Yellowing of white copper is generally caused by three factors:
1. Oxidation
When white copper is exposed to air, especially in high humidity, metals such as copper and nickel on its surface slowly oxidize with oxygen, forming a thin oxide film. Over time, this oxide layer gradually thickens, causing the surface to yellow.
Exposing white copper to high temperatures accelerates the oxidation reaction. For example, exposure to heat near a heat source or during processing can cause the surface to yellow more quickly.
2. Corrosion
White copper may come into contact with chemicals such as acids, alkalis, and salts. These chemicals react with the surface, destroying its structure and causing yellowing. For example, in environments containing chlorides, white copper is prone to corrosion, producing yellow corrosion products.
When white copper comes into contact with other metals and is in a humid environment, electrochemical corrosion may occur. The potential difference between the dissimilar metals causes electrons to flow, accelerating the corrosion process and causing the surface to yellow.
3. Dirt and Contaminants
Airborne contaminants such as dust and grease can adhere to the surface of white copper. This dirt not only affects the appearance of the white copper but also interacts with moisture and other substances in the air, promoting oxidation and corrosion, resulting in yellowing.
Human fingerprints and sweat contain salt, grease, and other substances that leave marks when they come into contact with the white copper surface. If not cleaned promptly, these marks will gradually accumulate, causing the surface to yellow.
How to Treat Yellowing
Polishing:
Use fine sandpaper (e.g., 2000 grit or higher) to gently sand the white copper surface to remove the yellow oxide. Be careful to polish in a consistent direction to avoid scratches. Then, use a soft cloth dipped in polishing paste to restore the white copper surface's luster. For small white copper ornaments or parts, an electric polisher can be used, but be careful to control the time and pressure of polishing to avoid over-polishing and damaging the surface.
Preventative Coating:
Apply a clear protective coating, such as varnish or nail polish, to the white copper surface. These coatings isolate air and moisture, preventing oxidation and yellowing of the copper-nickel alloy surface. The coating should be even, thin, and transparent to avoid affecting the copper-nickel alloy's appearance. Alternatively, you can use specialized metal protective agents, such as anti-rust oil or paint. These agents offer enhanced protection and long-term preservation of the copper-nickel alloy surface.
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