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Black anodized parts changed color
I manage a machine shop that runs a lot of aluminum sheet metal and milled parts. We have a local plating shop do all of our black anodize. Recently we had a customer complain that our plater could not answer, perhaps you can help.
Twelve different parts that we fabricated and had anodized had been installed in an electronics control cabinet (Six circuit board bases and six covers- all black anodized aluminum). Three of the bases turned a copper-ish color while the other nine parts remained black.
What can cause this and are there any fumes or chemicals released when this happens that could effect the other components in the cabinet?
Thank you
Machine Shop - Camarillo, California, USA
2004
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If the coloring dye is organic and a high temperature area you may have a discoloration, like a paint . Mostly those colors are organic compounds, you can even use food coloring stuff. There are inorganic based colors which are more stable but less of gamma colors.  Khair Shishani aircraft maintenance - Al Ain, UAE Give us some more information. Alloy? Cleaning Cycle? Voltage, acid concentration, temperature? Sealing? Thickness of coating? Best initial guess is that the initial weathered oxide was not removed, the anodize film is of very low thickness, and there is NO seal.  Robert H Probert Robert H Probert Technical Services  Garner, North Carolina  Were all the parts processed at the same time? At first glance, this looks like a poor seal. If they were processed at the same time, perhaps the coater has an issue with the evenness of the heat in his seal bath. Another thought is heat.. are the parts that are discolored exposed to extreme heat (above 250f). Most black dyes will discolor at that temp. I'd go with the seal issue first.  Marc Green anodizer - Boise, Idaho |
If the parts are type II anodized it would appear that they were improperly sealed. A simple dye stain resistance test can verify a proper seal.
Bill Graysonmetal finishing - San Jose, California
2004
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