Hard anodized, full name hard anodic oxidation treatment. The main purpose of hard anodized of aluminum alloys is to improve the properties of aluminum and aluminum alloys, including corrosion resistance, wear resistance, weather resistance, insulation and adsorption. It is suitable for both deformed aluminum alloy and possibly for pressure casting aluminum alloy parts.
1. Process Introduction:
Generally, hard anodized film thickness is required to be 25-150um, However, mostly hard anodized film thickness is 50-80um. While film thickness is less than 25um, it is used for tooth bonds and spiral parts. The thickness of anodic oxide film for wear resistance or insulation is about 50um. In some special technological conditions, hard anodic oxide film with a thickness of more than 125um is required to be produced. But it must be noted that the thicker the anodic oxide film is, the lower the microhardness of its outer layer can be, and the roughness of the surface of the film layer can be increased. Hard anodized tank, generally, sulfuric acid solution and sulfuric acid added organic acids, such as oxalic acid, sulfamic acid, etc. In addition, the hard anodized treatment can be achieved by reducing the anodizing temperature or sulfuric acid concentration. Special anoDIzing measures may also be considered for deformed aluminium alloys with copper content greater than 5% or silicon content greater than 8%, or for high silicon pressure cast aluminium alloys. For example, for 2XXX series aluminum alloy, in order to avoid the aluminum alloy burned in the anodic oxidation process, 385g/L sulfuric acid plus 15g/L oxalic acid can be used as the electrolytic solution, the current density should also be increased to more than 2.5A/ DM.
2. Process method:
There are many methods of Hard anodized electrolysis, such as sulfuric acid, oxalic acid, propylene glycol, sulfosalicylic acid and other inorganic salts and organic acids. The power supply can be divided into DC, AC, AC and DC superposition, pulse and superposition pulse power supply, widely used in the following hard anodic oxidation.
(1) sulfuric acid hard anodized method;
(2) oxalic acid hard anodized method.
(3) Mixed acid type hard anodic oxidation
Among them, sulfuric acid method is widely used as a hard oxidation method.
3. Principle of hard anodized
There are no essential difference between simple sulfuric acid aluminum alloy hard anodic oxidation principle and ordinary anodic oxidation, but it will have some side reactions if mixed acid hard oxidation.
(1) cathode reaction: 4H+ + 4E =2H2↑
(2) Anodic reaction: 4OH-- 4E =2H2O+O2↑
(3) Aluminum oxidation: the oxygen precipitated on the anode is atomic state, more active than the molecular state of oxygen, easier to react with aluminum: 2Al+3O→Al2O3.
(4) Dynamic equilibrium of oxidation on dissolution of anodic film: Oxidation film thickens with the increase of conduction time and the increase of current. At the same time, due to the dual nature of (Al2O3) chemical properties, that is, it is alkaline oxide in acidic solution, acidic oxide in alkaline solution. There is no doubt that the oxide film solution is dissolved in sulfuric acid solution. Only when the formation rate of the oxide film is greater than its dissolution rate, the oxide film is likely to thicken. When the dissolution rate is equal to the formation rate, the oxide film will not thicken. When the oxidation rate is too much higher than the dissolution rate, the surface of aluminum and aluminum alloy is easy to form a powdery oxide film.
4. Technological requirements
In order to get better quality hard anodized film, and ensure the required size of the parts, it must be processed according to the following requirements.
Acute Angle round: The processed parts are not allowed to have sharp angles, burrs and other sharp corners because of hard oxidation, the general anodic oxidation time is very long, and the oxidation process (Al+O2→A12O3+ Q) itself is an exothermic reaction. And because the general parts of the edges and corners are often more concentrated parts of the current, so these parts are most likely to cause parts of the local overheating, so that the parts were burned. Therefore, all edges and corners of aluminum and aluminum alloys should be chamfered, and the radius of the chamfered Y circle should not be less than 0.5 mm.
Surface finish: The surface of the parts is changed after hard anodic oxidation finished. For a rough surface, after this treatment can appear than the original level off some, but for the original finish higher parts, often after this treatment, according to the smooth surface brightness decreases instead, lower amplitude in 1 ~ 2 magnitude.
Size allowance: Because the thickness of hard oxide film is higher, so if the aluminum parts need further processing or the parts need to be assembled, a certain processing allowance should be reserved in advance, and the specified clamping parts.
As the size of the parts need to change while hard anodized, therefore, in machining, it is necessary to predict in advance the possible thickness and size tolerance of the oxide film, and then determine the actual size of the parts before anodizing, so that after processing, in line with the prescribed tolerance range.
Generally speaking, the size of the part is roughly half of the thickness of the oxide film.
Jig: Because the hard anodized parts in the oxidation process need to withstand high voltage and high current, it is necessary to make sure that the fixture and the parts can maintain excellent contact, otherwise will cause breakdown or burn parts contact parts of the fault due to poor contact. So the requirements of different shapes of parts, and parts after oxidation of the specific requirements to design and manufacture special fixture.
Local protection: For example, in the same part, both ordinary anodic oxidation and hard anodic oxidation parts according to the finish and precision of the parts to arrange the specific process. Generally, the first ordinary anodizing, in the hard anodized, do not need to be hard anodized surface insulation, insulation method useful spray gun or brush, will be prepared to nitro gum or perhydrogen vinyl glue on the surface do not need to be treated, insulation layer to be coated thin and uniform. Each coating should be dried at a low temperature for 30 ~ 60 minutes for a total of 2 ~ 4 coats.
5. Technological characteristics
Hard anodized electrolyte is electrolyzed at a temperature of -10℃ ~ +5℃. Due to the high resistance of the oxide film produced by hard anodic oxidation, the oxidation of current intensity will be directly affected. For thicker oxide film, certainly will will increase the voltage, its purpose is to eliminate the influence of resistance is big, and keep the current density, but larger current will produce intense heating phenomenon, and generate the oxide film will release a lot of heat, making the parts around the electrolyte temperature sharp increase, the temperature rise will accelerate oxidation membrane dissolved, the oxide film not thickening. In addition, the heating phenomenon is the most serious in the contact between the film and the metal, if not solved in time, the local surface of the processing parts will be burned out due to the temperature rise.
Solution: the combination of cooling equipment and stirring. Cooling equipment to make the electrolyte forced cooling, stirring is to make the whole tank electrolyte temperature uniform, so as to obtain high quality hard oxide film.
6. Influence factor
Whether aluminum and aluminum alloy surface can generate high quality hard oxide film layer is mainly depends on the composition of the electrolyte concentration, temperature, current density, and the composition of raw materials.
Electrolyte concentration: When sulfuric acid electrolyte is used for hard anodizing, it is generally in the concentration range of 10% ~ 30%. When the concentration is low, the hardness of the oxide film is high, especially for pure aluminum, but the aluminum alloy with high copper content (CY12) is an exception. Because aluminum alloy with higher copper content is easy to produce CuAl2 compounds, which dissolve faster during oxidation and easily burn aluminum parts. Therefore, it is generally not suitable to use low concentration of sulfuric acid electrolyte, must be in high concentration (H2SO4 in 300 ~ 400g/L) in the oxidation treatment or the use of AC-DC superposition method.
Effect of temperature on film layer: The temperature of the electrolyte has a great influence on the wear resistance of the oxide film. Generally speaking, if the temperature drops, the wear resistance of the anodic oxidation film of aluminum and aluminum alloy will increase, which is caused by the decrease of the dissolution rate of the electrolyte for the film, in order to obtain a higher hardness of the oxide film, we should grasp the temperature in the range of ±2℃ for hard anodizing treatment.
