When anodizing, power is dissipated in the cell and heats the water/acid electrolyte. The heat has two components, electrical resistance heating of the anode’s barrier layer and heat of formation of the oxide film. For example, if the anode area is 16 square inches then the current required is 1.33 amps (12 amps/square foot) and the voltage will be about 14 volts so the the electrical resistance heating is 18.7 watts. The heat of formation of aluminum oxide film is 140 BTU/(sq. ft. * mil) so putting on 1 mil of oxide in 1 hour is another 4.6 watts. The total power that must be removed to keep the electrolyte at constant temperature is 23.3 watts. If this is not removed, then 24 oz. of electrolyte will rise 53 deg F in one hour, and the process will not be successful.
The above figures describe the limit of my small anodizing cells when resting in an icewater bath at about 35 deg F. The walls of the polypropylene jar are good insulators, and conduct only about 23 watts of heat when the electrolyte is at 68 deg F, for a temperatue difference of 33 deg F. Earlier posts (1 Aug 2011, 30 July 2012) on this blog show the water bath setup. Sixteen square inches is smaller than a complete reel frame, so I have had to anodize reels as subassemblies.
To overcome the limit, but more significantly to eliminate the bother of extracting ice cubes from the refrigerator, I decided to try water cooling the cathode. I live in a rural area and so have a well supplying ground water. Here in August, the cold water is emerging at 54 deg F, 14 degrees below the desired cell temperature. Below is the cell lid/cathode assembly that I built.
Here you see a cooling loop of 3003 tubing, a J shaped tube (also of 1/4 inch 3003 tube) for air bubbling, and a plugged straight tube (6061 aluminum) that is a well for a thermometer. These are electrically tied together to form the cathode. The total surface area of the cathode is much less than 16 square inches, but I learned from a reference book that anodizing cathodes are typically about one third the area of the anode. The 3003 tubing is soft enough to be easily bent by hand. Air bubbling keeps the electrolyte from becoming thermally stratified; air is provided by an aquarium pump.
Before making the complete cell, I tested the heat removal capacity by cooling some hot water from the household taps. The cell temperature should show an exponential decay of temperature with time.
Here I have plotted a log function of the cell temperature in order to turn the exponential curve into a straight line. From this plot, I deduce that the heat removal capacity of the new cathode is 53 watts, for a ground water temperature of 54 deg F. In my new reel design, I need 2.44 amps to anodize the complete frame plus the spool hub. The heat to be removed is 43 watts.
Here is a picture of the anodizing in process. The plastic “shoebox” is no longer a water bath, but just a safety basin for acid drips.
With water running steadily, the electrolyte temperature fell slowly, perhaps 5 deg F in 20 minutes. I regulate electrolyte temperature by turning cooling water on/off.
Here I have extracted the lid/electrode assembly after 1 hour. Note that the 6061 thermometer tube has a dark deposit (which can be wiped off) but the 3003 tubes, having less alloy metal content, stay bright.
I have no instrument for measuring the oxide film thickness. But there are two ways to obtain some assurance that a film did develop. First, the film is an insulator (touch with the probes of a multimeter set to its continuity test function). Second, rings of conductive aluminum can be seen where the reel pillars touched the endplates and the foot.