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Anodizing the inside of a tube

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  • Anodizing the inside of a tube

    Pictures of setup and product after anodization are in my personal gallery

    I have been attempting to anodize the inside of an aluminum tube, in my case my son’s paint ball marker barrel. I read through the Caswell manual "Use of Conforming Anodes" in my case; cathodes and got some great tips from Fibergeek.

    I purchased the parts from Home Depot, Pet smart and McMaster-Carr. Parts purchased where
    Home Depot
    1”X3” Schedule 80 threaded nipple
    1” Schedule 80 coupler
    1” Schedule 80 Flange
    ½” OD x 3/8” ID vinyl tubing
    3/8” OD x ¼” ID vinyl tubing

    McMaster- Carr
    GP aluminum Tubing .15 ID, 1/4” OD, .049” wall thickness

    Pet smart
    Fluval mag drive pump.

    One RSS6 silicone plug from the Argon masking kit.

    I’m not sure how long the pump will last in the acid bath. I tested it out for a few hours before attempting to anodize. I took it out of the bath, rinsed it and disassembled it. My inspection showed no signs of acid damaged so I went ahead and attempted to anodize a tube inside and out.

    The ¼” aluminum tube has small holes I drilled through it at various lengths. The tube was placed through a predrilled hole in the silicone plug and the plug is pushed into the PVC coupler. The bottom end of the tube has a 3/8” OD 1/4" ID vinyl tube coming out and connects to the output of the pump via 1/2" OD vinyl tube. This allows fluid to pass through the holes in the tubing so electrolyte can flow inside the tubing/barrel during anodization.

    The ¼” aluminum tube on the rig has a jumper cable running from the top of the tube to one of the tank cathodes. This makes the ¼” aluminum tubing an inside cathode. I placed a small section of vinyl tubing on the bottom of the inside cathode, I cut a small PVC disk slightly smaller than the inside diameter of the tube/barrel being anodized and a section of vinyl tubing at the top of the cathode tube. These PVC and vinyl pieces act as electrical insulators and spacers. The Caswell manual says if the inside diameter of the tube being anodized is not equal distant all the way around the cathode tube one side of the tubing will receive a thicker coating than the other side.

    I welded a piece of 16 AWG aluminum wire using my Pure Power Systems CDW welder to the inside neck of the barrel and placed the barrel on the rig with the front of the barrel facing down but left some space between the bottom of the barrel and the silicon plug to insure that the front of the barrel anodizes. I dropped the entire rig with pump in the tank and connected the tank cathode to the inside barrel cathode via jumper cable, tied off the 16 AWG welded aluminum wire to my anode bar and fired up the pump.

    I had tested the pump/cathode tube before hand in fresh water and made sure that the fluid was flowing through the small holes I drilled through the cathode tube.

    The piece I was anodizing was a section of an aluminum paintball barrel 9 inches long by ¾” in diameter. 2 x 3.14 x 0.5 x .75 x 9 = 21.195 x 2 (outside and inside diameter) = 42.39 sq inches. Using 6 amps per square foot 6/144sq inches = 0.0416 X 42.39 = 1.76 Amps.

    I set my CC power supply to 1.8 amps and timed it for 90 minutes. My voltage was in the 15 volt range for most of the time. The barrel dyed perfectly inside and out. I did a ohm test on the inside of the barrel and the meter read opened but I know this is not an accurate measurement and have no idea if I was able to place a proper coating on the inside of the barrel. Once again I am open to any and all advice, criticisms or improvements you have to offer.

    Pictures of setup and product after anodization are in my personal gallery

  • #2
    Very nice work, Scocklin.

    By the looks of your setup in the backround of your pictures, you are really getting into this.

    Your aquarium pump will be OK if you remove it after you're finished and rinse it out. People have experienced degradation with non-acid rated pumps if they leave them in the electrolye indefinately.