• Topic 55760 "Aluminum Anodizing flakes off when heated to 125 °C"

A. You have 2 options: Specify either 1) a heat resistant dye or 2) electrolytic (two-step) coloring. Dyes are organometallic molecules subject to degradation; electrolytic colors are inorganic, so more stable.

Heat resistance data are scarce but correspond fairly well with light (fade) resistance. MIL-A-8625F allows the customer to specify light fastness testing of Class 2 (dyed) coatings, although this is typically only done for initial qualification of a process. A Type II anodize coating thickness of at least 0.001" improves fade resistance. All dyed anodize is required by spec. to be properly sealed.

Probably, best to correlate actual past results with dye data from the anodizer(s). Your anodizer(s) has records on dyes used for at least the past year per MIL-A-8625F, maybe even 10 years if using a modern quality system. My opinion of the fade resistance of some common black dyes: Black CRO < BK < BK Super < MLW < HBL. A dye supplier such as U.S. Specialty Color Corporation will have more information.

The Aluminum Association has a "Designation System for Aluminum Finishes," publication DAF-45. Some designations for electrolytically dyed anodize are A-34 for Architectural Class 2 (10-18 microns) and A-44 for Architectural Class 1 ( >18 microns). Not all anodizing jobshops use these designations, however.

A. We do black anodize here. Some of the items I do are for photographic lighting which gets very hot - near 500 °F & our black dye has no problem with that temp.

There are many black dyes available for anodizers to purchase -- each one has a lightfast & heat-fast rating -- your anodizer may be buying a dye that is not heat-fast.

Also, even with a good dye it is important that you have a thick enough coating to get a good black color & a good seal.

If your anodize is being done by the extruder, they may be using 2 step electro coloring -- that should definitely not be fading.

Hope this helps.

A. The dye is covered above. Here are two more problems. At the temperature you bake the waters of hydration in the seal are driven off. Secondly, at your baking temperatures the coefficient of expansion of the underlying aluminum is 5 times that of the oxide coating -- so it cracks!

A. Tim

Try searching "effects of a simulated space environment on thermal radiation characteristics..."

Table 1 has some parameters, but not specs. I'm not sure if the sulfide will cause interference, but you might be able to test with a colorimeter typically used to analyze (waste) waters.

A. Hi. Alan has not specified whether it's 200 °F or 200 °C. Most of the world uses C, so that would be my assumption, but many posters use F in deference to this being a USA-based site where F is still the commonly understood temperature scale. Please get back to us with your situation, Alan.

Regards,

Excellent catch Ted! My response was under the assumption of 200 °F. If it's 200 °C, then dyed anodizing should not be the coating used.

A. Hi Alan,

200 °C is a lot to ask of any dyed system. You could try asking one of the dye manufacturers whether they have anything rated to that temperature (possibly might have an inorganic dye that might withstand the temperature).

As an aside, taking anodizing to 200 °C isn't the greatest idea. The coefficient of linear thermal expansion of the base aluminum is around five times that of the anodic layer, essentially meaning that any soak at temperature is likely to cause cracking. This is not a problem if the finish is aesthetic, but if you are expecting corrosion resistance then you are likely to have compromised the coating, such that you will significantly shorten the life of the part.