What is chromate anodizing in aluminum profile?
Publish Time: 2022-07-10 Origin: Site
What is chromate anodizing in aluminum profile?
Characteristics of chromate anodizing
Using 3%~10% chromic acid electrolyte to conduct anodizing treatment of aluminum and aluminum alloy or aluminum profiles under certain working conditions, the chromic acid oxide film is much thinner than that of sulfuric acid oxide film and oxalic acid oxide film. , The general thickness is only 2~5μm, which can maintain the accuracy and surface roughness of the original parts.
The film layer is soft and elastic, and basically does not reduce the fatigue strength of the raw material. But the wear resistance is not as good as the sulfuric acid anodized film. The film is opaque and the color ranges from off-white to dark gray or iridescent.
Since chromic acid has almost no cavities, it is generally not easy to dye. The film layer can be used without sealing, and its corrosion resistance is higher than that of the unsealed sulfuric acid oxide film under the same thickness. The chromic acid oxide film has a good bonding force with organic matter, and is a good primer for pain.
Since the solubility of chromic acid for aluminum is smaller than that in other electrolytes, this process is suitable for CNC machining parts, sheet metal parts, aluminum die casting parts, riveted parts, electric welding parts and the size of which is difficult to process with sulfuric acid method. Aluminum extrusion and aluminum profile parts with small differences and low surface roughness. Aluminum alloys containing more than 4% copper are generally not suitable for chromate anodizing.
Process flow for chromate anodizing
Aluminum extrusion parts → mechanical polishing → degreasing → alkali etching → light emission → cleaning → chromate anodizing → cleaning → drying → finished product.
Process specification for chromate anodizing(DC power supply)
The specifications of chromate anodizing process for two different formulations are shown in the following table:
Composition and process parameters | Formula 1
| Formula 2
|
Chromic anhydride Cr0 g·L- | 0~35 | 95~100 |
Temperature /℃ | 40±2 | 37±2 |
Anodizing time/min | 60 | 35 |
Cathode material | Lead or graphite | Lead or graphite |
Anode current density /A·dm2 | 0.2~0.6 | 0.3-0.5 |
Voltage / V | 0~40 | 0~40 |
pH value | 0.65-0.8 | <0.8 |
Cathode and anode area ratio | 3:1 | |
Scope of application | Anodizing treatment for polished parts with small tolerances | Suitable for surface oxidation of general parts, weldments or paint primers |
Electrolyte preparation method
Calculate the volume of the tank and the required chromic anhydride, add all of it into the tank, then add 4/5 volume of distilled or deionized water, stir vigorously, and after the chromic anhydride is completely dissolved, add distilled water to the working liquid level. After the analysis and debugging are qualified, it can be used. During the oxidation process, concentration analysis and timely addition of chromic anhydride should be carried out.
Influence and exclusion of harmful impurities
The common harmful impurities in the electrolyte are SO2-4, C1-, Cr⊃3;+ plasma, the oxide film becomes rough when the content of SO2-4 is greater than 0.5g/L and Cl3+- is greater than 0.2g/L. In addition, due to the reduction of hexavalent chromium to trivalent chromium during the oxidation process, the increase of Cl3+ will make the oxide film dark and dull, and the corrosion resistance will be reduced.
The trivalent chromium in the solution can be removed by electrolysis. With lead anode and steel cathode, the anode current density is 0.25A/dm⊃2;, and the cathode current density is 10A/dm⊃2;, so that Cr⊃3;+ is oxidized to Cr6+ in the anode.
If the SO2-4 content in the solution is too high, the anodic oxidation effect is not good. When the sulfate content exceeds 0.5%, 0.2~0.3g/L of Ba(OH)2 or BaCO3 can be added to form barium sulfate precipitation and removed by chemical precipitation. The chloride content in the solution should not exceed 0.2g/L. When the CI- content in the solution is too high, the solution must be diluted or replaced.
Maintenance and Adjustment of Electrolyte
Due to the dissolution of aluminum during the oxidation process, the amount of aluminum chromate [Al2(CrO4)3] and basic aluminum chromate [Al2(40H)CrO4] in the solution gradually increases, while the content of free chromic acid decreases, which makes the electrolyte The oxidizing ability is reduced, therefore, the electrolyte should be analyzed regularly and chromic anhydride should be added in a timely manner. The electrolyte can also be adjusted by measuring the unit conductivity or pH value of the electrolyte.
Process operation points for chromate anodizing
Within 15min of starting oxidation, the voltage was controlled at about 25V, and then the voltage was gradually adjusted to 40V for 45min, the power was cut off, and the aluminum parts were taken out.
As the anodizing process progresses, the current will drop. In order to maintain a certain current density, the voltage must be adjusted frequently and the pH value must be strictly controlled within the specified range.
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