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Cast Aluminum

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  • Cast Aluminum

    I know I was told by the forum moderators to leave the cast aluminum on the shelf for now but I have anodized quite a few pieces of the 6xxx bar stock and I even went out and purchased some aluminum stock at the hardware store and anodized them with the same good results. My setup is the standard Caswell deluxe kit using the 20 amp CC power supply sold by Caswell.

    The piece of cast aluminum was a large boat cleat with a total SF of 45 square inches. I decided to anodize this piece using 6 amps per square foot. My calculations arrived at 416mA per square inch x 45 square inches and I then set my constant current power supply for 1.9 amps. Please correct me if I have made any mistakes, I’m new to this procedure and seeking any help I can get.

    I decided to make voltage readings at ten minute intervals and they where as follows:

    Start 8.6V
    10 min. 9.9
    20 min. 10.3
    30 min. 10.5
    40 min. 10.8
    50 min. 10.9
    60 min. 11.0
    80 min. 11.0
    90 min. 11.0V

    I removed the cleat after 90 minutes rinsed it in sodium bicarbonate solution and then distilled water. I plan on just anodizing the cleats and polishing them but I wanted to check if this cleat would hold dye and to my surprise it did. The cleat is deep black in color with no signs yet of discoloration.

    On a normal run I watch and record the voltage, it usually drops through out the run. I could see on this run that the voltage immediately started climbing, was this my first experience with dissolution? Could you explain what happen during this run and how I could improve my results? Thanks again for your help.

  • #2
    More good work, you are a quick study.

    Your calculations are right, but you wound up with a decimal point in the wrong place; 41.6mA not 416mA.

    Did you use your Sputwelder? If you did, you avoided completely the normal connection hassles most every beginner has to live through. I'll bet the pieces of barstock all look exactly alike. You're certainly ready for something more difficult.

    Your voltage readings look textbook perfect for 6ASF and LCD electrolyte when operating in CC mode.
    The rapid rise in voltage in the first few minutes is what you see as the work, which has no anodic coating starts to form one. It's resistance starts very low (its bare aluminum) and increases as the anodic coating forms. The PS responds to this by a low voltage at the beginning which rises quickly. Now, that is what happens when the PS is operated in Constant Current Mode (CC). The PS is holding the current constant at 1.9A, throughout your entire anodization. The low voltage indicates low resistance, a very thin coating or no coating. The climbing voltage indicates higher resistance, as the coating grows thicker. You should see on the amp meter that the current stayed at 1.9A.

    Dissolution is the action of the sulfuric acid dissolving the coating that you have grown. It is accelerated by a rise in electrolyte temperature, and by an increase in acid concentration. It has nothing to do with current density, and is occurring the entire time the work is in the tank. It dissolves your pore structure from the inside out, making the pores larger as time progresses. Ironically, some dissolution is necessary, if you stopped it completely you would never form a pore structure at all.

    The problem arrises when the dissolution becomes excessive, and is eating the coating away too fast. If this was happening you would have seen the voltage rise rapidly as before, but it would then start to droop over time, slowly going down for the rest of the anodization. Dyeing and sealing problems are common with excessive dissolution, and the anodize coating has been weakened considerably. When the work is dry you commonly notice a fine powder, the color of the dye coming off the work. This is the outer layer of your anodize coating what was destroyed by excessive dissolution. A bad thing.

    On a normal run I watch and record the voltage, it usually drops through out the run.
    If this is right, are you sure that you set up your PS correctly for CC mode? When in CC mode, the current (amps) stays at the value you set it for, and the voltage changes. The voltage going down as the anodic layer forms is what would happen in Constant Voltage (CV) mode. This isn't good, the current density is also going down with the voltage in CV mode.

    Get into the habit of measuring and recording the electrolyte temperature just before you start, and again when you finish. You want it to stay between 70 and 75 deg. F. If it goes up much beyond that you will have your first experience with excessive dissolution.


    • #3
      Thank you for the encouragement. Yes I did use the sput welder and indeed all pieces dyed black are the same density. After reading your reply I had to go back and lay all of the pieces out in the same light and look at them.

      I will watch the decimal point next time.

      Yes I have been measuring the temperature before and after a run and it climbs on average about 5 degrees. I have 2 zip-lock bags that I fill with ice and place in the bath to drop the temperature between runs.

      I’m looking through my note book and in-fact the early run I made this morning was a 10.5 inch long square open tube with 1 inch sides. I calculated one side at h x l (10.5*1=10.5 square inches and multiplied it by 8 sides, 4 inside and 4 outside. Is this correct? I then took my total square inches of 84 and multiplied it by 30mA and got 2.5amps at a 4.5 CD. My starting voltage was 15.3 volts, at 5 minutes my voltage was 14.9 and at 90 minutes my voltage was 12.8 volts. Current stayed at 2.5 amps through out the entire run. Thanks again for all the information it is a great help!


      • #4
        Good. You are seeing the same thing Neilfj, M_D, and I have seen; the superior connection the Sputwelder provides makes a marked improvement in the uniformity of the work.

        What you describe with the square tube is indeed a mild case of dissolution. Your surface area calculations are a little off, but close enough. I'm assuming it wasn't bad enough to show any powder after it is dry. You probably don't know the alloy, if it was something in the 2xxx series, like 2024, it would have been better to use 6ASF rather than 4.5ASF. You can look at coating growth (controlled by the ASF) as a race against dissolution (controlled by temperature) you want coating growth to win the race (the curve slopes up and not down). Long hollow tubes can be difficult to clean and completely remove the native oxide from the inside, without a beadblaster (highly recommended).

        Any anodized and dyed sample will be real close to the right color and shade when its completely dry, without any grease, oil, wax, or anything else applied as a final step. If you don't have this you have a dissolution issue to some degree.

        Its time for you to take a look at some anodization curves; look for three threads here titled "Fibergeek's Anodization Curves". You will have some questions after you read them.


        • #5
          Thank You, I will have a look at your anodization curves this weekend. I really appreciate the ability to have my days work reviewed and critiqued all in the same day.


          • #6
            Speaking for myself (Pure Power Systems) and Caswell, Inc. who is footing the bill for this forum, we aim to support our customers, the best we possibly can. We can't always guarantee a critique the same day, but we will certainly try.