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Ask the Expert Question-and-Answer Archive

by Ron Joseph

December, 2004

Painting over Dichromated Aluminum

Q. We usually dip our aluminium in a dichromate bath before powder coating. We have had a request from local builders for aluminium that has been through the dichromate bath, but not powder coated so that they can paint it on site. Are there any environmental/health risks to anyone handling aluminium that has been dipped in dichromate, and is there a maximum period between when the aluminium is dichromate dipped and when it needs to be painted?

I don't expect you to experience any environmental hazards if you leave the dichromated aluminum on site prior to painting. The dichromate is a conversion coating that is tightly bonded (electrochemically) to the aluminum. If the aluminum were to be immersed in rainwater or in the soil, then perhaps some of the chromium could dissolve into the ground water, but I doubt that there would be sufficient to be measurable. If you are concerned, then store the aluminum on pallets, or place a plastic sheet under the aluminum to catch any rainwater that might come into contact with the aluminum.

I also do not anticipate any health risks, because the dichromate is well bonded to the aluminum. Thousands of electronic assembly line workers handle conversion coated aluminum every day and I have never heard that this poses a health problem. Again, if you want to play it safe, have the workers wear plastic or cotton gloves when handling the aluminum. In any case, that is good practice if you intend to paint over the aluminum after assembly.

The third question is more difficult to answer. The longer the aluminum remains uncoated the more likely it is that you might experience adhesion problems. Frankly, we always like conversion coated aluminum to be painted as soon as possible, and you might need to conduct your own wet and dry adhesion tests to answer your question.

Best wishes,

Ron Joseph

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