Anodizing Clinic: Painting Parts After Anodizing; Seal, or No Seal?
Q. In conversations with our anodizer on the best way to seal parts for adhesion prior to painting, he referred to a choice of either a final hot water immersion at around 120°F with no seal or a hot water seal at a much higher temperature. What is the difference between the two options and which one do you think is more advantageous for paint adhesion?
A. Your anodizer is correct. The best substrate for painting anodized aluminum tubes would be either no seal, which would be immersion in 120°F hot water, or sealing in near-boiling deionized (DI) water. Either way would work well for the parts. The dip in warm water is just to help the parts dry before they are unracked. The warm water dip can be used up to a temperature of about 160°F. The immersion time is just long enough to heat the parts; around 30 to 60 seconds.
The anodizer recognizes that a hot water sealing bath must have a temperature of at least 208°F. The closer to boiling, the better. High-quality DI water with a minimum specific conductance of less than 5 microSeimens is recommended. The higher the quality of water, the better the seal quality, all other conditions being equal.
Water quality is also measured in terms of electrical resistance. A reading of 200 K Ohms resistance is the same as 5 microSeimens (5 µmS) specific conductance. The lower the conductance, the higher the resistance, or impedance. For example, 1 µmS conductance is equal to 1 million Ohms impedance (impedance is the opposition of electrical current flow in a AC circuit). So as a result, 1 µmS water is of higher purity than 5 µmS water.
The pH of the bath is also important. The pH should be between 5.7 and 6.2. A pH buffer is usually added to the bath to help stabilize the pH. Otherwise, it is very difficult to keep the pH at the right value. A good pH buffer is sodium acetate added at about a half pound (approx. 0.25 kg) per 100 gallons (388 liters) of bath solution.
The caveat for any anodized surface that is going to be painted is that if the parts are sealed, the sealing bath must be free of any and all surfactants (wetting agents). Proprietary “mid-temp nickel acetate seal” can contain at least some amount of surfactant to inhibit sealing smut. If the sealing bath contains wetting agents, the paint will not adhere well and may eventually peel off.
If the parts are left unsealed, e.g., at a temperature of 120°F (49°C) hot water final rinse, it is recommended that the parts be stored in a dust-free environment and/or kept covered while they are waiting to be painted. It is best if unsealed parts can be painted within 24 hours of being anodized.
Originally published in the January 2017 issue.
The following anodizing process overviews are provided as a means of introduction to aerospace anodizing
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