Anodizing Hardness vs. temperature, current, and other parameters

A. Suspect one is that your agitation , air and / or liquid, has dropped. Two is the temperature control throughout the run is not consistent-possibly larger loads. Three is your temperature control has lost calibration. Four is your power supply has lost its calibration. five, someone changed some parameter of the tanks chemistry or times. Finally, your anodize tank has too much copper or aluminum in it.

A. If you used the exact same operating parameters, and the same aluminum, the hardness would not change, so you will have to look at your operating parameters and see which have changed. Let's take temperature for example. You say 13C, but there must be some operating range, an upper and lower control limit. If you were to anodize today at, say,11C and tomorrow at, say 15 C, the work you do tomorrow would likely be softer. Find out exactly what your parameters were when you did good work, duplicate them, and you'll do good work again.

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A. Amen to what Chris said.

By Faraday's Law, anodization is done by current, not voltage. The voltage will be whatever is required to establish the anodizing current, governed exactly by Ohm's Law. 23 volts is completely meaningless without knowing the total electrical resistance of the anodizing setup in question. Decent ammeters are now commonplace on anodizing rectifiers, there is no excuse for archaic (and grossly inaccurate) "voltage" anodizing.

If the intent here was a Type III coating; 13 °C = 55.4 °F, seems too warm, even with additives. A 200 g/l electrolyte also seems too strong, both will contribute to a softer coating.

Ed. note: it is our sad duty to advise of the passing of Paul Yursis in August 2005.
Here is a brief obituary by Mike Caswell.

A. Drop the sulfuric acid from 200 g/L to 165.3 g/L and the pore gets smaller and the surface gets harder.

A. Hi,
Before offering suggestions, will you confirm that you are anodizing for 2.5 hours and getting 10 - 15 microns. If not, what are the correct values.

A. "Normal" hardcoating parameters are 0 °C with constant current density of 3.5 to 4.0 A/dm2. The final voltage should be higher than 21 VDC. I would expect to achieve 10-15µm in about 20-30 minutes with a terminal voltage of 30-40 VDC.