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topic 9751

Effects of humidity on anodized aluminum


I need to know what the effects of humidity on clear anodized parts stored in plastic bags would have on the finish. I am experiencing little white specks that can be readily removed by wiping down with a cloth. We have been experiencing higher than normal humidity and the plating company we used seemed to think that the dampness caused the part to oxidize.

Ronald A. Mongeon
Brookfield Engineering Labs - Middleboro, Massachusetts

First of two simultaneous responses -- (2001)

Ronald, the plastic bags you are using to package the anodized aluminum seal any humidity present at the time of packaging into the bag. When the temperature of the air sealed in the bag drops, the humidity increases. The high humidity or condensed water vapor in the bags can cause corrosion problems.

The anodize coating on the aluminum is designed to protect the parts against corrosion. The formation of white spots could be due to soluble salt left in the anodize coating. Normally, there are a series of heat rinses at the end of the anodize coating process which remove all the soluble salts from the coating. If the rinse water contains high levels of salts or the rinsing time is too short, some salts could be left in the anodize coating. These salts appear to be coming out of the anodize coating when the parts are exposed to the high humidity in the plastic bags.

There are several ways to prevent the white spots. The first would be to remove more of the salts at the end of the anodizing process. The second would be to package the parts with air which contains less humidity (air condition the packaging area). The third possible solution is to enclose a desiccant.

Roy Nuss
Trevose, Pennsylvania, USA

Second of two simultaneous responses -- (2001)

I have doubt if the humidity causes oxidation and the subsequent white spots on your parts. The anodic coating itself if an oxidation product and very resistant to further oxidation.

I used to test corrosion resistance on anodic coating and usually it takes more than a week before it fails. The process requires continuous salty mist sprayed on them. In your case, assuming the humid air trapped inside the plastic cover is also salty, it should not be sufficient to cause coating failure.

Dado Macapagal
- Toronto, Ont

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