Letter 14078

Problem with soft anodic coating [New Zealand] 

Soft anodizing

I am experiencing soft anodic coatings. They tend to scratch easily. I am anodizing with a current density of 1.5amps per square decimeter, with the anodizing time of around 40-45 minutes which should be giving me about a 25 micron layer. My bath temperature varies between 18-21 degrees celcius. This is for type ii sulphuric anodize. I seal the anodic coating with straight de ionized water for about a hour. What am I doing wrong!

Steve Power
- Nelson, New Zealand

Steve...you didn't mention the alloy you are working with, or the electrolyte composition.. both of which could be contributing to your problem. Is this a new problem for you.. are you an experienced anodizer? Do you regularly monitor your DI water for conductivity/resistivity, and Ph? Is your Al content of your bath ok? There are alot of variables to check.

Marc Green anodizer - Boise, ID, USA

Your temps is correct, your current is correct, but probably your acid concentration is too high. Get it down to 15%/wt which is 12 fluid ounces per gal of 66 degree Baume Sulfuric Acid which is 165.3 grams per liter.

Robert H Probert Robert H Probert Technical Services Garner, NC, USA Editor's note:        Mr. Probert is the    author of Aluminum How-To

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