In order to prevent or minimize fading of the red copper color obtained from electrolytic coloring during subsequent electrophoresis and baking processes, appropriate protective measures can also be taken for the copper deposited within the oxide film to prevent it from easily undergoing anodic dissolution.
The following protective measures can effectively prevent the fading of copper color:
1. Semi-seal the purple copper-colored anodic oxide film, which can be done using cold sealing or medium-temperature sealing. However, the purple copper-colored anodic oxide film should not be fully sealed, because fully sealing it may severely affect the electrophoresis process while preventing the fading of the purple copper color, hindering the formation of the electrophoretic coating. Additionally, fully sealed purple copper-colored anodic oxide film has significantly reduced thermoplasticity, which may cause cracking of the oxide film during the baking of the electrophoretic coating.
2. Passivate the purple copper-colored anodic oxide film, so that a passivation layer forms on the copper deposited in the pores of the oxide film, to prevent copper from undergoing anodic dissolution during electrophoresis.
3. By taking both of the above prevention measures simultaneously, the anti-fading effect is more stable than using either measure alone.
4. Treat the purple copper-colored anodic oxide film with high-temperature hot water at 80–85°C. This is equivalent to the protective measures in (1) and (2), but it will affect the subsequent electrophoresis process and degrade the performance of the surface electrophoretic coating.
5. Based on (1) or (2), control the temperature of pre-electrophoresis hot water washing at 70–75°C and the washing time at 7–10 minutes (use the upper limit for higher temperatures, and the lower limit for lower temperatures), which will also make the anti-fading effect more stable than using either measure alone.
6. Perform secondary electrolytic coloring on the purple copper anodized film (that is, after coloring to purple copper and washing, carry out a second electrolytic coloring process). Either a single tin salt or a tin-nickel mixed salt coloring bath can be used, allowing the copper surface deposited in the pores of the oxide film to be protected by trace amounts of tin and nickel.
Preventive measures can more easily achieve a stable copper color and do not have a negative impact on suppressing the cracking of the baked film. Introducing a small amount of copper sulfate into the tin salt or tin-nickel mixed salt bath generally does not cause pollution because copper sulfate is usually reduced to elemental copper by the reducing agents present in the tin salt or tin-nickel mixed salt bath solution and settles at the bottom of the bath.
