Anodizing Temperature Control

DIY anodizing of aluminum parts is a practical process for home shop reel makers. These earlier posts provide background:
Technical Data on Anodizing.
Water Cooled Cathode
Cool water

I anodize reel parts in a 32 ounce polypropylene jar, and the lid assembly has several features. A reel spool is suspended from the lid by an aluminum “rack”, which both supports the spool and provides an electrical path.

Also visible here are 3 tubes: the big looped tube passes cooling water through the acid solution, an open ended tube is for bubbling air to keep the solution stirred, and a closed end tube is for temperature sensing. These three tubes are electrically connected together to serve as the cathode.

Here is the acid jar with lid and an auxiliary jar for ice and cold water. A small 12 volt dc pump circulates water between the jars.
Two additional connections are provided on the cold water jar, a normally plugged outlet for system drain and an overflow for excess melt water.

This is the complete setup, adding a current controlled power supply, an aquarium air pump for bubbling, my new temperature controller, and an IR temperature gun for casual monitoring.

Before I made the controller, I kept a glass thermometer in the closed end tube and turned on the pump when the temperature reached 21 deg C, and turned it off again at 19 deg C. This required constant vigilance during the one hour process.

This is a close-up of the controller, it is just a resistor bridge (one leg is a thermistor in the closed end tube), a voltage comparator, and a power transistor to switch the pump.
This automatic control makes running the process much more pleasant; I can leave my garage for a while (88 deg F in the summer) and just periodically check on whether ice should be added to the auxiliary jar.

When the process is started, the acid needs to already be at 20 deg C (68 deg F). I get it there by holding it in an insulated chest with some blue ice for about an hour.

Recently I had been getting some cosmetic failures of the process, dark smudges under the anodize coating. I have been able to eliminate these by switching from titanium to aluminum for the anode rack. Not sure why the smudges were developing; I was using grade 2 titanium and so not introducing rogue metals.

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