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Trouble Getting Al (Type III) Anodizing to work

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  • Trouble Getting Al (Type III) Anodizing to work

    Hello, I posted this in the "Hard Anodizing" pinned post but haven't gotten a response so figured I'd just make a new post. I'm a materials engineer familiar with electrochemical processes and surface etching but new to anodizing.

    I'm doing some R&D and trying to see if I can Type III anodize small aluminum parts in preparation to create some non-conductive gaskets and I keep getting partial success or complete failure. I'm hoping someone can clarify some things for me and point out my mistakes.

    My test samples are 1.25inx1.25inx0.62in cut from Aluminum sheet.

    I'm performing this in a fume hood and using either homemade or generic solutions. I am -currently- using SP degreaser, HCl 34% for Desmut (Nitric wasn't doing anything), and a 1:2 H2SO4:H2O bath which is in a polymer container and placed in a larger polymer container filled with water and frozen overnight to make sure the bath temp is 32F. Part is suspended using a titanium wire and rinsed in distilled water between each step and I soak in acetone to clean before each attempt. The present current is 0.1A for 120min. The process is conducted over a magnetic stirring plate for agitation.

    I started out following the instructions for standard Type III Anodizing Al from the Caswell Plating Manual Version X (3:1 Acid:water bath, Nitric DeOx/Desmut, 0.025A/sq in for 120 min using the 720 rule) and got partially passivated results, the current was at 0.028A and voltage would rise to about 35V. Then saw the/a founder of Caswell (Mike) respond to a post here: https://forum.caswellplating.com/for...nodizing-setup saying that you had to dilute the acid and increase Amps. So I've been doing 1:2 acid:water bath at higher amps and tried that using HCl as the DeOx/desmut, to less success. With 0.1A the voltage doesn't go above 3.24V.

    Please let me know your thoughts or corrections!

    Many thanks.

    -Perry

  • #2
    Type III anodizing is typically done at 24 amps/sqft. We see some shops do a "commercial hard coat" at 18ASF, and we also see runs as high as 36ASF, but the norm is 24ASF. Because of the high current density, you have to have a cold bath otherwise the part will burn. Other than the current density and temperature, Type III runs the same as Type II, only longer.

    The 720 rule says that running at 12 ASF for one hour will produce a 1 mil coating, so to get 2 mils (the typical Type III thickness) you will run at 24ASF for about an hour.

    Your part is 1.25in X 1.25in = 1.5625 sqin or (dividing by 144 sqin/sqft) you get 0.01085 sqft. If you are anodizing both sides, then double the area to get 0.0217 sqft. To run this at 24ASF you should be running at ~0.52 A.

    Regarding chemistry, as a general rule, you should never be using HCl, or any other chemical with chlorides or the parts will pit (this is one major reason not to use any tap water in your process). I suppose if it's simply a test run and you are using HCl prior to the anodizing bath, you would be ok, but for production, ditch the HCl.

    It's hard to know if your H2SO4 concentration is correct or not. Saying "1:2 H2SO4:H2O" doesn't give any indication of the sulfuric concentration unless you know the starting concentration of the acid. Normaly we state the acid concentration in terms of grams/Liter of H2SO4. A proper anodizing bath (either Type II or Type III) should be in the range of 180grams/Liter and there should be at least 3-4 grams/Liter of dissolved Al, although some would say this is not needed.

    The amount of voltage and voltage increase you see, depends on a number of factors including, bath temperature rises, your actual sulfuric concentration and the alloy of the part. A typical 6061 Type III run at 24 ASF should start somewhere around 14 volts and could climb as high as 60-70 or more.

    HTH and GL.
    Kevin
    Process control doesn't give you good quality, it gives you consistent quality.
    Good quality comes from consistently doing the right things.

    Process control systems for Anodizers
    If a post helps you out LIKE the post
    I'm not an Amateur Metal Finisher. I've just been around the industry for a dozen years or so helping and consulting when and where I can.

    Comment


    • #3
      Hey Kevin,

      Thanks so much this does clear up a lot! I was trying to find a middle ground between the two processes (Type II & III) described in the manual in terms of calculated ASF, but this tells me I was wrong in doing so. I know using HCl is generally not a great idea but wasn't finding much about non-nitric common systems and was using it as a test before the anodizing bath and dipped in DI water before beginning to anodize. But NaOH seems to be the go-to so I will switch to that. By 1:2 I mean 33% H2SO4 to 66% DI. But I will add the dissolved Al.

      I appreciate your help!

      -Perry

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      • #4
        What would you suggest to use as a de-ox/de-smut acid instead of Nitric?

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        • #5
          I can't speak too much about chemistries outside of the anodize bath, but I believe nitric is your best option.

          Just to be clear about the process, you should going into these tanks in order,
          Cleaner
          Rinse
          Etch (strong NaOH solution)
          Rinse

          At this point you should see a black sooty film on the parts. This is what you need to remove with the deox.
          Deox. Room temp ~3-4 minutes depending on the alloy.
          Rinse
          Ano
          Rinse
          Last edited by KevinB; 05-26-2021, 05:14 PM.
          Process control doesn't give you good quality, it gives you consistent quality.
          Good quality comes from consistently doing the right things.

          Process control systems for Anodizers
          If a post helps you out LIKE the post
          I'm not an Amateur Metal Finisher. I've just been around the industry for a dozen years or so helping and consulting when and where I can.

          Comment

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