Copper and copper alloy castings can be used directly after machining, but to enhance their wear resistance, corrosion resistance, or to achieve a specific aesthetic appearance—especially for decorative art pieces—surface treatment is often necessary. Common surface treatments include oxidation, passivation, and coloring. Each method serves a unique purpose and contributes to the durability and visual appeal of the final product.
1. **Oxidation Treatment**
After machining, copper and copper alloy castings can undergo chemical or electrochemical oxidation to form a black or deep blue oxide layer on their surfaces. This protective film typically has a thickness ranging from 0.5 to 2.5 micrometers. Once oxidized, the casting should be oiled or coated with clear paint to further protect the oxide layer from wear and environmental damage. The chemical composition and process conditions for both chemical and electrochemical oxidation are detailed in Table 10-3 and Table 10-4.
2. **Passivation Treatment**
Passivation is another effective method to improve the corrosion resistance of copper and copper alloy castings, especially when they will be exposed to harsh environments. This process involves acid washing, which removes surface impurities and enhances the material’s natural resistance to corrosion. Passivation is favored for its simplicity, efficiency, and low cost. The solution composition and process parameters for passivation are outlined in Table 10-5.
3. **Coloring**
Coloring copper and copper alloy castings is a popular technique used to create decorative effects, such as bronze tones, intricate patterns, or other desired hues. This is achieved through the combined use of chemicals, pigments, and artistic methods. The coloring process typically involves three main stages: pretreatment, dyeing, and post-treatment.
During pretreatment, the castings are polished, rinsed with hot and cold water, and sometimes treated with a cyanide solution to prepare the surface. In the dyeing stage, the cleaned parts are immersed in a colored solution to achieve the desired color. Afterward, the castings are washed with hot water, alkaline water, and alcohol, then dried using sawdust. Finally, a transparent resin coating is applied to enhance both the wear resistance and the visual impact of the finished piece.
These surface treatments not only improve the functional properties of copper and copper alloy castings but also allow for greater design flexibility and aesthetic appeal. Whether for industrial applications or artistic creations, proper surface treatment plays a crucial role in ensuring the longevity and beauty of the final product.
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