Aluminum and its alloys are light and cheap with good thermal and electrical properties and excellent mechanical properties and machinability, so they are widely used in various fields of the national economy such as mechanical industry, aerospace industry, daily necessities industry, electronic industry, building materials industry, etc.
Aluminum and its alloys are easily oxidized in the atmosphere because of their strong chemical activity, resulting in a thin oxide film on the surface, which is protective because of its good corrosion resistance in dry air, but is easily damaged in contaminated atmosphere, water, seawater, soil and various corrosive media. Therefore, surface treatment is usually applied to enhance the surface performance of aluminum and its alloys such as corrosion resistance, wear resistance and decorativeness.
Surface treatment methods of aluminum and its alloys usually include mechanical surface treatment (mechanical grinding), chemical surface treatment (chemical coating), electrochemical surface treatment (anode oxidation, electroplating), and physical surface treatment (coating, deposition), etc.
Anode oxidation is the most commonly used electrochemical surface treatment method, which allows aluminum and its alloys to form protective coatings with a major component of amorphous aluminum oxide and a thickness of several tens of micrometers under the action of an external electric field in an acidic electrolyte (e.g. sulfuric acid, oxalic acid, phosphoric acid, etc.). The increase in the temperature of electrolyte due to the electrical resistance heat during anode oxidation promotes the dissolution of the surface oxide film and decreases its thickness, which leads to the reduction of its mechanical properties and corrosion resistance. Therefore, in the anode oxidation process of aluminum and its alloys, it is necessary to cool the electrolyte with cooling equipment to prevent excessive rise in the temperature, which means consumption of a large amount of energy.
Kim Chang Sok, a researcher at the Science Engineering Institute, has proposed a surface treatment method for excellent surface properties and low energy consumption by applying wide temperature anode oxidation technique to the surface treatment of aluminum alloy 6061 commonly used as dry bulb.
Addition of polycarboxylic acid and inorganic salts to the sulphuric acid electrolyte of aluminum alloys can inhibit or retard the electrochemical and chemical dissolution of aluminum, resulting in an anodic oxide film with excellent surface properties even at wide temperatures, which can significantly reduce energy consumption and cost during anode oxidation. Considering it, he selected inexpensive oxalic acid as polycarboxylic acid and nickel sulfate which is widely used in the coloration process of aluminum alloy as inorganic salt, and conducted a study to increase the allowable electrolytic temperature of anodic oxidation.
The results showed that the surface characteristics of the samples anodized at 30℃ are almost the same as those of the samples anodized by the existing method at 20℃, so that the acceptable temperature range of anode oxidation can be increased by 10℃ and the energy consumed for cooling of electrolyte during anodization can be significantly reduced.
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