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topic 2794

Optimum coating weight for chromate conversion


Does anybody konw the optinum coating weight for chromate type conversion coating? The application is to use chromate conversion as a paint primer for aluminum. I have been told that the best coating weight is about 15 mg/ft2. Any information is very much appreicated.

S. Y. Yuen
- Hong Kong, China


Chris, My experience with high copper alloys is that they respond well to a post etch treatment in what is commonly referred to as a 'tri-acid' de-smut. I use a home brew of 25% BV sulfuric acid, 25% BV water, 50% BV nitric acid and 50g/l ammonium bifluoride. A 10-30 second dip usually does the job. This also works well for high silicon alloys like castings, etc. Good luck.

Keith Wicklund, CEF-SE
avionics - Minneapolis, Minnesota


The question about coating weights is a very general question looking for a specific answer. Coating weights of chromates can vary according to the application, the alloy, and with the specific chromate you are using. We sell clear Alodines, for example, to automotive wheel manufacturers who want coating weights of 3 to 5 mg/ft2 for clear painted wheels. Gold chromates usually run somewhat higher. 15 mg is not a bad coating weight, but one advantage of running a gold chromate is that you can see its color and can almost instantly tell whether you have a uniform coating. A range of 25-40 mg has a better gold color than 15. With accelerated chromates such as Alodine 1200S, 15 mg/ft2 might be difficult to achieve from the standpoint of requiring ver short immersion times, and 40 mg would be more realistic. It is more important to make sure the chromate is a good tight coating, never powdery or smutty. Incidentally, the trend these days is to use non-chrome treatments prior! to paint. The latest generation of our non-chrome Alodines matches the performance of chromates or chrome-phosphates in most painted applications.

phil johnson
Phil Johnson
- Madison Heights, Michigan

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