Aluminum sulfate anodizing and its treatment

Aluminum sulfate  is a relatively young metal and is called "20th century metal." Its annual output is second only to steel in the world, ranking second in metal materials. The reason why aluminum and aluminum alloys are widely used is that it has many characteristics. Used as Water Treatment Chemicals like Trichloroisocyanuric Acid.

The proportion of aluminum is 2.702, which is about 1/3 of that of copper (specific gravity 8.9) and iron (specific gravity 7.9). Its products are light in weight and can be used in automobiles, airplanes, railway vehicles, ships, and high-rise buildings.
The strength of pure aluminum is low, but after adding a small amount of copper, magnesium, manganese, zinc, silicon and other elements in aluminum to form aluminum alloy, the microhardness can reach 400-600kg/mm2, under special circumstances, it can reach 1200-15Ookg/mm2, The strength is better than carbon steel and it is comparable to special steel.
Aluminum and its alloys in the air will naturally generate a very thin oxide film with a thickness of 0.01-0.05 μm on the surface. Although this natural oxide film can prevent them from being exposed to atmospheric corrosion, the film is loose and porous, and when it comes to industrial gases, the corrosion resistance is greatly reduced. However, if the electrolytic oxidation process produces a hard and dense oxide film on the surface, many substances will not corrode it, making it suitable for use in industrial areas and coastal areas.
Due to the excellent ductility of aluminum, it is easy to process and shape, and after artificial oxidation and dyeing, various beautiful colors of aluminum products can be obtained. With the extensive application of aluminum oxide and its alloy surface protection and decorative oxidation technology, new oxidation processes and new technologies have emerged in recent years, such as imitation fireworks, transfer printing, infiltration, ice pattern, etc. The colorfulness of the flowers and the blooming of flowers make them pleasing to the eye. Therefore, at present, the construction industry uses a large number of doors, windows, pillars, frames, and other parts used in high-end hotels, which are both solid and firm and beautiful.
Since aluminum and its alloys also have good thermal and electrical conductivity, they have good reflectivity to light, heat, and waves, no magnetism, low temperature and chemical resistance, and sound absorption...etc. Therefore, its application is more and more extensive.
In order to ensure that the aluminum alloy has sufficient strength and high corrosion resistance, it must be oxidized.
The oxidation treatment of aluminum and aluminum alloys is divided into chemical oxidation and electrochemical oxidation. Chemical oxidation uses no external current to place the parts into the proper solution, creating an oxide film on the surface. In electrochemical oxidation, aluminum and its alloys are used as anodes, so it is also called anodization.
There are many methods of electrochemical oxidation. In this chapter, the problems of sulfuric acid, anodization of chromic acid, electrical oxidation of hard electrochemical oxides, and electrochemical oxidation of porcelain are described separately.
Sulfuric Acid Electrochemical Oxidation Faults and Treatment Methods Sulfuric Acid Electrochemical Oxidation is referred to as anodizing of sulfuric acid. The resulting oxide film color varies depending on the composition of the aluminum material and the oxidation process, and is generally colorless, yellowish, and gray. The thickness of the oxide film is approximately between 1 and 6 μm. Oxide film can be dyed, its protective performance is good, and it has a certain pressure resistance. The disadvantage of this process is that for parts with porosity, such as sanding, riveting and welding, after the anodizing by sulfuric acid, the pores are easily whitened. At present, there is no way to completely eliminate this problem.
I. Introduction of sulfuric acid electrochemical oxidation process Silica Electrochemical Oxidation Fluid Formulation and Operating Conditions The sulfuric acid electrochemical oxidation fluid formulation and operating conditions are as follows:
Sulfuric acid (specific gravity 1.84, CP) (g/l) 160 to 180
Temperature (°C) 15～25
Voltage (V) 12 to 20
Anode current density (A/dm2) 1 to 1.5
Time (min) 35～45
2. Process components on the rack → chemical degreasing → hot water cleaning → cold water cleaning → light → water washing → sulfuric acid anodizing → cold water washing → drying → dyeing → 100 °C hot water closed for 10 minutes.
a. Handling Precautions (1) When the dyeings are anodized, the concentration, temperature, voltage, and current density should be avoided from the upper limit, and the time should be appropriately extended.
(2) In addition to dyeing black, when other colors need to be dyed, pure aluminum, rust-proof aluminum LF2, and hard aluminum LY11, LY12 should be used for the parts. Those with high decorative requirements should preferably use high-purity aluminum or high-purity aluminum-magnesium alloys.
Second, faults and processing methods Symptom 1
Thin oxide film or red ash, poor corrosion resistance.
Cause Analysis The reasons for the above failures are many.
(1) Sulfuric acid content and operating conditions are inconsistent with the process specification Electrochemical oxidation is the loss of electrons on the surface of the aluminum part to become aluminum ions, and then combined with the nascent oxygen generated on the anode to form an oxide film. The chemical dissolution of the oxide film also occurs during the film formation during the oxidation. The formation rate of the oxide film is accelerated with the increase of the anode current density, the voltage, the concentration of the solution, and the increase of the temperature, and decreases with the increase of the anodization time. Of course, the chemical dissolution rate of the oxide film also increases with the increase of these factors. The formation and dissolution of oxide films are the opposite of one another and are mutually restrictive. Therefore, the specified process specification should be based on the requirements of the parts, and the better parameters summarized through practice.
If the potential density is low, the voltage is low, and the time is short, there will be less dissolved aluminum ions per unit area and the resulting oxide film will be thinner. If the concentration of the solution is low, the ions in the solution are small, and the conductivity of the solution is small. To achieve the same current density, the cell voltage of the solution with a low concentration is higher than the cell voltage with a high concentration, and the electrochemical oxidation of sulfuric acid is generally a control voltage. Therefore, when the concentration of the solution is low, the current density is small at the specified voltage, and the resulting oxide film is also thin. When the temperature of the solution is low, the viscosity of the solution is large, the movement of ions is slow, and the conductivity of the solution is small. Under the specified voltage, the current density is small, and the resulting oxide film is, of course, thin. On the contrary, the oxide film is thick, but the dissolution of the oxide film is also accelerated, and a loose powdery oxide film is formed.
(2) The parts are too loose, and the fixture and the conductive rod are in poor contact. If the operating conditions for the analysis of the sulfuric acid content are all in accordance with the process specification, the mounting of the parts should be checked. If the parts are clamped too loose and have been displaced, poor conductivity will result, resulting in a thin oxide film.
(3) The old oxide film on the fixture is not removed. If the sulfuric acid content and operating conditions are in accordance with the process specification, the fixture is also normal. At this time, the fixture should be carefully checked. If the old oxide film on the fixture is not completely removed, the part is clamped. In the jig having a thin oxide film, the conductivity is poor, and the oxide film formation speed becomes slow, making the oxide film thin.
(4) Excessive treatment of copper ions in the oxidizing solution For reason 1, the operator must strictly control the process specification.
For Cause 2, the solution is to remove the oxide film and re-anodize it, but this method has a certain effect on the finish. If the part has accuracy requirements, it can be clamped on the non-work surface, and the oxide film on the part can be lightly wiped off with a jig, so that the clamp can be clamped on the part where the oxide film is removed, and then the oxidation can be performed.
For the reason 3, the parts can be dismantled, the oxide film on the net jig and the oxide film on the parts can be removed, or the oxide film at the clamped part can be wiped off with a jig, and the oxidation can be performed again to bring about the normal oxide film.
For reason 4, only updating the electrolyte can eliminate the ills.
Symptom 2
Oxide film loose powder.
There are two reasons for the cause of loose oxide film:
(1) The control of the process specification is not the current surface. It has been pointed out that the formation and dissolution of the oxide film during the oxidation process are performed at the same time. The chemical dissolution of the oxide film can cause loosening of the oxide film and even loose oxide oxide film. The dissolution rate of the oxide film has a great influence on the temperature of the solution and it accelerates as the temperature of the solution increases. In addition, a long oxidation time and a high concentration of the solution also produce a loose oxide film. When the current density is too high or the voltage is too high, the Joule heat generated will cause the part to generate heat, causing the temperature of the solution around the part to rise, accelerating the chemical dissolution of the oxide film, causing the oxide film to loosen, or generating a powdery oxide film. Therefore, strict control of the process specification is an important factor in preventing the oxide film from becoming loose.
(2) After the addition of sulfuric acid, the solution is not uniformly anodized in the same tank. Some oxide films are too thin, and some oxide films are loose and powdery. This failure usually occurs after adding sulfuric acid, which is caused by the uneven distribution of sulfuric acid in the solution.
The treatment method strictly follows the specification of the process. After sulfuric acid is added, compression and anger are fully stirred, and power failure can be eliminated.
Failure phenomenon 3
Oxidized black produces red or blue.
Cause Analysis There are two reasons for the redness or blueness of oxidized black.
(1) Poor absorption of oxide film When the black acid ATT dye is used to dye black, it will cause red and blue defects. Acidic black ATT dyes are composed of 70% of Acid Blue Black 10B and 30% of Acid Orange II. Therefore, when the oxide film has small pores and poor absorption performance, the small molecule of Acid Orange II is easily absorbed by the oxide film. Acid Orange II in the film exceeds the dye ratio, so the color is red. In this case, the anodizing time can be appropriately extended to increase the porosity of the oxide film so that the large acid blue-black 10B can be smoothly absorbed by the oxide film. The dye in the film is proportioned, and the dyed color is black. .
For plates and extruded parts, the parts should be alkali-etched prior to anodization to remove surface crusts to obtain a uniform black color, otherwise red or black flowers will be obtained.
Parts with high silicon content can be immersed for about 20s in a solution of 40-50ml/l nitric acid and about 10ml/l hydrofluoric acid to dissolve impurities, leaving a thin layer of purer aluminum to improve the properties of the oxide film. Make it have good adsorption performance, get a uniform black.
(2) Improper pH of the dyeing solution The pH value of the dyeing solution has a greater influence on the dyeing because the absorption properties of the various dyes have different pH values. For example, when the pH is close to 7, the acid blue-black 10B has good adsorption performance, while the acid orange II has poor adsorption performance, so the color is bluish. When the pH is about 4.5, the adsorption capacity of the acid blue-black 10B is poor and the sour orange II is adsorbed. Strong ability, so the color is reddish. Therefore, the pH should be kept away from the value of 7 or 4.5 when dyeing, which is generally controlled at about 3.6 or 5-6.
Symptom 4
Sometimes there are no oxide films locally in the production.
Cause Analysis (1) Improper clamping position Owing to improper positioning of the clamp, oxygen does not escape from the localized surface when anodized, resulting in stifling gas and no oxide film. When mounting, care should be taken to select an appropriate position so that the concave portion of the part is tilted upward or upward so that no oxide film failure can be avoided.
(2) Adherent on the surface of the component If the surface of the component before the anodization is made of a transparent viscous material such as adhesive tape, it does not come off during the anodization, and it falls off when it is closed. As a result, there is no oxide film on the adsorbent, and the operator In the pretreatment, care should be taken to remove such substances.
(3) Too dilute oxidizing solution component The solution component is too dilute, and there is no oxide film where the local current is too small. The concentration of sulfuric acid is analyzed and increased, and the failure is eliminated.