Anodized and Dyed Black Extrusions Turning Green and Purple

Ask an Expert From: Products Finishing, from Anodizing Technologies

Posted on: 2/18/2011

We notice that the black extrusions have a greenish cast to them when we receive them from the anodizer. When the parts are exposed to plant lighting and to indirect sunlight, they turn a purplish color after six months or less.

 Q. We send aluminum extrusions to our anodizer to be anodized and dyed black. We notice that the black extrusions have a greenish cast to them when we receive them from the anodizer. When the parts are exposed to plant lighting and to indirect sunlight, they turn a purplish color after six months or less. 

We would like to know what is causing the parts to become purple. And why do the parts have a greenish cast to them right out of the box? What is the correct finish to specify in order to get black parts that stay black? We did test the coating thickness and found it to be in the 10–15 µ (0.4–0.7 mil) range.  P.D.

 
 
 
A. Most organic dyes for aluminum are not lightfast; black included. Some black dyes are more lightfast than others, however. There are one or two black dyes that will maintain their black color for a long time as long as they are processed properly. More about that later. You can ask your anodizer to contact the dye supplier to find out which black dyes are most lightfast.
 
There are many “colors” of black dye. In addition to being “black” some black dyes have a red tinge, some are bluish, some greenish, even brownish in appearance. There are also a few black-black organic dyes. It might be best for the anodizer to choose either the most lightfast, or the black-black. However, all black dyes will fade to a purplish color over time, especially if exposed to UV. There might be argument from the dye suppliers on this. Two typical sources of UV are sunlight and fluorescent lighting. 
 
For any organic dye to maintain its color over a reasonable length of time, the anodized, dyed parts must be processed in accordance with rigid specifications. Briefly, these include the following:
 
The anodic coating thickness should be minimum 0.7 mil (15µ). 1.0 mil (25µ) is better. This is because heavier anodic coatings absorb more dye and, therefore, they have a better chance of maintaining their color longer.
All steps in the anodizing process should be optimized so that processing takes place at the maximum conditions for each step in the process. The use of deionized water rinsing before dying and sealing is important in order to remove contaminants from the parts. This helps keep the dying and sealing baths free of contaminants.
 
Quality of seal is just as important as the anodizing and dying steps.  The anodizer should also know about this. Sealing quality must be superior. Your anodizer undoubtedly knows how to do this. If you absolutely need a fade-resistant black color, then the black color should be obtained by electrolytic coloring instead of dying.  This is a totally different way of coloring and it is lightfast and permanent.  Your anodizer either has the electrolytic coloring process, or he doesn’t. If your product requires electrolytic coloring instead of organic dying, perhaps you can enlist the help of your present anodizer to suggest an anodizer who does this process.
 


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