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Chromate conversion ambient conditions

(-----) 2007

We are brush/swab applying chromate conversion coatings to 2000 series aluminium alloys and have done successfully for many years. We did this in a bay inside the facility. It was shared with a steam wash, but the processes never ran concurrently. This bay was changed to a covered external bay where there is no control over the temperature. It worked ok in the summer but now we have come into winter we are getting a speckled finish on the coatings. Could this be due to the lower temperature or the higher level of condensation?
Is there any way around it, such as force drying or do we have to bring the process back inside?

Ciaron Murphy
Chemist - South Wales, UK


As a chemist, you should be aware that for most reactions, a 10 °C drop in temperature will require twice as long to get the same reaction. With a brush application, this can be difficult trying to keep all of the surface completely and uniformly covered. Since aluminum is not always a perfectly homogeneous alloy with its heat treat, portions of it will not react the same with the given chemical composition at less than optimum conditions, thus your spots. A nearly constant temperature of the metal and the chemical is very necessary for a good outcome. Your QC/QA procedures should demand this if it is done to any normal customer or industrial specification.

James Watts
- Navarre, Florida

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