China Aluminum Industry Network – Based on the causes of yellowing in electrophoretic aluminum, the following key factors have been identified: issues with the electrophoretic painting process itself, poor conductivity during oxidation, insufficient rinsing before electrophoresis, excessive curing, and contamination of the oxidation bath solution by nitric acid. Understanding these causes is essential for preventing discoloration and ensuring high-quality surface finishes.
1. Yellowing Caused by Poor Conductivity During Oxidation
When the contact between the aluminum profile and the conductive rod is not good, resistance at the contact point increases significantly. This can lead to overheating at the tip of the profile, causing the oxide film to form too quickly and potentially burn or even break down. The resulting oxide film appears cloudy and takes on a yellowish hue. If electrophoretic painting is performed under such conditions, the yellowing becomes very noticeable. This type of defect usually occurs only in a few sections, typically at the end of the profile. To prevent this, it's crucial to ensure consistent and secure contact between the profile and the conductive rod throughout the oxidation process.
2. Yellowing Due to Incomplete Rinsing Before Electrophoresis
The porous structure of the oxide film allows sulfuric acid to remain trapped within its pores. If the profile isn't thoroughly rinsed before electrophoresis, this residual acid can react with the paint, leading to yellowing. However, it's important to note that this yellowing affects the oxide film itself, not just the paint layer. A properly formed oxide film should be clear and transparent. When sulfate remains in the pores, it reacts with the oxide film at high temperatures, causing cloudiness and reducing transparency. Combined with the high reflectivity of the electrophoretic paint, this effect becomes more pronounced, resulting in visible yellowing. Therefore, multiple rinsing steps are necessary, along with proper water quality, temperature, and duration to ensure complete removal of residues.
3. Yellowing Caused by Nitric Acid Contamination in the Oxidation Bath
To improve the cleaning efficiency, some facilities add a certain amount of nitric acid to the neutralization tank. However, if the rinsing after neutralization is inadequate, nitric acid can enter the oxidation bath, leading to the accumulation of nitrates. These nitrates can damage the oxide film by etching its structure, thinning the barrier layer, and deepening the pores. This weakens the adhesion of the oxide film to the aluminum substrate and makes it harder to remove residual sulfate. As a result, yellowing may occur even after proper washing. To avoid this, it’s recommended to adjust the pH of the rinse water to 8–9.5 using a neutralizer before entering the hot tank, and allow for 2–3 minutes of rinsing. This helps neutralize any remaining sulfate and reduces the risk of yellowing during electrophoretic painting.
4. Yellowing from Over-Curing
Most electrophoretic paints are cured at around 180°C for 30 minutes. Under normal conditions, this doesn’t cause significant yellowing. However, some manufacturers experience uneven furnace temperatures, where local areas may exceed 210°C, or their temperature control systems are inaccurate, leading to over-curing. In such cases, lower-quality electrophoretic paints tend to yellow more easily, even turning slightly discolored. High-quality paints, on the other hand, can withstand higher temperatures without yellowing. To prevent this issue, it’s essential to maintain uniform furnace temperatures and use reliable, high-quality electrophoretic paints that are resistant to thermal degradation.
5. Yellowing Caused by the Paint Itself
Anodic electrophoretic paint typically consists of acrylic resin and amine-based resins. During baking, these components cross-link to form a smooth, transparent coating. However, some manufacturers use substandard raw materials or immature production processes, which can result in an improper curing range. If the baking is too short, the film may lack hardness, while over-baking can cause yellowing. This poses challenges in production management. To minimize this risk, it's advisable to choose a paint supplier with a proven track record of consistent quality and reliability.
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