Copper and copper alloy castings can often be used directly after machining, but to enhance their wear resistance, corrosion resistance, or to alter the color for artistic purposes, surface treatment is typically required. Common surface treatments include oxidation, passivation, and coloring. Each method offers unique benefits and is applied based on the desired outcome.
1. **Oxidation Treatment**
After machining, copper and copper alloy castings can undergo chemical or electrochemical oxidation to form a black or dark blue oxide layer on the surface. This oxide film usually has a thickness of around 0.5 micrometers. Once oxidized, it's recommended to apply oil or clear paint to protect the film 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, which provide specific solutions and parameters for different applications.
2. **Passivation Treatment**
Passivation is an effective way to improve the corrosion resistance of copper and copper alloy castings, especially when they will be used in more demanding environments. This treatment involves acid washing and is known for its simplicity, high efficiency, and low cost. The solution composition and process conditions for passivation are outlined in Table 10-5, offering guidance for optimal results.
3. **Coloring**
Coloring is a popular method for enhancing the aesthetic appeal of copper and copper alloy castings. It involves using chemicals, pigments, and artistic techniques to create a variety of colors such as bronze, decorative patterns, or other desired hues. The coloring process typically includes pretreatment, dyeing, and post-treatment steps. First, the parts are polished and rinsed with hot and cold water, followed by treatment in a cyanide solution. Then, the cleaned parts are immersed in a dye solution to achieve the desired color. Afterward, they are washed with hot water, alkaline solution, cold water, and alcohol, then dried with sawdust. Finally, a transparent resin coating is applied to improve both durability and visual appeal.
These surface treatments not only enhance the functional properties of copper castings but also expand their use in both industrial and decorative applications.
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