Hard oxidation, as a special type of electrochemical film-forming method in aluminum alloy anodizing, occupies an important position in aluminum surface treatment processes. As a technical indicator of hard oxidation, one is that the oxide film layer is relatively thick, generally around 25 μ Above m; The second is that the cross-sectional microhardness of the oxide film is above 350HV. Although the hardness and oxide film thickness have been improved, there is no essential difference between the oxidation process and the ordinary anode process.
The main process differences are as follows:
① The temperature of the hard oxidation tank is very low, it is recommended to be around 0 ℃, which requires a powerful freezer and stirring circulation device. The lower the temperature, the harder the quality of the oxide film.
② The concentration of sulfuric acid for hard oxidation is relatively low, recommended to be around 130g/L, and the aluminum ion content should be controlled within the range of 5-10g/L.
③ The current density and voltage of hard oxidation are relatively high. It is recommended that the voltage be above 30V and the current density be above 2.5A/dm2. The specific electrical parameters need to be determined comprehensively in conjunction with the tank temperature, sulfuric acid concentration, alloy material, etc.
④ Using a pulse power supply or a special waveform power supply, gradually increasing the voltage during the boosting stage can avoid material burning under high voltage and promote the normal growth of the oxide film.
⑤ Hard oxidation can also add some organic acids to the tank solution. Organic acids can improve the hardness and toughness of oxide films, and can also be directly used in oxidation processes using organic acid solutions for some special occasions of component oxidation.
Characteristics of hard oxidation of different aluminum alloy series:
① 1 series pure aluminum alloy is most prone to hard anodizing and obtains a high insulation and high hardness oxide film.
② 2 series aluminum copper alloys have difficulty in hard oxidation. Mainly intermetallic compounds containing copper are preferentially dissolved during the oxidation process, resulting in discontinuous oxide films or needle-like defects. The general solution is to use AC/DC superposition or pulse power supply, change electrolyte composition, electrical parameters, etc. to avoid hard oxygen defects in 2-series alloys.
③ The technical difficulty of hard oxidation for 5-series aluminum-magnesium alloys is average, and the hardness of the film layer is slightly lower than that of 6-series alloys, and there are problems such as burns and excessive film thickness.
④ The difficulty of hard oxidation is average for 6-series aluminum magnesium silicon alloys, which belong to alloys that are relatively easy to obtain high-quality oxidation film layers. The components that play a significant role in the film layer quality are generally caused by copper and iron.
⑤ The 7 series aluminum zinc alloy has a higher difficulty in hard oxidation, and there are oxidation defects such as pinholes. The hardness and wear resistance of the film layer are also slightly worse than those of the 6 series alloy.
Usually, in terms of the film-forming efficiency and oxide film quality of hard oxygen, the order from best to worst is
1 Series → 5 Series → 3 Series → 6 Series → 7 Series → 2 Series
3、 Hard oxidation process and parameters of sulfuric acid solution
Free sulfuric acid concentration: 100-150g/L
Aluminum ion content: 1-5g/L
Temperature of oxidation tank solution: (0 ± 2) ℃
Current density: (3.5 ± 0.5) A/dm2
Oxidation time: 60-120 minutes
Tank liquid stirring: Tank liquid strong circulation stirring
Anode oxide film thickness: 50-100 μ M
Sealing treatment: Generally, sealing treatment is not required, but when sealing treatment is needed, it is not recommended to use medium-temperature, medium-high-temperature, high-temperature nickel (nickel-free) sealing, or pure water boiling water sealing. It is recommended to use nickel at room temperature or nickel-free sealing at room temperature, which not only enhances the corrosion resistance of the oxide film but also does not lead to a decrease in its wear resistance.
Overview of the performance of hard anodized films
① Appearance and color
Usually, hard oxide films are relatively rough with microcracks, and the color of the film layer is closely related to the type and thickness of the aluminum alloy. Overall, there is a transition from opaque light gray to dark gray.
② Film thickness
The thickness of hard oxide film is generally above 50 microns. The larger the thickness of the oxide film, the more appearance defects and cracks there are, and the poorer the uniformity and continuity of the film thickness.
③ Hardness
The hardness of hard oxide film depends on the alloy composition and hard oxygen process, and the microhardness value is even related to the relative position between the cross-section of the film and the substrate. The microhardness value is higher as it is closer to the substrate. According to the regulations on the qualified value of Vickers microhardness in the national standard, 250HV is required for 2-series alloys, while 400HV is required for other deformed aluminum alloys except for 2-series, and 300HV is required for high magnesium (mg ≥ 2%) 5-series and 7-series.
④ Wear resistance
Wear resistance is generally carried out on an unsealed oxide film, as after high-temperature sealing, the wear resistance will decrease by about 50%. The wear resistance of the oxide film of hard oxygen is usually 1-2 times higher than that of ordinary oxygen (except for 2-series alloys).
⑤ Electrical insulation
Anodic oxide films are non-conductive, and the breakdown voltage of hard anodic oxide films can even reach over 1000V. If it is necessary to further increase the breakdown voltage, the applied voltage during hard oxygen should be increased.
⑥ Corrosion resistance
The corrosion resistance of hard oxygen films is generally stronger than that of conventional oxide films, but hard oxygen films are more prone to microcracks. Therefore, after hard oxygen, those sealed with or without nickel at room temperature are often more resistant to salt spray testing. At the same time, adding coatings, paraffin, mineral oil, etc. after hard oxygen can also effectively improve the corrosion resistance of the film layer.
⑦ Heat resistance
Hard oxide film is a good “black body” for heat dissipation, and its thermal emissivity increases rapidly with the thickness of the oxide film. By utilizing this characteristic, it can eliminate hot spots in heating components and be applied to some cookware materials.
⑧ Mechanical properties
Hard oxygen generally does not affect the mechanical properties of materials, but may lead to a decrease in their ductility and fatigue strength, and the thicker the film layer, the greater the impact
