The Causes of Fading and Preventive Measures for Electrophoretic Aluminum Profiles in Reddish-Brown Copper Color

The following protective measures can effectively prevent the fading of the copper color:  

1. Semi-seal the copper-colored anodized film, which can be done using cold sealing or medium-temperature sealing. However, the copper-colored anodized film cannot be fully sealed, because while fully sealing can inhibit the fading of copper-colored electrophoretic coating, it will severely affect the electrophoresis process, hinder the formation of the electrophoretic coating, and the fully sealed copper-colored anodized film will have significantly reduced thermoplasticity, inevitably causing cracking of the anodized film during the baking of the electrophoretic coating.

2. Perform passivation treatment on the reddish-brown anodized film to form a passivation layer on the copper surface deposited inside the oxide film pores, thereby preventing copper from undergoing anodic dissolution during electrophoresis;

3. At the same time, adopting the two prevention measures above results in a more stable anti-fading effect than using either one alone;

4. Treat the reddish-brown anodized film with high-temperature hot water at 80–85°C. This is equivalent to the protective measures in (1) and (2), but it affects the subsequent electrophoresis process and worsens the performance of the surface electrophoretic coating;

5. Based on (1) or (2), control the pre-electrophoresis hot water wash temperature at 70–75°C and wash time at 7–10 minutes (using the upper limit for higher temperatures and the lower limit for lower temperatures). This also results in a more stable anti-fading effect than using a single measure alone;

6. Perform secondary electrolytic coloring on the reddish-brown anodized film (i.e., after reddish-brown coloring and water washing, carry out a second electrolytic coloring treatment). Using either a single tin salt or a tin-nickel mixed salt coloring bath is acceptable, which provides micro amounts of tin and nickel to protect the copper surface deposited inside the oxide film pores.

This preventive measure is relatively easy to achieve a stable copper color and does not negatively affect the suppression of baking film cracks. Introducing a small amount of copper sulfate into the tin salt or tin-nickel mixed salt bath generally does not cause contamination, because copper sulfate is usually reduced to elemental copper by the reducing agents present in the tin salt or tin-nickel mixed salt bath and settles at the bottom of the bath.