretreatment before finishing, and mechanical treatment is one of the main methods of surface pretreatment.
Mechanical treatment is generally divided into:
Polishing (polishing, polishing, polishing or mirror polishing), sandblasting (shot), brushing, rolling and other methods
Purpose of mechanical treatment:
1) Provide good apparent conditions to improve surface finishing quality
2) Improve product grade
3) Reduce the impact of welding
4) Produce decorative effect
5) Get a clean surface
Chapter 3 Chemical polishing and electrochemical polishing
Bright anodizing only by using special chemical polishing and electrochemical polishing treatment, in order to ensure high mirror surface quality after anodizing.
Common chemical polishing process: phosphoric acid - sulfuric acid - nitric acid
Common electrochemical polishing processes: sulfuric acid - chromic acid, phosphoric acid - sulfuric acid - chromic acid and sodium carbonate - trisodium phosphate, etc
Chapter 4 Chemical cleaning and etching
Purpose of chemical cleaning:
Because the use of lubricant, rolling oil in the production process of aluminum, the use of polishing paste in mechanical polishing, the use of anti-rust oil and other necessary oils in the presence of semi-finished products; In addition, aluminum in the operation process and the transport process is slightly not paid attention to, it is likely to adhere to unnecessary mechanical equipment lubrication, dust, impurity particles of pollutants, serious will form dirt. After the aluminum surface comes into contact with grease or pollutants, it will prevent the surface from fully contacting with the treatment solution, and can not get a uniform wetting surface. If the grease or pollutants are brought into the tank, the composition of the tank will be destroyed. Therefore, the aluminum surface treatment process must first carefully chemical cleaning, remove the surface of grease, pollutants, dirt, aluminum surface oxide film, etc., so that the aluminum to get a smooth and uniform clean surface.
Purpose of etching:
The etched surface that causes the surface of the aluminum to produce uniform scattering is usually called the matte surface.
Chapter 5 Aluminum anodizing and anodizing film
5. Aluminum anodizing process
The polarization behavior of aluminum as an anode in various electrolytic solutions can be divided into at least 5 cases, of which the barrier type anodizing film and the porous type anodizing film belong to the scope of aluminum anodizing defined by national standards, and the following is only a brief introduction to the two anodizing processes.
5.1.1 Anodizing of barrier type
In the case that the electrolytic solution is basically insoluble to the anodic oxide film, in the neutral salt, the initial voltage rises rapidly with the anodic oxidation time to a relatively high voltage, and if this voltage rises beyond the breakdown voltage, the oxide film is broken down. If this voltage does not reach the breakdown voltage, then at this voltage, the current quickly drops to close to zero or a very small so-called leakage current value, at which point the electrochemical reaction actually stops. The so-called "leakage current value" may mainly be the electronic current from defects, impurities or local films in the film. At this time, a barrier type anodic oxide film is generated.
5.1.2 Hole anodizing
In the case of "limited" dissolution of the electrolytic solution to the anodic oxidation film, the medium is generally used in acid and other solutions, and the voltage change is similar to the "barrier type anodic oxidation" at the beginning, but the decline has not reached a minimum value, and then rise to a relatively constant stable voltage, maintaining the electrochemical reaction of anodic oxidation. At this time, a porous anodic oxide film is generated.
Others have a point of corrosion anodizing (can not generate a complete anodizing film), only electrolytic polishing or electrolytic etching in strong acid media.
5.2 Structure and line appearance of anodizing
The pores of the porous anodic oxide film are regular holes perpendicular to the metal surface, and the density of the holes is very large, generally reaching 76 billion /CM2 5.3 The thickness, structure and composition of the porous anodic oxide film is composed of two parts, that is, the barrier layer and the porous layer.
The thickness of anodic oxide film δa=KIt
K: is a proportional constant
I: Anode current density (A/dm2)
t: Anodizing time (min)
Please note that the thickness of the anodized film is a limit value, and the above formula does not mean that the thickness of the anodized film will increase without limit over time.
The barrier layer of anodic oxide film is a dense non-porous amorphous oxide, and the porous layer is composed of amorphous alumina.
Chapter 6 Anodic oxidation process
6. Sulfuric acid anodizing process
The process of forming an anodizing film on the surface of aluminum by electrolyzing aluminum in sulfuric acid electrolyte with aluminum as anode is called aluminum-sulfuric acid anodizing.
The oxide film generated by direct anodizing of sulfuric acid has the following characteristics:
6.1.1 Low production cost
6.1.2 High transparency of the film. Generally, the sulfuric acid oxide film is colorless and transparent, the purest aluminum, the better the film transparency, alloying elements Si, Fe, Mn will reduce the transparency, but Mg has no effect on the transparency, and is most suitable for bright anodizing after polishing.
6.1.3 Good corrosion resistance and wear resistance
6.1.4 Electrolytic coloring and chemical coloring are easy. Sulfuric acid oxide film is porous, colorless and transparent, not affected by the color, in the process of electrolytic coloring, metal ions can precipitate from the bottom of the hole and hair color, so that the color is beautiful, light and weather resistance is good; In chemical dyeing, the porous membrane has strong adsorption force, and it is easy to penetrate the dyeing liquid into the membrane hole, and chemical or physical effects occur, dyeing into a variety of bright colors.
6.2 Other acid anodizing processes
Chromic acid anodizing process, oxalic acid anodizing process, phosphoric acid anodizing process
Chapter 7 Hard anodizing
The hard anodizing technology of aluminum is an anodizing technology with the hardness and wear resistance of the anodizing film as its primary characteristics. This film is generally used for general engineering applications or military applications, and the film thickness is usually greater than 25um.
7.1 Relationship between hard anodizing and material
The performance of hard anodizing process and hard anodizing film is greatly affected by the type and production process of aluminum alloy, in addition to the model of aluminum alloy, the shape of aluminum alloy also has an impact on hard anodizing, deformed aluminum alloy shape has sheet, sheet, extrusion material, forgings and castings, due to the aluminum alloy structure and grain size and shape of the different, Hard anodizing processes are also different.
7.1.1 The thin plate has fine crystal structure, and the grain is elongated in the rolling direction. The main problem is that the burning tendency of narrow section is large.
7.1.2 The main problem of the extrusion material is that there may be a coarse crystal band in the extrusion direction, which is in the middle of the surface of the hollow profile, or concentrated in some special parts of the profile. This situation is most common in 6061 alloy, and 6063 alloy is slightly. The defects of this structure come from the anisotropy of the extrusion alloy, and the anodizing rate of different grain orientations is different, which may affect the uneven thickness of the oxide film on the surface of aluminum.
7.1.3 The forged original surface often has a thick thermal oxidation film, which needs to be removed by special methods, and the forging may be commonly used to strip away a large amount of surface by mechanical processing, at which time the internal coarse crystal structure is revealed after hard anodizing, which sometimes occurs in extruded materials.
7.1.4 Casting is not a deformed alloy, generally contains a high silicon content, sometimes containing about 5% Cu, the main goal of the casting is not allowed to exist in the internal cavity. It is difficult to anodize high silicon aluminum alloy castings.
7.2 Properties of hard anodic oxide film
High hardness and high wear resistance, and due to the relatively high density and low porosity, the film has high electrical insulation and corrosion resistance.
Chapter 8 Coloring and sealing of anodizing
After anodizing aluminum and alloy, a layer of porous oxide film is formed on its surface. After coloring and sealing treatment, various colors can be obtained, and the corrosion resistance and wear resistance of the film can be improved.
8. Coloring with inorganic pigments
The coloring mechanism of inorganic pigments is mainly physical adsorption, that is, inorganic pigment molecules are adsorbed on the surface of the micropores of the film layer for filling. The color color of the method is not bright, the binding force with the substrate is poor, but the fast light is better
8.2 Organic dye coloring
The coloring mechanism of organic dyes is more complex, and it is generally believed that there are physical adsorption and chemical reactions. Organic dyes have bright color and wide range of colors, but poor light fastness.
8.3 Electrolytic Coloring
Electrolytic coloring is to put the anodized aluminum and its alloy into the electrolyte containing metal salt for electrolysis. Through electrochemical reaction, the heavy metal ions entering the oxide film micropores are reduced to metal atoms and deposited on the non-porous layer at the bottom of the hole for coloring. The color oxide film obtained by electrolytic coloring process has the advantages of good wear resistance, sun resistance, heat resistance, corrosion resistance and color stability, etc. At present, building profiles are widely used.
After anodizing aluminum and alloy, whether it is colored or not, it needs to be closed in time, the purpose is to fix the dye in the microhole, prevent seepage, and improve the wear resistance, sun resistance, corrosion resistance and insulation of the film. Sealing methods include hot water sealing method, water steam sealing method, bichromate sealing method, hydrolysis sealing method and filling sealing method.