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The difference between anodic oxidation and chemical oxidation
Aug 26,2019
The concept of anodizing: The process by which aluminum and its alloys form an oxide film on aluminum products (anodes) under the action of an applied current in the corresponding electrolyte and specific process conditions. Anodizing, if not specifically specified, usually refers to sulfuric acid anodizing
1. The role of anodic oxidation
☆ Protection
☆ Decorative
☆ Insulation
☆ Improve the adhesion with organic layers
☆ Improve the adhesion with inorganic coatings
☆ Other functions in development
2. Chemical conversion coating treatment of aluminum alloys (chemical oxidation, passivation, chromization)
The chemical conversion film of aluminum alloy is obtained through chemical oxidation, which can refer to the US military standard MIL-C-5541.
★ Why do we need to perform chemical conversion coating treatment on aluminum alloys
☆ Strengthen the rust prevention ability of aluminum alloys.
☆ It can have a stable contact with the cathode. (Once a customer's product required conductive oxidation, with the aim of stabilizing contact resistance and conductivity)
The conversion film is relatively thin (about 0.5-4um), soft, conductive, porous, and has good adsorption ability. It is usually used as the bottom layer of paint or other coatings.
Do not change the mechanical properties of the material.
☆ The equipment is simple, easy to operate, and the price is cheap.
☆ Does not affect the size of the workpiece.
★ Conversion film thickness
The chemical conversion film on the aluminum alloy surface is about 0.5~4um thin. The conversion film is a kind of gel, which is difficult to measure directly. Usually, it is just to weigh the weight of the workpiece before and after chemical oxidation, or to judge the corrosion resistance of the oxide film by the surface color and salt spray test.
★ Anti corrosion function after scratches
The chemical conversion film on the surface of aluminum alloy is a kind of gel. This gel can move after the conversion film is scratched. The gel around the scratch will move to the scratched surface and combine together to continue and prevent the corrosion of aluminum alloy. It still has the anti-corrosion function.
★ Color
The color of aluminum alloy chemical conversion film includes gray, white, grass green, golden yellow and Iridescence. The final color of conversion film is related to the applied conversion film solution and operating process conditions.
3. The difference between anodic oxidation and conductive oxidation
1) Anodizing is an electrochemical reaction process carried out under high voltage; Conductive oxidation (also known as chemical oxidation) does not require electrification, but only immersion in medicinal solution. It is a pure chemical reaction.
2) Anodizing takes a long time, often tens of minutes, while conductive oxidation only takes a few seconds.
3) The film generated by anodic oxidation ranges from several micrometers to tens of micrometers, and is hard and wear-resistant, while the film generated by conductive oxidation is only around 0.01-0.15 micrometers. The wear resistance is not very good, but it can conduct electricity and resist atmospheric corrosion, which is its advantage.
4) The oxide film is originally non-conductive, but because the film generated by conductive oxidation is really thin, it is conductive.
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