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topic 655

Amp-hour meters for anodizing control


(1996)

Will an Ampere Hour meter help me with my architectural aluminium anodizing process? I understand that the thickness of the anodic coat does not strictly follow Faraday's law.

Currently, we use 35 minutes for a 15 micron coat and 60 minutes for a 25 micron coat. However, I am led to believe that this is a 'rule of thumb' approach and that there are more accurate ways to ensure consistent coating thickness.

Thanks,

Sanjay A. Bulchandani
- Bombay, India


(1996)

Hi Sanjay, My feeling is that an amp hour meter is another piece of information if you keep an accurate log. I do not think that it can truly be used as a definition of thickness because nothing stays constant in the solution. The amount of acid, aluminum, copper and drag in is changing with every load. With all other things nearly constant, temperature of the solution will play a significant factor in thickness and pore structure. Things like how clean your cathode and all wire connections are is probably very significant. The AmpHr log will help you trace at what time something changed significantly (only if the log includes the exact sq. ft of that load). Micrometers and a thickness gauge appropriate for aluminum will give you a very quick idea of your coating thickness for each lot. I bet you will be surprised at how much less anodize you will find on some portion of some loads because it does not have good solution flow or is partially shielded by other parts. Several books have charts for the expected coating thickness for selected alloys/ temperature/ voltages/ acid concentration. Is it worth it? Only if you are having problems or want the extra data and it is used.

James Watts
- Navarre, Florida


December 8, 2008

Hi, Sanjay. There is a "720 rule" for anodizing, which you can read more about in letter 6701, 14520, and 30512 for starters. But that doesn't negate anything that James Watts has said.

Regards,

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



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