Anodizing: Adhesion of Organic Coatings on Black Anodized Parts

Q. Our product uses aluminum alloy 6061-T6 extrusions of varying lengths, which are sent to our anodizer to be finished per MIL-A-8625, Type II, Class 2, color: black.

The dye is deep black HBL and the process includes a nickel acetate seal. The parts are then dipped in a clear organic coating. We have been experiencing intermittent adhesion problems with the organic coating peeling away from the anodized parts.

We realize that the surfactants in proprietary nickel acetate seals could be the source of the problem. We are experimenting with no seal with a warm water immersion (120°F) and with hot water sealing (clean hot DI water at 208°F).

Our customer has raised some questions about these two sealing processes. They are concerned about the fastness of the black anodize color and if there is a possibility of the black dye leaching from the parts.

Would you please elaborate on the sealing aspect? Would this be an issue if the tubes were not subsequently coated with the clear organic finish? In your opinion, would this still be an issue even if the tubes are clear coated? Is there an alternate black anodized finish to the black dye method?

A. I’m not sure why your anodized and black dyed parts need to have a clear organic coating applied. There are high quality black dyes that, when properly processed, are lightfast. The dye manufacturer can tell you which black dye is the best to use for your particular application. There must be a good reason why the organic coatings you mentioned are required. I get the sense from your question, however, that lightfastness could be an issue.

Of the methods for sealing the Type II and black dyed extrusions that you mention, the hot water seal is the best way to go as long as the anodizer does not add anything other than a pH buffer to the hot DI water sealing bath. The proprietary nickel acetate seal could cause adhesion problems with the organic coating applied after sealing. The hot DI water seal should provide a very good seal and will protect the parts from staining or fading. Not sealing the parts would not be the best way to go. Not sealing the black dyed parts and then applying the organic coatings would work, theoretically, but there are many things that could happen in between the dying and the application of the organic coatings that might be detrimental. It would be better to seal the parts so there are no concerns. Prior to applying the organic coating, the parts should be handled with clean white cotton gloves and wiped down with a solvent such as acetone prior to the clear coat dip.

A word of caution about hot water sealing: If the parts have any sealing smut as a result of an “out of balance” hot water sealing process, there could be adhesion problems. The main causes of sealing smut are 1) low quality, or just dirty (overused) DI water, 2) pH of the sealing bath not under control, 3) sealing too long even when the other two parameters are under control. Your anodizer knows about these causes.

You asked about other high quality black anodized finishes. Anodizing followed by an electrolytic coloring step can be an option to dying the parts with an organic dye. Electrolytic coloring is a process that electrolytically deposits a metal such as tin (most commonly used) into the pores of the anodic coating. This produces a range of colors from a champagne color through bronze and finally black. Time in the coloring bath is the main variable that determines the final color. When the final coating is an applied organic, the parts should be sealed in near boiling DI water instead of a proprietary nickel acetate product.

The coloring bath would be a metal salt; in the case of tin it would be tin sulfate (stannous sulfate). The tin is deposited at the bottom of the anodic pores using an A.C. current. After the coating is properly sealed, the tin color is lightfast essentially for the life of the product. The anodic coating thickness required for black electrolytic coloring is greater than the thickness required for black dye in most cases. The electrolytic coloring process might be more expensive to produce than the black dye process. Of course, the organic clear coat adds some cost to the dyed finish. I do not know how the two would compare in cost.

High quality anodizing, dying and sealing plus a U/V inhibited organic clear coat should provide acceptable lightfastness.

Originally published in the December 2015 issue.